gcc 文档
[root@rockylinux docs]# man gcc
GCC(1) GNU GCC(1)
NAME
gcc - GNU project C and C++ compiler
SYNOPSIS
gcc [-c|-S|-E] [-std=standard]
[-g] [-pg] [-Olevel]
[-Wwarn...] [-Wpedantic]
[-Idir...] [-Ldir...]
[-Dmacro[=defn]...] [-Umacro]
[-foption...] [-mmachine-option...]
[-o outfile] [@file] infile...
Only the most useful options are listed here; see below for the remainder. g++
accepts mostly the same options as gcc.
DESCRIPTION
When you invoke GCC, it normally does preprocessing, compilation, assembly and
linking. The "overall options" allow you to stop this process at an intermediate
stage. For example, the -c option says not to run the linker. Then the output
consists of object files output by the assembler.
Other options are passed on to one or more stages of processing. Some options
control the preprocessor and others the compiler itself. Yet other options control
the assembler and linker; most of these are not documented here, since you rarely
need to use any of them.
Most of the command-line options that you can use with GCC are useful for C
programs; when an option is only useful with another language (usually C++), the
explanation says so explicitly. If the description for a particular option does
not mention a source language, you can use that option with all supported
languages.
The usual way to run GCC is to run the executable called gcc, or machine-gcc when
cross-compiling, or machine-gcc-version to run a specific version of GCC. When you
compile C++ programs, you should invoke GCC as g++ instead.
The gcc program accepts options and file names as operands. Many options have
multi-letter names; therefore multiple single-letter options may not be grouped:
-dv is very different from -d -v.
You can mix options and other arguments. For the most part, the order you use
doesn't matter. Order does matter when you use several options of the same kind;
for example, if you specify -L more than once, the directories are searched in the
order specified. Also, the placement of the -l option is significant.
Many options have long names starting with -f or with -W---for example,
-fmove-loop-invariants, -Wformat and so on. Most of these have both positive and
negative forms; the negative form of -ffoo is -fno-foo. This manual documents only
one of these two forms, whichever one is not the default.
OPTIONS
Option Summary
Here is a summary of all the options, grouped by type. Explanations are in the
following sections.
Overall Options
-c -S -E -o file -x language -v -### --help[=class[,...]] --target-help
--version -pass-exit-codes -pipe -specs=file -wrapper @file
-ffile-prefix-map=old=new -fplugin=file -fplugin-arg-name=arg
-fdump-ada-spec[-slim] -fada-spec-parent=unit -fdump-go-spec=file
C Language Options
-ansi -std=standard -fgnu89-inline -fpermitted-flt-eval-methods=standard
-aux-info filename -fallow-parameterless-variadic-functions -fno-asm
-fno-builtin -fno-builtin-function -fgimple -fhosted -ffreestanding
-fopenacc -fopenmp -fopenmp-simd -fms-extensions -fplan9-extensions
-fsso-struct=endianness -fallow-single-precision -fcond-mismatch
-flax-vector-conversions -fsigned-bitfields -fsigned-char -funsigned-bitfields
-funsigned-char
C++ Language Options
-fabi-version=n -fno-access-control -faligned-new=n -fargs-in-order=n
-fcheck-new -fconstexpr-depth=n -fconstexpr-loop-limit=n -ffriend-injection
-fno-elide-constructors -fno-enforce-eh-specs -ffor-scope -fno-for-scope
-fno-gnu-keywords -fno-implicit-templates -fno-implicit-inline-templates
-fno-implement-inlines -fms-extensions -fnew-inheriting-ctors
-fnew-ttp-matching -fno-nonansi-builtins -fnothrow-opt -fno-operator-names
-fno-optional-diags -fpermissive -fno-pretty-templates -frepo -fno-rtti
-fsized-deallocation -ftemplate-backtrace-limit=n -ftemplate-depth=n
-fno-threadsafe-statics -fuse-cxa-atexit -fno-weak -nostdinc++
-fvisibility-inlines-hidden -fvisibility-ms-compat -fext-numeric-literals
-Wabi=n -Wabi-tag -Wconversion-null -Wctor-dtor-privacy
-Wdelete-non-virtual-dtor -Wliteral-suffix -Wmultiple-inheritance
-Wnamespaces -Wnarrowing -Wnoexcept -Wnoexcept-type -Wclass-memaccess
-Wnon-virtual-dtor -Wreorder -Wregister -Weffc++ -Wstrict-null-sentinel
-Wtemplates -Wno-non-template-friend -Wold-style-cast -Woverloaded-virtual
-Wno-pmf-conversions -Wsign-promo -Wvirtual-inheritance
Objective-C and Objective-C++ Language Options
-fconstant-string-class=class-name -fgnu-runtime -fnext-runtime
-fno-nil-receivers -fobjc-abi-version=n -fobjc-call-cxx-cdtors
-fobjc-direct-dispatch -fobjc-exceptions -fobjc-gc -fobjc-nilcheck
-fobjc-std=objc1 -fno-local-ivars
-fivar-visibility=[public|protected|private|package] -freplace-objc-classes
-fzero-link -gen-decls -Wassign-intercept -Wno-protocol -Wselector
-Wstrict-selector-match -Wundeclared-selector
Diagnostic Message Formatting Options
-fmessage-length=n -fdiagnostics-show-location=[once|every-line]
-fdiagnostics-color=[auto|never|always] -fno-diagnostics-show-option
-fno-diagnostics-show-caret -fdiagnostics-parseable-fixits
-fdiagnostics-generate-patch -fdiagnostics-show-template-tree -fno-elide-type
-fno-show-column
Warning Options
-fsyntax-only -fmax-errors=n -Wpedantic -pedantic-errors -w -Wextra -Wall
-Waddress -Waggregate-return -Waligned-new -Walloc-zero
-Walloc-size-larger-than=n -Walloca -Walloca-larger-than=n
-Wno-aggressive-loop-optimizations -Warray-bounds -Warray-bounds=n
-Wno-attributes -Wbidirectional=[none|unpaired|any] -Wbool-compare
-Wbool-operation -Wno-builtin-declaration-mismatch -Wno-builtin-macro-redefined
-Wc90-c99-compat -Wc99-c11-compat -Wc++-compat -Wc++11-compat -Wc++14-compat
-Wcast-align -Wcast-align=strict -Wcast-function-type -Wcast-qual
-Wchar-subscripts -Wchkp -Wcatch-value -Wcatch-value=n -Wclobbered
-Wcomment -Wconditionally-supported -Wconversion -Wcoverage-mismatch
-Wno-cpp -Wdangling-else -Wdate-time -Wdelete-incomplete -Wno-deprecated
-Wno-deprecated-declarations -Wno-designated-init -Wdisabled-optimization
-Wno-discarded-qualifiers -Wno-discarded-array-qualifiers -Wno-div-by-zero
-Wdouble-promotion -Wduplicated-branches -Wduplicated-cond -Wempty-body
-Wenum-compare -Wno-endif-labels -Wexpansion-to-defined -Werror -Werror=*
-Wextra-semi -Wfatal-errors -Wfloat-equal -Wformat -Wformat=2
-Wno-format-contains-nul -Wno-format-extra-args -Wformat-nonliteral
-Wformat-overflow=n -Wformat-security -Wformat-signedness
-Wformat-truncation=n -Wformat-y2k -Wframe-address -Wframe-larger-than=len
-Wno-free-nonheap-object -Wjump-misses-init -Wif-not-aligned
-Wignored-qualifiers -Wignored-attributes -Wincompatible-pointer-types
-Wimplicit -Wimplicit-fallthrough -Wimplicit-fallthrough=n
-Wimplicit-function-declaration -Wimplicit-int -Winit-self -Winline
-Wno-int-conversion -Wint-in-bool-context -Wno-int-to-pointer-cast
-Winvalid-memory-model -Wno-invalid-offsetof -Winvalid-pch -Wlarger-than=len
-Wlogical-op -Wlogical-not-parentheses -Wlong-long -Wmain
-Wmaybe-uninitialized -Wmemset-elt-size -Wmemset-transposed-args
-Wmisleading-indentation -Wmissing-attributes -Wmissing-braces
-Wmissing-field-initializers -Wmissing-include-dirs -Wno-multichar
-Wmultistatement-macros -Wnonnull -Wnonnull-compare
-Wnormalized=[none|id|nfc|nfkc] -Wnull-dereference -Wodr -Wno-overflow
-Wopenmp-simd -Woverride-init-side-effects -Woverlength-strings -Wpacked
-Wpacked-bitfield-compat -Wpacked-not-aligned -Wpadded -Wparentheses
-Wno-pedantic-ms-format -Wplacement-new -Wplacement-new=n -Wpointer-arith
-Wpointer-compare -Wno-pointer-to-int-cast -Wno-pragmas -Wredundant-decls
-Wrestrict -Wno-return-local-addr -Wreturn-type -Wsequence-point -Wshadow
-Wno-shadow-ivar -Wshadow=global, -Wshadow=local, -Wshadow=compatible-local
-Wshift-overflow -Wshift-overflow=n -Wshift-count-negative
-Wshift-count-overflow -Wshift-negative-value -Wsign-compare
-Wsign-conversion -Wfloat-conversion -Wno-scalar-storage-order
-Wsizeof-pointer-div -Wsizeof-pointer-memaccess -Wsizeof-array-argument
-Wstack-protector -Wstack-usage=len -Wstrict-aliasing -Wstrict-aliasing=n
-Wstrict-overflow -Wstrict-overflow=n -Wstringop-overflow=n
-Wstringop-truncation -Wsuggest-attribute=[pure|const|noreturn|format|malloc]
-Wsuggest-final-types -Wsuggest-final-methods -Wsuggest-override
-Wmissing-format-attribute -Wsubobject-linkage -Wswitch -Wswitch-bool
-Wswitch-default -Wswitch-enum -Wswitch-unreachable -Wsync-nand
-Wsystem-headers -Wtautological-compare -Wtrampolines -Wtrigraphs
-Wtype-limits -Wundef -Wuninitialized -Wunknown-pragmas
-Wunsuffixed-float-constants -Wunused -Wunused-function -Wunused-label
-Wunused-local-typedefs -Wunused-macros -Wunused-parameter -Wno-unused-result
-Wunused-value -Wunused-variable -Wunused-const-variable
-Wunused-const-variable=n -Wunused-but-set-parameter -Wunused-but-set-variable
-Wuseless-cast -Wvariadic-macros -Wvector-operation-performance -Wvla
-Wvla-larger-than=n -Wvolatile-register-var -Wwrite-strings
-Wzero-as-null-pointer-constant -Whsa
C and Objective-C-only Warning Options
-Wbad-function-cast -Wmissing-declarations -Wmissing-parameter-type
-Wmissing-prototypes -Wnested-externs -Wold-style-declaration
-Wold-style-definition -Wstrict-prototypes -Wtraditional
-Wtraditional-conversion -Wdeclaration-after-statement -Wpointer-sign
Debugging Options
-g -glevel -gdwarf -gdwarf-version -ggdb -grecord-gcc-switches
-gno-record-gcc-switches -gstabs -gstabs+ -gstrict-dwarf -gno-strict-dwarf
-gas-loc-support -gno-as-loc-support -gas-locview-support
-gno-as-locview-support -gcolumn-info -gno-column-info -gstatement-frontiers
-gno-statement-frontiers -gvariable-location-views
-gno-variable-location-views -ginternal-reset-location-views
-gno-internal-reset-location-views -ginline-points -gno-inline-points -gvms
-gxcoff -gxcoff+ -gz[=type] -fdebug-prefix-map=old=new -fdebug-types-section
-fno-eliminate-unused-debug-types -femit-struct-debug-baseonly
-femit-struct-debug-reduced -femit-struct-debug-detailed[=spec-list]
-feliminate-unused-debug-symbols -femit-class-debug-always
-fno-merge-debug-strings -fno-dwarf2-cfi-asm -fvar-tracking
-fvar-tracking-assignments
Optimization Options
-faggressive-loop-optimizations -falign-functions[=n] -falign-jumps[=n]
-falign-labels[=n] -falign-loops[=n] -fassociative-math -fauto-profile
-fauto-profile[=path] -fauto-inc-dec -fbranch-probabilities
-fbranch-target-load-optimize -fbranch-target-load-optimize2
-fbtr-bb-exclusive -fcaller-saves -fcombine-stack-adjustments
-fconserve-stack -fcompare-elim -fcprop-registers -fcrossjumping
-fcse-follow-jumps -fcse-skip-blocks -fcx-fortran-rules -fcx-limited-range
-fdata-sections -fdce -fdelayed-branch -fdelete-null-pointer-checks
-fdevirtualize -fdevirtualize-speculatively -fdevirtualize-at-ltrans -fdse
-fearly-inlining -fipa-sra -fexpensive-optimizations -ffat-lto-objects
-ffast-math -ffinite-math-only -ffloat-store -fexcess-precision=style
-fforward-propagate -ffp-contract=style -ffunction-sections -fgcse
-fgcse-after-reload -fgcse-las -fgcse-lm -fgraphite-identity -fgcse-sm
-fhoist-adjacent-loads -fif-conversion -fif-conversion2 -findirect-inlining
-finline-functions -finline-functions-called-once -finline-limit=n
-finline-small-functions -fipa-cp -fipa-cp-clone -fipa-bit-cp -fipa-vrp
-fipa-pta -fipa-profile -fipa-pure-const -fipa-reference -fipa-icf
-fira-algorithm=algorithm -flive-patching=level -fira-region=region
-fira-hoist-pressure -fira-loop-pressure -fno-ira-share-save-slots
-fno-ira-share-spill-slots -fisolate-erroneous-paths-dereference
-fisolate-erroneous-paths-attribute -fivopts -fkeep-inline-functions
-fkeep-static-functions -fkeep-static-consts -flimit-function-alignment
-flive-range-shrinkage -floop-block -floop-interchange -floop-strip-mine
-floop-unroll-and-jam -floop-nest-optimize -floop-parallelize-all -flra-remat
-flto -flto-compression-level -flto-partition=alg -fmerge-all-constants
-fmerge-constants -fmodulo-sched -fmodulo-sched-allow-regmoves
-fmove-loop-invariants -fno-branch-count-reg -fno-defer-pop
-fno-fp-int-builtin-inexact -fno-function-cse -fno-guess-branch-probability
-fno-inline -fno-math-errno -fno-peephole -fno-peephole2
-fno-printf-return-value -fno-sched-interblock -fno-sched-spec
-fno-signed-zeros -fno-toplevel-reorder -fno-trapping-math
-fno-zero-initialized-in-bss -fomit-frame-pointer -foptimize-sibling-calls
-fpartial-inlining -fpeel-loops -fpredictive-commoning -fprefetch-loop-arrays
-fprofile-correction -fprofile-use -fprofile-use=path -fprofile-values
-fprofile-reorder-functions -freciprocal-math -free -frename-registers
-freorder-blocks -freorder-blocks-algorithm=algorithm
-freorder-blocks-and-partition -freorder-functions -frerun-cse-after-loop
-freschedule-modulo-scheduled-loops -frounding-math -fsched2-use-superblocks
-fsched-pressure -fsched-spec-load -fsched-spec-load-dangerous
-fsched-stalled-insns-dep[=n] -fsched-stalled-insns[=n]
-fsched-group-heuristic -fsched-critical-path-heuristic
-fsched-spec-insn-heuristic -fsched-rank-heuristic -fsched-last-insn-heuristic
-fsched-dep-count-heuristic -fschedule-fusion -fschedule-insns
-fschedule-insns2 -fsection-anchors -fselective-scheduling
-fselective-scheduling2 -fsel-sched-pipelining
-fsel-sched-pipelining-outer-loops -fsemantic-interposition -fshrink-wrap
-fshrink-wrap-separate -fsignaling-nans -fsingle-precision-constant
-fsplit-ivs-in-unroller -fsplit-loops -fsplit-paths -fsplit-wide-types
-fssa-backprop -fssa-phiopt -fstdarg-opt -fstore-merging -fstrict-aliasing
-fthread-jumps -ftracer -ftree-bit-ccp -ftree-builtin-call-dce -ftree-ccp
-ftree-ch -ftree-coalesce-vars -ftree-copy-prop -ftree-dce
-ftree-dominator-opts -ftree-dse -ftree-forwprop -ftree-fre -fcode-hoisting
-ftree-loop-if-convert -ftree-loop-im -ftree-phiprop -ftree-loop-distribution
-ftree-loop-distribute-patterns -ftree-loop-ivcanon -ftree-loop-linear
-ftree-loop-optimize -ftree-loop-vectorize -ftree-parallelize-loops=n
-ftree-pre -ftree-partial-pre -ftree-pta -ftree-reassoc -ftree-sink
-ftree-slsr -ftree-sra -ftree-switch-conversion -ftree-tail-merge -ftree-ter
-ftree-vectorize -ftree-vrp -funconstrained-commons -funit-at-a-time
-funroll-all-loops -funroll-loops -funsafe-math-optimizations
-funswitch-loops -fipa-ra -fvariable-expansion-in-unroller -fvect-cost-model
-fvpt -fweb -fwhole-program -fwpa -fuse-linker-plugin --param name=value -O
-O0 -O1 -O2 -O3 -Os -Ofast -Og
Program Instrumentation Options
-p -pg -fprofile-arcs --coverage -ftest-coverage -fprofile-abs-path
-fprofile-dir=path -fprofile-generate -fprofile-generate=path
-fsanitize=style -fsanitize-recover -fsanitize-recover=style
-fasan-shadow-offset=number -fsanitize-sections=s1,s2,...
-fsanitize-undefined-trap-on-error -fbounds-check -fcheck-pointer-bounds
-fchkp-check-incomplete-type -fchkp-first-field-has-own-bounds
-fchkp-narrow-bounds -fchkp-narrow-to-innermost-array -fchkp-optimize
-fchkp-use-fast-string-functions -fchkp-use-nochk-string-functions
-fchkp-use-static-bounds -fchkp-use-static-const-bounds
-fchkp-treat-zero-dynamic-size-as-infinite -fchkp-check-read -fchkp-check-read
-fchkp-check-write -fchkp-store-bounds -fchkp-instrument-calls
-fchkp-instrument-marked-only -fchkp-use-wrappers
-fchkp-flexible-struct-trailing-arrays
-fcf-protection=[full|branch|return|none] -fstack-protector
-fstack-protector-all -fstack-protector-strong -fstack-protector-explicit
-fstack-check -fstack-limit-register=reg -fstack-limit-symbol=sym
-fno-stack-limit -fsplit-stack -fvtable-verify=[std|preinit|none] -fvtv-counts
-fvtv-debug -finstrument-functions
-finstrument-functions-exclude-function-list=sym,sym,...
-finstrument-functions-exclude-file-list=file,file,...
Preprocessor Options
-Aquestion=answer -A-question[=answer] -C -CC -Dmacro[=defn] -dD -dI -dM
-dN -dU -fdebug-cpp -fdirectives-only -fdollars-in-identifiers
-fexec-charset=charset -fextended-identifiers -finput-charset=charset
-fmacro-prefix-map=old=new -fno-canonical-system-headers -fpch-deps
-fpch-preprocess -fpreprocessed -ftabstop=width -ftrack-macro-expansion
-fwide-exec-charset=charset -fworking-directory -H -imacros file -include
file -M -MD -MF -MG -MM -MMD -MP -MQ -MT -no-integrated-cpp -P
-pthread -remap -traditional -traditional-cpp -trigraphs -Umacro -undef
-Wp,option -Xpreprocessor option
Assembler Options
-Wa,option -Xassembler option
Linker Options
object-file-name -fuse-ld=linker -llibrary -nostartfiles -nodefaultlibs
-nostdlib -pie -pthread -rdynamic -s -static -static-pie -static-libgcc
-static-libstdc++ -static-libasan -static-libtsan -static-liblsan
-static-libubsan -static-libmpx -static-libmpxwrappers -shared -shared-libgcc
-symbolic -T script -Wl,option -Xlinker option -u symbol -z keyword
Directory Options
-Bprefix -Idir -I- -idirafter dir -imacros file -imultilib dir
-iplugindir=dir -iprefix file -iquote dir -isysroot dir -isystem dir
-iwithprefix dir -iwithprefixbefore dir -Ldir -no-canonical-prefixes
--no-sysroot-suffix -nostdinc -nostdinc++ --sysroot=dir
Code Generation Options
-fcall-saved-reg -fcall-used-reg -ffixed-reg -fexceptions
-fnon-call-exceptions -fdelete-dead-exceptions -funwind-tables
-fasynchronous-unwind-tables -fno-gnu-unique -finhibit-size-directive
-fno-common -fno-ident -fpcc-struct-return -fpic -fPIC -fpie -fPIE
-fno-plt -fno-jump-tables -frecord-gcc-switches -freg-struct-return
-fshort-enums -fshort-wchar -fverbose-asm -fpack-struct[=n]
-fleading-underscore -ftls-model=model -fstack-reuse=reuse_level -ftrampolines
-ftrapv -fwrapv -fvisibility=[default|internal|hidden|protected]
-fstrict-volatile-bitfields -fsync-libcalls
Developer Options
-dletters -dumpspecs -dumpmachine -dumpversion -dumpfullversion -fchecking
-fchecking=n -fdbg-cnt-list -fdbg-cnt=counter-value-list
-fdisable-ipa-pass_name -fdisable-rtl-pass_name -fdisable-rtl-pass-name=range-
list -fdisable-tree-pass_name -fdisable-tree-pass-name=range-list -fdump-noaddr
-fdump-unnumbered -fdump-unnumbered-links -fdump-final-insns[=file]
-fdump-ipa-all -fdump-ipa-cgraph -fdump-ipa-inline -fdump-lang-all
-fdump-lang-switch -fdump-lang-switch-options
-fdump-lang-switch-options=filename -fdump-passes -fdump-rtl-pass
-fdump-rtl-pass=filename -fdump-statistics -fdump-tree-all -fdump-tree-switch
-fdump-tree-switch-options -fdump-tree-switch-options=filename
-fcompare-debug[=opts] -fcompare-debug-second -fenable-kind-pass
-fenable-kind-pass=range-list -fira-verbose=n -flto-report -flto-report-wpa
-fmem-report-wpa -fmem-report -fpre-ipa-mem-report -fpost-ipa-mem-report
-fopt-info -fopt-info-options[=file] -fprofile-report -frandom-seed=string
-fsched-verbose=n -fsel-sched-verbose -fsel-sched-dump-cfg
-fsel-sched-pipelining-verbose -fstats -fstack-usage -ftime-report
-ftime-report-details -fvar-tracking-assignments-toggle -gtoggle
-print-file-name=library -print-libgcc-file-name -print-multi-directory
-print-multi-lib -print-multi-os-directory -print-prog-name=program
-print-search-dirs -Q -print-sysroot -print-sysroot-headers-suffix
-save-temps -save-temps=cwd -save-temps=obj -time[=file]
Machine-Dependent Options
AArch64 Options -mabi=name -mbig-endian -mlittle-endian -mgeneral-regs-only
-mcmodel=tiny -mcmodel=small -mcmodel=large -mstrict-align
-momit-leaf-frame-pointer -mtls-dialect=desc -mtls-dialect=traditional
-mtls-size=size -mfix-cortex-a53-835769 -mfix-cortex-a53-843419
-mlow-precision-recip-sqrt -mlow-precision-sqrt -mlow-precision-div
-mpc-relative-literal-loads -msign-return-address=scope -march=name -mcpu=name
-mtune=name -moverride=string -mverbose-cost-dump -moutline-atomics
Adapteva Epiphany Options -mhalf-reg-file -mprefer-short-insn-regs
-mbranch-cost=num -mcmove -mnops=num -msoft-cmpsf -msplit-lohi -mpost-inc
-mpost-modify -mstack-offset=num -mround-nearest -mlong-calls -mshort-calls
-msmall16 -mfp-mode=mode -mvect-double -max-vect-align=num
-msplit-vecmove-early -m1reg-reg
ARC Options -mbarrel-shifter -mjli-always -mcpu=cpu -mA6 -mARC600 -mA7
-mARC700 -mdpfp -mdpfp-compact -mdpfp-fast -mno-dpfp-lrsr -mea -mno-mpy
-mmul32x16 -mmul64 -matomic -mnorm -mspfp -mspfp-compact -mspfp-fast
-msimd -msoft-float -mswap -mcrc -mdsp-packa -mdvbf -mlock -mmac-d16
-mmac-24 -mrtsc -mswape -mtelephony -mxy -misize -mannotate-align
-marclinux -marclinux_prof -mlong-calls -mmedium-calls -msdata
-mirq-ctrl-saved -mrgf-banked-regs -mlpc-width=width -G num -mvolatile-cache
-mtp-regno=regno -malign-call -mauto-modify-reg -mbbit-peephole -mno-brcc
-mcase-vector-pcrel -mcompact-casesi -mno-cond-exec -mearly-cbranchsi
-mexpand-adddi -mindexed-loads -mlra -mlra-priority-none
-mlra-priority-compact mlra-priority-noncompact -mno-millicode -mmixed-code
-mq-class -mRcq -mRcw -msize-level=level -mtune=cpu -mmultcost=num
-munalign-prob-threshold=probability -mmpy-option=multo -mdiv-rem
-mcode-density -mll64 -mfpu=fpu -mrf16
ARM Options -mapcs-frame -mno-apcs-frame -mabi=name -mapcs-stack-check
-mno-apcs-stack-check -mapcs-reentrant -mno-apcs-reentrant -msched-prolog
-mno-sched-prolog -mlittle-endian -mbig-endian -mbe8 -mbe32 -mfloat-abi=name
-mfp16-format=name -mthumb-interwork -mno-thumb-interwork -mcpu=name
-march=name -mfpu=name -mtune=name -mprint-tune-info
-mstructure-size-boundary=n -mabort-on-noreturn -mlong-calls -mno-long-calls
-msingle-pic-base -mno-single-pic-base -mpic-register=reg -mnop-fun-dllimport
-mpoke-function-name -mthumb -marm -mflip-thumb -mtpcs-frame
-mtpcs-leaf-frame -mcaller-super-interworking -mcallee-super-interworking
-mtp=name -mtls-dialect=dialect -mword-relocations -mfix-cortex-m3-ldrd
-munaligned-access -mneon-for-64bits -mslow-flash-data -masm-syntax-unified
-mrestrict-it -mverbose-cost-dump -mpure-code -mcmse
AVR Options -mmcu=mcu -mabsdata -maccumulate-args -mbranch-cost=cost
-mcall-prologues -mgas-isr-prologues -mint8 -mn_flash=size -mno-interrupts
-mmain-is-OS_task -mrelax -mrmw -mstrict-X -mtiny-stack
-mfract-convert-truncate -mshort-calls -nodevicelib -nodevicespecs
-Waddr-space-convert -Wmisspelled-isr
Blackfin Options -mcpu=cpu[-sirevision] -msim -momit-leaf-frame-pointer
-mno-omit-leaf-frame-pointer -mspecld-anomaly -mno-specld-anomaly
-mcsync-anomaly -mno-csync-anomaly -mlow-64k -mno-low64k -mstack-check-l1
-mid-shared-library -mno-id-shared-library -mshared-library-id=n
-mleaf-id-shared-library -mno-leaf-id-shared-library -msep-data -mno-sep-data
-mlong-calls -mno-long-calls -mfast-fp -minline-plt -mmulticore -mcorea
-mcoreb -msdram -micplb
C6X Options -mbig-endian -mlittle-endian -march=cpu -msim -msdata=sdata-type
CRIS Options -mcpu=cpu -march=cpu -mtune=cpu -mmax-stack-frame=n
-melinux-stacksize=n -metrax4 -metrax100 -mpdebug -mcc-init
-mno-side-effects -mstack-align -mdata-align -mconst-align -m32-bit -m16-bit
-m8-bit -mno-prologue-epilogue -mno-gotplt -melf -maout -melinux -mlinux
-sim -sim2 -mmul-bug-workaround -mno-mul-bug-workaround
CR16 Options -mmac -mcr16cplus -mcr16c -msim -mint32 -mbit-ops
-mdata-model=model
Darwin Options -all_load -allowable_client -arch -arch_errors_fatal
-arch_only -bind_at_load -bundle -bundle_loader -client_name
-compatibility_version -current_version -dead_strip -dependency-file
-dylib_file -dylinker_install_name -dynamic -dynamiclib
-exported_symbols_list -filelist -flat_namespace -force_cpusubtype_ALL
-force_flat_namespace -headerpad_max_install_names -iframework -image_base
-init -install_name -keep_private_externs -multi_module -multiply_defined
-multiply_defined_unused -noall_load -no_dead_strip_inits_and_terms
-nofixprebinding -nomultidefs -noprebind -noseglinkedit -pagezero_size
-prebind -prebind_all_twolevel_modules -private_bundle -read_only_relocs
-sectalign -sectobjectsymbols -whyload -seg1addr -sectcreate
-sectobjectsymbols -sectorder -segaddr -segs_read_only_addr
-segs_read_write_addr -seg_addr_table -seg_addr_table_filename -seglinkedit
-segprot -segs_read_only_addr -segs_read_write_addr -single_module -static
-sub_library -sub_umbrella -twolevel_namespace -umbrella -undefined
-unexported_symbols_list -weak_reference_mismatches -whatsloaded -F -gused
-gfull -mmacosx-version-min=version -mkernel -mone-byte-bool
DEC Alpha Options -mno-fp-regs -msoft-float -mieee -mieee-with-inexact
-mieee-conformant -mfp-trap-mode=mode -mfp-rounding-mode=mode
-mtrap-precision=mode -mbuild-constants -mcpu=cpu-type -mtune=cpu-type -mbwx
-mmax -mfix -mcix -mfloat-vax -mfloat-ieee -mexplicit-relocs -msmall-data
-mlarge-data -msmall-text -mlarge-text -mmemory-latency=time
FR30 Options -msmall-model -mno-lsim
FT32 Options -msim -mlra -mnodiv -mft32b -mcompress -mnopm
FRV Options -mgpr-32 -mgpr-64 -mfpr-32 -mfpr-64 -mhard-float -msoft-float
-malloc-cc -mfixed-cc -mdword -mno-dword -mdouble -mno-double -mmedia
-mno-media -mmuladd -mno-muladd -mfdpic -minline-plt -mgprel-ro
-multilib-library-pic -mlinked-fp -mlong-calls -malign-labels -mlibrary-pic
-macc-4 -macc-8 -mpack -mno-pack -mno-eflags -mcond-move -mno-cond-move
-moptimize-membar -mno-optimize-membar -mscc -mno-scc -mcond-exec
-mno-cond-exec -mvliw-branch -mno-vliw-branch -mmulti-cond-exec
-mno-multi-cond-exec -mnested-cond-exec -mno-nested-cond-exec -mtomcat-stats
-mTLS -mtls -mcpu=cpu
GNU/Linux Options -mglibc -muclibc -mmusl -mbionic -mandroid
-tno-android-cc -tno-android-ld
H8/300 Options -mrelax -mh -ms -mn -mexr -mno-exr -mint32 -malign-300
HPPA Options -march=architecture-type -mcaller-copies -mdisable-fpregs
-mdisable-indexing -mfast-indirect-calls -mgas -mgnu-ld -mhp-ld
-mfixed-range=register-range -mjump-in-delay -mlinker-opt -mlong-calls
-mlong-load-store -mno-disable-fpregs -mno-disable-indexing
-mno-fast-indirect-calls -mno-gas -mno-jump-in-delay -mno-long-load-store
-mno-portable-runtime -mno-soft-float -mno-space-regs -msoft-float
-mpa-risc-1-0 -mpa-risc-1-1 -mpa-risc-2-0 -mportable-runtime -mschedule=cpu-
type -mspace-regs -msio -mwsio -munix=unix-std -nolibdld -static -threads
IA-64 Options -mbig-endian -mlittle-endian -mgnu-as -mgnu-ld -mno-pic
-mvolatile-asm-stop -mregister-names -msdata -mno-sdata -mconstant-gp
-mauto-pic -mfused-madd -minline-float-divide-min-latency
-minline-float-divide-max-throughput -mno-inline-float-divide
-minline-int-divide-min-latency -minline-int-divide-max-throughput
-mno-inline-int-divide -minline-sqrt-min-latency -minline-sqrt-max-throughput
-mno-inline-sqrt -mdwarf2-asm -mearly-stop-bits -mfixed-range=register-range
-mtls-size=tls-size -mtune=cpu-type -milp32 -mlp64 -msched-br-data-spec
-msched-ar-data-spec -msched-control-spec -msched-br-in-data-spec
-msched-ar-in-data-spec -msched-in-control-spec -msched-spec-ldc
-msched-spec-control-ldc -msched-prefer-non-data-spec-insns
-msched-prefer-non-control-spec-insns -msched-stop-bits-after-every-cycle
-msched-count-spec-in-critical-path -msel-sched-dont-check-control-spec
-msched-fp-mem-deps-zero-cost -msched-max-memory-insns-hard-limit
-msched-max-memory-insns=max-insns
LM32 Options -mbarrel-shift-enabled -mdivide-enabled -mmultiply-enabled
-msign-extend-enabled -muser-enabled
M32R/D Options -m32r2 -m32rx -m32r -mdebug -malign-loops -mno-align-loops
-missue-rate=number -mbranch-cost=number -mmodel=code-size-model-type
-msdata=sdata-type -mno-flush-func -mflush-func=name -mno-flush-trap
-mflush-trap=number -G num
M32C Options -mcpu=cpu -msim -memregs=number
M680x0 Options -march=arch -mcpu=cpu -mtune=tune -m68000 -m68020 -m68020-40
-m68020-60 -m68030 -m68040 -m68060 -mcpu32 -m5200 -m5206e -m528x -m5307
-m5407 -mcfv4e -mbitfield -mno-bitfield -mc68000 -mc68020 -mnobitfield
-mrtd -mno-rtd -mdiv -mno-div -mshort -mno-short -mhard-float -m68881
-msoft-float -mpcrel -malign-int -mstrict-align -msep-data -mno-sep-data
-mshared-library-id=n -mid-shared-library -mno-id-shared-library -mxgot
-mno-xgot -mlong-jump-table-offsets
MCore Options -mhardlit -mno-hardlit -mdiv -mno-div -mrelax-immediates
-mno-relax-immediates -mwide-bitfields -mno-wide-bitfields -m4byte-functions
-mno-4byte-functions -mcallgraph-data -mno-callgraph-data -mslow-bytes
-mno-slow-bytes -mno-lsim -mlittle-endian -mbig-endian -m210 -m340
-mstack-increment
MeP Options -mabsdiff -mall-opts -maverage -mbased=n -mbitops -mc=n -mclip
-mconfig=name -mcop -mcop32 -mcop64 -mivc2 -mdc -mdiv -meb -mel
-mio-volatile -ml -mleadz -mm -mminmax -mmult -mno-opts -mrepeat -ms
-msatur -msdram -msim -msimnovec -mtf -mtiny=n
MicroBlaze Options -msoft-float -mhard-float -msmall-divides -mcpu=cpu
-mmemcpy -mxl-soft-mul -mxl-soft-div -mxl-barrel-shift -mxl-pattern-compare
-mxl-stack-check -mxl-gp-opt -mno-clearbss -mxl-multiply-high
-mxl-float-convert -mxl-float-sqrt -mbig-endian -mlittle-endian -mxl-reorder
-mxl-mode-app-model
MIPS Options -EL -EB -march=arch -mtune=arch -mips1 -mips2 -mips3 -mips4
-mips32 -mips32r2 -mips32r3 -mips32r5 -mips32r6 -mips64 -mips64r2
-mips64r3 -mips64r5 -mips64r6 -mips16 -mno-mips16 -mflip-mips16
-minterlink-compressed -mno-interlink-compressed -minterlink-mips16
-mno-interlink-mips16 -mabi=abi -mabicalls -mno-abicalls -mshared
-mno-shared -mplt -mno-plt -mxgot -mno-xgot -mgp32 -mgp64 -mfp32 -mfpxx
-mfp64 -mhard-float -msoft-float -mno-float -msingle-float -mdouble-float
-modd-spreg -mno-odd-spreg -mabs=mode -mnan=encoding -mdsp -mno-dsp -mdspr2
-mno-dspr2 -mmcu -mmno-mcu -meva -mno-eva -mvirt -mno-virt -mxpa -mno-xpa
-mmicromips -mno-micromips -mmsa -mno-msa -mfpu=fpu-type -msmartmips
-mno-smartmips -mpaired-single -mno-paired-single -mdmx -mno-mdmx -mips3d
-mno-mips3d -mmt -mno-mt -mllsc -mno-llsc -mlong64 -mlong32 -msym32
-mno-sym32 -Gnum -mlocal-sdata -mno-local-sdata -mextern-sdata
-mno-extern-sdata -mgpopt -mno-gopt -membedded-data -mno-embedded-data
-muninit-const-in-rodata -mno-uninit-const-in-rodata -mcode-readable=setting
-msplit-addresses -mno-split-addresses -mexplicit-relocs -mno-explicit-relocs
-mcheck-zero-division -mno-check-zero-division -mdivide-traps -mdivide-breaks
-mload-store-pairs -mno-load-store-pairs -mmemcpy -mno-memcpy -mlong-calls
-mno-long-calls -mmad -mno-mad -mimadd -mno-imadd -mfused-madd
-mno-fused-madd -nocpp -mfix-24k -mno-fix-24k -mfix-r4000 -mno-fix-r4000
-mfix-r4400 -mno-fix-r4400 -mfix-r10000 -mno-fix-r10000 -mfix-rm7000
-mno-fix-rm7000 -mfix-vr4120 -mno-fix-vr4120 -mfix-vr4130 -mno-fix-vr4130
-mfix-sb1 -mno-fix-sb1 -mflush-func=func -mno-flush-func -mbranch-cost=num
-mbranch-likely -mno-branch-likely -mcompact-branches=policy -mfp-exceptions
-mno-fp-exceptions -mvr4130-align -mno-vr4130-align -msynci -mno-synci
-mlxc1-sxc1 -mno-lxc1-sxc1 -mmadd4 -mno-madd4 -mrelax-pic-calls
-mno-relax-pic-calls -mmcount-ra-address -mframe-header-opt
-mno-frame-header-opt
MMIX Options -mlibfuncs -mno-libfuncs -mepsilon -mno-epsilon -mabi=gnu
-mabi=mmixware -mzero-extend -mknuthdiv -mtoplevel-symbols -melf
-mbranch-predict -mno-branch-predict -mbase-addresses -mno-base-addresses
-msingle-exit -mno-single-exit
MN10300 Options -mmult-bug -mno-mult-bug -mno-am33 -mam33 -mam33-2 -mam34
-mtune=cpu-type -mreturn-pointer-on-d0 -mno-crt0 -mrelax -mliw -msetlb
Moxie Options -meb -mel -mmul.x -mno-crt0
MSP430 Options -msim -masm-hex -mmcu= -mcpu= -mlarge -msmall -mrelax
-mwarn-mcu -mcode-region= -mdata-region= -msilicon-errata=
-msilicon-errata-warn= -mhwmult= -minrt
NDS32 Options -mbig-endian -mlittle-endian -mreduced-regs -mfull-regs -mcmov
-mno-cmov -mext-perf -mno-ext-perf -mext-perf2 -mno-ext-perf2 -mext-string
-mno-ext-string -mv3push -mno-v3push -m16bit -mno-16bit -misr-vector-size=num
-mcache-block-size=num -march=arch -mcmodel=code-model -mctor-dtor -mrelax
Nios II Options -G num -mgpopt=option -mgpopt -mno-gpopt -mgprel-sec=regexp
-mr0rel-sec=regexp -mel -meb -mno-bypass-cache -mbypass-cache
-mno-cache-volatile -mcache-volatile -mno-fast-sw-div -mfast-sw-div -mhw-mul
-mno-hw-mul -mhw-mulx -mno-hw-mulx -mno-hw-div -mhw-div -mcustom-insn=N
-mno-custom-insn -mcustom-fpu-cfg=name -mhal -msmallc -msys-crt0=name
-msys-lib=name -march=arch -mbmx -mno-bmx -mcdx -mno-cdx
Nvidia PTX Options -m32 -m64 -mmainkernel -moptimize
PDP-11 Options -mfpu -msoft-float -mac0 -mno-ac0 -m40 -m45 -m10 -mbcopy
-mbcopy-builtin -mint32 -mno-int16 -mint16 -mno-int32 -mfloat32
-mno-float64 -mfloat64 -mno-float32 -mabshi -mno-abshi -mbranch-expensive
-mbranch-cheap -munix-asm -mdec-asm
picoChip Options -mae=ae_type -mvliw-lookahead=N -msymbol-as-address
-mno-inefficient-warnings
PowerPC Options See RS/6000 and PowerPC Options.
PowerPC SPE Options -mcpu=cpu-type -mtune=cpu-type -mmfcrf -mno-mfcrf
-mpopcntb -mno-popcntb -mfull-toc -mminimal-toc -mno-fp-in-toc
-mno-sum-in-toc -m32 -mxl-compat -mno-xl-compat -malign-power
-malign-natural -msoft-float -mhard-float -mmultiple -mno-multiple
-msingle-float -mdouble-float -mupdate -mno-update -mavoid-indexed-addresses
-mno-avoid-indexed-addresses -mstrict-align -mno-strict-align -mrelocatable
-mno-relocatable -mrelocatable-lib -mno-relocatable-lib -mtoc -mno-toc
-mlittle -mlittle-endian -mbig -mbig-endian -msingle-pic-base
-mprioritize-restricted-insns=priority -msched-costly-dep=dependence_type
-minsert-sched-nops=scheme -mcall-sysv -mcall-netbsd -maix-struct-return
-msvr4-struct-return -mabi=abi-type -msecure-plt -mbss-plt
-mblock-move-inline-limit=num -misel -mno-isel -misel=yes -misel=no -mspe
-mno-spe -mspe=yes -mspe=no -mfloat-gprs=yes -mfloat-gprs=no
-mfloat-gprs=single -mfloat-gprs=double -mprototype -mno-prototype -msim
-mmvme -mads -myellowknife -memb -msdata -msdata=opt -mvxworks -G num
-mrecip -mrecip=opt -mno-recip -mrecip-precision -mno-recip-precision
-mpointers-to-nested-functions -mno-pointers-to-nested-functions
-msave-toc-indirect -mno-save-toc-indirect -mcompat-align-parm
-mno-compat-align-parm -mfloat128 -mno-float128 -mgnu-attribute
-mno-gnu-attribute -mstack-protector-guard=guard
-mstack-protector-guard-reg=reg -mstack-protector-guard-offset=offset
RISC-V Options -mbranch-cost=N-instruction -mplt -mno-plt -mabi=ABI-string
-mfdiv -mno-fdiv -mdiv -mno-div -march=ISA-string -mtune=processor-string
-mpreferred-stack-boundary=num -msmall-data-limit=N-bytes -msave-restore
-mno-save-restore -mstrict-align -mno-strict-align -mcmodel=medlow
-mcmodel=medany -mexplicit-relocs -mno-explicit-relocs -mrelax -mno-relax
RL78 Options -msim -mmul=none -mmul=g13 -mmul=g14 -mallregs -mcpu=g10
-mcpu=g13 -mcpu=g14 -mg10 -mg13 -mg14 -m64bit-doubles -m32bit-doubles
-msave-mduc-in-interrupts
RS/6000 and PowerPC Options -mcpu=cpu-type -mtune=cpu-type -mcmodel=code-model
-mpowerpc64 -maltivec -mno-altivec -mpowerpc-gpopt -mno-powerpc-gpopt
-mpowerpc-gfxopt -mno-powerpc-gfxopt -mmfcrf -mno-mfcrf -mpopcntb
-mno-popcntb -mpopcntd -mno-popcntd -mfprnd -mno-fprnd -mcmpb -mno-cmpb
-mmfpgpr -mno-mfpgpr -mhard-dfp -mno-hard-dfp -mfull-toc -mminimal-toc
-mno-fp-in-toc -mno-sum-in-toc -m64 -m32 -mxl-compat -mno-xl-compat -mpe
-malign-power -malign-natural -msoft-float -mhard-float -mmultiple
-mno-multiple -msingle-float -mdouble-float -msimple-fpu -mupdate
-mno-update -mavoid-indexed-addresses -mno-avoid-indexed-addresses
-mfused-madd -mno-fused-madd -mbit-align -mno-bit-align -mstrict-align
-mno-strict-align -mrelocatable -mno-relocatable -mrelocatable-lib
-mno-relocatable-lib -mtoc -mno-toc -mlittle -mlittle-endian -mbig
-mbig-endian -mdynamic-no-pic -maltivec -mswdiv -msingle-pic-base
-mprioritize-restricted-insns=priority -msched-costly-dep=dependence_type
-minsert-sched-nops=scheme -mcall-aixdesc -mcall-eabi -mcall-freebsd
-mcall-linux -mcall-netbsd -mcall-openbsd -mcall-sysv -mcall-sysv-eabi
-mcall-sysv-noeabi -mtraceback=traceback_type -maix-struct-return
-msvr4-struct-return -mabi=abi-type -msecure-plt -mbss-plt
-mblock-move-inline-limit=num -mblock-compare-inline-limit=num
-mblock-compare-inline-loop-limit=num -mstring-compare-inline-limit=num -misel
-mno-isel -misel=yes -misel=no -mpaired -mvrsave -mno-vrsave -mmulhw
-mno-mulhw -mdlmzb -mno-dlmzb -mprototype -mno-prototype -msim -mmvme -mads
-myellowknife -memb -msdata -msdata=opt -mreadonly-in-sdata -mvxworks -G
num -mrecip -mrecip=opt -mno-recip -mrecip-precision -mno-recip-precision
-mveclibabi=type -mfriz -mno-friz -mpointers-to-nested-functions
-mno-pointers-to-nested-functions -msave-toc-indirect -mno-save-toc-indirect
-mpower8-fusion -mno-mpower8-fusion -mpower8-vector -mno-power8-vector
-mcrypto -mno-crypto -mhtm -mno-htm -mquad-memory -mno-quad-memory
-mquad-memory-atomic -mno-quad-memory-atomic -mcompat-align-parm
-mno-compat-align-parm -mfloat128 -mno-float128 -mfloat128-hardware
-mno-float128-hardware -mgnu-attribute -mno-gnu-attribute
-mstack-protector-guard=guard -mstack-protector-guard-reg=reg
-mstack-protector-guard-offset=offset
RX Options -m64bit-doubles -m32bit-doubles -fpu -nofpu -mcpu=
-mbig-endian-data -mlittle-endian-data -msmall-data -msim -mno-sim
-mas100-syntax -mno-as100-syntax -mrelax -mmax-constant-size= -mint-register=
-mpid -mallow-string-insns -mno-allow-string-insns -mjsr
-mno-warn-multiple-fast-interrupts -msave-acc-in-interrupts
S/390 and zSeries Options -mtune=cpu-type -march=cpu-type -mhard-float
-msoft-float -mhard-dfp -mno-hard-dfp -mlong-double-64 -mlong-double-128
-mbackchain -mno-backchain -mpacked-stack -mno-packed-stack -msmall-exec
-mno-small-exec -mmvcle -mno-mvcle -m64 -m31 -mdebug -mno-debug -mesa
-mzarch -mhtm -mvx -mzvector -mtpf-trace -mno-tpf-trace -mfused-madd
-mno-fused-madd -mwarn-framesize -mwarn-dynamicstack -mstack-size
-mstack-guard -mhotpatch=halfwords,halfwords
Score Options -meb -mel -mnhwloop -muls -mmac -mscore5 -mscore5u -mscore7
-mscore7d
SH Options -m1 -m2 -m2e -m2a-nofpu -m2a-single-only -m2a-single -m2a -m3
-m3e -m4-nofpu -m4-single-only -m4-single -m4 -m4a-nofpu -m4a-single-only
-m4a-single -m4a -m4al -mb -ml -mdalign -mrelax -mbigtable -mfmovd
-mrenesas -mno-renesas -mnomacsave -mieee -mno-ieee -mbitops -misize
-minline-ic_invalidate -mpadstruct -mprefergot -musermode -multcost=number
-mdiv=strategy -mdivsi3_libfunc=name -mfixed-range=register-range
-maccumulate-outgoing-args -matomic-model=atomic-model -mbranch-cost=num
-mzdcbranch -mno-zdcbranch -mcbranch-force-delay-slot -mfused-madd
-mno-fused-madd -mfsca -mno-fsca -mfsrra -mno-fsrra -mpretend-cmove -mtas
Solaris 2 Options -mclear-hwcap -mno-clear-hwcap -mimpure-text
-mno-impure-text -pthreads
SPARC Options -mcpu=cpu-type -mtune=cpu-type -mcmodel=code-model
-mmemory-model=mem-model -m32 -m64 -mapp-regs -mno-app-regs -mfaster-structs
-mno-faster-structs -mflat -mno-flat -mfpu -mno-fpu -mhard-float
-msoft-float -mhard-quad-float -msoft-quad-float -mstack-bias -mno-stack-bias
-mstd-struct-return -mno-std-struct-return -munaligned-doubles
-mno-unaligned-doubles -muser-mode -mno-user-mode -mv8plus -mno-v8plus -mvis
-mno-vis -mvis2 -mno-vis2 -mvis3 -mno-vis3 -mvis4 -mno-vis4 -mvis4b
-mno-vis4b -mcbcond -mno-cbcond -mfmaf -mno-fmaf -mfsmuld -mno-fsmuld
-mpopc -mno-popc -msubxc -mno-subxc -mfix-at697f -mfix-ut699 -mfix-ut700
-mfix-gr712rc -mlra -mno-lra
SPU Options -mwarn-reloc -merror-reloc -msafe-dma -munsafe-dma -mbranch-hints
-msmall-mem -mlarge-mem -mstdmain -mfixed-range=register-range -mea32 -mea64
-maddress-space-conversion -mno-address-space-conversion -mcache-size=cache-
size -matomic-updates -mno-atomic-updates
System V Options -Qy -Qn -YP,paths -Ym,dir
TILE-Gx Options -mcpu=CPU -m32 -m64 -mbig-endian -mlittle-endian
-mcmodel=code-model
TILEPro Options -mcpu=cpu -m32
V850 Options -mlong-calls -mno-long-calls -mep -mno-ep -mprolog-function
-mno-prolog-function -mspace -mtda=n -msda=n -mzda=n -mapp-regs
-mno-app-regs -mdisable-callt -mno-disable-callt -mv850e2v3 -mv850e2
-mv850e1 -mv850es -mv850e -mv850 -mv850e3v5 -mloop -mrelax -mlong-jumps
-msoft-float -mhard-float -mgcc-abi -mrh850-abi -mbig-switch
VAX Options -mg -mgnu -munix
Visium Options -mdebug -msim -mfpu -mno-fpu -mhard-float -msoft-float
-mcpu=cpu-type -mtune=cpu-type -msv-mode -muser-mode
VMS Options -mvms-return-codes -mdebug-main=prefix -mmalloc64
-mpointer-size=size
VxWorks Options -mrtp -non-static -Bstatic -Bdynamic -Xbind-lazy -Xbind-now
x86 Options -mtune=cpu-type -march=cpu-type -mtune-ctrl=feature-list
-mdump-tune-features -mno-default -mfpmath=unit -masm=dialect
-mno-fancy-math-387 -mno-fp-ret-in-387 -m80387 -mhard-float -msoft-float
-mno-wide-multiply -mrtd -malign-double -mpreferred-stack-boundary=num
-mincoming-stack-boundary=num -mcld -mcx16 -msahf -mmovbe -mcrc32 -mrecip
-mrecip=opt -mvzeroupper -mprefer-avx128 -mprefer-vector-width=opt -mmmx
-msse -msse2 -msse3 -mssse3 -msse4.1 -msse4.2 -msse4 -mavx -mavx2
-mavx512f -mavx512pf -mavx512er -mavx512cd -mavx512vl -mavx512bw
-mavx512dq -mavx512ifma -mavx512vbmi -msha -maes -mpclmul -mfsgsbase
-mrdrnd -mf16c -mfma -mpconfig -mwbnoinvd -mprefetchwt1 -mclflushopt -mclwb
-mxsavec -mxsaves -msse4a -m3dnow -m3dnowa -mpopcnt -mabm -mbmi -mtbm
-mfma4 -mxop -madx -mlzcnt -mbmi2 -mfxsr -mxsave -mxsaveopt -mrtm -mlwp
-mmpx -mmwaitx -mclzero -mpku -mthreads -mgfni -mvaes -mshstk
-mforce-indirect-call -mavx512vbmi2 -mvpclmulqdq -mavx512bitalg -mmovdiri
-mmovdir64b -mavx512vpopcntdq -mavx5124fmaps -mavx512vnni -mavx5124vnniw
-mprfchw -mrdpid -mrdseed -msgx -mms-bitfields -mno-align-stringops
-minline-all-stringops -minline-stringops-dynamically -mstringop-strategy=alg
-mmemcpy-strategy=strategy -mmemset-strategy=strategy -mpush-args
-maccumulate-outgoing-args -m128bit-long-double -m96bit-long-double
-mlong-double-64 -mlong-double-80 -mlong-double-128 -mregparm=num
-msseregparm -mveclibabi=type -mvect8-ret-in-mem -mpc32 -mpc64 -mpc80
-mstackrealign -momit-leaf-frame-pointer -mno-red-zone
-mno-tls-direct-seg-refs -mcmodel=code-model -mabi=name -maddress-mode=mode
-m32 -m64 -mx32 -m16 -miamcu -mlarge-data-threshold=num -msse2avx
-mfentry -mrecord-mcount -mnop-mcount -m8bit-idiv
-mavx256-split-unaligned-load -mavx256-split-unaligned-store -malign-data=type
-mstack-protector-guard=guard -mstack-protector-guard-reg=reg
-mstack-protector-guard-offset=offset -mstack-protector-guard-symbol=symbol
-mmitigate-rop -mgeneral-regs-only -mcall-ms2sysv-xlogues
-mindirect-branch=choice -mfunction-return=choice -mindirect-branch-register
x86 Windows Options -mconsole -mcygwin -mno-cygwin -mdll -mnop-fun-dllimport
-mthread -municode -mwin32 -mwindows -fno-set-stack-executable
Xstormy16 Options -msim
Xtensa Options -mconst16 -mno-const16 -mfused-madd -mno-fused-madd
-mforce-no-pic -mserialize-volatile -mno-serialize-volatile
-mtext-section-literals -mno-text-section-literals -mauto-litpools
-mno-auto-litpools -mtarget-align -mno-target-align -mlongcalls
-mno-longcalls
zSeries Options See S/390 and zSeries Options.
Options Controlling the Kind of Output
Compilation can involve up to four stages: preprocessing, compilation proper,
assembly and linking, always in that order. GCC is capable of preprocessing and
compiling several files either into several assembler input files, or into one
assembler input file; then each assembler input file produces an object file, and
linking combines all the object files (those newly compiled, and those specified as
input) into an executable file.
For any given input file, the file name suffix determines what kind of compilation
is done:
file.c
C source code that must be preprocessed.
file.i
C source code that should not be preprocessed.
file.ii
C++ source code that should not be preprocessed.
file.m
Objective-C source code. Note that you must link with the libobjc library to
make an Objective-C program work.
file.mi
Objective-C source code that should not be preprocessed.
file.mm
file.M
Objective-C++ source code. Note that you must link with the libobjc library to
make an Objective-C++ program work. Note that .M refers to a literal capital
M.
file.mii
Objective-C++ source code that should not be preprocessed.
file.h
C, C++, Objective-C or Objective-C++ header file to be turned into a
precompiled header (default), or C, C++ header file to be turned into an Ada
spec (via the -fdump-ada-spec switch).
file.cc
file.cp
file.cxx
file.cpp
file.CPP
file.c++
file.C
C++ source code that must be preprocessed. Note that in .cxx, the last two
letters must both be literally x. Likewise, .C refers to a literal capital C.
file.mm
file.M
Objective-C++ source code that must be preprocessed.
file.mii
Objective-C++ source code that should not be preprocessed.
file.hh
file.H
file.hp
file.hxx
file.hpp
file.HPP
file.h++
file.tcc
C++ header file to be turned into a precompiled header or Ada spec.
file.f
file.for
file.ftn
Fixed form Fortran source code that should not be preprocessed.
file.F
file.FOR
file.fpp
file.FPP
file.FTN
Fixed form Fortran source code that must be preprocessed (with the traditional
preprocessor).
file.f90
file.f95
file.f03
file.f08
Free form Fortran source code that should not be preprocessed.
file.F90
file.F95
file.F03
file.F08
Free form Fortran source code that must be preprocessed (with the traditional
preprocessor).
file.go
Go source code.
file.brig
BRIG files (binary representation of HSAIL).
file.ads
Ada source code file that contains a library unit declaration (a declaration of
a package, subprogram, or generic, or a generic instantiation), or a library
unit renaming declaration (a package, generic, or subprogram renaming
declaration). Such files are also called specs.
file.adb
Ada source code file containing a library unit body (a subprogram or package
body). Such files are also called bodies.
file.s
Assembler code.
file.S
file.sx
Assembler code that must be preprocessed.
other
An object file to be fed straight into linking. Any file name with no
recognized suffix is treated this way.
You can specify the input language explicitly with the -x option:
-x language
Specify explicitly the language for the following input files (rather than
letting the compiler choose a default based on the file name suffix). This
option applies to all following input files until the next -x option. Possible
values for language are:
c c-header cpp-output
c++ c++-header c++-cpp-output
objective-c objective-c-header objective-c-cpp-output
objective-c++ objective-c++-header objective-c++-cpp-output
assembler assembler-with-cpp
ada
f77 f77-cpp-input f95 f95-cpp-input
go
brig
-x none
Turn off any specification of a language, so that subsequent files are handled
according to their file name suffixes (as they are if -x has not been used at
all).
If you only want some of the stages of compilation, you can use -x (or filename
suffixes) to tell gcc where to start, and one of the options -c, -S, or -E to say
where gcc is to stop. Note that some combinations (for example, -x cpp-output -E)
instruct gcc to do nothing at all.
-c Compile or assemble the source files, but do not link. The linking stage
simply is not done. The ultimate output is in the form of an object file for
each source file.
By default, the object file name for a source file is made by replacing the
suffix .c, .i, .s, etc., with .o.
Unrecognized input files, not requiring compilation or assembly, are ignored.
-S Stop after the stage of compilation proper; do not assemble. The output is in
the form of an assembler code file for each non-assembler input file specified.
By default, the assembler file name for a source file is made by replacing the
suffix .c, .i, etc., with .s.
Input files that don't require compilation are ignored.
-E Stop after the preprocessing stage; do not run the compiler proper. The output
is in the form of preprocessed source code, which is sent to the standard
output.
Input files that don't require preprocessing are ignored.
-o file
Place output in file file. This applies to whatever sort of output is being
produced, whether it be an executable file, an object file, an assembler file
or preprocessed C code.
If -o is not specified, the default is to put an executable file in a.out, the
object file for source.suffix in source.o, its assembler file in source.s, a
precompiled header file in source.suffix.gch, and all preprocessed C source on
standard output.
-v Print (on standard error output) the commands executed to run the stages of
compilation. Also print the version number of the compiler driver program and
of the preprocessor and the compiler proper.
-###
Like -v except the commands are not executed and arguments are quoted unless
they contain only alphanumeric characters or "./-_". This is useful for shell
scripts to capture the driver-generated command lines.
--help
Print (on the standard output) a description of the command-line options
understood by gcc. If the -v option is also specified then --help is also
passed on to the various processes invoked by gcc, so that they can display the
command-line options they accept. If the -Wextra option has also been
specified (prior to the --help option), then command-line options that have no
documentation associated with them are also displayed.
--target-help
Print (on the standard output) a description of target-specific command-line
options for each tool. For some targets extra target-specific information may
also be printed.
--help={class|[^]qualifier}[,...]
Print (on the standard output) a description of the command-line options
understood by the compiler that fit into all specified classes and qualifiers.
These are the supported classes:
optimizers
Display all of the optimization options supported by the compiler.
warnings
Display all of the options controlling warning messages produced by the
compiler.
target
Display target-specific options. Unlike the --target-help option however,
target-specific options of the linker and assembler are not displayed.
This is because those tools do not currently support the extended --help=
syntax.
params
Display the values recognized by the --param option.
language
Display the options supported for language, where language is the name of
one of the languages supported in this version of GCC.
common
Display the options that are common to all languages.
These are the supported qualifiers:
undocumented
Display only those options that are undocumented.
joined
Display options taking an argument that appears after an equal sign in the
same continuous piece of text, such as: --help=target.
separate
Display options taking an argument that appears as a separate word
following the original option, such as: -o output-file.
Thus for example to display all the undocumented target-specific switches
supported by the compiler, use:
--help=target,undocumented
The sense of a qualifier can be inverted by prefixing it with the ^ character,
so for example to display all binary warning options (i.e., ones that are
either on or off and that do not take an argument) that have a description,
use:
--help=warnings,^joined,^undocumented
The argument to --help= should not consist solely of inverted qualifiers.
Combining several classes is possible, although this usually restricts the
output so much that there is nothing to display. One case where it does work,
however, is when one of the classes is target. For example, to display all the
target-specific optimization options, use:
--help=target,optimizers
The --help= option can be repeated on the command line. Each successive use
displays its requested class of options, skipping those that have already been
displayed.
If the -Q option appears on the command line before the --help= option, then
the descriptive text displayed by --help= is changed. Instead of describing
the displayed options, an indication is given as to whether the option is
enabled, disabled or set to a specific value (assuming that the compiler knows
this at the point where the --help= option is used).
Here is a truncated example from the ARM port of gcc:
% gcc -Q -mabi=2 --help=target -c
The following options are target specific:
-mabi= 2
-mabort-on-noreturn [disabled]
-mapcs [disabled]
The output is sensitive to the effects of previous command-line options, so for
example it is possible to find out which optimizations are enabled at -O2 by
using:
-Q -O2 --help=optimizers
Alternatively you can discover which binary optimizations are enabled by -O3 by
using:
gcc -c -Q -O3 --help=optimizers > /tmp/O3-opts
gcc -c -Q -O2 --help=optimizers > /tmp/O2-opts
diff /tmp/O2-opts /tmp/O3-opts | grep enabled
--version
Display the version number and copyrights of the invoked GCC.
-pass-exit-codes
Normally the gcc program exits with the code of 1 if any phase of the compiler
returns a non-success return code. If you specify -pass-exit-codes, the gcc
program instead returns with the numerically highest error produced by any
phase returning an error indication. The C, C++, and Fortran front ends return
4 if an internal compiler error is encountered.
-pipe
Use pipes rather than temporary files for communication between the various
stages of compilation. This fails to work on some systems where the assembler
is unable to read from a pipe; but the GNU assembler has no trouble.
-specs=file
Process file after the compiler reads in the standard specs file, in order to
override the defaults which the gcc driver program uses when determining what
switches to pass to cc1, cc1plus, as, ld, etc. More than one -specs=file can
be specified on the command line, and they are processed in order, from left to
right.
-wrapper
Invoke all subcommands under a wrapper program. The name of the wrapper
program and its parameters are passed as a comma separated list.
gcc -c t.c -wrapper gdb,--args
This invokes all subprograms of gcc under gdb --args, thus the invocation of
cc1 is gdb --args cc1 ....
-ffile-prefix-map=old=new
When compiling files residing in directory old, record any references to them
in the result of the compilation as if the files resided in directory new
instead. Specifying this option is equivalent to specifying all the individual
-f*-prefix-map options. This can be used to make reproducible builds that are
location independent. See also -fmacro-prefix-map and -fdebug-prefix-map.
-fplugin=name.so
Load the plugin code in file name.so, assumed to be a shared object to be
dlopen'd by the compiler. The base name of the shared object file is used to
identify the plugin for the purposes of argument parsing (See
-fplugin-arg-name-key=value below). Each plugin should define the callback
functions specified in the Plugins API.
-fplugin-arg-name-key=value
Define an argument called key with a value of value for the plugin called name.
-fdump-ada-spec[-slim]
For C and C++ source and include files, generate corresponding Ada specs.
-fada-spec-parent=unit
In conjunction with -fdump-ada-spec[-slim] above, generate Ada specs as child
units of parent unit.
-fdump-go-spec=file
For input files in any language, generate corresponding Go declarations in
file. This generates Go "const", "type", "var", and "func" declarations which
may be a useful way to start writing a Go interface to code written in some
other language.
@file
Read command-line options from file. The options read are inserted in place of
the original @file option. If file does not exist, or cannot be read, then the
option will be treated literally, and not removed.
Options in file are separated by whitespace. A whitespace character may be
included in an option by surrounding the entire option in either single or
double quotes. Any character (including a backslash) may be included by
prefixing the character to be included with a backslash. The file may itself
contain additional @file options; any such options will be processed
recursively.
Compiling C++ Programs
C++ source files conventionally use one of the suffixes .C, .cc, .cpp, .CPP, .c++,
.cp, or .cxx; C++ header files often use .hh, .hpp, .H, or (for shared template
code) .tcc; and preprocessed C++ files use the suffix .ii. GCC recognizes files
with these names and compiles them as C++ programs even if you call the compiler
the same way as for compiling C programs (usually with the name gcc).
However, the use of gcc does not add the C++ library. g++ is a program that calls
GCC and automatically specifies linking against the C++ library. It treats .c, .h
and .i files as C++ source files instead of C source files unless -x is used. This
program is also useful when precompiling a C header file with a .h extension for
use in C++ compilations. On many systems, g++ is also installed with the name c++.
When you compile C++ programs, you may specify many of the same command-line
options that you use for compiling programs in any language; or command-line
options meaningful for C and related languages; or options that are meaningful only
for C++ programs.
Options Controlling C Dialect
The following options control the dialect of C (or languages derived from C, such
as C++, Objective-C and Objective-C++) that the compiler accepts:
-ansi
In C mode, this is equivalent to -std=c90. In C++ mode, it is equivalent to
-std=c++98.
This turns off certain features of GCC that are incompatible with ISO C90 (when
compiling C code), or of standard C++ (when compiling C++ code), such as the
"asm" and "typeof" keywords, and predefined macros such as "unix" and "vax"
that identify the type of system you are using. It also enables the
undesirable and rarely used ISO trigraph feature. For the C compiler, it
disables recognition of C++ style // comments as well as the "inline" keyword.
The alternate keywords "__asm__", "__extension__", "__inline__" and
"__typeof__" continue to work despite -ansi. You would not want to use them in
an ISO C program, of course, but it is useful to put them in header files that
might be included in compilations done with -ansi. Alternate predefined macros
such as "__unix__" and "__vax__" are also available, with or without -ansi.
The -ansi option does not cause non-ISO programs to be rejected gratuitously.
For that, -Wpedantic is required in addition to -ansi.
The macro "__STRICT_ANSI__" is predefined when the -ansi option is used. Some
header files may notice this macro and refrain from declaring certain functions
or defining certain macros that the ISO standard doesn't call for; this is to
avoid interfering with any programs that might use these names for other
things.
Functions that are normally built in but do not have semantics defined by ISO C
(such as "alloca" and "ffs") are not built-in functions when -ansi is used.
-std=
Determine the language standard. This option is currently only supported when
compiling C or C++.
The compiler can accept several base standards, such as c90 or c++98, and GNU
dialects of those standards, such as gnu90 or gnu++98. When a base standard is
specified, the compiler accepts all programs following that standard plus those
using GNU extensions that do not contradict it. For example, -std=c90 turns
off certain features of GCC that are incompatible with ISO C90, such as the
"asm" and "typeof" keywords, but not other GNU extensions that do not have a
meaning in ISO C90, such as omitting the middle term of a "?:" expression. On
the other hand, when a GNU dialect of a standard is specified, all features
supported by the compiler are enabled, even when those features change the
meaning of the base standard. As a result, some strict-conforming programs may
be rejected. The particular standard is used by -Wpedantic to identify which
features are GNU extensions given that version of the standard. For example
-std=gnu90 -Wpedantic warns about C++ style // comments, while -std=gnu99
-Wpedantic does not.
A value for this option must be provided; possible values are
c90
c89
iso9899:1990
Support all ISO C90 programs (certain GNU extensions that conflict with ISO
C90 are disabled). Same as -ansi for C code.
iso9899:199409
ISO C90 as modified in amendment 1.
c99
c9x
iso9899:1999
iso9899:199x
ISO C99. This standard is substantially completely supported, modulo bugs
and floating-point issues (mainly but not entirely relating to optional C99
features from Annexes F and G). See <http://gcc.gnu.org/c99status.html>
for more information. The names c9x and iso9899:199x are deprecated.
c11
c1x
iso9899:2011
ISO C11, the 2011 revision of the ISO C standard. This standard is
substantially completely supported, modulo bugs, floating-point issues
(mainly but not entirely relating to optional C11 features from Annexes F
and G) and the optional Annexes K (Bounds-checking interfaces) and L
(Analyzability). The name c1x is deprecated.
c17
c18
iso9899:2017
iso9899:2018
ISO C17, the 2017 revision of the ISO C standard (expected to be published
in 2018). This standard is same as C11 except for corrections of defects
(all of which are also applied with -std=c11) and a new value of
"__STDC_VERSION__", and so is supported to the same extent as C11.
gnu90
gnu89
GNU dialect of ISO C90 (including some C99 features).
gnu99
gnu9x
GNU dialect of ISO C99. The name gnu9x is deprecated.
gnu11
gnu1x
GNU dialect of ISO C11. The name gnu1x is deprecated.
gnu17
gnu18
GNU dialect of ISO C17. This is the default for C code.
c++98
c++03
The 1998 ISO C++ standard plus the 2003 technical corrigendum and some
additional defect reports. Same as -ansi for C++ code.
gnu++98
gnu++03
GNU dialect of -std=c++98.
c++11
c++0x
The 2011 ISO C++ standard plus amendments. The name c++0x is deprecated.
gnu++11
gnu++0x
GNU dialect of -std=c++11. The name gnu++0x is deprecated.
c++14
c++1y
The 2014 ISO C++ standard plus amendments. The name c++1y is deprecated.
gnu++14
gnu++1y
GNU dialect of -std=c++14. This is the default for C++ code. The name
gnu++1y is deprecated.
c++17
c++1z
The 2017 ISO C++ standard plus amendments. The name c++1z is deprecated.
gnu++17
gnu++1z
GNU dialect of -std=c++17. The name gnu++1z is deprecated.
c++2a
The next revision of the ISO C++ standard, tentatively planned for 2020.
Support is highly experimental, and will almost certainly change in
incompatible ways in future releases.
gnu++2a
GNU dialect of -std=c++2a. Support is highly experimental, and will almost
certainly change in incompatible ways in future releases.
-fgnu89-inline
The option -fgnu89-inline tells GCC to use the traditional GNU semantics for
"inline" functions when in C99 mode.
Using this option is roughly equivalent to adding the "gnu_inline" function
attribute to all inline functions.
The option -fno-gnu89-inline explicitly tells GCC to use the C99 semantics for
"inline" when in C99 or gnu99 mode (i.e., it specifies the default behavior).
This option is not supported in -std=c90 or -std=gnu90 mode.
The preprocessor macros "__GNUC_GNU_INLINE__" and "__GNUC_STDC_INLINE__" may be
used to check which semantics are in effect for "inline" functions.
-fpermitted-flt-eval-methods=style
ISO/IEC TS 18661-3 defines new permissible values for "FLT_EVAL_METHOD" that
indicate that operations and constants with a semantic type that is an
interchange or extended format should be evaluated to the precision and range
of that type. These new values are a superset of those permitted under
C99/C11, which does not specify the meaning of other positive values of
"FLT_EVAL_METHOD". As such, code conforming to C11 may not have been written
expecting the possibility of the new values.
-fpermitted-flt-eval-methods specifies whether the compiler should allow only
the values of "FLT_EVAL_METHOD" specified in C99/C11, or the extended set of
values specified in ISO/IEC TS 18661-3.
style is either "c11" or "ts-18661-3" as appropriate.
The default when in a standards compliant mode (-std=c11 or similar) is
-fpermitted-flt-eval-methods=c11. The default when in a GNU dialect
(-std=gnu11 or similar) is -fpermitted-flt-eval-methods=ts-18661-3.
-aux-info filename
Output to the given filename prototyped declarations for all functions declared
and/or defined in a translation unit, including those in header files. This
option is silently ignored in any language other than C.
Besides declarations, the file indicates, in comments, the origin of each
declaration (source file and line), whether the declaration was implicit,
prototyped or unprototyped (I, N for new or O for old, respectively, in the
first character after the line number and the colon), and whether it came from
a declaration or a definition (C or F, respectively, in the following
character). In the case of function definitions, a K&R-style list of arguments
followed by their declarations is also provided, inside comments, after the
declaration.
-fallow-parameterless-variadic-functions
Accept variadic functions without named parameters.
Although it is possible to define such a function, this is not very useful as
it is not possible to read the arguments. This is only supported for C as this
construct is allowed by C++.
-fno-asm
Do not recognize "asm", "inline" or "typeof" as a keyword, so that code can use
these words as identifiers. You can use the keywords "__asm__", "__inline__"
and "__typeof__" instead. -ansi implies -fno-asm.
In C++, this switch only affects the "typeof" keyword, since "asm" and "inline"
are standard keywords. You may want to use the -fno-gnu-keywords flag instead,
which has the same effect. In C99 mode (-std=c99 or -std=gnu99), this switch
only affects the "asm" and "typeof" keywords, since "inline" is a standard
keyword in ISO C99.
-fno-builtin
-fno-builtin-function
Don't recognize built-in functions that do not begin with __builtin_ as prefix.
GCC normally generates special code to handle certain built-in functions more
efficiently; for instance, calls to "alloca" may become single instructions
which adjust the stack directly, and calls to "memcpy" may become inline copy
loops. The resulting code is often both smaller and faster, but since the
function calls no longer appear as such, you cannot set a breakpoint on those
calls, nor can you change the behavior of the functions by linking with a
different library. In addition, when a function is recognized as a built-in
function, GCC may use information about that function to warn about problems
with calls to that function, or to generate more efficient code, even if the
resulting code still contains calls to that function. For example, warnings
are given with -Wformat for bad calls to "printf" when "printf" is built in and
"strlen" is known not to modify global memory.
With the -fno-builtin-function option only the built-in function function is
disabled. function must not begin with __builtin_. If a function is named
that is not built-in in this version of GCC, this option is ignored. There is
no corresponding -fbuiltin-function option; if you wish to enable built-in
functions selectively when using -fno-builtin or -ffreestanding, you may define
macros such as:
#define abs(n) __builtin_abs ((n))
#define strcpy(d, s) __builtin_strcpy ((d), (s))
-fgimple
Enable parsing of function definitions marked with "__GIMPLE". This is an
experimental feature that allows unit testing of GIMPLE passes.
-fhosted
Assert that compilation targets a hosted environment. This implies -fbuiltin.
A hosted environment is one in which the entire standard library is available,
and in which "main" has a return type of "int". Examples are nearly everything
except a kernel. This is equivalent to -fno-freestanding.
-ffreestanding
Assert that compilation targets a freestanding environment. This implies
-fno-builtin. A freestanding environment is one in which the standard library
may not exist, and program startup may not necessarily be at "main". The most
obvious example is an OS kernel. This is equivalent to -fno-hosted.
-fopenacc
Enable handling of OpenACC directives "#pragma acc" in C/C++ and "!$acc" in
Fortran. When -fopenacc is specified, the compiler generates accelerated code
according to the OpenACC Application Programming Interface v2.0
<https://www.openacc.org>. This option implies -pthread, and thus is only
supported on targets that have support for -pthread.
-fopenacc-dim=geom
Specify default compute dimensions for parallel offload regions that do not
explicitly specify. The geom value is a triple of ':'-separated sizes, in
order 'gang', 'worker' and, 'vector'. A size can be omitted, to use a target-
specific default value.
-fopenmp
Enable handling of OpenMP directives "#pragma omp" in C/C++ and "!$omp" in
Fortran. When -fopenmp is specified, the compiler generates parallel code
according to the OpenMP Application Program Interface v4.5
<http://www.openmp.org/>. This option implies -pthread, and thus is only
supported on targets that have support for -pthread. -fopenmp implies
-fopenmp-simd.
-fopenmp-simd
Enable handling of OpenMP's SIMD directives with "#pragma omp" in C/C++ and
"!$omp" in Fortran. Other OpenMP directives are ignored.
-fgnu-tm
When the option -fgnu-tm is specified, the compiler generates code for the
Linux variant of Intel's current Transactional Memory ABI specification
document (Revision 1.1, May 6 2009). This is an experimental feature whose
interface may change in future versions of GCC, as the official specification
changes. Please note that not all architectures are supported for this
feature.
For more information on GCC's support for transactional memory,
Note that the transactional memory feature is not supported with non-call
exceptions (-fnon-call-exceptions).
-fms-extensions
Accept some non-standard constructs used in Microsoft header files.
In C++ code, this allows member names in structures to be similar to previous
types declarations.
typedef int UOW;
struct ABC {
UOW UOW;
};
Some cases of unnamed fields in structures and unions are only accepted with
this option.
Note that this option is off for all targets but x86 targets using ms-abi.
-fplan9-extensions
Accept some non-standard constructs used in Plan 9 code.
This enables -fms-extensions, permits passing pointers to structures with
anonymous fields to functions that expect pointers to elements of the type of
the field, and permits referring to anonymous fields declared using a typedef.
This is only supported for C, not C++.
-fcond-mismatch
Allow conditional expressions with mismatched types in the second and third
arguments. The value of such an expression is void. This option is not
supported for C++.
-flax-vector-conversions
Allow implicit conversions between vectors with differing numbers of elements
and/or incompatible element types. This option should not be used for new
code.
-funsigned-char
Let the type "char" be unsigned, like "unsigned char".
Each kind of machine has a default for what "char" should be. It is either
like "unsigned char" by default or like "signed char" by default.
Ideally, a portable program should always use "signed char" or "unsigned char"
when it depends on the signedness of an object. But many programs have been
written to use plain "char" and expect it to be signed, or expect it to be
unsigned, depending on the machines they were written for. This option, and
its inverse, let you make such a program work with the opposite default.
The type "char" is always a distinct type from each of "signed char" or
"unsigned char", even though its behavior is always just like one of those two.
-fsigned-char
Let the type "char" be signed, like "signed char".
Note that this is equivalent to -fno-unsigned-char, which is the negative form
of -funsigned-char. Likewise, the option -fno-signed-char is equivalent to
-funsigned-char.
-fsigned-bitfields
-funsigned-bitfields
-fno-signed-bitfields
-fno-unsigned-bitfields
These options control whether a bit-field is signed or unsigned, when the
declaration does not use either "signed" or "unsigned". By default, such a
bit-field is signed, because this is consistent: the basic integer types such
as "int" are signed types.
-fsso-struct=endianness
Set the default scalar storage order of structures and unions to the specified
endianness. The accepted values are big-endian, little-endian and native for
the native endianness of the target (the default). This option is not
supported for C++.
Warning: the -fsso-struct switch causes GCC to generate code that is not binary
compatible with code generated without it if the specified endianness is not
the native endianness of the target.
Options Controlling C++ Dialect
This section describes the command-line options that are only meaningful for C++
programs. You can also use most of the GNU compiler options regardless of what
language your program is in. For example, you might compile a file firstClass.C
like this:
g++ -g -fstrict-enums -O -c firstClass.C
In this example, only -fstrict-enums is an option meant only for C++ programs; you
can use the other options with any language supported by GCC.
Some options for compiling C programs, such as -std, are also relevant for C++
programs.
Here is a list of options that are only for compiling C++ programs:
-fabi-version=n
Use version n of the C++ ABI. The default is version 0.
Version 0 refers to the version conforming most closely to the C++ ABI
specification. Therefore, the ABI obtained using version 0 will change in
different versions of G++ as ABI bugs are fixed.
Version 1 is the version of the C++ ABI that first appeared in G++ 3.2.
Version 2 is the version of the C++ ABI that first appeared in G++ 3.4, and was
the default through G++ 4.9.
Version 3 corrects an error in mangling a constant address as a template
argument.
Version 4, which first appeared in G++ 4.5, implements a standard mangling for
vector types.
Version 5, which first appeared in G++ 4.6, corrects the mangling of attribute
const/volatile on function pointer types, decltype of a plain decl, and use of
a function parameter in the declaration of another parameter.
Version 6, which first appeared in G++ 4.7, corrects the promotion behavior of
C++11 scoped enums and the mangling of template argument packs,
const/static_cast, prefix ++ and --, and a class scope function used as a
template argument.
Version 7, which first appeared in G++ 4.8, that treats nullptr_t as a builtin
type and corrects the mangling of lambdas in default argument scope.
Version 8, which first appeared in G++ 4.9, corrects the substitution behavior
of function types with function-cv-qualifiers.
Version 9, which first appeared in G++ 5.2, corrects the alignment of
"nullptr_t".
Version 10, which first appeared in G++ 6.1, adds mangling of attributes that
affect type identity, such as ia32 calling convention attributes (e.g.
stdcall).
Version 11, which first appeared in G++ 7, corrects the mangling of sizeof...
expressions and operator names. For multiple entities with the same name
within a function, that are declared in different scopes, the mangling now
changes starting with the twelfth occurrence. It also implies
-fnew-inheriting-ctors.
Version 12, which first appeared in G++ 8, corrects the calling conventions for
empty classes on the x86_64 target and for classes with only deleted copy/move
constructors. It accidentally changes the calling convention for classes with
a deleted copy constructor and a trivial move constructor.
Version 13, which first appeared in G++ 8.2, fixes the accidental change in
version 12.
See also -Wabi.
-fabi-compat-version=n
On targets that support strong aliases, G++ works around mangling changes by
creating an alias with the correct mangled name when defining a symbol with an
incorrect mangled name. This switch specifies which ABI version to use for the
alias.
With -fabi-version=0 (the default), this defaults to 11 (GCC 7 compatibility).
If another ABI version is explicitly selected, this defaults to 0. For
compatibility with GCC versions 3.2 through 4.9, use -fabi-compat-version=2.
If this option is not provided but -Wabi=n is, that version is used for
compatibility aliases. If this option is provided along with -Wabi (without
the version), the version from this option is used for the warning.
-fno-access-control
Turn off all access checking. This switch is mainly useful for working around
bugs in the access control code.
-faligned-new
Enable support for C++17 "new" of types that require more alignment than "void*
::operator new(std::size_t)" provides. A numeric argument such as
"-faligned-new=32" can be used to specify how much alignment (in bytes) is
provided by that function, but few users will need to override the default of
"alignof(std::max_align_t)".
This flag is enabled by default for -std=c++17.
-fcheck-new
Check that the pointer returned by "operator new" is non-null before attempting
to modify the storage allocated. This check is normally unnecessary because
the C++ standard specifies that "operator new" only returns 0 if it is declared
"throw()", in which case the compiler always checks the return value even
without this option. In all other cases, when "operator new" has a non-empty
exception specification, memory exhaustion is signalled by throwing
"std::bad_alloc". See also new (nothrow).
-fconcepts
Enable support for the C++ Extensions for Concepts Technical Specification, ISO
19217 (2015), which allows code like
template <class T> concept bool Addable = requires (T t) { t + t; };
template <Addable T> T add (T a, T b) { return a + b; }
-fconstexpr-depth=n
Set the maximum nested evaluation depth for C++11 constexpr functions to n. A
limit is needed to detect endless recursion during constant expression
evaluation. The minimum specified by the standard is 512.
-fconstexpr-loop-limit=n
Set the maximum number of iterations for a loop in C++14 constexpr functions to
n. A limit is needed to detect infinite loops during constant expression
evaluation. The default is 262144 (1<<18).
-fdeduce-init-list
Enable deduction of a template type parameter as "std::initializer_list" from a
brace-enclosed initializer list, i.e.
template <class T> auto forward(T t) -> decltype (realfn (t))
{
return realfn (t);
}
void f()
{
forward({1,2}); // call forward<std::initializer_list<int>>
}
This deduction was implemented as a possible extension to the originally
proposed semantics for the C++11 standard, but was not part of the final
standard, so it is disabled by default. This option is deprecated, and may be
removed in a future version of G++.
-ffriend-injection
Inject friend functions into the enclosing namespace, so that they are visible
outside the scope of the class in which they are declared. Friend functions
were documented to work this way in the old Annotated C++ Reference Manual.
However, in ISO C++ a friend function that is not declared in an enclosing
scope can only be found using argument dependent lookup. GCC defaults to the
standard behavior.
This option is deprecated and will be removed.
-fno-elide-constructors
The C++ standard allows an implementation to omit creating a temporary that is
only used to initialize another object of the same type. Specifying this
option disables that optimization, and forces G++ to call the copy constructor
in all cases. This option also causes G++ to call trivial member functions
which otherwise would be expanded inline.
In C++17, the compiler is required to omit these temporaries, but this option
still affects trivial member functions.
-fno-enforce-eh-specs
Don't generate code to check for violation of exception specifications at run
time. This option violates the C++ standard, but may be useful for reducing
code size in production builds, much like defining "NDEBUG". This does not
give user code permission to throw exceptions in violation of the exception
specifications; the compiler still optimizes based on the specifications, so
throwing an unexpected exception results in undefined behavior at run time.
-fextern-tls-init
-fno-extern-tls-init
The C++11 and OpenMP standards allow "thread_local" and "threadprivate"
variables to have dynamic (runtime) initialization. To support this, any use
of such a variable goes through a wrapper function that performs any necessary
initialization. When the use and definition of the variable are in the same
translation unit, this overhead can be optimized away, but when the use is in a
different translation unit there is significant overhead even if the variable
doesn't actually need dynamic initialization. If the programmer can be sure
that no use of the variable in a non-defining TU needs to trigger dynamic
initialization (either because the variable is statically initialized, or a use
of the variable in the defining TU will be executed before any uses in another
TU), they can avoid this overhead with the -fno-extern-tls-init option.
On targets that support symbol aliases, the default is -fextern-tls-init. On
targets that do not support symbol aliases, the default is
-fno-extern-tls-init.
-ffor-scope
-fno-for-scope
If -ffor-scope is specified, the scope of variables declared in a for-init-
statement is limited to the "for" loop itself, as specified by the C++
standard. If -fno-for-scope is specified, the scope of variables declared in a
for-init-statement extends to the end of the enclosing scope, as was the case
in old versions of G++, and other (traditional) implementations of C++.
This option is deprecated and the associated non-standard functionality will be
removed.
-fno-gnu-keywords
Do not recognize "typeof" as a keyword, so that code can use this word as an
identifier. You can use the keyword "__typeof__" instead. This option is
implied by the strict ISO C++ dialects: -ansi, -std=c++98, -std=c++11, etc.
-fno-implicit-templates
Never emit code for non-inline templates that are instantiated implicitly (i.e.
by use); only emit code for explicit instantiations.
-fno-implicit-inline-templates
Don't emit code for implicit instantiations of inline templates, either. The
default is to handle inlines differently so that compiles with and without
optimization need the same set of explicit instantiations.
-fno-implement-inlines
To save space, do not emit out-of-line copies of inline functions controlled by
"#pragma implementation". This causes linker errors if these functions are not
inlined everywhere they are called.
-fms-extensions
Disable Wpedantic warnings about constructs used in MFC, such as implicit int
and getting a pointer to member function via non-standard syntax.
-fnew-inheriting-ctors
Enable the P0136 adjustment to the semantics of C++11 constructor inheritance.
This is part of C++17 but also considered to be a Defect Report against C++11
and C++14. This flag is enabled by default unless -fabi-version=10 or lower is
specified.
-fnew-ttp-matching
Enable the P0522 resolution to Core issue 150, template template parameters and
default arguments: this allows a template with default template arguments as an
argument for a template template parameter with fewer template parameters.
This flag is enabled by default for -std=c++17.
-fno-nonansi-builtins
Disable built-in declarations of functions that are not mandated by ANSI/ISO C.
These include "ffs", "alloca", "_exit", "index", "bzero", "conjf", and other
related functions.
-fnothrow-opt
Treat a "throw()" exception specification as if it were a "noexcept"
specification to reduce or eliminate the text size overhead relative to a
function with no exception specification. If the function has local variables
of types with non-trivial destructors, the exception specification actually
makes the function smaller because the EH cleanups for those variables can be
optimized away. The semantic effect is that an exception thrown out of a
function with such an exception specification results in a call to "terminate"
rather than "unexpected".
-fno-operator-names
Do not treat the operator name keywords "and", "bitand", "bitor", "compl",
"not", "or" and "xor" as synonyms as keywords.
-fno-optional-diags
Disable diagnostics that the standard says a compiler does not need to issue.
Currently, the only such diagnostic issued by G++ is the one for a name having
multiple meanings within a class.
-fpermissive
Downgrade some diagnostics about nonconformant code from errors to warnings.
Thus, using -fpermissive allows some nonconforming code to compile.
-fno-pretty-templates
When an error message refers to a specialization of a function template, the
compiler normally prints the signature of the template followed by the template
arguments and any typedefs or typenames in the signature (e.g. "void f(T) [with
T = int]" rather than "void f(int)") so that it's clear which template is
involved. When an error message refers to a specialization of a class
template, the compiler omits any template arguments that match the default
template arguments for that template. If either of these behaviors make it
harder to understand the error message rather than easier, you can use
-fno-pretty-templates to disable them.
-frepo
Enable automatic template instantiation at link time. This option also implies
-fno-implicit-templates.
-fno-rtti
Disable generation of information about every class with virtual functions for
use by the C++ run-time type identification features ("dynamic_cast" and
"typeid"). If you don't use those parts of the language, you can save some
space by using this flag. Note that exception handling uses the same
information, but G++ generates it as needed. The "dynamic_cast" operator can
still be used for casts that do not require run-time type information, i.e.
casts to "void *" or to unambiguous base classes.
-fsized-deallocation
Enable the built-in global declarations
void operator delete (void *, std::size_t) noexcept;
void operator delete[] (void *, std::size_t) noexcept;
as introduced in C++14. This is useful for user-defined replacement
deallocation functions that, for example, use the size of the object to make
deallocation faster. Enabled by default under -std=c++14 and above. The flag
-Wsized-deallocation warns about places that might want to add a definition.
-fstrict-enums
Allow the compiler to optimize using the assumption that a value of enumerated
type can only be one of the values of the enumeration (as defined in the C++
standard; basically, a value that can be represented in the minimum number of
bits needed to represent all the enumerators). This assumption may not be
valid if the program uses a cast to convert an arbitrary integer value to the
enumerated type.
-fstrong-eval-order
Evaluate member access, array subscripting, and shift expressions in left-to-
right order, and evaluate assignment in right-to-left order, as adopted for
C++17. Enabled by default with -std=c++17. -fstrong-eval-order=some enables
just the ordering of member access and shift expressions, and is the default
without -std=c++17.
-ftemplate-backtrace-limit=n
Set the maximum number of template instantiation notes for a single warning or
error to n. The default value is 10.
-ftemplate-depth=n
Set the maximum instantiation depth for template classes to n. A limit on the
template instantiation depth is needed to detect endless recursions during
template class instantiation. ANSI/ISO C++ conforming programs must not rely
on a maximum depth greater than 17 (changed to 1024 in C++11). The default
value is 900, as the compiler can run out of stack space before hitting 1024 in
some situations.
-fno-threadsafe-statics
Do not emit the extra code to use the routines specified in the C++ ABI for
thread-safe initialization of local statics. You can use this option to reduce
code size slightly in code that doesn't need to be thread-safe.
-fuse-cxa-atexit
Register destructors for objects with static storage duration with the
"__cxa_atexit" function rather than the "atexit" function. This option is
required for fully standards-compliant handling of static destructors, but only
works if your C library supports "__cxa_atexit".
-fno-use-cxa-get-exception-ptr
Don't use the "__cxa_get_exception_ptr" runtime routine. This causes
"std::uncaught_exception" to be incorrect, but is necessary if the runtime
routine is not available.
-fvisibility-inlines-hidden
This switch declares that the user does not attempt to compare pointers to
inline functions or methods where the addresses of the two functions are taken
in different shared objects.
The effect of this is that GCC may, effectively, mark inline methods with
"__attribute__ ((visibility ("hidden")))" so that they do not appear in the
export table of a DSO and do not require a PLT indirection when used within the
DSO. Enabling this option can have a dramatic effect on load and link times of
a DSO as it massively reduces the size of the dynamic export table when the
library makes heavy use of templates.
The behavior of this switch is not quite the same as marking the methods as
hidden directly, because it does not affect static variables local to the
function or cause the compiler to deduce that the function is defined in only
one shared object.
You may mark a method as having a visibility explicitly to negate the effect of
the switch for that method. For example, if you do want to compare pointers to
a particular inline method, you might mark it as having default visibility.
Marking the enclosing class with explicit visibility has no effect.
Explicitly instantiated inline methods are unaffected by this option as their
linkage might otherwise cross a shared library boundary.
-fvisibility-ms-compat
This flag attempts to use visibility settings to make GCC's C++ linkage model
compatible with that of Microsoft Visual Studio.
The flag makes these changes to GCC's linkage model:
1. It sets the default visibility to "hidden", like -fvisibility=hidden.
2. Types, but not their members, are not hidden by default.
3. The One Definition Rule is relaxed for types without explicit visibility
specifications that are defined in more than one shared object: those
declarations are permitted if they are permitted when this option is not
used.
In new code it is better to use -fvisibility=hidden and export those classes
that are intended to be externally visible. Unfortunately it is possible for
code to rely, perhaps accidentally, on the Visual Studio behavior.
Among the consequences of these changes are that static data members of the
same type with the same name but defined in different shared objects are
different, so changing one does not change the other; and that pointers to
function members defined in different shared objects may not compare equal.
When this flag is given, it is a violation of the ODR to define types with the
same name differently.
-fno-weak
Do not use weak symbol support, even if it is provided by the linker. By
default, G++ uses weak symbols if they are available. This option exists only
for testing, and should not be used by end-users; it results in inferior code
and has no benefits. This option may be removed in a future release of G++.
-nostdinc++
Do not search for header files in the standard directories specific to C++, but
do still search the other standard directories. (This option is used when
building the C++ library.)
In addition, these optimization, warning, and code generation options have meanings
only for C++ programs:
-Wabi (C, Objective-C, C++ and Objective-C++ only)
Warn when G++ it generates code that is probably not compatible with the
vendor-neutral C++ ABI. Since G++ now defaults to updating the ABI with each
major release, normally -Wabi will warn only if there is a check added later in
a release series for an ABI issue discovered since the initial release. -Wabi
will warn about more things if an older ABI version is selected (with
-fabi-version=n).
-Wabi can also be used with an explicit version number to warn about
compatibility with a particular -fabi-version level, e.g. -Wabi=2 to warn about
changes relative to -fabi-version=2.
If an explicit version number is provided and -fabi-compat-version is not
specified, the version number from this option is used for compatibility
aliases. If no explicit version number is provided with this option, but
-fabi-compat-version is specified, that version number is used for ABI
warnings.
Although an effort has been made to warn about all such cases, there are
probably some cases that are not warned about, even though G++ is generating
incompatible code. There may also be cases where warnings are emitted even
though the code that is generated is compatible.
You should rewrite your code to avoid these warnings if you are concerned about
the fact that code generated by G++ may not be binary compatible with code
generated by other compilers.
Known incompatibilities in -fabi-version=2 (which was the default from GCC 3.4
to 4.9) include:
* A template with a non-type template parameter of reference type was mangled
incorrectly:
extern int N;
template <int &> struct S {};
void n (S<N>) {2}
This was fixed in -fabi-version=3.
* SIMD vector types declared using "__attribute ((vector_size))" were mangled
in a non-standard way that does not allow for overloading of functions
taking vectors of different sizes.
The mangling was changed in -fabi-version=4.
* "__attribute ((const))" and "noreturn" were mangled as type qualifiers, and
"decltype" of a plain declaration was folded away.
These mangling issues were fixed in -fabi-version=5.
* Scoped enumerators passed as arguments to a variadic function are promoted
like unscoped enumerators, causing "va_arg" to complain. On most targets
this does not actually affect the parameter passing ABI, as there is no way
to pass an argument smaller than "int".
Also, the ABI changed the mangling of template argument packs,
"const_cast", "static_cast", prefix increment/decrement, and a class scope
function used as a template argument.
These issues were corrected in -fabi-version=6.
* Lambdas in default argument scope were mangled incorrectly, and the ABI
changed the mangling of "nullptr_t".
These issues were corrected in -fabi-version=7.
* When mangling a function type with function-cv-qualifiers, the un-qualified
function type was incorrectly treated as a substitution candidate.
This was fixed in -fabi-version=8, the default for GCC 5.1.
* "decltype(nullptr)" incorrectly had an alignment of 1, leading to unaligned
accesses. Note that this did not affect the ABI of a function with a
"nullptr_t" parameter, as parameters have a minimum alignment.
This was fixed in -fabi-version=9, the default for GCC 5.2.
* Target-specific attributes that affect the identity of a type, such as ia32
calling conventions on a function type (stdcall, regparm, etc.), did not
affect the mangled name, leading to name collisions when function pointers
were used as template arguments.
This was fixed in -fabi-version=10, the default for GCC 6.1.
It also warns about psABI-related changes. The known psABI changes at this
point include:
* For SysV/x86-64, unions with "long double" members are passed in memory as
specified in psABI. For example:
union U {
long double ld;
int i;
};
"union U" is always passed in memory.
-Wabi-tag (C++ and Objective-C++ only)
Warn when a type with an ABI tag is used in a context that does not have that
ABI tag. See C++ Attributes for more information about ABI tags.
-Wctor-dtor-privacy (C++ and Objective-C++ only)
Warn when a class seems unusable because all the constructors or destructors in
that class are private, and it has neither friends nor public static member
functions. Also warn if there are no non-private methods, and there's at least
one private member function that isn't a constructor or destructor.
-Wdelete-non-virtual-dtor (C++ and Objective-C++ only)
Warn when "delete" is used to destroy an instance of a class that has virtual
functions and non-virtual destructor. It is unsafe to delete an instance of a
derived class through a pointer to a base class if the base class does not have
a virtual destructor. This warning is enabled by -Wall.
-Wliteral-suffix (C++ and Objective-C++ only)
Warn when a string or character literal is followed by a ud-suffix which does
not begin with an underscore. As a conforming extension, GCC treats such
suffixes as separate preprocessing tokens in order to maintain backwards
compatibility with code that uses formatting macros from "<inttypes.h>". For
example:
#define __STDC_FORMAT_MACROS
#include <inttypes.h>
#include <stdio.h>
int main() {
int64_t i64 = 123;
printf("My int64: %" PRId64"\n", i64);
}
In this case, "PRId64" is treated as a separate preprocessing token.
Additionally, warn when a user-defined literal operator is declared with a
literal suffix identifier that doesn't begin with an underscore. Literal suffix
identifiers that don't begin with an underscore are reserved for future
standardization.
This warning is enabled by default.
-Wlto-type-mismatch
During the link-time optimization warn about type mismatches in global
declarations from different compilation units. Requires -flto to be enabled.
Enabled by default.
-Wno-narrowing (C++ and Objective-C++ only)
For C++11 and later standards, narrowing conversions are diagnosed by default,
as required by the standard. A narrowing conversion from a constant produces
an error, and a narrowing conversion from a non-constant produces a warning,
but -Wno-narrowing suppresses the diagnostic. Note that this does not affect
the meaning of well-formed code; narrowing conversions are still considered
ill-formed in SFINAE contexts.
With -Wnarrowing in C++98, warn when a narrowing conversion prohibited by C++11
occurs within { }, e.g.
int i = { 2.2 }; // error: narrowing from double to int
This flag is included in -Wall and -Wc++11-compat.
-Wnoexcept (C++ and Objective-C++ only)
Warn when a noexcept-expression evaluates to false because of a call to a
function that does not have a non-throwing exception specification (i.e.
"throw()" or "noexcept") but is known by the compiler to never throw an
exception.
-Wnoexcept-type (C++ and Objective-C++ only)
Warn if the C++17 feature making "noexcept" part of a function type changes the
mangled name of a symbol relative to C++14. Enabled by -Wabi and
-Wc++17-compat.
As an example:
template <class T> void f(T t) { t(); };
void g() noexcept;
void h() { f(g); }
In C++14, "f" calls "f<void(*)()>", but in C++17 it calls
"f<void(*)()noexcept>".
-Wclass-memaccess (C++ and Objective-C++ only)
Warn when the destination of a call to a raw memory function such as "memset"
or "memcpy" is an object of class type, and when writing into such an object
might bypass the class non-trivial or deleted constructor or copy assignment,
violate const-correctness or encapsulation, or corrupt virtual table pointers.
Modifying the representation of such objects may violate invariants maintained
by member functions of the class. For example, the call to "memset" below is
undefined because it modifies a non-trivial class object and is, therefore,
diagnosed. The safe way to either initialize or clear the storage of objects
of such types is by using the appropriate constructor or assignment operator,
if one is available.
std::string str = "abc";
memset (&str, 0, sizeof str);
The -Wclass-memaccess option is enabled by -Wall. Explicitly casting the
pointer to the class object to "void *" or to a type that can be safely
accessed by the raw memory function suppresses the warning.
-Wnon-virtual-dtor (C++ and Objective-C++ only)
Warn when a class has virtual functions and an accessible non-virtual
destructor itself or in an accessible polymorphic base class, in which case it
is possible but unsafe to delete an instance of a derived class through a
pointer to the class itself or base class. This warning is automatically
enabled if -Weffc++ is specified.
-Wregister (C++ and Objective-C++ only)
Warn on uses of the "register" storage class specifier, except when it is part
of the GNU Explicit Register Variables extension. The use of the "register"
keyword as storage class specifier has been deprecated in C++11 and removed in
C++17. Enabled by default with -std=c++17.
-Wreorder (C++ and Objective-C++ only)
Warn when the order of member initializers given in the code does not match the
order in which they must be executed. For instance:
struct A {
int i;
int j;
A(): j (0), i (1) { }
};
The compiler rearranges the member initializers for "i" and "j" to match the
declaration order of the members, emitting a warning to that effect. This
warning is enabled by -Wall.
-fext-numeric-literals (C++ and Objective-C++ only)
Accept imaginary, fixed-point, or machine-defined literal number suffixes as
GNU extensions. When this option is turned off these suffixes are treated as
C++11 user-defined literal numeric suffixes. This is on by default for all
pre-C++11 dialects and all GNU dialects: -std=c++98, -std=gnu++98,
-std=gnu++11, -std=gnu++14. This option is off by default for ISO C++11
onwards (-std=c++11, ...).
The following -W... options are not affected by -Wall.
-Weffc++ (C++ and Objective-C++ only)
Warn about violations of the following style guidelines from Scott Meyers'
Effective C++ series of books:
* Define a copy constructor and an assignment operator for classes with
dynamically-allocated memory.
* Prefer initialization to assignment in constructors.
* Have "operator=" return a reference to *this.
* Don't try to return a reference when you must return an object.
* Distinguish between prefix and postfix forms of increment and decrement
operators.
* Never overload "&&", "||", or ",".
This option also enables -Wnon-virtual-dtor, which is also one of the effective
C++ recommendations. However, the check is extended to warn about the lack of
virtual destructor in accessible non-polymorphic bases classes too.
When selecting this option, be aware that the standard library headers do not
obey all of these guidelines; use grep -v to filter out those warnings.
-Wstrict-null-sentinel (C++ and Objective-C++ only)
Warn about the use of an uncasted "NULL" as sentinel. When compiling only with
GCC this is a valid sentinel, as "NULL" is defined to "__null". Although it is
a null pointer constant rather than a null pointer, it is guaranteed to be of
the same size as a pointer. But this use is not portable across different
compilers.
-Wno-non-template-friend (C++ and Objective-C++ only)
Disable warnings when non-template friend functions are declared within a
template. In very old versions of GCC that predate implementation of the ISO
standard, declarations such as friend int foo(int), where the name of the
friend is an unqualified-id, could be interpreted as a particular
specialization of a template function; the warning exists to diagnose
compatibility problems, and is enabled by default.
-Wold-style-cast (C++ and Objective-C++ only)
Warn if an old-style (C-style) cast to a non-void type is used within a C++
program. The new-style casts ("dynamic_cast", "static_cast",
"reinterpret_cast", and "const_cast") are less vulnerable to unintended effects
and much easier to search for.
-Woverloaded-virtual (C++ and Objective-C++ only)
Warn when a function declaration hides virtual functions from a base class.
For example, in:
struct A {
virtual void f();
};
struct B: public A {
void f(int);
};
the "A" class version of "f" is hidden in "B", and code like:
B* b;
b->f();
fails to compile.
-Wno-pmf-conversions (C++ and Objective-C++ only)
Disable the diagnostic for converting a bound pointer to member function to a
plain pointer.
-Wsign-promo (C++ and Objective-C++ only)
Warn when overload resolution chooses a promotion from unsigned or enumerated
type to a signed type, over a conversion to an unsigned type of the same size.
Previous versions of G++ tried to preserve unsignedness, but the standard
mandates the current behavior.
-Wtemplates (C++ and Objective-C++ only)
Warn when a primary template declaration is encountered. Some coding rules
disallow templates, and this may be used to enforce that rule. The warning is
inactive inside a system header file, such as the STL, so one can still use the
STL. One may also instantiate or specialize templates.
-Wmultiple-inheritance (C++ and Objective-C++ only)
Warn when a class is defined with multiple direct base classes. Some coding
rules disallow multiple inheritance, and this may be used to enforce that rule.
The warning is inactive inside a system header file, such as the STL, so one
can still use the STL. One may also define classes that indirectly use
multiple inheritance.
-Wvirtual-inheritance
Warn when a class is defined with a virtual direct base class. Some coding
rules disallow multiple inheritance, and this may be used to enforce that rule.
The warning is inactive inside a system header file, such as the STL, so one
can still use the STL. One may also define classes that indirectly use virtual
inheritance.
-Wnamespaces
Warn when a namespace definition is opened. Some coding rules disallow
namespaces, and this may be used to enforce that rule. The warning is inactive
inside a system header file, such as the STL, so one can still use the STL.
One may also use using directives and qualified names.
-Wno-terminate (C++ and Objective-C++ only)
Disable the warning about a throw-expression that will immediately result in a
call to "terminate".
Options Controlling Objective-C and Objective-C++ Dialects
(NOTE: This manual does not describe the Objective-C and Objective-C++ languages
themselves.
This section describes the command-line options that are only meaningful for
Objective-C and Objective-C++ programs. You can also use most of the language-
independent GNU compiler options. For example, you might compile a file
some_class.m like this:
gcc -g -fgnu-runtime -O -c some_class.m
In this example, -fgnu-runtime is an option meant only for Objective-C and
Objective-C++ programs; you can use the other options with any language supported
by GCC.
Note that since Objective-C is an extension of the C language, Objective-C
compilations may also use options specific to the C front-end (e.g.,
-Wtraditional). Similarly, Objective-C++ compilations may use C++-specific options
(e.g., -Wabi).
Here is a list of options that are only for compiling Objective-C and Objective-C++
programs:
-fconstant-string-class=class-name
Use class-name as the name of the class to instantiate for each literal string
specified with the syntax "@"..."". The default class name is
"NXConstantString" if the GNU runtime is being used, and "NSConstantString" if
the NeXT runtime is being used (see below). The -fconstant-cfstrings option,
if also present, overrides the -fconstant-string-class setting and cause
"@"..."" literals to be laid out as constant CoreFoundation strings.
-fgnu-runtime
Generate object code compatible with the standard GNU Objective-C runtime.
This is the default for most types of systems.
-fnext-runtime
Generate output compatible with the NeXT runtime. This is the default for
NeXT-based systems, including Darwin and Mac OS X. The macro
"__NEXT_RUNTIME__" is predefined if (and only if) this option is used.
-fno-nil-receivers
Assume that all Objective-C message dispatches ("[receiver message:arg]") in
this translation unit ensure that the receiver is not "nil". This allows for
more efficient entry points in the runtime to be used. This option is only
available in conjunction with the NeXT runtime and ABI version 0 or 1.
-fobjc-abi-version=n
Use version n of the Objective-C ABI for the selected runtime. This option is
currently supported only for the NeXT runtime. In that case, Version 0 is the
traditional (32-bit) ABI without support for properties and other Objective-C
2.0 additions. Version 1 is the traditional (32-bit) ABI with support for
properties and other Objective-C 2.0 additions. Version 2 is the modern
(64-bit) ABI. If nothing is specified, the default is Version 0 on 32-bit
target machines, and Version 2 on 64-bit target machines.
-fobjc-call-cxx-cdtors
For each Objective-C class, check if any of its instance variables is a C++
object with a non-trivial default constructor. If so, synthesize a special "-
(id) .cxx_construct" instance method which runs non-trivial default
constructors on any such instance variables, in order, and then return "self".
Similarly, check if any instance variable is a C++ object with a non-trivial
destructor, and if so, synthesize a special "- (void) .cxx_destruct" method
which runs all such default destructors, in reverse order.
The "- (id) .cxx_construct" and "- (void) .cxx_destruct" methods thusly
generated only operate on instance variables declared in the current Objective-
C class, and not those inherited from superclasses. It is the responsibility
of the Objective-C runtime to invoke all such methods in an object's
inheritance hierarchy. The "- (id) .cxx_construct" methods are invoked by the
runtime immediately after a new object instance is allocated; the "- (void)
.cxx_destruct" methods are invoked immediately before the runtime deallocates
an object instance.
As of this writing, only the NeXT runtime on Mac OS X 10.4 and later has
support for invoking the "- (id) .cxx_construct" and "- (void) .cxx_destruct"
methods.
-fobjc-direct-dispatch
Allow fast jumps to the message dispatcher. On Darwin this is accomplished via
the comm page.
-fobjc-exceptions
Enable syntactic support for structured exception handling in Objective-C,
similar to what is offered by C++. This option is required to use the
Objective-C keywords @try, @throw, @catch, @finally and @synchronized. This
option is available with both the GNU runtime and the NeXT runtime (but not
available in conjunction with the NeXT runtime on Mac OS X 10.2 and earlier).
-fobjc-gc
Enable garbage collection (GC) in Objective-C and Objective-C++ programs. This
option is only available with the NeXT runtime; the GNU runtime has a different
garbage collection implementation that does not require special compiler flags.
-fobjc-nilcheck
For the NeXT runtime with version 2 of the ABI, check for a nil receiver in
method invocations before doing the actual method call. This is the default
and can be disabled using -fno-objc-nilcheck. Class methods and super calls
are never checked for nil in this way no matter what this flag is set to.
Currently this flag does nothing when the GNU runtime, or an older version of
the NeXT runtime ABI, is used.
-fobjc-std=objc1
Conform to the language syntax of Objective-C 1.0, the language recognized by
GCC 4.0. This only affects the Objective-C additions to the C/C++ language; it
does not affect conformance to C/C++ standards, which is controlled by the
separate C/C++ dialect option flags. When this option is used with the
Objective-C or Objective-C++ compiler, any Objective-C syntax that is not
recognized by GCC 4.0 is rejected. This is useful if you need to make sure
that your Objective-C code can be compiled with older versions of GCC.
-freplace-objc-classes
Emit a special marker instructing ld(1) not to statically link in the resulting
object file, and allow dyld(1) to load it in at run time instead. This is used
in conjunction with the Fix-and-Continue debugging mode, where the object file
in question may be recompiled and dynamically reloaded in the course of program
execution, without the need to restart the program itself. Currently, Fix-and-
Continue functionality is only available in conjunction with the NeXT runtime
on Mac OS X 10.3 and later.
-fzero-link
When compiling for the NeXT runtime, the compiler ordinarily replaces calls to
"objc_getClass("...")" (when the name of the class is known at compile time)
with static class references that get initialized at load time, which improves
run-time performance. Specifying the -fzero-link flag suppresses this behavior
and causes calls to "objc_getClass("...")" to be retained. This is useful in
Zero-Link debugging mode, since it allows for individual class implementations
to be modified during program execution. The GNU runtime currently always
retains calls to "objc_get_class("...")" regardless of command-line options.
-fno-local-ivars
By default instance variables in Objective-C can be accessed as if they were
local variables from within the methods of the class they're declared in. This
can lead to shadowing between instance variables and other variables declared
either locally inside a class method or globally with the same name.
Specifying the -fno-local-ivars flag disables this behavior thus avoiding
variable shadowing issues.
-fivar-visibility=[public|protected|private|package]
Set the default instance variable visibility to the specified option so that
instance variables declared outside the scope of any access modifier directives
default to the specified visibility.
-gen-decls
Dump interface declarations for all classes seen in the source file to a file
named sourcename.decl.
-Wassign-intercept (Objective-C and Objective-C++ only)
Warn whenever an Objective-C assignment is being intercepted by the garbage
collector.
-Wno-protocol (Objective-C and Objective-C++ only)
If a class is declared to implement a protocol, a warning is issued for every
method in the protocol that is not implemented by the class. The default
behavior is to issue a warning for every method not explicitly implemented in
the class, even if a method implementation is inherited from the superclass.
If you use the -Wno-protocol option, then methods inherited from the superclass
are considered to be implemented, and no warning is issued for them.
-Wselector (Objective-C and Objective-C++ only)
Warn if multiple methods of different types for the same selector are found
during compilation. The check is performed on the list of methods in the final
stage of compilation. Additionally, a check is performed for each selector
appearing in a "@selector(...)" expression, and a corresponding method for
that selector has been found during compilation. Because these checks scan the
method table only at the end of compilation, these warnings are not produced if
the final stage of compilation is not reached, for example because an error is
found during compilation, or because the -fsyntax-only option is being used.
-Wstrict-selector-match (Objective-C and Objective-C++ only)
Warn if multiple methods with differing argument and/or return types are found
for a given selector when attempting to send a message using this selector to a
receiver of type "id" or "Class". When this flag is off (which is the default
behavior), the compiler omits such warnings if any differences found are
confined to types that share the same size and alignment.
-Wundeclared-selector (Objective-C and Objective-C++ only)
Warn if a "@selector(...)" expression referring to an undeclared selector is
found. A selector is considered undeclared if no method with that name has
been declared before the "@selector(...)" expression, either explicitly in an
@interface or @protocol declaration, or implicitly in an @implementation
section. This option always performs its checks as soon as a "@selector(...)"
expression is found, while -Wselector only performs its checks in the final
stage of compilation. This also enforces the coding style convention that
methods and selectors must be declared before being used.
-print-objc-runtime-info
Generate C header describing the largest structure that is passed by value, if
any.
Options to Control Diagnostic Messages Formatting
Traditionally, diagnostic messages have been formatted irrespective of the output
device's aspect (e.g. its width, ...). You can use the options described below to
control the formatting algorithm for diagnostic messages, e.g. how many characters
per line, how often source location information should be reported. Note that some
language front ends may not honor these options.
-fmessage-length=n
Try to format error messages so that they fit on lines of about n characters.
If n is zero, then no line-wrapping is done; each error message appears on a
single line. This is the default for all front ends.
-fdiagnostics-show-location=once
Only meaningful in line-wrapping mode. Instructs the diagnostic messages
reporter to emit source location information once; that is, in case the message
is too long to fit on a single physical line and has to be wrapped, the source
location won't be emitted (as prefix) again, over and over, in subsequent
continuation lines. This is the default behavior.
-fdiagnostics-show-location=every-line
Only meaningful in line-wrapping mode. Instructs the diagnostic messages
reporter to emit the same source location information (as prefix) for physical
lines that result from the process of breaking a message which is too long to
fit on a single line.
-fdiagnostics-color[=WHEN]
-fno-diagnostics-color
Use color in diagnostics. WHEN is never, always, or auto. The default depends
on how the compiler has been configured, it can be any of the above WHEN
options or also never if GCC_COLORS environment variable isn't present in the
environment, and auto otherwise. auto means to use color only when the
standard error is a terminal. The forms -fdiagnostics-color and
-fno-diagnostics-color are aliases for -fdiagnostics-color=always and
-fdiagnostics-color=never, respectively.
The colors are defined by the environment variable GCC_COLORS. Its value is a
colon-separated list of capabilities and Select Graphic Rendition (SGR)
substrings. SGR commands are interpreted by the terminal or terminal emulator.
(See the section in the documentation of your text terminal for permitted
values and their meanings as character attributes.) These substring values are
integers in decimal representation and can be concatenated with semicolons.
Common values to concatenate include 1 for bold, 4 for underline, 5 for blink,
7 for inverse, 39 for default foreground color, 30 to 37 for foreground colors,
90 to 97 for 16-color mode foreground colors, 38;5;0 to 38;5;255 for 88-color
and 256-color modes foreground colors, 49 for default background color, 40 to
47 for background colors, 100 to 107 for 16-color mode background colors, and
48;5;0 to 48;5;255 for 88-color and 256-color modes background colors.
The default GCC_COLORS is
error=01;31:warning=01;35:note=01;36:range1=32:range2=34:locus=01:\
quote=01:fixit-insert=32:fixit-delete=31:\
diff-filename=01:diff-hunk=32:diff-delete=31:diff-insert=32:\
type-diff=01;32
where 01;31 is bold red, 01;35 is bold magenta, 01;36 is bold cyan, 32 is
green, 34 is blue, 01 is bold, and 31 is red. Setting GCC_COLORS to the empty
string disables colors. Supported capabilities are as follows.
"error="
SGR substring for error: markers.
"warning="
SGR substring for warning: markers.
"note="
SGR substring for note: markers.
"range1="
SGR substring for first additional range.
"range2="
SGR substring for second additional range.
"locus="
SGR substring for location information, file:line or file:line:column etc.
"quote="
SGR substring for information printed within quotes.
"fixit-insert="
SGR substring for fix-it hints suggesting text to be inserted or replaced.
"fixit-delete="
SGR substring for fix-it hints suggesting text to be deleted.
"diff-filename="
SGR substring for filename headers within generated patches.
"diff-hunk="
SGR substring for the starts of hunks within generated patches.
"diff-delete="
SGR substring for deleted lines within generated patches.
"diff-insert="
SGR substring for inserted lines within generated patches.
"type-diff="
SGR substring for highlighting mismatching types within template arguments
in the C++ frontend.
-fno-diagnostics-show-option
By default, each diagnostic emitted includes text indicating the command-line
option that directly controls the diagnostic (if such an option is known to the
diagnostic machinery). Specifying the -fno-diagnostics-show-option flag
suppresses that behavior.
-fno-diagnostics-show-caret
By default, each diagnostic emitted includes the original source line and a
caret ^ indicating the column. This option suppresses this information. The
source line is truncated to n characters, if the -fmessage-length=n option is
given. When the output is done to the terminal, the width is limited to the
width given by the COLUMNS environment variable or, if not set, to the terminal
width.
-fdiagnostics-parseable-fixits
Emit fix-it hints in a machine-parseable format, suitable for consumption by
IDEs. For each fix-it, a line will be printed after the relevant diagnostic,
starting with the string "fix-it:". For example:
fix-it:"test.c":{45:3-45:21}:"gtk_widget_show_all"
The location is expressed as a half-open range, expressed as a count of bytes,
starting at byte 1 for the initial column. In the above example, bytes 3
through 20 of line 45 of "test.c" are to be replaced with the given string:
00000000011111111112222222222
12345678901234567890123456789
gtk_widget_showall (dlg);
^^^^^^^^^^^^^^^^^^
gtk_widget_show_all
The filename and replacement string escape backslash as "\\", tab as "\t",
newline as "\n", double quotes as "\"", non-printable characters as octal (e.g.
vertical tab as "\013").
An empty replacement string indicates that the given range is to be removed.
An empty range (e.g. "45:3-45:3") indicates that the string is to be inserted
at the given position.
-fdiagnostics-generate-patch
Print fix-it hints to stderr in unified diff format, after any diagnostics are
printed. For example:
--- test.c
+++ test.c
@ -42,5 +42,5 @
void show_cb(GtkDialog *dlg)
{
- gtk_widget_showall(dlg);
+ gtk_widget_show_all(dlg);
}
The diff may or may not be colorized, following the same rules as for
diagnostics (see -fdiagnostics-color).
-fdiagnostics-show-template-tree
In the C++ frontend, when printing diagnostics showing mismatching template
types, such as:
could not convert 'std::map<int, std::vector<double> >()'
from 'map<[...],vector<double>>' to 'map<[...],vector<float>>
the -fdiagnostics-show-template-tree flag enables printing a tree-like
structure showing the common and differing parts of the types, such as:
map<
[...],
vector<
[double != float]>>
The parts that differ are highlighted with color ("double" and "float" in this
case).
-fno-elide-type
By default when the C++ frontend prints diagnostics showing mismatching
template types, common parts of the types are printed as "[...]" to simplify
the error message. For example:
could not convert 'std::map<int, std::vector<double> >()'
from 'map<[...],vector<double>>' to 'map<[...],vector<float>>
Specifying the -fno-elide-type flag suppresses that behavior. This flag also
affects the output of the -fdiagnostics-show-template-tree flag.
-fno-show-column
Do not print column numbers in diagnostics. This may be necessary if
diagnostics are being scanned by a program that does not understand the column
numbers, such as dejagnu.
Options to Request or Suppress Warnings
Warnings are diagnostic messages that report constructions that are not inherently
erroneous but that are risky or suggest there may have been an error.
The following language-independent options do not enable specific warnings but
control the kinds of diagnostics produced by GCC.
-fsyntax-only
Check the code for syntax errors, but don't do anything beyond that.
-fmax-errors=n
Limits the maximum number of error messages to n, at which point GCC bails out
rather than attempting to continue processing the source code. If n is 0 (the
default), there is no limit on the number of error messages produced. If
-Wfatal-errors is also specified, then -Wfatal-errors takes precedence over
this option.
-w Inhibit all warning messages.
-Werror
Make all warnings into errors.
-Werror=
Make the specified warning into an error. The specifier for a warning is
appended; for example -Werror=switch turns the warnings controlled by -Wswitch
into errors. This switch takes a negative form, to be used to negate -Werror
for specific warnings; for example -Wno-error=switch makes -Wswitch warnings
not be errors, even when -Werror is in effect.
The warning message for each controllable warning includes the option that
controls the warning. That option can then be used with -Werror= and
-Wno-error= as described above. (Printing of the option in the warning message
can be disabled using the -fno-diagnostics-show-option flag.)
Note that specifying -Werror=foo automatically implies -Wfoo. However,
-Wno-error=foo does not imply anything.
-Wfatal-errors
This option causes the compiler to abort compilation on the first error
occurred rather than trying to keep going and printing further error messages.
You can request many specific warnings with options beginning with -W, for example
-Wimplicit to request warnings on implicit declarations. Each of these specific
warning options also has a negative form beginning -Wno- to turn off warnings; for
example, -Wno-implicit. This manual lists only one of the two forms, whichever is
not the default. For further language-specific options also refer to C++ Dialect
Options and Objective-C and Objective-C++ Dialect Options.
Some options, such as -Wall and -Wextra, turn on other options, such as -Wunused,
which may turn on further options, such as -Wunused-value. The combined effect of
positive and negative forms is that more specific options have priority over less
specific ones, independently of their position in the command-line. For options of
the same specificity, the last one takes effect. Options enabled or disabled via
pragmas take effect as if they appeared at the end of the command-line.
When an unrecognized warning option is requested (e.g., -Wunknown-warning), GCC
emits a diagnostic stating that the option is not recognized. However, if the
-Wno- form is used, the behavior is slightly different: no diagnostic is produced
for -Wno-unknown-warning unless other diagnostics are being produced. This allows
the use of new -Wno- options with old compilers, but if something goes wrong, the
compiler warns that an unrecognized option is present.
The effectiveness of some warnings depends on optimizations also being enabled. For
example -Wsuggest-final-types is more effective with link-time optimization and
-Wmaybe-uninitialized will not warn at all unless optimization is enabled.
-Wpedantic
-pedantic
Issue all the warnings demanded by strict ISO C and ISO C++; reject all
programs that use forbidden extensions, and some other programs that do not
follow ISO C and ISO C++. For ISO C, follows the version of the ISO C standard
specified by any -std option used.
Valid ISO C and ISO C++ programs should compile properly with or without this
option (though a rare few require -ansi or a -std option specifying the
required version of ISO C). However, without this option, certain GNU
extensions and traditional C and C++ features are supported as well. With this
option, they are rejected.
-Wpedantic does not cause warning messages for use of the alternate keywords
whose names begin and end with __. Pedantic warnings are also disabled in the
expression that follows "__extension__". However, only system header files
should use these escape routes; application programs should avoid them.
Some users try to use -Wpedantic to check programs for strict ISO C
conformance. They soon find that it does not do quite what they want: it finds
some non-ISO practices, but not all---only those for which ISO C requires a
diagnostic, and some others for which diagnostics have been added.
A feature to report any failure to conform to ISO C might be useful in some
instances, but would require considerable additional work and would be quite
different from -Wpedantic. We don't have plans to support such a feature in
the near future.
Where the standard specified with -std represents a GNU extended dialect of C,
such as gnu90 or gnu99, there is a corresponding base standard, the version of
ISO C on which the GNU extended dialect is based. Warnings from -Wpedantic are
given where they are required by the base standard. (It does not make sense
for such warnings to be given only for features not in the specified GNU C
dialect, since by definition the GNU dialects of C include all features the
compiler supports with the given option, and there would be nothing to warn
about.)
-pedantic-errors
Give an error whenever the base standard (see -Wpedantic) requires a
diagnostic, in some cases where there is undefined behavior at compile-time and
in some other cases that do not prevent compilation of programs that are valid
according to the standard. This is not equivalent to -Werror=pedantic, since
there are errors enabled by this option and not enabled by the latter and vice
versa.
-Wall
This enables all the warnings about constructions that some users consider
questionable, and that are easy to avoid (or modify to prevent the warning),
even in conjunction with macros. This also enables some language-specific
warnings described in C++ Dialect Options and Objective-C and Objective-C++
Dialect Options.
-Wall turns on the following warning flags:
-Waddress -Warray-bounds=1 (only with -O2) -Wbool-compare -Wbool-operation
-Wc++11-compat -Wc++14-compat -Wcatch-value (C++ and Objective-C++ only)
-Wchar-subscripts -Wcomment -Wduplicate-decl-specifier (C and Objective-C only)
-Wenum-compare (in C/ObjC; this is on by default in C++) -Wformat
-Wint-in-bool-context -Wimplicit (C and Objective-C only) -Wimplicit-int (C and
Objective-C only) -Wimplicit-function-declaration (C and Objective-C only)
-Winit-self (only for C++) -Wlogical-not-parentheses -Wmain (only for C/ObjC
and unless -ffreestanding) -Wmaybe-uninitialized -Wmemset-elt-size
-Wmemset-transposed-args -Wmisleading-indentation (only for C/C++)
-Wmissing-attributes -Wmissing-braces (only for C/ObjC) -Wmultistatement-macros
-Wnarrowing (only for C++) -Wnonnull -Wnonnull-compare -Wopenmp-simd
-Wparentheses -Wpointer-sign -Wreorder -Wrestrict -Wreturn-type
-Wsequence-point -Wsign-compare (only in C++) -Wsizeof-pointer-div
-Wsizeof-pointer-memaccess -Wstrict-aliasing -Wstrict-overflow=1
-Wstringop-truncation -Wswitch -Wtautological-compare -Wtrigraphs
-Wuninitialized -Wunknown-pragmas -Wunused-function -Wunused-label
-Wunused-value -Wunused-variable -Wvolatile-register-var
Note that some warning flags are not implied by -Wall. Some of them warn about
constructions that users generally do not consider questionable, but which
occasionally you might wish to check for; others warn about constructions that
are necessary or hard to avoid in some cases, and there is no simple way to
modify the code to suppress the warning. Some of them are enabled by -Wextra
but many of them must be enabled individually.
-Wextra
This enables some extra warning flags that are not enabled by -Wall. (This
option used to be called -W. The older name is still supported, but the newer
name is more descriptive.)
-Wclobbered -Wcast-function-type -Wempty-body -Wignored-qualifiers
-Wimplicit-fallthrough=3 -Wmissing-field-initializers -Wmissing-parameter-type
(C only) -Wold-style-declaration (C only) -Woverride-init -Wsign-compare (C
only) -Wtype-limits -Wuninitialized -Wshift-negative-value (in C++03 and in C99
and newer) -Wunused-parameter (only with -Wunused or -Wall)
-Wunused-but-set-parameter (only with -Wunused or -Wall)
The option -Wextra also prints warning messages for the following cases:
* A pointer is compared against integer zero with "<", "<=", ">", or ">=".
* (C++ only) An enumerator and a non-enumerator both appear in a conditional
expression.
* (C++ only) Ambiguous virtual bases.
* (C++ only) Subscripting an array that has been declared "register".
* (C++ only) Taking the address of a variable that has been declared
"register".
* (C++ only) A base class is not initialized in the copy constructor of a
derived class.
-Wchar-subscripts
Warn if an array subscript has type "char". This is a common cause of error,
as programmers often forget that this type is signed on some machines. This
warning is enabled by -Wall.
-Wchkp
Warn about an invalid memory access that is found by Pointer Bounds Checker
(-fcheck-pointer-bounds).
-Wno-coverage-mismatch
Warn if feedback profiles do not match when using the -fprofile-use option. If
a source file is changed between compiling with -fprofile-gen and with
-fprofile-use, the files with the profile feedback can fail to match the source
file and GCC cannot use the profile feedback information. By default, this
warning is enabled and is treated as an error. -Wno-coverage-mismatch can be
used to disable the warning or -Wno-error=coverage-mismatch can be used to
disable the error. Disabling the error for this warning can result in poorly
optimized code and is useful only in the case of very minor changes such as bug
fixes to an existing code-base. Completely disabling the warning is not
recommended.
-Wno-cpp
(C, Objective-C, C++, Objective-C++ and Fortran only)
Suppress warning messages emitted by "#warning" directives.
-Wdouble-promotion (C, C++, Objective-C and Objective-C++ only)
Give a warning when a value of type "float" is implicitly promoted to "double".
CPUs with a 32-bit "single-precision" floating-point unit implement "float" in
hardware, but emulate "double" in software. On such a machine, doing
computations using "double" values is much more expensive because of the
overhead required for software emulation.
It is easy to accidentally do computations with "double" because floating-point
literals are implicitly of type "double". For example, in:
float area(float radius)
{
return 3.14159 * radius * radius;
}
the compiler performs the entire computation with "double" because the
floating-point literal is a "double".
-Wduplicate-decl-specifier (C and Objective-C only)
Warn if a declaration has duplicate "const", "volatile", "restrict" or
"_Atomic" specifier. This warning is enabled by -Wall.
-Wformat
-Wformat=n
Check calls to "printf" and "scanf", etc., to make sure that the arguments
supplied have types appropriate to the format string specified, and that the
conversions specified in the format string make sense. This includes standard
functions, and others specified by format attributes, in the "printf", "scanf",
"strftime" and "strfmon" (an X/Open extension, not in the C standard) families
(or other target-specific families). Which functions are checked without
format attributes having been specified depends on the standard version
selected, and such checks of functions without the attribute specified are
disabled by -ffreestanding or -fno-builtin.
The formats are checked against the format features supported by GNU libc
version 2.2. These include all ISO C90 and C99 features, as well as features
from the Single Unix Specification and some BSD and GNU extensions. Other
library implementations may not support all these features; GCC does not
support warning about features that go beyond a particular library's
limitations. However, if -Wpedantic is used with -Wformat, warnings are given
about format features not in the selected standard version (but not for
"strfmon" formats, since those are not in any version of the C standard).
-Wformat=1
-Wformat
Option -Wformat is equivalent to -Wformat=1, and -Wno-format is equivalent
to -Wformat=0. Since -Wformat also checks for null format arguments for
several functions, -Wformat also implies -Wnonnull. Some aspects of this
level of format checking can be disabled by the options:
-Wno-format-contains-nul, -Wno-format-extra-args, and
-Wno-format-zero-length. -Wformat is enabled by -Wall.
-Wno-format-contains-nul
If -Wformat is specified, do not warn about format strings that contain NUL
bytes.
-Wno-format-extra-args
If -Wformat is specified, do not warn about excess arguments to a "printf"
or "scanf" format function. The C standard specifies that such arguments
are ignored.
Where the unused arguments lie between used arguments that are specified
with $ operand number specifications, normally warnings are still given,
since the implementation could not know what type to pass to "va_arg" to
skip the unused arguments. However, in the case of "scanf" formats, this
option suppresses the warning if the unused arguments are all pointers,
since the Single Unix Specification says that such unused arguments are
allowed.
-Wformat-overflow
-Wformat-overflow=level
Warn about calls to formatted input/output functions such as "sprintf" and
"vsprintf" that might overflow the destination buffer. When the exact
number of bytes written by a format directive cannot be determined at
compile-time it is estimated based on heuristics that depend on the level
argument and on optimization. While enabling optimization will in most
cases improve the accuracy of the warning, it may also result in false
positives.
-Wformat-overflow
-Wformat-overflow=1
Level 1 of -Wformat-overflow enabled by -Wformat employs a conservative
approach that warns only about calls that most likely overflow the
buffer. At this level, numeric arguments to format directives with
unknown values are assumed to have the value of one, and strings of
unknown length to be empty. Numeric arguments that are known to be
bounded to a subrange of their type, or string arguments whose output
is bounded either by their directive's precision or by a finite set of
string literals, are assumed to take on the value within the range that
results in the most bytes on output. For example, the call to
"sprintf" below is diagnosed because even with both a and b equal to
zero, the terminating NUL character ('\0') appended by the function to
the destination buffer will be written past its end. Increasing the
size of the buffer by a single byte is sufficient to avoid the warning,
though it may not be sufficient to avoid the overflow.
void f (int a, int b)
{
char buf [13];
sprintf (buf, "a = %i, b = %i\n", a, b);
}
-Wformat-overflow=2
Level 2 warns also about calls that might overflow the destination
buffer given an argument of sufficient length or magnitude. At level
2, unknown numeric arguments are assumed to have the minimum
representable value for signed types with a precision greater than 1,
and the maximum representable value otherwise. Unknown string
arguments whose length cannot be assumed to be bounded either by the
directive's precision, or by a finite set of string literals they may
evaluate to, or the character array they may point to, are assumed to
be 1 character long.
At level 2, the call in the example above is again diagnosed, but this
time because with a equal to a 32-bit "INT_MIN" the first %i directive
will write some of its digits beyond the end of the destination buffer.
To make the call safe regardless of the values of the two variables,
the size of the destination buffer must be increased to at least 34
bytes. GCC includes the minimum size of the buffer in an informational
note following the warning.
An alternative to increasing the size of the destination buffer is to
constrain the range of formatted values. The maximum length of string
arguments can be bounded by specifying the precision in the format
directive. When numeric arguments of format directives can be assumed
to be bounded by less than the precision of their type, choosing an
appropriate length modifier to the format specifier will reduce the
required buffer size. For example, if a and b in the example above can
be assumed to be within the precision of the "short int" type then
using either the %hi format directive or casting the argument to
"short" reduces the maximum required size of the buffer to 24 bytes.
void f (int a, int b)
{
char buf [23];
sprintf (buf, "a = %hi, b = %i\n", a, (short)b);
}
-Wno-format-zero-length
If -Wformat is specified, do not warn about zero-length formats. The C
standard specifies that zero-length formats are allowed.
-Wformat=2
Enable -Wformat plus additional format checks. Currently equivalent to
-Wformat -Wformat-nonliteral -Wformat-security -Wformat-y2k.
-Wformat-nonliteral
If -Wformat is specified, also warn if the format string is not a string
literal and so cannot be checked, unless the format function takes its
format arguments as a "va_list".
-Wformat-security
If -Wformat is specified, also warn about uses of format functions that
represent possible security problems. At present, this warns about calls
to "printf" and "scanf" functions where the format string is not a string
literal and there are no format arguments, as in "printf (foo);". This may
be a security hole if the format string came from untrusted input and
contains %n. (This is currently a subset of what -Wformat-nonliteral warns
about, but in future warnings may be added to -Wformat-security that are
not included in -Wformat-nonliteral.)
-Wformat-signedness
If -Wformat is specified, also warn if the format string requires an
unsigned argument and the argument is signed and vice versa.
-Wformat-truncation
-Wformat-truncation=level
Warn about calls to formatted input/output functions such as "snprintf" and
"vsnprintf" that might result in output truncation. When the exact number
of bytes written by a format directive cannot be determined at compile-time
it is estimated based on heuristics that depend on the level argument and
on optimization. While enabling optimization will in most cases improve
the accuracy of the warning, it may also result in false positives. Except
as noted otherwise, the option uses the same logic -Wformat-overflow.
-Wformat-truncation
-Wformat-truncation=1
Level 1 of -Wformat-truncation enabled by -Wformat employs a
conservative approach that warns only about calls to bounded functions
whose return value is unused and that will most likely result in output
truncation.
-Wformat-truncation=2
Level 2 warns also about calls to bounded functions whose return value
is used and that might result in truncation given an argument of
sufficient length or magnitude.
-Wformat-y2k
If -Wformat is specified, also warn about "strftime" formats that may yield
only a two-digit year.
-Wnonnull
Warn about passing a null pointer for arguments marked as requiring a non-null
value by the "nonnull" function attribute.
-Wnonnull is included in -Wall and -Wformat. It can be disabled with the
-Wno-nonnull option.
-Wnonnull-compare
Warn when comparing an argument marked with the "nonnull" function attribute
against null inside the function.
-Wnonnull-compare is included in -Wall. It can be disabled with the
-Wno-nonnull-compare option.
-Wnull-dereference
Warn if the compiler detects paths that trigger erroneous or undefined behavior
due to dereferencing a null pointer. This option is only active when
-fdelete-null-pointer-checks is active, which is enabled by optimizations in
most targets. The precision of the warnings depends on the optimization
options used.
-Winit-self (C, C++, Objective-C and Objective-C++ only)
Warn about uninitialized variables that are initialized with themselves. Note
this option can only be used with the -Wuninitialized option.
For example, GCC warns about "i" being uninitialized in the following snippet
only when -Winit-self has been specified:
int f()
{
int i = i;
return i;
}
This warning is enabled by -Wall in C++.
-Wimplicit-int (C and Objective-C only)
Warn when a declaration does not specify a type. This warning is enabled by
-Wall.
-Wimplicit-function-declaration (C and Objective-C only)
Give a warning whenever a function is used before being declared. In C99 mode
(-std=c99 or -std=gnu99), this warning is enabled by default and it is made
into an error by -pedantic-errors. This warning is also enabled by -Wall.
-Wimplicit (C and Objective-C only)
Same as -Wimplicit-int and -Wimplicit-function-declaration. This warning is
enabled by -Wall.
-Wimplicit-fallthrough
-Wimplicit-fallthrough is the same as -Wimplicit-fallthrough=3 and
-Wno-implicit-fallthrough is the same as -Wimplicit-fallthrough=0.
-Wimplicit-fallthrough=n
Warn when a switch case falls through. For example:
switch (cond)
{
case 1:
a = 1;
break;
case 2:
a = 2;
case 3:
a = 3;
break;
}
This warning does not warn when the last statement of a case cannot fall
through, e.g. when there is a return statement or a call to function declared
with the noreturn attribute. -Wimplicit-fallthrough= also takes into account
control flow statements, such as ifs, and only warns when appropriate. E.g.
switch (cond)
{
case 1:
if (i > 3) {
bar (5);
break;
} else if (i < 1) {
bar (0);
} else
return;
default:
...
}
Since there are occasions where a switch case fall through is desirable, GCC
provides an attribute, "__attribute__ ((fallthrough))", that is to be used
along with a null statement to suppress this warning that would normally occur:
switch (cond)
{
case 1:
bar (0);
__attribute__ ((fallthrough));
default:
...
}
C++17 provides a standard way to suppress the -Wimplicit-fallthrough warning
using "[[fallthrough]];" instead of the GNU attribute. In C++11 or C++14 users
can use "[[gnu::fallthrough]];", which is a GNU extension. Instead of these
attributes, it is also possible to add a fallthrough comment to silence the
warning. The whole body of the C or C++ style comment should match the given
regular expressions listed below. The option argument n specifies what kind of
comments are accepted:
*<-Wimplicit-fallthrough=0 disables the warning altogether.>
*<-Wimplicit-fallthrough=1 matches ".*" regular>
expression, any comment is used as fallthrough comment.
*<-Wimplicit-fallthrough=2 case insensitively matches>
".*falls?[ \t-]*thr(ough|u).*" regular expression.
*<-Wimplicit-fallthrough=3 case sensitively matches one of the>
following regular expressions:
*<"-fallthrough">
*<"@fallthrough@">
*<"lint -fallthrough[ \t]*">
*<"[ \t.!]*(ELSE,? |INTENTIONAL(LY)? )?FALL(S | |-)?THR(OUGH|U)[
\t.!]*(-[^\n\r]*)?">
*<"[ \t.!]*(Else,? |Intentional(ly)? )?Fall((s | |-)[Tt]|t)hr(ough|u)[
\t.!]*(-[^\n\r]*)?">
*<"[ \t.!]*([Ee]lse,? |[Ii]ntentional(ly)? )?fall(s | |-)?thr(ough|u)[
\t.!]*(-[^\n\r]*)?">
*<-Wimplicit-fallthrough=4 case sensitively matches one of the>
following regular expressions:
*<"-fallthrough">
*<"@fallthrough@">
*<"lint -fallthrough[ \t]*">
*<"[ \t]*FALLTHR(OUGH|U)[ \t]*">
*<-Wimplicit-fallthrough=5 doesn't recognize any comments as>
fallthrough comments, only attributes disable the warning.
The comment needs to be followed after optional whitespace and other comments
by "case" or "default" keywords or by a user label that precedes some "case" or
"default" label.
switch (cond)
{
case 1:
bar (0);
/* FALLTHRU */
default:
...
}
The -Wimplicit-fallthrough=3 warning is enabled by -Wextra.
-Wif-not-aligned (C, C++, Objective-C and Objective-C++ only)
Control if warning triggered by the "warn_if_not_aligned" attribute should be
issued. This is enabled by default. Use -Wno-if-not-aligned to disable it.
-Wignored-qualifiers (C and C++ only)
Warn if the return type of a function has a type qualifier such as "const".
For ISO C such a type qualifier has no effect, since the value returned by a
function is not an lvalue. For C++, the warning is only emitted for scalar
types or "void". ISO C prohibits qualified "void" return types on function
definitions, so such return types always receive a warning even without this
option.
This warning is also enabled by -Wextra.
-Wignored-attributes (C and C++ only)
Warn when an attribute is ignored. This is different from the -Wattributes
option in that it warns whenever the compiler decides to drop an attribute, not
that the attribute is either unknown, used in a wrong place, etc. This warning
is enabled by default.
-Wmain
Warn if the type of "main" is suspicious. "main" should be a function with
external linkage, returning int, taking either zero arguments, two, or three
arguments of appropriate types. This warning is enabled by default in C++ and
is enabled by either -Wall or -Wpedantic.
-Wmisleading-indentation (C and C++ only)
Warn when the indentation of the code does not reflect the block structure.
Specifically, a warning is issued for "if", "else", "while", and "for" clauses
with a guarded statement that does not use braces, followed by an unguarded
statement with the same indentation.
In the following example, the call to "bar" is misleadingly indented as if it
were guarded by the "if" conditional.
if (some_condition ())
foo ();
bar (); /* Gotcha: this is not guarded by the "if". */
In the case of mixed tabs and spaces, the warning uses the -ftabstop= option to
determine if the statements line up (defaulting to 8).
The warning is not issued for code involving multiline preprocessor logic such
as the following example.
if (flagA)
foo (0);
#if SOME_CONDITION_THAT_DOES_NOT_HOLD
if (flagB)
#endif
foo (1);
The warning is not issued after a "#line" directive, since this typically
indicates autogenerated code, and no assumptions can be made about the layout
of the file that the directive references.
This warning is enabled by -Wall in C and C++.
-Wmissing-attributes
Warn when a declaration of a function is missing one or more attributes that a
related function is declared with and whose absence may adversely affect the
correctness or efficiency of generated code. For example, in C++, the warning
is issued when an explicit specialization of a primary template declared with
attribute "alloc_align", "alloc_size", "assume_aligned", "format",
"format_arg", "malloc", or "nonnull" is declared without it. Attributes
"deprecated", "error", and "warning" suppress the warning..
-Wmissing-attributes is enabled by -Wall.
For example, since the declaration of the primary function template below makes
use of both attribute "malloc" and "alloc_size" the declaration of the explicit
specialization of the template is diagnosed because it is missing one of the
attributes.
template <class T>
T* __attribute__ ((malloc, alloc_size (1)))
allocate (size_t);
template <>
void* __attribute__ ((malloc)) // missing alloc_size
allocate<void> (size_t);
-Wmissing-braces
Warn if an aggregate or union initializer is not fully bracketed. In the
following example, the initializer for "a" is not fully bracketed, but that for
"b" is fully bracketed. This warning is enabled by -Wall in C.
int a[2][2] = { 0, 1, 2, 3 };
int b[2][2] = { { 0, 1 }, { 2, 3 } };
This warning is enabled by -Wall.
-Wmissing-include-dirs (C, C++, Objective-C and Objective-C++ only)
Warn if a user-supplied include directory does not exist.
-Wmultistatement-macros
Warn about unsafe multiple statement macros that appear to be guarded by a
clause such as "if", "else", "for", "switch", or "while", in which only the
first statement is actually guarded after the macro is expanded.
For example:
#define DOIT x++; y++
if (c)
DOIT;
will increment "y" unconditionally, not just when "c" holds. The can usually
be fixed by wrapping the macro in a do-while loop:
#define DOIT do { x++; y++; } while (0)
if (c)
DOIT;
This warning is enabled by -Wall in C and C++.
-Wparentheses
Warn if parentheses are omitted in certain contexts, such as when there is an
assignment in a context where a truth value is expected, or when operators are
nested whose precedence people often get confused about.
Also warn if a comparison like "x<=y<=z" appears; this is equivalent to "(x<=y
? 1 : 0) <= z", which is a different interpretation from that of ordinary
mathematical notation.
Also warn for dangerous uses of the GNU extension to "?:" with omitted middle
operand. When the condition in the "?": operator is a boolean expression, the
omitted value is always 1. Often programmers expect it to be a value computed
inside the conditional expression instead.
For C++ this also warns for some cases of unnecessary parentheses in
declarations, which can indicate an attempt at a function call instead of a
declaration:
{
// Declares a local variable called mymutex.
std::unique_lock<std::mutex> (mymutex);
// User meant std::unique_lock<std::mutex> lock (mymutex);
}
This warning is enabled by -Wall.
-Wsequence-point
Warn about code that may have undefined semantics because of violations of
sequence point rules in the C and C++ standards.
The C and C++ standards define the order in which expressions in a C/C++
program are evaluated in terms of sequence points, which represent a partial
ordering between the execution of parts of the program: those executed before
the sequence point, and those executed after it. These occur after the
evaluation of a full expression (one which is not part of a larger expression),
after the evaluation of the first operand of a "&&", "||", "? :" or "," (comma)
operator, before a function is called (but after the evaluation of its
arguments and the expression denoting the called function), and in certain
other places. Other than as expressed by the sequence point rules, the order
of evaluation of subexpressions of an expression is not specified. All these
rules describe only a partial order rather than a total order, since, for
example, if two functions are called within one expression with no sequence
point between them, the order in which the functions are called is not
specified. However, the standards committee have ruled that function calls do
not overlap.
It is not specified when between sequence points modifications to the values of
objects take effect. Programs whose behavior depends on this have undefined
behavior; the C and C++ standards specify that "Between the previous and next
sequence point an object shall have its stored value modified at most once by
the evaluation of an expression. Furthermore, the prior value shall be read
only to determine the value to be stored.". If a program breaks these rules,
the results on any particular implementation are entirely unpredictable.
Examples of code with undefined behavior are "a = a++;", "a[n] = b[n++]" and
"a[i++] = i;". Some more complicated cases are not diagnosed by this option,
and it may give an occasional false positive result, but in general it has been
found fairly effective at detecting this sort of problem in programs.
The C++17 standard will define the order of evaluation of operands in more
cases: in particular it requires that the right-hand side of an assignment be
evaluated before the left-hand side, so the above examples are no longer
undefined. But this warning will still warn about them, to help people avoid
writing code that is undefined in C and earlier revisions of C++.
The standard is worded confusingly, therefore there is some debate over the
precise meaning of the sequence point rules in subtle cases. Links to
discussions of the problem, including proposed formal definitions, may be found
on the GCC readings page, at <http://gcc.gnu.org/readings.html>.
This warning is enabled by -Wall for C and C++.
-Wno-return-local-addr
Do not warn about returning a pointer (or in C++, a reference) to a variable
that goes out of scope after the function returns.
-Wreturn-type
Warn whenever a function is defined with a return type that defaults to "int".
Also warn about any "return" statement with no return value in a function whose
return type is not "void" (falling off the end of the function body is
considered returning without a value).
For C only, warn about a "return" statement with an expression in a function
whose return type is "void", unless the expression type is also "void". As a
GNU extension, the latter case is accepted without a warning unless -Wpedantic
is used.
For C++, a function without return type always produces a diagnostic message,
even when -Wno-return-type is specified. The only exceptions are "main" and
functions defined in system headers.
This warning is enabled by default for C++ and is enabled by -Wall.
-Wshift-count-negative
Warn if shift count is negative. This warning is enabled by default.
-Wshift-count-overflow
Warn if shift count >= width of type. This warning is enabled by default.
-Wshift-negative-value
Warn if left shifting a negative value. This warning is enabled by -Wextra in
C99 and C++11 modes (and newer).
-Wshift-overflow
-Wshift-overflow=n
Warn about left shift overflows. This warning is enabled by default in C99 and
C++11 modes (and newer).
-Wshift-overflow=1
This is the warning level of -Wshift-overflow and is enabled by default in
C99 and C++11 modes (and newer). This warning level does not warn about
left-shifting 1 into the sign bit. (However, in C, such an overflow is
still rejected in contexts where an integer constant expression is
required.)
-Wshift-overflow=2
This warning level also warns about left-shifting 1 into the sign bit,
unless C++14 mode is active.
-Wswitch
Warn whenever a "switch" statement has an index of enumerated type and lacks a
"case" for one or more of the named codes of that enumeration. (The presence
of a "default" label prevents this warning.) "case" labels outside the
enumeration range also provoke warnings when this option is used (even if there
is a "default" label). This warning is enabled by -Wall.
-Wswitch-default
Warn whenever a "switch" statement does not have a "default" case.
-Wswitch-enum
Warn whenever a "switch" statement has an index of enumerated type and lacks a
"case" for one or more of the named codes of that enumeration. "case" labels
outside the enumeration range also provoke warnings when this option is used.
The only difference between -Wswitch and this option is that this option gives
a warning about an omitted enumeration code even if there is a "default" label.
-Wswitch-bool
Warn whenever a "switch" statement has an index of boolean type and the case
values are outside the range of a boolean type. It is possible to suppress
this warning by casting the controlling expression to a type other than "bool".
For example:
switch ((int) (a == 4))
{
...
}
This warning is enabled by default for C and C++ programs.
-Wswitch-unreachable
Warn whenever a "switch" statement contains statements between the controlling
expression and the first case label, which will never be executed. For
example:
switch (cond)
{
i = 15;
...
case 5:
...
}
-Wswitch-unreachable does not warn if the statement between the controlling
expression and the first case label is just a declaration:
switch (cond)
{
int i;
...
case 5:
i = 5;
...
}
This warning is enabled by default for C and C++ programs.
-Wsync-nand (C and C++ only)
Warn when "__sync_fetch_and_nand" and "__sync_nand_and_fetch" built-in
functions are used. These functions changed semantics in GCC 4.4.
-Wunused-but-set-parameter
Warn whenever a function parameter is assigned to, but otherwise unused (aside
from its declaration).
To suppress this warning use the "unused" attribute.
This warning is also enabled by -Wunused together with -Wextra.
-Wunused-but-set-variable
Warn whenever a local variable is assigned to, but otherwise unused (aside from
its declaration). This warning is enabled by -Wall.
To suppress this warning use the "unused" attribute.
This warning is also enabled by -Wunused, which is enabled by -Wall.
-Wunused-function
Warn whenever a static function is declared but not defined or a non-inline
static function is unused. This warning is enabled by -Wall.
-Wunused-label
Warn whenever a label is declared but not used. This warning is enabled by
-Wall.
To suppress this warning use the "unused" attribute.
-Wunused-local-typedefs (C, Objective-C, C++ and Objective-C++ only)
Warn when a typedef locally defined in a function is not used. This warning is
enabled by -Wall.
-Wunused-parameter
Warn whenever a function parameter is unused aside from its declaration.
To suppress this warning use the "unused" attribute.
-Wno-unused-result
Do not warn if a caller of a function marked with attribute
"warn_unused_result" does not use its return value. The default is
-Wunused-result.
-Wunused-variable
Warn whenever a local or static variable is unused aside from its declaration.
This option implies -Wunused-const-variable=1 for C, but not for C++. This
warning is enabled by -Wall.
To suppress this warning use the "unused" attribute.
-Wunused-const-variable
-Wunused-const-variable=n
Warn whenever a constant static variable is unused aside from its declaration.
-Wunused-const-variable=1 is enabled by -Wunused-variable for C, but not for
C++. In C this declares variable storage, but in C++ this is not an error since
const variables take the place of "#define"s.
To suppress this warning use the "unused" attribute.
-Wunused-const-variable=1
This is the warning level that is enabled by -Wunused-variable for C. It
warns only about unused static const variables defined in the main
compilation unit, but not about static const variables declared in any
header included.
-Wunused-const-variable=2
This warning level also warns for unused constant static variables in
headers (excluding system headers). This is the warning level of
-Wunused-const-variable and must be explicitly requested since in C++ this
isn't an error and in C it might be harder to clean up all headers
included.
-Wunused-value
Warn whenever a statement computes a result that is explicitly not used. To
suppress this warning cast the unused expression to "void". This includes an
expression-statement or the left-hand side of a comma expression that contains
no side effects. For example, an expression such as "x[i,j]" causes a warning,
while "x[(void)i,j]" does not.
This warning is enabled by -Wall.
-Wunused
All the above -Wunused options combined.
In order to get a warning about an unused function parameter, you must either
specify -Wextra -Wunused (note that -Wall implies -Wunused), or separately
specify -Wunused-parameter.
-Wuninitialized
Warn if an automatic variable is used without first being initialized or if a
variable may be clobbered by a "setjmp" call. In C++, warn if a non-static
reference or non-static "const" member appears in a class without constructors.
If you want to warn about code that uses the uninitialized value of the
variable in its own initializer, use the -Winit-self option.
These warnings occur for individual uninitialized or clobbered elements of
structure, union or array variables as well as for variables that are
uninitialized or clobbered as a whole. They do not occur for variables or
elements declared "volatile". Because these warnings depend on optimization,
the exact variables or elements for which there are warnings depends on the
precise optimization options and version of GCC used.
Note that there may be no warning about a variable that is used only to compute
a value that itself is never used, because such computations may be deleted by
data flow analysis before the warnings are printed.
-Winvalid-memory-model
Warn for invocations of __atomic Builtins, __sync Builtins, and the C11 atomic
generic functions with a memory consistency argument that is either invalid for
the operation or outside the range of values of the "memory_order" enumeration.
For example, since the "__atomic_store" and "__atomic_store_n" built-ins are
only defined for the relaxed, release, and sequentially consistent memory
orders the following code is diagnosed:
void store (int *i)
{
__atomic_store_n (i, 0, memory_order_consume);
}
-Winvalid-memory-model is enabled by default.
-Wmaybe-uninitialized
For an automatic (i.e. local) variable, if there exists a path from the
function entry to a use of the variable that is initialized, but there exist
some other paths for which the variable is not initialized, the compiler emits
a warning if it cannot prove the uninitialized paths are not executed at run
time.
These warnings are only possible in optimizing compilation, because otherwise
GCC does not keep track of the state of variables.
These warnings are made optional because GCC may not be able to determine when
the code is correct in spite of appearing to have an error. Here is one
example of how this can happen:
{
int x;
switch (y)
{
case 1: x = 1;
break;
case 2: x = 4;
break;
case 3: x = 5;
}
foo (x);
}
If the value of "y" is always 1, 2 or 3, then "x" is always initialized, but
GCC doesn't know this. To suppress the warning, you need to provide a default
case with assert(0) or similar code.
This option also warns when a non-volatile automatic variable might be changed
by a call to "longjmp". The compiler sees only the calls to "setjmp". It
cannot know where "longjmp" will be called; in fact, a signal handler could
call it at any point in the code. As a result, you may get a warning even when
there is in fact no problem because "longjmp" cannot in fact be called at the
place that would cause a problem.
Some spurious warnings can be avoided if you declare all the functions you use
that never return as "noreturn".
This warning is enabled by -Wall or -Wextra.
-Wunknown-pragmas
Warn when a "#pragma" directive is encountered that is not understood by GCC.
If this command-line option is used, warnings are even issued for unknown
pragmas in system header files. This is not the case if the warnings are only
enabled by the -Wall command-line option.
-Wno-pragmas
Do not warn about misuses of pragmas, such as incorrect parameters, invalid
syntax, or conflicts between pragmas. See also -Wunknown-pragmas.
-Wstrict-aliasing
This option is only active when -fstrict-aliasing is active. It warns about
code that might break the strict aliasing rules that the compiler is using for
optimization. The warning does not catch all cases, but does attempt to catch
the more common pitfalls. It is included in -Wall. It is equivalent to
-Wstrict-aliasing=3
-Wstrict-aliasing=n
This option is only active when -fstrict-aliasing is active. It warns about
code that might break the strict aliasing rules that the compiler is using for
optimization. Higher levels correspond to higher accuracy (fewer false
positives). Higher levels also correspond to more effort, similar to the way
-O works. -Wstrict-aliasing is equivalent to -Wstrict-aliasing=3.
Level 1: Most aggressive, quick, least accurate. Possibly useful when higher
levels do not warn but -fstrict-aliasing still breaks the code, as it has very
few false negatives. However, it has many false positives. Warns for all
pointer conversions between possibly incompatible types, even if never
dereferenced. Runs in the front end only.
Level 2: Aggressive, quick, not too precise. May still have many false
positives (not as many as level 1 though), and few false negatives (but
possibly more than level 1). Unlike level 1, it only warns when an address is
taken. Warns about incomplete types. Runs in the front end only.
Level 3 (default for -Wstrict-aliasing): Should have very few false positives
and few false negatives. Slightly slower than levels 1 or 2 when optimization
is enabled. Takes care of the common pun+dereference pattern in the front end:
"*(int*)&some_float". If optimization is enabled, it also runs in the back
end, where it deals with multiple statement cases using flow-sensitive points-
to information. Only warns when the converted pointer is dereferenced. Does
not warn about incomplete types.
-Wstrict-overflow
-Wstrict-overflow=n
This option is only active when signed overflow is undefined. It warns about
cases where the compiler optimizes based on the assumption that signed overflow
does not occur. Note that it does not warn about all cases where the code
might overflow: it only warns about cases where the compiler implements some
optimization. Thus this warning depends on the optimization level.
An optimization that assumes that signed overflow does not occur is perfectly
safe if the values of the variables involved are such that overflow never does,
in fact, occur. Therefore this warning can easily give a false positive: a
warning about code that is not actually a problem. To help focus on important
issues, several warning levels are defined. No warnings are issued for the use
of undefined signed overflow when estimating how many iterations a loop
requires, in particular when determining whether a loop will be executed at
all.
-Wstrict-overflow=1
Warn about cases that are both questionable and easy to avoid. For example
the compiler simplifies "x + 1 > x" to 1. This level of -Wstrict-overflow
is enabled by -Wall; higher levels are not, and must be explicitly
requested.
-Wstrict-overflow=2
Also warn about other cases where a comparison is simplified to a constant.
For example: "abs (x) >= 0". This can only be simplified when signed
integer overflow is undefined, because "abs (INT_MIN)" overflows to
"INT_MIN", which is less than zero. -Wstrict-overflow (with no level) is
the same as -Wstrict-overflow=2.
-Wstrict-overflow=3
Also warn about other cases where a comparison is simplified. For example:
"x + 1 > 1" is simplified to "x > 0".
-Wstrict-overflow=4
Also warn about other simplifications not covered by the above cases. For
example: "(x * 10) / 5" is simplified to "x * 2".
-Wstrict-overflow=5
Also warn about cases where the compiler reduces the magnitude of a
constant involved in a comparison. For example: "x + 2 > y" is simplified
to "x + 1 >= y". This is reported only at the highest warning level
because this simplification applies to many comparisons, so this warning
level gives a very large number of false positives.
-Wstringop-overflow
-Wstringop-overflow=type
Warn for calls to string manipulation functions such as "memcpy" and "strcpy"
that are determined to overflow the destination buffer. The optional argument
is one greater than the type of Object Size Checking to perform to determine
the size of the destination. The argument is meaningful only for functions
that operate on character arrays but not for raw memory functions like "memcpy"
which always make use of Object Size type-0. The option also warns for calls
that specify a size in excess of the largest possible object or at most
"SIZE_MAX / 2" bytes. The option produces the best results with optimization
enabled but can detect a small subset of simple buffer overflows even without
optimization in calls to the GCC built-in functions like "__builtin_memcpy"
that correspond to the standard functions. In any case, the option warns about
just a subset of buffer overflows detected by the corresponding overflow
checking built-ins. For example, the option will issue a warning for the
"strcpy" call below because it copies at least 5 characters (the string "blue"
including the terminating NUL) into the buffer of size 4.
enum Color { blue, purple, yellow };
const char* f (enum Color clr)
{
static char buf [4];
const char *str;
switch (clr)
{
case blue: str = "blue"; break;
case purple: str = "purple"; break;
case yellow: str = "yellow"; break;
}
return strcpy (buf, str); // warning here
}
Option -Wstringop-overflow=2 is enabled by default.
-Wstringop-overflow
-Wstringop-overflow=1
The -Wstringop-overflow=1 option uses type-zero Object Size Checking to
determine the sizes of destination objects. This is the default setting of
the option. At this setting the option will not warn for writes past the
end of subobjects of larger objects accessed by pointers unless the size of
the largest surrounding object is known. When the destination may be one
of several objects it is assumed to be the largest one of them. On Linux
systems, when optimization is enabled at this setting the option warns for
the same code as when the "_FORTIFY_SOURCE" macro is defined to a non-zero
value.
-Wstringop-overflow=2
The -Wstringop-overflow=2 option uses type-one Object Size Checking to
determine the sizes of destination objects. At this setting the option
will warn about overflows when writing to members of the largest complete
objects whose exact size is known. It will, however, not warn for
excessive writes to the same members of unknown objects referenced by
pointers since they may point to arrays containing unknown numbers of
elements.
-Wstringop-overflow=3
The -Wstringop-overflow=3 option uses type-two Object Size Checking to
determine the sizes of destination objects. At this setting the option
warns about overflowing the smallest object or data member. This is the
most restrictive setting of the option that may result in warnings for safe
code.
-Wstringop-overflow=4
The -Wstringop-overflow=4 option uses type-three Object Size Checking to
determine the sizes of destination objects. At this setting the option
will warn about overflowing any data members, and when the destination is
one of several objects it uses the size of the largest of them to decide
whether to issue a warning. Similarly to -Wstringop-overflow=3 this
setting of the option may result in warnings for benign code.
-Wstringop-truncation
Warn for calls to bounded string manipulation functions such as "strncat",
"strncpy", and "stpncpy" that may either truncate the copied string or leave
the destination unchanged.
In the following example, the call to "strncat" specifies a bound that is less
than the length of the source string. As a result, the copy of the source will
be truncated and so the call is diagnosed. To avoid the warning use "bufsize -
strlen (buf) - 1)" as the bound.
void append (char *buf, size_t bufsize)
{
strncat (buf, ".txt", 3);
}
As another example, the following call to "strncpy" results in copying to "d"
just the characters preceding the terminating NUL, without appending the NUL to
the end. Assuming the result of "strncpy" is necessarily a NUL-terminated
string is a common mistake, and so the call is diagnosed. To avoid the warning
when the result is not expected to be NUL-terminated, call "memcpy" instead.
void copy (char *d, const char *s)
{
strncpy (d, s, strlen (s));
}
In the following example, the call to "strncpy" specifies the size of the
destination buffer as the bound. If the length of the source string is equal
to or greater than this size the result of the copy will not be NUL-terminated.
Therefore, the call is also diagnosed. To avoid the warning, specify "sizeof
buf - 1" as the bound and set the last element of the buffer to "NUL".
void copy (const char *s)
{
char buf[80];
strncpy (buf, s, sizeof buf);
...
}
In situations where a character array is intended to store a sequence of bytes
with no terminating "NUL" such an array may be annotated with attribute
"nonstring" to avoid this warning. Such arrays, however, are not suitable
arguments to functions that expect "NUL"-terminated strings. To help detect
accidental misuses of such arrays GCC issues warnings unless it can prove that
the use is safe.
Option -Wstringop-truncation is enabled by -Wall.
-Wsuggest-attribute=[pure|const|noreturn|format|cold|malloc]
Warn for cases where adding an attribute may be beneficial. The attributes
currently supported are listed below.
-Wsuggest-attribute=pure
-Wsuggest-attribute=const
-Wsuggest-attribute=noreturn
-Wsuggest-attribute=malloc
Warn about functions that might be candidates for attributes "pure",
"const" or "noreturn" or "malloc". The compiler only warns for functions
visible in other compilation units or (in the case of "pure" and "const")
if it cannot prove that the function returns normally. A function returns
normally if it doesn't contain an infinite loop or return abnormally by
throwing, calling "abort" or trapping. This analysis requires option
-fipa-pure-const, which is enabled by default at -O and higher. Higher
optimization levels improve the accuracy of the analysis.
-Wsuggest-attribute=format
-Wmissing-format-attribute
Warn about function pointers that might be candidates for "format"
attributes. Note these are only possible candidates, not absolute ones.
GCC guesses that function pointers with "format" attributes that are used
in assignment, initialization, parameter passing or return statements
should have a corresponding "format" attribute in the resulting type. I.e.
the left-hand side of the assignment or initialization, the type of the
parameter variable, or the return type of the containing function
respectively should also have a "format" attribute to avoid the warning.
GCC also warns about function definitions that might be candidates for
"format" attributes. Again, these are only possible candidates. GCC
guesses that "format" attributes might be appropriate for any function that
calls a function like "vprintf" or "vscanf", but this might not always be
the case, and some functions for which "format" attributes are appropriate
may not be detected.
-Wsuggest-attribute=cold
Warn about functions that might be candidates for "cold" attribute. This
is based on static detection and generally will only warn about functions
which always leads to a call to another "cold" function such as wrappers of
C++ "throw" or fatal error reporting functions leading to "abort".
-Wsuggest-final-types
Warn about types with virtual methods where code quality would be improved if
the type were declared with the C++11 "final" specifier, or, if possible,
declared in an anonymous namespace. This allows GCC to more aggressively
devirtualize the polymorphic calls. This warning is more effective with link
time optimization, where the information about the class hierarchy graph is
more complete.
-Wsuggest-final-methods
Warn about virtual methods where code quality would be improved if the method
were declared with the C++11 "final" specifier, or, if possible, its type were
declared in an anonymous namespace or with the "final" specifier. This warning
is more effective with link-time optimization, where the information about the
class hierarchy graph is more complete. It is recommended to first consider
suggestions of -Wsuggest-final-types and then rebuild with new annotations.
-Wsuggest-override
Warn about overriding virtual functions that are not marked with the override
keyword.
-Walloc-zero
Warn about calls to allocation functions decorated with attribute "alloc_size"
that specify zero bytes, including those to the built-in forms of the functions
"aligned_alloc", "alloca", "calloc", "malloc", and "realloc". Because the
behavior of these functions when called with a zero size differs among
implementations (and in the case of "realloc" has been deprecated) relying on
it may result in subtle portability bugs and should be avoided.
-Walloc-size-larger-than=n
Warn about calls to functions decorated with attribute "alloc_size" that
attempt to allocate objects larger than the specified number of bytes, or where
the result of the size computation in an integer type with infinite precision
would exceed "SIZE_MAX / 2". The option argument n may end in one of the
standard suffixes designating a multiple of bytes such as "kB" and "KiB" for
kilobyte and kibibyte, respectively, "MB" and "MiB" for megabyte and mebibyte,
and so on. -Walloc-size-larger-than=PTRDIFF_MAX is enabled by default.
Warnings controlled by the option can be disabled by specifying n of SIZE_MAX
or more.
-Walloca
This option warns on all uses of "alloca" in the source.
-Walloca-larger-than=n
This option warns on calls to "alloca" that are not bounded by a controlling
predicate limiting its argument of integer type to at most n bytes, or calls to
"alloca" where the bound is unknown. Arguments of non-integer types are
considered unbounded even if they appear to be constrained to the expected
range.
For example, a bounded case of "alloca" could be:
void func (size_t n)
{
void *p;
if (n <= 1000)
p = alloca (n);
else
p = malloc (n);
f (p);
}
In the above example, passing "-Walloca-larger-than=1000" would not issue a
warning because the call to "alloca" is known to be at most 1000 bytes.
However, if "-Walloca-larger-than=500" were passed, the compiler would emit a
warning.
Unbounded uses, on the other hand, are uses of "alloca" with no controlling
predicate constraining its integer argument. For example:
void func ()
{
void *p = alloca (n);
f (p);
}
If "-Walloca-larger-than=500" were passed, the above would trigger a warning,
but this time because of the lack of bounds checking.
Note, that even seemingly correct code involving signed integers could cause a
warning:
void func (signed int n)
{
if (n < 500)
{
p = alloca (n);
f (p);
}
}
In the above example, n could be negative, causing a larger than expected
argument to be implicitly cast into the "alloca" call.
This option also warns when "alloca" is used in a loop.
This warning is not enabled by -Wall, and is only active when -ftree-vrp is
active (default for -O2 and above).
See also -Wvla-larger-than=n.
-Warray-bounds
-Warray-bounds=n
This option is only active when -ftree-vrp is active (default for -O2 and
above). It warns about subscripts to arrays that are always out of bounds. This
warning is enabled by -Wall.
-Warray-bounds=1
This is the warning level of -Warray-bounds and is enabled by -Wall; higher
levels are not, and must be explicitly requested.
-Warray-bounds=2
This warning level also warns about out of bounds access for arrays at the
end of a struct and for arrays accessed through pointers. This warning
level may give a larger number of false positives and is deactivated by
default.
-Wattribute-alias
Warn about declarations using the "alias" and similar attributes whose target
is incompatible with the type of the alias.
-Wbidirectional=[none|unpaired|any]
Warn about UTF-8 bidirectional characters. Such characters can change left-to-
right writing direction into right-to-left (and vice versa), which can cause
confusion between the logical order and visual order. This may be dangerous;
for instance, it may seem that a piece of code is not commented out, whereas it
in fact is.
There are three levels of warning supported by GCC. The default is
-Wbidirectional=unpaired, which warns about improperly terminated bidi
contexts. -Wbidirectional=none turns the warning off. -Wbidirectional=any
warns about any use of bidirectional characters.
-Wbool-compare
Warn about boolean expression compared with an integer value different from
"true"/"false". For instance, the following comparison is always false:
int n = 5;
...
if ((n > 1) == 2) { ... }
This warning is enabled by -Wall.
-Wbool-operation
Warn about suspicious operations on expressions of a boolean type. For
instance, bitwise negation of a boolean is very likely a bug in the program.
For C, this warning also warns about incrementing or decrementing a boolean,
which rarely makes sense. (In C++, decrementing a boolean is always invalid.
Incrementing a boolean is invalid in C++17, and deprecated otherwise.)
This warning is enabled by -Wall.
-Wduplicated-branches
Warn when an if-else has identical branches. This warning detects cases like
if (p != NULL)
return 0;
else
return 0;
It doesn't warn when both branches contain just a null statement. This warning
also warn for conditional operators:
int i = x ? *p : *p;
-Wduplicated-cond
Warn about duplicated conditions in an if-else-if chain. For instance, warn
for the following code:
if (p->q != NULL) { ... }
else if (p->q != NULL) { ... }
-Wframe-address
Warn when the __builtin_frame_address or __builtin_return_address is called
with an argument greater than 0. Such calls may return indeterminate values or
crash the program. The warning is included in -Wall.
-Wno-discarded-qualifiers (C and Objective-C only)
Do not warn if type qualifiers on pointers are being discarded. Typically, the
compiler warns if a "const char *" variable is passed to a function that takes
a "char *" parameter. This option can be used to suppress such a warning.
-Wno-discarded-array-qualifiers (C and Objective-C only)
Do not warn if type qualifiers on arrays which are pointer targets are being
discarded. Typically, the compiler warns if a "const int (*)[]" variable is
passed to a function that takes a "int (*)[]" parameter. This option can be
used to suppress such a warning.
-Wno-incompatible-pointer-types (C and Objective-C only)
Do not warn when there is a conversion between pointers that have incompatible
types. This warning is for cases not covered by -Wno-pointer-sign, which warns
for pointer argument passing or assignment with different signedness.
-Wno-int-conversion (C and Objective-C only)
Do not warn about incompatible integer to pointer and pointer to integer
conversions. This warning is about implicit conversions; for explicit
conversions the warnings -Wno-int-to-pointer-cast and -Wno-pointer-to-int-cast
may be used.
-Wno-div-by-zero
Do not warn about compile-time integer division by zero. Floating-point
division by zero is not warned about, as it can be a legitimate way of
obtaining infinities and NaNs.
-Wsystem-headers
Print warning messages for constructs found in system header files. Warnings
from system headers are normally suppressed, on the assumption that they
usually do not indicate real problems and would only make the compiler output
harder to read. Using this command-line option tells GCC to emit warnings from
system headers as if they occurred in user code. However, note that using
-Wall in conjunction with this option does not warn about unknown pragmas in
system headers---for that, -Wunknown-pragmas must also be used.
-Wtautological-compare
Warn if a self-comparison always evaluates to true or false. This warning
detects various mistakes such as:
int i = 1;
...
if (i > i) { ... }
This warning also warns about bitwise comparisons that always evaluate to true
or false, for instance:
if ((a & 16) == 10) { ... }
will always be false.
This warning is enabled by -Wall.
-Wtrampolines
Warn about trampolines generated for pointers to nested functions. A
trampoline is a small piece of data or code that is created at run time on the
stack when the address of a nested function is taken, and is used to call the
nested function indirectly. For some targets, it is made up of data only and
thus requires no special treatment. But, for most targets, it is made up of
code and thus requires the stack to be made executable in order for the program
to work properly.
-Wfloat-equal
Warn if floating-point values are used in equality comparisons.
The idea behind this is that sometimes it is convenient (for the programmer) to
consider floating-point values as approximations to infinitely precise real
numbers. If you are doing this, then you need to compute (by analyzing the
code, or in some other way) the maximum or likely maximum error that the
computation introduces, and allow for it when performing comparisons (and when
producing output, but that's a different problem). In particular, instead of
testing for equality, you should check to see whether the two values have
ranges that overlap; and this is done with the relational operators, so
equality comparisons are probably mistaken.
-Wtraditional (C and Objective-C only)
Warn about certain constructs that behave differently in traditional and ISO C.
Also warn about ISO C constructs that have no traditional C equivalent, and/or
problematic constructs that should be avoided.
* Macro parameters that appear within string literals in the macro body. In
traditional C macro replacement takes place within string literals, but in
ISO C it does not.
* In traditional C, some preprocessor directives did not exist. Traditional
preprocessors only considered a line to be a directive if the # appeared in
column 1 on the line. Therefore -Wtraditional warns about directives that
traditional C understands but ignores because the # does not appear as the
first character on the line. It also suggests you hide directives like
"#pragma" not understood by traditional C by indenting them. Some
traditional implementations do not recognize "#elif", so this option
suggests avoiding it altogether.
* A function-like macro that appears without arguments.
* The unary plus operator.
* The U integer constant suffix, or the F or L floating-point constant
suffixes. (Traditional C does support the L suffix on integer constants.)
Note, these suffixes appear in macros defined in the system headers of most
modern systems, e.g. the _MIN/_MAX macros in "<limits.h>". Use of these
macros in user code might normally lead to spurious warnings, however GCC's
integrated preprocessor has enough context to avoid warning in these cases.
* A function declared external in one block and then used after the end of
the block.
* A "switch" statement has an operand of type "long".
* A non-"static" function declaration follows a "static" one. This construct
is not accepted by some traditional C compilers.
* The ISO type of an integer constant has a different width or signedness
from its traditional type. This warning is only issued if the base of the
constant is ten. I.e. hexadecimal or octal values, which typically
represent bit patterns, are not warned about.
* Usage of ISO string concatenation is detected.
* Initialization of automatic aggregates.
* Identifier conflicts with labels. Traditional C lacks a separate namespace
for labels.
* Initialization of unions. If the initializer is zero, the warning is
omitted. This is done under the assumption that the zero initializer in
user code appears conditioned on e.g. "__STDC__" to avoid missing
initializer warnings and relies on default initialization to zero in the
traditional C case.
* Conversions by prototypes between fixed/floating-point values and vice
versa. The absence of these prototypes when compiling with traditional C
causes serious problems. This is a subset of the possible conversion
warnings; for the full set use -Wtraditional-conversion.
* Use of ISO C style function definitions. This warning intentionally is not
issued for prototype declarations or variadic functions because these ISO C
features appear in your code when using libiberty's traditional C
compatibility macros, "PARAMS" and "VPARAMS". This warning is also
bypassed for nested functions because that feature is already a GCC
extension and thus not relevant to traditional C compatibility.
-Wtraditional-conversion (C and Objective-C only)
Warn if a prototype causes a type conversion that is different from what would
happen to the same argument in the absence of a prototype. This includes
conversions of fixed point to floating and vice versa, and conversions changing
the width or signedness of a fixed-point argument except when the same as the
default promotion.
-Wdeclaration-after-statement (C and Objective-C only)
Warn when a declaration is found after a statement in a block. This construct,
known from C++, was introduced with ISO C99 and is by default allowed in GCC.
It is not supported by ISO C90.
-Wshadow
Warn whenever a local variable or type declaration shadows another variable,
parameter, type, class member (in C++), or instance variable (in Objective-C)
or whenever a built-in function is shadowed. Note that in C++, the compiler
warns if a local variable shadows an explicit typedef, but not if it shadows a
struct/class/enum. Same as -Wshadow=global.
-Wno-shadow-ivar (Objective-C only)
Do not warn whenever a local variable shadows an instance variable in an
Objective-C method.
-Wshadow=global
The default for -Wshadow. Warns for any (global) shadowing.
-Wshadow=local
Warn when a local variable shadows another local variable or parameter. This
warning is enabled by -Wshadow=global.
-Wshadow=compatible-local
Warn when a local variable shadows another local variable or parameter whose
type is compatible with that of the shadowing variable. In C++, type
compatibility here means the type of the shadowing variable can be converted to
that of the shadowed variable. The creation of this flag (in addition to
-Wshadow=local) is based on the idea that when a local variable shadows another
one of incompatible type, it is most likely intentional, not a bug or typo, as
shown in the following example:
for (SomeIterator i = SomeObj.begin(); i != SomeObj.end(); ++i)
{
for (int i = 0; i < N; ++i)
{
...
}
...
}
Since the two variable "i" in the example above have incompatible types,
enabling only -Wshadow=compatible-local will not emit a warning. Because their
types are incompatible, if a programmer accidentally uses one in place of the
other, type checking will catch that and emit an error or warning. So not
warning (about shadowing) in this case will not lead to undetected bugs. Use of
this flag instead of -Wshadow=local can possibly reduce the number of warnings
triggered by intentional shadowing.
This warning is enabled by -Wshadow=local.
-Wlarger-than=len
Warn whenever an object of larger than len bytes is defined.
-Wframe-larger-than=len
Warn if the size of a function frame is larger than len bytes. The computation
done to determine the stack frame size is approximate and not conservative.
The actual requirements may be somewhat greater than len even if you do not get
a warning. In addition, any space allocated via "alloca", variable-length
arrays, or related constructs is not included by the compiler when determining
whether or not to issue a warning.
-Wno-free-nonheap-object
Do not warn when attempting to free an object that was not allocated on the
heap.
-Wstack-usage=len
Warn if the stack usage of a function might be larger than len bytes. The
computation done to determine the stack usage is conservative. Any space
allocated via "alloca", variable-length arrays, or related constructs is
included by the compiler when determining whether or not to issue a warning.
The message is in keeping with the output of -fstack-usage.
* If the stack usage is fully static but exceeds the specified amount, it's:
warning: stack usage is 1120 bytes
* If the stack usage is (partly) dynamic but bounded, it's:
warning: stack usage might be 1648 bytes
* If the stack usage is (partly) dynamic and not bounded, it's:
warning: stack usage might be unbounded
-Wno-pedantic-ms-format (MinGW targets only)
When used in combination with -Wformat and -pedantic without GNU extensions,
this option disables the warnings about non-ISO "printf" / "scanf" format width
specifiers "I32", "I64", and "I" used on Windows targets, which depend on the
MS runtime.
-Waligned-new
Warn about a new-expression of a type that requires greater alignment than the
"alignof(std::max_align_t)" but uses an allocation function without an explicit
alignment parameter. This option is enabled by -Wall.
Normally this only warns about global allocation functions, but
-Waligned-new=all also warns about class member allocation functions.
-Wplacement-new
-Wplacement-new=n
Warn about placement new expressions with undefined behavior, such as
constructing an object in a buffer that is smaller than the type of the object.
For example, the placement new expression below is diagnosed because it
attempts to construct an array of 64 integers in a buffer only 64 bytes large.
char buf [64];
new (buf) int[64];
This warning is enabled by default.
-Wplacement-new=1
This is the default warning level of -Wplacement-new. At this level the
warning is not issued for some strictly undefined constructs that GCC
allows as extensions for compatibility with legacy code. For example, the
following "new" expression is not diagnosed at this level even though it
has undefined behavior according to the C++ standard because it writes past
the end of the one-element array.
struct S { int n, a[1]; };
S *s = (S *)malloc (sizeof *s + 31 * sizeof s->a[0]);
new (s->a)int [32]();
-Wplacement-new=2
At this level, in addition to diagnosing all the same constructs as at
level 1, a diagnostic is also issued for placement new expressions that
construct an object in the last member of structure whose type is an array
of a single element and whose size is less than the size of the object
being constructed. While the previous example would be diagnosed, the
following construct makes use of the flexible member array extension to
avoid the warning at level 2.
struct S { int n, a[]; };
S *s = (S *)malloc (sizeof *s + 32 * sizeof s->a[0]);
new (s->a)int [32]();
-Wpointer-arith
Warn about anything that depends on the "size of" a function type or of "void".
GNU C assigns these types a size of 1, for convenience in calculations with
"void *" pointers and pointers to functions. In C++, warn also when an
arithmetic operation involves "NULL". This warning is also enabled by
-Wpedantic.
-Wpointer-compare
Warn if a pointer is compared with a zero character constant. This usually
means that the pointer was meant to be dereferenced. For example:
const char *p = foo ();
if (p == '\0')
return 42;
Note that the code above is invalid in C++11.
This warning is enabled by default.
-Wtype-limits
Warn if a comparison is always true or always false due to the limited range of
the data type, but do not warn for constant expressions. For example, warn if
an unsigned variable is compared against zero with "<" or ">=". This warning
is also enabled by -Wextra.
-Wcomment
-Wcomments
Warn whenever a comment-start sequence /* appears in a /* comment, or whenever
a backslash-newline appears in a // comment. This warning is enabled by -Wall.
-Wtrigraphs
Warn if any trigraphs are encountered that might change the meaning of the
program. Trigraphs within comments are not warned about, except those that
would form escaped newlines.
This option is implied by -Wall. If -Wall is not given, this option is still
enabled unless trigraphs are enabled. To get trigraph conversion without
warnings, but get the other -Wall warnings, use -trigraphs -Wall
-Wno-trigraphs.
-Wundef
Warn if an undefined identifier is evaluated in an "#if" directive. Such
identifiers are replaced with zero.
-Wexpansion-to-defined
Warn whenever defined is encountered in the expansion of a macro (including the
case where the macro is expanded by an #if directive). Such usage is not
portable. This warning is also enabled by -Wpedantic and -Wextra.
-Wunused-macros
Warn about macros defined in the main file that are unused. A macro is used if
it is expanded or tested for existence at least once. The preprocessor also
warns if the macro has not been used at the time it is redefined or undefined.
Built-in macros, macros defined on the command line, and macros defined in
include files are not warned about.
Note: If a macro is actually used, but only used in skipped conditional blocks,
then the preprocessor reports it as unused. To avoid the warning in such a
case, you might improve the scope of the macro's definition by, for example,
moving it into the first skipped block. Alternatively, you could provide a
dummy use with something like:
#if defined the_macro_causing_the_warning
#endif
-Wno-endif-labels
Do not warn whenever an "#else" or an "#endif" are followed by text. This
sometimes happens in older programs with code of the form
#if FOO
...
#else FOO
...
#endif FOO
The second and third "FOO" should be in comments. This warning is on by
default.
-Wbad-function-cast (C and Objective-C only)
Warn when a function call is cast to a non-matching type. For example, warn if
a call to a function returning an integer type is cast to a pointer type.
-Wc90-c99-compat (C and Objective-C only)
Warn about features not present in ISO C90, but present in ISO C99. For
instance, warn about use of variable length arrays, "long long" type, "bool"
type, compound literals, designated initializers, and so on. This option is
independent of the standards mode. Warnings are disabled in the expression
that follows "__extension__".
-Wc99-c11-compat (C and Objective-C only)
Warn about features not present in ISO C99, but present in ISO C11. For
instance, warn about use of anonymous structures and unions, "_Atomic" type
qualifier, "_Thread_local" storage-class specifier, "_Alignas" specifier,
"Alignof" operator, "_Generic" keyword, and so on. This option is independent
of the standards mode. Warnings are disabled in the expression that follows
"__extension__".
-Wc++-compat (C and Objective-C only)
Warn about ISO C constructs that are outside of the common subset of ISO C and
ISO C++, e.g. request for implicit conversion from "void *" to a pointer to
non-"void" type.
-Wc++11-compat (C++ and Objective-C++ only)
Warn about C++ constructs whose meaning differs between ISO C++ 1998 and ISO
C++ 2011, e.g., identifiers in ISO C++ 1998 that are keywords in ISO C++ 2011.
This warning turns on -Wnarrowing and is enabled by -Wall.
-Wc++14-compat (C++ and Objective-C++ only)
Warn about C++ constructs whose meaning differs between ISO C++ 2011 and ISO
C++ 2014. This warning is enabled by -Wall.
-Wc++17-compat (C++ and Objective-C++ only)
Warn about C++ constructs whose meaning differs between ISO C++ 2014 and ISO
C++ 2017. This warning is enabled by -Wall.
-Wcast-qual
Warn whenever a pointer is cast so as to remove a type qualifier from the
target type. For example, warn if a "const char *" is cast to an ordinary
"char *".
Also warn when making a cast that introduces a type qualifier in an unsafe way.
For example, casting "char **" to "const char **" is unsafe, as in this
example:
/* p is char ** value. */
const char **q = (const char **) p;
/* Assignment of readonly string to const char * is OK. */
*q = "string";
/* Now char** pointer points to read-only memory. */
**p = 'b';
-Wcast-align
Warn whenever a pointer is cast such that the required alignment of the target
is increased. For example, warn if a "char *" is cast to an "int *" on
machines where integers can only be accessed at two- or four-byte boundaries.
-Wcast-align=strict
Warn whenever a pointer is cast such that the required alignment of the target
is increased. For example, warn if a "char *" is cast to an "int *" regardless
of the target machine.
-Wcast-function-type
Warn when a function pointer is cast to an incompatible function pointer. In a
cast involving function types with a variable argument list only the types of
initial arguments that are provided are considered. Any parameter of pointer-
type matches any other pointer-type. Any benign differences in integral types
are ignored, like "int" vs. "long" on ILP32 targets. Likewise type qualifiers
are ignored. The function type "void (*) (void)" is special and matches
everything, which can be used to suppress this warning. In a cast involving
pointer to member types this warning warns whenever the type cast is changing
the pointer to member type. This warning is enabled by -Wextra.
-Wwrite-strings
When compiling C, give string constants the type "const char[length]" so that
copying the address of one into a non-"const" "char *" pointer produces a
warning. These warnings help you find at compile time code that can try to
write into a string constant, but only if you have been very careful about
using "const" in declarations and prototypes. Otherwise, it is just a
nuisance. This is why we did not make -Wall request these warnings.
When compiling C++, warn about the deprecated conversion from string literals
to "char *". This warning is enabled by default for C++ programs.
-Wcatch-value
-Wcatch-value=n (C++ and Objective-C++ only)
Warn about catch handlers that do not catch via reference. With
-Wcatch-value=1 (or -Wcatch-value for short) warn about polymorphic class types
that are caught by value. With -Wcatch-value=2 warn about all class types that
are caught by value. With -Wcatch-value=3 warn about all types that are not
caught by reference. -Wcatch-value is enabled by -Wall.
-Wclobbered
Warn for variables that might be changed by "longjmp" or "vfork". This warning
is also enabled by -Wextra.
-Wconditionally-supported (C++ and Objective-C++ only)
Warn for conditionally-supported (C++11 [intro.defs]) constructs.
-Wconversion
Warn for implicit conversions that may alter a value. This includes conversions
between real and integer, like "abs (x)" when "x" is "double"; conversions
between signed and unsigned, like "unsigned ui = -1"; and conversions to
smaller types, like "sqrtf (M_PI)". Do not warn for explicit casts like "abs
((int) x)" and "ui = (unsigned) -1", or if the value is not changed by the
conversion like in "abs (2.0)". Warnings about conversions between signed and
unsigned integers can be disabled by using -Wno-sign-conversion.
For C++, also warn for confusing overload resolution for user-defined
conversions; and conversions that never use a type conversion operator:
conversions to "void", the same type, a base class or a reference to them.
Warnings about conversions between signed and unsigned integers are disabled by
default in C++ unless -Wsign-conversion is explicitly enabled.
-Wno-conversion-null (C++ and Objective-C++ only)
Do not warn for conversions between "NULL" and non-pointer types.
-Wconversion-null is enabled by default.
-Wzero-as-null-pointer-constant (C++ and Objective-C++ only)
Warn when a literal 0 is used as null pointer constant. This can be useful to
facilitate the conversion to "nullptr" in C++11.
-Wsubobject-linkage (C++ and Objective-C++ only)
Warn if a class type has a base or a field whose type uses the anonymous
namespace or depends on a type with no linkage. If a type A depends on a type
B with no or internal linkage, defining it in multiple translation units would
be an ODR violation because the meaning of B is different in each translation
unit. If A only appears in a single translation unit, the best way to silence
the warning is to give it internal linkage by putting it in an anonymous
namespace as well. The compiler doesn't give this warning for types defined in
the main .C file, as those are unlikely to have multiple definitions.
-Wsubobject-linkage is enabled by default.
-Wdangling-else
Warn about constructions where there may be confusion to which "if" statement
an "else" branch belongs. Here is an example of such a case:
{
if (a)
if (b)
foo ();
else
bar ();
}
In C/C++, every "else" branch belongs to the innermost possible "if" statement,
which in this example is "if (b)". This is often not what the programmer
expected, as illustrated in the above example by indentation the programmer
chose. When there is the potential for this confusion, GCC issues a warning
when this flag is specified. To eliminate the warning, add explicit braces
around the innermost "if" statement so there is no way the "else" can belong to
the enclosing "if". The resulting code looks like this:
{
if (a)
{
if (b)
foo ();
else
bar ();
}
}
This warning is enabled by -Wparentheses.
-Wdate-time
Warn when macros "__TIME__", "__DATE__" or "__TIMESTAMP__" are encountered as
they might prevent bit-wise-identical reproducible compilations.
-Wdelete-incomplete (C++ and Objective-C++ only)
Warn when deleting a pointer to incomplete type, which may cause undefined
behavior at runtime. This warning is enabled by default.
-Wuseless-cast (C++ and Objective-C++ only)
Warn when an expression is casted to its own type.
-Wempty-body
Warn if an empty body occurs in an "if", "else" or "do while" statement. This
warning is also enabled by -Wextra.
-Wenum-compare
Warn about a comparison between values of different enumerated types. In C++
enumerated type mismatches in conditional expressions are also diagnosed and
the warning is enabled by default. In C this warning is enabled by -Wall.
-Wextra-semi (C++, Objective-C++ only)
Warn about redundant semicolon after in-class function definition.
-Wjump-misses-init (C, Objective-C only)
Warn if a "goto" statement or a "switch" statement jumps forward across the
initialization of a variable, or jumps backward to a label after the variable
has been initialized. This only warns about variables that are initialized
when they are declared. This warning is only supported for C and Objective-C;
in C++ this sort of branch is an error in any case.
-Wjump-misses-init is included in -Wc++-compat. It can be disabled with the
-Wno-jump-misses-init option.
-Wsign-compare
Warn when a comparison between signed and unsigned values could produce an
incorrect result when the signed value is converted to unsigned. In C++, this
warning is also enabled by -Wall. In C, it is also enabled by -Wextra.
-Wsign-conversion
Warn for implicit conversions that may change the sign of an integer value,
like assigning a signed integer expression to an unsigned integer variable. An
explicit cast silences the warning. In C, this option is enabled also by
-Wconversion.
-Wfloat-conversion
Warn for implicit conversions that reduce the precision of a real value. This
includes conversions from real to integer, and from higher precision real to
lower precision real values. This option is also enabled by -Wconversion.
-Wno-scalar-storage-order
Do not warn on suspicious constructs involving reverse scalar storage order.
-Wsized-deallocation (C++ and Objective-C++ only)
Warn about a definition of an unsized deallocation function
void operator delete (void *) noexcept;
void operator delete[] (void *) noexcept;
without a definition of the corresponding sized deallocation function
void operator delete (void *, std::size_t) noexcept;
void operator delete[] (void *, std::size_t) noexcept;
or vice versa. Enabled by -Wextra along with -fsized-deallocation.
-Wsizeof-pointer-div
Warn for suspicious divisions of two sizeof expressions that divide the pointer
size by the element size, which is the usual way to compute the array size but
won't work out correctly with pointers. This warning warns e.g. about "sizeof
(ptr) / sizeof (ptr[0])" if "ptr" is not an array, but a pointer. This warning
is enabled by -Wall.
-Wsizeof-pointer-memaccess
Warn for suspicious length parameters to certain string and memory built-in
functions if the argument uses "sizeof". This warning triggers for example for
"memset (ptr, 0, sizeof (ptr));" if "ptr" is not an array, but a pointer, and
suggests a possible fix, or about "memcpy (&foo, ptr, sizeof (&foo));".
-Wsizeof-pointer-memaccess also warns about calls to bounded string copy
functions like "strncat" or "strncpy" that specify as the bound a "sizeof"
expression of the source array. For example, in the following function the
call to "strncat" specifies the size of the source string as the bound. That
is almost certainly a mistake and so the call is diagnosed.
void make_file (const char *name)
{
char path[PATH_MAX];
strncpy (path, name, sizeof path - 1);
strncat (path, ".text", sizeof ".text");
...
}
The -Wsizeof-pointer-memaccess option is enabled by -Wall.
-Wsizeof-array-argument
Warn when the "sizeof" operator is applied to a parameter that is declared as
an array in a function definition. This warning is enabled by default for C
and C++ programs.
-Wmemset-elt-size
Warn for suspicious calls to the "memset" built-in function, if the first
argument references an array, and the third argument is a number equal to the
number of elements, but not equal to the size of the array in memory. This
indicates that the user has omitted a multiplication by the element size. This
warning is enabled by -Wall.
-Wmemset-transposed-args
Warn for suspicious calls to the "memset" built-in function, if the second
argument is not zero and the third argument is zero. This warns e.g.@ about
"memset (buf, sizeof buf, 0)" where most probably "memset (buf, 0, sizeof buf)"
was meant instead. The diagnostics is only emitted if the third argument is
literal zero. If it is some expression that is folded to zero, a cast of zero
to some type, etc., it is far less likely that the user has mistakenly
exchanged the arguments and no warning is emitted. This warning is enabled by
-Wall.
-Waddress
Warn about suspicious uses of memory addresses. These include using the address
of a function in a conditional expression, such as "void func(void); if
(func)", and comparisons against the memory address of a string literal, such
as "if (x == "abc")". Such uses typically indicate a programmer error: the
address of a function always evaluates to true, so their use in a conditional
usually indicate that the programmer forgot the parentheses in a function call;
and comparisons against string literals result in unspecified behavior and are
not portable in C, so they usually indicate that the programmer intended to use
"strcmp". This warning is enabled by -Wall.
-Wlogical-op
Warn about suspicious uses of logical operators in expressions. This includes
using logical operators in contexts where a bit-wise operator is likely to be
expected. Also warns when the operands of a logical operator are the same:
extern int a;
if (a < 0 && a < 0) { ... }
-Wlogical-not-parentheses
Warn about logical not used on the left hand side operand of a comparison.
This option does not warn if the right operand is considered to be a boolean
expression. Its purpose is to detect suspicious code like the following:
int a;
...
if (!a > 1) { ... }
It is possible to suppress the warning by wrapping the LHS into parentheses:
if ((!a) > 1) { ... }
This warning is enabled by -Wall.
-Waggregate-return
Warn if any functions that return structures or unions are defined or called.
(In languages where you can return an array, this also elicits a warning.)
-Wno-aggressive-loop-optimizations
Warn if in a loop with constant number of iterations the compiler detects
undefined behavior in some statement during one or more of the iterations.
-Wno-attributes
Do not warn if an unexpected "__attribute__" is used, such as unrecognized
attributes, function attributes applied to variables, etc. This does not stop
errors for incorrect use of supported attributes.
-Wno-builtin-declaration-mismatch
Warn if a built-in function is declared with the wrong signature or as non-
function. This warning is enabled by default.
-Wno-builtin-macro-redefined
Do not warn if certain built-in macros are redefined. This suppresses warnings
for redefinition of "__TIMESTAMP__", "__TIME__", "__DATE__", "__FILE__", and
"__BASE_FILE__".
-Wstrict-prototypes (C and Objective-C only)
Warn if a function is declared or defined without specifying the argument
types. (An old-style function definition is permitted without a warning if
preceded by a declaration that specifies the argument types.)
-Wold-style-declaration (C and Objective-C only)
Warn for obsolescent usages, according to the C Standard, in a declaration. For
example, warn if storage-class specifiers like "static" are not the first
things in a declaration. This warning is also enabled by -Wextra.
-Wold-style-definition (C and Objective-C only)
Warn if an old-style function definition is used. A warning is given even if
there is a previous prototype.
-Wmissing-parameter-type (C and Objective-C only)
A function parameter is declared without a type specifier in K&R-style
functions:
void foo(bar) { }
This warning is also enabled by -Wextra.
-Wmissing-prototypes (C and Objective-C only)
Warn if a global function is defined without a previous prototype declaration.
This warning is issued even if the definition itself provides a prototype. Use
this option to detect global functions that do not have a matching prototype
declaration in a header file. This option is not valid for C++ because all
function declarations provide prototypes and a non-matching declaration
declares an overload rather than conflict with an earlier declaration. Use
-Wmissing-declarations to detect missing declarations in C++.
-Wmissing-declarations
Warn if a global function is defined without a previous declaration. Do so
even if the definition itself provides a prototype. Use this option to detect
global functions that are not declared in header files. In C, no warnings are
issued for functions with previous non-prototype declarations; use
-Wmissing-prototypes to detect missing prototypes. In C++, no warnings are
issued for function templates, or for inline functions, or for functions in
anonymous namespaces.
-Wmissing-field-initializers
Warn if a structure's initializer has some fields missing. For example, the
following code causes such a warning, because "x.h" is implicitly zero:
struct s { int f, g, h; };
struct s x = { 3, 4 };
This option does not warn about designated initializers, so the following
modification does not trigger a warning:
struct s { int f, g, h; };
struct s x = { .f = 3, .g = 4 };
In C this option does not warn about the universal zero initializer { 0 }:
struct s { int f, g, h; };
struct s x = { 0 };
Likewise, in C++ this option does not warn about the empty { } initializer, for
example:
struct s { int f, g, h; };
s x = { };
This warning is included in -Wextra. To get other -Wextra warnings without
this one, use -Wextra -Wno-missing-field-initializers.
-Wno-multichar
Do not warn if a multicharacter constant ('FOOF') is used. Usually they
indicate a typo in the user's code, as they have implementation-defined values,
and should not be used in portable code.
-Wnormalized=[none|id|nfc|nfkc]
In ISO C and ISO C++, two identifiers are different if they are different
sequences of characters. However, sometimes when characters outside the basic
ASCII character set are used, you can have two different character sequences
that look the same. To avoid confusion, the ISO 10646 standard sets out some
normalization rules which when applied ensure that two sequences that look the
same are turned into the same sequence. GCC can warn you if you are using
identifiers that have not been normalized; this option controls that warning.
There are four levels of warning supported by GCC. The default is
-Wnormalized=nfc, which warns about any identifier that is not in the ISO 10646
"C" normalized form, NFC. NFC is the recommended form for most uses. It is
equivalent to -Wnormalized.
Unfortunately, there are some characters allowed in identifiers by ISO C and
ISO C++ that, when turned into NFC, are not allowed in identifiers. That is,
there's no way to use these symbols in portable ISO C or C++ and have all your
identifiers in NFC. -Wnormalized=id suppresses the warning for these
characters. It is hoped that future versions of the standards involved will
correct this, which is why this option is not the default.
You can switch the warning off for all characters by writing -Wnormalized=none
or -Wno-normalized. You should only do this if you are using some other
normalization scheme (like "D"), because otherwise you can easily create bugs
that are literally impossible to see.
Some characters in ISO 10646 have distinct meanings but look identical in some
fonts or display methodologies, especially once formatting has been applied.
For instance "\u207F", "SUPERSCRIPT LATIN SMALL LETTER N", displays just like a
regular "n" that has been placed in a superscript. ISO 10646 defines the NFKC
normalization scheme to convert all these into a standard form as well, and GCC
warns if your code is not in NFKC if you use -Wnormalized=nfkc. This warning
is comparable to warning about every identifier that contains the letter O
because it might be confused with the digit 0, and so is not the default, but
may be useful as a local coding convention if the programming environment
cannot be fixed to display these characters distinctly.
-Wno-deprecated
Do not warn about usage of deprecated features.
-Wno-deprecated-declarations
Do not warn about uses of functions, variables, and types marked as deprecated
by using the "deprecated" attribute.
-Wno-overflow
Do not warn about compile-time overflow in constant expressions.
-Wno-odr
Warn about One Definition Rule violations during link-time optimization.
Requires -flto-odr-type-merging to be enabled. Enabled by default.
-Wopenmp-simd
Warn if the vectorizer cost model overrides the OpenMP simd directive set by
user. The -fsimd-cost-model=unlimited option can be used to relax the cost
model.
-Woverride-init (C and Objective-C only)
Warn if an initialized field without side effects is overridden when using
designated initializers.
This warning is included in -Wextra. To get other -Wextra warnings without
this one, use -Wextra -Wno-override-init.
-Woverride-init-side-effects (C and Objective-C only)
Warn if an initialized field with side effects is overridden when using
designated initializers. This warning is enabled by default.
-Wpacked
Warn if a structure is given the packed attribute, but the packed attribute has
no effect on the layout or size of the structure. Such structures may be mis-
aligned for little benefit. For instance, in this code, the variable "f.x" in
"struct bar" is misaligned even though "struct bar" does not itself have the
packed attribute:
struct foo {
int x;
char a, b, c, d;
} __attribute__((packed));
struct bar {
char z;
struct foo f;
};
-Wpacked-bitfield-compat
The 4.1, 4.2 and 4.3 series of GCC ignore the "packed" attribute on bit-fields
of type "char". This has been fixed in GCC 4.4 but the change can lead to
differences in the structure layout. GCC informs you when the offset of such a
field has changed in GCC 4.4. For example there is no longer a 4-bit padding
between field "a" and "b" in this structure:
struct foo
{
char a:4;
char b:8;
} __attribute__ ((packed));
This warning is enabled by default. Use -Wno-packed-bitfield-compat to disable
this warning.
-Wpacked-not-aligned (C, C++, Objective-C and Objective-C++ only)
Warn if a structure field with explicitly specified alignment in a packed
struct or union is misaligned. For example, a warning will be issued on
"struct S", like, "warning: alignment 1 of 'struct S' is less than 8", in this
code:
struct __attribute__ ((aligned (8))) S8 { char a[8]; };
struct __attribute__ ((packed)) S {
struct S8 s8;
};
This warning is enabled by -Wall.
-Wpadded
Warn if padding is included in a structure, either to align an element of the
structure or to align the whole structure. Sometimes when this happens it is
possible to rearrange the fields of the structure to reduce the padding and so
make the structure smaller.
-Wredundant-decls
Warn if anything is declared more than once in the same scope, even in cases
where multiple declaration is valid and changes nothing.
-Wno-restrict
Warn when an object referenced by a "restrict"-qualified parameter (or, in C++,
a "__restrict"-qualified parameter) is aliased by another argument, or when
copies between such objects overlap. For example, the call to the "strcpy"
function below attempts to truncate the string by replacing its initial
characters with the last four. However, because the call writes the
terminating NUL into "a[4]", the copies overlap and the call is diagnosed.
void foo (void)
{
char a[] = "abcd1234";
strcpy (a, a + 4);
...
}
The -Wrestrict option detects some instances of simple overlap even without
optimization but works best at -O2 and above. It is included in -Wall.
-Wnested-externs (C and Objective-C only)
Warn if an "extern" declaration is encountered within a function.
-Wno-inherited-variadic-ctor
Suppress warnings about use of C++11 inheriting constructors when the base
class inherited from has a C variadic constructor; the warning is on by default
because the ellipsis is not inherited.
-Winline
Warn if a function that is declared as inline cannot be inlined. Even with
this option, the compiler does not warn about failures to inline functions
declared in system headers.
The compiler uses a variety of heuristics to determine whether or not to inline
a function. For example, the compiler takes into account the size of the
function being inlined and the amount of inlining that has already been done in
the current function. Therefore, seemingly insignificant changes in the source
program can cause the warnings produced by -Winline to appear or disappear.
-Wno-invalid-offsetof (C++ and Objective-C++ only)
Suppress warnings from applying the "offsetof" macro to a non-POD type.
According to the 2014 ISO C++ standard, applying "offsetof" to a non-standard-
layout type is undefined. In existing C++ implementations, however, "offsetof"
typically gives meaningful results. This flag is for users who are aware that
they are writing nonportable code and who have deliberately chosen to ignore
the warning about it.
The restrictions on "offsetof" may be relaxed in a future version of the C++
standard.
-Wint-in-bool-context
Warn for suspicious use of integer values where boolean values are expected,
such as conditional expressions (?:) using non-boolean integer constants in
boolean context, like "if (a <= b ? 2 : 3)". Or left shifting of signed
integers in boolean context, like "for (a = 0; 1 << a; a++);". Likewise for
all kinds of multiplications regardless of the data type. This warning is
enabled by -Wall.
-Wno-int-to-pointer-cast
Suppress warnings from casts to pointer type of an integer of a different size.
In C++, casting to a pointer type of smaller size is an error. Wint-to-pointer-
cast is enabled by default.
-Wno-pointer-to-int-cast (C and Objective-C only)
Suppress warnings from casts from a pointer to an integer type of a different
size.
-Winvalid-pch
Warn if a precompiled header is found in the search path but cannot be used.
-Wlong-long
Warn if "long long" type is used. This is enabled by either -Wpedantic or
-Wtraditional in ISO C90 and C++98 modes. To inhibit the warning messages, use
-Wno-long-long.
-Wvariadic-macros
Warn if variadic macros are used in ISO C90 mode, or if the GNU alternate
syntax is used in ISO C99 mode. This is enabled by either -Wpedantic or
-Wtraditional. To inhibit the warning messages, use -Wno-variadic-macros.
-Wvarargs
Warn upon questionable usage of the macros used to handle variable arguments
like "va_start". This is default. To inhibit the warning messages, use
-Wno-varargs.
-Wvector-operation-performance
Warn if vector operation is not implemented via SIMD capabilities of the
architecture. Mainly useful for the performance tuning. Vector operation can
be implemented "piecewise", which means that the scalar operation is performed
on every vector element; "in parallel", which means that the vector operation
is implemented using scalars of wider type, which normally is more performance
efficient; and "as a single scalar", which means that vector fits into a scalar
type.
-Wno-virtual-move-assign
Suppress warnings about inheriting from a virtual base with a non-trivial C++11
move assignment operator. This is dangerous because if the virtual base is
reachable along more than one path, it is moved multiple times, which can mean
both objects end up in the moved-from state. If the move assignment operator
is written to avoid moving from a moved-from object, this warning can be
disabled.
-Wvla
Warn if a variable-length array is used in the code. -Wno-vla prevents the
-Wpedantic warning of the variable-length array.
-Wvla-larger-than=n
If this option is used, the compiler will warn on uses of variable-length
arrays where the size is either unbounded, or bounded by an argument that can
be larger than n bytes. This is similar to how -Walloca-larger-than=n works,
but with variable-length arrays.
Note that GCC may optimize small variable-length arrays of a known value into
plain arrays, so this warning may not get triggered for such arrays.
This warning is not enabled by -Wall, and is only active when -ftree-vrp is
active (default for -O2 and above).
See also -Walloca-larger-than=n.
-Wvolatile-register-var
Warn if a register variable is declared volatile. The volatile modifier does
not inhibit all optimizations that may eliminate reads and/or writes to
register variables. This warning is enabled by -Wall.
-Wdisabled-optimization
Warn if a requested optimization pass is disabled. This warning does not
generally indicate that there is anything wrong with your code; it merely
indicates that GCC's optimizers are unable to handle the code effectively.
Often, the problem is that your code is too big or too complex; GCC refuses to
optimize programs when the optimization itself is likely to take inordinate
amounts of time.
-Wpointer-sign (C and Objective-C only)
Warn for pointer argument passing or assignment with different signedness.
This option is only supported for C and Objective-C. It is implied by -Wall
and by -Wpedantic, which can be disabled with -Wno-pointer-sign.
-Wstack-protector
This option is only active when -fstack-protector is active. It warns about
functions that are not protected against stack smashing.
-Woverlength-strings
Warn about string constants that are longer than the "minimum maximum" length
specified in the C standard. Modern compilers generally allow string constants
that are much longer than the standard's minimum limit, but very portable
programs should avoid using longer strings.
The limit applies after string constant concatenation, and does not count the
trailing NUL. In C90, the limit was 509 characters; in C99, it was raised to
4095. C++98 does not specify a normative minimum maximum, so we do not
diagnose overlength strings in C++.
This option is implied by -Wpedantic, and can be disabled with
-Wno-overlength-strings.
-Wunsuffixed-float-constants (C and Objective-C only)
Issue a warning for any floating constant that does not have a suffix. When
used together with -Wsystem-headers it warns about such constants in system
header files. This can be useful when preparing code to use with the
"FLOAT_CONST_DECIMAL64" pragma from the decimal floating-point extension to
C99.
-Wno-designated-init (C and Objective-C only)
Suppress warnings when a positional initializer is used to initialize a
structure that has been marked with the "designated_init" attribute.
-Whsa
Issue a warning when HSAIL cannot be emitted for the compiled function or
OpenMP construct.
Options for Debugging Your Program
To tell GCC to emit extra information for use by a debugger, in almost all cases
you need only to add -g to your other options.
GCC allows you to use -g with -O. The shortcuts taken by optimized code may
occasionally be surprising: some variables you declared may not exist at all; flow
of control may briefly move where you did not expect it; some statements may not be
executed because they compute constant results or their values are already at hand;
some statements may execute in different places because they have been moved out of
loops. Nevertheless it is possible to debug optimized output. This makes it
reasonable to use the optimizer for programs that might have bugs.
If you are not using some other optimization option, consider using -Og with -g.
With no -O option at all, some compiler passes that collect information useful for
debugging do not run at all, so that -Og may result in a better debugging
experience.
-g Produce debugging information in the operating system's native format (stabs,
COFF, XCOFF, or DWARF). GDB can work with this debugging information.
On most systems that use stabs format, -g enables use of extra debugging
information that only GDB can use; this extra information makes debugging work
better in GDB but probably makes other debuggers crash or refuse to read the
program. If you want to control for certain whether to generate the extra
information, use -gstabs+, -gstabs, -gxcoff+, -gxcoff, or -gvms (see below).
-ggdb
Produce debugging information for use by GDB. This means to use the most
expressive format available (DWARF, stabs, or the native format if neither of
those are supported), including GDB extensions if at all possible.
-gdwarf
-gdwarf-version
Produce debugging information in DWARF format (if that is supported). The
value of version may be either 2, 3, 4 or 5; the default version for most
targets is 4. DWARF Version 5 is only experimental.
Note that with DWARF Version 2, some ports require and always use some non-
conflicting DWARF 3 extensions in the unwind tables.
Version 4 may require GDB 7.0 and -fvar-tracking-assignments for maximum
benefit.
GCC no longer supports DWARF Version 1, which is substantially different than
Version 2 and later. For historical reasons, some other DWARF-related options
such as -fno-dwarf2-cfi-asm) retain a reference to DWARF Version 2 in their
names, but apply to all currently-supported versions of DWARF.
-gstabs
Produce debugging information in stabs format (if that is supported), without
GDB extensions. This is the format used by DBX on most BSD systems. On MIPS,
Alpha and System V Release 4 systems this option produces stabs debugging
output that is not understood by DBX. On System V Release 4 systems this
option requires the GNU assembler.
-gstabs+
Produce debugging information in stabs format (if that is supported), using GNU
extensions understood only by the GNU debugger (GDB). The use of these
extensions is likely to make other debuggers crash or refuse to read the
program.
-gxcoff
Produce debugging information in XCOFF format (if that is supported). This is
the format used by the DBX debugger on IBM RS/6000 systems.
-gxcoff+
Produce debugging information in XCOFF format (if that is supported), using GNU
extensions understood only by the GNU debugger (GDB). The use of these
extensions is likely to make other debuggers crash or refuse to read the
program, and may cause assemblers other than the GNU assembler (GAS) to fail
with an error.
-gvms
Produce debugging information in Alpha/VMS debug format (if that is supported).
This is the format used by DEBUG on Alpha/VMS systems.
-glevel
-ggdblevel
-gstabslevel
-gxcofflevel
-gvmslevel
Request debugging information and also use level to specify how much
information. The default level is 2.
Level 0 produces no debug information at all. Thus, -g0 negates -g.
Level 1 produces minimal information, enough for making backtraces in parts of
the program that you don't plan to debug. This includes descriptions of
functions and external variables, and line number tables, but no information
about local variables.
Level 3 includes extra information, such as all the macro definitions present
in the program. Some debuggers support macro expansion when you use -g3.
-gdwarf does not accept a concatenated debug level, to avoid confusion with
-gdwarf-level. Instead use an additional -glevel option to change the debug
level for DWARF.
-feliminate-unused-debug-symbols
Produce debugging information in stabs format (if that is supported), for only
symbols that are actually used.
-femit-class-debug-always
Instead of emitting debugging information for a C++ class in only one object
file, emit it in all object files using the class. This option should be used
only with debuggers that are unable to handle the way GCC normally emits
debugging information for classes because using this option increases the size
of debugging information by as much as a factor of two.
-fno-merge-debug-strings
Direct the linker to not merge together strings in the debugging information
that are identical in different object files. Merging is not supported by all
assemblers or linkers. Merging decreases the size of the debug information in
the output file at the cost of increasing link processing time. Merging is
enabled by default.
-fdebug-prefix-map=old=new
When compiling files residing in directory old, record debugging information
describing them as if the files resided in directory new instead. This can be
used to replace a build-time path with an install-time path in the debug info.
It can also be used to change an absolute path to a relative path by using .
for new. This can give more reproducible builds, which are location
independent, but may require an extra command to tell GDB where to find the
source files. See also -ffile-prefix-map.
-fvar-tracking
Run variable tracking pass. It computes where variables are stored at each
position in code. Better debugging information is then generated (if the
debugging information format supports this information).
It is enabled by default when compiling with optimization (-Os, -O, -O2, ...),
debugging information (-g) and the debug info format supports it.
-fvar-tracking-assignments
Annotate assignments to user variables early in the compilation and attempt to
carry the annotations over throughout the compilation all the way to the end,
in an attempt to improve debug information while optimizing. Use of -gdwarf-4
is recommended along with it.
It can be enabled even if var-tracking is disabled, in which case annotations
are created and maintained, but discarded at the end. By default, this flag is
enabled together with -fvar-tracking, except when selective scheduling is
enabled.
-gsplit-dwarf
Separate as much DWARF debugging information as possible into a separate output
file with the extension .dwo. This option allows the build system to avoid
linking files with debug information. To be useful, this option requires a
debugger capable of reading .dwo files.
-gpubnames
Generate DWARF ".debug_pubnames" and ".debug_pubtypes" sections.
-ggnu-pubnames
Generate ".debug_pubnames" and ".debug_pubtypes" sections in a format suitable
for conversion into a GDB index. This option is only useful with a linker that
can produce GDB index version 7.
-fdebug-types-section
When using DWARF Version 4 or higher, type DIEs can be put into their own
".debug_types" section instead of making them part of the ".debug_info"
section. It is more efficient to put them in a separate comdat sections since
the linker can then remove duplicates. But not all DWARF consumers support
".debug_types" sections yet and on some objects ".debug_types" produces larger
instead of smaller debugging information.
-grecord-gcc-switches
-gno-record-gcc-switches
This switch causes the command-line options used to invoke the compiler that
may affect code generation to be appended to the DW_AT_producer attribute in
DWARF debugging information. The options are concatenated with spaces
separating them from each other and from the compiler version. It is enabled
by default. See also -frecord-gcc-switches for another way of storing compiler
options into the object file.
-gstrict-dwarf
Disallow using extensions of later DWARF standard version than selected with
-gdwarf-version. On most targets using non-conflicting DWARF extensions from
later standard versions is allowed.
-gno-strict-dwarf
Allow using extensions of later DWARF standard version than selected with
-gdwarf-version.
-gas-loc-support
Inform the compiler that the assembler supports ".loc" directives. It may then
use them for the assembler to generate DWARF2+ line number tables.
This is generally desirable, because assembler-generated line-number tables are
a lot more compact than those the compiler can generate itself.
This option will be enabled by default if, at GCC configure time, the assembler
was found to support such directives.
-gno-as-loc-support
Force GCC to generate DWARF2+ line number tables internally, if DWARF2+ line
number tables are to be generated.
gas-locview-support
Inform the compiler that the assembler supports "view" assignment and reset
assertion checking in ".loc" directives.
This option will be enabled by default if, at GCC configure time, the assembler
was found to support them.
gno-as-locview-support
Force GCC to assign view numbers internally, if -gvariable-location-views are
explicitly requested.
-gcolumn-info
-gno-column-info
Emit location column information into DWARF debugging information, rather than
just file and line. This option is enabled by default.
-gstatement-frontiers
-gno-statement-frontiers
This option causes GCC to create markers in the internal representation at the
beginning of statements, and to keep them roughly in place throughout
compilation, using them to guide the output of "is_stmt" markers in the line
number table. This is enabled by default when compiling with optimization
(-Os, -O, -O2, ...), and outputting DWARF 2 debug information at the normal
level.
-gvariable-location-views
-gvariable-location-views=incompat5
-gno-variable-location-views
Augment variable location lists with progressive view numbers implied from the
line number table. This enables debug information consumers to inspect state
at certain points of the program, even if no instructions associated with the
corresponding source locations are present at that point. If the assembler
lacks support for view numbers in line number tables, this will cause the
compiler to emit the line number table, which generally makes them somewhat
less compact. The augmented line number tables and location lists are fully
backward-compatible, so they can be consumed by debug information consumers
that are not aware of these augmentations, but they won't derive any benefit
from them either.
This is enabled by default when outputting DWARF 2 debug information at the
normal level, as long as there is assembler support, -fvar-tracking-assignments
is enabled and -gstrict-dwarf is not. When assembler support is not available,
this may still be enabled, but it will force GCC to output internal line number
tables, and if -ginternal-reset-location-views is not enabled, that will most
certainly lead to silently mismatching location views.
There is a proposed representation for view numbers that is not backward
compatible with the location list format introduced in DWARF 5, that can be
enabled with -gvariable-location-views=incompat5. This option may be removed
in the future, is only provided as a reference implementation of the proposed
representation. Debug information consumers are not expected to support this
extended format, and they would be rendered unable to decode location lists
using it.
-ginternal-reset-location-views
-gno-internal-reset-location-views
Attempt to determine location views that can be omitted from location view
lists. This requires the compiler to have very accurate insn length estimates,
which isn't always the case, and it may cause incorrect view lists to be
generated silently when using an assembler that does not support location view
lists. The GNU assembler will flag any such error as a "view number mismatch".
This is only enabled on ports that define a reliable estimation function.
-ginline-points
-gno-inline-points
Generate extended debug information for inlined functions. Location view
tracking markers are inserted at inlined entry points, so that address and view
numbers can be computed and output in debug information. This can be enabled
independently of location views, in which case the view numbers won't be
output, but it can only be enabled along with statement frontiers, and it is
only enabled by default if location views are enabled.
-gz[=type]
Produce compressed debug sections in DWARF format, if that is supported. If
type is not given, the default type depends on the capabilities of the
assembler and linker used. type may be one of none (don't compress debug
sections), zlib (use zlib compression in ELF gABI format), or zlib-gnu (use
zlib compression in traditional GNU format). If the linker doesn't support
writing compressed debug sections, the option is rejected. Otherwise, if the
assembler does not support them, -gz is silently ignored when producing object
files.
-femit-struct-debug-baseonly
Emit debug information for struct-like types only when the base name of the
compilation source file matches the base name of file in which the struct is
defined.
This option substantially reduces the size of debugging information, but at
significant potential loss in type information to the debugger. See
-femit-struct-debug-reduced for a less aggressive option. See
-femit-struct-debug-detailed for more detailed control.
This option works only with DWARF debug output.
-femit-struct-debug-reduced
Emit debug information for struct-like types only when the base name of the
compilation source file matches the base name of file in which the type is
defined, unless the struct is a template or defined in a system header.
This option significantly reduces the size of debugging information, with some
potential loss in type information to the debugger. See
-femit-struct-debug-baseonly for a more aggressive option. See
-femit-struct-debug-detailed for more detailed control.
This option works only with DWARF debug output.
-femit-struct-debug-detailed[=spec-list]
Specify the struct-like types for which the compiler generates debug
information. The intent is to reduce duplicate struct debug information
between different object files within the same program.
This option is a detailed version of -femit-struct-debug-reduced and
-femit-struct-debug-baseonly, which serves for most needs.
A specification has the syntax[dir:|ind:][ord:|gen:](any|sys|base|none)
The optional first word limits the specification to structs that are used
directly (dir:) or used indirectly (ind:). A struct type is used directly when
it is the type of a variable, member. Indirect uses arise through pointers to
structs. That is, when use of an incomplete struct is valid, the use is
indirect. An example is struct one direct; struct two * indirect;.
The optional second word limits the specification to ordinary structs (ord:) or
generic structs (gen:). Generic structs are a bit complicated to explain. For
C++, these are non-explicit specializations of template classes, or non-
template classes within the above. Other programming languages have generics,
but -femit-struct-debug-detailed does not yet implement them.
The third word specifies the source files for those structs for which the
compiler should emit debug information. The values none and any have the
normal meaning. The value base means that the base of name of the file in
which the type declaration appears must match the base of the name of the main
compilation file. In practice, this means that when compiling foo.c, debug
information is generated for types declared in that file and foo.h, but not
other header files. The value sys means those types satisfying base or
declared in system or compiler headers.
You may need to experiment to determine the best settings for your application.
The default is -femit-struct-debug-detailed=all.
This option works only with DWARF debug output.
-fno-dwarf2-cfi-asm
Emit DWARF unwind info as compiler generated ".eh_frame" section instead of
using GAS ".cfi_*" directives.
-fno-eliminate-unused-debug-types
Normally, when producing DWARF output, GCC avoids producing debug symbol output
for types that are nowhere used in the source file being compiled. Sometimes
it is useful to have GCC emit debugging information for all types declared in a
compilation unit, regardless of whether or not they are actually used in that
compilation unit, for example if, in the debugger, you want to cast a value to
a type that is not actually used in your program (but is declared). More
often, however, this results in a significant amount of wasted space.
Options That Control Optimization
These options control various sorts of optimizations.
Without any optimization option, the compiler's goal is to reduce the cost of
compilation and to make debugging produce the expected results. Statements are
independent: if you stop the program with a breakpoint between statements, you can
then assign a new value to any variable or change the program counter to any other
statement in the function and get exactly the results you expect from the source
code.
Turning on optimization flags makes the compiler attempt to improve the performance
and/or code size at the expense of compilation time and possibly the ability to
debug the program.
The compiler performs optimization based on the knowledge it has of the program.
Compiling multiple files at once to a single output file mode allows the compiler
to use information gained from all of the files when compiling each of them.
Not all optimizations are controlled directly by a flag. Only optimizations that
have a flag are listed in this section.
Most optimizations are only enabled if an -O level is set on the command line.
Otherwise they are disabled, even if individual optimization flags are specified.
Depending on the target and how GCC was configured, a slightly different set of
optimizations may be enabled at each -O level than those listed here. You can
invoke GCC with -Q --help=optimizers to find out the exact set of optimizations
that are enabled at each level.
-O
-O1 Optimize. Optimizing compilation takes somewhat more time, and a lot more
memory for a large function.
With -O, the compiler tries to reduce code size and execution time, without
performing any optimizations that take a great deal of compilation time.
-O turns on the following optimization flags:
-fauto-inc-dec -fbranch-count-reg -fcombine-stack-adjustments -fcompare-elim
-fcprop-registers -fdce -fdefer-pop -fdelayed-branch -fdse -fforward-propagate
-fguess-branch-probability -fif-conversion2 -fif-conversion
-finline-functions-called-once -fipa-pure-const -fipa-profile -fipa-reference
-fmerge-constants -fmove-loop-invariants -fomit-frame-pointer -freorder-blocks
-fshrink-wrap -fshrink-wrap-separate -fsplit-wide-types -fssa-backprop
-fssa-phiopt -ftree-bit-ccp -ftree-ccp -ftree-ch -ftree-coalesce-vars
-ftree-copy-prop -ftree-dce -ftree-dominator-opts -ftree-dse -ftree-forwprop
-ftree-fre -ftree-phiprop -ftree-sink -ftree-slsr -ftree-sra -ftree-pta
-ftree-ter -funit-at-a-time
-O2 Optimize even more. GCC performs nearly all supported optimizations that do
not involve a space-speed tradeoff. As compared to -O, this option increases
both compilation time and the performance of the generated code.
-O2 turns on all optimization flags specified by -O. It also turns on the
following optimization flags: -fthread-jumps -falign-functions -falign-jumps
-falign-loops -falign-labels -fcaller-saves -fcrossjumping -fcse-follow-jumps
-fcse-skip-blocks -fdelete-null-pointer-checks -fdevirtualize
-fdevirtualize-speculatively -fexpensive-optimizations -fgcse -fgcse-lm
-fhoist-adjacent-loads -finline-small-functions -findirect-inlining -fipa-cp
-fipa-bit-cp -fipa-vrp -fipa-sra -fipa-icf
-fisolate-erroneous-paths-dereference -flra-remat -foptimize-sibling-calls
-foptimize-strlen -fpartial-inlining -fpeephole2 -freorder-blocks-algorithm=stc
-freorder-blocks-and-partition -freorder-functions -frerun-cse-after-loop
-fsched-interblock -fsched-spec -fschedule-insns -fschedule-insns2
-fstore-merging -fstrict-aliasing -ftree-builtin-call-dce
-ftree-switch-conversion -ftree-tail-merge -fcode-hoisting -ftree-pre
-ftree-vrp -fipa-ra
Please note the warning under -fgcse about invoking -O2 on programs that use
computed gotos.
-O3 Optimize yet more. -O3 turns on all optimizations specified by -O2 and also
turns on the following optimization flags: -finline-functions -funswitch-loops
-fpredictive-commoning -fgcse-after-reload -ftree-loop-vectorize
-ftree-loop-distribution -ftree-loop-distribute-patterns -floop-interchange
-floop-unroll-and-jam -fsplit-paths -ftree-slp-vectorize -fvect-cost-model
-ftree-partial-pre -fpeel-loops -fipa-cp-clone
-O0 Reduce compilation time and make debugging produce the expected results. This
is the default.
-Os Optimize for size. -Os enables all -O2 optimizations that do not typically
increase code size.
-Os disables the following optimization flags: -falign-functions -falign-jumps
-falign-loops -falign-labels -fprefetch-loop-arrays
It also enables -finline-functions, causes the compiler to tune for code size
rather than execution speed, and performs further optimizations designed to
reduce code size.
-Ofast
Disregard strict standards compliance. -Ofast enables all -O3 optimizations.
It also enables optimizations that are not valid for all standard-compliant
programs. It turns on -ffast-math and the Fortran-specific -fstack-arrays,
unless -fmax-stack-var-size is specified, and -fno-protect-parens.
-Og Optimize debugging experience. -Og enables optimizations that do not interfere
with debugging. It should be the optimization level of choice for the standard
edit-compile-debug cycle, offering a reasonable level of optimization while
maintaining fast compilation and a good debugging experience.
If you use multiple -O options, with or without level numbers, the last such option
is the one that is effective.
Options of the form -fflag specify machine-independent flags. Most flags have both
positive and negative forms; the negative form of -ffoo is -fno-foo. In the table
below, only one of the forms is listed---the one you typically use. You can figure
out the other form by either removing no- or adding it.
The following options control specific optimizations. They are either activated by
-O options or are related to ones that are. You can use the following flags in the
rare cases when "fine-tuning" of optimizations to be performed is desired.
-fno-defer-pop
Always pop the arguments to each function call as soon as that function
returns. For machines that must pop arguments after a function call, the
compiler normally lets arguments accumulate on the stack for several function
calls and pops them all at once.
Disabled at levels -O, -O2, -O3, -Os.
-fforward-propagate
Perform a forward propagation pass on RTL. The pass tries to combine two
instructions and checks if the result can be simplified. If loop unrolling is
active, two passes are performed and the second is scheduled after loop
unrolling.
This option is enabled by default at optimization levels -O, -O2, -O3, -Os.
-ffp-contract=style
-ffp-contract=off disables floating-point expression contraction.
-ffp-contract=fast enables floating-point expression contraction such as
forming of fused multiply-add operations if the target has native support for
them. -ffp-contract=on enables floating-point expression contraction if
allowed by the language standard. This is currently not implemented and
treated equal to -ffp-contract=off.
The default is -ffp-contract=fast.
-fomit-frame-pointer
Omit the frame pointer in functions that don't need one. This avoids the
instructions to save, set up and restore the frame pointer; on many targets it
also makes an extra register available.
On some targets this flag has no effect because the standard calling sequence
always uses a frame pointer, so it cannot be omitted.
Note that -fno-omit-frame-pointer doesn't guarantee the frame pointer is used
in all functions. Several targets always omit the frame pointer in leaf
functions.
Enabled by default at -O and higher.
-foptimize-sibling-calls
Optimize sibling and tail recursive calls.
Enabled at levels -O2, -O3, -Os.
-foptimize-strlen
Optimize various standard C string functions (e.g. "strlen", "strchr" or
"strcpy") and their "_FORTIFY_SOURCE" counterparts into faster alternatives.
Enabled at levels -O2, -O3.
-fno-inline
Do not expand any functions inline apart from those marked with the
"always_inline" attribute. This is the default when not optimizing.
Single functions can be exempted from inlining by marking them with the
"noinline" attribute.
-finline-small-functions
Integrate functions into their callers when their body is smaller than expected
function call code (so overall size of program gets smaller). The compiler
heuristically decides which functions are simple enough to be worth integrating
in this way. This inlining applies to all functions, even those not declared
inline.
Enabled at levels -O2, -O3, -Os.
-findirect-inlining
Inline also indirect calls that are discovered to be known at compile time
thanks to previous inlining. This option has any effect only when inlining
itself is turned on by the -finline-functions or -finline-small-functions
options.
Enabled at levels -O3, -Os. Also enabled by -fprofile-use and -fauto-profile.
-finline-functions
Consider all functions for inlining, even if they are not declared inline. The
compiler heuristically decides which functions are worth integrating in this
way.
If all calls to a given function are integrated, and the function is declared
"static", then the function is normally not output as assembler code in its own
right.
Enabled at levels -O2, -O3, -Os.
-finline-functions-called-once
Consider all "static" functions called once for inlining into their caller even
if they are not marked "inline". If a call to a given function is integrated,
then the function is not output as assembler code in its own right.
Enabled at levels -O1, -O2, -O3 and -Os.
-fearly-inlining
Inline functions marked by "always_inline" and functions whose body seems
smaller than the function call overhead early before doing -fprofile-generate
instrumentation and real inlining pass. Doing so makes profiling significantly
cheaper and usually inlining faster on programs having large chains of nested
wrapper functions.
Enabled by default.
-fipa-sra
Perform interprocedural scalar replacement of aggregates, removal of unused
parameters and replacement of parameters passed by reference by parameters
passed by value.
Enabled at levels -O2, -O3 and -Os.
-finline-limit=n
By default, GCC limits the size of functions that can be inlined. This flag
allows coarse control of this limit. n is the size of functions that can be
inlined in number of pseudo instructions.
Inlining is actually controlled by a number of parameters, which may be
specified individually by using --param name=value. The -finline-limit=n
option sets some of these parameters as follows:
max-inline-insns-single
is set to n/2.
max-inline-insns-auto
is set to n/2.
See below for a documentation of the individual parameters controlling inlining
and for the defaults of these parameters.
Note: there may be no value to -finline-limit that results in default behavior.
Note: pseudo instruction represents, in this particular context, an abstract
measurement of function's size. In no way does it represent a count of
assembly instructions and as such its exact meaning might change from one
release to an another.
-fno-keep-inline-dllexport
This is a more fine-grained version of -fkeep-inline-functions, which applies
only to functions that are declared using the "dllexport" attribute or
declspec.
-fkeep-inline-functions
In C, emit "static" functions that are declared "inline" into the object file,
even if the function has been inlined into all of its callers. This switch
does not affect functions using the "extern inline" extension in GNU C90. In
C++, emit any and all inline functions into the object file.
-fkeep-static-functions
Emit "static" functions into the object file, even if the function is never
used.
-fkeep-static-consts
Emit variables declared "static const" when optimization isn't turned on, even
if the variables aren't referenced.
GCC enables this option by default. If you want to force the compiler to check
if a variable is referenced, regardless of whether or not optimization is
turned on, use the -fno-keep-static-consts option.
-fmerge-constants
Attempt to merge identical constants (string constants and floating-point
constants) across compilation units.
This option is the default for optimized compilation if the assembler and
linker support it. Use -fno-merge-constants to inhibit this behavior.
Enabled at levels -O, -O2, -O3, -Os.
-fmerge-all-constants
Attempt to merge identical constants and identical variables.
This option implies -fmerge-constants. In addition to -fmerge-constants this
considers e.g. even constant initialized arrays or initialized constant
variables with integral or floating-point types. Languages like C or C++
require each variable, including multiple instances of the same variable in
recursive calls, to have distinct locations, so using this option results in
non-conforming behavior.
-fmodulo-sched
Perform swing modulo scheduling immediately before the first scheduling pass.
This pass looks at innermost loops and reorders their instructions by
overlapping different iterations.
-fmodulo-sched-allow-regmoves
Perform more aggressive SMS-based modulo scheduling with register moves
allowed. By setting this flag certain anti-dependences edges are deleted,
which triggers the generation of reg-moves based on the life-range analysis.
This option is effective only with -fmodulo-sched enabled.
-fno-branch-count-reg
Avoid running a pass scanning for opportunities to use "decrement and branch"
instructions on a count register instead of generating sequences of
instructions that decrement a register, compare it against zero, and then
branch based upon the result. This option is only meaningful on architectures
that support such instructions, which include x86, PowerPC, IA-64 and S/390.
Note that the -fno-branch-count-reg option doesn't remove the decrement and
branch instructions from the generated instruction stream introduced by other
optimization passes.
Enabled by default at -O1 and higher.
The default is -fbranch-count-reg.
-fno-function-cse
Do not put function addresses in registers; make each instruction that calls a
constant function contain the function's address explicitly.
This option results in less efficient code, but some strange hacks that alter
the assembler output may be confused by the optimizations performed when this
option is not used.
The default is -ffunction-cse
-fno-zero-initialized-in-bss
If the target supports a BSS section, GCC by default puts variables that are
initialized to zero into BSS. This can save space in the resulting code.
This option turns off this behavior because some programs explicitly rely on
variables going to the data section---e.g., so that the resulting executable
can find the beginning of that section and/or make assumptions based on that.
The default is -fzero-initialized-in-bss.
-fthread-jumps
Perform optimizations that check to see if a jump branches to a location where
another comparison subsumed by the first is found. If so, the first branch is
redirected to either the destination of the second branch or a point
immediately following it, depending on whether the condition is known to be
true or false.
Enabled at levels -O2, -O3, -Os.
-fsplit-wide-types
When using a type that occupies multiple registers, such as "long long" on a
32-bit system, split the registers apart and allocate them independently. This
normally generates better code for those types, but may make debugging more
difficult.
Enabled at levels -O, -O2, -O3, -Os.
-fcse-follow-jumps
In common subexpression elimination (CSE), scan through jump instructions when
the target of the jump is not reached by any other path. For example, when CSE
encounters an "if" statement with an "else" clause, CSE follows the jump when
the condition tested is false.
Enabled at levels -O2, -O3, -Os.
-fcse-skip-blocks
This is similar to -fcse-follow-jumps, but causes CSE to follow jumps that
conditionally skip over blocks. When CSE encounters a simple "if" statement
with no else clause, -fcse-skip-blocks causes CSE to follow the jump around the
body of the "if".
Enabled at levels -O2, -O3, -Os.
-frerun-cse-after-loop
Re-run common subexpression elimination after loop optimizations are performed.
Enabled at levels -O2, -O3, -Os.
-fgcse
Perform a global common subexpression elimination pass. This pass also
performs global constant and copy propagation.
Note: When compiling a program using computed gotos, a GCC extension, you may
get better run-time performance if you disable the global common subexpression
elimination pass by adding -fno-gcse to the command line.
Enabled at levels -O2, -O3, -Os.
-fgcse-lm
When -fgcse-lm is enabled, global common subexpression elimination attempts to
move loads that are only killed by stores into themselves. This allows a loop
containing a load/store sequence to be changed to a load outside the loop, and
a copy/store within the loop.
Enabled by default when -fgcse is enabled.
-fgcse-sm
When -fgcse-sm is enabled, a store motion pass is run after global common
subexpression elimination. This pass attempts to move stores out of loops.
When used in conjunction with -fgcse-lm, loops containing a load/store sequence
can be changed to a load before the loop and a store after the loop.
Not enabled at any optimization level.
-fgcse-las
When -fgcse-las is enabled, the global common subexpression elimination pass
eliminates redundant loads that come after stores to the same memory location
(both partial and full redundancies).
Not enabled at any optimization level.
-fgcse-after-reload
When -fgcse-after-reload is enabled, a redundant load elimination pass is
performed after reload. The purpose of this pass is to clean up redundant
spilling.
-faggressive-loop-optimizations
This option tells the loop optimizer to use language constraints to derive
bounds for the number of iterations of a loop. This assumes that loop code
does not invoke undefined behavior by for example causing signed integer
overflows or out-of-bound array accesses. The bounds for the number of
iterations of a loop are used to guide loop unrolling and peeling and loop exit
test optimizations. This option is enabled by default.
-funconstrained-commons
This option tells the compiler that variables declared in common blocks (e.g.
Fortran) may later be overridden with longer trailing arrays. This prevents
certain optimizations that depend on knowing the array bounds.
-fcrossjumping
Perform cross-jumping transformation. This transformation unifies equivalent
code and saves code size. The resulting code may or may not perform better
than without cross-jumping.
Enabled at levels -O2, -O3, -Os.
-fauto-inc-dec
Combine increments or decrements of addresses with memory accesses. This pass
is always skipped on architectures that do not have instructions to support
this. Enabled by default at -O and higher on architectures that support this.
-fdce
Perform dead code elimination (DCE) on RTL. Enabled by default at -O and
higher.
-fdse
Perform dead store elimination (DSE) on RTL. Enabled by default at -O and
higher.
-fif-conversion
Attempt to transform conditional jumps into branch-less equivalents. This
includes use of conditional moves, min, max, set flags and abs instructions,
and some tricks doable by standard arithmetics. The use of conditional
execution on chips where it is available is controlled by -fif-conversion2.
Enabled at levels -O, -O2, -O3, -Os.
-fif-conversion2
Use conditional execution (where available) to transform conditional jumps into
branch-less equivalents.
Enabled at levels -O, -O2, -O3, -Os.
-fdeclone-ctor-dtor
The C++ ABI requires multiple entry points for constructors and destructors:
one for a base subobject, one for a complete object, and one for a virtual
destructor that calls operator delete afterwards. For a hierarchy with virtual
bases, the base and complete variants are clones, which means two copies of the
function. With this option, the base and complete variants are changed to be
thunks that call a common implementation.
Enabled by -Os.
-fdelete-null-pointer-checks
Assume that programs cannot safely dereference null pointers, and that no code
or data element resides at address zero. This option enables simple constant
folding optimizations at all optimization levels. In addition, other
optimization passes in GCC use this flag to control global dataflow analyses
that eliminate useless checks for null pointers; these assume that a memory
access to address zero always results in a trap, so that if a pointer is
checked after it has already been dereferenced, it cannot be null.
Note however that in some environments this assumption is not true. Use
-fno-delete-null-pointer-checks to disable this optimization for programs that
depend on that behavior.
This option is enabled by default on most targets. On Nios II ELF, it defaults
to off. On AVR, CR16, and MSP430, this option is completely disabled.
Passes that use the dataflow information are enabled independently at different
optimization levels.
-fdevirtualize
Attempt to convert calls to virtual functions to direct calls. This is done
both within a procedure and interprocedurally as part of indirect inlining
(-findirect-inlining) and interprocedural constant propagation (-fipa-cp).
Enabled at levels -O2, -O3, -Os.
-fdevirtualize-speculatively
Attempt to convert calls to virtual functions to speculative direct calls.
Based on the analysis of the type inheritance graph, determine for a given call
the set of likely targets. If the set is small, preferably of size 1, change
the call into a conditional deciding between direct and indirect calls. The
speculative calls enable more optimizations, such as inlining. When they seem
useless after further optimization, they are converted back into original form.
-fdevirtualize-at-ltrans
Stream extra information needed for aggressive devirtualization when running
the link-time optimizer in local transformation mode. This option enables more
devirtualization but significantly increases the size of streamed data. For
this reason it is disabled by default.
-fexpensive-optimizations
Perform a number of minor optimizations that are relatively expensive.
Enabled at levels -O2, -O3, -Os.
-free
Attempt to remove redundant extension instructions. This is especially helpful
for the x86-64 architecture, which implicitly zero-extends in 64-bit registers
after writing to their lower 32-bit half.
Enabled for Alpha, AArch64 and x86 at levels -O2, -O3, -Os.
-fno-lifetime-dse
In C++ the value of an object is only affected by changes within its lifetime:
when the constructor begins, the object has an indeterminate value, and any
changes during the lifetime of the object are dead when the object is
destroyed. Normally dead store elimination will take advantage of this; if
your code relies on the value of the object storage persisting beyond the
lifetime of the object, you can use this flag to disable this optimization. To
preserve stores before the constructor starts (e.g. because your operator new
clears the object storage) but still treat the object as dead after the
destructor you, can use -flifetime-dse=1. The default behavior can be
explicitly selected with -flifetime-dse=2. -flifetime-dse=0 is equivalent to
-fno-lifetime-dse.
-flive-range-shrinkage
Attempt to decrease register pressure through register live range shrinkage.
This is helpful for fast processors with small or moderate size register sets.
-fira-algorithm=algorithm
Use the specified coloring algorithm for the integrated register allocator.
The algorithm argument can be priority, which specifies Chow's priority
coloring, or CB, which specifies Chaitin-Briggs coloring. Chaitin-Briggs
coloring is not implemented for all architectures, but for those targets that
do support it, it is the default because it generates better code.
-fira-region=region
Use specified regions for the integrated register allocator. The region
argument should be one of the following:
all Use all loops as register allocation regions. This can give the best
results for machines with a small and/or irregular register set.
mixed
Use all loops except for loops with small register pressure as the regions.
This value usually gives the best results in most cases and for most
architectures, and is enabled by default when compiling with optimization
for speed (-O, -O2, ...).
one Use all functions as a single region. This typically results in the
smallest code size, and is enabled by default for -Os or -O0.
-fira-hoist-pressure
Use IRA to evaluate register pressure in the code hoisting pass for decisions
to hoist expressions. This option usually results in smaller code, but it can
slow the compiler down.
This option is enabled at level -Os for all targets.
-fira-loop-pressure
Use IRA to evaluate register pressure in loops for decisions to move loop
invariants. This option usually results in generation of faster and smaller
code on machines with large register files (>= 32 registers), but it can slow
the compiler down.
This option is enabled at level -O3 for some targets.
-fno-ira-share-save-slots
Disable sharing of stack slots used for saving call-used hard registers living
through a call. Each hard register gets a separate stack slot, and as a result
function stack frames are larger.
-fno-ira-share-spill-slots
Disable sharing of stack slots allocated for pseudo-registers. Each pseudo-
register that does not get a hard register gets a separate stack slot, and as a
result function stack frames are larger.
-flra-remat
Enable CFG-sensitive rematerialization in LRA. Instead of loading values of
spilled pseudos, LRA tries to rematerialize (recalculate) values if it is
profitable.
Enabled at levels -O2, -O3, -Os.
-fdelayed-branch
If supported for the target machine, attempt to reorder instructions to exploit
instruction slots available after delayed branch instructions.
Enabled at levels -O, -O2, -O3, -Os.
-fschedule-insns
If supported for the target machine, attempt to reorder instructions to
eliminate execution stalls due to required data being unavailable. This helps
machines that have slow floating point or memory load instructions by allowing
other instructions to be issued until the result of the load or floating-point
instruction is required.
Enabled at levels -O2, -O3.
-fschedule-insns2
Similar to -fschedule-insns, but requests an additional pass of instruction
scheduling after register allocation has been done. This is especially useful
on machines with a relatively small number of registers and where memory load
instructions take more than one cycle.
Enabled at levels -O2, -O3, -Os.
-fno-sched-interblock
Don't schedule instructions across basic blocks. This is normally enabled by
default when scheduling before register allocation, i.e. with -fschedule-insns
or at -O2 or higher.
-fno-sched-spec
Don't allow speculative motion of non-load instructions. This is normally
enabled by default when scheduling before register allocation, i.e. with
-fschedule-insns or at -O2 or higher.
-fsched-pressure
Enable register pressure sensitive insn scheduling before register allocation.
This only makes sense when scheduling before register allocation is enabled,
i.e. with -fschedule-insns or at -O2 or higher. Usage of this option can
improve the generated code and decrease its size by preventing register
pressure increase above the number of available hard registers and subsequent
spills in register allocation.
-fsched-spec-load
Allow speculative motion of some load instructions. This only makes sense when
scheduling before register allocation, i.e. with -fschedule-insns or at -O2 or
higher.
-fsched-spec-load-dangerous
Allow speculative motion of more load instructions. This only makes sense when
scheduling before register allocation, i.e. with -fschedule-insns or at -O2 or
higher.
-fsched-stalled-insns
-fsched-stalled-insns=n
Define how many insns (if any) can be moved prematurely from the queue of
stalled insns into the ready list during the second scheduling pass.
-fno-sched-stalled-insns means that no insns are moved prematurely,
-fsched-stalled-insns=0 means there is no limit on how many queued insns can be
moved prematurely. -fsched-stalled-insns without a value is equivalent to
-fsched-stalled-insns=1.
-fsched-stalled-insns-dep
-fsched-stalled-insns-dep=n
Define how many insn groups (cycles) are examined for a dependency on a stalled
insn that is a candidate for premature removal from the queue of stalled insns.
This has an effect only during the second scheduling pass, and only if
-fsched-stalled-insns is used. -fno-sched-stalled-insns-dep is equivalent to
-fsched-stalled-insns-dep=0. -fsched-stalled-insns-dep without a value is
equivalent to -fsched-stalled-insns-dep=1.
-fsched2-use-superblocks
When scheduling after register allocation, use superblock scheduling. This
allows motion across basic block boundaries, resulting in faster schedules.
This option is experimental, as not all machine descriptions used by GCC model
the CPU closely enough to avoid unreliable results from the algorithm.
This only makes sense when scheduling after register allocation, i.e. with
-fschedule-insns2 or at -O2 or higher.
-fsched-group-heuristic
Enable the group heuristic in the scheduler. This heuristic favors the
instruction that belongs to a schedule group. This is enabled by default when
scheduling is enabled, i.e. with -fschedule-insns or -fschedule-insns2 or at
-O2 or higher.
-fsched-critical-path-heuristic
Enable the critical-path heuristic in the scheduler. This heuristic favors
instructions on the critical path. This is enabled by default when scheduling
is enabled, i.e. with -fschedule-insns or -fschedule-insns2 or at -O2 or
higher.
-fsched-spec-insn-heuristic
Enable the speculative instruction heuristic in the scheduler. This heuristic
favors speculative instructions with greater dependency weakness. This is
enabled by default when scheduling is enabled, i.e. with -fschedule-insns or
-fschedule-insns2 or at -O2 or higher.
-fsched-rank-heuristic
Enable the rank heuristic in the scheduler. This heuristic favors the
instruction belonging to a basic block with greater size or frequency. This is
enabled by default when scheduling is enabled, i.e. with -fschedule-insns or
-fschedule-insns2 or at -O2 or higher.
-fsched-last-insn-heuristic
Enable the last-instruction heuristic in the scheduler. This heuristic favors
the instruction that is less dependent on the last instruction scheduled. This
is enabled by default when scheduling is enabled, i.e. with -fschedule-insns or
-fschedule-insns2 or at -O2 or higher.
-fsched-dep-count-heuristic
Enable the dependent-count heuristic in the scheduler. This heuristic favors
the instruction that has more instructions depending on it. This is enabled by
default when scheduling is enabled, i.e. with -fschedule-insns or
-fschedule-insns2 or at -O2 or higher.
-freschedule-modulo-scheduled-loops
Modulo scheduling is performed before traditional scheduling. If a loop is
modulo scheduled, later scheduling passes may change its schedule. Use this
option to control that behavior.
-fselective-scheduling
Schedule instructions using selective scheduling algorithm. Selective
scheduling runs instead of the first scheduler pass.
-fselective-scheduling2
Schedule instructions using selective scheduling algorithm. Selective
scheduling runs instead of the second scheduler pass.
-fsel-sched-pipelining
Enable software pipelining of innermost loops during selective scheduling.
This option has no effect unless one of -fselective-scheduling or
-fselective-scheduling2 is turned on.
-fsel-sched-pipelining-outer-loops
When pipelining loops during selective scheduling, also pipeline outer loops.
This option has no effect unless -fsel-sched-pipelining is turned on.
-fsemantic-interposition
Some object formats, like ELF, allow interposing of symbols by the dynamic
linker. This means that for symbols exported from the DSO, the compiler cannot
perform interprocedural propagation, inlining and other optimizations in
anticipation that the function or variable in question may change. While this
feature is useful, for example, to rewrite memory allocation functions by a
debugging implementation, it is expensive in the terms of code quality. With
-fno-semantic-interposition the compiler assumes that if interposition happens
for functions the overwriting function will have precisely the same semantics
(and side effects). Similarly if interposition happens for variables, the
constructor of the variable will be the same. The flag has no effect for
functions explicitly declared inline (where it is never allowed for
interposition to change semantics) and for symbols explicitly declared weak.
-fshrink-wrap
Emit function prologues only before parts of the function that need it, rather
than at the top of the function. This flag is enabled by default at -O and
higher.
-fshrink-wrap-separate
Shrink-wrap separate parts of the prologue and epilogue separately, so that
those parts are only executed when needed. This option is on by default, but
has no effect unless -fshrink-wrap is also turned on and the target supports
this.
-fcaller-saves
Enable allocation of values to registers that are clobbered by function calls,
by emitting extra instructions to save and restore the registers around such
calls. Such allocation is done only when it seems to result in better code.
This option is always enabled by default on certain machines, usually those
which have no call-preserved registers to use instead.
Enabled at levels -O2, -O3, -Os.
-fcombine-stack-adjustments
Tracks stack adjustments (pushes and pops) and stack memory references and then
tries to find ways to combine them.
Enabled by default at -O1 and higher.
-fipa-ra
Use caller save registers for allocation if those registers are not used by any
called function. In that case it is not necessary to save and restore them
around calls. This is only possible if called functions are part of same
compilation unit as current function and they are compiled before it.
Enabled at levels -O2, -O3, -Os, however the option is disabled if generated
code will be instrumented for profiling (-p, or -pg) or if callee's register
usage cannot be known exactly (this happens on targets that do not expose
prologues and epilogues in RTL).
-fconserve-stack
Attempt to minimize stack usage. The compiler attempts to use less stack
space, even if that makes the program slower. This option implies setting the
large-stack-frame parameter to 100 and the large-stack-frame-growth parameter
to 400.
-ftree-reassoc
Perform reassociation on trees. This flag is enabled by default at -O and
higher.
-fcode-hoisting
Perform code hoisting. Code hoisting tries to move the evaluation of
expressions executed on all paths to the function exit as early as possible.
This is especially useful as a code size optimization, but it often helps for
code speed as well. This flag is enabled by default at -O2 and higher.
-ftree-pre
Perform partial redundancy elimination (PRE) on trees. This flag is enabled by
default at -O2 and -O3.
-ftree-partial-pre
Make partial redundancy elimination (PRE) more aggressive. This flag is
enabled by default at -O3.
-ftree-forwprop
Perform forward propagation on trees. This flag is enabled by default at -O
and higher.
-ftree-fre
Perform full redundancy elimination (FRE) on trees. The difference between FRE
and PRE is that FRE only considers expressions that are computed on all paths
leading to the redundant computation. This analysis is faster than PRE, though
it exposes fewer redundancies. This flag is enabled by default at -O and
higher.
-ftree-phiprop
Perform hoisting of loads from conditional pointers on trees. This pass is
enabled by default at -O and higher.
-fhoist-adjacent-loads
Speculatively hoist loads from both branches of an if-then-else if the loads
are from adjacent locations in the same structure and the target architecture
has a conditional move instruction. This flag is enabled by default at -O2 and
higher.
-ftree-copy-prop
Perform copy propagation on trees. This pass eliminates unnecessary copy
operations. This flag is enabled by default at -O and higher.
-fipa-pure-const
Discover which functions are pure or constant. Enabled by default at -O and
higher.
-fipa-reference
Discover which static variables do not escape the compilation unit. Enabled by
default at -O and higher.
-fipa-pta
Perform interprocedural pointer analysis and interprocedural modification and
reference analysis. This option can cause excessive memory and compile-time
usage on large compilation units. It is not enabled by default at any
optimization level.
-fipa-profile
Perform interprocedural profile propagation. The functions called only from
cold functions are marked as cold. Also functions executed once (such as
"cold", "noreturn", static constructors or destructors) are identified. Cold
functions and loop less parts of functions executed once are then optimized for
size. Enabled by default at -O and higher.
-fipa-cp
Perform interprocedural constant propagation. This optimization analyzes the
program to determine when values passed to functions are constants and then
optimizes accordingly. This optimization can substantially increase
performance if the application has constants passed to functions. This flag is
enabled by default at -O2, -Os and -O3.
-fipa-cp-clone
Perform function cloning to make interprocedural constant propagation stronger.
When enabled, interprocedural constant propagation performs function cloning
when externally visible function can be called with constant arguments.
Because this optimization can create multiple copies of functions, it may
significantly increase code size (see --param ipcp-unit-growth=value). This
flag is enabled by default at -O3.
-fipa-bit-cp
When enabled, perform interprocedural bitwise constant propagation. This flag
is enabled by default at -O2. It requires that -fipa-cp is enabled.
-fipa-vrp
When enabled, perform interprocedural propagation of value ranges. This flag is
enabled by default at -O2. It requires that -fipa-cp is enabled.
-fipa-icf
Perform Identical Code Folding for functions and read-only variables. The
optimization reduces code size and may disturb unwind stacks by replacing a
function by equivalent one with a different name. The optimization works more
effectively with link-time optimization enabled.
Nevertheless the behavior is similar to Gold Linker ICF optimization, GCC ICF
works on different levels and thus the optimizations are not same - there are
equivalences that are found only by GCC and equivalences found only by Gold.
This flag is enabled by default at -O2 and -Os.
-flive-patching=level
Control GCC's optimizations to produce output suitable for live-patching.
If the compiler's optimization uses a function's body or information extracted
from its body to optimize/change another function, the latter is called an
impacted function of the former. If a function is patched, its impacted
functions should be patched too.
The impacted functions are determined by the compiler's interprocedural
optimizations. For example, a caller is impacted when inlining a function into
its caller, cloning a function and changing its caller to call this new clone,
or extracting a function's pureness/constness information to optimize its
direct or indirect callers, etc.
Usually, the more IPA optimizations enabled, the larger the number of impacted
functions for each function. In order to control the number of impacted
functions and more easily compute the list of impacted function, IPA
optimizations can be partially enabled at two different levels.
The level argument should be one of the following:
inline-clone
Only enable inlining and cloning optimizations, which includes inlining,
cloning, interprocedural scalar replacement of aggregates and partial
inlining. As a result, when patching a function, all its callers and its
clones' callers are impacted, therefore need to be patched as well.
-flive-patching=inline-clone disables the following optimization flags:
-fwhole-program -fipa-pta -fipa-reference -fipa-ra -fipa-icf
-fipa-icf-functions -fipa-icf-variables -fipa-bit-cp -fipa-vrp
-fipa-pure-const -fipa-reference-addressable -fipa-stack-alignment
inline-only-static
Only enable inlining of static functions. As a result, when patching a
static function, all its callers are impacted and so need to be patched as
well.
In addition to all the flags that -flive-patching=inline-clone disables,
-flive-patching=inline-only-static disables the following additional
optimization flags: -fipa-cp-clone -fipa-sra -fpartial-inlining -fipa-cp
When -flive-patching is specified without any value, the default value is
inline-clone.
This flag is disabled by default.
Note that -flive-patching is not supported with link-time optimization (-flto).
-fisolate-erroneous-paths-dereference
Detect paths that trigger erroneous or undefined behavior due to dereferencing
a null pointer. Isolate those paths from the main control flow and turn the
statement with erroneous or undefined behavior into a trap. This flag is
enabled by default at -O2 and higher and depends on
-fdelete-null-pointer-checks also being enabled.
-fisolate-erroneous-paths-attribute
Detect paths that trigger erroneous or undefined behavior due to a null value
being used in a way forbidden by a "returns_nonnull" or "nonnull" attribute.
Isolate those paths from the main control flow and turn the statement with
erroneous or undefined behavior into a trap. This is not currently enabled,
but may be enabled by -O2 in the future.
-ftree-sink
Perform forward store motion on trees. This flag is enabled by default at -O
and higher.
-ftree-bit-ccp
Perform sparse conditional bit constant propagation on trees and propagate
pointer alignment information. This pass only operates on local scalar
variables and is enabled by default at -O and higher. It requires that
-ftree-ccp is enabled.
-ftree-ccp
Perform sparse conditional constant propagation (CCP) on trees. This pass only
operates on local scalar variables and is enabled by default at -O and higher.
-fssa-backprop
Propagate information about uses of a value up the definition chain in order to
simplify the definitions. For example, this pass strips sign operations if the
sign of a value never matters. The flag is enabled by default at -O and
higher.
-fssa-phiopt
Perform pattern matching on SSA PHI nodes to optimize conditional code. This
pass is enabled by default at -O and higher.
-ftree-switch-conversion
Perform conversion of simple initializations in a switch to initializations
from a scalar array. This flag is enabled by default at -O2 and higher.
-ftree-tail-merge
Look for identical code sequences. When found, replace one with a jump to the
other. This optimization is known as tail merging or cross jumping. This flag
is enabled by default at -O2 and higher. The compilation time in this pass can
be limited using max-tail-merge-comparisons parameter and max-tail-merge-
iterations parameter.
-ftree-dce
Perform dead code elimination (DCE) on trees. This flag is enabled by default
at -O and higher.
-ftree-builtin-call-dce
Perform conditional dead code elimination (DCE) for calls to built-in functions
that may set "errno" but are otherwise free of side effects. This flag is
enabled by default at -O2 and higher if -Os is not also specified.
-ftree-dominator-opts
Perform a variety of simple scalar cleanups (constant/copy propagation,
redundancy elimination, range propagation and expression simplification) based
on a dominator tree traversal. This also performs jump threading (to reduce
jumps to jumps). This flag is enabled by default at -O and higher.
-ftree-dse
Perform dead store elimination (DSE) on trees. A dead store is a store into a
memory location that is later overwritten by another store without any
intervening loads. In this case the earlier store can be deleted. This flag
is enabled by default at -O and higher.
-ftree-ch
Perform loop header copying on trees. This is beneficial since it increases
effectiveness of code motion optimizations. It also saves one jump. This flag
is enabled by default at -O and higher. It is not enabled for -Os, since it
usually increases code size.
-ftree-loop-optimize
Perform loop optimizations on trees. This flag is enabled by default at -O and
higher.
-ftree-loop-linear
-floop-strip-mine
-floop-block
Perform loop nest optimizations. Same as -floop-nest-optimize. To use this
code transformation, GCC has to be configured with --with-isl to enable the
Graphite loop transformation infrastructure.
-fgraphite-identity
Enable the identity transformation for graphite. For every SCoP we generate
the polyhedral representation and transform it back to gimple. Using
-fgraphite-identity we can check the costs or benefits of the GIMPLE ->
GRAPHITE -> GIMPLE transformation. Some minimal optimizations are also
performed by the code generator isl, like index splitting and dead code
elimination in loops.
-floop-nest-optimize
Enable the isl based loop nest optimizer. This is a generic loop nest
optimizer based on the Pluto optimization algorithms. It calculates a loop
structure optimized for data-locality and parallelism. This option is
experimental.
-floop-parallelize-all
Use the Graphite data dependence analysis to identify loops that can be
parallelized. Parallelize all the loops that can be analyzed to not contain
loop carried dependences without checking that it is profitable to parallelize
the loops.
-ftree-coalesce-vars
While transforming the program out of the SSA representation, attempt to reduce
copying by coalescing versions of different user-defined variables, instead of
just compiler temporaries. This may severely limit the ability to debug an
optimized program compiled with -fno-var-tracking-assignments. In the negated
form, this flag prevents SSA coalescing of user variables. This option is
enabled by default if optimization is enabled, and it does very little
otherwise.
-ftree-loop-if-convert
Attempt to transform conditional jumps in the innermost loops to branch-less
equivalents. The intent is to remove control-flow from the innermost loops in
order to improve the ability of the vectorization pass to handle these loops.
This is enabled by default if vectorization is enabled.
-ftree-loop-distribution
Perform loop distribution. This flag can improve cache performance on big loop
bodies and allow further loop optimizations, like parallelization or
vectorization, to take place. For example, the loop
DO I = 1, N
A(I) = B(I) + C
D(I) = E(I) * F
ENDDO
is transformed to
DO I = 1, N
A(I) = B(I) + C
ENDDO
DO I = 1, N
D(I) = E(I) * F
ENDDO
-ftree-loop-distribute-patterns
Perform loop distribution of patterns that can be code generated with calls to
a library. This flag is enabled by default at -O3.
This pass distributes the initialization loops and generates a call to memset
zero. For example, the loop
DO I = 1, N
A(I) = 0
B(I) = A(I) + I
ENDDO
is transformed to
DO I = 1, N
A(I) = 0
ENDDO
DO I = 1, N
B(I) = A(I) + I
ENDDO
and the initialization loop is transformed into a call to memset zero.
-floop-interchange
Perform loop interchange outside of graphite. This flag can improve cache
performance on loop nest and allow further loop optimizations, like
vectorization, to take place. For example, the loop
for (int i = 0; i < N; i++)
for (int j = 0; j < N; j++)
for (int k = 0; k < N; k++)
c[i][j] = c[i][j] + a[i][k]*b[k][j];
is transformed to
for (int i = 0; i < N; i++)
for (int k = 0; k < N; k++)
for (int j = 0; j < N; j++)
c[i][j] = c[i][j] + a[i][k]*b[k][j];
This flag is enabled by default at -O3.
-floop-unroll-and-jam
Apply unroll and jam transformations on feasible loops. In a loop nest this
unrolls the outer loop by some factor and fuses the resulting multiple inner
loops. This flag is enabled by default at -O3.
-ftree-loop-im
Perform loop invariant motion on trees. This pass moves only invariants that
are hard to handle at RTL level (function calls, operations that expand to
nontrivial sequences of insns). With -funswitch-loops it also moves operands
of conditions that are invariant out of the loop, so that we can use just
trivial invariantness analysis in loop unswitching. The pass also includes
store motion.
-ftree-loop-ivcanon
Create a canonical counter for number of iterations in loops for which
determining number of iterations requires complicated analysis. Later
optimizations then may determine the number easily. Useful especially in
connection with unrolling.
-fivopts
Perform induction variable optimizations (strength reduction, induction
variable merging and induction variable elimination) on trees.
-ftree-parallelize-loops=n
Parallelize loops, i.e., split their iteration space to run in n threads. This
is only possible for loops whose iterations are independent and can be
arbitrarily reordered. The optimization is only profitable on multiprocessor
machines, for loops that are CPU-intensive, rather than constrained e.g. by
memory bandwidth. This option implies -pthread, and thus is only supported on
targets that have support for -pthread.
-ftree-pta
Perform function-local points-to analysis on trees. This flag is enabled by
default at -O and higher.
-ftree-sra
Perform scalar replacement of aggregates. This pass replaces structure
references with scalars to prevent committing structures to memory too early.
This flag is enabled by default at -O and higher.
-fstore-merging
Perform merging of narrow stores to consecutive memory addresses. This pass
merges contiguous stores of immediate values narrower than a word into fewer
wider stores to reduce the number of instructions. This is enabled by default
at -O2 and higher as well as -Os.
-ftree-ter
Perform temporary expression replacement during the SSA->normal phase. Single
use/single def temporaries are replaced at their use location with their
defining expression. This results in non-GIMPLE code, but gives the expanders
much more complex trees to work on resulting in better RTL generation. This is
enabled by default at -O and higher.
-ftree-slsr
Perform straight-line strength reduction on trees. This recognizes related
expressions involving multiplications and replaces them by less expensive
calculations when possible. This is enabled by default at -O and higher.
-ftree-vectorize
Perform vectorization on trees. This flag enables -ftree-loop-vectorize and
-ftree-slp-vectorize if not explicitly specified.
-ftree-loop-vectorize
Perform loop vectorization on trees. This flag is enabled by default at -O3 and
when -ftree-vectorize is enabled.
-ftree-slp-vectorize
Perform basic block vectorization on trees. This flag is enabled by default at
-O3 and when -ftree-vectorize is enabled.
-fvect-cost-model=model
Alter the cost model used for vectorization. The model argument should be one
of unlimited, dynamic or cheap. With the unlimited model the vectorized code-
path is assumed to be profitable while with the dynamic model a runtime check
guards the vectorized code-path to enable it only for iteration counts that
will likely execute faster than when executing the original scalar loop. The
cheap model disables vectorization of loops where doing so would be cost
prohibitive for example due to required runtime checks for data dependence or
alignment but otherwise is equal to the dynamic model. The default cost model
depends on other optimization flags and is either dynamic or cheap.
-fsimd-cost-model=model
Alter the cost model used for vectorization of loops marked with the OpenMP
simd directive. The model argument should be one of unlimited, dynamic, cheap.
All values of model have the same meaning as described in -fvect-cost-model and
by default a cost model defined with -fvect-cost-model is used.
-ftree-vrp
Perform Value Range Propagation on trees. This is similar to the constant
propagation pass, but instead of values, ranges of values are propagated. This
allows the optimizers to remove unnecessary range checks like array bound
checks and null pointer checks. This is enabled by default at -O2 and higher.
Null pointer check elimination is only done if -fdelete-null-pointer-checks is
enabled.
-fsplit-paths
Split paths leading to loop backedges. This can improve dead code elimination
and common subexpression elimination. This is enabled by default at -O2 and
above.
-fsplit-ivs-in-unroller
Enables expression of values of induction variables in later iterations of the
unrolled loop using the value in the first iteration. This breaks long
dependency chains, thus improving efficiency of the scheduling passes.
A combination of -fweb and CSE is often sufficient to obtain the same effect.
However, that is not reliable in cases where the loop body is more complicated
than a single basic block. It also does not work at all on some architectures
due to restrictions in the CSE pass.
This optimization is enabled by default.
-fvariable-expansion-in-unroller
With this option, the compiler creates multiple copies of some local variables
when unrolling a loop, which can result in superior code.
-fpartial-inlining
Inline parts of functions. This option has any effect only when inlining
itself is turned on by the -finline-functions or -finline-small-functions
options.
Enabled at levels -O2, -O3, -Os.
-fpredictive-commoning
Perform predictive commoning optimization, i.e., reusing computations
(especially memory loads and stores) performed in previous iterations of loops.
This option is enabled at level -O3.
-fprefetch-loop-arrays
If supported by the target machine, generate instructions to prefetch memory to
improve the performance of loops that access large arrays.
This option may generate better or worse code; results are highly dependent on
the structure of loops within the source code.
Disabled at level -Os.
-fno-printf-return-value
Do not substitute constants for known return value of formatted output
functions such as "sprintf", "snprintf", "vsprintf", and "vsnprintf" (but not
"printf" of "fprintf"). This transformation allows GCC to optimize or even
eliminate branches based on the known return value of these functions called
with arguments that are either constant, or whose values are known to be in a
range that makes determining the exact return value possible. For example,
when -fprintf-return-value is in effect, both the branch and the body of the
"if" statement (but not the call to "snprint") can be optimized away when "i"
is a 32-bit or smaller integer because the return value is guaranteed to be at
most 8.
char buf[9];
if (snprintf (buf, "%08x", i) >= sizeof buf)
...
The -fprintf-return-value option relies on other optimizations and yields best
results with -O2 and above. It works in tandem with the -Wformat-overflow and
-Wformat-truncation options. The -fprintf-return-value option is enabled by
default.
-fno-peephole
-fno-peephole2
Disable any machine-specific peephole optimizations. The difference between
-fno-peephole and -fno-peephole2 is in how they are implemented in the
compiler; some targets use one, some use the other, a few use both.
-fpeephole is enabled by default. -fpeephole2 enabled at levels -O2, -O3, -Os.
-fno-guess-branch-probability
Do not guess branch probabilities using heuristics.
GCC uses heuristics to guess branch probabilities if they are not provided by
profiling feedback (-fprofile-arcs). These heuristics are based on the control
flow graph. If some branch probabilities are specified by "__builtin_expect",
then the heuristics are used to guess branch probabilities for the rest of the
control flow graph, taking the "__builtin_expect" info into account. The
interactions between the heuristics and "__builtin_expect" can be complex, and
in some cases, it may be useful to disable the heuristics so that the effects
of "__builtin_expect" are easier to understand.
The default is -fguess-branch-probability at levels -O, -O2, -O3, -Os.
-freorder-blocks
Reorder basic blocks in the compiled function in order to reduce number of
taken branches and improve code locality.
Enabled at levels -O, -O2, -O3, -Os.
-freorder-blocks-algorithm=algorithm
Use the specified algorithm for basic block reordering. The algorithm argument
can be simple, which does not increase code size (except sometimes due to
secondary effects like alignment), or stc, the "software trace cache"
algorithm, which tries to put all often executed code together, minimizing the
number of branches executed by making extra copies of code.
The default is simple at levels -O, -Os, and stc at levels -O2, -O3.
-freorder-blocks-and-partition
In addition to reordering basic blocks in the compiled function, in order to
reduce number of taken branches, partitions hot and cold basic blocks into
separate sections of the assembly and .o files, to improve paging and cache
locality performance.
This optimization is automatically turned off in the presence of exception
handling or unwind tables (on targets using setjump/longjump or target specific
scheme), for linkonce sections, for functions with a user-defined section
attribute and on any architecture that does not support named sections. When
-fsplit-stack is used this option is not enabled by default (to avoid linker
errors), but may be enabled explicitly (if using a working linker).
Enabled for x86 at levels -O2, -O3, -Os.
-freorder-functions
Reorder functions in the object file in order to improve code locality. This
is implemented by using special subsections ".text.hot" for most frequently
executed functions and ".text.unlikely" for unlikely executed functions.
Reordering is done by the linker so object file format must support named
sections and linker must place them in a reasonable way.
Also profile feedback must be available to make this option effective. See
-fprofile-arcs for details.
Enabled at levels -O2, -O3, -Os.
-fstrict-aliasing
Allow the compiler to assume the strictest aliasing rules applicable to the
language being compiled. For C (and C++), this activates optimizations based
on the type of expressions. In particular, an object of one type is assumed
never to reside at the same address as an object of a different type, unless
the types are almost the same. For example, an "unsigned int" can alias an
"int", but not a "void*" or a "double". A character type may alias any other
type.
Pay special attention to code like this:
union a_union {
int i;
double d;
};
int f() {
union a_union t;
t.d = 3.0;
return t.i;
}
The practice of reading from a different union member than the one most
recently written to (called "type-punning") is common. Even with
-fstrict-aliasing, type-punning is allowed, provided the memory is accessed
through the union type. So, the code above works as expected. However, this
code might not:
int f() {
union a_union t;
int* ip;
t.d = 3.0;
ip = &t.i;
return *ip;
}
Similarly, access by taking the address, casting the resulting pointer and
dereferencing the result has undefined behavior, even if the cast uses a union
type, e.g.:
int f() {
double d = 3.0;
return ((union a_union *) &d)->i;
}
The -fstrict-aliasing option is enabled at levels -O2, -O3, -Os.
-falign-functions
-falign-functions=n
Align the start of functions to the next power-of-two greater than n, skipping
up to n bytes. For instance, -falign-functions=32 aligns functions to the next
32-byte boundary, but -falign-functions=24 aligns to the next 32-byte boundary
only if this can be done by skipping 23 bytes or less.
-fno-align-functions and -falign-functions=1 are equivalent and mean that
functions are not aligned.
Some assemblers only support this flag when n is a power of two; in that case,
it is rounded up.
If n is not specified or is zero, use a machine-dependent default. The maximum
allowed n option value is 65536.
Enabled at levels -O2, -O3.
-flimit-function-alignment
If this option is enabled, the compiler tries to avoid unnecessarily
overaligning functions. It attempts to instruct the assembler to align by the
amount specified by -falign-functions, but not to skip more bytes than the size
of the function.
-falign-labels
-falign-labels=n
Align all branch targets to a power-of-two boundary, skipping up to n bytes
like -falign-functions. This option can easily make code slower, because it
must insert dummy operations for when the branch target is reached in the usual
flow of the code.
-fno-align-labels and -falign-labels=1 are equivalent and mean that labels are
not aligned.
If -falign-loops or -falign-jumps are applicable and are greater than this
value, then their values are used instead.
If n is not specified or is zero, use a machine-dependent default which is very
likely to be 1, meaning no alignment. The maximum allowed n option value is
65536.
Enabled at levels -O2, -O3.
-falign-loops
-falign-loops=n
Align loops to a power-of-two boundary, skipping up to n bytes like
-falign-functions. If the loops are executed many times, this makes up for any
execution of the dummy operations.
-fno-align-loops and -falign-loops=1 are equivalent and mean that loops are not
aligned. The maximum allowed n option value is 65536.
If n is not specified or is zero, use a machine-dependent default.
Enabled at levels -O2, -O3.
-falign-jumps
-falign-jumps=n
Align branch targets to a power-of-two boundary, for branch targets where the
targets can only be reached by jumping, skipping up to n bytes like
-falign-functions. In this case, no dummy operations need be executed.
-fno-align-jumps and -falign-jumps=1 are equivalent and mean that loops are not
aligned.
If n is not specified or is zero, use a machine-dependent default. The maximum
allowed n option value is 65536.
Enabled at levels -O2, -O3.
-funit-at-a-time
This option is left for compatibility reasons. -funit-at-a-time has no effect,
while -fno-unit-at-a-time implies -fno-toplevel-reorder and
-fno-section-anchors.
Enabled by default.
-fno-toplevel-reorder
Do not reorder top-level functions, variables, and "asm" statements. Output
them in the same order that they appear in the input file. When this option is
used, unreferenced static variables are not removed. This option is intended
to support existing code that relies on a particular ordering. For new code,
it is better to use attributes when possible.
Enabled at level -O0. When disabled explicitly, it also implies
-fno-section-anchors, which is otherwise enabled at -O0 on some targets.
-fweb
Constructs webs as commonly used for register allocation purposes and assign
each web individual pseudo register. This allows the register allocation pass
to operate on pseudos directly, but also strengthens several other optimization
passes, such as CSE, loop optimizer and trivial dead code remover. It can,
however, make debugging impossible, since variables no longer stay in a "home
register".
Enabled by default with -funroll-loops.
-fwhole-program
Assume that the current compilation unit represents the whole program being
compiled. All public functions and variables with the exception of "main" and
those merged by attribute "externally_visible" become static functions and in
effect are optimized more aggressively by interprocedural optimizers.
This option should not be used in combination with -flto. Instead relying on a
linker plugin should provide safer and more precise information.
-flto[=n]
This option runs the standard link-time optimizer. When invoked with source
code, it generates GIMPLE (one of GCC's internal representations) and writes it
to special ELF sections in the object file. When the object files are linked
together, all the function bodies are read from these ELF sections and
instantiated as if they had been part of the same translation unit.
To use the link-time optimizer, -flto and optimization options should be
specified at compile time and during the final link. It is recommended that
you compile all the files participating in the same link with the same options
and also specify those options at link time. For example:
gcc -c -O2 -flto foo.c
gcc -c -O2 -flto bar.c
gcc -o myprog -flto -O2 foo.o bar.o
The first two invocations to GCC save a bytecode representation of GIMPLE into
special ELF sections inside foo.o and bar.o. The final invocation reads the
GIMPLE bytecode from foo.o and bar.o, merges the two files into a single
internal image, and compiles the result as usual. Since both foo.o and bar.o
are merged into a single image, this causes all the interprocedural analyses
and optimizations in GCC to work across the two files as if they were a single
one. This means, for example, that the inliner is able to inline functions in
bar.o into functions in foo.o and vice-versa.
Another (simpler) way to enable link-time optimization is:
gcc -o myprog -flto -O2 foo.c bar.c
The above generates bytecode for foo.c and bar.c, merges them together into a
single GIMPLE representation and optimizes them as usual to produce myprog.
The only important thing to keep in mind is that to enable link-time
optimizations you need to use the GCC driver to perform the link step. GCC
then automatically performs link-time optimization if any of the objects
involved were compiled with the -flto command-line option. You generally
should specify the optimization options to be used for link-time optimization
though GCC tries to be clever at guessing an optimization level to use from the
options used at compile time if you fail to specify one at link time. You can
always override the automatic decision to do link-time optimization by passing
-fno-lto to the link command.
To make whole program optimization effective, it is necessary to make certain
whole program assumptions. The compiler needs to know what functions and
variables can be accessed by libraries and runtime outside of the link-time
optimized unit. When supported by the linker, the linker plugin (see
-fuse-linker-plugin) passes information to the compiler about used and
externally visible symbols. When the linker plugin is not available,
-fwhole-program should be used to allow the compiler to make these assumptions,
which leads to more aggressive optimization decisions.
When -fuse-linker-plugin is not enabled, when a file is compiled with -flto,
the generated object file is larger than a regular object file because it
contains GIMPLE bytecodes and the usual final code (see -ffat-lto-objects.
This means that object files with LTO information can be linked as normal
object files; if -fno-lto is passed to the linker, no interprocedural
optimizations are applied. Note that when -fno-fat-lto-objects is enabled the
compile stage is faster but you cannot perform a regular, non-LTO link on them.
Additionally, the optimization flags used to compile individual files are not
necessarily related to those used at link time. For instance,
gcc -c -O0 -ffat-lto-objects -flto foo.c
gcc -c -O0 -ffat-lto-objects -flto bar.c
gcc -o myprog -O3 foo.o bar.o
This produces individual object files with unoptimized assembler code, but the
resulting binary myprog is optimized at -O3. If, instead, the final binary is
generated with -fno-lto, then myprog is not optimized.
When producing the final binary, GCC only applies link-time optimizations to
those files that contain bytecode. Therefore, you can mix and match object
files and libraries with GIMPLE bytecodes and final object code. GCC
automatically selects which files to optimize in LTO mode and which files to
link without further processing.
There are some code generation flags preserved by GCC when generating
bytecodes, as they need to be used during the final link stage. Generally
options specified at link time override those specified at compile time.
If you do not specify an optimization level option -O at link time, then GCC
uses the highest optimization level used when compiling the object files.
Currently, the following options and their settings are taken from the first
object file that explicitly specifies them: -fPIC, -fpic, -fpie, -fcommon,
-fexceptions, -fnon-call-exceptions, -fgnu-tm and all the -m target flags.
Certain ABI-changing flags are required to match in all compilation units, and
trying to override this at link time with a conflicting value is ignored. This
includes options such as -freg-struct-return and -fpcc-struct-return.
Other options such as -ffp-contract, -fno-strict-overflow, -fwrapv, -fno-trapv
or -fno-strict-aliasing are passed through to the link stage and merged
conservatively for conflicting translation units. Specifically
-fno-strict-overflow, -fwrapv and -fno-trapv take precedence; and for example
-ffp-contract=off takes precedence over -ffp-contract=fast. You can override
them at link time.
If LTO encounters objects with C linkage declared with incompatible types in
separate translation units to be linked together (undefined behavior according
to ISO C99 6.2.7), a non-fatal diagnostic may be issued. The behavior is still
undefined at run time. Similar diagnostics may be raised for other languages.
Another feature of LTO is that it is possible to apply interprocedural
optimizations on files written in different languages:
gcc -c -flto foo.c
g++ -c -flto bar.cc
gfortran -c -flto baz.f90
g++ -o myprog -flto -O3 foo.o bar.o baz.o -lgfortran
Notice that the final link is done with g++ to get the C++ runtime libraries
and -lgfortran is added to get the Fortran runtime libraries. In general, when
mixing languages in LTO mode, you should use the same link command options as
when mixing languages in a regular (non-LTO) compilation.
If object files containing GIMPLE bytecode are stored in a library archive, say
libfoo.a, it is possible to extract and use them in an LTO link if you are
using a linker with plugin support. To create static libraries suitable for
LTO, use gcc-ar and gcc-ranlib instead of ar and ranlib; to show the symbols of
object files with GIMPLE bytecode, use gcc-nm. Those commands require that ar,
ranlib and nm have been compiled with plugin support. At link time, use the
flag -fuse-linker-plugin to ensure that the library participates in the LTO
optimization process:
gcc -o myprog -O2 -flto -fuse-linker-plugin a.o b.o -lfoo
With the linker plugin enabled, the linker extracts the needed GIMPLE files
from libfoo.a and passes them on to the running GCC to make them part of the
aggregated GIMPLE image to be optimized.
If you are not using a linker with plugin support and/or do not enable the
linker plugin, then the objects inside libfoo.a are extracted and linked as
usual, but they do not participate in the LTO optimization process. In order
to make a static library suitable for both LTO optimization and usual linkage,
compile its object files with -flto -ffat-lto-objects.
Link-time optimizations do not require the presence of the whole program to
operate. If the program does not require any symbols to be exported, it is
possible to combine -flto and -fwhole-program to allow the interprocedural
optimizers to use more aggressive assumptions which may lead to improved
optimization opportunities. Use of -fwhole-program is not needed when linker
plugin is active (see -fuse-linker-plugin).
The current implementation of LTO makes no attempt to generate bytecode that is
portable between different types of hosts. The bytecode files are versioned
and there is a strict version check, so bytecode files generated in one version
of GCC do not work with an older or newer version of GCC.
Link-time optimization does not work well with generation of debugging
information on systems other than those using a combination of ELF and DWARF.
If you specify the optional n, the optimization and code generation done at
link time is executed in parallel using n parallel jobs by utilizing an
installed make program. The environment variable MAKE may be used to override
the program used. The default value for n is 1.
You can also specify -flto=jobserver to use GNU make's job server mode to
determine the number of parallel jobs. This is useful when the Makefile calling
GCC is already executing in parallel. You must prepend a + to the command
recipe in the parent Makefile for this to work. This option likely only works
if MAKE is GNU make.
-flto-partition=alg
Specify the partitioning algorithm used by the link-time optimizer. The value
is either 1to1 to specify a partitioning mirroring the original source files or
balanced to specify partitioning into equally sized chunks (whenever possible)
or max to create new partition for every symbol where possible. Specifying
none as an algorithm disables partitioning and streaming completely. The
default value is balanced. While 1to1 can be used as an workaround for various
code ordering issues, the max partitioning is intended for internal testing
only. The value one specifies that exactly one partition should be used while
the value none bypasses partitioning and executes the link-time optimization
step directly from the WPA phase.
-flto-odr-type-merging
Enable streaming of mangled types names of C++ types and their unification at
link time. This increases size of LTO object files, but enables diagnostics
about One Definition Rule violations.
-flto-compression-level=n
This option specifies the level of compression used for intermediate language
written to LTO object files, and is only meaningful in conjunction with LTO
mode (-flto). Valid values are 0 (no compression) to 9 (maximum compression).
Values outside this range are clamped to either 0 or 9. If the option is not
given, a default balanced compression setting is used.
-fuse-linker-plugin
Enables the use of a linker plugin during link-time optimization. This option
relies on plugin support in the linker, which is available in gold or in GNU ld
2.21 or newer.
This option enables the extraction of object files with GIMPLE bytecode out of
library archives. This improves the quality of optimization by exposing more
code to the link-time optimizer. This information specifies what symbols can
be accessed externally (by non-LTO object or during dynamic linking).
Resulting code quality improvements on binaries (and shared libraries that use
hidden visibility) are similar to -fwhole-program. See -flto for a description
of the effect of this flag and how to use it.
This option is enabled by default when LTO support in GCC is enabled and GCC
was configured for use with a linker supporting plugins (GNU ld 2.21 or newer
or gold).
-ffat-lto-objects
Fat LTO objects are object files that contain both the intermediate language
and the object code. This makes them usable for both LTO linking and normal
linking. This option is effective only when compiling with -flto and is ignored
at link time.
-fno-fat-lto-objects improves compilation time over plain LTO, but requires the
complete toolchain to be aware of LTO. It requires a linker with linker plugin
support for basic functionality. Additionally, nm, ar and ranlib need to
support linker plugins to allow a full-featured build environment (capable of
building static libraries etc). GCC provides the gcc-ar, gcc-nm, gcc-ranlib
wrappers to pass the right options to these tools. With non fat LTO makefiles
need to be modified to use them.
Note that modern binutils provide plugin auto-load mechanism. Installing the
linker plugin into $libdir/bfd-plugins has the same effect as usage of the
command wrappers (gcc-ar, gcc-nm and gcc-ranlib).
The default is -fno-fat-lto-objects on targets with linker plugin support.
-fcompare-elim
After register allocation and post-register allocation instruction splitting,
identify arithmetic instructions that compute processor flags similar to a
comparison operation based on that arithmetic. If possible, eliminate the
explicit comparison operation.
This pass only applies to certain targets that cannot explicitly represent the
comparison operation before register allocation is complete.
Enabled at levels -O, -O2, -O3, -Os.
-fcprop-registers
After register allocation and post-register allocation instruction splitting,
perform a copy-propagation pass to try to reduce scheduling dependencies and
occasionally eliminate the copy.
Enabled at levels -O, -O2, -O3, -Os.
-fprofile-correction
Profiles collected using an instrumented binary for multi-threaded programs may
be inconsistent due to missed counter updates. When this option is specified,
GCC uses heuristics to correct or smooth out such inconsistencies. By default,
GCC emits an error message when an inconsistent profile is detected.
-fprofile-use
-fprofile-use=path
Enable profile feedback-directed optimizations, and the following optimizations
which are generally profitable only with profile feedback available:
-fbranch-probabilities, -fvpt, -funroll-loops, -fpeel-loops, -ftracer,
-ftree-vectorize, and ftree-loop-distribute-patterns.
Before you can use this option, you must first generate profiling information.
By default, GCC emits an error message if the feedback profiles do not match
the source code. This error can be turned into a warning by using
-Wcoverage-mismatch. Note this may result in poorly optimized code.
If path is specified, GCC looks at the path to find the profile feedback data
files. See -fprofile-dir.
-fauto-profile
-fauto-profile=path
Enable sampling-based feedback-directed optimizations, and the following
optimizations which are generally profitable only with profile feedback
available: -fbranch-probabilities, -fvpt, -funroll-loops, -fpeel-loops,
-ftracer, -ftree-vectorize, -finline-functions, -fipa-cp, -fipa-cp-clone,
-fpredictive-commoning, -funswitch-loops, -fgcse-after-reload, and
-ftree-loop-distribute-patterns.
path is the name of a file containing AutoFDO profile information. If omitted,
it defaults to fbdata.afdo in the current directory.
Producing an AutoFDO profile data file requires running your program with the
perf utility on a supported GNU/Linux target system. For more information, see
<https://perf.wiki.kernel.org/>.
E.g.
perf record -e br_inst_retired:near_taken -b -o perf.data \
-- your_program
Then use the create_gcov tool to convert the raw profile data to a format that
can be used by GCC. You must also supply the unstripped binary for your
program to this tool. See <https://github.com/google/autofdo>.
E.g.
create_gcov --binary=your_program.unstripped --profile=perf.data \
--gcov=profile.afdo
The following options control compiler behavior regarding floating-point
arithmetic. These options trade off between speed and correctness. All must be
specifically enabled.
-ffloat-store
Do not store floating-point variables in registers, and inhibit other options
that might change whether a floating-point value is taken from a register or
memory.
This option prevents undesirable excess precision on machines such as the 68000
where the floating registers (of the 68881) keep more precision than a "double"
is supposed to have. Similarly for the x86 architecture. For most programs,
the excess precision does only good, but a few programs rely on the precise
definition of IEEE floating point. Use -ffloat-store for such programs, after
modifying them to store all pertinent intermediate computations into variables.
-fexcess-precision=style
This option allows further control over excess precision on machines where
floating-point operations occur in a format with more precision or range than
the IEEE standard and interchange floating-point types. By default,
-fexcess-precision=fast is in effect; this means that operations may be carried
out in a wider precision than the types specified in the source if that would
result in faster code, and it is unpredictable when rounding to the types
specified in the source code takes place. When compiling C, if
-fexcess-precision=standard is specified then excess precision follows the
rules specified in ISO C99; in particular, both casts and assignments cause
values to be rounded to their semantic types (whereas -ffloat-store only
affects assignments). This option is enabled by default for C if a strict
conformance option such as -std=c99 is used. -ffast-math enables
-fexcess-precision=fast by default regardless of whether a strict conformance
option is used.
-fexcess-precision=standard is not implemented for languages other than C. On
the x86, it has no effect if -mfpmath=sse or -mfpmath=sse+387 is specified; in
the former case, IEEE semantics apply without excess precision, and in the
latter, rounding is unpredictable.
-ffast-math
Sets the options -fno-math-errno, -funsafe-math-optimizations,
-ffinite-math-only, -fno-rounding-math, -fno-signaling-nans, -fcx-limited-range
and -fexcess-precision=fast.
This option causes the preprocessor macro "__FAST_MATH__" to be defined.
This option is not turned on by any -O option besides -Ofast since it can
result in incorrect output for programs that depend on an exact implementation
of IEEE or ISO rules/specifications for math functions. It may, however, yield
faster code for programs that do not require the guarantees of these
specifications.
-fno-math-errno
Do not set "errno" after calling math functions that are executed with a single
instruction, e.g., "sqrt". A program that relies on IEEE exceptions for math
error handling may want to use this flag for speed while maintaining IEEE
arithmetic compatibility.
This option is not turned on by any -O option since it can result in incorrect
output for programs that depend on an exact implementation of IEEE or ISO
rules/specifications for math functions. It may, however, yield faster code for
programs that do not require the guarantees of these specifications.
The default is -fmath-errno.
On Darwin systems, the math library never sets "errno". There is therefore no
reason for the compiler to consider the possibility that it might, and
-fno-math-errno is the default.
-funsafe-math-optimizations
Allow optimizations for floating-point arithmetic that (a) assume that
arguments and results are valid and (b) may violate IEEE or ANSI standards.
When used at link time, it may include libraries or startup files that change
the default FPU control word or other similar optimizations.
This option is not turned on by any -O option since it can result in incorrect
output for programs that depend on an exact implementation of IEEE or ISO
rules/specifications for math functions. It may, however, yield faster code for
programs that do not require the guarantees of these specifications. Enables
-fno-signed-zeros, -fno-trapping-math, -fassociative-math and
-freciprocal-math.
The default is -fno-unsafe-math-optimizations.
-fassociative-math
Allow re-association of operands in series of floating-point operations. This
violates the ISO C and C++ language standard by possibly changing computation
result. NOTE: re-ordering may change the sign of zero as well as ignore NaNs
and inhibit or create underflow or overflow (and thus cannot be used on code
that relies on rounding behavior like "(x + 2**52) - 2**52". May also reorder
floating-point comparisons and thus may not be used when ordered comparisons
are required. This option requires that both -fno-signed-zeros and
-fno-trapping-math be in effect. Moreover, it doesn't make much sense with
-frounding-math. For Fortran the option is automatically enabled when both
-fno-signed-zeros and -fno-trapping-math are in effect.
The default is -fno-associative-math.
-freciprocal-math
Allow the reciprocal of a value to be used instead of dividing by the value if
this enables optimizations. For example "x / y" can be replaced with "x *
(1/y)", which is useful if "(1/y)" is subject to common subexpression
elimination. Note that this loses precision and increases the number of flops
operating on the value.
The default is -fno-reciprocal-math.
-ffinite-math-only
Allow optimizations for floating-point arithmetic that assume that arguments
and results are not NaNs or +-Infs.
This option is not turned on by any -O option since it can result in incorrect
output for programs that depend on an exact implementation of IEEE or ISO
rules/specifications for math functions. It may, however, yield faster code for
programs that do not require the guarantees of these specifications.
The default is -fno-finite-math-only.
-fno-signed-zeros
Allow optimizations for floating-point arithmetic that ignore the signedness of
zero. IEEE arithmetic specifies the behavior of distinct +0.0 and -0.0 values,
which then prohibits simplification of expressions such as x+0.0 or 0.0*x (even
with -ffinite-math-only). This option implies that the sign of a zero result
isn't significant.
The default is -fsigned-zeros.
-fno-trapping-math
Compile code assuming that floating-point operations cannot generate user-
visible traps. These traps include division by zero, overflow, underflow,
inexact result and invalid operation. This option requires that
-fno-signaling-nans be in effect. Setting this option may allow faster code if
one relies on "non-stop" IEEE arithmetic, for example.
This option should never be turned on by any -O option since it can result in
incorrect output for programs that depend on an exact implementation of IEEE or
ISO rules/specifications for math functions.
The default is -ftrapping-math.
-frounding-math
Disable transformations and optimizations that assume default floating-point
rounding behavior. This is round-to-zero for all floating point to integer
conversions, and round-to-nearest for all other arithmetic truncations. This
option should be specified for programs that change the FP rounding mode
dynamically, or that may be executed with a non-default rounding mode. This
option disables constant folding of floating-point expressions at compile time
(which may be affected by rounding mode) and arithmetic transformations that
are unsafe in the presence of sign-dependent rounding modes.
The default is -fno-rounding-math.
This option is experimental and does not currently guarantee to disable all GCC
optimizations that are affected by rounding mode. Future versions of GCC may
provide finer control of this setting using C99's "FENV_ACCESS" pragma. This
command-line option will be used to specify the default state for
"FENV_ACCESS".
-fsignaling-nans
Compile code assuming that IEEE signaling NaNs may generate user-visible traps
during floating-point operations. Setting this option disables optimizations
that may change the number of exceptions visible with signaling NaNs. This
option implies -ftrapping-math.
This option causes the preprocessor macro "__SUPPORT_SNAN__" to be defined.
The default is -fno-signaling-nans.
This option is experimental and does not currently guarantee to disable all GCC
optimizations that affect signaling NaN behavior.
-fno-fp-int-builtin-inexact
Do not allow the built-in functions "ceil", "floor", "round" and "trunc", and
their "float" and "long double" variants, to generate code that raises the
"inexact" floating-point exception for noninteger arguments. ISO C99 and C11
allow these functions to raise the "inexact" exception, but ISO/IEC TS
18661-1:2014, the C bindings to IEEE 754-2008, does not allow these functions
to do so.
The default is -ffp-int-builtin-inexact, allowing the exception to be raised.
This option does nothing unless -ftrapping-math is in effect.
Even if -fno-fp-int-builtin-inexact is used, if the functions generate a call
to a library function then the "inexact" exception may be raised if the library
implementation does not follow TS 18661.
-fsingle-precision-constant
Treat floating-point constants as single precision instead of implicitly
converting them to double-precision constants.
-fcx-limited-range
When enabled, this option states that a range reduction step is not needed when
performing complex division. Also, there is no checking whether the result of
a complex multiplication or division is "NaN + I*NaN", with an attempt to
rescue the situation in that case. The default is -fno-cx-limited-range, but
is enabled by -ffast-math.
This option controls the default setting of the ISO C99 "CX_LIMITED_RANGE"
pragma. Nevertheless, the option applies to all languages.
-fcx-fortran-rules
Complex multiplication and division follow Fortran rules. Range reduction is
done as part of complex division, but there is no checking whether the result
of a complex multiplication or division is "NaN + I*NaN", with an attempt to
rescue the situation in that case.
The default is -fno-cx-fortran-rules.
The following options control optimizations that may improve performance, but are
not enabled by any -O options. This section includes experimental options that may
produce broken code.
-fbranch-probabilities
After running a program compiled with -fprofile-arcs, you can compile it a
second time using -fbranch-probabilities, to improve optimizations based on the
number of times each branch was taken. When a program compiled with
-fprofile-arcs exits, it saves arc execution counts to a file called
sourcename.gcda for each source file. The information in this data file is
very dependent on the structure of the generated code, so you must use the same
source code and the same optimization options for both compilations.
With -fbranch-probabilities, GCC puts a REG_BR_PROB note on each JUMP_INSN and
CALL_INSN. These can be used to improve optimization. Currently, they are
only used in one place: in reorg.c, instead of guessing which path a branch is
most likely to take, the REG_BR_PROB values are used to exactly determine which
path is taken more often.
-fprofile-values
If combined with -fprofile-arcs, it adds code so that some data about values of
expressions in the program is gathered.
With -fbranch-probabilities, it reads back the data gathered from profiling
values of expressions for usage in optimizations.
Enabled with -fprofile-generate and -fprofile-use.
-fprofile-reorder-functions
Function reordering based on profile instrumentation collects first time of
execution of a function and orders these functions in ascending order.
Enabled with -fprofile-use.
-fvpt
If combined with -fprofile-arcs, this option instructs the compiler to add code
to gather information about values of expressions.
With -fbranch-probabilities, it reads back the data gathered and actually
performs the optimizations based on them. Currently the optimizations include
specialization of division operations using the knowledge about the value of
the denominator.
-frename-registers
Attempt to avoid false dependencies in scheduled code by making use of
registers left over after register allocation. This optimization most benefits
processors with lots of registers. Depending on the debug information format
adopted by the target, however, it can make debugging impossible, since
variables no longer stay in a "home register".
Enabled by default with -funroll-loops.
-fschedule-fusion
Performs a target dependent pass over the instruction stream to schedule
instructions of same type together because target machine can execute them more
efficiently if they are adjacent to each other in the instruction flow.
Enabled at levels -O2, -O3, -Os.
-ftracer
Perform tail duplication to enlarge superblock size. This transformation
simplifies the control flow of the function allowing other optimizations to do
a better job.
Enabled with -fprofile-use.
-funroll-loops
Unroll loops whose number of iterations can be determined at compile time or
upon entry to the loop. -funroll-loops implies -frerun-cse-after-loop, -fweb
and -frename-registers. It also turns on complete loop peeling (i.e. complete
removal of loops with a small constant number of iterations). This option
makes code larger, and may or may not make it run faster.
Enabled with -fprofile-use.
-funroll-all-loops
Unroll all loops, even if their number of iterations is uncertain when the loop
is entered. This usually makes programs run more slowly. -funroll-all-loops
implies the same options as -funroll-loops.
-fpeel-loops
Peels loops for which there is enough information that they do not roll much
(from profile feedback or static analysis). It also turns on complete loop
peeling (i.e. complete removal of loops with small constant number of
iterations).
Enabled with -O3 and/or -fprofile-use.
-fmove-loop-invariants
Enables the loop invariant motion pass in the RTL loop optimizer. Enabled at
level -O1
-fsplit-loops
Split a loop into two if it contains a condition that's always true for one
side of the iteration space and false for the other.
-funswitch-loops
Move branches with loop invariant conditions out of the loop, with duplicates
of the loop on both branches (modified according to result of the condition).
-ffunction-sections
-fdata-sections
Place each function or data item into its own section in the output file if the
target supports arbitrary sections. The name of the function or the name of
the data item determines the section's name in the output file.
Use these options on systems where the linker can perform optimizations to
improve locality of reference in the instruction space. Most systems using the
ELF object format have linkers with such optimizations. On AIX, the linker
rearranges sections (CSECTs) based on the call graph. The performance impact
varies.
Together with a linker garbage collection (linker --gc-sections option) these
options may lead to smaller statically-linked executables (after stripping).
On ELF/DWARF systems these options do not degenerate the quality of the debug
information. There could be issues with other object files/debug info formats.
Only use these options when there are significant benefits from doing so. When
you specify these options, the assembler and linker create larger object and
executable files and are also slower. These options affect code generation.
They prevent optimizations by the compiler and assembler using relative
locations inside a translation unit since the locations are unknown until link
time. An example of such an optimization is relaxing calls to short call
instructions.
-fbranch-target-load-optimize
Perform branch target register load optimization before prologue / epilogue
threading. The use of target registers can typically be exposed only during
reload, thus hoisting loads out of loops and doing inter-block scheduling needs
a separate optimization pass.
-fbranch-target-load-optimize2
Perform branch target register load optimization after prologue / epilogue
threading.
-fbtr-bb-exclusive
When performing branch target register load optimization, don't reuse branch
target registers within any basic block.
-fstdarg-opt
Optimize the prologue of variadic argument functions with respect to usage of
those arguments.
-fsection-anchors
Try to reduce the number of symbolic address calculations by using shared
"anchor" symbols to address nearby objects. This transformation can help to
reduce the number of GOT entries and GOT accesses on some targets.
For example, the implementation of the following function "foo":
static int a, b, c;
int foo (void) { return a + b + c; }
usually calculates the addresses of all three variables, but if you compile it
with -fsection-anchors, it accesses the variables from a common anchor point
instead. The effect is similar to the following pseudocode (which isn't valid
C):
int foo (void)
{
register int *xr = &x;
return xr[&a - &x] + xr[&b - &x] + xr[&c - &x];
}
Not all targets support this option.
--param name=value
In some places, GCC uses various constants to control the amount of
optimization that is done. For example, GCC does not inline functions that
contain more than a certain number of instructions. You can control some of
these constants on the command line using the --param option.
The names of specific parameters, and the meaning of the values, are tied to
the internals of the compiler, and are subject to change without notice in
future releases.
In each case, the value is an integer. The allowable choices for name are:
predictable-branch-outcome
When branch is predicted to be taken with probability lower than this
threshold (in percent), then it is considered well predictable. The default
is 10.
max-rtl-if-conversion-insns
RTL if-conversion tries to remove conditional branches around a block and
replace them with conditionally executed instructions. This parameter
gives the maximum number of instructions in a block which should be
considered for if-conversion. The default is 10, though the compiler will
also use other heuristics to decide whether if-conversion is likely to be
profitable.
max-rtl-if-conversion-predictable-cost
max-rtl-if-conversion-unpredictable-cost
RTL if-conversion will try to remove conditional branches around a block
and replace them with conditionally executed instructions. These
parameters give the maximum permissible cost for the sequence that would be
generated by if-conversion depending on whether the branch is statically
determined to be predictable or not. The units for this parameter are the
same as those for the GCC internal seq_cost metric. The compiler will try
to provide a reasonable default for this parameter using the BRANCH_COST
target macro.
max-crossjump-edges
The maximum number of incoming edges to consider for cross-jumping. The
algorithm used by -fcrossjumping is O(N^2) in the number of edges incoming
to each block. Increasing values mean more aggressive optimization, making
the compilation time increase with probably small improvement in executable
size.
min-crossjump-insns
The minimum number of instructions that must be matched at the end of two
blocks before cross-jumping is performed on them. This value is ignored in
the case where all instructions in the block being cross-jumped from are
matched. The default value is 5.
max-grow-copy-bb-insns
The maximum code size expansion factor when copying basic blocks instead of
jumping. The expansion is relative to a jump instruction. The default
value is 8.
max-goto-duplication-insns
The maximum number of instructions to duplicate to a block that jumps to a
computed goto. To avoid O(N^2) behavior in a number of passes, GCC factors
computed gotos early in the compilation process, and unfactors them as late
as possible. Only computed jumps at the end of a basic blocks with no more
than max-goto-duplication-insns are unfactored. The default value is 8.
max-delay-slot-insn-search
The maximum number of instructions to consider when looking for an
instruction to fill a delay slot. If more than this arbitrary number of
instructions are searched, the time savings from filling the delay slot are
minimal, so stop searching. Increasing values mean more aggressive
optimization, making the compilation time increase with probably small
improvement in execution time.
max-delay-slot-live-search
When trying to fill delay slots, the maximum number of instructions to
consider when searching for a block with valid live register information.
Increasing this arbitrarily chosen value means more aggressive
optimization, increasing the compilation time. This parameter should be
removed when the delay slot code is rewritten to maintain the control-flow
graph.
max-gcse-memory
The approximate maximum amount of memory that can be allocated in order to
perform the global common subexpression elimination optimization. If more
memory than specified is required, the optimization is not done.
max-gcse-insertion-ratio
If the ratio of expression insertions to deletions is larger than this
value for any expression, then RTL PRE inserts or removes the expression
and thus leaves partially redundant computations in the instruction stream.
The default value is 20.
max-pending-list-length
The maximum number of pending dependencies scheduling allows before
flushing the current state and starting over. Large functions with few
branches or calls can create excessively large lists which needlessly
consume memory and resources.
max-modulo-backtrack-attempts
The maximum number of backtrack attempts the scheduler should make when
modulo scheduling a loop. Larger values can exponentially increase
compilation time.
max-inline-insns-single
Several parameters control the tree inliner used in GCC. This number sets
the maximum number of instructions (counted in GCC's internal
representation) in a single function that the tree inliner considers for
inlining. This only affects functions declared inline and methods
implemented in a class declaration (C++). The default value is 400.
max-inline-insns-auto
When you use -finline-functions (included in -O3), a lot of functions that
would otherwise not be considered for inlining by the compiler are
investigated. To those functions, a different (more restrictive) limit
compared to functions declared inline can be applied. The default value is
30.
inline-min-speedup
When estimated performance improvement of caller + callee runtime exceeds
this threshold (in percent), the function can be inlined regardless of the
limit on --param max-inline-insns-single and --param max-inline-insns-auto.
The default value is 15.
large-function-insns
The limit specifying really large functions. For functions larger than
this limit after inlining, inlining is constrained by --param large-
function-growth. This parameter is useful primarily to avoid extreme
compilation time caused by non-linear algorithms used by the back end. The
default value is 2700.
large-function-growth
Specifies maximal growth of large function caused by inlining in percents.
The default value is 100 which limits large function growth to 2.0 times
the original size.
large-unit-insns
The limit specifying large translation unit. Growth caused by inlining of
units larger than this limit is limited by --param inline-unit-growth. For
small units this might be too tight. For example, consider a unit
consisting of function A that is inline and B that just calls A three
times. If B is small relative to A, the growth of unit is 300\% and yet
such inlining is very sane. For very large units consisting of small
inlineable functions, however, the overall unit growth limit is needed to
avoid exponential explosion of code size. Thus for smaller units, the size
is increased to --param large-unit-insns before applying --param inline-
unit-growth. The default is 10000.
inline-unit-growth
Specifies maximal overall growth of the compilation unit caused by
inlining. The default value is 20 which limits unit growth to 1.2 times
the original size. Cold functions (either marked cold via an attribute or
by profile feedback) are not accounted into the unit size.
ipcp-unit-growth
Specifies maximal overall growth of the compilation unit caused by
interprocedural constant propagation. The default value is 10 which limits
unit growth to 1.1 times the original size.
large-stack-frame
The limit specifying large stack frames. While inlining the algorithm is
trying to not grow past this limit too much. The default value is 256
bytes.
large-stack-frame-growth
Specifies maximal growth of large stack frames caused by inlining in
percents. The default value is 1000 which limits large stack frame growth
to 11 times the original size.
max-inline-insns-recursive
max-inline-insns-recursive-auto
Specifies the maximum number of instructions an out-of-line copy of a self-
recursive inline function can grow into by performing recursive inlining.
--param max-inline-insns-recursive applies to functions declared inline.
For functions not declared inline, recursive inlining happens only when
-finline-functions (included in -O3) is enabled; --param max-inline-insns-
recursive-auto applies instead. The default value is 450.
max-inline-recursive-depth
max-inline-recursive-depth-auto
Specifies the maximum recursion depth used for recursive inlining.
--param max-inline-recursive-depth applies to functions declared inline.
For functions not declared inline, recursive inlining happens only when
-finline-functions (included in -O3) is enabled; --param max-inline-
recursive-depth-auto applies instead. The default value is 8.
min-inline-recursive-probability
Recursive inlining is profitable only for function having deep recursion in
average and can hurt for function having little recursion depth by
increasing the prologue size or complexity of function body to other
optimizers.
When profile feedback is available (see -fprofile-generate) the actual
recursion depth can be guessed from the probability that function recurses
via a given call expression. This parameter limits inlining only to call
expressions whose probability exceeds the given threshold (in percents).
The default value is 10.
early-inlining-insns
Specify growth that the early inliner can make. In effect it increases the
amount of inlining for code having a large abstraction penalty. The
default value is 14.
max-early-inliner-iterations
Limit of iterations of the early inliner. This basically bounds the number
of nested indirect calls the early inliner can resolve. Deeper chains are
still handled by late inlining.
comdat-sharing-probability
Probability (in percent) that C++ inline function with comdat visibility
are shared across multiple compilation units. The default value is 20.
profile-func-internal-id
A parameter to control whether to use function internal id in profile
database lookup. If the value is 0, the compiler uses an id that is based
on function assembler name and filename, which makes old profile data more
tolerant to source changes such as function reordering etc. The default
value is 0.
min-vect-loop-bound
The minimum number of iterations under which loops are not vectorized when
-ftree-vectorize is used. The number of iterations after vectorization
needs to be greater than the value specified by this option to allow
vectorization. The default value is 0.
gcse-cost-distance-ratio
Scaling factor in calculation of maximum distance an expression can be
moved by GCSE optimizations. This is currently supported only in the code
hoisting pass. The bigger the ratio, the more aggressive code hoisting is
with simple expressions, i.e., the expressions that have cost less than
gcse-unrestricted-cost. Specifying 0 disables hoisting of simple
expressions. The default value is 10.
gcse-unrestricted-cost
Cost, roughly measured as the cost of a single typical machine instruction,
at which GCSE optimizations do not constrain the distance an expression can
travel. This is currently supported only in the code hoisting pass. The
lesser the cost, the more aggressive code hoisting is. Specifying 0 allows
all expressions to travel unrestricted distances. The default value is 3.
max-hoist-depth
The depth of search in the dominator tree for expressions to hoist. This
is used to avoid quadratic behavior in hoisting algorithm. The value of 0
does not limit on the search, but may slow down compilation of huge
functions. The default value is 30.
max-tail-merge-comparisons
The maximum amount of similar bbs to compare a bb with. This is used to
avoid quadratic behavior in tree tail merging. The default value is 10.
max-tail-merge-iterations
The maximum amount of iterations of the pass over the function. This is
used to limit compilation time in tree tail merging. The default value is
2.
store-merging-allow-unaligned
Allow the store merging pass to introduce unaligned stores if it is legal
to do so. The default value is 1.
max-stores-to-merge
The maximum number of stores to attempt to merge into wider stores in the
store merging pass. The minimum value is 2 and the default is 64.
max-unrolled-insns
The maximum number of instructions that a loop may have to be unrolled. If
a loop is unrolled, this parameter also determines how many times the loop
code is unrolled.
max-average-unrolled-insns
The maximum number of instructions biased by probabilities of their
execution that a loop may have to be unrolled. If a loop is unrolled, this
parameter also determines how many times the loop code is unrolled.
max-unroll-times
The maximum number of unrollings of a single loop.
max-peeled-insns
The maximum number of instructions that a loop may have to be peeled. If a
loop is peeled, this parameter also determines how many times the loop code
is peeled.
max-peel-times
The maximum number of peelings of a single loop.
max-peel-branches
The maximum number of branches on the hot path through the peeled sequence.
max-completely-peeled-insns
The maximum number of insns of a completely peeled loop.
max-completely-peel-times
The maximum number of iterations of a loop to be suitable for complete
peeling.
max-completely-peel-loop-nest-depth
The maximum depth of a loop nest suitable for complete peeling.
max-unswitch-insns
The maximum number of insns of an unswitched loop.
max-unswitch-level
The maximum number of branches unswitched in a single loop.
max-loop-headers-insns
The maximum number of insns in loop header duplicated by the copy loop
headers pass.
lim-expensive
The minimum cost of an expensive expression in the loop invariant motion.
iv-consider-all-candidates-bound
Bound on number of candidates for induction variables, below which all
candidates are considered for each use in induction variable optimizations.
If there are more candidates than this, only the most relevant ones are
considered to avoid quadratic time complexity.
iv-max-considered-uses
The induction variable optimizations give up on loops that contain more
induction variable uses.
iv-always-prune-cand-set-bound
If the number of candidates in the set is smaller than this value, always
try to remove unnecessary ivs from the set when adding a new one.
avg-loop-niter
Average number of iterations of a loop.
dse-max-object-size
Maximum size (in bytes) of objects tracked bytewise by dead store
elimination. Larger values may result in larger compilation times.
scev-max-expr-size
Bound on size of expressions used in the scalar evolutions analyzer. Large
expressions slow the analyzer.
scev-max-expr-complexity
Bound on the complexity of the expressions in the scalar evolutions
analyzer. Complex expressions slow the analyzer.
max-tree-if-conversion-phi-args
Maximum number of arguments in a PHI supported by TREE if conversion unless
the loop is marked with simd pragma.
vect-max-version-for-alignment-checks
The maximum number of run-time checks that can be performed when doing loop
versioning for alignment in the vectorizer.
vect-max-version-for-alias-checks
The maximum number of run-time checks that can be performed when doing loop
versioning for alias in the vectorizer.
vect-max-peeling-for-alignment
The maximum number of loop peels to enhance access alignment for
vectorizer. Value -1 means no limit.
max-iterations-to-track
The maximum number of iterations of a loop the brute-force algorithm for
analysis of the number of iterations of the loop tries to evaluate.
hot-bb-count-ws-permille
A basic block profile count is considered hot if it contributes to the
given permillage (i.e. 0...1000) of the entire profiled execution.
hot-bb-frequency-fraction
Select fraction of the entry block frequency of executions of basic block
in function given basic block needs to have to be considered hot.
max-predicted-iterations
The maximum number of loop iterations we predict statically. This is
useful in cases where a function contains a single loop with known bound
and another loop with unknown bound. The known number of iterations is
predicted correctly, while the unknown number of iterations average to
roughly 10. This means that the loop without bounds appears artificially
cold relative to the other one.
builtin-expect-probability
Control the probability of the expression having the specified value. This
parameter takes a percentage (i.e. 0 ... 100) as input. The default
probability of 90 is obtained empirically.
align-threshold
Select fraction of the maximal frequency of executions of a basic block in
a function to align the basic block.
align-loop-iterations
A loop expected to iterate at least the selected number of iterations is
aligned.
tracer-dynamic-coverage
tracer-dynamic-coverage-feedback
This value is used to limit superblock formation once the given percentage
of executed instructions is covered. This limits unnecessary code size
expansion.
The tracer-dynamic-coverage-feedback parameter is used only when profile
feedback is available. The real profiles (as opposed to statically
estimated ones) are much less balanced allowing the threshold to be larger
value.
tracer-max-code-growth
Stop tail duplication once code growth has reached given percentage. This
is a rather artificial limit, as most of the duplicates are eliminated
later in cross jumping, so it may be set to much higher values than is the
desired code growth.
tracer-min-branch-ratio
Stop reverse growth when the reverse probability of best edge is less than
this threshold (in percent).
tracer-min-branch-probability
tracer-min-branch-probability-feedback
Stop forward growth if the best edge has probability lower than this
threshold.
Similarly to tracer-dynamic-coverage two parameters are provided. tracer-
min-branch-probability-feedback is used for compilation with profile
feedback and tracer-min-branch-probability compilation without. The value
for compilation with profile feedback needs to be more conservative
(higher) in order to make tracer effective.
stack-clash-protection-guard-size
Specify the size of the operating system provided stack guard as 2 raised
to num bytes. The default value is 12 (4096 bytes). Acceptable values are
between 12 and 30. Higher values may reduce the number of explicit probes,
but a value larger than the operating system provided guard will leave code
vulnerable to stack clash style attacks.
stack-clash-protection-probe-interval
Stack clash protection involves probing stack space as it is allocated.
This param controls the maximum distance between probes into the stack as 2
raised to num bytes. Acceptable values are between 10 and 16 and defaults
to 12. Higher values may reduce the number of explicit probes, but a value
larger than the operating system provided guard will leave code vulnerable
to stack clash style attacks.
max-cse-path-length
The maximum number of basic blocks on path that CSE considers. The default
is 10.
max-cse-insns
The maximum number of instructions CSE processes before flushing. The
default is 1000.
ggc-min-expand
GCC uses a garbage collector to manage its own memory allocation. This
parameter specifies the minimum percentage by which the garbage collector's
heap should be allowed to expand between collections. Tuning this may
improve compilation speed; it has no effect on code generation.
The default is 30% + 70% * (RAM/1GB) with an upper bound of 100% when RAM
>= 1GB. If "getrlimit" is available, the notion of "RAM" is the smallest
of actual RAM and "RLIMIT_DATA" or "RLIMIT_AS". If GCC is not able to
calculate RAM on a particular platform, the lower bound of 30% is used.
Setting this parameter and ggc-min-heapsize to zero causes a full
collection to occur at every opportunity. This is extremely slow, but can
be useful for debugging.
ggc-min-heapsize
Minimum size of the garbage collector's heap before it begins bothering to
collect garbage. The first collection occurs after the heap expands by
ggc-min-expand% beyond ggc-min-heapsize. Again, tuning this may improve
compilation speed, and has no effect on code generation.
The default is the smaller of RAM/8, RLIMIT_RSS, or a limit that tries to
ensure that RLIMIT_DATA or RLIMIT_AS are not exceeded, but with a lower
bound of 4096 (four megabytes) and an upper bound of 131072 (128
megabytes). If GCC is not able to calculate RAM on a particular platform,
the lower bound is used. Setting this parameter very large effectively
disables garbage collection. Setting this parameter and ggc-min-expand to
zero causes a full collection to occur at every opportunity.
max-reload-search-insns
The maximum number of instruction reload should look backward for
equivalent register. Increasing values mean more aggressive optimization,
making the compilation time increase with probably slightly better
performance. The default value is 100.
max-cselib-memory-locations
The maximum number of memory locations cselib should take into account.
Increasing values mean more aggressive optimization, making the compilation
time increase with probably slightly better performance. The default value
is 500.
max-sched-ready-insns
The maximum number of instructions ready to be issued the scheduler should
consider at any given time during the first scheduling pass. Increasing
values mean more thorough searches, making the compilation time increase
with probably little benefit. The default value is 100.
max-sched-region-blocks
The maximum number of blocks in a region to be considered for interblock
scheduling. The default value is 10.
max-pipeline-region-blocks
The maximum number of blocks in a region to be considered for pipelining in
the selective scheduler. The default value is 15.
max-sched-region-insns
The maximum number of insns in a region to be considered for interblock
scheduling. The default value is 100.
max-pipeline-region-insns
The maximum number of insns in a region to be considered for pipelining in
the selective scheduler. The default value is 200.
min-spec-prob
The minimum probability (in percents) of reaching a source block for
interblock speculative scheduling. The default value is 40.
max-sched-extend-regions-iters
The maximum number of iterations through CFG to extend regions. A value of
0 (the default) disables region extensions.
max-sched-insn-conflict-delay
The maximum conflict delay for an insn to be considered for speculative
motion. The default value is 3.
sched-spec-prob-cutoff
The minimal probability of speculation success (in percents), so that
speculative insns are scheduled. The default value is 40.
sched-state-edge-prob-cutoff
The minimum probability an edge must have for the scheduler to save its
state across it. The default value is 10.
sched-mem-true-dep-cost
Minimal distance (in CPU cycles) between store and load targeting same
memory locations. The default value is 1.
selsched-max-lookahead
The maximum size of the lookahead window of selective scheduling. It is a
depth of search for available instructions. The default value is 50.
selsched-max-sched-times
The maximum number of times that an instruction is scheduled during
selective scheduling. This is the limit on the number of iterations
through which the instruction may be pipelined. The default value is 2.
selsched-insns-to-rename
The maximum number of best instructions in the ready list that are
considered for renaming in the selective scheduler. The default value is
2.
sms-min-sc
The minimum value of stage count that swing modulo scheduler generates.
The default value is 2.
max-last-value-rtl
The maximum size measured as number of RTLs that can be recorded in an
expression in combiner for a pseudo register as last known value of that
register. The default is 10000.
max-combine-insns
The maximum number of instructions the RTL combiner tries to combine. The
default value is 2 at -Og and 4 otherwise.
integer-share-limit
Small integer constants can use a shared data structure, reducing the
compiler's memory usage and increasing its speed. This sets the maximum
value of a shared integer constant. The default value is 256.
ssp-buffer-size
The minimum size of buffers (i.e. arrays) that receive stack smashing
protection when -fstack-protection is used.
min-size-for-stack-sharing
The minimum size of variables taking part in stack slot sharing when not
optimizing. The default value is 32.
max-jump-thread-duplication-stmts
Maximum number of statements allowed in a block that needs to be duplicated
when threading jumps.
max-fields-for-field-sensitive
Maximum number of fields in a structure treated in a field sensitive manner
during pointer analysis. The default is zero for -O0 and -O1, and 100 for
-Os, -O2, and -O3.
prefetch-latency
Estimate on average number of instructions that are executed before
prefetch finishes. The distance prefetched ahead is proportional to this
constant. Increasing this number may also lead to less streams being
prefetched (see simultaneous-prefetches).
simultaneous-prefetches
Maximum number of prefetches that can run at the same time.
l1-cache-line-size
The size of cache line in L1 cache, in bytes.
l1-cache-size
The size of L1 cache, in kilobytes.
l2-cache-size
The size of L2 cache, in kilobytes.
loop-interchange-max-num-stmts
The maximum number of stmts in a loop to be interchanged.
loop-interchange-stride-ratio
The minimum ratio between stride of two loops for interchange to be
profitable.
min-insn-to-prefetch-ratio
The minimum ratio between the number of instructions and the number of
prefetches to enable prefetching in a loop.
prefetch-min-insn-to-mem-ratio
The minimum ratio between the number of instructions and the number of
memory references to enable prefetching in a loop.
use-canonical-types
Whether the compiler should use the "canonical" type system. By default,
this should always be 1, which uses a more efficient internal mechanism for
comparing types in C++ and Objective-C++. However, if bugs in the
canonical type system are causing compilation failures, set this value to 0
to disable canonical types.
switch-conversion-max-branch-ratio
Switch initialization conversion refuses to create arrays that are bigger
than switch-conversion-max-branch-ratio times the number of branches in the
switch.
max-partial-antic-length
Maximum length of the partial antic set computed during the tree partial
redundancy elimination optimization (-ftree-pre) when optimizing at -O3 and
above. For some sorts of source code the enhanced partial redundancy
elimination optimization can run away, consuming all of the memory
available on the host machine. This parameter sets a limit on the length
of the sets that are computed, which prevents the runaway behavior.
Setting a value of 0 for this parameter allows an unlimited set length.
sccvn-max-scc-size
Maximum size of a strongly connected component (SCC) during SCCVN
processing. If this limit is hit, SCCVN processing for the whole function
is not done and optimizations depending on it are disabled. The default
maximum SCC size is 10000.
sccvn-max-alias-queries-per-access
Maximum number of alias-oracle queries we perform when looking for
redundancies for loads and stores. If this limit is hit the search is
aborted and the load or store is not considered redundant. The number of
queries is algorithmically limited to the number of stores on all paths
from the load to the function entry. The default maximum number of queries
is 1000.
ira-max-loops-num
IRA uses regional register allocation by default. If a function contains
more loops than the number given by this parameter, only at most the given
number of the most frequently-executed loops form regions for regional
register allocation. The default value of the parameter is 100.
ira-max-conflict-table-size
Although IRA uses a sophisticated algorithm to compress the conflict table,
the table can still require excessive amounts of memory for huge functions.
If the conflict table for a function could be more than the size in MB
given by this parameter, the register allocator instead uses a faster,
simpler, and lower-quality algorithm that does not require building a
pseudo-register conflict table. The default value of the parameter is
2000.
ira-loop-reserved-regs
IRA can be used to evaluate more accurate register pressure in loops for
decisions to move loop invariants (see -O3). The number of available
registers reserved for some other purposes is given by this parameter. The
default value of the parameter is 2, which is the minimal number of
registers needed by typical instructions. This value is the best found
from numerous experiments.
lra-inheritance-ebb-probability-cutoff
LRA tries to reuse values reloaded in registers in subsequent insns. This
optimization is called inheritance. EBB is used as a region to do this
optimization. The parameter defines a minimal fall-through edge
probability in percentage used to add BB to inheritance EBB in LRA. The
default value of the parameter is 40. The value was chosen from numerous
runs of SPEC2000 on x86-64.
loop-invariant-max-bbs-in-loop
Loop invariant motion can be very expensive, both in compilation time and
in amount of needed compile-time memory, with very large loops. Loops with
more basic blocks than this parameter won't have loop invariant motion
optimization performed on them. The default value of the parameter is 1000
for -O1 and 10000 for -O2 and above.
loop-max-datarefs-for-datadeps
Building data dependencies is expensive for very large loops. This
parameter limits the number of data references in loops that are considered
for data dependence analysis. These large loops are no handled by the
optimizations using loop data dependencies. The default value is 1000.
max-vartrack-size
Sets a maximum number of hash table slots to use during variable tracking
dataflow analysis of any function. If this limit is exceeded with variable
tracking at assignments enabled, analysis for that function is retried
without it, after removing all debug insns from the function. If the limit
is exceeded even without debug insns, var tracking analysis is completely
disabled for the function. Setting the parameter to zero makes it
unlimited.
max-vartrack-expr-depth
Sets a maximum number of recursion levels when attempting to map variable
names or debug temporaries to value expressions. This trades compilation
time for more complete debug information. If this is set too low, value
expressions that are available and could be represented in debug
information may end up not being used; setting this higher may enable the
compiler to find more complex debug expressions, but compile time and
memory use may grow. The default is 12.
max-debug-marker-count
Sets a threshold on the number of debug markers (e.g. begin stmt markers)
to avoid complexity explosion at inlining or expanding to RTL. If a
function has more such gimple stmts than the set limit, such stmts will be
dropped from the inlined copy of a function, and from its RTL expansion.
The default is 100000.
min-nondebug-insn-uid
Use uids starting at this parameter for nondebug insns. The range below
the parameter is reserved exclusively for debug insns created by
-fvar-tracking-assignments, but debug insns may get (non-overlapping) uids
above it if the reserved range is exhausted.
ipa-sra-ptr-growth-factor
IPA-SRA replaces a pointer to an aggregate with one or more new parameters
only when their cumulative size is less or equal to ipa-sra-ptr-growth-
factor times the size of the original pointer parameter.
sra-max-scalarization-size-Ospeed
sra-max-scalarization-size-Osize
The two Scalar Reduction of Aggregates passes (SRA and IPA-SRA) aim to
replace scalar parts of aggregates with uses of independent scalar
variables. These parameters control the maximum size, in storage units, of
aggregate which is considered for replacement when compiling for speed
(sra-max-scalarization-size-Ospeed) or size (sra-max-scalarization-size-
Osize) respectively.
sra-max-propagations
The maximum number of artificial accesses that Scalar Replacement of
Aggregates (SRA) will track, per one local variable, in order to facilitate
copy propagation.
tm-max-aggregate-size
When making copies of thread-local variables in a transaction, this
parameter specifies the size in bytes after which variables are saved with
the logging functions as opposed to save/restore code sequence pairs. This
option only applies when using -fgnu-tm.
graphite-max-nb-scop-params
To avoid exponential effects in the Graphite loop transforms, the number of
parameters in a Static Control Part (SCoP) is bounded. The default value
is 10 parameters, a value of zero can be used to lift the bound. A
variable whose value is unknown at compilation time and defined outside a
SCoP is a parameter of the SCoP.
loop-block-tile-size
Loop blocking or strip mining transforms, enabled with -floop-block or
-floop-strip-mine, strip mine each loop in the loop nest by a given number
of iterations. The strip length can be changed using the loop-block-tile-
size parameter. The default value is 51 iterations.
loop-unroll-jam-size
Specify the unroll factor for the -floop-unroll-and-jam option. The
default value is 4.
loop-unroll-jam-depth
Specify the dimension to be unrolled (counting from the most inner loop)
for the -floop-unroll-and-jam. The default value is 2.
ipa-cp-value-list-size
IPA-CP attempts to track all possible values and types passed to a
function's parameter in order to propagate them and perform
devirtualization. ipa-cp-value-list-size is the maximum number of values
and types it stores per one formal parameter of a function.
ipa-cp-eval-threshold
IPA-CP calculates its own score of cloning profitability heuristics and
performs those cloning opportunities with scores that exceed ipa-cp-eval-
threshold.
ipa-cp-recursion-penalty
Percentage penalty the recursive functions will receive when they are
evaluated for cloning.
ipa-cp-single-call-penalty
Percentage penalty functions containing a single call to another function
will receive when they are evaluated for cloning.
ipa-max-agg-items
IPA-CP is also capable to propagate a number of scalar values passed in an
aggregate. ipa-max-agg-items controls the maximum number of such values per
one parameter.
ipa-cp-loop-hint-bonus
When IPA-CP determines that a cloning candidate would make the number of
iterations of a loop known, it adds a bonus of ipa-cp-loop-hint-bonus to
the profitability score of the candidate.
ipa-cp-array-index-hint-bonus
When IPA-CP determines that a cloning candidate would make the index of an
array access known, it adds a bonus of ipa-cp-array-index-hint-bonus to the
profitability score of the candidate.
ipa-max-aa-steps
During its analysis of function bodies, IPA-CP employs alias analysis in
order to track values pointed to by function parameters. In order not
spend too much time analyzing huge functions, it gives up and consider all
memory clobbered after examining ipa-max-aa-steps statements modifying
memory.
lto-partitions
Specify desired number of partitions produced during WHOPR compilation.
The number of partitions should exceed the number of CPUs used for
compilation. The default value is 32.
lto-min-partition
Size of minimal partition for WHOPR (in estimated instructions). This
prevents expenses of splitting very small programs into too many
partitions.
lto-max-partition
Size of max partition for WHOPR (in estimated instructions). to provide an
upper bound for individual size of partition. Meant to be used only with
balanced partitioning.
cxx-max-namespaces-for-diagnostic-help
The maximum number of namespaces to consult for suggestions when C++ name
lookup fails for an identifier. The default is 1000.
sink-frequency-threshold
The maximum relative execution frequency (in percents) of the target block
relative to a statement's original block to allow statement sinking of a
statement. Larger numbers result in more aggressive statement sinking.
The default value is 75. A small positive adjustment is applied for
statements with memory operands as those are even more profitable so sink.
max-stores-to-sink
The maximum number of conditional store pairs that can be sunk. Set to 0
if either vectorization (-ftree-vectorize) or if-conversion
(-ftree-loop-if-convert) is disabled. The default is 2.
allow-store-data-races
Allow optimizers to introduce new data races on stores. Set to 1 to allow,
otherwise to 0. This option is enabled by default at optimization level
-Ofast.
case-values-threshold
The smallest number of different values for which it is best to use a jump-
table instead of a tree of conditional branches. If the value is 0, use
the default for the machine. The default is 0.
tree-reassoc-width
Set the maximum number of instructions executed in parallel in reassociated
tree. This parameter overrides target dependent heuristics used by default
if has non zero value.
sched-pressure-algorithm
Choose between the two available implementations of -fsched-pressure.
Algorithm 1 is the original implementation and is the more likely to
prevent instructions from being reordered. Algorithm 2 was designed to be
a compromise between the relatively conservative approach taken by
algorithm 1 and the rather aggressive approach taken by the default
scheduler. It relies more heavily on having a regular register file and
accurate register pressure classes. See haifa-sched.c in the GCC sources
for more details.
The default choice depends on the target.
max-slsr-cand-scan
Set the maximum number of existing candidates that are considered when
seeking a basis for a new straight-line strength reduction candidate.
asan-globals
Enable buffer overflow detection for global objects. This kind of
protection is enabled by default if you are using -fsanitize=address
option. To disable global objects protection use --param asan-globals=0.
asan-stack
Enable buffer overflow detection for stack objects. This kind of
protection is enabled by default when using -fsanitize=address. To disable
stack protection use --param asan-stack=0 option.
asan-instrument-reads
Enable buffer overflow detection for memory reads. This kind of protection
is enabled by default when using -fsanitize=address. To disable memory
reads protection use --param asan-instrument-reads=0.
asan-instrument-writes
Enable buffer overflow detection for memory writes. This kind of
protection is enabled by default when using -fsanitize=address. To disable
memory writes protection use --param asan-instrument-writes=0 option.
asan-memintrin
Enable detection for built-in functions. This kind of protection is
enabled by default when using -fsanitize=address. To disable built-in
functions protection use --param asan-memintrin=0.
asan-use-after-return
Enable detection of use-after-return. This kind of protection is enabled
by default when using the -fsanitize=address option. To disable it use
--param asan-use-after-return=0.
Note: By default the check is disabled at run time. To enable it, add
"detect_stack_use_after_return=1" to the environment variable ASAN_OPTIONS.
asan-instrumentation-with-call-threshold
If number of memory accesses in function being instrumented is greater or
equal to this number, use callbacks instead of inline checks. E.g. to
disable inline code use --param asan-instrumentation-with-call-threshold=0.
use-after-scope-direct-emission-threshold
If the size of a local variable in bytes is smaller or equal to this
number, directly poison (or unpoison) shadow memory instead of using run-
time callbacks. The default value is 256.
chkp-max-ctor-size
Static constructors generated by Pointer Bounds Checker may become very
large and significantly increase compile time at optimization level -O1 and
higher. This parameter is a maximum number of statements in a single
generated constructor. Default value is 5000.
max-fsm-thread-path-insns
Maximum number of instructions to copy when duplicating blocks on a finite
state automaton jump thread path. The default is 100.
max-fsm-thread-length
Maximum number of basic blocks on a finite state automaton jump thread
path. The default is 10.
max-fsm-thread-paths
Maximum number of new jump thread paths to create for a finite state
automaton. The default is 50.
parloops-chunk-size
Chunk size of omp schedule for loops parallelized by parloops. The default
is 0.
parloops-schedule
Schedule type of omp schedule for loops parallelized by parloops (static,
dynamic, guided, auto, runtime). The default is static.
parloops-min-per-thread
The minimum number of iterations per thread of an innermost parallelized
loop for which the parallelized variant is prefered over the single
threaded one. The default is 100. Note that for a parallelized loop nest
the minimum number of iterations of the outermost loop per thread is two.
max-ssa-name-query-depth
Maximum depth of recursion when querying properties of SSA names in things
like fold routines. One level of recursion corresponds to following a use-
def chain.
hsa-gen-debug-stores
Enable emission of special debug stores within HSA kernels which are then
read and reported by libgomp plugin. Generation of these stores is
disabled by default, use --param hsa-gen-debug-stores=1 to enable it.
max-speculative-devirt-maydefs
The maximum number of may-defs we analyze when looking for a must-def
specifying the dynamic type of an object that invokes a virtual call we may
be able to devirtualize speculatively.
max-vrp-switch-assertions
The maximum number of assertions to add along the default edge of a switch
statement during VRP. The default is 10.
unroll-jam-min-percent
The minimum percentage of memory references that must be optimized away for
the unroll-and-jam transformation to be considered profitable.
unroll-jam-max-unroll
The maximum number of times the outer loop should be unrolled by the
unroll-and-jam transformation.
Program Instrumentation Options
GCC supports a number of command-line options that control adding run-time
instrumentation to the code it normally generates. For example, one purpose of
instrumentation is collect profiling statistics for use in finding program hot
spots, code coverage analysis, or profile-guided optimizations. Another class of
program instrumentation is adding run-time checking to detect programming errors
like invalid pointer dereferences or out-of-bounds array accesses, as well as
deliberately hostile attacks such as stack smashing or C++ vtable hijacking. There
is also a general hook which can be used to implement other forms of tracing or
function-level instrumentation for debug or program analysis purposes.
-p Generate extra code to write profile information suitable for the analysis
program prof. You must use this option when compiling the source files you
want data about, and you must also use it when linking.
-pg Generate extra code to write profile information suitable for the analysis
program gprof. You must use this option when compiling the source files you
want data about, and you must also use it when linking.
-fprofile-arcs
Add code so that program flow arcs are instrumented. During execution the
program records how many times each branch and call is executed and how many
times it is taken or returns. On targets that support constructors with
priority support, profiling properly handles constructors, destructors and C++
constructors (and destructors) of classes which are used as a type of a global
variable.
When the compiled program exits it saves this data to a file called
auxname.gcda for each source file. The data may be used for profile-directed
optimizations (-fbranch-probabilities), or for test coverage analysis
(-ftest-coverage). Each object file's auxname is generated from the name of
the output file, if explicitly specified and it is not the final executable,
otherwise it is the basename of the source file. In both cases any suffix is
removed (e.g. foo.gcda for input file dir/foo.c, or dir/foo.gcda for output
file specified as -o dir/foo.o).
--coverage
This option is used to compile and link code instrumented for coverage
analysis. The option is a synonym for -fprofile-arcs -ftest-coverage (when
compiling) and -lgcov (when linking). See the documentation for those options
for more details.
* Compile the source files with -fprofile-arcs plus optimization and code
generation options. For test coverage analysis, use the additional
-ftest-coverage option. You do not need to profile every source file in a
program.
* Compile the source files additionally with -fprofile-abs-path to create
absolute path names in the .gcno files. This allows gcov to find the
correct sources in projects where compilations occur with different working
directories.
* Link your object files with -lgcov or -fprofile-arcs (the latter implies
the former).
* Run the program on a representative workload to generate the arc profile
information. This may be repeated any number of times. You can run
concurrent instances of your program, and provided that the file system
supports locking, the data files will be correctly updated. Unless a
strict ISO C dialect option is in effect, "fork" calls are detected and
correctly handled without double counting.
* For profile-directed optimizations, compile the source files again with the
same optimization and code generation options plus -fbranch-probabilities.
* For test coverage analysis, use gcov to produce human readable information
from the .gcno and .gcda files. Refer to the gcov documentation for
further information.
With -fprofile-arcs, for each function of your program GCC creates a program
flow graph, then finds a spanning tree for the graph. Only arcs that are not
on the spanning tree have to be instrumented: the compiler adds code to count
the number of times that these arcs are executed. When an arc is the only exit
or only entrance to a block, the instrumentation code can be added to the
block; otherwise, a new basic block must be created to hold the instrumentation
code.
-ftest-coverage
Produce a notes file that the gcov code-coverage utility can use to show
program coverage. Each source file's note file is called auxname.gcno. Refer
to the -fprofile-arcs option above for a description of auxname and
instructions on how to generate test coverage data. Coverage data matches the
source files more closely if you do not optimize.
-fprofile-abs-path
Automatically convert relative source file names to absolute path names in the
.gcno files. This allows gcov to find the correct sources in projects where
compilations occur with different working directories.
-fprofile-dir=path
Set the directory to search for the profile data files in to path. This option
affects only the profile data generated by -fprofile-generate, -ftest-coverage,
-fprofile-arcs and used by -fprofile-use and -fbranch-probabilities and its
related options. Both absolute and relative paths can be used. By default,
GCC uses the current directory as path, thus the profile data file appears in
the same directory as the object file.
-fprofile-generate
-fprofile-generate=path
Enable options usually used for instrumenting application to produce profile
useful for later recompilation with profile feedback based optimization. You
must use -fprofile-generate both when compiling and when linking your program.
The following options are enabled: -fprofile-arcs, -fprofile-values, -fvpt.
If path is specified, GCC looks at the path to find the profile feedback data
files. See -fprofile-dir.
To optimize the program based on the collected profile information, use
-fprofile-use.
-fprofile-update=method
Alter the update method for an application instrumented for profile feedback
based optimization. The method argument should be one of single, atomic or
prefer-atomic. The first one is useful for single-threaded applications, while
the second one prevents profile corruption by emitting thread-safe code.
Warning: When an application does not properly join all threads (or creates an
detached thread), a profile file can be still corrupted.
Using prefer-atomic would be transformed either to atomic, when supported by a
target, or to single otherwise. The GCC driver automatically selects prefer-
atomic when -pthread is present in the command line.
-fsanitize=address
Enable AddressSanitizer, a fast memory error detector. Memory access
instructions are instrumented to detect out-of-bounds and use-after-free bugs.
The option enables -fsanitize-address-use-after-scope. See
<https://github.com/google/sanitizers/wiki/AddressSanitizer> for more details.
The run-time behavior can be influenced using the ASAN_OPTIONS environment
variable. When set to "help=1", the available options are shown at startup of
the instrumented program. See
<https://github.com/google/sanitizers/wiki/AddressSanitizerFlags#run-time-flags>
for a list of supported options. The option cannot be combined with
-fsanitize=thread and/or -fcheck-pointer-bounds.
-fsanitize=kernel-address
Enable AddressSanitizer for Linux kernel. See
<https://github.com/google/kasan/wiki> for more details. The option cannot be
combined with -fcheck-pointer-bounds.
-fsanitize=pointer-compare
Instrument comparison operation (<, <=, >, >=) with pointer operands. The
option must be combined with either -fsanitize=kernel-address or
-fsanitize=address The option cannot be combined with -fsanitize=thread and/or
-fcheck-pointer-bounds. Note: By default the check is disabled at run time.
To enable it, add "detect_invalid_pointer_pairs=2" to the environment variable
ASAN_OPTIONS. Using "detect_invalid_pointer_pairs=1" detects invalid operation
only when both pointers are non-null.
-fsanitize=pointer-subtract
Instrument subtraction with pointer operands. The option must be combined with
either -fsanitize=kernel-address or -fsanitize=address The option cannot be
combined with -fsanitize=thread and/or -fcheck-pointer-bounds. Note: By
default the check is disabled at run time. To enable it, add
"detect_invalid_pointer_pairs=2" to the environment variable ASAN_OPTIONS.
Using "detect_invalid_pointer_pairs=1" detects invalid operation only when both
pointers are non-null.
-fsanitize=thread
Enable ThreadSanitizer, a fast data race detector. Memory access instructions
are instrumented to detect data race bugs. See
<https://github.com/google/sanitizers/wiki#threadsanitizer> for more details.
The run-time behavior can be influenced using the TSAN_OPTIONS environment
variable; see <https://github.com/google/sanitizers/wiki/ThreadSanitizerFlags>
for a list of supported options. The option cannot be combined with
-fsanitize=address, -fsanitize=leak and/or -fcheck-pointer-bounds.
Note that sanitized atomic builtins cannot throw exceptions when operating on
invalid memory addresses with non-call exceptions (-fnon-call-exceptions).
-fsanitize=leak
Enable LeakSanitizer, a memory leak detector. This option only matters for
linking of executables and the executable is linked against a library that
overrides "malloc" and other allocator functions. See
<https://github.com/google/sanitizers/wiki/AddressSanitizerLeakSanitizer> for
more details. The run-time behavior can be influenced using the LSAN_OPTIONS
environment variable. The option cannot be combined with -fsanitize=thread.
-fsanitize=undefined
Enable UndefinedBehaviorSanitizer, a fast undefined behavior detector. Various
computations are instrumented to detect undefined behavior at runtime. Current
suboptions are:
-fsanitize=shift
This option enables checking that the result of a shift operation is not
undefined. Note that what exactly is considered undefined differs slightly
between C and C++, as well as between ISO C90 and C99, etc. This option
has two suboptions, -fsanitize=shift-base and -fsanitize=shift-exponent.
-fsanitize=shift-exponent
This option enables checking that the second argument of a shift operation
is not negative and is smaller than the precision of the promoted first
argument.
-fsanitize=shift-base
If the second argument of a shift operation is within range, check that the
result of a shift operation is not undefined. Note that what exactly is
considered undefined differs slightly between C and C++, as well as between
ISO C90 and C99, etc.
-fsanitize=integer-divide-by-zero
Detect integer division by zero as well as "INT_MIN / -1" division.
-fsanitize=unreachable
With this option, the compiler turns the "__builtin_unreachable" call into
a diagnostics message call instead. When reaching the
"__builtin_unreachable" call, the behavior is undefined.
-fsanitize=vla-bound
This option instructs the compiler to check that the size of a variable
length array is positive.
-fsanitize=null
This option enables pointer checking. Particularly, the application built
with this option turned on will issue an error message when it tries to
dereference a NULL pointer, or if a reference (possibly an rvalue
reference) is bound to a NULL pointer, or if a method is invoked on an
object pointed by a NULL pointer.
-fsanitize=return
This option enables return statement checking. Programs built with this
option turned on will issue an error message when the end of a non-void
function is reached without actually returning a value. This option works
in C++ only.
-fsanitize=signed-integer-overflow
This option enables signed integer overflow checking. We check that the
result of "+", "*", and both unary and binary "-" does not overflow in the
signed arithmetics. Note, integer promotion rules must be taken into
account. That is, the following is not an overflow:
signed char a = SCHAR_MAX;
a++;
-fsanitize=bounds
This option enables instrumentation of array bounds. Various out of bounds
accesses are detected. Flexible array members, flexible array member-like
arrays, and initializers of variables with static storage are not
instrumented. The option cannot be combined with -fcheck-pointer-bounds.
-fsanitize=bounds-strict
This option enables strict instrumentation of array bounds. Most out of
bounds accesses are detected, including flexible array members and flexible
array member-like arrays. Initializers of variables with static storage
are not instrumented. The option cannot be combined with
-fcheck-pointer-bounds.
-fsanitize=alignment
This option enables checking of alignment of pointers when they are
dereferenced, or when a reference is bound to insufficiently aligned
target, or when a method or constructor is invoked on insufficiently
aligned object.
-fsanitize=object-size
This option enables instrumentation of memory references using the
"__builtin_object_size" function. Various out of bounds pointer accesses
are detected.
-fsanitize=float-divide-by-zero
Detect floating-point division by zero. Unlike other similar options,
-fsanitize=float-divide-by-zero is not enabled by -fsanitize=undefined,
since floating-point division by zero can be a legitimate way of obtaining
infinities and NaNs.
-fsanitize=float-cast-overflow
This option enables floating-point type to integer conversion checking. We
check that the result of the conversion does not overflow. Unlike other
similar options, -fsanitize=float-cast-overflow is not enabled by
-fsanitize=undefined. This option does not work well with "FE_INVALID"
exceptions enabled.
-fsanitize=nonnull-attribute
This option enables instrumentation of calls, checking whether null values
are not passed to arguments marked as requiring a non-null value by the
"nonnull" function attribute.
-fsanitize=returns-nonnull-attribute
This option enables instrumentation of return statements in functions
marked with "returns_nonnull" function attribute, to detect returning of
null values from such functions.
-fsanitize=bool
This option enables instrumentation of loads from bool. If a value other
than 0/1 is loaded, a run-time error is issued.
-fsanitize=enum
This option enables instrumentation of loads from an enum type. If a value
outside the range of values for the enum type is loaded, a run-time error
is issued.
-fsanitize=vptr
This option enables instrumentation of C++ member function calls, member
accesses and some conversions between pointers to base and derived classes,
to verify the referenced object has the correct dynamic type.
-fsanitize=pointer-overflow
This option enables instrumentation of pointer arithmetics. If the pointer
arithmetics overflows, a run-time error is issued.
-fsanitize=builtin
This option enables instrumentation of arguments to selected builtin
functions. If an invalid value is passed to such arguments, a run-time
error is issued. E.g. passing 0 as the argument to "__builtin_ctz" or
"__builtin_clz" invokes undefined behavior and is diagnosed by this option.
While -ftrapv causes traps for signed overflows to be emitted,
-fsanitize=undefined gives a diagnostic message. This currently works only for
the C family of languages.
-fno-sanitize=all
This option disables all previously enabled sanitizers. -fsanitize=all is not
allowed, as some sanitizers cannot be used together.
-fasan-shadow-offset=number
This option forces GCC to use custom shadow offset in AddressSanitizer checks.
It is useful for experimenting with different shadow memory layouts in Kernel
AddressSanitizer.
-fsanitize-sections=s1,s2,...
Sanitize global variables in selected user-defined sections. si may contain
wildcards.
-fsanitize-recover[=opts]
-fsanitize-recover= controls error recovery mode for sanitizers mentioned in
comma-separated list of opts. Enabling this option for a sanitizer component
causes it to attempt to continue running the program as if no error happened.
This means multiple runtime errors can be reported in a single program run, and
the exit code of the program may indicate success even when errors have been
reported. The -fno-sanitize-recover= option can be used to alter this
behavior: only the first detected error is reported and program then exits with
a non-zero exit code.
Currently this feature only works for -fsanitize=undefined (and its suboptions
except for -fsanitize=unreachable and -fsanitize=return),
-fsanitize=float-cast-overflow, -fsanitize=float-divide-by-zero,
-fsanitize=bounds-strict, -fsanitize=kernel-address and -fsanitize=address.
For these sanitizers error recovery is turned on by default, except
-fsanitize=address, for which this feature is experimental.
-fsanitize-recover=all and -fno-sanitize-recover=all is also accepted, the
former enables recovery for all sanitizers that support it, the latter disables
recovery for all sanitizers that support it.
Even if a recovery mode is turned on the compiler side, it needs to be also
enabled on the runtime library side, otherwise the failures are still fatal.
The runtime library defaults to "halt_on_error=0" for ThreadSanitizer and
UndefinedBehaviorSanitizer, while default value for AddressSanitizer is
"halt_on_error=1". This can be overridden through setting the "halt_on_error"
flag in the corresponding environment variable.
Syntax without an explicit opts parameter is deprecated. It is equivalent to
specifying an opts list of:
undefined,float-cast-overflow,float-divide-by-zero,bounds-strict
-fsanitize-address-use-after-scope
Enable sanitization of local variables to detect use-after-scope bugs. The
option sets -fstack-reuse to none.
-fsanitize-undefined-trap-on-error
The -fsanitize-undefined-trap-on-error option instructs the compiler to report
undefined behavior using "__builtin_trap" rather than a "libubsan" library
routine. The advantage of this is that the "libubsan" library is not needed
and is not linked in, so this is usable even in freestanding environments.
-fsanitize-coverage=trace-pc
Enable coverage-guided fuzzing code instrumentation. Inserts a call to
"__sanitizer_cov_trace_pc" into every basic block.
-fsanitize-coverage=trace-cmp
Enable dataflow guided fuzzing code instrumentation. Inserts a call to
"__sanitizer_cov_trace_cmp1", "__sanitizer_cov_trace_cmp2",
"__sanitizer_cov_trace_cmp4" or "__sanitizer_cov_trace_cmp8" for integral
comparison with both operands variable or "__sanitizer_cov_trace_const_cmp1",
"__sanitizer_cov_trace_const_cmp2", "__sanitizer_cov_trace_const_cmp4" or
"__sanitizer_cov_trace_const_cmp8" for integral comparison with one operand
constant, "__sanitizer_cov_trace_cmpf" or "__sanitizer_cov_trace_cmpd" for
float or double comparisons and "__sanitizer_cov_trace_switch" for switch
statements.
-fbounds-check
For front ends that support it, generate additional code to check that indices
used to access arrays are within the declared range. This is currently only
supported by the Fortran front end, where this option defaults to false.
-fcheck-pointer-bounds
Enable Pointer Bounds Checker instrumentation. Each memory reference is
instrumented with checks of the pointer used for memory access against bounds
associated with that pointer.
Currently there is only an implementation for Intel MPX available, thus x86
GNU/Linux target and -mmpx are required to enable this feature. MPX-based
instrumentation requires a runtime library to enable MPX in hardware and handle
bounds violation signals. By default when -fcheck-pointer-bounds and -mmpx
options are used to link a program, the GCC driver links against the libmpx and
libmpxwrappers libraries. Bounds checking on calls to dynamic libraries
requires a linker with -z bndplt support; if GCC was configured with a linker
without support for this option (including the Gold linker and older versions
of ld), a warning is given if you link with -mmpx without also specifying
-static, since the overall effectiveness of the bounds checking protection is
reduced. See also -static-libmpxwrappers.
MPX-based instrumentation may be used for debugging and also may be included in
production code to increase program security. Depending on usage, you may have
different requirements for the runtime library. The current version of the MPX
runtime library is more oriented for use as a debugging tool. MPX runtime
library usage implies -lpthread. See also -static-libmpx. The runtime library
behavior can be influenced using various CHKP_RT_* environment variables. See
<https://gcc.gnu.org/wiki/Intel%20MPX%20support%20in%20the%20GCC%20compiler>
for more details.
Generated instrumentation may be controlled by various -fchkp-* options and by
the "bnd_variable_size" structure field attribute and "bnd_legacy", and
"bnd_instrument" function attributes. GCC also provides a number of built-in
functions for controlling the Pointer Bounds Checker.
-fchkp-check-incomplete-type
Generate pointer bounds checks for variables with incomplete type. Enabled by
default.
-fchkp-narrow-bounds
Controls bounds used by Pointer Bounds Checker for pointers to object fields.
If narrowing is enabled then field bounds are used. Otherwise object bounds
are used. See also -fchkp-narrow-to-innermost-array and
-fchkp-first-field-has-own-bounds. Enabled by default.
-fchkp-first-field-has-own-bounds
Forces Pointer Bounds Checker to use narrowed bounds for the address of the
first field in the structure. By default a pointer to the first field has the
same bounds as a pointer to the whole structure.
-fchkp-flexible-struct-trailing-arrays
Forces Pointer Bounds Checker to treat all trailing arrays in structures as
possibly flexible. By default only array fields with zero length or that are
marked with attribute bnd_variable_size are treated as flexible.
-fchkp-narrow-to-innermost-array
Forces Pointer Bounds Checker to use bounds of the innermost arrays in case of
nested static array access. By default this option is disabled and bounds of
the outermost array are used.
-fchkp-optimize
Enables Pointer Bounds Checker optimizations. Enabled by default at
optimization levels -O, -O2, -O3.
-fchkp-use-fast-string-functions
Enables use of *_nobnd versions of string functions (not copying bounds) by
Pointer Bounds Checker. Disabled by default.
-fchkp-use-nochk-string-functions
Enables use of *_nochk versions of string functions (not checking bounds) by
Pointer Bounds Checker. Disabled by default.
-fchkp-use-static-bounds
Allow Pointer Bounds Checker to generate static bounds holding bounds of static
variables. Enabled by default.
-fchkp-use-static-const-bounds
Use statically-initialized bounds for constant bounds instead of generating
them each time they are required. By default enabled when
-fchkp-use-static-bounds is enabled.
-fchkp-treat-zero-dynamic-size-as-infinite
With this option, objects with incomplete type whose dynamically-obtained size
is zero are treated as having infinite size instead by Pointer Bounds Checker.
This option may be helpful if a program is linked with a library missing size
information for some symbols. Disabled by default.
-fchkp-check-read
Instructs Pointer Bounds Checker to generate checks for all read accesses to
memory. Enabled by default.
-fchkp-check-write
Instructs Pointer Bounds Checker to generate checks for all write accesses to
memory. Enabled by default.
-fchkp-store-bounds
Instructs Pointer Bounds Checker to generate bounds stores for pointer writes.
Enabled by default.
-fchkp-instrument-calls
Instructs Pointer Bounds Checker to pass pointer bounds to calls. Enabled by
default.
-fchkp-instrument-marked-only
Instructs Pointer Bounds Checker to instrument only functions marked with the
"bnd_instrument" attribute. Disabled by default.
-fchkp-use-wrappers
Allows Pointer Bounds Checker to replace calls to built-in functions with calls
to wrapper functions. When -fchkp-use-wrappers is used to link a program, the
GCC driver automatically links against libmpxwrappers. See also
-static-libmpxwrappers. Enabled by default.
-fcf-protection=[full|branch|return|none]
Enable code instrumentation of control-flow transfers to increase program
security by checking that target addresses of control-flow transfer
instructions (such as indirect function call, function return, indirect jump)
are valid. This prevents diverting the flow of control to an unexpected
target. This is intended to protect against such threats as Return-oriented
Programming (ROP), and similarly call/jmp-oriented programming (COP/JOP).
The value "branch" tells the compiler to implement checking of validity of
control-flow transfer at the point of indirect branch instructions, i.e.
call/jmp instructions. The value "return" implements checking of validity at
the point of returning from a function. The value "full" is an alias for
specifying both "branch" and "return". The value "none" turns off
instrumentation.
The macro "__CET__" is defined when -fcf-protection is used. The first bit of
"__CET__" is set to 1 for the value "branch" and the second bit of "__CET__" is
set to 1 for the "return".
You can also use the "nocf_check" attribute to identify which functions and
calls should be skipped from instrumentation.
Currently the x86 GNU/Linux target provides an implementation based on Intel
Control-flow Enforcement Technology (CET).
-fstack-protector
Emit extra code to check for buffer overflows, such as stack smashing attacks.
This is done by adding a guard variable to functions with vulnerable objects.
This includes functions that call "alloca", and functions with buffers larger
than 8 bytes. The guards are initialized when a function is entered and then
checked when the function exits. If a guard check fails, an error message is
printed and the program exits.
-fstack-protector-all
Like -fstack-protector except that all functions are protected.
-fstack-protector-strong
Like -fstack-protector but includes additional functions to be protected ---
those that have local array definitions, or have references to local frame
addresses.
-fstack-protector-explicit
Like -fstack-protector but only protects those functions which have the
"stack_protect" attribute.
-fstack-check
Generate code to verify that you do not go beyond the boundary of the stack.
You should specify this flag if you are running in an environment with multiple
threads, but you only rarely need to specify it in a single-threaded
environment since stack overflow is automatically detected on nearly all
systems if there is only one stack.
Note that this switch does not actually cause checking to be done; the
operating system or the language runtime must do that. The switch causes
generation of code to ensure that they see the stack being extended.
You can additionally specify a string parameter: no means no checking, generic
means force the use of old-style checking, specific means use the best checking
method and is equivalent to bare -fstack-check.
Old-style checking is a generic mechanism that requires no specific target
support in the compiler but comes with the following drawbacks:
1. Modified allocation strategy for large objects: they are always allocated
dynamically if their size exceeds a fixed threshold. Note this may change
the semantics of some code.
2. Fixed limit on the size of the static frame of functions: when it is topped
by a particular function, stack checking is not reliable and a warning is
issued by the compiler.
3. Inefficiency: because of both the modified allocation strategy and the
generic implementation, code performance is hampered.
Note that old-style stack checking is also the fallback method for specific if
no target support has been added in the compiler.
-fstack-check= is designed for Ada's needs to detect infinite recursion and
stack overflows. specific is an excellent choice when compiling Ada code. It
is not generally sufficient to protect against stack-clash attacks. To protect
against those you want -fstack-clash-protection.
-fstack-clash-protection
Generate code to prevent stack clash style attacks. When this option is
enabled, the compiler will only allocate one page of stack space at a time and
each page is accessed immediately after allocation. Thus, it prevents
allocations from jumping over any stack guard page provided by the operating
system.
Most targets do not fully support stack clash protection. However, on those
targets -fstack-clash-protection will protect dynamic stack allocations.
-fstack-clash-protection may also provide limited protection for static stack
allocations if the target supports -fstack-check=specific.
-fstack-limit-register=reg
-fstack-limit-symbol=sym
-fno-stack-limit
Generate code to ensure that the stack does not grow beyond a certain value,
either the value of a register or the address of a symbol. If a larger stack
is required, a signal is raised at run time. For most targets, the signal is
raised before the stack overruns the boundary, so it is possible to catch the
signal without taking special precautions.
For instance, if the stack starts at absolute address 0x80000000 and grows
downwards, you can use the flags -fstack-limit-symbol=__stack_limit and
-Wl,--defsym,__stack_limit=0x7ffe0000 to enforce a stack limit of 128KB. Note
that this may only work with the GNU linker.
You can locally override stack limit checking by using the "no_stack_limit"
function attribute.
-fsplit-stack
Generate code to automatically split the stack before it overflows. The
resulting program has a discontiguous stack which can only overflow if the
program is unable to allocate any more memory. This is most useful when
running threaded programs, as it is no longer necessary to calculate a good
stack size to use for each thread. This is currently only implemented for the
x86 targets running GNU/Linux.
When code compiled with -fsplit-stack calls code compiled without
-fsplit-stack, there may not be much stack space available for the latter code
to run. If compiling all code, including library code, with -fsplit-stack is
not an option, then the linker can fix up these calls so that the code compiled
without -fsplit-stack always has a large stack. Support for this is
implemented in the gold linker in GNU binutils release 2.21 and later.
-fvtable-verify=[std|preinit|none]
This option is only available when compiling C++ code. It turns on (or off, if
using -fvtable-verify=none) the security feature that verifies at run time, for
every virtual call, that the vtable pointer through which the call is made is
valid for the type of the object, and has not been corrupted or overwritten.
If an invalid vtable pointer is detected at run time, an error is reported and
execution of the program is immediately halted.
This option causes run-time data structures to be built at program startup,
which are used for verifying the vtable pointers. The options std and preinit
control the timing of when these data structures are built. In both cases the
data structures are built before execution reaches "main". Using
-fvtable-verify=std causes the data structures to be built after shared
libraries have been loaded and initialized. -fvtable-verify=preinit causes
them to be built before shared libraries have been loaded and initialized.
If this option appears multiple times in the command line with different values
specified, none takes highest priority over both std and preinit; preinit takes
priority over std.
-fvtv-debug
When used in conjunction with -fvtable-verify=std or -fvtable-verify=preinit,
causes debug versions of the runtime functions for the vtable verification
feature to be called. This flag also causes the compiler to log information
about which vtable pointers it finds for each class. This information is
written to a file named vtv_set_ptr_data.log in the directory named by the
environment variable VTV_LOGS_DIR if that is defined or the current working
directory otherwise.
Note: This feature appends data to the log file. If you want a fresh log file,
be sure to delete any existing one.
-fvtv-counts
This is a debugging flag. When used in conjunction with -fvtable-verify=std or
-fvtable-verify=preinit, this causes the compiler to keep track of the total
number of virtual calls it encounters and the number of verifications it
inserts. It also counts the number of calls to certain run-time library
functions that it inserts and logs this information for each compilation unit.
The compiler writes this information to a file named vtv_count_data.log in the
directory named by the environment variable VTV_LOGS_DIR if that is defined or
the current working directory otherwise. It also counts the size of the vtable
pointer sets for each class, and writes this information to
vtv_class_set_sizes.log in the same directory.
Note: This feature appends data to the log files. To get fresh log files, be
sure to delete any existing ones.
-finstrument-functions
Generate instrumentation calls for entry and exit to functions. Just after
function entry and just before function exit, the following profiling functions
are called with the address of the current function and its call site. (On
some platforms, "__builtin_return_address" does not work beyond the current
function, so the call site information may not be available to the profiling
functions otherwise.)
void __cyg_profile_func_enter (void *this_fn,
void *call_site);
void __cyg_profile_func_exit (void *this_fn,
void *call_site);
The first argument is the address of the start of the current function, which
may be looked up exactly in the symbol table.
This instrumentation is also done for functions expanded inline in other
functions. The profiling calls indicate where, conceptually, the inline
function is entered and exited. This means that addressable versions of such
functions must be available. If all your uses of a function are expanded
inline, this may mean an additional expansion of code size. If you use "extern
inline" in your C code, an addressable version of such functions must be
provided. (This is normally the case anyway, but if you get lucky and the
optimizer always expands the functions inline, you might have gotten away
without providing static copies.)
A function may be given the attribute "no_instrument_function", in which case
this instrumentation is not done. This can be used, for example, for the
profiling functions listed above, high-priority interrupt routines, and any
functions from which the profiling functions cannot safely be called (perhaps
signal handlers, if the profiling routines generate output or allocate memory).
-finstrument-functions-exclude-file-list=file,file,...
Set the list of functions that are excluded from instrumentation (see the
description of -finstrument-functions). If the file that contains a function
definition matches with one of file, then that function is not instrumented.
The match is done on substrings: if the file parameter is a substring of the
file name, it is considered to be a match.
For example:
-finstrument-functions-exclude-file-list=/bits/stl,include/sys
excludes any inline function defined in files whose pathnames contain /bits/stl
or include/sys.
If, for some reason, you want to include letter , in one of sym, write ,. For
example, -finstrument-functions-exclude-file-list=',,tmp' (note the single
quote surrounding the option).
-finstrument-functions-exclude-function-list=sym,sym,...
This is similar to -finstrument-functions-exclude-file-list, but this option
sets the list of function names to be excluded from instrumentation. The
function name to be matched is its user-visible name, such as "vector<int>
blah(const vector<int> &)", not the internal mangled name (e.g.,
"_Z4blahRSt6vectorIiSaIiEE"). The match is done on substrings: if the sym
parameter is a substring of the function name, it is considered to be a match.
For C99 and C++ extended identifiers, the function name must be given in UTF-8,
not using universal character names.
-fpatchable-function-entry=N[,M]
Generate N NOPs right at the beginning of each function, with the function
entry point before the Mth NOP. If M is omitted, it defaults to 0 so the
function entry points to the address just at the first NOP. The NOP
instructions reserve extra space which can be used to patch in any desired
instrumentation at run time, provided that the code segment is writable. The
amount of space is controllable indirectly via the number of NOPs; the NOP
instruction used corresponds to the instruction emitted by the internal GCC
back-end interface "gen_nop". This behavior is target-specific and may also
depend on the architecture variant and/or other compilation options.
For run-time identification, the starting addresses of these areas, which
correspond to their respective function entries minus M, are additionally
collected in the "__patchable_function_entries" section of the resulting
binary.
Note that the value of "__attribute__ ((patchable_function_entry (N,M)))" takes
precedence over command-line option -fpatchable-function-entry=N,M. This can
be used to increase the area size or to remove it completely on a single
function. If "N=0", no pad location is recorded.
The NOP instructions are inserted at---and maybe before, depending on M---the
function entry address, even before the prologue.
Options Controlling the Preprocessor
These options control the C preprocessor, which is run on each C source file before
actual compilation.
If you use the -E option, nothing is done except preprocessing. Some of these
options make sense only together with -E because they cause the preprocessor output
to be unsuitable for actual compilation.
In addition to the options listed here, there are a number of options to control
search paths for include files documented in Directory Options. Options to control
preprocessor diagnostics are listed in Warning Options.
-D name
Predefine name as a macro, with definition 1.
-D name=definition
The contents of definition are tokenized and processed as if they appeared
during translation phase three in a #define directive. In particular, the
definition is truncated by embedded newline characters.
If you are invoking the preprocessor from a shell or shell-like program you may
need to use the shell's quoting syntax to protect characters such as spaces
that have a meaning in the shell syntax.
If you wish to define a function-like macro on the command line, write its
argument list with surrounding parentheses before the equals sign (if any).
Parentheses are meaningful to most shells, so you should quote the option.
With sh and csh, -D'name(args...)=definition' works.
-D and -U options are processed in the order they are given on the command
line. All -imacros file and -include file options are processed after all -D
and -U options.
-U name
Cancel any previous definition of name, either built in or provided with a -D
option.
-include file
Process file as if "#include "file"" appeared as the first line of the primary
source file. However, the first directory searched for file is the
preprocessor's working directory instead of the directory containing the main
source file. If not found there, it is searched for in the remainder of the
"#include "..."" search chain as normal.
If multiple -include options are given, the files are included in the order
they appear on the command line.
-imacros file
Exactly like -include, except that any output produced by scanning file is
thrown away. Macros it defines remain defined. This allows you to acquire all
the macros from a header without also processing its declarations.
All files specified by -imacros are processed before all files specified by
-include.
-undef
Do not predefine any system-specific or GCC-specific macros. The standard
predefined macros remain defined.
-pthread
Define additional macros required for using the POSIX threads library. You
should use this option consistently for both compilation and linking. This
option is supported on GNU/Linux targets, most other Unix derivatives, and also
on x86 Cygwin and MinGW targets.
-M Instead of outputting the result of preprocessing, output a rule suitable for
make describing the dependencies of the main source file. The preprocessor
outputs one make rule containing the object file name for that source file, a
colon, and the names of all the included files, including those coming from
-include or -imacros command-line options.
Unless specified explicitly (with -MT or -MQ), the object file name consists of
the name of the source file with any suffix replaced with object file suffix
and with any leading directory parts removed. If there are many included files
then the rule is split into several lines using \-newline. The rule has no
commands.
This option does not suppress the preprocessor's debug output, such as -dM. To
avoid mixing such debug output with the dependency rules you should explicitly
specify the dependency output file with -MF, or use an environment variable
like DEPENDENCIES_OUTPUT. Debug output is still sent to the regular output
stream as normal.
Passing -M to the driver implies -E, and suppresses warnings with an implicit
-w.
-MM Like -M but do not mention header files that are found in system header
directories, nor header files that are included, directly or indirectly, from
such a header.
This implies that the choice of angle brackets or double quotes in an #include
directive does not in itself determine whether that header appears in -MM
dependency output.
-MF file
When used with -M or -MM, specifies a file to write the dependencies to. If no
-MF switch is given the preprocessor sends the rules to the same place it would
send preprocessed output.
When used with the driver options -MD or -MMD, -MF overrides the default
dependency output file.
If file is -, then the dependencies are written to stdout.
-MG In conjunction with an option such as -M requesting dependency generation, -MG
assumes missing header files are generated files and adds them to the
dependency list without raising an error. The dependency filename is taken
directly from the "#include" directive without prepending any path. -MG also
suppresses preprocessed output, as a missing header file renders this useless.
This feature is used in automatic updating of makefiles.
-MP This option instructs CPP to add a phony target for each dependency other than
the main file, causing each to depend on nothing. These dummy rules work
around errors make gives if you remove header files without updating the
Makefile to match.
This is typical output:
test.o: test.c test.h
test.h:
-MT target
Change the target of the rule emitted by dependency generation. By default CPP
takes the name of the main input file, deletes any directory components and any
file suffix such as .c, and appends the platform's usual object suffix. The
result is the target.
An -MT option sets the target to be exactly the string you specify. If you
want multiple targets, you can specify them as a single argument to -MT, or use
multiple -MT options.
For example, -MT '$(objpfx)foo.o' might give
$(objpfx)foo.o: foo.c
-MQ target
Same as -MT, but it quotes any characters which are special to Make.
-MQ '$(objpfx)foo.o' gives
$$(objpfx)foo.o: foo.c
The default target is automatically quoted, as if it were given with -MQ.
-MD -MD is equivalent to -M -MF file, except that -E is not implied. The driver
determines file based on whether an -o option is given. If it is, the driver
uses its argument but with a suffix of .d, otherwise it takes the name of the
input file, removes any directory components and suffix, and applies a .d
suffix.
If -MD is used in conjunction with -E, any -o switch is understood to specify
the dependency output file, but if used without -E, each -o is understood to
specify a target object file.
Since -E is not implied, -MD can be used to generate a dependency output file
as a side effect of the compilation process.
-MMD
Like -MD except mention only user header files, not system header files.
-fpreprocessed
Indicate to the preprocessor that the input file has already been preprocessed.
This suppresses things like macro expansion, trigraph conversion, escaped
newline splicing, and processing of most directives. The preprocessor still
recognizes and removes comments, so that you can pass a file preprocessed with
-C to the compiler without problems. In this mode the integrated preprocessor
is little more than a tokenizer for the front ends.
-fpreprocessed is implicit if the input file has one of the extensions .i, .ii
or .mi. These are the extensions that GCC uses for preprocessed files created
by -save-temps.
-fdirectives-only
When preprocessing, handle directives, but do not expand macros.
The option's behavior depends on the -E and -fpreprocessed options.
With -E, preprocessing is limited to the handling of directives such as
"#define", "#ifdef", and "#error". Other preprocessor operations, such as
macro expansion and trigraph conversion are not performed. In addition, the
-dD option is implicitly enabled.
With -fpreprocessed, predefinition of command line and most builtin macros is
disabled. Macros such as "__LINE__", which are contextually dependent, are
handled normally. This enables compilation of files previously preprocessed
with "-E -fdirectives-only".
With both -E and -fpreprocessed, the rules for -fpreprocessed take precedence.
This enables full preprocessing of files previously preprocessed with "-E
-fdirectives-only".
-fdollars-in-identifiers
Accept $ in identifiers.
-fextended-identifiers
Accept universal character names in identifiers. This option is enabled by
default for C99 (and later C standard versions) and C++.
-fno-canonical-system-headers
When preprocessing, do not shorten system header paths with canonicalization.
-ftabstop=width
Set the distance between tab stops. This helps the preprocessor report correct
column numbers in warnings or errors, even if tabs appear on the line. If the
value is less than 1 or greater than 100, the option is ignored. The default
is 8.
-ftrack-macro-expansion[=level]
Track locations of tokens across macro expansions. This allows the compiler to
emit diagnostic about the current macro expansion stack when a compilation
error occurs in a macro expansion. Using this option makes the preprocessor and
the compiler consume more memory. The level parameter can be used to choose the
level of precision of token location tracking thus decreasing the memory
consumption if necessary. Value 0 of level de-activates this option. Value 1
tracks tokens locations in a degraded mode for the sake of minimal memory
overhead. In this mode all tokens resulting from the expansion of an argument
of a function-like macro have the same location. Value 2 tracks tokens
locations completely. This value is the most memory hungry. When this option
is given no argument, the default parameter value is 2.
Note that "-ftrack-macro-expansion=2" is activated by default.
-fmacro-prefix-map=old=new
When preprocessing files residing in directory old, expand the "__FILE__" and
"__BASE_FILE__" macros as if the files resided in directory new instead. This
can be used to change an absolute path to a relative path by using . for new
which can result in more reproducible builds that are location independent.
This option also affects "__builtin_FILE()" during compilation. See also
-ffile-prefix-map.
-fexec-charset=charset
Set the execution character set, used for string and character constants. The
default is UTF-8. charset can be any encoding supported by the system's
"iconv" library routine.
-fwide-exec-charset=charset
Set the wide execution character set, used for wide string and character
constants. The default is UTF-32 or UTF-16, whichever corresponds to the width
of "wchar_t". As with -fexec-charset, charset can be any encoding supported by
the system's "iconv" library routine; however, you will have problems with
encodings that do not fit exactly in "wchar_t".
-finput-charset=charset
Set the input character set, used for translation from the character set of the
input file to the source character set used by GCC. If the locale does not
specify, or GCC cannot get this information from the locale, the default is
UTF-8. This can be overridden by either the locale or this command-line
option. Currently the command-line option takes precedence if there's a
conflict. charset can be any encoding supported by the system's "iconv"
library routine.
-fpch-deps
When using precompiled headers, this flag causes the dependency-output flags to
also list the files from the precompiled header's dependencies. If not
specified, only the precompiled header are listed and not the files that were
used to create it, because those files are not consulted when a precompiled
header is used.
-fpch-preprocess
This option allows use of a precompiled header together with -E. It inserts a
special "#pragma", "#pragma GCC pch_preprocess "filename"" in the output to
mark the place where the precompiled header was found, and its filename. When
-fpreprocessed is in use, GCC recognizes this "#pragma" and loads the PCH.
This option is off by default, because the resulting preprocessed output is
only really suitable as input to GCC. It is switched on by -save-temps.
You should not write this "#pragma" in your own code, but it is safe to edit
the filename if the PCH file is available in a different location. The
filename may be absolute or it may be relative to GCC's current directory.
-fworking-directory
Enable generation of linemarkers in the preprocessor output that let the
compiler know the current working directory at the time of preprocessing. When
this option is enabled, the preprocessor emits, after the initial linemarker, a
second linemarker with the current working directory followed by two slashes.
GCC uses this directory, when it's present in the preprocessed input, as the
directory emitted as the current working directory in some debugging
information formats. This option is implicitly enabled if debugging
information is enabled, but this can be inhibited with the negated form
-fno-working-directory. If the -P flag is present in the command line, this
option has no effect, since no "#line" directives are emitted whatsoever.
-A predicate=answer
Make an assertion with the predicate predicate and answer answer. This form is
preferred to the older form -A predicate(answer), which is still supported,
because it does not use shell special characters.
-A -predicate=answer
Cancel an assertion with the predicate predicate and answer answer.
-C Do not discard comments. All comments are passed through to the output file,
except for comments in processed directives, which are deleted along with the
directive.
You should be prepared for side effects when using -C; it causes the
preprocessor to treat comments as tokens in their own right. For example,
comments appearing at the start of what would be a directive line have the
effect of turning that line into an ordinary source line, since the first token
on the line is no longer a #.
-CC Do not discard comments, including during macro expansion. This is like -C,
except that comments contained within macros are also passed through to the
output file where the macro is expanded.
In addition to the side effects of the -C option, the -CC option causes all
C++-style comments inside a macro to be converted to C-style comments. This is
to prevent later use of that macro from inadvertently commenting out the
remainder of the source line.
The -CC option is generally used to support lint comments.
-P Inhibit generation of linemarkers in the output from the preprocessor. This
might be useful when running the preprocessor on something that is not C code,
and will be sent to a program which might be confused by the linemarkers.
-traditional
-traditional-cpp
Try to imitate the behavior of pre-standard C preprocessors, as opposed to ISO
C preprocessors. See the GNU CPP manual for details.
Note that GCC does not otherwise attempt to emulate a pre-standard C compiler,
and these options are only supported with the -E switch, or when invoking CPP
explicitly.
-trigraphs
Support ISO C trigraphs. These are three-character sequences, all starting
with ??, that are defined by ISO C to stand for single characters. For
example, ??/ stands for \, so '??/n' is a character constant for a newline.
The nine trigraphs and their replacements are
Trigraph: ??( ??) ??< ??> ??= ??/ ??' ??! ??-
Replacement: [ ] { } # \ ^ | ~
By default, GCC ignores trigraphs, but in standard-conforming modes it converts
them. See the -std and -ansi options.
-remap
Enable special code to work around file systems which only permit very short
file names, such as MS-DOS.
-H Print the name of each header file used, in addition to other normal
activities. Each name is indented to show how deep in the #include stack it
is. Precompiled header files are also printed, even if they are found to be
invalid; an invalid precompiled header file is printed with ...x and a valid
one with ...! .
-dletters
Says to make debugging dumps during compilation as specified by letters. The
flags documented here are those relevant to the preprocessor. Other letters
are interpreted by the compiler proper, or reserved for future versions of GCC,
and so are silently ignored. If you specify letters whose behavior conflicts,
the result is undefined.
-dM Instead of the normal output, generate a list of #define directives for all
the macros defined during the execution of the preprocessor, including
predefined macros. This gives you a way of finding out what is predefined
in your version of the preprocessor. Assuming you have no file foo.h, the
command
touch foo.h; cpp -dM foo.h
shows all the predefined macros.
If you use -dM without the -E option, -dM is interpreted as a synonym for
-fdump-rtl-mach.
-dD Like -dM except in two respects: it does not include the predefined macros,
and it outputs both the #define directives and the result of preprocessing.
Both kinds of output go to the standard output file.
-dN Like -dD, but emit only the macro names, not their expansions.
-dI Output #include directives in addition to the result of preprocessing.
-dU Like -dD except that only macros that are expanded, or whose definedness is
tested in preprocessor directives, are output; the output is delayed until
the use or test of the macro; and #undef directives are also output for
macros tested but undefined at the time.
-fdebug-cpp
This option is only useful for debugging GCC. When used from CPP or with -E,
it dumps debugging information about location maps. Every token in the output
is preceded by the dump of the map its location belongs to.
When used from GCC without -E, this option has no effect.
-Wp,option
You can use -Wp,option to bypass the compiler driver and pass option directly
through to the preprocessor. If option contains commas, it is split into
multiple options at the commas. However, many options are modified, translated
or interpreted by the compiler driver before being passed to the preprocessor,
and -Wp forcibly bypasses this phase. The preprocessor's direct interface is
undocumented and subject to change, so whenever possible you should avoid using
-Wp and let the driver handle the options instead.
-Xpreprocessor option
Pass option as an option to the preprocessor. You can use this to supply
system-specific preprocessor options that GCC does not recognize.
If you want to pass an option that takes an argument, you must use
-Xpreprocessor twice, once for the option and once for the argument.
-no-integrated-cpp
Perform preprocessing as a separate pass before compilation. By default, GCC
performs preprocessing as an integrated part of input tokenization and parsing.
If this option is provided, the appropriate language front end (cc1, cc1plus,
or cc1obj for C, C++, and Objective-C, respectively) is instead invoked twice,
once for preprocessing only and once for actual compilation of the preprocessed
input. This option may be useful in conjunction with the -B or -wrapper
options to specify an alternate preprocessor or perform additional processing
of the program source between normal preprocessing and compilation.
Passing Options to the Assembler
You can pass options to the assembler.
-Wa,option
Pass option as an option to the assembler. If option contains commas, it is
split into multiple options at the commas.
-Xassembler option
Pass option as an option to the assembler. You can use this to supply system-
specific assembler options that GCC does not recognize.
If you want to pass an option that takes an argument, you must use -Xassembler
twice, once for the option and once for the argument.
Options for Linking
These options come into play when the compiler links object files into an
executable output file. They are meaningless if the compiler is not doing a link
step.
object-file-name
A file name that does not end in a special recognized suffix is considered to
name an object file or library. (Object files are distinguished from libraries
by the linker according to the file contents.) If linking is done, these
object files are used as input to the linker.
-c
-S
-E If any of these options is used, then the linker is not run, and object file
names should not be used as arguments.
-fuse-ld=bfd
Use the bfd linker instead of the default linker.
-fuse-ld=gold
Use the gold linker instead of the default linker.
-fuse-ld=lld
Use the LLVM lld linker instead of the default linker.
-llibrary
-l library
Search the library named library when linking. (The second alternative with
the library as a separate argument is only for POSIX compliance and is not
recommended.)
It makes a difference where in the command you write this option; the linker
searches and processes libraries and object files in the order they are
specified. Thus, foo.o -lz bar.o searches library z after file foo.o but
before bar.o. If bar.o refers to functions in z, those functions may not be
loaded.
The linker searches a standard list of directories for the library, which is
actually a file named liblibrary.a. The linker then uses this file as if it
had been specified precisely by name.
The directories searched include several standard system directories plus any
that you specify with -L.
Normally the files found this way are library files---archive files whose
members are object files. The linker handles an archive file by scanning
through it for members which define symbols that have so far been referenced
but not defined. But if the file that is found is an ordinary object file, it
is linked in the usual fashion. The only difference between using an -l option
and specifying a file name is that -l surrounds library with lib and .a and
searches several directories.
-lobjc
You need this special case of the -l option in order to link an Objective-C or
Objective-C++ program.
-nostartfiles
Do not use the standard system startup files when linking. The standard system
libraries are used normally, unless -nostdlib or -nodefaultlibs is used.
-nodefaultlibs
Do not use the standard system libraries when linking. Only the libraries you
specify are passed to the linker, and options specifying linkage of the system
libraries, such as -static-libgcc or -shared-libgcc, are ignored. The standard
startup files are used normally, unless -nostartfiles is used.
The compiler may generate calls to "memcmp", "memset", "memcpy" and "memmove".
These entries are usually resolved by entries in libc. These entry points
should be supplied through some other mechanism when this option is specified.
-nostdlib
Do not use the standard system startup files or libraries when linking. No
startup files and only the libraries you specify are passed to the linker, and
options specifying linkage of the system libraries, such as -static-libgcc or
-shared-libgcc, are ignored.
The compiler may generate calls to "memcmp", "memset", "memcpy" and "memmove".
These entries are usually resolved by entries in libc. These entry points
should be supplied through some other mechanism when this option is specified.
One of the standard libraries bypassed by -nostdlib and -nodefaultlibs is
libgcc.a, a library of internal subroutines which GCC uses to overcome
shortcomings of particular machines, or special needs for some languages.
In most cases, you need libgcc.a even when you want to avoid other standard
libraries. In other words, when you specify -nostdlib or -nodefaultlibs you
should usually specify -lgcc as well. This ensures that you have no unresolved
references to internal GCC library subroutines. (An example of such an
internal subroutine is "__main", used to ensure C++ constructors are called.)
-pie
Produce a dynamically linked position independent executable on targets that
support it. For predictable results, you must also specify the same set of
options used for compilation (-fpie, -fPIE, or model suboptions) when you
specify this linker option.
-no-pie
Don't produce a dynamically linked position independent executable.
-static-pie
Produce a static position independent executable on targets that support it. A
static position independent executable is similar to a static executable, but
can be loaded at any address without a dynamic linker. For predictable
results, you must also specify the same set of options used for compilation
(-fpie, -fPIE, or model suboptions) when you specify this linker option.
-pthread
Link with the POSIX threads library. This option is supported on GNU/Linux
targets, most other Unix derivatives, and also on x86 Cygwin and MinGW targets.
On some targets this option also sets flags for the preprocessor, so it should
be used consistently for both compilation and linking.
-rdynamic
Pass the flag -export-dynamic to the ELF linker, on targets that support it.
This instructs the linker to add all symbols, not only used ones, to the
dynamic symbol table. This option is needed for some uses of "dlopen" or to
allow obtaining backtraces from within a program.
-s Remove all symbol table and relocation information from the executable.
-static
On systems that support dynamic linking, this overrides -pie and prevents
linking with the shared libraries. On other systems, this option has no
effect.
-shared
Produce a shared object which can then be linked with other objects to form an
executable. Not all systems support this option. For predictable results, you
must also specify the same set of options used for compilation (-fpic, -fPIC,
or model suboptions) when you specify this linker option.[1]
-shared-libgcc
-static-libgcc
On systems that provide libgcc as a shared library, these options force the use
of either the shared or static version, respectively. If no shared version of
libgcc was built when the compiler was configured, these options have no
effect.
There are several situations in which an application should use the shared
libgcc instead of the static version. The most common of these is when the
application wishes to throw and catch exceptions across different shared
libraries. In that case, each of the libraries as well as the application
itself should use the shared libgcc.
Therefore, the G++ driver automatically adds -shared-libgcc whenever you build
a shared library or a main executable, because C++ programs typically use
exceptions, so this is the right thing to do.
If, instead, you use the GCC driver to create shared libraries, you may find
that they are not always linked with the shared libgcc. If GCC finds, at its
configuration time, that you have a non-GNU linker or a GNU linker that does
not support option --eh-frame-hdr, it links the shared version of libgcc into
shared libraries by default. Otherwise, it takes advantage of the linker and
optimizes away the linking with the shared version of libgcc, linking with the
static version of libgcc by default. This allows exceptions to propagate
through such shared libraries, without incurring relocation costs at library
load time.
However, if a library or main executable is supposed to throw or catch
exceptions, you must link it using the G++ driver, or using the option
-shared-libgcc, such that it is linked with the shared libgcc.
-static-libasan
When the -fsanitize=address option is used to link a program, the GCC driver
automatically links against libasan. If libasan is available as a shared
library, and the -static option is not used, then this links against the shared
version of libasan. The -static-libasan option directs the GCC driver to link
libasan statically, without necessarily linking other libraries statically.
-static-libtsan
When the -fsanitize=thread option is used to link a program, the GCC driver
automatically links against libtsan. If libtsan is available as a shared
library, and the -static option is not used, then this links against the shared
version of libtsan. The -static-libtsan option directs the GCC driver to link
libtsan statically, without necessarily linking other libraries statically.
-static-liblsan
When the -fsanitize=leak option is used to link a program, the GCC driver
automatically links against liblsan. If liblsan is available as a shared
library, and the -static option is not used, then this links against the shared
version of liblsan. The -static-liblsan option directs the GCC driver to link
liblsan statically, without necessarily linking other libraries statically.
-static-libubsan
When the -fsanitize=undefined option is used to link a program, the GCC driver
automatically links against libubsan. If libubsan is available as a shared
library, and the -static option is not used, then this links against the shared
version of libubsan. The -static-libubsan option directs the GCC driver to
link libubsan statically, without necessarily linking other libraries
statically.
-static-libmpx
When the -fcheck-pointer bounds and -mmpx options are used to link a program,
the GCC driver automatically links against libmpx. If libmpx is available as a
shared library, and the -static option is not used, then this links against the
shared version of libmpx. The -static-libmpx option directs the GCC driver to
link libmpx statically, without necessarily linking other libraries statically.
-static-libmpxwrappers
When the -fcheck-pointer bounds and -mmpx options are used to link a program
without also using -fno-chkp-use-wrappers, the GCC driver automatically links
against libmpxwrappers. If libmpxwrappers is available as a shared library,
and the -static option is not used, then this links against the shared version
of libmpxwrappers. The -static-libmpxwrappers option directs the GCC driver to
link libmpxwrappers statically, without necessarily linking other libraries
statically.
-static-libstdc++
When the g++ program is used to link a C++ program, it normally automatically
links against libstdc++. If libstdc++ is available as a shared library, and
the -static option is not used, then this links against the shared version of
libstdc++. That is normally fine. However, it is sometimes useful to freeze
the version of libstdc++ used by the program without going all the way to a
fully static link. The -static-libstdc++ option directs the g++ driver to link
libstdc++ statically, without necessarily linking other libraries statically.
-symbolic
Bind references to global symbols when building a shared object. Warn about
any unresolved references (unless overridden by the link editor option -Xlinker
-z -Xlinker defs). Only a few systems support this option.
-T script
Use script as the linker script. This option is supported by most systems
using the GNU linker. On some targets, such as bare-board targets without an
operating system, the -T option may be required when linking to avoid
references to undefined symbols.
-Xlinker option
Pass option as an option to the linker. You can use this to supply system-
specific linker options that GCC does not recognize.
If you want to pass an option that takes a separate argument, you must use
-Xlinker twice, once for the option and once for the argument. For example, to
pass -assert definitions, you must write -Xlinker -assert -Xlinker definitions.
It does not work to write -Xlinker "-assert definitions", because this passes
the entire string as a single argument, which is not what the linker expects.
When using the GNU linker, it is usually more convenient to pass arguments to
linker options using the option=value syntax than as separate arguments. For
example, you can specify -Xlinker -Map=output.map rather than -Xlinker -Map
-Xlinker output.map. Other linkers may not support this syntax for command-
line options.
-Wl,option
Pass option as an option to the linker. If option contains commas, it is split
into multiple options at the commas. You can use this syntax to pass an
argument to the option. For example, -Wl,-Map,output.map passes -Map
output.map to the linker. When using the GNU linker, you can also get the same
effect with -Wl,-Map=output.map.
-u symbol
Pretend the symbol symbol is undefined, to force linking of library modules to
define it. You can use -u multiple times with different symbols to force
loading of additional library modules.
-z keyword
-z is passed directly on to the linker along with the keyword keyword. See the
section in the documentation of your linker for permitted values and their
meanings.
Options for Directory Search
These options specify directories to search for header files, for libraries and for
parts of the compiler:
-I dir
-iquote dir
-isystem dir
-idirafter dir
Add the directory dir to the list of directories to be searched for header
files during preprocessing. If dir begins with = or $SYSROOT, then the = or
$SYSROOT is replaced by the sysroot prefix; see --sysroot and -isysroot.
Directories specified with -iquote apply only to the quote form of the
directive, "#include "file"". Directories specified with -I, -isystem, or
-idirafter apply to lookup for both the "#include "file"" and "#include <file>"
directives.
You can specify any number or combination of these options on the command line
to search for header files in several directories. The lookup order is as
follows:
1. For the quote form of the include directive, the directory of the current
file is searched first.
2. For the quote form of the include directive, the directories specified by
-iquote options are searched in left-to-right order, as they appear on the
command line.
3. Directories specified with -I options are scanned in left-to-right order.
4. Directories specified with -isystem options are scanned in left-to-right
order.
5. Standard system directories are scanned.
6. Directories specified with -idirafter options are scanned in left-to-right
order.
You can use -I to override a system header file, substituting your own version,
since these directories are searched before the standard system header file
directories. However, you should not use this option to add directories that
contain vendor-supplied system header files; use -isystem for that.
The -isystem and -idirafter options also mark the directory as a system
directory, so that it gets the same special treatment that is applied to the
standard system directories.
If a standard system include directory, or a directory specified with -isystem,
is also specified with -I, the -I option is ignored. The directory is still
searched but as a system directory at its normal position in the system include
chain. This is to ensure that GCC's procedure to fix buggy system headers and
the ordering for the "#include_next" directive are not inadvertently changed.
If you really need to change the search order for system directories, use the
-nostdinc and/or -isystem options.
-I- Split the include path. This option has been deprecated. Please use -iquote
instead for -I directories before the -I- and remove the -I- option.
Any directories specified with -I options before -I- are searched only for
headers requested with "#include "file""; they are not searched for
"#include <file>". If additional directories are specified with -I options
after the -I-, those directories are searched for all #include directives.
In addition, -I- inhibits the use of the directory of the current file
directory as the first search directory for "#include "file"". There is no way
to override this effect of -I-.
-iprefix prefix
Specify prefix as the prefix for subsequent -iwithprefix options. If the
prefix represents a directory, you should include the final /.
-iwithprefix dir
-iwithprefixbefore dir
Append dir to the prefix specified previously with -iprefix, and add the
resulting directory to the include search path. -iwithprefixbefore puts it in
the same place -I would; -iwithprefix puts it where -idirafter would.
-isysroot dir
This option is like the --sysroot option, but applies only to header files
(except for Darwin targets, where it applies to both header files and
libraries). See the --sysroot option for more information.
-imultilib dir
Use dir as a subdirectory of the directory containing target-specific C++
headers.
-nostdinc
Do not search the standard system directories for header files. Only the
directories explicitly specified with -I, -iquote, -isystem, and/or -idirafter
options (and the directory of the current file, if appropriate) are searched.
-nostdinc++
Do not search for header files in the C++-specific standard directories, but do
still search the other standard directories. (This option is used when
building the C++ library.)
-iplugindir=dir
Set the directory to search for plugins that are passed by -fplugin=name
instead of -fplugin=path/name.so. This option is not meant to be used by the
user, but only passed by the driver.
-Ldir
Add directory dir to the list of directories to be searched for -l.
-Bprefix
This option specifies where to find the executables, libraries, include files,
and data files of the compiler itself.
The compiler driver program runs one or more of the subprograms cpp, cc1, as
and ld. It tries prefix as a prefix for each program it tries to run, both
with and without machine/version/ for the corresponding target machine and
compiler version.
For each subprogram to be run, the compiler driver first tries the -B prefix,
if any. If that name is not found, or if -B is not specified, the driver tries
two standard prefixes, /usr/lib/gcc/ and /usr/local/lib/gcc/. If neither of
those results in a file name that is found, the unmodified program name is
searched for using the directories specified in your PATH environment variable.
The compiler checks to see if the path provided by -B refers to a directory,
and if necessary it adds a directory separator character at the end of the
path.
-B prefixes that effectively specify directory names also apply to libraries in
the linker, because the compiler translates these options into -L options for
the linker. They also apply to include files in the preprocessor, because the
compiler translates these options into -isystem options for the preprocessor.
In this case, the compiler appends include to the prefix.
The runtime support file libgcc.a can also be searched for using the -B prefix,
if needed. If it is not found there, the two standard prefixes above are
tried, and that is all. The file is left out of the link if it is not found by
those means.
Another way to specify a prefix much like the -B prefix is to use the
environment variable GCC_EXEC_PREFIX.
As a special kludge, if the path provided by -B is [dir/]stageN/, where N is a
number in the range 0 to 9, then it is replaced by [dir/]include. This is to
help with boot-strapping the compiler.
-no-canonical-prefixes
Do not expand any symbolic links, resolve references to /../ or /./, or make
the path absolute when generating a relative prefix.
--sysroot=dir
Use dir as the logical root directory for headers and libraries. For example,
if the compiler normally searches for headers in /usr/include and libraries in
/usr/lib, it instead searches dir/usr/include and dir/usr/lib.
If you use both this option and the -isysroot option, then the --sysroot option
applies to libraries, but the -isysroot option applies to header files.
The GNU linker (beginning with version 2.16) has the necessary support for this
option. If your linker does not support this option, the header file aspect of
--sysroot still works, but the library aspect does not.
--no-sysroot-suffix
For some targets, a suffix is added to the root directory specified with
--sysroot, depending on the other options used, so that headers may for example
be found in dir/suffix/usr/include instead of dir/usr/include. This option
disables the addition of such a suffix.
Options for Code Generation Conventions
These machine-independent options control the interface conventions used in code
generation.
Most of them have both positive and negative forms; the negative form of -ffoo is
-fno-foo. In the table below, only one of the forms is listed---the one that is
not the default. You can figure out the other form by either removing no- or
adding it.
-fstack-reuse=reuse-level
This option controls stack space reuse for user declared local/auto variables
and compiler generated temporaries. reuse_level can be all, named_vars, or
none. all enables stack reuse for all local variables and temporaries,
named_vars enables the reuse only for user defined local variables with names,
and none disables stack reuse completely. The default value is all. The option
is needed when the program extends the lifetime of a scoped local variable or a
compiler generated temporary beyond the end point defined by the language.
When a lifetime of a variable ends, and if the variable lives in memory, the
optimizing compiler has the freedom to reuse its stack space with other
temporaries or scoped local variables whose live range does not overlap with
it. Legacy code extending local lifetime is likely to break with the stack
reuse optimization.
For example,
int *p;
{
int local1;
p = &local1;
local1 = 10;
....
}
{
int local2;
local2 = 20;
...
}
if (*p == 10) // out of scope use of local1
{
}
Another example:
struct A
{
A(int k) : i(k), j(k) { }
int i;
int j;
};
A *ap;
void foo(const A& ar)
{
ap = &ar;
}
void bar()
{
foo(A(10)); // temp object's lifetime ends when foo returns
{
A a(20);
....
}
ap->i+= 10; // ap references out of scope temp whose space
// is reused with a. What is the value of ap->i?
}
The lifetime of a compiler generated temporary is well defined by the C++
standard. When a lifetime of a temporary ends, and if the temporary lives in
memory, the optimizing compiler has the freedom to reuse its stack space with
other temporaries or scoped local variables whose live range does not overlap
with it. However some of the legacy code relies on the behavior of older
compilers in which temporaries' stack space is not reused, the aggressive stack
reuse can lead to runtime errors. This option is used to control the temporary
stack reuse optimization.
-ftrapv
This option generates traps for signed overflow on addition, subtraction,
multiplication operations. The options -ftrapv and -fwrapv override each
other, so using -ftrapv -fwrapv on the command-line results in -fwrapv being
effective. Note that only active options override, so using -ftrapv -fwrapv
-fno-wrapv on the command-line results in -ftrapv being effective.
-fwrapv
This option instructs the compiler to assume that signed arithmetic overflow of
addition, subtraction and multiplication wraps around using twos-complement
representation. This flag enables some optimizations and disables others. The
options -ftrapv and -fwrapv override each other, so using -ftrapv -fwrapv on
the command-line results in -fwrapv being effective. Note that only active
options override, so using -ftrapv -fwrapv -fno-wrapv on the command-line
results in -ftrapv being effective.
-fwrapv-pointer
This option instructs the compiler to assume that pointer arithmetic overflow
on addition and subtraction wraps around using twos-complement representation.
This flag disables some optimizations which assume pointer overflow is invalid.
-fstrict-overflow
This option implies -fno-wrapv -fno-wrapv-pointer and when negated implies
-fwrapv -fwrapv-pointer.
-fexceptions
Enable exception handling. Generates extra code needed to propagate
exceptions. For some targets, this implies GCC generates frame unwind
information for all functions, which can produce significant data size
overhead, although it does not affect execution. If you do not specify this
option, GCC enables it by default for languages like C++ that normally require
exception handling, and disables it for languages like C that do not normally
require it. However, you may need to enable this option when compiling C code
that needs to interoperate properly with exception handlers written in C++.
You may also wish to disable this option if you are compiling older C++
programs that don't use exception handling.
-fnon-call-exceptions
Generate code that allows trapping instructions to throw exceptions. Note that
this requires platform-specific runtime support that does not exist everywhere.
Moreover, it only allows trapping instructions to throw exceptions, i.e. memory
references or floating-point instructions. It does not allow exceptions to be
thrown from arbitrary signal handlers such as "SIGALRM".
-fdelete-dead-exceptions
Consider that instructions that may throw exceptions but don't otherwise
contribute to the execution of the program can be optimized away. This option
is enabled by default for the Ada front end, as permitted by the Ada language
specification. Optimization passes that cause dead exceptions to be removed
are enabled independently at different optimization levels.
-funwind-tables
Similar to -fexceptions, except that it just generates any needed static data,
but does not affect the generated code in any other way. You normally do not
need to enable this option; instead, a language processor that needs this
handling enables it on your behalf.
-fasynchronous-unwind-tables
Generate unwind table in DWARF format, if supported by target machine. The
table is exact at each instruction boundary, so it can be used for stack
unwinding from asynchronous events (such as debugger or garbage collector).
-fno-gnu-unique
On systems with recent GNU assembler and C library, the C++ compiler uses the
"STB_GNU_UNIQUE" binding to make sure that definitions of template static data
members and static local variables in inline functions are unique even in the
presence of "RTLD_LOCAL"; this is necessary to avoid problems with a library
used by two different "RTLD_LOCAL" plugins depending on a definition in one of
them and therefore disagreeing with the other one about the binding of the
symbol. But this causes "dlclose" to be ignored for affected DSOs; if your
program relies on reinitialization of a DSO via "dlclose" and "dlopen", you can
use -fno-gnu-unique.
-fpcc-struct-return
Return "short" "struct" and "union" values in memory like longer ones, rather
than in registers. This convention is less efficient, but it has the advantage
of allowing intercallability between GCC-compiled files and files compiled with
other compilers, particularly the Portable C Compiler (pcc).
The precise convention for returning structures in memory depends on the target
configuration macros.
Short structures and unions are those whose size and alignment match that of
some integer type.
Warning: code compiled with the -fpcc-struct-return switch is not binary
compatible with code compiled with the -freg-struct-return switch. Use it to
conform to a non-default application binary interface.
-freg-struct-return
Return "struct" and "union" values in registers when possible. This is more
efficient for small structures than -fpcc-struct-return.
If you specify neither -fpcc-struct-return nor -freg-struct-return, GCC
defaults to whichever convention is standard for the target. If there is no
standard convention, GCC defaults to -fpcc-struct-return, except on targets
where GCC is the principal compiler. In those cases, we can choose the
standard, and we chose the more efficient register return alternative.
Warning: code compiled with the -freg-struct-return switch is not binary
compatible with code compiled with the -fpcc-struct-return switch. Use it to
conform to a non-default application binary interface.
-fshort-enums
Allocate to an "enum" type only as many bytes as it needs for the declared
range of possible values. Specifically, the "enum" type is equivalent to the
smallest integer type that has enough room.
Warning: the -fshort-enums switch causes GCC to generate code that is not
binary compatible with code generated without that switch. Use it to conform
to a non-default application binary interface.
-fshort-wchar
Override the underlying type for "wchar_t" to be "short unsigned int" instead
of the default for the target. This option is useful for building programs to
run under WINE.
Warning: the -fshort-wchar switch causes GCC to generate code that is not
binary compatible with code generated without that switch. Use it to conform
to a non-default application binary interface.
-fno-common
In C code, this option controls the placement of global variables defined
without an initializer, known as tentative definitions in the C standard.
Tentative definitions are distinct from declarations of a variable with the
"extern" keyword, which do not allocate storage.
Unix C compilers have traditionally allocated storage for uninitialized global
variables in a common block. This allows the linker to resolve all tentative
definitions of the same variable in different compilation units to the same
object, or to a non-tentative definition. This is the behavior specified by
-fcommon, and is the default for GCC on most targets. On the other hand, this
behavior is not required by ISO C, and on some targets may carry a speed or
code size penalty on variable references.
The -fno-common option specifies that the compiler should instead place
uninitialized global variables in the data section of the object file. This
inhibits the merging of tentative definitions by the linker so you get a
multiple-definition error if the same variable is defined in more than one
compilation unit. Compiling with -fno-common is useful on targets for which it
provides better performance, or if you wish to verify that the program will
work on other systems that always treat uninitialized variable definitions this
way.
-fno-ident
Ignore the "#ident" directive.
-finhibit-size-directive
Don't output a ".size" assembler directive, or anything else that would cause
trouble if the function is split in the middle, and the two halves are placed
at locations far apart in memory. This option is used when compiling
crtstuff.c; you should not need to use it for anything else.
-fverbose-asm
Put extra commentary information in the generated assembly code to make it more
readable. This option is generally only of use to those who actually need to
read the generated assembly code (perhaps while debugging the compiler itself).
-fno-verbose-asm, the default, causes the extra information to be omitted and
is useful when comparing two assembler files.
The added comments include:
* information on the compiler version and command-line options,
* the source code lines associated with the assembly instructions, in the
form FILENAME:LINENUMBER:CONTENT OF LINE,
* hints on which high-level expressions correspond to the various assembly
instruction operands.
For example, given this C source file:
int test (int n)
{
int i;
int total = 0;
for (i = 0; i < n; i++)
total += i * i;
return total;
}
compiling to (x86_64) assembly via -S and emitting the result direct to stdout
via -o -
gcc -S test.c -fverbose-asm -Os -o -
gives output similar to this:
.file "test.c"
# GNU C11 (GCC) version 7.0.0 20160809 (experimental) (x86_64-pc-linux-gnu)
[...snip...]
# options passed:
[...snip...]
.text
.globl test
.type test, @function
test:
.LFB0:
.cfi_startproc
# test.c:4: int total = 0;
xorl %eax, %eax # <retval>
# test.c:6: for (i = 0; i < n; i++)
xorl %edx, %edx # i
.L2:
# test.c:6: for (i = 0; i < n; i++)
cmpl %edi, %edx # n, i
jge .L5 #,
# test.c:7: total += i * i;
movl %edx, %ecx # i, tmp92
imull %edx, %ecx # i, tmp92
# test.c:6: for (i = 0; i < n; i++)
incl %edx # i
# test.c:7: total += i * i;
addl %ecx, %eax # tmp92, <retval>
jmp .L2 #
.L5:
# test.c:10: }
ret
.cfi_endproc
.LFE0:
.size test, .-test
.ident "GCC: (GNU) 7.0.0 20160809 (experimental)"
.section .note.GNU-stack,"",@progbits
The comments are intended for humans rather than machines and hence the precise
format of the comments is subject to change.
-frecord-gcc-switches
This switch causes the command line used to invoke the compiler to be recorded
into the object file that is being created. This switch is only implemented on
some targets and the exact format of the recording is target and binary file
format dependent, but it usually takes the form of a section containing ASCII
text. This switch is related to the -fverbose-asm switch, but that switch only
records information in the assembler output file as comments, so it never
reaches the object file. See also -grecord-gcc-switches for another way of
storing compiler options into the object file.
-fpic
Generate position-independent code (PIC) suitable for use in a shared library,
if supported for the target machine. Such code accesses all constant addresses
through a global offset table (GOT). The dynamic loader resolves the GOT
entries when the program starts (the dynamic loader is not part of GCC; it is
part of the operating system). If the GOT size for the linked executable
exceeds a machine-specific maximum size, you get an error message from the
linker indicating that -fpic does not work; in that case, recompile with -fPIC
instead. (These maximums are 8k on the SPARC, 28k on AArch64 and 32k on the
m68k and RS/6000. The x86 has no such limit.)
Position-independent code requires special support, and therefore works only on
certain machines. For the x86, GCC supports PIC for System V but not for the
Sun 386i. Code generated for the IBM RS/6000 is always position-independent.
When this flag is set, the macros "__pic__" and "__PIC__" are defined to 1.
-fPIC
If supported for the target machine, emit position-independent code, suitable
for dynamic linking and avoiding any limit on the size of the global offset
table. This option makes a difference on AArch64, m68k, PowerPC and SPARC.
Position-independent code requires special support, and therefore works only on
certain machines.
When this flag is set, the macros "__pic__" and "__PIC__" are defined to 2.
-fpie
-fPIE
These options are similar to -fpic and -fPIC, but generated position
independent code can be only linked into executables. Usually these options
are used when -pie GCC option is used during linking.
-fpie and -fPIE both define the macros "__pie__" and "__PIE__". The macros
have the value 1 for -fpie and 2 for -fPIE.
-fno-plt
Do not use the PLT for external function calls in position-independent code.
Instead, load the callee address at call sites from the GOT and branch to it.
This leads to more efficient code by eliminating PLT stubs and exposing GOT
loads to optimizations. On architectures such as 32-bit x86 where PLT stubs
expect the GOT pointer in a specific register, this gives more register
allocation freedom to the compiler. Lazy binding requires use of the PLT; with
-fno-plt all external symbols are resolved at load time.
Alternatively, the function attribute "noplt" can be used to avoid calls
through the PLT for specific external functions.
In position-dependent code, a few targets also convert calls to functions that
are marked to not use the PLT to use the GOT instead.
-fno-jump-tables
Do not use jump tables for switch statements even where it would be more
efficient than other code generation strategies. This option is of use in
conjunction with -fpic or -fPIC for building code that forms part of a dynamic
linker and cannot reference the address of a jump table. On some targets, jump
tables do not require a GOT and this option is not needed.
-ffixed-reg
Treat the register named reg as a fixed register; generated code should never
refer to it (except perhaps as a stack pointer, frame pointer or in some other
fixed role).
reg must be the name of a register. The register names accepted are machine-
specific and are defined in the "REGISTER_NAMES" macro in the machine
description macro file.
This flag does not have a negative form, because it specifies a three-way
choice.
-fcall-used-reg
Treat the register named reg as an allocable register that is clobbered by
function calls. It may be allocated for temporaries or variables that do not
live across a call. Functions compiled this way do not save and restore the
register reg.
It is an error to use this flag with the frame pointer or stack pointer. Use
of this flag for other registers that have fixed pervasive roles in the
machine's execution model produces disastrous results.
This flag does not have a negative form, because it specifies a three-way
choice.
-fcall-saved-reg
Treat the register named reg as an allocable register saved by functions. It
may be allocated even for temporaries or variables that live across a call.
Functions compiled this way save and restore the register reg if they use it.
It is an error to use this flag with the frame pointer or stack pointer. Use
of this flag for other registers that have fixed pervasive roles in the
machine's execution model produces disastrous results.
A different sort of disaster results from the use of this flag for a register
in which function values may be returned.
This flag does not have a negative form, because it specifies a three-way
choice.
-fpack-struct[=n]
Without a value specified, pack all structure members together without holes.
When a value is specified (which must be a small power of two), pack structure
members according to this value, representing the maximum alignment (that is,
objects with default alignment requirements larger than this are output
potentially unaligned at the next fitting location.
Warning: the -fpack-struct switch causes GCC to generate code that is not
binary compatible with code generated without that switch. Additionally, it
makes the code suboptimal. Use it to conform to a non-default application
binary interface.
-fleading-underscore
This option and its counterpart, -fno-leading-underscore, forcibly change the
way C symbols are represented in the object file. One use is to help link with
legacy assembly code.
Warning: the -fleading-underscore switch causes GCC to generate code that is
not binary compatible with code generated without that switch. Use it to
conform to a non-default application binary interface. Not all targets provide
complete support for this switch.
-ftls-model=model
Alter the thread-local storage model to be used. The model argument should be
one of global-dynamic, local-dynamic, initial-exec or local-exec. Note that
the choice is subject to optimization: the compiler may use a more efficient
model for symbols not visible outside of the translation unit, or if -fpic is
not given on the command line.
The default without -fpic is initial-exec; with -fpic the default is global-
dynamic.
-ftrampolines
For targets that normally need trampolines for nested functions, always
generate them instead of using descriptors. Otherwise, for targets that do not
need them, like for example HP-PA or IA-64, do nothing.
A trampoline is a small piece of code that is created at run time on the stack
when the address of a nested function is taken, and is used to call the nested
function indirectly. Therefore, it requires the stack to be made executable in
order for the program to work properly.
-fno-trampolines is enabled by default on a language by language basis to let
the compiler avoid generating them, if it computes that this is safe, and
replace them with descriptors. Descriptors are made up of data only, but the
generated code must be prepared to deal with them. As of this writing,
-fno-trampolines is enabled by default only for Ada.
Moreover, code compiled with -ftrampolines and code compiled with
-fno-trampolines are not binary compatible if nested functions are present.
This option must therefore be used on a program-wide basis and be manipulated
with extreme care.
-fvisibility=[default|internal|hidden|protected]
Set the default ELF image symbol visibility to the specified option---all
symbols are marked with this unless overridden within the code. Using this
feature can very substantially improve linking and load times of shared object
libraries, produce more optimized code, provide near-perfect API export and
prevent symbol clashes. It is strongly recommended that you use this in any
shared objects you distribute.
Despite the nomenclature, default always means public; i.e., available to be
linked against from outside the shared object. protected and internal are
pretty useless in real-world usage so the only other commonly used option is
hidden. The default if -fvisibility isn't specified is default, i.e., make
every symbol public.
A good explanation of the benefits offered by ensuring ELF symbols have the
correct visibility is given by "How To Write Shared Libraries" by Ulrich
Drepper (which can be found at <https://www.akkadia.org/drepper/>)---however a
superior solution made possible by this option to marking things hidden when
the default is public is to make the default hidden and mark things public.
This is the norm with DLLs on Windows and with -fvisibility=hidden and
"__attribute__ ((visibility("default")))" instead of "__declspec(dllexport)"
you get almost identical semantics with identical syntax. This is a great boon
to those working with cross-platform projects.
For those adding visibility support to existing code, you may find "#pragma GCC
visibility" of use. This works by you enclosing the declarations you wish to
set visibility for with (for example) "#pragma GCC visibility push(hidden)" and
"#pragma GCC visibility pop". Bear in mind that symbol visibility should be
viewed as part of the API interface contract and thus all new code should
always specify visibility when it is not the default; i.e., declarations only
for use within the local DSO should always be marked explicitly as hidden as so
to avoid PLT indirection overheads---making this abundantly clear also aids
readability and self-documentation of the code. Note that due to ISO C++
specification requirements, "operator new" and "operator delete" must always be
of default visibility.
Be aware that headers from outside your project, in particular system headers
and headers from any other library you use, may not be expecting to be compiled
with visibility other than the default. You may need to explicitly say
"#pragma GCC visibility push(default)" before including any such headers.
"extern" declarations are not affected by -fvisibility, so a lot of code can be
recompiled with -fvisibility=hidden with no modifications. However, this means
that calls to "extern" functions with no explicit visibility use the PLT, so it
is more effective to use "__attribute ((visibility))" and/or "#pragma GCC
visibility" to tell the compiler which "extern" declarations should be treated
as hidden.
Note that -fvisibility does affect C++ vague linkage entities. This means that,
for instance, an exception class that is be thrown between DSOs must be
explicitly marked with default visibility so that the type_info nodes are
unified between the DSOs.
An overview of these techniques, their benefits and how to use them is at
<http://gcc.gnu.org/wiki/Visibility>.
-fstrict-volatile-bitfields
This option should be used if accesses to volatile bit-fields (or other
structure fields, although the compiler usually honors those types anyway)
should use a single access of the width of the field's type, aligned to a
natural alignment if possible. For example, targets with memory-mapped
peripheral registers might require all such accesses to be 16 bits wide; with
this flag you can declare all peripheral bit-fields as "unsigned short"
(assuming short is 16 bits on these targets) to force GCC to use 16-bit
accesses instead of, perhaps, a more efficient 32-bit access.
If this option is disabled, the compiler uses the most efficient instruction.
In the previous example, that might be a 32-bit load instruction, even though
that accesses bytes that do not contain any portion of the bit-field, or
memory-mapped registers unrelated to the one being updated.
In some cases, such as when the "packed" attribute is applied to a structure
field, it may not be possible to access the field with a single read or write
that is correctly aligned for the target machine. In this case GCC falls back
to generating multiple accesses rather than code that will fault or truncate
the result at run time.
Note: Due to restrictions of the C/C++11 memory model, write accesses are not
allowed to touch non bit-field members. It is therefore recommended to define
all bits of the field's type as bit-field members.
The default value of this option is determined by the application binary
interface for the target processor.
-fsync-libcalls
This option controls whether any out-of-line instance of the "__sync" family of
functions may be used to implement the C++11 "__atomic" family of functions.
The default value of this option is enabled, thus the only useful form of the
option is -fno-sync-libcalls. This option is used in the implementation of the
libatomic runtime library.
GCC Developer Options
This section describes command-line options that are primarily of interest to GCC
developers, including options to support compiler testing and investigation of
compiler bugs and compile-time performance problems. This includes options that
produce debug dumps at various points in the compilation; that print statistics
such as memory use and execution time; and that print information about GCC's
configuration, such as where it searches for libraries. You should rarely need to
use any of these options for ordinary compilation and linking tasks.
-dletters
-fdump-rtl-pass
-fdump-rtl-pass=filename
Says to make debugging dumps during compilation at times specified by letters.
This is used for debugging the RTL-based passes of the compiler. The file
names for most of the dumps are made by appending a pass number and a word to
the dumpname, and the files are created in the directory of the output file.
In case of =filename option, the dump is output on the given file instead of
the pass numbered dump files. Note that the pass number is assigned as passes
are registered into the pass manager. Most passes are registered in the order
that they will execute and for these passes the number corresponds to the pass
execution order. However, passes registered by plugins, passes specific to
compilation targets, or passes that are otherwise registered after all the
other passes are numbered higher than a pass named "final", even if they are
executed earlier. dumpname is generated from the name of the output file if
explicitly specified and not an executable, otherwise it is the basename of the
source file.
Some -dletters switches have different meaning when -E is used for
preprocessing.
Debug dumps can be enabled with a -fdump-rtl switch or some -d option letters.
Here are the possible letters for use in pass and letters, and their meanings:
-fdump-rtl-alignments
Dump after branch alignments have been computed.
-fdump-rtl-asmcons
Dump after fixing rtl statements that have unsatisfied in/out constraints.
-fdump-rtl-auto_inc_dec
Dump after auto-inc-dec discovery. This pass is only run on architectures
that have auto inc or auto dec instructions.
-fdump-rtl-barriers
Dump after cleaning up the barrier instructions.
-fdump-rtl-bbpart
Dump after partitioning hot and cold basic blocks.
-fdump-rtl-bbro
Dump after block reordering.
-fdump-rtl-btl1
-fdump-rtl-btl2
-fdump-rtl-btl1 and -fdump-rtl-btl2 enable dumping after the two branch
target load optimization passes.
-fdump-rtl-bypass
Dump after jump bypassing and control flow optimizations.
-fdump-rtl-combine
Dump after the RTL instruction combination pass.
-fdump-rtl-compgotos
Dump after duplicating the computed gotos.
-fdump-rtl-ce1
-fdump-rtl-ce2
-fdump-rtl-ce3
-fdump-rtl-ce1, -fdump-rtl-ce2, and -fdump-rtl-ce3 enable dumping after the
three if conversion passes.
-fdump-rtl-cprop_hardreg
Dump after hard register copy propagation.
-fdump-rtl-csa
Dump after combining stack adjustments.
-fdump-rtl-cse1
-fdump-rtl-cse2
-fdump-rtl-cse1 and -fdump-rtl-cse2 enable dumping after the two common
subexpression elimination passes.
-fdump-rtl-dce
Dump after the standalone dead code elimination passes.
-fdump-rtl-dbr
Dump after delayed branch scheduling.
-fdump-rtl-dce1
-fdump-rtl-dce2
-fdump-rtl-dce1 and -fdump-rtl-dce2 enable dumping after the two dead store
elimination passes.
-fdump-rtl-eh
Dump after finalization of EH handling code.
-fdump-rtl-eh_ranges
Dump after conversion of EH handling range regions.
-fdump-rtl-expand
Dump after RTL generation.
-fdump-rtl-fwprop1
-fdump-rtl-fwprop2
-fdump-rtl-fwprop1 and -fdump-rtl-fwprop2 enable dumping after the two
forward propagation passes.
-fdump-rtl-gcse1
-fdump-rtl-gcse2
-fdump-rtl-gcse1 and -fdump-rtl-gcse2 enable dumping after global common
subexpression elimination.
-fdump-rtl-init-regs
Dump after the initialization of the registers.
-fdump-rtl-initvals
Dump after the computation of the initial value sets.
-fdump-rtl-into_cfglayout
Dump after converting to cfglayout mode.
-fdump-rtl-ira
Dump after iterated register allocation.
-fdump-rtl-jump
Dump after the second jump optimization.
-fdump-rtl-loop2
-fdump-rtl-loop2 enables dumping after the rtl loop optimization passes.
-fdump-rtl-mach
Dump after performing the machine dependent reorganization pass, if that
pass exists.
-fdump-rtl-mode_sw
Dump after removing redundant mode switches.
-fdump-rtl-rnreg
Dump after register renumbering.
-fdump-rtl-outof_cfglayout
Dump after converting from cfglayout mode.
-fdump-rtl-peephole2
Dump after the peephole pass.
-fdump-rtl-postreload
Dump after post-reload optimizations.
-fdump-rtl-pro_and_epilogue
Dump after generating the function prologues and epilogues.
-fdump-rtl-sched1
-fdump-rtl-sched2
-fdump-rtl-sched1 and -fdump-rtl-sched2 enable dumping after the basic
block scheduling passes.
-fdump-rtl-ree
Dump after sign/zero extension elimination.
-fdump-rtl-seqabstr
Dump after common sequence discovery.
-fdump-rtl-shorten
Dump after shortening branches.
-fdump-rtl-sibling
Dump after sibling call optimizations.
-fdump-rtl-split1
-fdump-rtl-split2
-fdump-rtl-split3
-fdump-rtl-split4
-fdump-rtl-split5
These options enable dumping after five rounds of instruction splitting.
-fdump-rtl-sms
Dump after modulo scheduling. This pass is only run on some architectures.
-fdump-rtl-stack
Dump after conversion from GCC's "flat register file" registers to the
x87's stack-like registers. This pass is only run on x86 variants.
-fdump-rtl-subreg1
-fdump-rtl-subreg2
-fdump-rtl-subreg1 and -fdump-rtl-subreg2 enable dumping after the two
subreg expansion passes.
-fdump-rtl-unshare
Dump after all rtl has been unshared.
-fdump-rtl-vartrack
Dump after variable tracking.
-fdump-rtl-vregs
Dump after converting virtual registers to hard registers.
-fdump-rtl-web
Dump after live range splitting.
-fdump-rtl-regclass
-fdump-rtl-subregs_of_mode_init
-fdump-rtl-subregs_of_mode_finish
-fdump-rtl-dfinit
-fdump-rtl-dfinish
These dumps are defined but always produce empty files.
-da
-fdump-rtl-all
Produce all the dumps listed above.
-dA Annotate the assembler output with miscellaneous debugging information.
-dD Dump all macro definitions, at the end of preprocessing, in addition to
normal output.
-dH Produce a core dump whenever an error occurs.
-dp Annotate the assembler output with a comment indicating which pattern and
alternative is used. The length and cost of each instruction are also
printed.
-dP Dump the RTL in the assembler output as a comment before each instruction.
Also turns on -dp annotation.
-dx Just generate RTL for a function instead of compiling it. Usually used
with -fdump-rtl-expand.
-fdump-noaddr
When doing debugging dumps, suppress address output. This makes it more
feasible to use diff on debugging dumps for compiler invocations with different
compiler binaries and/or different text / bss / data / heap / stack / dso start
locations.
-freport-bug
Collect and dump debug information into a temporary file if an internal
compiler error (ICE) occurs.
-fdump-unnumbered
When doing debugging dumps, suppress instruction numbers and address output.
This makes it more feasible to use diff on debugging dumps for compiler
invocations with different options, in particular with and without -g.
-fdump-unnumbered-links
When doing debugging dumps (see -d option above), suppress instruction numbers
for the links to the previous and next instructions in a sequence.
-fdump-ipa-switch
Control the dumping at various stages of inter-procedural analysis language
tree to a file. The file name is generated by appending a switch specific
suffix to the source file name, and the file is created in the same directory
as the output file. The following dumps are possible:
all Enables all inter-procedural analysis dumps.
cgraph
Dumps information about call-graph optimization, unused function removal,
and inlining decisions.
inline
Dump after function inlining.
-fdump-lang-all
-fdump-lang-switch
-fdump-lang-switch-options
-fdump-lang-switch-options=filename
Control the dumping of language-specific information. The options and filename
portions behave as described in the -fdump-tree option. The following switch
values are accepted:
all Enable all language-specific dumps.
class
Dump class hierarchy information. Virtual table information is emitted
unless 'slim' is specified. This option is applicable to C++ only.
raw Dump the raw internal tree data. This option is applicable to C++ only.
-fdump-passes
Print on stderr the list of optimization passes that are turned on and off by
the current command-line options.
-fdump-statistics-option
Enable and control dumping of pass statistics in a separate file. The file
name is generated by appending a suffix ending in .statistics to the source
file name, and the file is created in the same directory as the output file.
If the -option form is used, -stats causes counters to be summed over the whole
compilation unit while -details dumps every event as the passes generate them.
The default with no option is to sum counters for each function compiled.
-fdump-tree-all
-fdump-tree-switch
-fdump-tree-switch-options
-fdump-tree-switch-options=filename
Control the dumping at various stages of processing the intermediate language
tree to a file. The file name is generated by appending a switch-specific
suffix to the source file name, and the file is created in the same directory
as the output file. In case of =filename option, the dump is output on the
given file instead of the auto named dump files. If the -options form is used,
options is a list of - separated options which control the details of the dump.
Not all options are applicable to all dumps; those that are not meaningful are
ignored. The following options are available
address
Print the address of each node. Usually this is not meaningful as it
changes according to the environment and source file. Its primary use is
for tying up a dump file with a debug environment.
asmname
If "DECL_ASSEMBLER_NAME" has been set for a given decl, use that in the
dump instead of "DECL_NAME". Its primary use is ease of use working
backward from mangled names in the assembly file.
slim
When dumping front-end intermediate representations, inhibit dumping of
members of a scope or body of a function merely because that scope has been
reached. Only dump such items when they are directly reachable by some
other path.
When dumping pretty-printed trees, this option inhibits dumping the bodies
of control structures.
When dumping RTL, print the RTL in slim (condensed) form instead of the
default LISP-like representation.
raw Print a raw representation of the tree. By default, trees are pretty-
printed into a C-like representation.
details
Enable more detailed dumps (not honored by every dump option). Also include
information from the optimization passes.
stats
Enable dumping various statistics about the pass (not honored by every dump
option).
blocks
Enable showing basic block boundaries (disabled in raw dumps).
graph
For each of the other indicated dump files (-fdump-rtl-pass), dump a
representation of the control flow graph suitable for viewing with GraphViz
to file.passid.pass.dot. Each function in the file is pretty-printed as a
subgraph, so that GraphViz can render them all in a single plot.
This option currently only works for RTL dumps, and the RTL is always
dumped in slim form.
vops
Enable showing virtual operands for every statement.
lineno
Enable showing line numbers for statements.
uid Enable showing the unique ID ("DECL_UID") for each variable.
verbose
Enable showing the tree dump for each statement.
eh Enable showing the EH region number holding each statement.
scev
Enable showing scalar evolution analysis details.
optimized
Enable showing optimization information (only available in certain passes).
missed
Enable showing missed optimization information (only available in certain
passes).
note
Enable other detailed optimization information (only available in certain
passes).
=filename
Instead of an auto named dump file, output into the given file name. The
file names stdout and stderr are treated specially and are considered
already open standard streams. For example,
gcc -O2 -ftree-vectorize -fdump-tree-vect-blocks=foo.dump
-fdump-tree-pre=/dev/stderr file.c
outputs vectorizer dump into foo.dump, while the PRE dump is output on to
stderr. If two conflicting dump filenames are given for the same pass, then
the latter option overrides the earlier one.
all Turn on all options, except raw, slim, verbose and lineno.
optall
Turn on all optimization options, i.e., optimized, missed, and note.
To determine what tree dumps are available or find the dump for a pass of
interest follow the steps below.
1. Invoke GCC with -fdump-passes and in the stderr output look for a code that
corresponds to the pass you are interested in. For example, the codes
"tree-evrp", "tree-vrp1", and "tree-vrp2" correspond to the three Value
Range Propagation passes. The number at the end distinguishes distinct
invocations of the same pass.
2. To enable the creation of the dump file, append the pass code to the
-fdump- option prefix and invoke GCC with it. For example, to enable the
dump from the Early Value Range Propagation pass, invoke GCC with the
-fdump-tree-evrp option. Optionally, you may specify the name of the dump
file. If you don't specify one, GCC creates as described below.
3. Find the pass dump in a file whose name is composed of three components
separated by a period: the name of the source file GCC was invoked to
compile, a numeric suffix indicating the pass number followed by the letter
t for tree passes (and the letter r for RTL passes), and finally the pass
code. For example, the Early VRP pass dump might be in a file named
myfile.c.038t.evrp in the current working directory. Note that the numeric
codes are not stable and may change from one version of GCC to another.
-fopt-info
-fopt-info-options
-fopt-info-options=filename
Controls optimization dumps from various optimization passes. If the -options
form is used, options is a list of - separated option keywords to select the
dump details and optimizations.
The options can be divided into two groups: options describing the verbosity of
the dump, and options describing which optimizations should be included. The
options from both the groups can be freely mixed as they are non-overlapping.
However, in case of any conflicts, the later options override the earlier
options on the command line.
The following options control the dump verbosity:
optimized
Print information when an optimization is successfully applied. It is up to
a pass to decide which information is relevant. For example, the vectorizer
passes print the source location of loops which are successfully
vectorized.
missed
Print information about missed optimizations. Individual passes control
which information to include in the output.
note
Print verbose information about optimizations, such as certain
transformations, more detailed messages about decisions etc.
all Print detailed optimization information. This includes optimized, missed,
and note.
One or more of the following option keywords can be used to describe a group of
optimizations:
ipa Enable dumps from all interprocedural optimizations.
loop
Enable dumps from all loop optimizations.
inline
Enable dumps from all inlining optimizations.
omp Enable dumps from all OMP (Offloading and Multi Processing) optimizations.
vec Enable dumps from all vectorization optimizations.
optall
Enable dumps from all optimizations. This is a superset of the optimization
groups listed above.
If options is omitted, it defaults to optimized-optall, which means to dump all
info about successful optimizations from all the passes.
If the filename is provided, then the dumps from all the applicable
optimizations are concatenated into the filename. Otherwise the dump is output
onto stderr. Though multiple -fopt-info options are accepted, only one of them
can include a filename. If other filenames are provided then all but the first
such option are ignored.
Note that the output filename is overwritten in case of multiple translation
units. If a combined output from multiple translation units is desired, stderr
should be used instead.
In the following example, the optimization info is output to stderr:
gcc -O3 -fopt-info
This example:
gcc -O3 -fopt-info-missed=missed.all
outputs missed optimization report from all the passes into missed.all, and
this one:
gcc -O2 -ftree-vectorize -fopt-info-vec-missed
prints information about missed optimization opportunities from vectorization
passes on stderr. Note that -fopt-info-vec-missed is equivalent to
-fopt-info-missed-vec. The order of the optimization group names and message
types listed after -fopt-info does not matter.
As another example,
gcc -O3 -fopt-info-inline-optimized-missed=inline.txt
outputs information about missed optimizations as well as optimized locations
from all the inlining passes into inline.txt.
Finally, consider:
gcc -fopt-info-vec-missed=vec.miss -fopt-info-loop-optimized=loop.opt
Here the two output filenames vec.miss and loop.opt are in conflict since only
one output file is allowed. In this case, only the first option takes effect
and the subsequent options are ignored. Thus only vec.miss is produced which
contains dumps from the vectorizer about missed opportunities.
-fsched-verbose=n
On targets that use instruction scheduling, this option controls the amount of
debugging output the scheduler prints to the dump files.
For n greater than zero, -fsched-verbose outputs the same information as
-fdump-rtl-sched1 and -fdump-rtl-sched2. For n greater than one, it also
output basic block probabilities, detailed ready list information and unit/insn
info. For n greater than two, it includes RTL at abort point, control-flow and
regions info. And for n over four, -fsched-verbose also includes dependence
info.
-fenable-kind-pass
-fdisable-kind-pass=range-list
This is a set of options that are used to explicitly disable/enable
optimization passes. These options are intended for use for debugging GCC.
Compiler users should use regular options for enabling/disabling passes
instead.
-fdisable-ipa-pass
Disable IPA pass pass. pass is the pass name. If the same pass is
statically invoked in the compiler multiple times, the pass name should be
appended with a sequential number starting from 1.
-fdisable-rtl-pass
-fdisable-rtl-pass=range-list
Disable RTL pass pass. pass is the pass name. If the same pass is
statically invoked in the compiler multiple times, the pass name should be
appended with a sequential number starting from 1. range-list is a comma-
separated list of function ranges or assembler names. Each range is a
number pair separated by a colon. The range is inclusive in both ends. If
the range is trivial, the number pair can be simplified as a single number.
If the function's call graph node's uid falls within one of the specified
ranges, the pass is disabled for that function. The uid is shown in the
function header of a dump file, and the pass names can be dumped by using
option -fdump-passes.
-fdisable-tree-pass
-fdisable-tree-pass=range-list
Disable tree pass pass. See -fdisable-rtl for the description of option
arguments.
-fenable-ipa-pass
Enable IPA pass pass. pass is the pass name. If the same pass is
statically invoked in the compiler multiple times, the pass name should be
appended with a sequential number starting from 1.
-fenable-rtl-pass
-fenable-rtl-pass=range-list
Enable RTL pass pass. See -fdisable-rtl for option argument description
and examples.
-fenable-tree-pass
-fenable-tree-pass=range-list
Enable tree pass pass. See -fdisable-rtl for the description of option
arguments.
Here are some examples showing uses of these options.
# disable ccp1 for all functions
-fdisable-tree-ccp1
# disable complete unroll for function whose cgraph node uid is 1
-fenable-tree-cunroll=1
# disable gcse2 for functions at the following ranges [1,1],
# [300,400], and [400,1000]
# disable gcse2 for functions foo and foo2
-fdisable-rtl-gcse2=foo,foo2
# disable early inlining
-fdisable-tree-einline
# disable ipa inlining
-fdisable-ipa-inline
# enable tree full unroll
-fenable-tree-unroll
-fchecking
-fchecking=n
Enable internal consistency checking. The default depends on the compiler
configuration. -fchecking=2 enables further internal consistency checking that
might affect code generation.
-frandom-seed=string
This option provides a seed that GCC uses in place of random numbers in
generating certain symbol names that have to be different in every compiled
file. It is also used to place unique stamps in coverage data files and the
object files that produce them. You can use the -frandom-seed option to
produce reproducibly identical object files.
The string can either be a number (decimal, octal or hex) or an arbitrary
string (in which case it's converted to a number by computing CRC32).
The string should be different for every file you compile.
-save-temps
-save-temps=cwd
Store the usual "temporary" intermediate files permanently; place them in the
current directory and name them based on the source file. Thus, compiling
foo.c with -c -save-temps produces files foo.i and foo.s, as well as foo.o.
This creates a preprocessed foo.i output file even though the compiler now
normally uses an integrated preprocessor.
When used in combination with the -x command-line option, -save-temps is
sensible enough to avoid over writing an input source file with the same
extension as an intermediate file. The corresponding intermediate file may be
obtained by renaming the source file before using -save-temps.
If you invoke GCC in parallel, compiling several different source files that
share a common base name in different subdirectories or the same source file
compiled for multiple output destinations, it is likely that the different
parallel compilers will interfere with each other, and overwrite the temporary
files. For instance:
gcc -save-temps -o outdir1/foo.o indir1/foo.c&
gcc -save-temps -o outdir2/foo.o indir2/foo.c&
may result in foo.i and foo.o being written to simultaneously by both
compilers.
-save-temps=obj
Store the usual "temporary" intermediate files permanently. If the -o option
is used, the temporary files are based on the object file. If the -o option is
not used, the -save-temps=obj switch behaves like -save-temps.
For example:
gcc -save-temps=obj -c foo.c
gcc -save-temps=obj -c bar.c -o dir/xbar.o
gcc -save-temps=obj foobar.c -o dir2/yfoobar
creates foo.i, foo.s, dir/xbar.i, dir/xbar.s, dir2/yfoobar.i, dir2/yfoobar.s,
and dir2/yfoobar.o.
-time[=file]
Report the CPU time taken by each subprocess in the compilation sequence. For
C source files, this is the compiler proper and assembler (plus the linker if
linking is done).
Without the specification of an output file, the output looks like this:
# cc1 0.12 0.01
# as 0.00 0.01
The first number on each line is the "user time", that is time spent executing
the program itself. The second number is "system time", time spent executing
operating system routines on behalf of the program. Both numbers are in
seconds.
With the specification of an output file, the output is appended to the named
file, and it looks like this:
0.12 0.01 cc1 <options>
0.00 0.01 as <options>
The "user time" and the "system time" are moved before the program name, and
the options passed to the program are displayed, so that one can later tell
what file was being compiled, and with which options.
-fdump-final-insns[=file]
Dump the final internal representation (RTL) to file. If the optional argument
is omitted (or if file is "."), the name of the dump file is determined by
appending ".gkd" to the compilation output file name.
-fcompare-debug[=opts]
If no error occurs during compilation, run the compiler a second time, adding
opts and -fcompare-debug-second to the arguments passed to the second
compilation. Dump the final internal representation in both compilations, and
print an error if they differ.
If the equal sign is omitted, the default -gtoggle is used.
The environment variable GCC_COMPARE_DEBUG, if defined, non-empty and nonzero,
implicitly enables -fcompare-debug. If GCC_COMPARE_DEBUG is defined to a
string starting with a dash, then it is used for opts, otherwise the default
-gtoggle is used.
-fcompare-debug=, with the equal sign but without opts, is equivalent to
-fno-compare-debug, which disables the dumping of the final representation and
the second compilation, preventing even GCC_COMPARE_DEBUG from taking effect.
To verify full coverage during -fcompare-debug testing, set GCC_COMPARE_DEBUG
to say -fcompare-debug-not-overridden, which GCC rejects as an invalid option
in any actual compilation (rather than preprocessing, assembly or linking). To
get just a warning, setting GCC_COMPARE_DEBUG to -w%n-fcompare-debug not
overridden will do.
-fcompare-debug-second
This option is implicitly passed to the compiler for the second compilation
requested by -fcompare-debug, along with options to silence warnings, and
omitting other options that would cause the compiler to produce output to files
or to standard output as a side effect. Dump files and preserved temporary
files are renamed so as to contain the ".gk" additional extension during the
second compilation, to avoid overwriting those generated by the first.
When this option is passed to the compiler driver, it causes the first
compilation to be skipped, which makes it useful for little other than
debugging the compiler proper.
-gtoggle
Turn off generation of debug info, if leaving out this option generates it, or
turn it on at level 2 otherwise. The position of this argument in the command
line does not matter; it takes effect after all other options are processed,
and it does so only once, no matter how many times it is given. This is mainly
intended to be used with -fcompare-debug.
-fvar-tracking-assignments-toggle
Toggle -fvar-tracking-assignments, in the same way that -gtoggle toggles -g.
-Q Makes the compiler print out each function name as it is compiled, and print
some statistics about each pass when it finishes.
-ftime-report
Makes the compiler print some statistics about the time consumed by each pass
when it finishes.
-ftime-report-details
Record the time consumed by infrastructure parts separately for each pass.
-fira-verbose=n
Control the verbosity of the dump file for the integrated register allocator.
The default value is 5. If the value n is greater or equal to 10, the dump
output is sent to stderr using the same format as n minus 10.
-flto-report
Prints a report with internal details on the workings of the link-time
optimizer. The contents of this report vary from version to version. It is
meant to be useful to GCC developers when processing object files in LTO mode
(via -flto).
Disabled by default.
-flto-report-wpa
Like -flto-report, but only print for the WPA phase of Link Time Optimization.
-fmem-report
Makes the compiler print some statistics about permanent memory allocation when
it finishes.
-fmem-report-wpa
Makes the compiler print some statistics about permanent memory allocation for
the WPA phase only.
-fpre-ipa-mem-report
-fpost-ipa-mem-report
Makes the compiler print some statistics about permanent memory allocation
before or after interprocedural optimization.
-fprofile-report
Makes the compiler print some statistics about consistency of the (estimated)
profile and effect of individual passes.
-fstack-usage
Makes the compiler output stack usage information for the program, on a per-
function basis. The filename for the dump is made by appending .su to the
auxname. auxname is generated from the name of the output file, if explicitly
specified and it is not an executable, otherwise it is the basename of the
source file. An entry is made up of three fields:
* The name of the function.
* A number of bytes.
* One or more qualifiers: "static", "dynamic", "bounded".
The qualifier "static" means that the function manipulates the stack
statically: a fixed number of bytes are allocated for the frame on function
entry and released on function exit; no stack adjustments are otherwise made in
the function. The second field is this fixed number of bytes.
The qualifier "dynamic" means that the function manipulates the stack
dynamically: in addition to the static allocation described above, stack
adjustments are made in the body of the function, for example to push/pop
arguments around function calls. If the qualifier "bounded" is also present,
the amount of these adjustments is bounded at compile time and the second field
is an upper bound of the total amount of stack used by the function. If it is
not present, the amount of these adjustments is not bounded at compile time and
the second field only represents the bounded part.
-fstats
Emit statistics about front-end processing at the end of the compilation. This
option is supported only by the C++ front end, and the information is generally
only useful to the G++ development team.
-fdbg-cnt-list
Print the name and the counter upper bound for all debug counters.
-fdbg-cnt=counter-value-list
Set the internal debug counter upper bound. counter-value-list is a comma-
separated list of name:value pairs which sets the upper bound of each debug
counter name to value. All debug counters have the initial upper bound of
"UINT_MAX"; thus "dbg_cnt" returns true always unless the upper bound is set by
this option. For example, with -fdbg-cnt=dce:10,tail_call:0, "dbg_cnt(dce)"
returns true only for first 10 invocations.
-print-file-name=library
Print the full absolute name of the library file library that would be used
when linking---and don't do anything else. With this option, GCC does not
compile or link anything; it just prints the file name.
-print-multi-directory
Print the directory name corresponding to the multilib selected by any other
switches present in the command line. This directory is supposed to exist in
GCC_EXEC_PREFIX.
-print-multi-lib
Print the mapping from multilib directory names to compiler switches that
enable them. The directory name is separated from the switches by ;, and each
switch starts with an @ instead of the -, without spaces between multiple
switches. This is supposed to ease shell processing.
-print-multi-os-directory
Print the path to OS libraries for the selected multilib, relative to some lib
subdirectory. If OS libraries are present in the lib subdirectory and no
multilibs are used, this is usually just ., if OS libraries are present in
libsuffix sibling directories this prints e.g. ../lib64, ../lib or ../lib32, or
if OS libraries are present in lib/subdir subdirectories it prints e.g. amd64,
sparcv9 or ev6.
-print-multiarch
Print the path to OS libraries for the selected multiarch, relative to some lib
subdirectory.
-print-prog-name=program
Like -print-file-name, but searches for a program such as cpp.
-print-libgcc-file-name
Same as -print-file-name=libgcc.a.
This is useful when you use -nostdlib or -nodefaultlibs but you do want to link
with libgcc.a. You can do:
gcc -nostdlib <files>... `gcc -print-libgcc-file-name`
-print-search-dirs
Print the name of the configured installation directory and a list of program
and library directories gcc searches---and don't do anything else.
This is useful when gcc prints the error message installation problem, cannot
exec cpp0: No such file or directory. To resolve this you either need to put
cpp0 and the other compiler components where gcc expects to find them, or you
can set the environment variable GCC_EXEC_PREFIX to the directory where you
installed them. Don't forget the trailing /.
-print-sysroot
Print the target sysroot directory that is used during compilation. This is
the target sysroot specified either at configure time or using the --sysroot
option, possibly with an extra suffix that depends on compilation options. If
no target sysroot is specified, the option prints nothing.
-print-sysroot-headers-suffix
Print the suffix added to the target sysroot when searching for headers, or
give an error if the compiler is not configured with such a suffix---and don't
do anything else.
-dumpmachine
Print the compiler's target machine (for example, i686-pc-linux-gnu)---and
don't do anything else.
-dumpversion
Print the compiler version (for example, 3.0, 6.3.0 or 7)---and don't do
anything else. This is the compiler version used in filesystem paths, specs,
can be depending on how the compiler has been configured just a single number
(major version), two numbers separated by dot (major and minor version) or
three numbers separated by dots (major, minor and patchlevel version).
-dumpfullversion
Print the full compiler version, always 3 numbers separated by dots, major,
minor and patchlevel version.
-dumpspecs
Print the compiler's built-in specs---and don't do anything else. (This is
used when GCC itself is being built.)
Machine-Dependent Options
Each target machine supported by GCC can have its own options---for example, to
allow you to compile for a particular processor variant or ABI, or to control
optimizations specific to that machine. By convention, the names of machine-
specific options start with -m.
Some configurations of the compiler also support additional target-specific
options, usually for compatibility with other compilers on the same platform.
AArch64 Options
These options are defined for AArch64 implementations:
-mabi=name
Generate code for the specified data model. Permissible values are ilp32 for
SysV-like data model where int, long int and pointers are 32 bits, and lp64 for
SysV-like data model where int is 32 bits, but long int and pointers are 64
bits.
The default depends on the specific target configuration. Note that the LP64
and ILP32 ABIs are not link-compatible; you must compile your entire program
with the same ABI, and link with a compatible set of libraries.
-mbig-endian
Generate big-endian code. This is the default when GCC is configured for an
aarch64_be-*-* target.
-mgeneral-regs-only
Generate code which uses only the general-purpose registers. This will prevent
the compiler from using floating-point and Advanced SIMD registers but will not
impose any restrictions on the assembler.
-mlittle-endian
Generate little-endian code. This is the default when GCC is configured for an
aarch64-*-* but not an aarch64_be-*-* target.
-mcmodel=tiny
Generate code for the tiny code model. The program and its statically defined
symbols must be within 1MB of each other. Programs can be statically or
dynamically linked.
-mcmodel=small
Generate code for the small code model. The program and its statically defined
symbols must be within 4GB of each other. Programs can be statically or
dynamically linked. This is the default code model.
-mcmodel=large
Generate code for the large code model. This makes no assumptions about
addresses and sizes of sections. Programs can be statically linked only.
-mstrict-align
Avoid generating memory accesses that may not be aligned on a natural object
boundary as described in the architecture specification.
-momit-leaf-frame-pointer
-mno-omit-leaf-frame-pointer
Omit or keep the frame pointer in leaf functions. The former behavior is the
default.
-mtls-dialect=desc
Use TLS descriptors as the thread-local storage mechanism for dynamic accesses
of TLS variables. This is the default.
-mtls-dialect=traditional
Use traditional TLS as the thread-local storage mechanism for dynamic accesses
of TLS variables.
-mtls-size=size
Specify bit size of immediate TLS offsets. Valid values are 12, 24, 32, 48.
This option requires binutils 2.26 or newer.
-mfix-cortex-a53-835769
-mno-fix-cortex-a53-835769
Enable or disable the workaround for the ARM Cortex-A53 erratum number 835769.
This involves inserting a NOP instruction between memory instructions and
64-bit integer multiply-accumulate instructions.
-mfix-cortex-a53-843419
-mno-fix-cortex-a53-843419
Enable or disable the workaround for the ARM Cortex-A53 erratum number 843419.
This erratum workaround is made at link time and this will only pass the
corresponding flag to the linker.
-mlow-precision-recip-sqrt
-mno-low-precision-recip-sqrt
Enable or disable the reciprocal square root approximation. This option only
has an effect if -ffast-math or -funsafe-math-optimizations is used as well.
Enabling this reduces precision of reciprocal square root results to about 16
bits for single precision and to 32 bits for double precision.
-mlow-precision-sqrt
-mno-low-precision-sqrt
Enable or disable the square root approximation. This option only has an
effect if -ffast-math or -funsafe-math-optimizations is used as well. Enabling
this reduces precision of square root results to about 16 bits for single
precision and to 32 bits for double precision. If enabled, it implies
-mlow-precision-recip-sqrt.
-mlow-precision-div
-mno-low-precision-div
Enable or disable the division approximation. This option only has an effect
if -ffast-math or -funsafe-math-optimizations is used as well. Enabling this
reduces precision of division results to about 16 bits for single precision and
to 32 bits for double precision.
-moutline-atomics
-mno-outline-atomics
Enable or disable calls to out-of-line helpers to implement atomic operations.
These helpers will, at runtime, determine if the LSE instructions from
ARMv8.1-A can be used; if not, they will use the load/store-exclusive
instructions that are present in the base ARMv8.0 ISA.
This option is only applicable when compiling for the base ARMv8.0 instruction
set. If using a later revision, e.g. -march=armv8.1-a or -march=armv8-a+lse,
the ARMv8.1-Atomics instructions will be used directly. The same applies when
using -mcpu= when the selected cpu supports the lse feature.
-march=name
Specify the name of the target architecture and, optionally, one or more
feature modifiers. This option has the form -march=arch{+[no]feature}*.
The permissible values for arch are armv8-a, armv8.1-a, armv8.2-a, armv8.3-a or
armv8.4-a or native.
The value armv8.4-a implies armv8.3-a and enables compiler support for the
ARMv8.4-A architecture extensions.
The value armv8.3-a implies armv8.2-a and enables compiler support for the
ARMv8.3-A architecture extensions.
The value armv8.2-a implies armv8.1-a and enables compiler support for the
ARMv8.2-A architecture extensions.
The value armv8.1-a implies armv8-a and enables compiler support for the
ARMv8.1-A architecture extension. In particular, it enables the +crc, +lse,
and +rdma features.
The value native is available on native AArch64 GNU/Linux and causes the
compiler to pick the architecture of the host system. This option has no
effect if the compiler is unable to recognize the architecture of the host
system,
The permissible values for feature are listed in the sub-section on
aarch64-feature-modifiers,,-march and -mcpu Feature Modifiers. Where
conflicting feature modifiers are specified, the right-most feature is used.
GCC uses name to determine what kind of instructions it can emit when
generating assembly code. If -march is specified without either of -mtune or
-mcpu also being specified, the code is tuned to perform well across a range of
target processors implementing the target architecture.
-mtune=name
Specify the name of the target processor for which GCC should tune the
performance of the code. Permissible values for this option are: generic,
cortex-a35, cortex-a53, cortex-a55, cortex-a57, cortex-a72, cortex-a73,
cortex-a75, cortex-a76, ares, neoverse-n1, neoverse-n2, neoverse-v1, zeus,
exynos-m1, falkor, qdf24xx, saphira, xgene1, vulcan, thunderx, thunderxt88,
thunderxt88p1, thunderxt81, thunderxt83, thunderx2t99, cortex-a57.cortex-a53,
cortex-a72.cortex-a53, cortex-a73.cortex-a35, cortex-a73.cortex-a53,
cortex-a75.cortex-a55, native.
The values cortex-a57.cortex-a53, cortex-a72.cortex-a53, cortex-a73.cortex-a35,
cortex-a73.cortex-a53, cortex-a75.cortex-a55 specify that GCC should tune for a
big.LITTLE system.
Additionally on native AArch64 GNU/Linux systems the value native tunes
performance to the host system. This option has no effect if the compiler is
unable to recognize the processor of the host system.
Where none of -mtune=, -mcpu= or -march= are specified, the code is tuned to
perform well across a range of target processors.
This option cannot be suffixed by feature modifiers.
-mcpu=name
Specify the name of the target processor, optionally suffixed by one or more
feature modifiers. This option has the form -mcpu=cpu{+[no]feature}*, where
the permissible values for cpu are the same as those available for -mtune. The
permissible values for feature are documented in the sub-section on
aarch64-feature-modifiers,,-march and -mcpu Feature Modifiers. Where
conflicting feature modifiers are specified, the right-most feature is used.
GCC uses name to determine what kind of instructions it can emit when
generating assembly code (as if by -march) and to determine the target
processor for which to tune for performance (as if by -mtune). Where this
option is used in conjunction with -march or -mtune, those options take
precedence over the appropriate part of this option.
-moverride=string
Override tuning decisions made by the back-end in response to a -mtune= switch.
The syntax, semantics, and accepted values for string in this option are not
guaranteed to be consistent across releases.
This option is only intended to be useful when developing GCC.
-mverbose-cost-dump
Enable verbose cost model dumping in the debug dump files. This option is
provided for use in debugging the compiler.
-mpc-relative-literal-loads
-mno-pc-relative-literal-loads
Enable or disable PC-relative literal loads. With this option literal pools
are accessed using a single instruction and emitted after each function. This
limits the maximum size of functions to 1MB. This is enabled by default for
-mcmodel=tiny.
-msign-return-address=scope
Select the function scope on which return address signing will be applied.
Permissible values are none, which disables return address signing, non-leaf,
which enables pointer signing for functions which are not leaf functions, and
all, which enables pointer signing for all functions. The default value is
none.
-msve-vector-bits=bits
Specify the number of bits in an SVE vector register. This option only has an
effect when SVE is enabled.
GCC supports two forms of SVE code generation: "vector-length agnostic" output
that works with any size of vector register and "vector-length specific" output
that allows GCC to make assumptions about the vector length when it is useful
for optimization reasons. The possible values of bits are: scalable, 128, 256,
512, 1024 and 2048. Specifying scalable selects vector-length agnostic output.
At present -msve-vector-bits=128 also generates vector-length agnostic output.
All other values generate vector-length specific code. The behavior of these
values may change in future releases and no value except scalable should be
relied on for producing code that is portable across different hardware SVE
vector lengths.
The default is -msve-vector-bits=scalable, which produces vector-length
agnostic code.
-march and -mcpu Feature Modifiers
Feature modifiers used with -march and -mcpu can be any of the following and their
inverses nofeature:
crc Enable CRC extension. This is on by default for -march=armv8.1-a.
crypto
Enable Crypto extension. This also enables Advanced SIMD and floating-point
instructions.
fp Enable floating-point instructions. This is on by default for all possible
values for options -march and -mcpu.
simd
Enable Advanced SIMD instructions. This also enables floating-point
instructions. This is on by default for all possible values for options -march
and -mcpu.
sve Enable Scalable Vector Extension instructions. This also enables Advanced SIMD
and floating-point instructions.
lse Enable Large System Extension instructions. This is on by default for
-march=armv8.1-a.
rdma
Enable Round Double Multiply Accumulate instructions. This is on by default
for -march=armv8.1-a.
fp16
Enable FP16 extension. This also enables floating-point instructions.
fp16fml
Enable FP16 fmla extension. This also enables FP16 extensions and floating-
point instructions. This option is enabled by default for -march=armv8.4-a. Use
of this option with architectures prior to Armv8.2-A is not supported.
rcpc
Enable the RcPc extension. This does not change code generation from GCC, but
is passed on to the assembler, enabling inline asm statements to use
instructions from the RcPc extension.
dotprod
Enable the Dot Product extension. This also enables Advanced SIMD
instructions.
aes Enable the Armv8-a aes and pmull crypto extension. This also enables Advanced
SIMD instructions.
sha2
Enable the Armv8-a sha2 crypto extension. This also enables Advanced SIMD
instructions.
sha3
Enable the sha512 and sha3 crypto extension. This also enables Advanced SIMD
instructions. Use of this option with architectures prior to Armv8.2-A is not
supported.
sm4 Enable the sm3 and sm4 crypto extension. This also enables Advanced SIMD
instructions. Use of this option with architectures prior to Armv8.2-A is not
supported.
Feature crypto implies aes, sha2, and simd, which implies fp. Conversely, nofp
implies nosimd, which implies nocrypto, noaes and nosha2.
Adapteva Epiphany Options
These -m options are defined for Adapteva Epiphany:
-mhalf-reg-file
Don't allocate any register in the range "r32"..."r63". That allows code to
run on hardware variants that lack these registers.
-mprefer-short-insn-regs
Preferentially allocate registers that allow short instruction generation.
This can result in increased instruction count, so this may either reduce or
increase overall code size.
-mbranch-cost=num
Set the cost of branches to roughly num "simple" instructions. This cost is
only a heuristic and is not guaranteed to produce consistent results across
releases.
-mcmove
Enable the generation of conditional moves.
-mnops=num
Emit num NOPs before every other generated instruction.
-mno-soft-cmpsf
For single-precision floating-point comparisons, emit an "fsub" instruction and
test the flags. This is faster than a software comparison, but can get
incorrect results in the presence of NaNs, or when two different small numbers
are compared such that their difference is calculated as zero. The default is
-msoft-cmpsf, which uses slower, but IEEE-compliant, software comparisons.
-mstack-offset=num
Set the offset between the top of the stack and the stack pointer. E.g., a
value of 8 means that the eight bytes in the range "sp+0...sp+7" can be used by
leaf functions without stack allocation. Values other than 8 or 16 are
untested and unlikely to work. Note also that this option changes the ABI;
compiling a program with a different stack offset than the libraries have been
compiled with generally does not work. This option can be useful if you want
to evaluate if a different stack offset would give you better code, but to
actually use a different stack offset to build working programs, it is
recommended to configure the toolchain with the appropriate
--with-stack-offset=num option.
-mno-round-nearest
Make the scheduler assume that the rounding mode has been set to truncating.
The default is -mround-nearest.
-mlong-calls
If not otherwise specified by an attribute, assume all calls might be beyond
the offset range of the "b" / "bl" instructions, and therefore load the
function address into a register before performing a (otherwise direct) call.
This is the default.
-mshort-calls
If not otherwise specified by an attribute, assume all direct calls are in the
range of the "b" / "bl" instructions, so use these instructions for direct
calls. The default is -mlong-calls.
-msmall16
Assume addresses can be loaded as 16-bit unsigned values. This does not apply
to function addresses for which -mlong-calls semantics are in effect.
-mfp-mode=mode
Set the prevailing mode of the floating-point unit. This determines the
floating-point mode that is provided and expected at function call and return
time. Making this mode match the mode you predominantly need at function start
can make your programs smaller and faster by avoiding unnecessary mode
switches.
mode can be set to one the following values:
caller
Any mode at function entry is valid, and retained or restored when the
function returns, and when it calls other functions. This mode is useful
for compiling libraries or other compilation units you might want to
incorporate into different programs with different prevailing FPU modes,
and the convenience of being able to use a single object file outweighs the
size and speed overhead for any extra mode switching that might be needed,
compared with what would be needed with a more specific choice of
prevailing FPU mode.
truncate
This is the mode used for floating-point calculations with truncating (i.e.
round towards zero) rounding mode. That includes conversion from floating
point to integer.
round-nearest
This is the mode used for floating-point calculations with round-to-
nearest-or-even rounding mode.
int This is the mode used to perform integer calculations in the FPU, e.g.
integer multiply, or integer multiply-and-accumulate.
The default is -mfp-mode=caller
-mnosplit-lohi
-mno-postinc
-mno-postmodify
Code generation tweaks that disable, respectively, splitting of 32-bit loads,
generation of post-increment addresses, and generation of post-modify
addresses. The defaults are msplit-lohi, -mpost-inc, and -mpost-modify.
-mnovect-double
Change the preferred SIMD mode to SImode. The default is -mvect-double, which
uses DImode as preferred SIMD mode.
-max-vect-align=num
The maximum alignment for SIMD vector mode types. num may be 4 or 8. The
default is 8. Note that this is an ABI change, even though many library
function interfaces are unaffected if they don't use SIMD vector modes in
places that affect size and/or alignment of relevant types.
-msplit-vecmove-early
Split vector moves into single word moves before reload. In theory this can
give better register allocation, but so far the reverse seems to be generally
the case.
-m1reg-reg
Specify a register to hold the constant -1, which makes loading small negative
constants and certain bitmasks faster. Allowable values for reg are r43 and
r63, which specify use of that register as a fixed register, and none, which
means that no register is used for this purpose. The default is -m1reg-none.
ARC Options
The following options control the architecture variant for which code is being
compiled:
-mbarrel-shifter
Generate instructions supported by barrel shifter. This is the default unless
-mcpu=ARC601 or -mcpu=ARCEM is in effect.
-mjli-always
Force to call a function using jli_s instruction. This option is valid only
for ARCv2 architecture.
-mcpu=cpu
Set architecture type, register usage, and instruction scheduling parameters
for cpu. There are also shortcut alias options available for backward
compatibility and convenience. Supported values for cpu are
arc600
Compile for ARC600. Aliases: -mA6, -mARC600.
arc601
Compile for ARC601. Alias: -mARC601.
arc700
Compile for ARC700. Aliases: -mA7, -mARC700. This is the default when
configured with --with-cpu=arc700.
arcem
Compile for ARC EM.
archs
Compile for ARC HS.
em Compile for ARC EM CPU with no hardware extensions.
em4 Compile for ARC EM4 CPU.
em4_dmips
Compile for ARC EM4 DMIPS CPU.
em4_fpus
Compile for ARC EM4 DMIPS CPU with the single-precision floating-point
extension.
em4_fpuda
Compile for ARC EM4 DMIPS CPU with single-precision floating-point and
double assist instructions.
hs Compile for ARC HS CPU with no hardware extensions except the atomic
instructions.
hs34
Compile for ARC HS34 CPU.
hs38
Compile for ARC HS38 CPU.
hs38_linux
Compile for ARC HS38 CPU with all hardware extensions on.
arc600_norm
Compile for ARC 600 CPU with "norm" instructions enabled.
arc600_mul32x16
Compile for ARC 600 CPU with "norm" and 32x16-bit multiply instructions
enabled.
arc600_mul64
Compile for ARC 600 CPU with "norm" and "mul64"-family instructions
enabled.
arc601_norm
Compile for ARC 601 CPU with "norm" instructions enabled.
arc601_mul32x16
Compile for ARC 601 CPU with "norm" and 32x16-bit multiply instructions
enabled.
arc601_mul64
Compile for ARC 601 CPU with "norm" and "mul64"-family instructions
enabled.
nps400
Compile for ARC 700 on NPS400 chip.
em_mini
Compile for ARC EM minimalist configuration featuring reduced register set.
-mdpfp
-mdpfp-compact
Generate double-precision FPX instructions, tuned for the compact
implementation.
-mdpfp-fast
Generate double-precision FPX instructions, tuned for the fast implementation.
-mno-dpfp-lrsr
Disable "lr" and "sr" instructions from using FPX extension aux registers.
-mea
Generate extended arithmetic instructions. Currently only "divaw", "adds",
"subs", and "sat16" are supported. This is always enabled for -mcpu=ARC700.
-mno-mpy
Do not generate "mpy"-family instructions for ARC700. This option is
deprecated.
-mmul32x16
Generate 32x16-bit multiply and multiply-accumulate instructions.
-mmul64
Generate "mul64" and "mulu64" instructions. Only valid for -mcpu=ARC600.
-mnorm
Generate "norm" instructions. This is the default if -mcpu=ARC700 is in
effect.
-mspfp
-mspfp-compact
Generate single-precision FPX instructions, tuned for the compact
implementation.
-mspfp-fast
Generate single-precision FPX instructions, tuned for the fast implementation.
-msimd
Enable generation of ARC SIMD instructions via target-specific builtins. Only
valid for -mcpu=ARC700.
-msoft-float
This option ignored; it is provided for compatibility purposes only. Software
floating-point code is emitted by default, and this default can overridden by
FPX options; -mspfp, -mspfp-compact, or -mspfp-fast for single precision, and
-mdpfp, -mdpfp-compact, or -mdpfp-fast for double precision.
-mswap
Generate "swap" instructions.
-matomic
This enables use of the locked load/store conditional extension to implement
atomic memory built-in functions. Not available for ARC 6xx or ARC EM cores.
-mdiv-rem
Enable "div" and "rem" instructions for ARCv2 cores.
-mcode-density
Enable code density instructions for ARC EM. This option is on by default for
ARC HS.
-mll64
Enable double load/store operations for ARC HS cores.
-mtp-regno=regno
Specify thread pointer register number.
-mmpy-option=multo
Compile ARCv2 code with a multiplier design option. You can specify the option
using either a string or numeric value for multo. wlh1 is the default value.
The recognized values are:
0
none
No multiplier available.
1
w 16x16 multiplier, fully pipelined. The following instructions are enabled:
"mpyw" and "mpyuw".
2
wlh1
32x32 multiplier, fully pipelined (1 stage). The following instructions
are additionally enabled: "mpy", "mpyu", "mpym", "mpymu", and "mpy_s".
3
wlh2
32x32 multiplier, fully pipelined (2 stages). The following instructions
are additionally enabled: "mpy", "mpyu", "mpym", "mpymu", and "mpy_s".
4
wlh3
Two 16x16 multipliers, blocking, sequential. The following instructions
are additionally enabled: "mpy", "mpyu", "mpym", "mpymu", and "mpy_s".
5
wlh4
One 16x16 multiplier, blocking, sequential. The following instructions are
additionally enabled: "mpy", "mpyu", "mpym", "mpymu", and "mpy_s".
6
wlh5
One 32x4 multiplier, blocking, sequential. The following instructions are
additionally enabled: "mpy", "mpyu", "mpym", "mpymu", and "mpy_s".
7
plus_dmpy
ARC HS SIMD support.
8
plus_macd
ARC HS SIMD support.
9
plus_qmacw
ARC HS SIMD support.
This option is only available for ARCv2 cores.
-mfpu=fpu
Enables support for specific floating-point hardware extensions for ARCv2
cores. Supported values for fpu are:
fpus
Enables support for single-precision floating-point hardware extensions.
fpud
Enables support for double-precision floating-point hardware extensions.
The single-precision floating-point extension is also enabled. Not
available for ARC EM.
fpuda
Enables support for double-precision floating-point hardware extensions
using double-precision assist instructions. The single-precision floating-
point extension is also enabled. This option is only available for ARC EM.
fpuda_div
Enables support for double-precision floating-point hardware extensions
using double-precision assist instructions. The single-precision floating-
point, square-root, and divide extensions are also enabled. This option is
only available for ARC EM.
fpuda_fma
Enables support for double-precision floating-point hardware extensions
using double-precision assist instructions. The single-precision floating-
point and fused multiply and add hardware extensions are also enabled.
This option is only available for ARC EM.
fpuda_all
Enables support for double-precision floating-point hardware extensions
using double-precision assist instructions. All single-precision floating-
point hardware extensions are also enabled. This option is only available
for ARC EM.
fpus_div
Enables support for single-precision floating-point, square-root and divide
hardware extensions.
fpud_div
Enables support for double-precision floating-point, square-root and divide
hardware extensions. This option includes option fpus_div. Not available
for ARC EM.
fpus_fma
Enables support for single-precision floating-point and fused multiply and
add hardware extensions.
fpud_fma
Enables support for double-precision floating-point and fused multiply and
add hardware extensions. This option includes option fpus_fma. Not
available for ARC EM.
fpus_all
Enables support for all single-precision floating-point hardware
extensions.
fpud_all
Enables support for all single- and double-precision floating-point
hardware extensions. Not available for ARC EM.
-mirq-ctrl-saved=register-range, blink, lp_count
Specifies general-purposes registers that the processor automatically
saves/restores on interrupt entry and exit. register-range is specified as two
registers separated by a dash. The register range always starts with "r0", the
upper limit is "fp" register. blink and lp_count are optional. This option is
only valid for ARC EM and ARC HS cores.
-mrgf-banked-regs=number
Specifies the number of registers replicated in second register bank on entry
to fast interrupt. Fast interrupts are interrupts with the highest priority
level P0. These interrupts save only PC and STATUS32 registers to avoid memory
transactions during interrupt entry and exit sequences. Use this option when
you are using fast interrupts in an ARC V2 family processor. Permitted values
are 4, 8, 16, and 32.
-mlpc-width=width
Specify the width of the "lp_count" register. Valid values for width are 8,
16, 20, 24, 28 and 32 bits. The default width is fixed to 32 bits. If the
width is less than 32, the compiler does not attempt to transform loops in your
program to use the zero-delay loop mechanism unless it is known that the
"lp_count" register can hold the required loop-counter value. Depending on the
width specified, the compiler and run-time library might continue to use the
loop mechanism for various needs. This option defines macro
"__ARC_LPC_WIDTH__" with the value of width.
-mrf16
This option instructs the compiler to generate code for a 16-entry register
file. This option defines the "__ARC_RF16__" preprocessor macro.
The following options are passed through to the assembler, and also define
preprocessor macro symbols.
-mdsp-packa
Passed down to the assembler to enable the DSP Pack A extensions. Also sets
the preprocessor symbol "__Xdsp_packa". This option is deprecated.
-mdvbf
Passed down to the assembler to enable the dual Viterbi butterfly extension.
Also sets the preprocessor symbol "__Xdvbf". This option is deprecated.
-mlock
Passed down to the assembler to enable the locked load/store conditional
extension. Also sets the preprocessor symbol "__Xlock".
-mmac-d16
Passed down to the assembler. Also sets the preprocessor symbol "__Xxmac_d16".
This option is deprecated.
-mmac-24
Passed down to the assembler. Also sets the preprocessor symbol "__Xxmac_24".
This option is deprecated.
-mrtsc
Passed down to the assembler to enable the 64-bit time-stamp counter extension
instruction. Also sets the preprocessor symbol "__Xrtsc". This option is
deprecated.
-mswape
Passed down to the assembler to enable the swap byte ordering extension
instruction. Also sets the preprocessor symbol "__Xswape".
-mtelephony
Passed down to the assembler to enable dual- and single-operand instructions
for telephony. Also sets the preprocessor symbol "__Xtelephony". This option
is deprecated.
-mxy
Passed down to the assembler to enable the XY memory extension. Also sets the
preprocessor symbol "__Xxy".
The following options control how the assembly code is annotated:
-misize
Annotate assembler instructions with estimated addresses.
-mannotate-align
Explain what alignment considerations lead to the decision to make an
instruction short or long.
The following options are passed through to the linker:
-marclinux
Passed through to the linker, to specify use of the "arclinux" emulation. This
option is enabled by default in tool chains built for "arc-linux-uclibc" and
"arceb-linux-uclibc" targets when profiling is not requested.
-marclinux_prof
Passed through to the linker, to specify use of the "arclinux_prof" emulation.
This option is enabled by default in tool chains built for "arc-linux-uclibc"
and "arceb-linux-uclibc" targets when profiling is requested.
The following options control the semantics of generated code:
-mlong-calls
Generate calls as register indirect calls, thus providing access to the full
32-bit address range.
-mmedium-calls
Don't use less than 25-bit addressing range for calls, which is the offset
available for an unconditional branch-and-link instruction. Conditional
execution of function calls is suppressed, to allow use of the 25-bit range,
rather than the 21-bit range with conditional branch-and-link. This is the
default for tool chains built for "arc-linux-uclibc" and "arceb-linux-uclibc"
targets.
-G num
Put definitions of externally-visible data in a small data section if that data
is no bigger than num bytes. The default value of num is 4 for any ARC
configuration, or 8 when we have double load/store operations.
-mno-sdata
Do not generate sdata references. This is the default for tool chains built
for "arc-linux-uclibc" and "arceb-linux-uclibc" targets.
-mvolatile-cache
Use ordinarily cached memory accesses for volatile references. This is the
default.
-mno-volatile-cache
Enable cache bypass for volatile references.
The following options fine tune code generation:
-malign-call
Do alignment optimizations for call instructions.
-mauto-modify-reg
Enable the use of pre/post modify with register displacement.
-mbbit-peephole
Enable bbit peephole2.
-mno-brcc
This option disables a target-specific pass in arc_reorg to generate compare-
and-branch ("brcc") instructions. It has no effect on generation of these
instructions driven by the combiner pass.
-mcase-vector-pcrel
Use PC-relative switch case tables to enable case table shortening. This is
the default for -Os.
-mcompact-casesi
Enable compact "casesi" pattern. This is the default for -Os, and only
available for ARCv1 cores.
-mno-cond-exec
Disable the ARCompact-specific pass to generate conditional execution
instructions.
Due to delay slot scheduling and interactions between operand numbers, literal
sizes, instruction lengths, and the support for conditional execution, the
target-independent pass to generate conditional execution is often lacking, so
the ARC port has kept a special pass around that tries to find more conditional
execution generation opportunities after register allocation, branch
shortening, and delay slot scheduling have been done. This pass generally, but
not always, improves performance and code size, at the cost of extra
compilation time, which is why there is an option to switch it off. If you
have a problem with call instructions exceeding their allowable offset range
because they are conditionalized, you should consider using -mmedium-calls
instead.
-mearly-cbranchsi
Enable pre-reload use of the "cbranchsi" pattern.
-mexpand-adddi
Expand "adddi3" and "subdi3" at RTL generation time into "add.f", "adc" etc.
This option is deprecated.
-mindexed-loads
Enable the use of indexed loads. This can be problematic because some
optimizers then assume that indexed stores exist, which is not the case.
-mlra
Enable Local Register Allocation. This is still experimental for ARC, so by
default the compiler uses standard reload (i.e. -mno-lra).
-mlra-priority-none
Don't indicate any priority for target registers.
-mlra-priority-compact
Indicate target register priority for r0..r3 / r12..r15.
-mlra-priority-noncompact
Reduce target register priority for r0..r3 / r12..r15.
-mno-millicode
When optimizing for size (using -Os), prologues and epilogues that have to save
or restore a large number of registers are often shortened by using call to a
special function in libgcc; this is referred to as a millicode call. As these
calls can pose performance issues, and/or cause linking issues when linking in
a nonstandard way, this option is provided to turn off millicode call
generation.
-mmixed-code
Tweak register allocation to help 16-bit instruction generation. This
generally has the effect of decreasing the average instruction size while
increasing the instruction count.
-mq-class
Enable q instruction alternatives. This is the default for -Os.
-mRcq
Enable Rcq constraint handling. Most short code generation depends on this.
This is the default.
-mRcw
Enable Rcw constraint handling. Most ccfsm condexec mostly depends on this.
This is the default.
-msize-level=level
Fine-tune size optimization with regards to instruction lengths and alignment.
The recognized values for level are:
0 No size optimization. This level is deprecated and treated like 1.
1 Short instructions are used opportunistically.
2 In addition, alignment of loops and of code after barriers are dropped.
3 In addition, optional data alignment is dropped, and the option Os is
enabled.
This defaults to 3 when -Os is in effect. Otherwise, the behavior when this is
not set is equivalent to level 1.
-mtune=cpu
Set instruction scheduling parameters for cpu, overriding any implied by
-mcpu=.
Supported values for cpu are
ARC600
Tune for ARC600 CPU.
ARC601
Tune for ARC601 CPU.
ARC700
Tune for ARC700 CPU with standard multiplier block.
ARC700-xmac
Tune for ARC700 CPU with XMAC block.
ARC725D
Tune for ARC725D CPU.
ARC750D
Tune for ARC750D CPU.
-mmultcost=num
Cost to assume for a multiply instruction, with 4 being equal to a normal
instruction.
-munalign-prob-threshold=probability
Set probability threshold for unaligning branches. When tuning for ARC700 and
optimizing for speed, branches without filled delay slot are preferably emitted
unaligned and long, unless profiling indicates that the probability for the
branch to be taken is below probability. The default is (REG_BR_PROB_BASE/2),
i.e. 5000.
The following options are maintained for backward compatibility, but are now
deprecated and will be removed in a future release:
-margonaut
Obsolete FPX.
-mbig-endian
-EB Compile code for big-endian targets. Use of these options is now deprecated.
Big-endian code is supported by configuring GCC to build "arceb-elf32" and
"arceb-linux-uclibc" targets, for which big endian is the default.
-mlittle-endian
-EL Compile code for little-endian targets. Use of these options is now
deprecated. Little-endian code is supported by configuring GCC to build
"arc-elf32" and "arc-linux-uclibc" targets, for which little endian is the
default.
-mbarrel_shifter
Replaced by -mbarrel-shifter.
-mdpfp_compact
Replaced by -mdpfp-compact.
-mdpfp_fast
Replaced by -mdpfp-fast.
-mdsp_packa
Replaced by -mdsp-packa.
-mEA
Replaced by -mea.
-mmac_24
Replaced by -mmac-24.
-mmac_d16
Replaced by -mmac-d16.
-mspfp_compact
Replaced by -mspfp-compact.
-mspfp_fast
Replaced by -mspfp-fast.
-mtune=cpu
Values arc600, arc601, arc700 and arc700-xmac for cpu are replaced by ARC600,
ARC601, ARC700 and ARC700-xmac respectively.
-multcost=num
Replaced by -mmultcost.
ARM Options
These -m options are defined for the ARM port:
-mabi=name
Generate code for the specified ABI. Permissible values are: apcs-gnu, atpcs,
aapcs, aapcs-linux and iwmmxt.
-mapcs-frame
Generate a stack frame that is compliant with the ARM Procedure Call Standard
for all functions, even if this is not strictly necessary for correct execution
of the code. Specifying -fomit-frame-pointer with this option causes the stack
frames not to be generated for leaf functions. The default is -mno-apcs-frame.
This option is deprecated.
-mapcs
This is a synonym for -mapcs-frame and is deprecated.
-mthumb-interwork
Generate code that supports calling between the ARM and Thumb instruction sets.
Without this option, on pre-v5 architectures, the two instruction sets cannot
be reliably used inside one program. The default is -mno-thumb-interwork,
since slightly larger code is generated when -mthumb-interwork is specified.
In AAPCS configurations this option is meaningless.
-mno-sched-prolog
Prevent the reordering of instructions in the function prologue, or the merging
of those instruction with the instructions in the function's body. This means
that all functions start with a recognizable set of instructions (or in fact
one of a choice from a small set of different function prologues), and this
information can be used to locate the start of functions inside an executable
piece of code. The default is -msched-prolog.
-mfloat-abi=name
Specifies which floating-point ABI to use. Permissible values are: soft,
softfp and hard.
Specifying soft causes GCC to generate output containing library calls for
floating-point operations. softfp allows the generation of code using hardware
floating-point instructions, but still uses the soft-float calling conventions.
hard allows generation of floating-point instructions and uses FPU-specific
calling conventions.
The default depends on the specific target configuration. Note that the hard-
float and soft-float ABIs are not link-compatible; you must compile your entire
program with the same ABI, and link with a compatible set of libraries.
-mlittle-endian
Generate code for a processor running in little-endian mode. This is the
default for all standard configurations.
-mbig-endian
Generate code for a processor running in big-endian mode; the default is to
compile code for a little-endian processor.
-mbe8
-mbe32
When linking a big-endian image select between BE8 and BE32 formats. The
option has no effect for little-endian images and is ignored. The default is
dependent on the selected target architecture. For ARMv6 and later
architectures the default is BE8, for older architectures the default is BE32.
BE32 format has been deprecated by ARM.
-march=name[+extension...]
This specifies the name of the target ARM architecture. GCC uses this name to
determine what kind of instructions it can emit when generating assembly code.
This option can be used in conjunction with or instead of the -mcpu= option.
Permissible names are: armv4t, armv5t, armv5te, armv6, armv6j, armv6k, armv6kz,
armv6t2, armv6z, armv6zk, armv7, armv7-a, armv7ve, armv8-a, armv8.1-a,
armv8.2-a, armv8.3-a, armv8.4-a, armv7-r, armv8-r, armv6-m, armv6s-m, armv7-m,
armv7e-m, armv8-m.base, armv8-m.main, iwmmxt and iwmmxt2.
Additionally, the following architectures, which lack support for the Thumb
execution state, are recognized but support is deprecated: armv2, armv2a,
armv3, armv3m, armv4, armv5 and armv5e.
Many of the architectures support extensions. These can be added by appending
+extension to the architecture name. Extension options are processed in order
and capabilities accumulate. An extension will also enable any necessary base
extensions upon which it depends. For example, the +crypto extension will
always enable the +simd extension. The exception to the additive construction
is for extensions that are prefixed with +no...: these extensions disable the
specified option and any other extensions that may depend on the presence of
that extension.
For example, -march=armv7-a+simd+nofp+vfpv4 is equivalent to writing
-march=armv7-a+vfpv4 since the +simd option is entirely disabled by the +nofp
option that follows it.
Most extension names are generically named, but have an effect that is
dependent upon the architecture to which it is applied. For example, the +simd
option can be applied to both armv7-a and armv8-a architectures, but will
enable the original ARMv7-A Advanced SIMD (Neon) extensions for armv7-a and the
ARMv8-A variant for armv8-a.
The table below lists the supported extensions for each architecture.
Architectures not mentioned do not support any extensions.
armv5e
armv5te
armv6
armv6j
armv6k
armv6kz
armv6t2
armv6z
armv6zk
+fp The VFPv2 floating-point instructions. The extension +vfpv2 can be
used as an alias for this extension.
+nofp
Disable the floating-point instructions.
armv7
The common subset of the ARMv7-A, ARMv7-R and ARMv7-M architectures.
+fp The VFPv3 floating-point instructions, with 16 double-precision
registers. The extension +vfpv3-d16 can be used as an alias for this
extension. Note that floating-point is not supported by the base
ARMv7-M architecture, but is compatible with both the ARMv7-A and
ARMv7-R architectures.
+nofp
Disable the floating-point instructions.
armv7-a
+mp The multiprocessing extension.
+sec
The security extension.
+fp The VFPv3 floating-point instructions, with 16 double-precision
registers. The extension +vfpv3-d16 can be used as an alias for this
extension.
+simd
The Advanced SIMD (Neon) v1 and the VFPv3 floating-point instructions.
The extensions +neon and +neon-vfpv3 can be used as aliases for this
extension.
+vfpv3
The VFPv3 floating-point instructions, with 32 double-precision
registers.
+vfpv3-d16-fp16
The VFPv3 floating-point instructions, with 16 double-precision
registers and the half-precision floating-point conversion operations.
+vfpv3-fp16
The VFPv3 floating-point instructions, with 32 double-precision
registers and the half-precision floating-point conversion operations.
+vfpv4-d16
The VFPv4 floating-point instructions, with 16 double-precision
registers.
+vfpv4
The VFPv4 floating-point instructions, with 32 double-precision
registers.
+neon-fp16
The Advanced SIMD (Neon) v1 and the VFPv3 floating-point instructions,
with the half-precision floating-point conversion operations.
+neon-vfpv4
The Advanced SIMD (Neon) v2 and the VFPv4 floating-point instructions.
+nosimd
Disable the Advanced SIMD instructions (does not disable floating
point).
+nofp
Disable the floating-point and Advanced SIMD instructions.
armv7ve
The extended version of the ARMv7-A architecture with support for
virtualization.
+fp The VFPv4 floating-point instructions, with 16 double-precision
registers. The extension +vfpv4-d16 can be used as an alias for this
extension.
+simd
The Advanced SIMD (Neon) v2 and the VFPv4 floating-point instructions.
The extension +neon-vfpv4 can be used as an alias for this extension.
+vfpv3-d16
The VFPv3 floating-point instructions, with 16 double-precision
registers.
+vfpv3
The VFPv3 floating-point instructions, with 32 double-precision
registers.
+vfpv3-d16-fp16
The VFPv3 floating-point instructions, with 16 double-precision
registers and the half-precision floating-point conversion operations.
+vfpv3-fp16
The VFPv3 floating-point instructions, with 32 double-precision
registers and the half-precision floating-point conversion operations.
+vfpv4-d16
The VFPv4 floating-point instructions, with 16 double-precision
registers.
+vfpv4
The VFPv4 floating-point instructions, with 32 double-precision
registers.
+neon
The Advanced SIMD (Neon) v1 and the VFPv3 floating-point instructions.
The extension +neon-vfpv3 can be used as an alias for this extension.
+neon-fp16
The Advanced SIMD (Neon) v1 and the VFPv3 floating-point instructions,
with the half-precision floating-point conversion operations.
+nosimd
Disable the Advanced SIMD instructions (does not disable floating
point).
+nofp
Disable the floating-point and Advanced SIMD instructions.
armv8-a
+crc
The Cyclic Redundancy Check (CRC) instructions.
+simd
The ARMv8-A Advanced SIMD and floating-point instructions.
+crypto
The cryptographic instructions.
+nocrypto
Disable the cryptographic instructions.
+nofp
Disable the floating-point, Advanced SIMD and cryptographic
instructions.
armv8.1-a
+simd
The ARMv8.1-A Advanced SIMD and floating-point instructions.
+crypto
The cryptographic instructions. This also enables the Advanced SIMD
and floating-point instructions.
+nocrypto
Disable the cryptographic instructions.
+nofp
Disable the floating-point, Advanced SIMD and cryptographic
instructions.
armv8.2-a
armv8.3-a
+fp16
The half-precision floating-point data processing instructions. This
also enables the Advanced SIMD and floating-point instructions.
+fp16fml
The half-precision floating-point fmla extension. This also enables
the half-precision floating-point extension and Advanced SIMD and
floating-point instructions.
+simd
The ARMv8.1-A Advanced SIMD and floating-point instructions.
+crypto
The cryptographic instructions. This also enables the Advanced SIMD
and floating-point instructions.
+dotprod
Enable the Dot Product extension. This also enables Advanced SIMD
instructions.
+nocrypto
Disable the cryptographic extension.
+nofp
Disable the floating-point, Advanced SIMD and cryptographic
instructions.
armv8.4-a
+fp16
The half-precision floating-point data processing instructions. This
also enables the Advanced SIMD and floating-point instructions as well
as the Dot Product extension and the half-precision floating-point fmla
extension.
+simd
The ARMv8.3-A Advanced SIMD and floating-point instructions as well as
the Dot Product extension.
+crypto
The cryptographic instructions. This also enables the Advanced SIMD
and floating-point instructions as well as the Dot Product extension.
+nocrypto
Disable the cryptographic extension.
+nofp
Disable the floating-point, Advanced SIMD and cryptographic
instructions.
armv7-r
+fp.sp
The single-precision VFPv3 floating-point instructions. The extension
+vfpv3xd can be used as an alias for this extension.
+fp The VFPv3 floating-point instructions with 16 double-precision
registers. The extension +vfpv3-d16 can be used as an alias for this
extension.
+vfpv3xd-d16-fp16
The single-precision VFPv3 floating-point instructions with 16 double-
precision registers and the half-precision floating-point conversion
operations.
+vfpv3-d16-fp16
The VFPv3 floating-point instructions, with 16 double-precision
registers and the half-precision floating-point conversion operations.
+nofp
Disable the floating-point extension.
+idiv
The ARM-state integer division instructions.
+noidiv
Disable the ARM-state integer division extension.
armv7e-m
+fp The single-precision VFPv4 floating-point instructions.
+fpv5
The single-precision FPv5 floating-point instructions.
+fp.dp
The single- and double-precision FPv5 floating-point instructions.
+nofp
Disable the floating-point extensions.
armv8-m.main
+dsp
The DSP instructions.
+nodsp
Disable the DSP extension.
+fp The single-precision floating-point instructions.
+fp.dp
The single- and double-precision floating-point instructions.
+nofp
Disable the floating-point extension.
armv8-r
+crc
The Cyclic Redundancy Check (CRC) instructions.
+fp.sp
The single-precision FPv5 floating-point instructions.
+simd
The ARMv8-A Advanced SIMD and floating-point instructions.
+crypto
The cryptographic instructions.
+nocrypto
Disable the cryptographic instructions.
+nofp
Disable the floating-point, Advanced SIMD and cryptographic
instructions.
-march=native causes the compiler to auto-detect the architecture of the build
computer. At present, this feature is only supported on GNU/Linux, and not all
architectures are recognized. If the auto-detect is unsuccessful the option
has no effect.
-mtune=name
This option specifies the name of the target ARM processor for which GCC should
tune the performance of the code. For some ARM implementations better
performance can be obtained by using this option. Permissible names are: arm2,
arm250, arm3, arm6, arm60, arm600, arm610, arm620, arm7, arm7m, arm7d, arm7dm,
arm7di, arm7dmi, arm70, arm700, arm700i, arm710, arm710c, arm7100, arm720,
arm7500, arm7500fe, arm7tdmi, arm7tdmi-s, arm710t, arm720t, arm740t, strongarm,
strongarm110, strongarm1100, strongarm1110, arm8, arm810, arm9, arm9e, arm920,
arm920t, arm922t, arm946e-s, arm966e-s, arm968e-s, arm926ej-s, arm940t,
arm9tdmi, arm10tdmi, arm1020t, arm1026ej-s, arm10e, arm1020e, arm1022e,
arm1136j-s, arm1136jf-s, mpcore, mpcorenovfp, arm1156t2-s, arm1156t2f-s,
arm1176jz-s, arm1176jzf-s, generic-armv7-a, cortex-a5, cortex-a7, cortex-a8,
cortex-a9, cortex-a12, cortex-a15, cortex-a17, cortex-a32, cortex-a35,
cortex-a53, cortex-a55, cortex-a57, cortex-a72, cortex-a73, cortex-a75,
neoverse-v1, neoverse-n2, cortex-r4, cortex-r4f, cortex-r5, cortex-r7,
cortex-r8, cortex-r52, cortex-m33, cortex-m23, cortex-m7, cortex-m4, cortex-m3,
cortex-m1, cortex-m0, cortex-m0plus, cortex-m1.small-multiply,
cortex-m0.small-multiply, cortex-m0plus.small-multiply, exynos-m1, marvell-pj4,
xscale, iwmmxt, iwmmxt2, ep9312, fa526, fa626, fa606te, fa626te, fmp626,
fa726te, xgene1.
Additionally, this option can specify that GCC should tune the performance of
the code for a big.LITTLE system. Permissible names are: cortex-a15.cortex-a7,
cortex-a17.cortex-a7, cortex-a57.cortex-a53, cortex-a72.cortex-a53,
cortex-a72.cortex-a35, cortex-a73.cortex-a53, cortex-a75.cortex-a55.
-mtune=generic-arch specifies that GCC should tune the performance for a blend
of processors within architecture arch. The aim is to generate code that run
well on the current most popular processors, balancing between optimizations
that benefit some CPUs in the range, and avoiding performance pitfalls of other
CPUs. The effects of this option may change in future GCC versions as CPU
models come and go.
-mtune permits the same extension options as -mcpu, but the extension options
do not affect the tuning of the generated code.
-mtune=native causes the compiler to auto-detect the CPU of the build computer.
At present, this feature is only supported on GNU/Linux, and not all
architectures are recognized. If the auto-detect is unsuccessful the option
has no effect.
-mcpu=name[+extension...]
This specifies the name of the target ARM processor. GCC uses this name to
derive the name of the target ARM architecture (as if specified by -march) and
the ARM processor type for which to tune for performance (as if specified by
-mtune). Where this option is used in conjunction with -march or -mtune, those
options take precedence over the appropriate part of this option.
Many of the supported CPUs implement optional architectural extensions. Where
this is so the architectural extensions are normally enabled by default. If
implementations that lack the extension exist, then the extension syntax can be
used to disable those extensions that have been omitted. For floating-point
and Advanced SIMD (Neon) instructions, the settings of the options -mfloat-abi
and -mfpu must also be considered: floating-point and Advanced SIMD
instructions will only be used if -mfloat-abi is not set to soft; and any
setting of -mfpu other than auto will override the available floating-point and
SIMD extension instructions.
For example, cortex-a9 can be found in three major configurations: integer
only, with just a floating-point unit or with floating-point and Advanced SIMD.
The default is to enable all the instructions, but the extensions +nosimd and
+nofp can be used to disable just the SIMD or both the SIMD and floating-point
instructions respectively.
Permissible names for this option are the same as those for -mtune.
The following extension options are common to the listed CPUs:
+nodsp
Disable the DSP instructions on cortex-m33.
+nofp
Disables the floating-point instructions on arm9e, arm946e-s, arm966e-s,
arm968e-s, arm10e, arm1020e, arm1022e, arm926ej-s, arm1026ej-s, cortex-r5,
cortex-r7, cortex-r8, cortex-m4, cortex-m7 and cortex-m33. Disables the
floating-point and SIMD instructions on generic-armv7-a, cortex-a5,
cortex-a7, cortex-a8, cortex-a9, cortex-a12, cortex-a15, cortex-a17,
cortex-a15.cortex-a7, cortex-a17.cortex-a7, cortex-a32, cortex-a35,
cortex-a53 and cortex-a55.
+nofp.dp
Disables the double-precision component of the floating-point instructions
on cortex-r5, cortex-r7, cortex-r8, cortex-r52 and cortex-m7.
+nosimd
Disables the SIMD (but not floating-point) instructions on generic-armv7-a,
cortex-a5, cortex-a7 and cortex-a9.
+crypto
Enables the cryptographic instructions on cortex-a32, cortex-a35,
cortex-a53, cortex-a55, cortex-a57, cortex-a72, cortex-a73, cortex-a75,
exynos-m1, xgene1, cortex-a57.cortex-a53, cortex-a72.cortex-a53,
cortex-a73.cortex-a35, cortex-a73.cortex-a53 and cortex-a75.cortex-a55.
Additionally the generic-armv7-a pseudo target defaults to VFPv3 with 16
double-precision registers. It supports the following extension options: mp,
sec, vfpv3-d16, vfpv3, vfpv3-d16-fp16, vfpv3-fp16, vfpv4-d16, vfpv4, neon,
neon-vfpv3, neon-fp16, neon-vfpv4. The meanings are the same as for the
extensions to -march=armv7-a.
-mcpu=generic-arch is also permissible, and is equivalent to -march=arch
-mtune=generic-arch. See -mtune for more information.
-mcpu=native causes the compiler to auto-detect the CPU of the build computer.
At present, this feature is only supported on GNU/Linux, and not all
architectures are recognized. If the auto-detect is unsuccessful the option
has no effect.
-mfpu=name
This specifies what floating-point hardware (or hardware emulation) is
available on the target. Permissible names are: auto, vfpv2, vfpv3,
vfpv3-fp16, vfpv3-d16, vfpv3-d16-fp16, vfpv3xd, vfpv3xd-fp16, neon-vfpv3,
neon-fp16, vfpv4, vfpv4-d16, fpv4-sp-d16, neon-vfpv4, fpv5-d16, fpv5-sp-d16,
fp-armv8, neon-fp-armv8 and crypto-neon-fp-armv8. Note that neon is an alias
for neon-vfpv3 and vfp is an alias for vfpv2.
The setting auto is the default and is special. It causes the compiler to
select the floating-point and Advanced SIMD instructions based on the settings
of -mcpu and -march.
If the selected floating-point hardware includes the NEON extension (e.g.
-mfpu=neon), note that floating-point operations are not generated by GCC's
auto-vectorization pass unless -funsafe-math-optimizations is also specified.
This is because NEON hardware does not fully implement the IEEE 754 standard
for floating-point arithmetic (in particular denormal values are treated as
zero), so the use of NEON instructions may lead to a loss of precision.
You can also set the fpu name at function level by using the "target("fpu=")"
function attributes or pragmas.
-mfp16-format=name
Specify the format of the "__fp16" half-precision floating-point type.
Permissible names are none, ieee, and alternative; the default is none, in
which case the "__fp16" type is not defined.
-mstructure-size-boundary=n
The sizes of all structures and unions are rounded up to a multiple of the
number of bits set by this option. Permissible values are 8, 32 and 64. The
default value varies for different toolchains. For the COFF targeted toolchain
the default value is 8. A value of 64 is only allowed if the underlying ABI
supports it.
Specifying a larger number can produce faster, more efficient code, but can
also increase the size of the program. Different values are potentially
incompatible. Code compiled with one value cannot necessarily expect to work
with code or libraries compiled with another value, if they exchange
information using structures or unions.
This option is deprecated.
-mabort-on-noreturn
Generate a call to the function "abort" at the end of a "noreturn" function.
It is executed if the function tries to return.
-mlong-calls
-mno-long-calls
Tells the compiler to perform function calls by first loading the address of
the function into a register and then performing a subroutine call on this
register. This switch is needed if the target function lies outside of the
64-megabyte addressing range of the offset-based version of subroutine call
instruction.
Even if this switch is enabled, not all function calls are turned into long
calls. The heuristic is that static functions, functions that have the
"short_call" attribute, functions that are inside the scope of a "#pragma
no_long_calls" directive, and functions whose definitions have already been
compiled within the current compilation unit are not turned into long calls.
The exceptions to this rule are that weak function definitions, functions with
the "long_call" attribute or the "section" attribute, and functions that are
within the scope of a "#pragma long_calls" directive are always turned into
long calls.
This feature is not enabled by default. Specifying -mno-long-calls restores
the default behavior, as does placing the function calls within the scope of a
"#pragma long_calls_off" directive. Note these switches have no effect on how
the compiler generates code to handle function calls via function pointers.
-msingle-pic-base
Treat the register used for PIC addressing as read-only, rather than loading it
in the prologue for each function. The runtime system is responsible for
initializing this register with an appropriate value before execution begins.
-mpic-register=reg
Specify the register to be used for PIC addressing. For standard PIC base
case, the default is any suitable register determined by compiler. For single
PIC base case, the default is R9 if target is EABI based or stack-checking is
enabled, otherwise the default is R10.
-mpic-data-is-text-relative
Assume that the displacement between the text and data segments is fixed at
static link time. This permits using PC-relative addressing operations to
access data known to be in the data segment. For non-VxWorks RTP targets, this
option is enabled by default. When disabled on such targets, it will enable
-msingle-pic-base by default.
-mpoke-function-name
Write the name of each function into the text section, directly preceding the
function prologue. The generated code is similar to this:
t0
.ascii "arm_poke_function_name", 0
.align
t1
.word 0xff000000 + (t1 - t0)
arm_poke_function_name
mov ip, sp
stmfd sp!, {fp, ip, lr, pc}
sub fp, ip, #4
When performing a stack backtrace, code can inspect the value of "pc" stored at
"fp + 0". If the trace function then looks at location "pc - 12" and the top 8
bits are set, then we know that there is a function name embedded immediately
preceding this location and has length "((pc[-3]) & 0xff000000)".
-mthumb
-marm
Select between generating code that executes in ARM and Thumb states. The
default for most configurations is to generate code that executes in ARM state,
but the default can be changed by configuring GCC with the --with-mode=state
configure option.
You can also override the ARM and Thumb mode for each function by using the
"target("thumb")" and "target("arm")" function attributes or pragmas.
-mflip-thumb
Switch ARM/Thumb modes on alternating functions. This option is provided for
regression testing of mixed Thumb/ARM code generation, and is not intended for
ordinary use in compiling code.
-mtpcs-frame
Generate a stack frame that is compliant with the Thumb Procedure Call Standard
for all non-leaf functions. (A leaf function is one that does not call any
other functions.) The default is -mno-tpcs-frame.
-mtpcs-leaf-frame
Generate a stack frame that is compliant with the Thumb Procedure Call Standard
for all leaf functions. (A leaf function is one that does not call any other
functions.) The default is -mno-apcs-leaf-frame.
-mcallee-super-interworking
Gives all externally visible functions in the file being compiled an ARM
instruction set header which switches to Thumb mode before executing the rest
of the function. This allows these functions to be called from non-
interworking code. This option is not valid in AAPCS configurations because
interworking is enabled by default.
-mcaller-super-interworking
Allows calls via function pointers (including virtual functions) to execute
correctly regardless of whether the target code has been compiled for
interworking or not. There is a small overhead in the cost of executing a
function pointer if this option is enabled. This option is not valid in AAPCS
configurations because interworking is enabled by default.
-mtp=name
Specify the access model for the thread local storage pointer. The valid
models are soft, which generates calls to "__aeabi_read_tp", cp15, which
fetches the thread pointer from "cp15" directly (supported in the arm6k
architecture), and auto, which uses the best available method for the selected
processor. The default setting is auto.
-mtls-dialect=dialect
Specify the dialect to use for accessing thread local storage. Two dialects
are supported---gnu and gnu2. The gnu dialect selects the original GNU scheme
for supporting local and global dynamic TLS models. The gnu2 dialect selects
the GNU descriptor scheme, which provides better performance for shared
libraries. The GNU descriptor scheme is compatible with the original scheme,
but does require new assembler, linker and library support. Initial and local
exec TLS models are unaffected by this option and always use the original
scheme.
-mword-relocations
Only generate absolute relocations on word-sized values (i.e. R_ARM_ABS32).
This is enabled by default on targets (uClinux, SymbianOS) where the runtime
loader imposes this restriction, and when -fpic or -fPIC is specified.
-mfix-cortex-m3-ldrd
Some Cortex-M3 cores can cause data corruption when "ldrd" instructions with
overlapping destination and base registers are used. This option avoids
generating these instructions. This option is enabled by default when
-mcpu=cortex-m3 is specified.
-munaligned-access
-mno-unaligned-access
Enables (or disables) reading and writing of 16- and 32- bit values from
addresses that are not 16- or 32- bit aligned. By default unaligned access is
disabled for all pre-ARMv6, all ARMv6-M and for ARMv8-M Baseline architectures,
and enabled for all other architectures. If unaligned access is not enabled
then words in packed data structures are accessed a byte at a time.
The ARM attribute "Tag_CPU_unaligned_access" is set in the generated object
file to either true or false, depending upon the setting of this option. If
unaligned access is enabled then the preprocessor symbol
"__ARM_FEATURE_UNALIGNED" is also defined.
-mneon-for-64bits
Enables using Neon to handle scalar 64-bits operations. This is disabled by
default since the cost of moving data from core registers to Neon is high.
-mslow-flash-data
Assume loading data from flash is slower than fetching instruction. Therefore
literal load is minimized for better performance. This option is only
supported when compiling for ARMv7 M-profile and off by default.
-masm-syntax-unified
Assume inline assembler is using unified asm syntax. The default is currently
off which implies divided syntax. This option has no impact on Thumb2.
However, this may change in future releases of GCC. Divided syntax should be
considered deprecated.
-mrestrict-it
Restricts generation of IT blocks to conform to the rules of ARMv8-A. IT
blocks can only contain a single 16-bit instruction from a select set of
instructions. This option is on by default for ARMv8-A Thumb mode.
-mprint-tune-info
Print CPU tuning information as comment in assembler file. This is an option
used only for regression testing of the compiler and not intended for ordinary
use in compiling code. This option is disabled by default.
-mverbose-cost-dump
Enable verbose cost model dumping in the debug dump files. This option is
provided for use in debugging the compiler.
-mpure-code
Do not allow constant data to be placed in code sections. Additionally, when
compiling for ELF object format give all text sections the ELF processor-
specific section attribute "SHF_ARM_PURECODE". This option is only available
when generating non-pic code for M-profile targets with the MOVT instruction.
-mcmse
Generate secure code as per the "ARMv8-M Security Extensions: Requirements on
Development Tools Engineering Specification", which can be found on
<https://developer.arm.com/documentation/ecm0359818/latest/>.
AVR Options
These options are defined for AVR implementations:
-mmcu=mcu
Specify Atmel AVR instruction set architectures (ISA) or MCU type.
The default for this option is avr2.
GCC supports the following AVR devices and ISAs:
"avr2"
"Classic" devices with up to 8 KiB of program memory. mcu = "attiny22",
"attiny26", "at90s2313", "at90s2323", "at90s2333", "at90s2343",
"at90s4414", "at90s4433", "at90s4434", "at90c8534", "at90s8515",
"at90s8535".
"avr25"
"Classic" devices with up to 8 KiB of program memory and with the "MOVW"
instruction. mcu = "attiny13", "attiny13a", "attiny24", "attiny24a",
"attiny25", "attiny261", "attiny261a", "attiny2313", "attiny2313a",
"attiny43u", "attiny44", "attiny44a", "attiny45", "attiny48", "attiny441",
"attiny461", "attiny461a", "attiny4313", "attiny84", "attiny84a",
"attiny85", "attiny87", "attiny88", "attiny828", "attiny841", "attiny861",
"attiny861a", "ata5272", "ata6616c", "at86rf401".
"avr3"
"Classic" devices with 16 KiB up to 64 KiB of program memory. mcu =
"at76c711", "at43usb355".
"avr31"
"Classic" devices with 128 KiB of program memory. mcu = "atmega103",
"at43usb320".
"avr35"
"Classic" devices with 16 KiB up to 64 KiB of program memory and with the
"MOVW" instruction. mcu = "attiny167", "attiny1634", "atmega8u2",
"atmega16u2", "atmega32u2", "ata5505", "ata6617c", "ata664251",
"at90usb82", "at90usb162".
"avr4"
"Enhanced" devices with up to 8 KiB of program memory. mcu = "atmega48",
"atmega48a", "atmega48p", "atmega48pa", "atmega48pb", "atmega8",
"atmega8a", "atmega8hva", "atmega88", "atmega88a", "atmega88p",
"atmega88pa", "atmega88pb", "atmega8515", "atmega8535", "ata6285",
"ata6286", "ata6289", "ata6612c", "at90pwm1", "at90pwm2", "at90pwm2b",
"at90pwm3", "at90pwm3b", "at90pwm81".
"avr5"
"Enhanced" devices with 16 KiB up to 64 KiB of program memory. mcu =
"atmega16", "atmega16a", "atmega16hva", "atmega16hva2", "atmega16hvb",
"atmega16hvbrevb", "atmega16m1", "atmega16u4", "atmega161", "atmega162",
"atmega163", "atmega164a", "atmega164p", "atmega164pa", "atmega165",
"atmega165a", "atmega165p", "atmega165pa", "atmega168", "atmega168a",
"atmega168p", "atmega168pa", "atmega168pb", "atmega169", "atmega169a",
"atmega169p", "atmega169pa", "atmega32", "atmega32a", "atmega32c1",
"atmega32hvb", "atmega32hvbrevb", "atmega32m1", "atmega32u4", "atmega32u6",
"atmega323", "atmega324a", "atmega324p", "atmega324pa", "atmega325",
"atmega325a", "atmega325p", "atmega325pa", "atmega328", "atmega328p",
"atmega328pb", "atmega329", "atmega329a", "atmega329p", "atmega329pa",
"atmega3250", "atmega3250a", "atmega3250p", "atmega3250pa", "atmega3290",
"atmega3290a", "atmega3290p", "atmega3290pa", "atmega406", "atmega64",
"atmega64a", "atmega64c1", "atmega64hve", "atmega64hve2", "atmega64m1",
"atmega64rfr2", "atmega640", "atmega644", "atmega644a", "atmega644p",
"atmega644pa", "atmega644rfr2", "atmega645", "atmega645a", "atmega645p",
"atmega649", "atmega649a", "atmega649p", "atmega6450", "atmega6450a",
"atmega6450p", "atmega6490", "atmega6490a", "atmega6490p", "ata5795",
"ata5790", "ata5790n", "ata5791", "ata6613c", "ata6614q", "ata5782",
"ata5831", "ata8210", "ata8510", "ata5702m322", "at90pwm161", "at90pwm216",
"at90pwm316", "at90can32", "at90can64", "at90scr100", "at90usb646",
"at90usb647", "at94k", "m3000".
"avr51"
"Enhanced" devices with 128 KiB of program memory. mcu = "atmega128",
"atmega128a", "atmega128rfa1", "atmega128rfr2", "atmega1280", "atmega1281",
"atmega1284", "atmega1284p", "atmega1284rfr2", "at90can128", "at90usb1286",
"at90usb1287".
"avr6"
"Enhanced" devices with 3-byte PC, i.e. with more than 128 KiB of program
memory. mcu = "atmega256rfr2", "atmega2560", "atmega2561",
"atmega2564rfr2".
"avrxmega2"
"XMEGA" devices with more than 8 KiB and up to 64 KiB of program memory.
mcu = "atxmega8e5", "atxmega16a4", "atxmega16a4u", "atxmega16c4",
"atxmega16d4", "atxmega16e5", "atxmega32a4", "atxmega32a4u", "atxmega32c3",
"atxmega32c4", "atxmega32d3", "atxmega32d4", "atxmega32e5".
"avrxmega3"
"XMEGA" devices with up to 64 KiB of combined program memory and RAM, and
with program memory visible in the RAM address space. mcu = "attiny202",
"attiny204", "attiny212", "attiny214", "attiny402", "attiny404",
"attiny406", "attiny412", "attiny414", "attiny416", "attiny417",
"attiny804", "attiny806", "attiny807", "attiny814", "attiny816",
"attiny817", "attiny1604", "attiny1606", "attiny1607", "attiny1614",
"attiny1616", "attiny1617", "attiny3214", "attiny3216", "attiny3217",
"atmega808", "atmega809", "atmega1608", "atmega1609", "atmega3208",
"atmega3209", "atmega4808", "atmega4809".
"avrxmega4"
"XMEGA" devices with more than 64 KiB and up to 128 KiB of program memory.
mcu = "atxmega64a3", "atxmega64a3u", "atxmega64a4u", "atxmega64b1",
"atxmega64b3", "atxmega64c3", "atxmega64d3", "atxmega64d4".
"avrxmega5"
"XMEGA" devices with more than 64 KiB and up to 128 KiB of program memory
and more than 64 KiB of RAM. mcu = "atxmega64a1", "atxmega64a1u".
"avrxmega6"
"XMEGA" devices with more than 128 KiB of program memory. mcu =
"atxmega128a3", "atxmega128a3u", "atxmega128b1", "atxmega128b3",
"atxmega128c3", "atxmega128d3", "atxmega128d4", "atxmega192a3",
"atxmega192a3u", "atxmega192c3", "atxmega192d3", "atxmega256a3",
"atxmega256a3b", "atxmega256a3bu", "atxmega256a3u", "atxmega256c3",
"atxmega256d3", "atxmega384c3", "atxmega384d3".
"avrxmega7"
"XMEGA" devices with more than 128 KiB of program memory and more than 64
KiB of RAM. mcu = "atxmega128a1", "atxmega128a1u", "atxmega128a4u".
"avrtiny"
"TINY" Tiny core devices with 512 B up to 4 KiB of program memory. mcu =
"attiny4", "attiny5", "attiny9", "attiny10", "attiny20", "attiny40".
"avr1"
This ISA is implemented by the minimal AVR core and supported for assembler
only. mcu = "attiny11", "attiny12", "attiny15", "attiny28", "at90s1200".
-mabsdata
Assume that all data in static storage can be accessed by LDS / STS
instructions. This option has only an effect on reduced Tiny devices like
ATtiny40. See also the "absdata" AVR Variable Attributes,variable attribute.
-maccumulate-args
Accumulate outgoing function arguments and acquire/release the needed stack
space for outgoing function arguments once in function prologue/epilogue.
Without this option, outgoing arguments are pushed before calling a function
and popped afterwards.
Popping the arguments after the function call can be expensive on AVR so that
accumulating the stack space might lead to smaller executables because
arguments need not be removed from the stack after such a function call.
This option can lead to reduced code size for functions that perform several
calls to functions that get their arguments on the stack like calls to printf-
like functions.
-mbranch-cost=cost
Set the branch costs for conditional branch instructions to cost. Reasonable
values for cost are small, non-negative integers. The default branch cost is 0.
-mcall-prologues
Functions prologues/epilogues are expanded as calls to appropriate subroutines.
Code size is smaller.
-mgas-isr-prologues
Interrupt service routines (ISRs) may use the "__gcc_isr" pseudo instruction
supported by GNU Binutils. If this option is on, the feature can still be
disabled for individual ISRs by means of the AVR Function
Attributes,,"no_gccisr" function attribute. This feature is activated per
default if optimization is on (but not with -Og, @pxref{Optimize Options}), and
if GNU Binutils support PR21683 ("https://sourceware.org/PR21683").
-mint8
Assume "int" to be 8-bit integer. This affects the sizes of all types: a
"char" is 1 byte, an "int" is 1 byte, a "long" is 2 bytes, and "long long" is 4
bytes. Please note that this option does not conform to the C standards, but
it results in smaller code size.
-mmain-is-OS_task
Do not save registers in "main". The effect is the same like attaching
attribute AVR Function Attributes,,"OS_task" to "main". It is activated per
default if optimization is on.
-mn-flash=num
Assume that the flash memory has a size of num times 64 KiB.
-mno-interrupts
Generated code is not compatible with hardware interrupts. Code size is
smaller.
-mrelax
Try to replace "CALL" resp. "JMP" instruction by the shorter "RCALL" resp.
"RJMP" instruction if applicable. Setting -mrelax just adds the --mlink-relax
option to the assembler's command line and the --relax option to the linker's
command line.
Jump relaxing is performed by the linker because jump offsets are not known
before code is located. Therefore, the assembler code generated by the compiler
is the same, but the instructions in the executable may differ from
instructions in the assembler code.
Relaxing must be turned on if linker stubs are needed, see the section on
"EIND" and linker stubs below.
-mrmw
Assume that the device supports the Read-Modify-Write instructions "XCH",
"LAC", "LAS" and "LAT".
-mshort-calls
Assume that "RJMP" and "RCALL" can target the whole program memory.
This option is used internally for multilib selection. It is not an
optimization option, and you don't need to set it by hand.
-msp8
Treat the stack pointer register as an 8-bit register, i.e. assume the high
byte of the stack pointer is zero. In general, you don't need to set this
option by hand.
This option is used internally by the compiler to select and build multilibs
for architectures "avr2" and "avr25". These architectures mix devices with and
without "SPH". For any setting other than -mmcu=avr2 or -mmcu=avr25 the
compiler driver adds or removes this option from the compiler proper's command
line, because the compiler then knows if the device or architecture has an
8-bit stack pointer and thus no "SPH" register or not.
-mstrict-X
Use address register "X" in a way proposed by the hardware. This means that
"X" is only used in indirect, post-increment or pre-decrement addressing.
Without this option, the "X" register may be used in the same way as "Y" or "Z"
which then is emulated by additional instructions. For example, loading a
value with "X+const" addressing with a small non-negative "const < 64" to a
register Rn is performed as
adiw r26, const ; X += const
ld <Rn>, X ; <Rn> = *X
sbiw r26, const ; X -= const
-mtiny-stack
Only change the lower 8 bits of the stack pointer.
-mfract-convert-truncate
Allow to use truncation instead of rounding towards zero for fractional fixed-
point types.
-nodevicelib
Don't link against AVR-LibC's device specific library "lib<mcu>.a".
-nodevicespecs
Don't add -specs=device-specs/specs-<mcu> to the compiler driver's command
line. The user takes responsibility for supplying the sub-processes like
compiler proper, assembler and linker with appropriate command line options.
-Waddr-space-convert
Warn about conversions between address spaces in the case where the resulting
address space is not contained in the incoming address space.
-Wmisspelled-isr
Warn if the ISR is misspelled, i.e. without __vector prefix. Enabled by
default.
"EIND" and Devices with More Than 128 Ki Bytes of Flash
Pointers in the implementation are 16 bits wide. The address of a function or
label is represented as word address so that indirect jumps and calls can target
any code address in the range of 64 Ki words.
In order to facilitate indirect jump on devices with more than 128 Ki bytes of
program memory space, there is a special function register called "EIND" that
serves as most significant part of the target address when "EICALL" or "EIJMP"
instructions are used.
Indirect jumps and calls on these devices are handled as follows by the compiler
and are subject to some limitations:
* The compiler never sets "EIND".
* The compiler uses "EIND" implicitly in "EICALL"/"EIJMP" instructions or might
read "EIND" directly in order to emulate an indirect call/jump by means of a
"RET" instruction.
* The compiler assumes that "EIND" never changes during the startup code or
during the application. In particular, "EIND" is not saved/restored in function
or interrupt service routine prologue/epilogue.
* For indirect calls to functions and computed goto, the linker generates stubs.
Stubs are jump pads sometimes also called trampolines. Thus, the indirect
call/jump jumps to such a stub. The stub contains a direct jump to the desired
address.
* Linker relaxation must be turned on so that the linker generates the stubs
correctly in all situations. See the compiler option -mrelax and the linker
option --relax. There are corner cases where the linker is supposed to
generate stubs but aborts without relaxation and without a helpful error
message.
* The default linker script is arranged for code with "EIND = 0". If code is
supposed to work for a setup with "EIND != 0", a custom linker script has to be
used in order to place the sections whose name start with ".trampolines" into
the segment where "EIND" points to.
* The startup code from libgcc never sets "EIND". Notice that startup code is a
blend of code from libgcc and AVR-LibC. For the impact of AVR-LibC on "EIND",
see the AVR-LibC user manual ("http://nongnu.org/avr-libc/user-manual/").
* It is legitimate for user-specific startup code to set up "EIND" early, for
example by means of initialization code located in section ".init3". Such code
runs prior to general startup code that initializes RAM and calls constructors,
but after the bit of startup code from AVR-LibC that sets "EIND" to the segment
where the vector table is located.
#include <avr/io.h>
static void
__attribute__((section(".init3"),naked,used,no_instrument_function))
init3_set_eind (void)
{
__asm volatile ("ldi r24,pm_hh8(__trampolines_start)\n\t"
"out %i0,r24" :: "n" (&EIND) : "r24","memory");
}
The "__trampolines_start" symbol is defined in the linker script.
* Stubs are generated automatically by the linker if the following two conditions
are met:
-<The address of a label is taken by means of the "gs" modifier>
(short for generate stubs) like so:
LDI r24, lo8(gs(<func>))
LDI r25, hi8(gs(<func>))
-<The final location of that label is in a code segment>
outside the segment where the stubs are located.
* The compiler emits such "gs" modifiers for code labels in the following
situations:
-<Taking address of a function or code label.>
-<Computed goto.>
-<If prologue-save function is used, see -mcall-prologues>
command-line option.
-<Switch/case dispatch tables. If you do not want such dispatch>
tables you can specify the -fno-jump-tables command-line option.
-<C and C++ constructors/destructors called during startup/shutdown.>
-<If the tools hit a "gs()" modifier explained above.>
* Jumping to non-symbolic addresses like so is not supported:
int main (void)
{
/* Call function at word address 0x2 */
return ((int(*)(void)) 0x2)();
}
Instead, a stub has to be set up, i.e. the function has to be called through a
symbol ("func_4" in the example):
int main (void)
{
extern int func_4 (void);
/* Call function at byte address 0x4 */
return func_4();
}
and the application be linked with -Wl,--defsym,func_4=0x4. Alternatively,
"func_4" can be defined in the linker script.
Handling of the "RAMPD", "RAMPX", "RAMPY" and "RAMPZ" Special Function Registers
Some AVR devices support memories larger than the 64 KiB range that can be accessed
with 16-bit pointers. To access memory locations outside this 64 KiB range, the
content of a "RAMP" register is used as high part of the address: The "X", "Y", "Z"
address register is concatenated with the "RAMPX", "RAMPY", "RAMPZ" special
function register, respectively, to get a wide address. Similarly, "RAMPD" is used
together with direct addressing.
* The startup code initializes the "RAMP" special function registers with zero.
* If a AVR Named Address Spaces,named address space other than generic or
"__flash" is used, then "RAMPZ" is set as needed before the operation.
* If the device supports RAM larger than 64 KiB and the compiler needs to change
"RAMPZ" to accomplish an operation, "RAMPZ" is reset to zero after the
operation.
* If the device comes with a specific "RAMP" register, the ISR prologue/epilogue
saves/restores that SFR and initializes it with zero in case the ISR code might
(implicitly) use it.
* RAM larger than 64 KiB is not supported by GCC for AVR targets. If you use
inline assembler to read from locations outside the 16-bit address range and
change one of the "RAMP" registers, you must reset it to zero after the access.
AVR Built-in Macros
GCC defines several built-in macros so that the user code can test for the presence
or absence of features. Almost any of the following built-in macros are deduced
from device capabilities and thus triggered by the -mmcu= command-line option.
For even more AVR-specific built-in macros see AVR Named Address Spaces and AVR
Built-in Functions.
"__AVR_ARCH__"
Build-in macro that resolves to a decimal number that identifies the
architecture and depends on the -mmcu=mcu option. Possible values are:
2, 25, 3, 31, 35, 4, 5, 51, 6
for mcu="avr2", "avr25", "avr3", "avr31", "avr35", "avr4", "avr5", "avr51",
"avr6",
respectively and
100, 102, 103, 104, 105, 106, 107
for mcu="avrtiny", "avrxmega2", "avrxmega3", "avrxmega4", "avrxmega5",
"avrxmega6", "avrxmega7", respectively. If mcu specifies a device, this built-
in macro is set accordingly. For example, with -mmcu=atmega8 the macro is
defined to 4.
"__AVR_Device__"
Setting -mmcu=device defines this built-in macro which reflects the device's
name. For example, -mmcu=atmega8 defines the built-in macro "__AVR_ATmega8__",
-mmcu=attiny261a defines "__AVR_ATtiny261A__", etc.
The built-in macros' names follow the scheme "__AVR_Device__" where Device is
the device name as from the AVR user manual. The difference between Device in
the built-in macro and device in -mmcu=device is that the latter is always
lowercase.
If device is not a device but only a core architecture like avr51, this macro
is not defined.
"__AVR_DEVICE_NAME__"
Setting -mmcu=device defines this built-in macro to the device's name. For
example, with -mmcu=atmega8 the macro is defined to "atmega8".
If device is not a device but only a core architecture like avr51, this macro
is not defined.
"__AVR_XMEGA__"
The device / architecture belongs to the XMEGA family of devices.
"__AVR_HAVE_ELPM__"
The device has the "ELPM" instruction.
"__AVR_HAVE_ELPMX__"
The device has the "ELPM Rn,Z" and "ELPM Rn,Z+" instructions.
"__AVR_HAVE_MOVW__"
The device has the "MOVW" instruction to perform 16-bit register-register
moves.
"__AVR_HAVE_LPMX__"
The device has the "LPM Rn,Z" and "LPM Rn,Z+" instructions.
"__AVR_HAVE_MUL__"
The device has a hardware multiplier.
"__AVR_HAVE_JMP_CALL__"
The device has the "JMP" and "CALL" instructions. This is the case for devices
with more than 8 KiB of program memory.
"__AVR_HAVE_EIJMP_EICALL__"
"__AVR_3_BYTE_PC__"
The device has the "EIJMP" and "EICALL" instructions. This is the case for
devices with more than 128 KiB of program memory. This also means that the
program counter (PC) is 3 bytes wide.
"__AVR_2_BYTE_PC__"
The program counter (PC) is 2 bytes wide. This is the case for devices with up
to 128 KiB of program memory.
"__AVR_HAVE_8BIT_SP__"
"__AVR_HAVE_16BIT_SP__"
The stack pointer (SP) register is treated as 8-bit respectively 16-bit
register by the compiler. The definition of these macros is affected by
-mtiny-stack.
"__AVR_HAVE_SPH__"
"__AVR_SP8__"
The device has the SPH (high part of stack pointer) special function register
or has an 8-bit stack pointer, respectively. The definition of these macros is
affected by -mmcu= and in the cases of -mmcu=avr2 and -mmcu=avr25 also by
-msp8.
"__AVR_HAVE_RAMPD__"
"__AVR_HAVE_RAMPX__"
"__AVR_HAVE_RAMPY__"
"__AVR_HAVE_RAMPZ__"
The device has the "RAMPD", "RAMPX", "RAMPY", "RAMPZ" special function
register, respectively.
"__NO_INTERRUPTS__"
This macro reflects the -mno-interrupts command-line option.
"__AVR_ERRATA_SKIP__"
"__AVR_ERRATA_SKIP_JMP_CALL__"
Some AVR devices (AT90S8515, ATmega103) must not skip 32-bit instructions
because of a hardware erratum. Skip instructions are "SBRS", "SBRC", "SBIS",
"SBIC" and "CPSE". The second macro is only defined if "__AVR_HAVE_JMP_CALL__"
is also set.
"__AVR_ISA_RMW__"
The device has Read-Modify-Write instructions (XCH, LAC, LAS and LAT).
"__AVR_SFR_OFFSET__=offset"
Instructions that can address I/O special function registers directly like
"IN", "OUT", "SBI", etc. may use a different address as if addressed by an
instruction to access RAM like "LD" or "STS". This offset depends on the device
architecture and has to be subtracted from the RAM address in order to get the
respective I/O address.
"__AVR_SHORT_CALLS__"
The -mshort-calls command line option is set.
"__AVR_PM_BASE_ADDRESS__=addr"
Some devices support reading from flash memory by means of "LD*" instructions.
The flash memory is seen in the data address space at an offset of
"__AVR_PM_BASE_ADDRESS__". If this macro is not defined, this feature is not
available. If defined, the address space is linear and there is no need to put
".rodata" into RAM. This is handled by the default linker description file,
and is currently available for "avrtiny" and "avrxmega3". Even more
convenient, there is no need to use address spaces like "__flash" or features
like attribute "progmem" and "pgm_read_*".
"__WITH_AVRLIBC__"
The compiler is configured to be used together with AVR-Libc. See the
--with-avrlibc configure option.
Blackfin Options
-mcpu=cpu[-sirevision]
Specifies the name of the target Blackfin processor. Currently, cpu can be one
of bf512, bf514, bf516, bf518, bf522, bf523, bf524, bf525, bf526, bf527, bf531,
bf532, bf533, bf534, bf536, bf537, bf538, bf539, bf542, bf544, bf547, bf548,
bf549, bf542m, bf544m, bf547m, bf548m, bf549m, bf561, bf592.
The optional sirevision specifies the silicon revision of the target Blackfin
processor. Any workarounds available for the targeted silicon revision are
enabled. If sirevision is none, no workarounds are enabled. If sirevision is
any, all workarounds for the targeted processor are enabled. The
"__SILICON_REVISION__" macro is defined to two hexadecimal digits representing
the major and minor numbers in the silicon revision. If sirevision is none,
the "__SILICON_REVISION__" is not defined. If sirevision is any, the
"__SILICON_REVISION__" is defined to be 0xffff. If this optional sirevision is
not used, GCC assumes the latest known silicon revision of the targeted
Blackfin processor.
GCC defines a preprocessor macro for the specified cpu. For the bfin-elf
toolchain, this option causes the hardware BSP provided by libgloss to be
linked in if -msim is not given.
Without this option, bf532 is used as the processor by default.
Note that support for bf561 is incomplete. For bf561, only the preprocessor
macro is defined.
-msim
Specifies that the program will be run on the simulator. This causes the
simulator BSP provided by libgloss to be linked in. This option has effect
only for bfin-elf toolchain. Certain other options, such as
-mid-shared-library and -mfdpic, imply -msim.
-momit-leaf-frame-pointer
Don't keep the frame pointer in a register for leaf functions. This avoids the
instructions to save, set up and restore frame pointers and makes an extra
register available in leaf functions.
-mspecld-anomaly
When enabled, the compiler ensures that the generated code does not contain
speculative loads after jump instructions. If this option is used,
"__WORKAROUND_SPECULATIVE_LOADS" is defined.
-mno-specld-anomaly
Don't generate extra code to prevent speculative loads from occurring.
-mcsync-anomaly
When enabled, the compiler ensures that the generated code does not contain
CSYNC or SSYNC instructions too soon after conditional branches. If this
option is used, "__WORKAROUND_SPECULATIVE_SYNCS" is defined.
-mno-csync-anomaly
Don't generate extra code to prevent CSYNC or SSYNC instructions from occurring
too soon after a conditional branch.
-mlow-64k
When enabled, the compiler is free to take advantage of the knowledge that the
entire program fits into the low 64k of memory.
-mno-low-64k
Assume that the program is arbitrarily large. This is the default.
-mstack-check-l1
Do stack checking using information placed into L1 scratchpad memory by the
uClinux kernel.
-mid-shared-library
Generate code that supports shared libraries via the library ID method. This
allows for execute in place and shared libraries in an environment without
virtual memory management. This option implies -fPIC. With a bfin-elf target,
this option implies -msim.
-mno-id-shared-library
Generate code that doesn't assume ID-based shared libraries are being used.
This is the default.
-mleaf-id-shared-library
Generate code that supports shared libraries via the library ID method, but
assumes that this library or executable won't link against any other ID shared
libraries. That allows the compiler to use faster code for jumps and calls.
-mno-leaf-id-shared-library
Do not assume that the code being compiled won't link against any ID shared
libraries. Slower code is generated for jump and call insns.
-mshared-library-id=n
Specifies the identification number of the ID-based shared library being
compiled. Specifying a value of 0 generates more compact code; specifying
other values forces the allocation of that number to the current library but is
no more space- or time-efficient than omitting this option.
-msep-data
Generate code that allows the data segment to be located in a different area of
memory from the text segment. This allows for execute in place in an
environment without virtual memory management by eliminating relocations
against the text section.
-mno-sep-data
Generate code that assumes that the data segment follows the text segment.
This is the default.
-mlong-calls
-mno-long-calls
Tells the compiler to perform function calls by first loading the address of
the function into a register and then performing a subroutine call on this
register. This switch is needed if the target function lies outside of the
24-bit addressing range of the offset-based version of subroutine call
instruction.
This feature is not enabled by default. Specifying -mno-long-calls restores
the default behavior. Note these switches have no effect on how the compiler
generates code to handle function calls via function pointers.
-mfast-fp
Link with the fast floating-point library. This library relaxes some of the
IEEE floating-point standard's rules for checking inputs against Not-a-Number
(NAN), in the interest of performance.
-minline-plt
Enable inlining of PLT entries in function calls to functions that are not
known to bind locally. It has no effect without -mfdpic.
-mmulticore
Build a standalone application for multicore Blackfin processors. This option
causes proper start files and link scripts supporting multicore to be used, and
defines the macro "__BFIN_MULTICORE". It can only be used with
-mcpu=bf561[-sirevision].
This option can be used with -mcorea or -mcoreb, which selects the one-
application-per-core programming model. Without -mcorea or -mcoreb, the
single-application/dual-core programming model is used. In this model, the main
function of Core B should be named as "coreb_main".
If this option is not used, the single-core application programming model is
used.
-mcorea
Build a standalone application for Core A of BF561 when using the one-
application-per-core programming model. Proper start files and link scripts are
used to support Core A, and the macro "__BFIN_COREA" is defined. This option
can only be used in conjunction with -mmulticore.
-mcoreb
Build a standalone application for Core B of BF561 when using the one-
application-per-core programming model. Proper start files and link scripts are
used to support Core B, and the macro "__BFIN_COREB" is defined. When this
option is used, "coreb_main" should be used instead of "main". This option can
only be used in conjunction with -mmulticore.
-msdram
Build a standalone application for SDRAM. Proper start files and link scripts
are used to put the application into SDRAM, and the macro "__BFIN_SDRAM" is
defined. The loader should initialize SDRAM before loading the application.
-micplb
Assume that ICPLBs are enabled at run time. This has an effect on certain
anomaly workarounds. For Linux targets, the default is to assume ICPLBs are
enabled; for standalone applications the default is off.
C6X Options
-march=name
This specifies the name of the target architecture. GCC uses this name to
determine what kind of instructions it can emit when generating assembly code.
Permissible names are: c62x, c64x, c64x+, c67x, c67x+, c674x.
-mbig-endian
Generate code for a big-endian target.
-mlittle-endian
Generate code for a little-endian target. This is the default.
-msim
Choose startup files and linker script suitable for the simulator.
-msdata=default
Put small global and static data in the ".neardata" section, which is pointed
to by register "B14". Put small uninitialized global and static data in the
".bss" section, which is adjacent to the ".neardata" section. Put small read-
only data into the ".rodata" section. The corresponding sections used for
large pieces of data are ".fardata", ".far" and ".const".
-msdata=all
Put all data, not just small objects, into the sections reserved for small
data, and use addressing relative to the "B14" register to access them.
-msdata=none
Make no use of the sections reserved for small data, and use absolute addresses
to access all data. Put all initialized global and static data in the
".fardata" section, and all uninitialized data in the ".far" section. Put all
constant data into the ".const" section.
CRIS Options
These options are defined specifically for the CRIS ports.
-march=architecture-type
-mcpu=architecture-type
Generate code for the specified architecture. The choices for architecture-
type are v3, v8 and v10 for respectively ETRAX 4, ETRAX 100, and ETRAX 100 LX.
Default is v0 except for cris-axis-linux-gnu, where the default is v10.
-mtune=architecture-type
Tune to architecture-type everything applicable about the generated code,
except for the ABI and the set of available instructions. The choices for
architecture-type are the same as for -march=architecture-type.
-mmax-stack-frame=n
Warn when the stack frame of a function exceeds n bytes.
-metrax4
-metrax100
The options -metrax4 and -metrax100 are synonyms for -march=v3 and -march=v8
respectively.
-mmul-bug-workaround
-mno-mul-bug-workaround
Work around a bug in the "muls" and "mulu" instructions for CPU models where it
applies. This option is active by default.
-mpdebug
Enable CRIS-specific verbose debug-related information in the assembly code.
This option also has the effect of turning off the #NO_APP formatted-code
indicator to the assembler at the beginning of the assembly file.
-mcc-init
Do not use condition-code results from previous instruction; always emit
compare and test instructions before use of condition codes.
-mno-side-effects
Do not emit instructions with side effects in addressing modes other than post-
increment.
-mstack-align
-mno-stack-align
-mdata-align
-mno-data-align
-mconst-align
-mno-const-align
These options (no- options) arrange (eliminate arrangements) for the stack
frame, individual data and constants to be aligned for the maximum single data
access size for the chosen CPU model. The default is to arrange for 32-bit
alignment. ABI details such as structure layout are not affected by these
options.
-m32-bit
-m16-bit
-m8-bit
Similar to the stack- data- and const-align options above, these options
arrange for stack frame, writable data and constants to all be 32-bit, 16-bit
or 8-bit aligned. The default is 32-bit alignment.
-mno-prologue-epilogue
-mprologue-epilogue
With -mno-prologue-epilogue, the normal function prologue and epilogue which
set up the stack frame are omitted and no return instructions or return
sequences are generated in the code. Use this option only together with visual
inspection of the compiled code: no warnings or errors are generated when call-
saved registers must be saved, or storage for local variables needs to be
allocated.
-mno-gotplt
-mgotplt
With -fpic and -fPIC, don't generate (do generate) instruction sequences that
load addresses for functions from the PLT part of the GOT rather than
(traditional on other architectures) calls to the PLT. The default is
-mgotplt.
-melf
Legacy no-op option only recognized with the cris-axis-elf and cris-axis-linux-
gnu targets.
-mlinux
Legacy no-op option only recognized with the cris-axis-linux-gnu target.
-sim
This option, recognized for the cris-axis-elf, arranges to link with input-
output functions from a simulator library. Code, initialized data and zero-
initialized data are allocated consecutively.
-sim2
Like -sim, but pass linker options to locate initialized data at 0x40000000 and
zero-initialized data at 0x80000000.
CR16 Options
These options are defined specifically for the CR16 ports.
-mmac
Enable the use of multiply-accumulate instructions. Disabled by default.
-mcr16cplus
-mcr16c
Generate code for CR16C or CR16C+ architecture. CR16C+ architecture is default.
-msim
Links the library libsim.a which is in compatible with simulator. Applicable to
ELF compiler only.
-mint32
Choose integer type as 32-bit wide.
-mbit-ops
Generates "sbit"/"cbit" instructions for bit manipulations.
-mdata-model=model
Choose a data model. The choices for model are near, far or medium. medium is
default. However, far is not valid with -mcr16c, as the CR16C architecture
does not support the far data model.
Darwin Options
These options are defined for all architectures running the Darwin operating
system.
FSF GCC on Darwin does not create "fat" object files; it creates an object file for
the single architecture that GCC was built to target. Apple's GCC on Darwin does
create "fat" files if multiple -arch options are used; it does so by running the
compiler or linker multiple times and joining the results together with lipo.
The subtype of the file created (like ppc7400 or ppc970 or i686) is determined by
the flags that specify the ISA that GCC is targeting, like -mcpu or -march. The
-force_cpusubtype_ALL option can be used to override this.
The Darwin tools vary in their behavior when presented with an ISA mismatch. The
assembler, as, only permits instructions to be used that are valid for the subtype
of the file it is generating, so you cannot put 64-bit instructions in a ppc750
object file. The linker for shared libraries, /usr/bin/libtool, fails and prints
an error if asked to create a shared library with a less restrictive subtype than
its input files (for instance, trying to put a ppc970 object file in a ppc7400
library). The linker for executables, ld, quietly gives the executable the most
restrictive subtype of any of its input files.
-Fdir
Add the framework directory dir to the head of the list of directories to be
searched for header files. These directories are interleaved with those
specified by -I options and are scanned in a left-to-right order.
A framework directory is a directory with frameworks in it. A framework is a
directory with a Headers and/or PrivateHeaders directory contained directly in
it that ends in .framework. The name of a framework is the name of this
directory excluding the .framework. Headers associated with the framework are
found in one of those two directories, with Headers being searched first. A
subframework is a framework directory that is in a framework's Frameworks
directory. Includes of subframework headers can only appear in a header of a
framework that contains the subframework, or in a sibling subframework header.
Two subframeworks are siblings if they occur in the same framework. A
subframework should not have the same name as a framework; a warning is issued
if this is violated. Currently a subframework cannot have subframeworks; in
the future, the mechanism may be extended to support this. The standard
frameworks can be found in /System/Library/Frameworks and /Library/Frameworks.
An example include looks like "#include <Framework/header.h>", where Framework
denotes the name of the framework and header.h is found in the PrivateHeaders
or Headers directory.
-iframeworkdir
Like -F except the directory is a treated as a system directory. The main
difference between this -iframework and -F is that with -iframework the
compiler does not warn about constructs contained within header files found via
dir. This option is valid only for the C family of languages.
-gused
Emit debugging information for symbols that are used. For stabs debugging
format, this enables -feliminate-unused-debug-symbols. This is by default ON.
-gfull
Emit debugging information for all symbols and types.
-mmacosx-version-min=version
The earliest version of MacOS X that this executable will run on is version.
Typical values of version include 10.1, 10.2, and 10.3.9.
If the compiler was built to use the system's headers by default, then the
default for this option is the system version on which the compiler is running,
otherwise the default is to make choices that are compatible with as many
systems and code bases as possible.
-mkernel
Enable kernel development mode. The -mkernel option sets -static, -fno-common,
-fno-use-cxa-atexit, -fno-exceptions, -fno-non-call-exceptions, -fapple-kext,
-fno-weak and -fno-rtti where applicable. This mode also sets -mno-altivec,
-msoft-float, -fno-builtin and -mlong-branch for PowerPC targets.
-mone-byte-bool
Override the defaults for "bool" so that "sizeof(bool)==1". By default
"sizeof(bool)" is 4 when compiling for Darwin/PowerPC and 1 when compiling for
Darwin/x86, so this option has no effect on x86.
Warning: The -mone-byte-bool switch causes GCC to generate code that is not
binary compatible with code generated without that switch. Using this switch
may require recompiling all other modules in a program, including system
libraries. Use this switch to conform to a non-default data model.
-mfix-and-continue
-ffix-and-continue
-findirect-data
Generate code suitable for fast turnaround development, such as to allow GDB to
dynamically load .o files into already-running programs. -findirect-data and
-ffix-and-continue are provided for backwards compatibility.
-all_load
Loads all members of static archive libraries. See man ld(1) for more
information.
-arch_errors_fatal
Cause the errors having to do with files that have the wrong architecture to be
fatal.
-bind_at_load
Causes the output file to be marked such that the dynamic linker will bind all
undefined references when the file is loaded or launched.
-bundle
Produce a Mach-o bundle format file. See man ld(1) for more information.
-bundle_loader executable
This option specifies the executable that will load the build output file being
linked. See man ld(1) for more information.
-dynamiclib
When passed this option, GCC produces a dynamic library instead of an
executable when linking, using the Darwin libtool command.
-force_cpusubtype_ALL
This causes GCC's output file to have the ALL subtype, instead of one
controlled by the -mcpu or -march option.
-allowable_client client_name
-client_name
-compatibility_version
-current_version
-dead_strip
-dependency-file
-dylib_file
-dylinker_install_name
-dynamic
-exported_symbols_list
-filelist
-flat_namespace
-force_flat_namespace
-headerpad_max_install_names
-image_base
-init
-install_name
-keep_private_externs
-multi_module
-multiply_defined
-multiply_defined_unused
-noall_load
-no_dead_strip_inits_and_terms
-nofixprebinding
-nomultidefs
-noprebind
-noseglinkedit
-pagezero_size
-prebind
-prebind_all_twolevel_modules
-private_bundle
-read_only_relocs
-sectalign
-sectobjectsymbols
-whyload
-seg1addr
-sectcreate
-sectobjectsymbols
-sectorder
-segaddr
-segs_read_only_addr
-segs_read_write_addr
-seg_addr_table
-seg_addr_table_filename
-seglinkedit
-segprot
-segs_read_only_addr
-segs_read_write_addr
-single_module
-static
-sub_library
-sub_umbrella
-twolevel_namespace
-umbrella
-undefined
-unexported_symbols_list
-weak_reference_mismatches
-whatsloaded
These options are passed to the Darwin linker. The Darwin linker man page
describes them in detail.
DEC Alpha Options
These -m options are defined for the DEC Alpha implementations:
-mno-soft-float
-msoft-float
Use (do not use) the hardware floating-point instructions for floating-point
operations. When -msoft-float is specified, functions in libgcc.a are used to
perform floating-point operations. Unless they are replaced by routines that
emulate the floating-point operations, or compiled in such a way as to call
such emulations routines, these routines issue floating-point operations. If
you are compiling for an Alpha without floating-point operations, you must
ensure that the library is built so as not to call them.
Note that Alpha implementations without floating-point operations are required
to have floating-point registers.
-mfp-reg
-mno-fp-regs
Generate code that uses (does not use) the floating-point register set.
-mno-fp-regs implies -msoft-float. If the floating-point register set is not
used, floating-point operands are passed in integer registers as if they were
integers and floating-point results are passed in $0 instead of $f0. This is a
non-standard calling sequence, so any function with a floating-point argument
or return value called by code compiled with -mno-fp-regs must also be compiled
with that option.
A typical use of this option is building a kernel that does not use, and hence
need not save and restore, any floating-point registers.
-mieee
The Alpha architecture implements floating-point hardware optimized for maximum
performance. It is mostly compliant with the IEEE floating-point standard.
However, for full compliance, software assistance is required. This option
generates code fully IEEE-compliant code except that the inexact-flag is not
maintained (see below). If this option is turned on, the preprocessor macro
"_IEEE_FP" is defined during compilation. The resulting code is less efficient
but is able to correctly support denormalized numbers and exceptional IEEE
values such as not-a-number and plus/minus infinity. Other Alpha compilers
call this option -ieee_with_no_inexact.
-mieee-with-inexact
This is like -mieee except the generated code also maintains the IEEE inexact-
flag. Turning on this option causes the generated code to implement fully-
compliant IEEE math. In addition to "_IEEE_FP", "_IEEE_FP_EXACT" is defined as
a preprocessor macro. On some Alpha implementations the resulting code may
execute significantly slower than the code generated by default. Since there
is very little code that depends on the inexact-flag, you should normally not
specify this option. Other Alpha compilers call this option
-ieee_with_inexact.
-mfp-trap-mode=trap-mode
This option controls what floating-point related traps are enabled. Other
Alpha compilers call this option -fptm trap-mode. The trap mode can be set to
one of four values:
n This is the default (normal) setting. The only traps that are enabled are
the ones that cannot be disabled in software (e.g., division by zero trap).
u In addition to the traps enabled by n, underflow traps are enabled as well.
su Like u, but the instructions are marked to be safe for software completion
(see Alpha architecture manual for details).
sui Like su, but inexact traps are enabled as well.
-mfp-rounding-mode=rounding-mode
Selects the IEEE rounding mode. Other Alpha compilers call this option -fprm
rounding-mode. The rounding-mode can be one of:
n Normal IEEE rounding mode. Floating-point numbers are rounded towards the
nearest machine number or towards the even machine number in case of a tie.
m Round towards minus infinity.
c Chopped rounding mode. Floating-point numbers are rounded towards zero.
d Dynamic rounding mode. A field in the floating-point control register
(fpcr, see Alpha architecture reference manual) controls the rounding mode
in effect. The C library initializes this register for rounding towards
plus infinity. Thus, unless your program modifies the fpcr, d corresponds
to round towards plus infinity.
-mtrap-precision=trap-precision
In the Alpha architecture, floating-point traps are imprecise. This means
without software assistance it is impossible to recover from a floating trap
and program execution normally needs to be terminated. GCC can generate code
that can assist operating system trap handlers in determining the exact
location that caused a floating-point trap. Depending on the requirements of
an application, different levels of precisions can be selected:
p Program precision. This option is the default and means a trap handler can
only identify which program caused a floating-point exception.
f Function precision. The trap handler can determine the function that
caused a floating-point exception.
i Instruction precision. The trap handler can determine the exact
instruction that caused a floating-point exception.
Other Alpha compilers provide the equivalent options called -scope_safe and
-resumption_safe.
-mieee-conformant
This option marks the generated code as IEEE conformant. You must not use this
option unless you also specify -mtrap-precision=i and either -mfp-trap-mode=su
or -mfp-trap-mode=sui. Its only effect is to emit the line .eflag 48 in the
function prologue of the generated assembly file.
-mbuild-constants
Normally GCC examines a 32- or 64-bit integer constant to see if it can
construct it from smaller constants in two or three instructions. If it
cannot, it outputs the constant as a literal and generates code to load it from
the data segment at run time.
Use this option to require GCC to construct all integer constants using code,
even if it takes more instructions (the maximum is six).
You typically use this option to build a shared library dynamic loader. Itself
a shared library, it must relocate itself in memory before it can find the
variables and constants in its own data segment.
-mbwx
-mno-bwx
-mcix
-mno-cix
-mfix
-mno-fix
-mmax
-mno-max
Indicate whether GCC should generate code to use the optional BWX, CIX, FIX and
MAX instruction sets. The default is to use the instruction sets supported by
the CPU type specified via -mcpu= option or that of the CPU on which GCC was
built if none is specified.
-mfloat-vax
-mfloat-ieee
Generate code that uses (does not use) VAX F and G floating-point arithmetic
instead of IEEE single and double precision.
-mexplicit-relocs
-mno-explicit-relocs
Older Alpha assemblers provided no way to generate symbol relocations except
via assembler macros. Use of these macros does not allow optimal instruction
scheduling. GNU binutils as of version 2.12 supports a new syntax that allows
the compiler to explicitly mark which relocations should apply to which
instructions. This option is mostly useful for debugging, as GCC detects the
capabilities of the assembler when it is built and sets the default
accordingly.
-msmall-data
-mlarge-data
When -mexplicit-relocs is in effect, static data is accessed via gp-relative
relocations. When -msmall-data is used, objects 8 bytes long or smaller are
placed in a small data area (the ".sdata" and ".sbss" sections) and are
accessed via 16-bit relocations off of the $gp register. This limits the size
of the small data area to 64KB, but allows the variables to be directly
accessed via a single instruction.
The default is -mlarge-data. With this option the data area is limited to just
below 2GB. Programs that require more than 2GB of data must use "malloc" or
"mmap" to allocate the data in the heap instead of in the program's data
segment.
When generating code for shared libraries, -fpic implies -msmall-data and -fPIC
implies -mlarge-data.
-msmall-text
-mlarge-text
When -msmall-text is used, the compiler assumes that the code of the entire
program (or shared library) fits in 4MB, and is thus reachable with a branch
instruction. When -msmall-data is used, the compiler can assume that all local
symbols share the same $gp value, and thus reduce the number of instructions
required for a function call from 4 to 1.
The default is -mlarge-text.
-mcpu=cpu_type
Set the instruction set and instruction scheduling parameters for machine type
cpu_type. You can specify either the EV style name or the corresponding chip
number. GCC supports scheduling parameters for the EV4, EV5 and EV6 family of
processors and chooses the default values for the instruction set from the
processor you specify. If you do not specify a processor type, GCC defaults to
the processor on which the compiler was built.
Supported values for cpu_type are
ev4
ev45
21064
Schedules as an EV4 and has no instruction set extensions.
ev5
21164
Schedules as an EV5 and has no instruction set extensions.
ev56
21164a
Schedules as an EV5 and supports the BWX extension.
pca56
21164pc
21164PC
Schedules as an EV5 and supports the BWX and MAX extensions.
ev6
21264
Schedules as an EV6 and supports the BWX, FIX, and MAX extensions.
ev67
21264a
Schedules as an EV6 and supports the BWX, CIX, FIX, and MAX extensions.
Native toolchains also support the value native, which selects the best
architecture option for the host processor. -mcpu=native has no effect if GCC
does not recognize the processor.
-mtune=cpu_type
Set only the instruction scheduling parameters for machine type cpu_type. The
instruction set is not changed.
Native toolchains also support the value native, which selects the best
architecture option for the host processor. -mtune=native has no effect if GCC
does not recognize the processor.
-mmemory-latency=time
Sets the latency the scheduler should assume for typical memory references as
seen by the application. This number is highly dependent on the memory access
patterns used by the application and the size of the external cache on the
machine.
Valid options for time are
number
A decimal number representing clock cycles.
L1
L2
L3
main
The compiler contains estimates of the number of clock cycles for "typical"
EV4 & EV5 hardware for the Level 1, 2 & 3 caches (also called Dcache,
Scache, and Bcache), as well as to main memory. Note that L3 is only valid
for EV5.
FR30 Options
These options are defined specifically for the FR30 port.
-msmall-model
Use the small address space model. This can produce smaller code, but it does
assume that all symbolic values and addresses fit into a 20-bit range.
-mno-lsim
Assume that runtime support has been provided and so there is no need to
include the simulator library (libsim.a) on the linker command line.
FT32 Options
These options are defined specifically for the FT32 port.
-msim
Specifies that the program will be run on the simulator. This causes an
alternate runtime startup and library to be linked. You must not use this
option when generating programs that will run on real hardware; you must
provide your own runtime library for whatever I/O functions are needed.
-mlra
Enable Local Register Allocation. This is still experimental for FT32, so by
default the compiler uses standard reload.
-mnodiv
Do not use div and mod instructions.
-mft32b
Enable use of the extended instructions of the FT32B processor.
-mcompress
Compress all code using the Ft32B code compression scheme.
-mnopm
Do not generate code that reads program memory.
FRV Options
-mgpr-32
Only use the first 32 general-purpose registers.
-mgpr-64
Use all 64 general-purpose registers.
-mfpr-32
Use only the first 32 floating-point registers.
-mfpr-64
Use all 64 floating-point registers.
-mhard-float
Use hardware instructions for floating-point operations.
-msoft-float
Use library routines for floating-point operations.
-malloc-cc
Dynamically allocate condition code registers.
-mfixed-cc
Do not try to dynamically allocate condition code registers, only use "icc0"
and "fcc0".
-mdword
Change ABI to use double word insns.
-mno-dword
Do not use double word instructions.
-mdouble
Use floating-point double instructions.
-mno-double
Do not use floating-point double instructions.
-mmedia
Use media instructions.
-mno-media
Do not use media instructions.
-mmuladd
Use multiply and add/subtract instructions.
-mno-muladd
Do not use multiply and add/subtract instructions.
-mfdpic
Select the FDPIC ABI, which uses function descriptors to represent pointers to
functions. Without any PIC/PIE-related options, it implies -fPIE. With -fpic
or -fpie, it assumes GOT entries and small data are within a 12-bit range from
the GOT base address; with -fPIC or -fPIE, GOT offsets are computed with 32
bits. With a bfin-elf target, this option implies -msim.
-minline-plt
Enable inlining of PLT entries in function calls to functions that are not
known to bind locally. It has no effect without -mfdpic. It's enabled by
default if optimizing for speed and compiling for shared libraries (i.e., -fPIC
or -fpic), or when an optimization option such as -O3 or above is present in
the command line.
-mTLS
Assume a large TLS segment when generating thread-local code.
-mtls
Do not assume a large TLS segment when generating thread-local code.
-mgprel-ro
Enable the use of "GPREL" relocations in the FDPIC ABI for data that is known
to be in read-only sections. It's enabled by default, except for -fpic or
-fpie: even though it may help make the global offset table smaller, it trades
1 instruction for 4. With -fPIC or -fPIE, it trades 3 instructions for 4, one
of which may be shared by multiple symbols, and it avoids the need for a GOT
entry for the referenced symbol, so it's more likely to be a win. If it is
not, -mno-gprel-ro can be used to disable it.
-multilib-library-pic
Link with the (library, not FD) pic libraries. It's implied by -mlibrary-pic,
as well as by -fPIC and -fpic without -mfdpic. You should never have to use it
explicitly.
-mlinked-fp
Follow the EABI requirement of always creating a frame pointer whenever a stack
frame is allocated. This option is enabled by default and can be disabled with
-mno-linked-fp.
-mlong-calls
Use indirect addressing to call functions outside the current compilation unit.
This allows the functions to be placed anywhere within the 32-bit address
space.
-malign-labels
Try to align labels to an 8-byte boundary by inserting NOPs into the previous
packet. This option only has an effect when VLIW packing is enabled. It
doesn't create new packets; it merely adds NOPs to existing ones.
-mlibrary-pic
Generate position-independent EABI code.
-macc-4
Use only the first four media accumulator registers.
-macc-8
Use all eight media accumulator registers.
-mpack
Pack VLIW instructions.
-mno-pack
Do not pack VLIW instructions.
-mno-eflags
Do not mark ABI switches in e_flags.
-mcond-move
Enable the use of conditional-move instructions (default).
This switch is mainly for debugging the compiler and will likely be removed in
a future version.
-mno-cond-move
Disable the use of conditional-move instructions.
This switch is mainly for debugging the compiler and will likely be removed in
a future version.
-mscc
Enable the use of conditional set instructions (default).
This switch is mainly for debugging the compiler and will likely be removed in
a future version.
-mno-scc
Disable the use of conditional set instructions.
This switch is mainly for debugging the compiler and will likely be removed in
a future version.
-mcond-exec
Enable the use of conditional execution (default).
This switch is mainly for debugging the compiler and will likely be removed in
a future version.
-mno-cond-exec
Disable the use of conditional execution.
This switch is mainly for debugging the compiler and will likely be removed in
a future version.
-mvliw-branch
Run a pass to pack branches into VLIW instructions (default).
This switch is mainly for debugging the compiler and will likely be removed in
a future version.
-mno-vliw-branch
Do not run a pass to pack branches into VLIW instructions.
This switch is mainly for debugging the compiler and will likely be removed in
a future version.
-mmulti-cond-exec
Enable optimization of "&&" and "||" in conditional execution (default).
This switch is mainly for debugging the compiler and will likely be removed in
a future version.
-mno-multi-cond-exec
Disable optimization of "&&" and "||" in conditional execution.
This switch is mainly for debugging the compiler and will likely be removed in
a future version.
-mnested-cond-exec
Enable nested conditional execution optimizations (default).
This switch is mainly for debugging the compiler and will likely be removed in
a future version.
-mno-nested-cond-exec
Disable nested conditional execution optimizations.
This switch is mainly for debugging the compiler and will likely be removed in
a future version.
-moptimize-membar
This switch removes redundant "membar" instructions from the compiler-generated
code. It is enabled by default.
-mno-optimize-membar
This switch disables the automatic removal of redundant "membar" instructions
from the generated code.
-mtomcat-stats
Cause gas to print out tomcat statistics.
-mcpu=cpu
Select the processor type for which to generate code. Possible values are frv,
fr550, tomcat, fr500, fr450, fr405, fr400, fr300 and simple.
GNU/Linux Options
These -m options are defined for GNU/Linux targets:
-mglibc
Use the GNU C library. This is the default except on *-*-linux-*uclibc*,
*-*-linux-*musl* and *-*-linux-*android* targets.
-muclibc
Use uClibc C library. This is the default on *-*-linux-*uclibc* targets.
-mmusl
Use the musl C library. This is the default on *-*-linux-*musl* targets.
-mbionic
Use Bionic C library. This is the default on *-*-linux-*android* targets.
-mandroid
Compile code compatible with Android platform. This is the default on
*-*-linux-*android* targets.
When compiling, this option enables -mbionic, -fPIC, -fno-exceptions and
-fno-rtti by default. When linking, this option makes the GCC driver pass
Android-specific options to the linker. Finally, this option causes the
preprocessor macro "__ANDROID__" to be defined.
-tno-android-cc
Disable compilation effects of -mandroid, i.e., do not enable -mbionic, -fPIC,
-fno-exceptions and -fno-rtti by default.
-tno-android-ld
Disable linking effects of -mandroid, i.e., pass standard Linux linking options
to the linker.
H8/300 Options
These -m options are defined for the H8/300 implementations:
-mrelax
Shorten some address references at link time, when possible; uses the linker
option -relax.
-mh Generate code for the H8/300H.
-ms Generate code for the H8S.
-mn Generate code for the H8S and H8/300H in the normal mode. This switch must be
used either with -mh or -ms.
-ms2600
Generate code for the H8S/2600. This switch must be used with -ms.
-mexr
Extended registers are stored on stack before execution of function with
monitor attribute. Default option is -mexr. This option is valid only for H8S
targets.
-mno-exr
Extended registers are not stored on stack before execution of function with
monitor attribute. Default option is -mno-exr. This option is valid only for
H8S targets.
-mint32
Make "int" data 32 bits by default.
-malign-300
On the H8/300H and H8S, use the same alignment rules as for the H8/300. The
default for the H8/300H and H8S is to align longs and floats on 4-byte
boundaries. -malign-300 causes them to be aligned on 2-byte boundaries. This
option has no effect on the H8/300.
HPPA Options
These -m options are defined for the HPPA family of computers:
-march=architecture-type
Generate code for the specified architecture. The choices for architecture-
type are 1.0 for PA 1.0, 1.1 for PA 1.1, and 2.0 for PA 2.0 processors. Refer
to /usr/lib/sched.models on an HP-UX system to determine the proper
architecture option for your machine. Code compiled for lower numbered
architectures runs on higher numbered architectures, but not the other way
around.
-mpa-risc-1-0
-mpa-risc-1-1
-mpa-risc-2-0
Synonyms for -march=1.0, -march=1.1, and -march=2.0 respectively.
-mcaller-copies
The caller copies function arguments passed by hidden reference. This option
should be used with care as it is not compatible with the default 32-bit
runtime. However, only aggregates larger than eight bytes are passed by hidden
reference and the option provides better compatibility with OpenMP.
-mjump-in-delay
This option is ignored and provided for compatibility purposes only.
-mdisable-fpregs
Prevent floating-point registers from being used in any manner. This is
necessary for compiling kernels that perform lazy context switching of
floating-point registers. If you use this option and attempt to perform
floating-point operations, the compiler aborts.
-mdisable-indexing
Prevent the compiler from using indexing address modes. This avoids some
rather obscure problems when compiling MIG generated code under MACH.
-mno-space-regs
Generate code that assumes the target has no space registers. This allows GCC
to generate faster indirect calls and use unscaled index address modes.
Such code is suitable for level 0 PA systems and kernels.
-mfast-indirect-calls
Generate code that assumes calls never cross space boundaries. This allows GCC
to emit code that performs faster indirect calls.
This option does not work in the presence of shared libraries or nested
functions.
-mfixed-range=register-range
Generate code treating the given register range as fixed registers. A fixed
register is one that the register allocator cannot use. This is useful when
compiling kernel code. A register range is specified as two registers
separated by a dash. Multiple register ranges can be specified separated by a
comma.
-mlong-load-store
Generate 3-instruction load and store sequences as sometimes required by the
HP-UX 10 linker. This is equivalent to the +k option to the HP compilers.
-mportable-runtime
Use the portable calling conventions proposed by HP for ELF systems.
-mgas
Enable the use of assembler directives only GAS understands.
-mschedule=cpu-type
Schedule code according to the constraints for the machine type cpu-type. The
choices for cpu-type are 700 7100, 7100LC, 7200, 7300 and 8000. Refer to
/usr/lib/sched.models on an HP-UX system to determine the proper scheduling
option for your machine. The default scheduling is 8000.
-mlinker-opt
Enable the optimization pass in the HP-UX linker. Note this makes symbolic
debugging impossible. It also triggers a bug in the HP-UX 8 and HP-UX 9
linkers in which they give bogus error messages when linking some programs.
-msoft-float
Generate output containing library calls for floating point. Warning: the
requisite libraries are not available for all HPPA targets. Normally the
facilities of the machine's usual C compiler are used, but this cannot be done
directly in cross-compilation. You must make your own arrangements to provide
suitable library functions for cross-compilation.
-msoft-float changes the calling convention in the output file; therefore, it
is only useful if you compile all of a program with this option. In
particular, you need to compile libgcc.a, the library that comes with GCC, with
-msoft-float in order for this to work.
-msio
Generate the predefine, "_SIO", for server IO. The default is -mwsio. This
generates the predefines, "__hp9000s700", "__hp9000s700__" and "_WSIO", for
workstation IO. These options are available under HP-UX and HI-UX.
-mgnu-ld
Use options specific to GNU ld. This passes -shared to ld when building a
shared library. It is the default when GCC is configured, explicitly or
implicitly, with the GNU linker. This option does not affect which ld is
called; it only changes what parameters are passed to that ld. The ld that is
called is determined by the --with-ld configure option, GCC's program search
path, and finally by the user's PATH. The linker used by GCC can be printed
using which `gcc -print-prog-name=ld`. This option is only available on the
64-bit HP-UX GCC, i.e. configured with hppa*64*-*-hpux*.
-mhp-ld
Use options specific to HP ld. This passes -b to ld when building a shared
library and passes +Accept TypeMismatch to ld on all links. It is the default
when GCC is configured, explicitly or implicitly, with the HP linker. This
option does not affect which ld is called; it only changes what parameters are
passed to that ld. The ld that is called is determined by the --with-ld
configure option, GCC's program search path, and finally by the user's PATH.
The linker used by GCC can be printed using which `gcc -print-prog-name=ld`.
This option is only available on the 64-bit HP-UX GCC, i.e. configured with
hppa*64*-*-hpux*.
-mlong-calls
Generate code that uses long call sequences. This ensures that a call is
always able to reach linker generated stubs. The default is to generate long
calls only when the distance from the call site to the beginning of the
function or translation unit, as the case may be, exceeds a predefined limit
set by the branch type being used. The limits for normal calls are 7,600,000
and 240,000 bytes, respectively for the PA 2.0 and PA 1.X architectures.
Sibcalls are always limited at 240,000 bytes.
Distances are measured from the beginning of functions when using the
-ffunction-sections option, or when using the -mgas and -mno-portable-runtime
options together under HP-UX with the SOM linker.
It is normally not desirable to use this option as it degrades performance.
However, it may be useful in large applications, particularly when partial
linking is used to build the application.
The types of long calls used depends on the capabilities of the assembler and
linker, and the type of code being generated. The impact on systems that
support long absolute calls, and long pic symbol-difference or pc-relative
calls should be relatively small. However, an indirect call is used on 32-bit
ELF systems in pic code and it is quite long.
-munix=unix-std
Generate compiler predefines and select a startfile for the specified UNIX
standard. The choices for unix-std are 93, 95 and 98. 93 is supported on all
HP-UX versions. 95 is available on HP-UX 10.10 and later. 98 is available on
HP-UX 11.11 and later. The default values are 93 for HP-UX 10.00, 95 for HP-UX
10.10 though to 11.00, and 98 for HP-UX 11.11 and later.
-munix=93 provides the same predefines as GCC 3.3 and 3.4. -munix=95 provides
additional predefines for "XOPEN_UNIX" and "_XOPEN_SOURCE_EXTENDED", and the
startfile unix95.o. -munix=98 provides additional predefines for
"_XOPEN_UNIX", "_XOPEN_SOURCE_EXTENDED", "_INCLUDE__STDC_A1_SOURCE" and
"_INCLUDE_XOPEN_SOURCE_500", and the startfile unix98.o.
It is important to note that this option changes the interfaces for various
library routines. It also affects the operational behavior of the C library.
Thus, extreme care is needed in using this option.
Library code that is intended to operate with more than one UNIX standard must
test, set and restore the variable "__xpg4_extended_mask" as appropriate. Most
GNU software doesn't provide this capability.
-nolibdld
Suppress the generation of link options to search libdld.sl when the -static
option is specified on HP-UX 10 and later.
-static
The HP-UX implementation of setlocale in libc has a dependency on libdld.sl.
There isn't an archive version of libdld.sl. Thus, when the -static option is
specified, special link options are needed to resolve this dependency.
On HP-UX 10 and later, the GCC driver adds the necessary options to link with
libdld.sl when the -static option is specified. This causes the resulting
binary to be dynamic. On the 64-bit port, the linkers generate dynamic
binaries by default in any case. The -nolibdld option can be used to prevent
the GCC driver from adding these link options.
-threads
Add support for multithreading with the dce thread library under HP-UX. This
option sets flags for both the preprocessor and linker.
IA-64 Options
These are the -m options defined for the Intel IA-64 architecture.
-mbig-endian
Generate code for a big-endian target. This is the default for HP-UX.
-mlittle-endian
Generate code for a little-endian target. This is the default for AIX5 and
GNU/Linux.
-mgnu-as
-mno-gnu-as
Generate (or don't) code for the GNU assembler. This is the default.
-mgnu-ld
-mno-gnu-ld
Generate (or don't) code for the GNU linker. This is the default.
-mno-pic
Generate code that does not use a global pointer register. The result is not
position independent code, and violates the IA-64 ABI.
-mvolatile-asm-stop
-mno-volatile-asm-stop
Generate (or don't) a stop bit immediately before and after volatile asm
statements.
-mregister-names
-mno-register-names
Generate (or don't) in, loc, and out register names for the stacked registers.
This may make assembler output more readable.
-mno-sdata
-msdata
Disable (or enable) optimizations that use the small data section. This may be
useful for working around optimizer bugs.
-mconstant-gp
Generate code that uses a single constant global pointer value. This is useful
when compiling kernel code.
-mauto-pic
Generate code that is self-relocatable. This implies -mconstant-gp. This is
useful when compiling firmware code.
-minline-float-divide-min-latency
Generate code for inline divides of floating-point values using the minimum
latency algorithm.
-minline-float-divide-max-throughput
Generate code for inline divides of floating-point values using the maximum
throughput algorithm.
-mno-inline-float-divide
Do not generate inline code for divides of floating-point values.
-minline-int-divide-min-latency
Generate code for inline divides of integer values using the minimum latency
algorithm.
-minline-int-divide-max-throughput
Generate code for inline divides of integer values using the maximum throughput
algorithm.
-mno-inline-int-divide
Do not generate inline code for divides of integer values.
-minline-sqrt-min-latency
Generate code for inline square roots using the minimum latency algorithm.
-minline-sqrt-max-throughput
Generate code for inline square roots using the maximum throughput algorithm.
-mno-inline-sqrt
Do not generate inline code for "sqrt".
-mfused-madd
-mno-fused-madd
Do (don't) generate code that uses the fused multiply/add or multiply/subtract
instructions. The default is to use these instructions.
-mno-dwarf2-asm
-mdwarf2-asm
Don't (or do) generate assembler code for the DWARF line number debugging info.
This may be useful when not using the GNU assembler.
-mearly-stop-bits
-mno-early-stop-bits
Allow stop bits to be placed earlier than immediately preceding the instruction
that triggered the stop bit. This can improve instruction scheduling, but does
not always do so.
-mfixed-range=register-range
Generate code treating the given register range as fixed registers. A fixed
register is one that the register allocator cannot use. This is useful when
compiling kernel code. A register range is specified as two registers
separated by a dash. Multiple register ranges can be specified separated by a
comma.
-mtls-size=tls-size
Specify bit size of immediate TLS offsets. Valid values are 14, 22, and 64.
-mtune=cpu-type
Tune the instruction scheduling for a particular CPU, Valid values are itanium,
itanium1, merced, itanium2, and mckinley.
-milp32
-mlp64
Generate code for a 32-bit or 64-bit environment. The 32-bit environment sets
int, long and pointer to 32 bits. The 64-bit environment sets int to 32 bits
and long and pointer to 64 bits. These are HP-UX specific flags.
-mno-sched-br-data-spec
-msched-br-data-spec
(Dis/En)able data speculative scheduling before reload. This results in
generation of "ld.a" instructions and the corresponding check instructions
("ld.c" / "chk.a"). The default setting is disabled.
-msched-ar-data-spec
-mno-sched-ar-data-spec
(En/Dis)able data speculative scheduling after reload. This results in
generation of "ld.a" instructions and the corresponding check instructions
("ld.c" / "chk.a"). The default setting is enabled.
-mno-sched-control-spec
-msched-control-spec
(Dis/En)able control speculative scheduling. This feature is available only
during region scheduling (i.e. before reload). This results in generation of
the "ld.s" instructions and the corresponding check instructions "chk.s". The
default setting is disabled.
-msched-br-in-data-spec
-mno-sched-br-in-data-spec
(En/Dis)able speculative scheduling of the instructions that are dependent on
the data speculative loads before reload. This is effective only with
-msched-br-data-spec enabled. The default setting is enabled.
-msched-ar-in-data-spec
-mno-sched-ar-in-data-spec
(En/Dis)able speculative scheduling of the instructions that are dependent on
the data speculative loads after reload. This is effective only with
-msched-ar-data-spec enabled. The default setting is enabled.
-msched-in-control-spec
-mno-sched-in-control-spec
(En/Dis)able speculative scheduling of the instructions that are dependent on
the control speculative loads. This is effective only with
-msched-control-spec enabled. The default setting is enabled.
-mno-sched-prefer-non-data-spec-insns
-msched-prefer-non-data-spec-insns
If enabled, data-speculative instructions are chosen for schedule only if there
are no other choices at the moment. This makes the use of the data speculation
much more conservative. The default setting is disabled.
-mno-sched-prefer-non-control-spec-insns
-msched-prefer-non-control-spec-insns
If enabled, control-speculative instructions are chosen for schedule only if
there are no other choices at the moment. This makes the use of the control
speculation much more conservative. The default setting is disabled.
-mno-sched-count-spec-in-critical-path
-msched-count-spec-in-critical-path
If enabled, speculative dependencies are considered during computation of the
instructions priorities. This makes the use of the speculation a bit more
conservative. The default setting is disabled.
-msched-spec-ldc
Use a simple data speculation check. This option is on by default.
-msched-control-spec-ldc
Use a simple check for control speculation. This option is on by default.
-msched-stop-bits-after-every-cycle
Place a stop bit after every cycle when scheduling. This option is on by
default.
-msched-fp-mem-deps-zero-cost
Assume that floating-point stores and loads are not likely to cause a conflict
when placed into the same instruction group. This option is disabled by
default.
-msel-sched-dont-check-control-spec
Generate checks for control speculation in selective scheduling. This flag is
disabled by default.
-msched-max-memory-insns=max-insns
Limit on the number of memory insns per instruction group, giving lower
priority to subsequent memory insns attempting to schedule in the same
instruction group. Frequently useful to prevent cache bank conflicts. The
default value is 1.
-msched-max-memory-insns-hard-limit
Makes the limit specified by msched-max-memory-insns a hard limit, disallowing
more than that number in an instruction group. Otherwise, the limit is "soft",
meaning that non-memory operations are preferred when the limit is reached, but
memory operations may still be scheduled.
LM32 Options
These -m options are defined for the LatticeMico32 architecture:
-mbarrel-shift-enabled
Enable barrel-shift instructions.
-mdivide-enabled
Enable divide and modulus instructions.
-mmultiply-enabled
Enable multiply instructions.
-msign-extend-enabled
Enable sign extend instructions.
-muser-enabled
Enable user-defined instructions.
M32C Options
-mcpu=name
Select the CPU for which code is generated. name may be one of r8c for the
R8C/Tiny series, m16c for the M16C (up to /60) series, m32cm for the M16C/80
series, or m32c for the M32C/80 series.
-msim
Specifies that the program will be run on the simulator. This causes an
alternate runtime library to be linked in which supports, for example, file
I/O. You must not use this option when generating programs that will run on
real hardware; you must provide your own runtime library for whatever I/O
functions are needed.
-memregs=number
Specifies the number of memory-based pseudo-registers GCC uses during code
generation. These pseudo-registers are used like real registers, so there is a
tradeoff between GCC's ability to fit the code into available registers, and
the performance penalty of using memory instead of registers. Note that all
modules in a program must be compiled with the same value for this option.
Because of that, you must not use this option with GCC's default runtime
libraries.
M32R/D Options
These -m options are defined for Renesas M32R/D architectures:
-m32r2
Generate code for the M32R/2.
-m32rx
Generate code for the M32R/X.
-m32r
Generate code for the M32R. This is the default.
-mmodel=small
Assume all objects live in the lower 16MB of memory (so that their addresses
can be loaded with the "ld24" instruction), and assume all subroutines are
reachable with the "bl" instruction. This is the default.
The addressability of a particular object can be set with the "model"
attribute.
-mmodel=medium
Assume objects may be anywhere in the 32-bit address space (the compiler
generates "seth/add3" instructions to load their addresses), and assume all
subroutines are reachable with the "bl" instruction.
-mmodel=large
Assume objects may be anywhere in the 32-bit address space (the compiler
generates "seth/add3" instructions to load their addresses), and assume
subroutines may not be reachable with the "bl" instruction (the compiler
generates the much slower "seth/add3/jl" instruction sequence).
-msdata=none
Disable use of the small data area. Variables are put into one of ".data",
".bss", or ".rodata" (unless the "section" attribute has been specified). This
is the default.
The small data area consists of sections ".sdata" and ".sbss". Objects may be
explicitly put in the small data area with the "section" attribute using one of
these sections.
-msdata=sdata
Put small global and static data in the small data area, but do not generate
special code to reference them.
-msdata=use
Put small global and static data in the small data area, and generate special
instructions to reference them.
-G num
Put global and static objects less than or equal to num bytes into the small
data or BSS sections instead of the normal data or BSS sections. The default
value of num is 8. The -msdata option must be set to one of sdata or use for
this option to have any effect.
All modules should be compiled with the same -G num value. Compiling with
different values of num may or may not work; if it doesn't the linker gives an
error message---incorrect code is not generated.
-mdebug
Makes the M32R-specific code in the compiler display some statistics that might
help in debugging programs.
-malign-loops
Align all loops to a 32-byte boundary.
-mno-align-loops
Do not enforce a 32-byte alignment for loops. This is the default.
-missue-rate=number
Issue number instructions per cycle. number can only be 1 or 2.
-mbranch-cost=number
number can only be 1 or 2. If it is 1 then branches are preferred over
conditional code, if it is 2, then the opposite applies.
-mflush-trap=number
Specifies the trap number to use to flush the cache. The default is 12. Valid
numbers are between 0 and 15 inclusive.
-mno-flush-trap
Specifies that the cache cannot be flushed by using a trap.
-mflush-func=name
Specifies the name of the operating system function to call to flush the cache.
The default is _flush_cache, but a function call is only used if a trap is not
available.
-mno-flush-func
Indicates that there is no OS function for flushing the cache.
M680x0 Options
These are the -m options defined for M680x0 and ColdFire processors. The default
settings depend on which architecture was selected when the compiler was
configured; the defaults for the most common choices are given below.
-march=arch
Generate code for a specific M680x0 or ColdFire instruction set architecture.
Permissible values of arch for M680x0 architectures are: 68000, 68010, 68020,
68030, 68040, 68060 and cpu32. ColdFire architectures are selected according
to Freescale's ISA classification and the permissible values are: isaa,
isaaplus, isab and isac.
GCC defines a macro "__mcfarch__" whenever it is generating code for a ColdFire
target. The arch in this macro is one of the -march arguments given above.
When used together, -march and -mtune select code that runs on a family of
similar processors but that is optimized for a particular microarchitecture.
-mcpu=cpu
Generate code for a specific M680x0 or ColdFire processor. The M680x0 cpus
are: 68000, 68010, 68020, 68030, 68040, 68060, 68302, 68332 and cpu32. The
ColdFire cpus are given by the table below, which also classifies the CPUs into
families:
Family : -mcpu arguments
51 : 51 51ac 51ag 51cn 51em 51je 51jf 51jg 51jm 51mm 51qe 51qm
5206 : 5202 5204 5206
5206e : 5206e
5208 : 5207 5208
5211a : 5210a 5211a
5213 : 5211 5212 5213
5216 : 5214 5216
52235 : 52230 52231 52232 52233 52234 52235
5225 : 5224 5225
52259 : 52252 52254 52255 52256 52258 52259
5235 : 5232 5233 5234 5235 523x
5249 : 5249
5250 : 5250
5271 : 5270 5271
5272 : 5272
5275 : 5274 5275
5282 : 5280 5281 5282 528x
53017 : 53011 53012 53013 53014 53015 53016 53017
5307 : 5307
5329 : 5327 5328 5329 532x
5373 : 5372 5373 537x
5407 : 5407
5475 : 5470 5471 5472 5473 5474 5475 547x 5480 5481 5482 5483 5484 5485
-mcpu=cpu overrides -march=arch if arch is compatible with cpu. Other
combinations of -mcpu and -march are rejected.
GCC defines the macro "__mcf_cpu_cpu" when ColdFire target cpu is selected. It
also defines "__mcf_family_family", where the value of family is given by the
table above.
-mtune=tune
Tune the code for a particular microarchitecture within the constraints set by
-march and -mcpu. The M680x0 microarchitectures are: 68000, 68010, 68020,
68030, 68040, 68060 and cpu32. The ColdFire microarchitectures are: cfv1,
cfv2, cfv3, cfv4 and cfv4e.
You can also use -mtune=68020-40 for code that needs to run relatively well on
68020, 68030 and 68040 targets. -mtune=68020-60 is similar but includes 68060
targets as well. These two options select the same tuning decisions as
-m68020-40 and -m68020-60 respectively.
GCC defines the macros "__mcarch" and "__mcarch__" when tuning for 680x0
architecture arch. It also defines "mcarch" unless either -ansi or a non-GNU
-std option is used. If GCC is tuning for a range of architectures, as
selected by -mtune=68020-40 or -mtune=68020-60, it defines the macros for every
architecture in the range.
GCC also defines the macro "__muarch__" when tuning for ColdFire
microarchitecture uarch, where uarch is one of the arguments given above.
-m68000
-mc68000
Generate output for a 68000. This is the default when the compiler is
configured for 68000-based systems. It is equivalent to -march=68000.
Use this option for microcontrollers with a 68000 or EC000 core, including the
68008, 68302, 68306, 68307, 68322, 68328 and 68356.
-m68010
Generate output for a 68010. This is the default when the compiler is
configured for 68010-based systems. It is equivalent to -march=68010.
-m68020
-mc68020
Generate output for a 68020. This is the default when the compiler is
configured for 68020-based systems. It is equivalent to -march=68020.
-m68030
Generate output for a 68030. This is the default when the compiler is
configured for 68030-based systems. It is equivalent to -march=68030.
-m68040
Generate output for a 68040. This is the default when the compiler is
configured for 68040-based systems. It is equivalent to -march=68040.
This option inhibits the use of 68881/68882 instructions that have to be
emulated by software on the 68040. Use this option if your 68040 does not have
code to emulate those instructions.
-m68060
Generate output for a 68060. This is the default when the compiler is
configured for 68060-based systems. It is equivalent to -march=68060.
This option inhibits the use of 68020 and 68881/68882 instructions that have to
be emulated by software on the 68060. Use this option if your 68060 does not
have code to emulate those instructions.
-mcpu32
Generate output for a CPU32. This is the default when the compiler is
configured for CPU32-based systems. It is equivalent to -march=cpu32.
Use this option for microcontrollers with a CPU32 or CPU32+ core, including the
68330, 68331, 68332, 68333, 68334, 68336, 68340, 68341, 68349 and 68360.
-m5200
Generate output for a 520X ColdFire CPU. This is the default when the compiler
is configured for 520X-based systems. It is equivalent to -mcpu=5206, and is
now deprecated in favor of that option.
Use this option for microcontroller with a 5200 core, including the MCF5202,
MCF5203, MCF5204 and MCF5206.
-m5206e
Generate output for a 5206e ColdFire CPU. The option is now deprecated in
favor of the equivalent -mcpu=5206e.
-m528x
Generate output for a member of the ColdFire 528X family. The option is now
deprecated in favor of the equivalent -mcpu=528x.
-m5307
Generate output for a ColdFire 5307 CPU. The option is now deprecated in favor
of the equivalent -mcpu=5307.
-m5407
Generate output for a ColdFire 5407 CPU. The option is now deprecated in favor
of the equivalent -mcpu=5407.
-mcfv4e
Generate output for a ColdFire V4e family CPU (e.g. 547x/548x). This includes
use of hardware floating-point instructions. The option is equivalent to
-mcpu=547x, and is now deprecated in favor of that option.
-m68020-40
Generate output for a 68040, without using any of the new instructions. This
results in code that can run relatively efficiently on either a 68020/68881 or
a 68030 or a 68040. The generated code does use the 68881 instructions that
are emulated on the 68040.
The option is equivalent to -march=68020 -mtune=68020-40.
-m68020-60
Generate output for a 68060, without using any of the new instructions. This
results in code that can run relatively efficiently on either a 68020/68881 or
a 68030 or a 68040. The generated code does use the 68881 instructions that
are emulated on the 68060.
The option is equivalent to -march=68020 -mtune=68020-60.
-mhard-float
-m68881
Generate floating-point instructions. This is the default for 68020 and above,
and for ColdFire devices that have an FPU. It defines the macro
"__HAVE_68881__" on M680x0 targets and "__mcffpu__" on ColdFire targets.
-msoft-float
Do not generate floating-point instructions; use library calls instead. This
is the default for 68000, 68010, and 68832 targets. It is also the default for
ColdFire devices that have no FPU.
-mdiv
-mno-div
Generate (do not generate) ColdFire hardware divide and remainder instructions.
If -march is used without -mcpu, the default is "on" for ColdFire architectures
and "off" for M680x0 architectures. Otherwise, the default is taken from the
target CPU (either the default CPU, or the one specified by -mcpu). For
example, the default is "off" for -mcpu=5206 and "on" for -mcpu=5206e.
GCC defines the macro "__mcfhwdiv__" when this option is enabled.
-mshort
Consider type "int" to be 16 bits wide, like "short int". Additionally,
parameters passed on the stack are also aligned to a 16-bit boundary even on
targets whose API mandates promotion to 32-bit.
-mno-short
Do not consider type "int" to be 16 bits wide. This is the default.
-mnobitfield
-mno-bitfield
Do not use the bit-field instructions. The -m68000, -mcpu32 and -m5200 options
imply -mnobitfield.
-mbitfield
Do use the bit-field instructions. The -m68020 option implies -mbitfield.
This is the default if you use a configuration designed for a 68020.
-mrtd
Use a different function-calling convention, in which functions that take a
fixed number of arguments return with the "rtd" instruction, which pops their
arguments while returning. This saves one instruction in the caller since
there is no need to pop the arguments there.
This calling convention is incompatible with the one normally used on Unix, so
you cannot use it if you need to call libraries compiled with the Unix
compiler.
Also, you must provide function prototypes for all functions that take variable
numbers of arguments (including "printf"); otherwise incorrect code is
generated for calls to those functions.
In addition, seriously incorrect code results if you call a function with too
many arguments. (Normally, extra arguments are harmlessly ignored.)
The "rtd" instruction is supported by the 68010, 68020, 68030, 68040, 68060 and
CPU32 processors, but not by the 68000 or 5200.
-mno-rtd
Do not use the calling conventions selected by -mrtd. This is the default.
-malign-int
-mno-align-int
Control whether GCC aligns "int", "long", "long long", "float", "double", and
"long double" variables on a 32-bit boundary (-malign-int) or a 16-bit boundary
(-mno-align-int). Aligning variables on 32-bit boundaries produces code that
runs somewhat faster on processors with 32-bit busses at the expense of more
memory.
Warning: if you use the -malign-int switch, GCC aligns structures containing
the above types differently than most published application binary interface
specifications for the m68k.
-mpcrel
Use the pc-relative addressing mode of the 68000 directly, instead of using a
global offset table. At present, this option implies -fpic, allowing at most a
16-bit offset for pc-relative addressing. -fPIC is not presently supported
with -mpcrel, though this could be supported for 68020 and higher processors.
-mno-strict-align
-mstrict-align
Do not (do) assume that unaligned memory references are handled by the system.
-msep-data
Generate code that allows the data segment to be located in a different area of
memory from the text segment. This allows for execute-in-place in an
environment without virtual memory management. This option implies -fPIC.
-mno-sep-data
Generate code that assumes that the data segment follows the text segment.
This is the default.
-mid-shared-library
Generate code that supports shared libraries via the library ID method. This
allows for execute-in-place and shared libraries in an environment without
virtual memory management. This option implies -fPIC.
-mno-id-shared-library
Generate code that doesn't assume ID-based shared libraries are being used.
This is the default.
-mshared-library-id=n
Specifies the identification number of the ID-based shared library being
compiled. Specifying a value of 0 generates more compact code; specifying
other values forces the allocation of that number to the current library, but
is no more space- or time-efficient than omitting this option.
-mxgot
-mno-xgot
When generating position-independent code for ColdFire, generate code that
works if the GOT has more than 8192 entries. This code is larger and slower
than code generated without this option. On M680x0 processors, this option is
not needed; -fPIC suffices.
GCC normally uses a single instruction to load values from the GOT. While this
is relatively efficient, it only works if the GOT is smaller than about 64k.
Anything larger causes the linker to report an error such as:
relocation truncated to fit: R_68K_GOT16O foobar
If this happens, you should recompile your code with -mxgot. It should then
work with very large GOTs. However, code generated with -mxgot is less
efficient, since it takes 4 instructions to fetch the value of a global symbol.
Note that some linkers, including newer versions of the GNU linker, can create
multiple GOTs and sort GOT entries. If you have such a linker, you should only
need to use -mxgot when compiling a single object file that accesses more than
8192 GOT entries. Very few do.
These options have no effect unless GCC is generating position-independent
code.
-mlong-jump-table-offsets
Use 32-bit offsets in "switch" tables. The default is to use 16-bit offsets.
MCore Options
These are the -m options defined for the Motorola M*Core processors.
-mhardlit
-mno-hardlit
Inline constants into the code stream if it can be done in two instructions or
less.
-mdiv
-mno-div
Use the divide instruction. (Enabled by default).
-mrelax-immediate
-mno-relax-immediate
Allow arbitrary-sized immediates in bit operations.
-mwide-bitfields
-mno-wide-bitfields
Always treat bit-fields as "int"-sized.
-m4byte-functions
-mno-4byte-functions
Force all functions to be aligned to a 4-byte boundary.
-mcallgraph-data
-mno-callgraph-data
Emit callgraph information.
-mslow-bytes
-mno-slow-bytes
Prefer word access when reading byte quantities.
-mlittle-endian
-mbig-endian
Generate code for a little-endian target.
-m210
-m340
Generate code for the 210 processor.
-mno-lsim
Assume that runtime support has been provided and so omit the simulator library
(libsim.a) from the linker command line.
-mstack-increment=size
Set the maximum amount for a single stack increment operation. Large values
can increase the speed of programs that contain functions that need a large
amount of stack space, but they can also trigger a segmentation fault if the
stack is extended too much. The default value is 0x1000.
MeP Options
-mabsdiff
Enables the "abs" instruction, which is the absolute difference between two
registers.
-mall-opts
Enables all the optional instructions---average, multiply, divide, bit
operations, leading zero, absolute difference, min/max, clip, and saturation.
-maverage
Enables the "ave" instruction, which computes the average of two registers.
-mbased=n
Variables of size n bytes or smaller are placed in the ".based" section by
default. Based variables use the $tp register as a base register, and there is
a 128-byte limit to the ".based" section.
-mbitops
Enables the bit operation instructions---bit test ("btstm"), set ("bsetm"),
clear ("bclrm"), invert ("bnotm"), and test-and-set ("tas").
-mc=name
Selects which section constant data is placed in. name may be tiny, near, or
far.
-mclip
Enables the "clip" instruction. Note that -mclip is not useful unless you also
provide -mminmax.
-mconfig=name
Selects one of the built-in core configurations. Each MeP chip has one or more
modules in it; each module has a core CPU and a variety of coprocessors,
optional instructions, and peripherals. The "MeP-Integrator" tool, not part of
GCC, provides these configurations through this option; using this option is
the same as using all the corresponding command-line options. The default
configuration is default.
-mcop
Enables the coprocessor instructions. By default, this is a 32-bit
coprocessor. Note that the coprocessor is normally enabled via the -mconfig=
option.
-mcop32
Enables the 32-bit coprocessor's instructions.
-mcop64
Enables the 64-bit coprocessor's instructions.
-mivc2
Enables IVC2 scheduling. IVC2 is a 64-bit VLIW coprocessor.
-mdc
Causes constant variables to be placed in the ".near" section.
-mdiv
Enables the "div" and "divu" instructions.
-meb
Generate big-endian code.
-mel
Generate little-endian code.
-mio-volatile
Tells the compiler that any variable marked with the "io" attribute is to be
considered volatile.
-ml Causes variables to be assigned to the ".far" section by default.
-mleadz
Enables the "leadz" (leading zero) instruction.
-mm Causes variables to be assigned to the ".near" section by default.
-mminmax
Enables the "min" and "max" instructions.
-mmult
Enables the multiplication and multiply-accumulate instructions.
-mno-opts
Disables all the optional instructions enabled by -mall-opts.
-mrepeat
Enables the "repeat" and "erepeat" instructions, used for low-overhead looping.
-ms Causes all variables to default to the ".tiny" section. Note that there is a
65536-byte limit to this section. Accesses to these variables use the %gp base
register.
-msatur
Enables the saturation instructions. Note that the compiler does not currently
generate these itself, but this option is included for compatibility with other
tools, like "as".
-msdram
Link the SDRAM-based runtime instead of the default ROM-based runtime.
-msim
Link the simulator run-time libraries.
-msimnovec
Link the simulator runtime libraries, excluding built-in support for reset and
exception vectors and tables.
-mtf
Causes all functions to default to the ".far" section. Without this option,
functions default to the ".near" section.
-mtiny=n
Variables that are n bytes or smaller are allocated to the ".tiny" section.
These variables use the $gp base register. The default for this option is 4,
but note that there's a 65536-byte limit to the ".tiny" section.
MicroBlaze Options
-msoft-float
Use software emulation for floating point (default).
-mhard-float
Use hardware floating-point instructions.
-mmemcpy
Do not optimize block moves, use "memcpy".
-mno-clearbss
This option is deprecated. Use -fno-zero-initialized-in-bss instead.
-mcpu=cpu-type
Use features of, and schedule code for, the given CPU. Supported values are in
the format vX.YY.Z, where X is a major version, YY is the minor version, and Z
is compatibility code. Example values are v3.00.a, v4.00.b, v5.00.a, v5.00.b,
v6.00.a.
-mxl-soft-mul
Use software multiply emulation (default).
-mxl-soft-div
Use software emulation for divides (default).
-mxl-barrel-shift
Use the hardware barrel shifter.
-mxl-pattern-compare
Use pattern compare instructions.
-msmall-divides
Use table lookup optimization for small signed integer divisions.
-mxl-stack-check
This option is deprecated. Use -fstack-check instead.
-mxl-gp-opt
Use GP-relative ".sdata"/".sbss" sections.
-mxl-multiply-high
Use multiply high instructions for high part of 32x32 multiply.
-mxl-float-convert
Use hardware floating-point conversion instructions.
-mxl-float-sqrt
Use hardware floating-point square root instruction.
-mbig-endian
Generate code for a big-endian target.
-mlittle-endian
Generate code for a little-endian target.
-mxl-reorder
Use reorder instructions (swap and byte reversed load/store).
-mxl-mode-app-model
Select application model app-model. Valid models are
executable
normal executable (default), uses startup code crt0.o.
xmdstub
for use with Xilinx Microprocessor Debugger (XMD) based software intrusive
debug agent called xmdstub. This uses startup file crt1.o and sets the
start address of the program to 0x800.
bootstrap
for applications that are loaded using a bootloader. This model uses
startup file crt2.o which does not contain a processor reset vector
handler. This is suitable for transferring control on a processor reset to
the bootloader rather than the application.
novectors
for applications that do not require any of the MicroBlaze vectors. This
option may be useful for applications running within a monitoring
application. This model uses crt3.o as a startup file.
Option -xl-mode-app-model is a deprecated alias for -mxl-mode-app-model.
MIPS Options
-EB Generate big-endian code.
-EL Generate little-endian code. This is the default for mips*el-*-*
configurations.
-march=arch
Generate code that runs on arch, which can be the name of a generic MIPS ISA,
or the name of a particular processor. The ISA names are: mips1, mips2, mips3,
mips4, mips32, mips32r2, mips32r3, mips32r5, mips32r6, mips64, mips64r2,
mips64r3, mips64r5 and mips64r6. The processor names are: 4kc, 4km, 4kp, 4ksc,
4kec, 4kem, 4kep, 4ksd, 5kc, 5kf, 20kc, 24kc, 24kf2_1, 24kf1_1, 24kec,
24kef2_1, 24kef1_1, 34kc, 34kf2_1, 34kf1_1, 34kn, 74kc, 74kf2_1, 74kf1_1,
74kf3_2, 1004kc, 1004kf2_1, 1004kf1_1, i6400, interaptiv, loongson2e,
loongson2f, loongson3a, m4k, m14k, m14kc, m14ke, m14kec, m5100, m5101, octeon,
octeon+, octeon2, octeon3, orion, p5600, r2000, r3000, r3900, r4000, r4400,
r4600, r4650, r4700, r6000, r8000, rm7000, rm9000, r10000, r12000, r14000,
r16000, sb1, sr71000, vr4100, vr4111, vr4120, vr4130, vr4300, vr5000, vr5400,
vr5500, xlr and xlp. The special value from-abi selects the most compatible
architecture for the selected ABI (that is, mips1 for 32-bit ABIs and mips3 for
64-bit ABIs).
The native Linux/GNU toolchain also supports the value native, which selects
the best architecture option for the host processor. -march=native has no
effect if GCC does not recognize the processor.
In processor names, a final 000 can be abbreviated as k (for example,
-march=r2k). Prefixes are optional, and vr may be written r.
Names of the form nf2_1 refer to processors with FPUs clocked at half the rate
of the core, names of the form nf1_1 refer to processors with FPUs clocked at
the same rate as the core, and names of the form nf3_2 refer to processors with
FPUs clocked a ratio of 3:2 with respect to the core. For compatibility
reasons, nf is accepted as a synonym for nf2_1 while nx and bfx are accepted as
synonyms for nf1_1.
GCC defines two macros based on the value of this option. The first is
"_MIPS_ARCH", which gives the name of target architecture, as a string. The
second has the form "_MIPS_ARCH_foo", where foo is the capitalized value of
"_MIPS_ARCH". For example, -march=r2000 sets "_MIPS_ARCH" to "r2000" and
defines the macro "_MIPS_ARCH_R2000".
Note that the "_MIPS_ARCH" macro uses the processor names given above. In
other words, it has the full prefix and does not abbreviate 000 as k. In the
case of from-abi, the macro names the resolved architecture (either "mips1" or
"mips3"). It names the default architecture when no -march option is given.
-mtune=arch
Optimize for arch. Among other things, this option controls the way
instructions are scheduled, and the perceived cost of arithmetic operations.
The list of arch values is the same as for -march.
When this option is not used, GCC optimizes for the processor specified by
-march. By using -march and -mtune together, it is possible to generate code
that runs on a family of processors, but optimize the code for one particular
member of that family.
-mtune defines the macros "_MIPS_TUNE" and "_MIPS_TUNE_foo", which work in the
same way as the -march ones described above.
-mips1
Equivalent to -march=mips1.
-mips2
Equivalent to -march=mips2.
-mips3
Equivalent to -march=mips3.
-mips4
Equivalent to -march=mips4.
-mips32
Equivalent to -march=mips32.
-mips32r3
Equivalent to -march=mips32r3.
-mips32r5
Equivalent to -march=mips32r5.
-mips32r6
Equivalent to -march=mips32r6.
-mips64
Equivalent to -march=mips64.
-mips64r2
Equivalent to -march=mips64r2.
-mips64r3
Equivalent to -march=mips64r3.
-mips64r5
Equivalent to -march=mips64r5.
-mips64r6
Equivalent to -march=mips64r6.
-mips16
-mno-mips16
Generate (do not generate) MIPS16 code. If GCC is targeting a MIPS32 or MIPS64
architecture, it makes use of the MIPS16e ASE.
MIPS16 code generation can also be controlled on a per-function basis by means
of "mips16" and "nomips16" attributes.
-mflip-mips16
Generate MIPS16 code on alternating functions. This option is provided for
regression testing of mixed MIPS16/non-MIPS16 code generation, and is not
intended for ordinary use in compiling user code.
-minterlink-compressed
-mno-interlink-compressed
Require (do not require) that code using the standard (uncompressed) MIPS ISA
be link-compatible with MIPS16 and microMIPS code, and vice versa.
For example, code using the standard ISA encoding cannot jump directly to
MIPS16 or microMIPS code; it must either use a call or an indirect jump.
-minterlink-compressed therefore disables direct jumps unless GCC knows that
the target of the jump is not compressed.
-minterlink-mips16
-mno-interlink-mips16
Aliases of -minterlink-compressed and -mno-interlink-compressed. These options
predate the microMIPS ASE and are retained for backwards compatibility.
-mabi=32
-mabi=o64
-mabi=n32
-mabi=64
-mabi=eabi
Generate code for the given ABI.
Note that the EABI has a 32-bit and a 64-bit variant. GCC normally generates
64-bit code when you select a 64-bit architecture, but you can use -mgp32 to
get 32-bit code instead.
For information about the O64 ABI, see
<http://gcc.gnu.org/projects/mipso64-abi.html>.
GCC supports a variant of the o32 ABI in which floating-point registers are 64
rather than 32 bits wide. You can select this combination with -mabi=32
-mfp64. This ABI relies on the "mthc1" and "mfhc1" instructions and is
therefore only supported for MIPS32R2, MIPS32R3 and MIPS32R5 processors.
The register assignments for arguments and return values remain the same, but
each scalar value is passed in a single 64-bit register rather than a pair of
32-bit registers. For example, scalar floating-point values are returned in
$f0 only, not a $f0/$f1 pair. The set of call-saved registers also remains the
same in that the even-numbered double-precision registers are saved.
Two additional variants of the o32 ABI are supported to enable a transition
from 32-bit to 64-bit registers. These are FPXX (-mfpxx) and FP64A (-mfp64
-mno-odd-spreg). The FPXX extension mandates that all code must execute
correctly when run using 32-bit or 64-bit registers. The code can be
interlinked with either FP32 or FP64, but not both. The FP64A extension is
similar to the FP64 extension but forbids the use of odd-numbered single-
precision registers. This can be used in conjunction with the "FRE" mode of
FPUs in MIPS32R5 processors and allows both FP32 and FP64A code to interlink
and run in the same process without changing FPU modes.
-mabicalls
-mno-abicalls
Generate (do not generate) code that is suitable for SVR4-style dynamic
objects. -mabicalls is the default for SVR4-based systems.
-mshared
-mno-shared
Generate (do not generate) code that is fully position-independent, and that
can therefore be linked into shared libraries. This option only affects
-mabicalls.
All -mabicalls code has traditionally been position-independent, regardless of
options like -fPIC and -fpic. However, as an extension, the GNU toolchain
allows executables to use absolute accesses for locally-binding symbols. It
can also use shorter GP initialization sequences and generate direct calls to
locally-defined functions. This mode is selected by -mno-shared.
-mno-shared depends on binutils 2.16 or higher and generates objects that can
only be linked by the GNU linker. However, the option does not affect the ABI
of the final executable; it only affects the ABI of relocatable objects. Using
-mno-shared generally makes executables both smaller and quicker.
-mshared is the default.
-mplt
-mno-plt
Assume (do not assume) that the static and dynamic linkers support PLTs and
copy relocations. This option only affects -mno-shared -mabicalls. For the
n64 ABI, this option has no effect without -msym32.
You can make -mplt the default by configuring GCC with --with-mips-plt. The
default is -mno-plt otherwise.
-mxgot
-mno-xgot
Lift (do not lift) the usual restrictions on the size of the global offset
table.
GCC normally uses a single instruction to load values from the GOT. While this
is relatively efficient, it only works if the GOT is smaller than about 64k.
Anything larger causes the linker to report an error such as:
relocation truncated to fit: R_MIPS_GOT16 foobar
If this happens, you should recompile your code with -mxgot. This works with
very large GOTs, although the code is also less efficient, since it takes three
instructions to fetch the value of a global symbol.
Note that some linkers can create multiple GOTs. If you have such a linker,
you should only need to use -mxgot when a single object file accesses more than
64k's worth of GOT entries. Very few do.
These options have no effect unless GCC is generating position independent
code.
-mgp32
Assume that general-purpose registers are 32 bits wide.
-mgp64
Assume that general-purpose registers are 64 bits wide.
-mfp32
Assume that floating-point registers are 32 bits wide.
-mfp64
Assume that floating-point registers are 64 bits wide.
-mfpxx
Do not assume the width of floating-point registers.
-mhard-float
Use floating-point coprocessor instructions.
-msoft-float
Do not use floating-point coprocessor instructions. Implement floating-point
calculations using library calls instead.
-mno-float
Equivalent to -msoft-float, but additionally asserts that the program being
compiled does not perform any floating-point operations. This option is
presently supported only by some bare-metal MIPS configurations, where it may
select a special set of libraries that lack all floating-point support
(including, for example, the floating-point "printf" formats). If code
compiled with -mno-float accidentally contains floating-point operations, it is
likely to suffer a link-time or run-time failure.
-msingle-float
Assume that the floating-point coprocessor only supports single-precision
operations.
-mdouble-float
Assume that the floating-point coprocessor supports double-precision
operations. This is the default.
-modd-spreg
-mno-odd-spreg
Enable the use of odd-numbered single-precision floating-point registers for
the o32 ABI. This is the default for processors that are known to support
these registers. When using the o32 FPXX ABI, -mno-odd-spreg is set by
default.
-mabs=2008
-mabs=legacy
These options control the treatment of the special not-a-number (NaN) IEEE 754
floating-point data with the "abs.fmt" and "neg.fmt" machine instructions.
By default or when -mabs=legacy is used the legacy treatment is selected. In
this case these instructions are considered arithmetic and avoided where
correct operation is required and the input operand might be a NaN. A longer
sequence of instructions that manipulate the sign bit of floating-point datum
manually is used instead unless the -ffinite-math-only option has also been
specified.
The -mabs=2008 option selects the IEEE 754-2008 treatment. In this case these
instructions are considered non-arithmetic and therefore operating correctly in
all cases, including in particular where the input operand is a NaN. These
instructions are therefore always used for the respective operations.
-mnan=2008
-mnan=legacy
These options control the encoding of the special not-a-number (NaN) IEEE 754
floating-point data.
The -mnan=legacy option selects the legacy encoding. In this case quiet NaNs
(qNaNs) are denoted by the first bit of their trailing significand field being
0, whereas signaling NaNs (sNaNs) are denoted by the first bit of their
trailing significand field being 1.
The -mnan=2008 option selects the IEEE 754-2008 encoding. In this case qNaNs
are denoted by the first bit of their trailing significand field being 1,
whereas sNaNs are denoted by the first bit of their trailing significand field
being 0.
The default is -mnan=legacy unless GCC has been configured with
--with-nan=2008.
-mllsc
-mno-llsc
Use (do not use) ll, sc, and sync instructions to implement atomic memory
built-in functions. When neither option is specified, GCC uses the
instructions if the target architecture supports them.
-mllsc is useful if the runtime environment can emulate the instructions and
-mno-llsc can be useful when compiling for nonstandard ISAs. You can make
either option the default by configuring GCC with --with-llsc and
--without-llsc respectively. --with-llsc is the default for some
configurations; see the installation documentation for details.
-mdsp
-mno-dsp
Use (do not use) revision 1 of the MIPS DSP ASE.
This option defines the preprocessor macro "__mips_dsp". It also defines
"__mips_dsp_rev" to 1.
-mdspr2
-mno-dspr2
Use (do not use) revision 2 of the MIPS DSP ASE.
This option defines the preprocessor macros "__mips_dsp" and "__mips_dspr2".
It also defines "__mips_dsp_rev" to 2.
-msmartmips
-mno-smartmips
Use (do not use) the MIPS SmartMIPS ASE.
-mpaired-single
-mno-paired-single
Use (do not use) paired-single floating-point instructions.
This option requires hardware floating-point support to be enabled.
-mdmx
-mno-mdmx
Use (do not use) MIPS Digital Media Extension instructions. This option can
only be used when generating 64-bit code and requires hardware floating-point
support to be enabled.
-mips3d
-mno-mips3d
Use (do not use) the MIPS-3D ASE. The option -mips3d implies -mpaired-single.
-mmicromips
-mno-micromips
Generate (do not generate) microMIPS code.
MicroMIPS code generation can also be controlled on a per-function basis by
means of "micromips" and "nomicromips" attributes.
-mmt
-mno-mt
Use (do not use) MT Multithreading instructions.
-mmcu
-mno-mcu
Use (do not use) the MIPS MCU ASE instructions.
-meva
-mno-eva
Use (do not use) the MIPS Enhanced Virtual Addressing instructions.
-mvirt
-mno-virt
Use (do not use) the MIPS Virtualization (VZ) instructions.
-mxpa
-mno-xpa
Use (do not use) the MIPS eXtended Physical Address (XPA) instructions.
-mlong64
Force "long" types to be 64 bits wide. See -mlong32 for an explanation of the
default and the way that the pointer size is determined.
-mlong32
Force "long", "int", and pointer types to be 32 bits wide.
The default size of "int"s, "long"s and pointers depends on the ABI. All the
supported ABIs use 32-bit "int"s. The n64 ABI uses 64-bit "long"s, as does the
64-bit EABI; the others use 32-bit "long"s. Pointers are the same size as
"long"s, or the same size as integer registers, whichever is smaller.
-msym32
-mno-sym32
Assume (do not assume) that all symbols have 32-bit values, regardless of the
selected ABI. This option is useful in combination with -mabi=64 and
-mno-abicalls because it allows GCC to generate shorter and faster references
to symbolic addresses.
-G num
Put definitions of externally-visible data in a small data section if that data
is no bigger than num bytes. GCC can then generate more efficient accesses to
the data; see -mgpopt for details.
The default -G option depends on the configuration.
-mlocal-sdata
-mno-local-sdata
Extend (do not extend) the -G behavior to local data too, such as to static
variables in C. -mlocal-sdata is the default for all configurations.
If the linker complains that an application is using too much small data, you
might want to try rebuilding the less performance-critical parts with
-mno-local-sdata. You might also want to build large libraries with
-mno-local-sdata, so that the libraries leave more room for the main program.
-mextern-sdata
-mno-extern-sdata
Assume (do not assume) that externally-defined data is in a small data section
if the size of that data is within the -G limit. -mextern-sdata is the default
for all configurations.
If you compile a module Mod with -mextern-sdata -G num -mgpopt, and Mod
references a variable Var that is no bigger than num bytes, you must make sure
that Var is placed in a small data section. If Var is defined by another
module, you must either compile that module with a high-enough -G setting or
attach a "section" attribute to Var's definition. If Var is common, you must
link the application with a high-enough -G setting.
The easiest way of satisfying these restrictions is to compile and link every
module with the same -G option. However, you may wish to build a library that
supports several different small data limits. You can do this by compiling the
library with the highest supported -G setting and additionally using
-mno-extern-sdata to stop the library from making assumptions about externally-
defined data.
-mgpopt
-mno-gpopt
Use (do not use) GP-relative accesses for symbols that are known to be in a
small data section; see -G, -mlocal-sdata and -mextern-sdata. -mgpopt is the
default for all configurations.
-mno-gpopt is useful for cases where the $gp register might not hold the value
of "_gp". For example, if the code is part of a library that might be used in
a boot monitor, programs that call boot monitor routines pass an unknown value
in $gp. (In such situations, the boot monitor itself is usually compiled with
-G0.)
-mno-gpopt implies -mno-local-sdata and -mno-extern-sdata.
-membedded-data
-mno-embedded-data
Allocate variables to the read-only data section first if possible, then next
in the small data section if possible, otherwise in data. This gives slightly
slower code than the default, but reduces the amount of RAM required when
executing, and thus may be preferred for some embedded systems.
-muninit-const-in-rodata
-mno-uninit-const-in-rodata
Put uninitialized "const" variables in the read-only data section. This option
is only meaningful in conjunction with -membedded-data.
-mcode-readable=setting
Specify whether GCC may generate code that reads from executable sections.
There are three possible settings:
-mcode-readable=yes
Instructions may freely access executable sections. This is the default
setting.
-mcode-readable=pcrel
MIPS16 PC-relative load instructions can access executable sections, but
other instructions must not do so. This option is useful on 4KSc and 4KSd
processors when the code TLBs have the Read Inhibit bit set. It is also
useful on processors that can be configured to have a dual instruction/data
SRAM interface and that, like the M4K, automatically redirect PC-relative
loads to the instruction RAM.
-mcode-readable=no
Instructions must not access executable sections. This option can be
useful on targets that are configured to have a dual instruction/data SRAM
interface but that (unlike the M4K) do not automatically redirect PC-
relative loads to the instruction RAM.
-msplit-addresses
-mno-split-addresses
Enable (disable) use of the "%hi()" and "%lo()" assembler relocation operators.
This option has been superseded by -mexplicit-relocs but is retained for
backwards compatibility.
-mexplicit-relocs
-mno-explicit-relocs
Use (do not use) assembler relocation operators when dealing with symbolic
addresses. The alternative, selected by -mno-explicit-relocs, is to use
assembler macros instead.
-mexplicit-relocs is the default if GCC was configured to use an assembler that
supports relocation operators.
-mcheck-zero-division
-mno-check-zero-division
Trap (do not trap) on integer division by zero.
The default is -mcheck-zero-division.
-mdivide-traps
-mdivide-breaks
MIPS systems check for division by zero by generating either a conditional trap
or a break instruction. Using traps results in smaller code, but is only
supported on MIPS II and later. Also, some versions of the Linux kernel have a
bug that prevents trap from generating the proper signal ("SIGFPE"). Use
-mdivide-traps to allow conditional traps on architectures that support them
and -mdivide-breaks to force the use of breaks.
The default is usually -mdivide-traps, but this can be overridden at configure
time using --with-divide=breaks. Divide-by-zero checks can be completely
disabled using -mno-check-zero-division.
-mload-store-pairs
-mno-load-store-pairs
Enable (disable) an optimization that pairs consecutive load or store
instructions to enable load/store bonding. This option is enabled by default
but only takes effect when the selected architecture is known to support
bonding.
-mmemcpy
-mno-memcpy
Force (do not force) the use of "memcpy" for non-trivial block moves. The
default is -mno-memcpy, which allows GCC to inline most constant-sized copies.
-mlong-calls
-mno-long-calls
Disable (do not disable) use of the "jal" instruction. Calling functions using
"jal" is more efficient but requires the caller and callee to be in the same
256 megabyte segment.
This option has no effect on abicalls code. The default is -mno-long-calls.
-mmad
-mno-mad
Enable (disable) use of the "mad", "madu" and "mul" instructions, as provided
by the R4650 ISA.
-mimadd
-mno-imadd
Enable (disable) use of the "madd" and "msub" integer instructions. The
default is -mimadd on architectures that support "madd" and "msub" except for
the 74k architecture where it was found to generate slower code.
-mfused-madd
-mno-fused-madd
Enable (disable) use of the floating-point multiply-accumulate instructions,
when they are available. The default is -mfused-madd.
On the R8000 CPU when multiply-accumulate instructions are used, the
intermediate product is calculated to infinite precision and is not subject to
the FCSR Flush to Zero bit. This may be undesirable in some circumstances. On
other processors the result is numerically identical to the equivalent
computation using separate multiply, add, subtract and negate instructions.
-nocpp
Tell the MIPS assembler to not run its preprocessor over user assembler files
(with a .s suffix) when assembling them.
-mfix-24k
-mno-fix-24k
Work around the 24K E48 (lost data on stores during refill) errata. The
workarounds are implemented by the assembler rather than by GCC.
-mfix-r4000
-mno-fix-r4000
Work around certain R4000 CPU errata:
- A double-word or a variable shift may give an incorrect result if executed
immediately after starting an integer division.
- A double-word or a variable shift may give an incorrect result if executed
while an integer multiplication is in progress.
- An integer division may give an incorrect result if started in a delay slot
of a taken branch or a jump.
-mfix-r4400
-mno-fix-r4400
Work around certain R4400 CPU errata:
- A double-word or a variable shift may give an incorrect result if executed
immediately after starting an integer division.
-mfix-r10000
-mno-fix-r10000
Work around certain R10000 errata:
- "ll"/"sc" sequences may not behave atomically on revisions prior to 3.0.
They may deadlock on revisions 2.6 and earlier.
This option can only be used if the target architecture supports branch-likely
instructions. -mfix-r10000 is the default when -march=r10000 is used;
-mno-fix-r10000 is the default otherwise.
-mfix-rm7000
-mno-fix-rm7000
Work around the RM7000 "dmult"/"dmultu" errata. The workarounds are
implemented by the assembler rather than by GCC.
-mfix-vr4120
-mno-fix-vr4120
Work around certain VR4120 errata:
- "dmultu" does not always produce the correct result.
- "div" and "ddiv" do not always produce the correct result if one of the
operands is negative.
The workarounds for the division errata rely on special functions in libgcc.a.
At present, these functions are only provided by the "mips64vr*-elf"
configurations.
Other VR4120 errata require a NOP to be inserted between certain pairs of
instructions. These errata are handled by the assembler, not by GCC itself.
-mfix-vr4130
Work around the VR4130 "mflo"/"mfhi" errata. The workarounds are implemented
by the assembler rather than by GCC, although GCC avoids using "mflo" and
"mfhi" if the VR4130 "macc", "macchi", "dmacc" and "dmacchi" instructions are
available instead.
-mfix-sb1
-mno-fix-sb1
Work around certain SB-1 CPU core errata. (This flag currently works around
the SB-1 revision 2 "F1" and "F2" floating-point errata.)
-mr10k-cache-barrier=setting
Specify whether GCC should insert cache barriers to avoid the side effects of
speculation on R10K processors.
In common with many processors, the R10K tries to predict the outcome of a
conditional branch and speculatively executes instructions from the "taken"
branch. It later aborts these instructions if the predicted outcome is wrong.
However, on the R10K, even aborted instructions can have side effects.
This problem only affects kernel stores and, depending on the system, kernel
loads. As an example, a speculatively-executed store may load the target
memory into cache and mark the cache line as dirty, even if the store itself is
later aborted. If a DMA operation writes to the same area of memory before the
"dirty" line is flushed, the cached data overwrites the DMA-ed data. See the
R10K processor manual for a full description, including other potential
problems.
One workaround is to insert cache barrier instructions before every memory
access that might be speculatively executed and that might have side effects
even if aborted. -mr10k-cache-barrier=setting controls GCC's implementation of
this workaround. It assumes that aborted accesses to any byte in the following
regions does not have side effects:
1. the memory occupied by the current function's stack frame;
2. the memory occupied by an incoming stack argument;
3. the memory occupied by an object with a link-time-constant address.
It is the kernel's responsibility to ensure that speculative accesses to these
regions are indeed safe.
If the input program contains a function declaration such as:
void foo (void);
then the implementation of "foo" must allow "j foo" and "jal foo" to be
executed speculatively. GCC honors this restriction for functions it compiles
itself. It expects non-GCC functions (such as hand-written assembly code) to
do the same.
The option has three forms:
-mr10k-cache-barrier=load-store
Insert a cache barrier before a load or store that might be speculatively
executed and that might have side effects even if aborted.
-mr10k-cache-barrier=store
Insert a cache barrier before a store that might be speculatively executed
and that might have side effects even if aborted.
-mr10k-cache-barrier=none
Disable the insertion of cache barriers. This is the default setting.
-mflush-func=func
-mno-flush-func
Specifies the function to call to flush the I and D caches, or to not call any
such function. If called, the function must take the same arguments as the
common "_flush_func", that is, the address of the memory range for which the
cache is being flushed, the size of the memory range, and the number 3 (to
flush both caches). The default depends on the target GCC was configured for,
but commonly is either "_flush_func" or "__cpu_flush".
mbranch-cost=num
Set the cost of branches to roughly num "simple" instructions. This cost is
only a heuristic and is not guaranteed to produce consistent results across
releases. A zero cost redundantly selects the default, which is based on the
-mtune setting.
-mbranch-likely
-mno-branch-likely
Enable or disable use of Branch Likely instructions, regardless of the default
for the selected architecture. By default, Branch Likely instructions may be
generated if they are supported by the selected architecture. An exception is
for the MIPS32 and MIPS64 architectures and processors that implement those
architectures; for those, Branch Likely instructions are not be generated by
default because the MIPS32 and MIPS64 architectures specifically deprecate
their use.
-mcompact-branches=never
-mcompact-branches=optimal
-mcompact-branches=always
These options control which form of branches will be generated. The default is
-mcompact-branches=optimal.
The -mcompact-branches=never option ensures that compact branch instructions
will never be generated.
The -mcompact-branches=always option ensures that a compact branch instruction
will be generated if available. If a compact branch instruction is not
available, a delay slot form of the branch will be used instead.
This option is supported from MIPS Release 6 onwards.
The -mcompact-branches=optimal option will cause a delay slot branch to be used
if one is available in the current ISA and the delay slot is successfully
filled. If the delay slot is not filled, a compact branch will be chosen if
one is available.
-mfp-exceptions
-mno-fp-exceptions
Specifies whether FP exceptions are enabled. This affects how FP instructions
are scheduled for some processors. The default is that FP exceptions are
enabled.
For instance, on the SB-1, if FP exceptions are disabled, and we are emitting
64-bit code, then we can use both FP pipes. Otherwise, we can only use one FP
pipe.
-mvr4130-align
-mno-vr4130-align
The VR4130 pipeline is two-way superscalar, but can only issue two instructions
together if the first one is 8-byte aligned. When this option is enabled, GCC
aligns pairs of instructions that it thinks should execute in parallel.
This option only has an effect when optimizing for the VR4130. It normally
makes code faster, but at the expense of making it bigger. It is enabled by
default at optimization level -O3.
-msynci
-mno-synci
Enable (disable) generation of "synci" instructions on architectures that
support it. The "synci" instructions (if enabled) are generated when
"__builtin___clear_cache" is compiled.
This option defaults to -mno-synci, but the default can be overridden by
configuring GCC with --with-synci.
When compiling code for single processor systems, it is generally safe to use
"synci". However, on many multi-core (SMP) systems, it does not invalidate the
instruction caches on all cores and may lead to undefined behavior.
-mrelax-pic-calls
-mno-relax-pic-calls
Try to turn PIC calls that are normally dispatched via register $25 into direct
calls. This is only possible if the linker can resolve the destination at link
time and if the destination is within range for a direct call.
-mrelax-pic-calls is the default if GCC was configured to use an assembler and
a linker that support the ".reloc" assembly directive and -mexplicit-relocs is
in effect. With -mno-explicit-relocs, this optimization can be performed by
the assembler and the linker alone without help from the compiler.
-mmcount-ra-address
-mno-mcount-ra-address
Emit (do not emit) code that allows "_mcount" to modify the calling function's
return address. When enabled, this option extends the usual "_mcount"
interface with a new ra-address parameter, which has type "intptr_t *" and is
passed in register $12. "_mcount" can then modify the return address by doing
both of the following:
* Returning the new address in register $31.
* Storing the new address in "*ra-address", if ra-address is nonnull.
The default is -mno-mcount-ra-address.
-mframe-header-opt
-mno-frame-header-opt
Enable (disable) frame header optimization in the o32 ABI. When using the o32
ABI, calling functions will allocate 16 bytes on the stack for the called
function to write out register arguments. When enabled, this optimization will
suppress the allocation of the frame header if it can be determined that it is
unused.
This optimization is off by default at all optimization levels.
-mlxc1-sxc1
-mno-lxc1-sxc1
When applicable, enable (disable) the generation of "lwxc1", "swxc1", "ldxc1",
"sdxc1" instructions. Enabled by default.
-mmadd4
-mno-madd4
When applicable, enable (disable) the generation of 4-operand "madd.s",
"madd.d" and related instructions. Enabled by default.
MMIX Options
These options are defined for the MMIX:
-mlibfuncs
-mno-libfuncs
Specify that intrinsic library functions are being compiled, passing all values
in registers, no matter the size.
-mepsilon
-mno-epsilon
Generate floating-point comparison instructions that compare with respect to
the "rE" epsilon register.
-mabi=mmixware
-mabi=gnu
Generate code that passes function parameters and return values that (in the
called function) are seen as registers $0 and up, as opposed to the GNU ABI
which uses global registers $231 and up.
-mzero-extend
-mno-zero-extend
When reading data from memory in sizes shorter than 64 bits, use (do not use)
zero-extending load instructions by default, rather than sign-extending ones.
-mknuthdiv
-mno-knuthdiv
Make the result of a division yielding a remainder have the same sign as the
divisor. With the default, -mno-knuthdiv, the sign of the remainder follows
the sign of the dividend. Both methods are arithmetically valid, the latter
being almost exclusively used.
-mtoplevel-symbols
-mno-toplevel-symbols
Prepend (do not prepend) a : to all global symbols, so the assembly code can be
used with the "PREFIX" assembly directive.
-melf
Generate an executable in the ELF format, rather than the default mmo format
used by the mmix simulator.
-mbranch-predict
-mno-branch-predict
Use (do not use) the probable-branch instructions, when static branch
prediction indicates a probable branch.
-mbase-addresses
-mno-base-addresses
Generate (do not generate) code that uses base addresses. Using a base address
automatically generates a request (handled by the assembler and the linker) for
a constant to be set up in a global register. The register is used for one or
more base address requests within the range 0 to 255 from the value held in the
register. The generally leads to short and fast code, but the number of
different data items that can be addressed is limited. This means that a
program that uses lots of static data may require -mno-base-addresses.
-msingle-exit
-mno-single-exit
Force (do not force) generated code to have a single exit point in each
function.
MN10300 Options
These -m options are defined for Matsushita MN10300 architectures:
-mmult-bug
Generate code to avoid bugs in the multiply instructions for the MN10300
processors. This is the default.
-mno-mult-bug
Do not generate code to avoid bugs in the multiply instructions for the MN10300
processors.
-mam33
Generate code using features specific to the AM33 processor.
-mno-am33
Do not generate code using features specific to the AM33 processor. This is
the default.
-mam33-2
Generate code using features specific to the AM33/2.0 processor.
-mam34
Generate code using features specific to the AM34 processor.
-mtune=cpu-type
Use the timing characteristics of the indicated CPU type when scheduling
instructions. This does not change the targeted processor type. The CPU type
must be one of mn10300, am33, am33-2 or am34.
-mreturn-pointer-on-d0
When generating a function that returns a pointer, return the pointer in both
"a0" and "d0". Otherwise, the pointer is returned only in "a0", and attempts
to call such functions without a prototype result in errors. Note that this
option is on by default; use -mno-return-pointer-on-d0 to disable it.
-mno-crt0
Do not link in the C run-time initialization object file.
-mrelax
Indicate to the linker that it should perform a relaxation optimization pass to
shorten branches, calls and absolute memory addresses. This option only has an
effect when used on the command line for the final link step.
This option makes symbolic debugging impossible.
-mliw
Allow the compiler to generate Long Instruction Word instructions if the target
is the AM33 or later. This is the default. This option defines the
preprocessor macro "__LIW__".
-mnoliw
Do not allow the compiler to generate Long Instruction Word instructions. This
option defines the preprocessor macro "__NO_LIW__".
-msetlb
Allow the compiler to generate the SETLB and Lcc instructions if the target is
the AM33 or later. This is the default. This option defines the preprocessor
macro "__SETLB__".
-mnosetlb
Do not allow the compiler to generate SETLB or Lcc instructions. This option
defines the preprocessor macro "__NO_SETLB__".
Moxie Options
-meb
Generate big-endian code. This is the default for moxie-*-* configurations.
-mel
Generate little-endian code.
-mmul.x
Generate mul.x and umul.x instructions. This is the default for moxiebox-*-*
configurations.
-mno-crt0
Do not link in the C run-time initialization object file.
MSP430 Options
These options are defined for the MSP430:
-masm-hex
Force assembly output to always use hex constants. Normally such constants are
signed decimals, but this option is available for testsuite and/or aesthetic
purposes.
-mmcu=
Select the MCU to target. This is used to create a C preprocessor symbol based
upon the MCU name, converted to upper case and pre- and post-fixed with __.
This in turn is used by the msp430.h header file to select an MCU-specific
supplementary header file.
The option also sets the ISA to use. If the MCU name is one that is known to
only support the 430 ISA then that is selected, otherwise the 430X ISA is
selected. A generic MCU name of msp430 can also be used to select the 430 ISA.
Similarly the generic msp430x MCU name selects the 430X ISA.
In addition an MCU-specific linker script is added to the linker command line.
The script's name is the name of the MCU with .ld appended. Thus specifying
-mmcu=xxx on the gcc command line defines the C preprocessor symbol "__XXX__"
and cause the linker to search for a script called xxx.ld.
This option is also passed on to the assembler.
-mwarn-mcu
-mno-warn-mcu
This option enables or disables warnings about conflicts between the MCU name
specified by the -mmcu option and the ISA set by the -mcpu option and/or the
hardware multiply support set by the -mhwmult option. It also toggles warnings
about unrecognized MCU names. This option is on by default.
-mcpu=
Specifies the ISA to use. Accepted values are msp430, msp430x and msp430xv2.
This option is deprecated. The -mmcu= option should be used to select the ISA.
-msim
Link to the simulator runtime libraries and linker script. Overrides any
scripts that would be selected by the -mmcu= option.
-mlarge
Use large-model addressing (20-bit pointers, 32-bit "size_t").
-msmall
Use small-model addressing (16-bit pointers, 16-bit "size_t").
-mrelax
This option is passed to the assembler and linker, and allows the linker to
perform certain optimizations that cannot be done until the final link.
mhwmult=
Describes the type of hardware multiply supported by the target. Accepted
values are none for no hardware multiply, 16bit for the original 16-bit-only
multiply supported by early MCUs. 32bit for the 16/32-bit multiply supported
by later MCUs and f5series for the 16/32-bit multiply supported by F5-series
MCUs. A value of auto can also be given. This tells GCC to deduce the
hardware multiply support based upon the MCU name provided by the -mmcu option.
If no -mmcu option is specified or if the MCU name is not recognized then no
hardware multiply support is assumed. "auto" is the default setting.
Hardware multiplies are normally performed by calling a library routine. This
saves space in the generated code. When compiling at -O3 or higher however the
hardware multiplier is invoked inline. This makes for bigger, but faster code.
The hardware multiply routines disable interrupts whilst running and restore
the previous interrupt state when they finish. This makes them safe to use
inside interrupt handlers as well as in normal code.
-minrt
Enable the use of a minimum runtime environment - no static initializers or
constructors. This is intended for memory-constrained devices. The compiler
includes special symbols in some objects that tell the linker and runtime which
code fragments are required.
-mcode-region=
-mdata-region=
These options tell the compiler where to place functions and data that do not
have one of the "lower", "upper", "either" or "section" attributes. Possible
values are "lower", "upper", "either" or "any". The first three behave like
the corresponding attribute. The fourth possible value - "any" - is the
default. It leaves placement entirely up to the linker script and how it
assigns the standard sections (".text", ".data", etc) to the memory regions.
-msilicon-errata=
This option passes on a request to assembler to enable the fixes for the named
silicon errata.
-msilicon-errata-warn=
This option passes on a request to the assembler to enable warning messages
when a silicon errata might need to be applied.
NDS32 Options
These options are defined for NDS32 implementations:
-mbig-endian
Generate code in big-endian mode.
-mlittle-endian
Generate code in little-endian mode.
-mreduced-regs
Use reduced-set registers for register allocation.
-mfull-regs
Use full-set registers for register allocation.
-mcmov
Generate conditional move instructions.
-mno-cmov
Do not generate conditional move instructions.
-mext-perf
Generate performance extension instructions.
-mno-ext-perf
Do not generate performance extension instructions.
-mext-perf2
Generate performance extension 2 instructions.
-mno-ext-perf2
Do not generate performance extension 2 instructions.
-mext-string
Generate string extension instructions.
-mno-ext-string
Do not generate string extension instructions.
-mv3push
Generate v3 push25/pop25 instructions.
-mno-v3push
Do not generate v3 push25/pop25 instructions.
-m16-bit
Generate 16-bit instructions.
-mno-16-bit
Do not generate 16-bit instructions.
-misr-vector-size=num
Specify the size of each interrupt vector, which must be 4 or 16.
-mcache-block-size=num
Specify the size of each cache block, which must be a power of 2 between 4 and
512.
-march=arch
Specify the name of the target architecture.
-mcmodel=code-model
Set the code model to one of
small
All the data and read-only data segments must be within 512KB addressing
space. The text segment must be within 16MB addressing space.
medium
The data segment must be within 512KB while the read-only data segment can
be within 4GB addressing space. The text segment should be still within
16MB addressing space.
large
All the text and data segments can be within 4GB addressing space.
-mctor-dtor
Enable constructor/destructor feature.
-mrelax
Guide linker to relax instructions.
Nios II Options
These are the options defined for the Altera Nios II processor.
-G num
Put global and static objects less than or equal to num bytes into the small
data or BSS sections instead of the normal data or BSS sections. The default
value of num is 8.
-mgpopt=option
-mgpopt
-mno-gpopt
Generate (do not generate) GP-relative accesses. The following option names
are recognized:
none
Do not generate GP-relative accesses.
local
Generate GP-relative accesses for small data objects that are not external,
weak, or uninitialized common symbols. Also use GP-relative addressing for
objects that have been explicitly placed in a small data section via a
"section" attribute.
global
As for local, but also generate GP-relative accesses for small data objects
that are external, weak, or common. If you use this option, you must
ensure that all parts of your program (including libraries) are compiled
with the same -G setting.
data
Generate GP-relative accesses for all data objects in the program. If you
use this option, the entire data and BSS segments of your program must fit
in 64K of memory and you must use an appropriate linker script to allocate
them within the addressable range of the global pointer.
all Generate GP-relative addresses for function pointers as well as data
pointers. If you use this option, the entire text, data, and BSS segments
of your program must fit in 64K of memory and you must use an appropriate
linker script to allocate them within the addressable range of the global
pointer.
-mgpopt is equivalent to -mgpopt=local, and -mno-gpopt is equivalent to
-mgpopt=none.
The default is -mgpopt except when -fpic or -fPIC is specified to generate
position-independent code. Note that the Nios II ABI does not permit GP-
relative accesses from shared libraries.
You may need to specify -mno-gpopt explicitly when building programs that
include large amounts of small data, including large GOT data sections. In
this case, the 16-bit offset for GP-relative addressing may not be large enough
to allow access to the entire small data section.
-mgprel-sec=regexp
This option specifies additional section names that can be accessed via GP-
relative addressing. It is most useful in conjunction with "section"
attributes on variable declarations and a custom linker script. The regexp is
a POSIX Extended Regular Expression.
This option does not affect the behavior of the -G option, and the specified
sections are in addition to the standard ".sdata" and ".sbss" small-data
sections that are recognized by -mgpopt.
-mr0rel-sec=regexp
This option specifies names of sections that can be accessed via a 16-bit
offset from "r0"; that is, in the low 32K or high 32K of the 32-bit address
space. It is most useful in conjunction with "section" attributes on variable
declarations and a custom linker script. The regexp is a POSIX Extended
Regular Expression.
In contrast to the use of GP-relative addressing for small data, zero-based
addressing is never generated by default and there are no conventional section
names used in standard linker scripts for sections in the low or high areas of
memory.
-mel
-meb
Generate little-endian (default) or big-endian (experimental) code,
respectively.
-march=arch
This specifies the name of the target Nios II architecture. GCC uses this name
to determine what kind of instructions it can emit when generating assembly
code. Permissible names are: r1, r2.
The preprocessor macro "__nios2_arch__" is available to programs, with value 1
or 2, indicating the targeted ISA level.
-mbypass-cache
-mno-bypass-cache
Force all load and store instructions to always bypass cache by using I/O
variants of the instructions. The default is not to bypass the cache.
-mno-cache-volatile
-mcache-volatile
Volatile memory access bypass the cache using the I/O variants of the load and
store instructions. The default is not to bypass the cache.
-mno-fast-sw-div
-mfast-sw-div
Do not use table-based fast divide for small numbers. The default is to use the
fast divide at -O3 and above.
-mno-hw-mul
-mhw-mul
-mno-hw-mulx
-mhw-mulx
-mno-hw-div
-mhw-div
Enable or disable emitting "mul", "mulx" and "div" family of instructions by
the compiler. The default is to emit "mul" and not emit "div" and "mulx".
-mbmx
-mno-bmx
-mcdx
-mno-cdx
Enable or disable generation of Nios II R2 BMX (bit manipulation) and CDX (code
density) instructions. Enabling these instructions also requires -march=r2.
Since these instructions are optional extensions to the R2 architecture, the
default is not to emit them.
-mcustom-insn=N
-mno-custom-insn
Each -mcustom-insn=N option enables use of a custom instruction with encoding N
when generating code that uses insn. For example, -mcustom-fadds=253 generates
custom instruction 253 for single-precision floating-point add operations
instead of the default behavior of using a library call.
The following values of insn are supported. Except as otherwise noted,
floating-point operations are expected to be implemented with normal IEEE 754
semantics and correspond directly to the C operators or the equivalent GCC
built-in functions.
Single-precision floating point:
fadds, fsubs, fdivs, fmuls
Binary arithmetic operations.
fnegs
Unary negation.
fabss
Unary absolute value.
fcmpeqs, fcmpges, fcmpgts, fcmples, fcmplts, fcmpnes
Comparison operations.
fmins, fmaxs
Floating-point minimum and maximum. These instructions are only generated
if -ffinite-math-only is specified.
fsqrts
Unary square root operation.
fcoss, fsins, ftans, fatans, fexps, flogs
Floating-point trigonometric and exponential functions. These instructions
are only generated if -funsafe-math-optimizations is also specified.
Double-precision floating point:
faddd, fsubd, fdivd, fmuld
Binary arithmetic operations.
fnegd
Unary negation.
fabsd
Unary absolute value.
fcmpeqd, fcmpged, fcmpgtd, fcmpled, fcmpltd, fcmpned
Comparison operations.
fmind, fmaxd
Double-precision minimum and maximum. These instructions are only
generated if -ffinite-math-only is specified.
fsqrtd
Unary square root operation.
fcosd, fsind, ftand, fatand, fexpd, flogd
Double-precision trigonometric and exponential functions. These
instructions are only generated if -funsafe-math-optimizations is also
specified.
Conversions:
fextsd
Conversion from single precision to double precision.
ftruncds
Conversion from double precision to single precision.
fixsi, fixsu, fixdi, fixdu
Conversion from floating point to signed or unsigned integer types, with
truncation towards zero.
round
Conversion from single-precision floating point to signed integer, rounding
to the nearest integer and ties away from zero. This corresponds to the
"__builtin_lroundf" function when -fno-math-errno is used.
floatis, floatus, floatid, floatud
Conversion from signed or unsigned integer types to floating-point types.
In addition, all of the following transfer instructions for internal registers
X and Y must be provided to use any of the double-precision floating-point
instructions. Custom instructions taking two double-precision source operands
expect the first operand in the 64-bit register X. The other operand (or only
operand of a unary operation) is given to the custom arithmetic instruction
with the least significant half in source register src1 and the most
significant half in src2. A custom instruction that returns a double-precision
result returns the most significant 32 bits in the destination register and the
other half in 32-bit register Y. GCC automatically generates the necessary
code sequences to write register X and/or read register Y when double-precision
floating-point instructions are used.
fwrx
Write src1 into the least significant half of X and src2 into the most
significant half of X.
fwry
Write src1 into Y.
frdxhi, frdxlo
Read the most or least (respectively) significant half of X and store it in
dest.
frdy
Read the value of Y and store it into dest.
Note that you can gain more local control over generation of Nios II custom
instructions by using the "target("custom-insn=N")" and
"target("no-custom-insn")" function attributes or pragmas.
-mcustom-fpu-cfg=name
This option enables a predefined, named set of custom instruction encodings
(see -mcustom-insn above). Currently, the following sets are defined:
-mcustom-fpu-cfg=60-1 is equivalent to: -mcustom-fmuls=252 -mcustom-fadds=253
-mcustom-fsubs=254 -fsingle-precision-constant
-mcustom-fpu-cfg=60-2 is equivalent to: -mcustom-fmuls=252 -mcustom-fadds=253
-mcustom-fsubs=254 -mcustom-fdivs=255 -fsingle-precision-constant
-mcustom-fpu-cfg=72-3 is equivalent to: -mcustom-floatus=243 -mcustom-fixsi=244
-mcustom-floatis=245 -mcustom-fcmpgts=246 -mcustom-fcmples=249
-mcustom-fcmpeqs=250 -mcustom-fcmpnes=251 -mcustom-fmuls=252 -mcustom-fadds=253
-mcustom-fsubs=254 -mcustom-fdivs=255 -fsingle-precision-constant
Custom instruction assignments given by individual -mcustom-insn= options
override those given by -mcustom-fpu-cfg=, regardless of the order of the
options on the command line.
Note that you can gain more local control over selection of a FPU configuration
by using the "target("custom-fpu-cfg=name")" function attribute or pragma.
These additional -m options are available for the Altera Nios II ELF (bare-metal)
target:
-mhal
Link with HAL BSP. This suppresses linking with the GCC-provided C runtime
startup and termination code, and is typically used in conjunction with
-msys-crt0= to specify the location of the alternate startup code provided by
the HAL BSP.
-msmallc
Link with a limited version of the C library, -lsmallc, rather than Newlib.
-msys-crt0=startfile
startfile is the file name of the startfile (crt0) to use when linking. This
option is only useful in conjunction with -mhal.
-msys-lib=systemlib
systemlib is the library name of the library that provides low-level system
calls required by the C library, e.g. "read" and "write". This option is
typically used to link with a library provided by a HAL BSP.
Nvidia PTX Options
These options are defined for Nvidia PTX:
-m32
-m64
Generate code for 32-bit or 64-bit ABI.
-mmainkernel
Link in code for a __main kernel. This is for stand-alone instead of
offloading execution.
-moptimize
Apply partitioned execution optimizations. This is the default when any level
of optimization is selected.
-msoft-stack
Generate code that does not use ".local" memory directly for stack storage.
Instead, a per-warp stack pointer is maintained explicitly. This enables
variable-length stack allocation (with variable-length arrays or "alloca"), and
when global memory is used for underlying storage, makes it possible to access
automatic variables from other threads, or with atomic instructions. This code
generation variant is used for OpenMP offloading, but the option is exposed on
its own for the purpose of testing the compiler; to generate code suitable for
linking into programs using OpenMP offloading, use option -mgomp.
-muniform-simt
Switch to code generation variant that allows to execute all threads in each
warp, while maintaining memory state and side effects as if only one thread in
each warp was active outside of OpenMP SIMD regions. All atomic operations and
calls to runtime (malloc, free, vprintf) are conditionally executed (iff
current lane index equals the master lane index), and the register being
assigned is copied via a shuffle instruction from the master lane. Outside of
SIMD regions lane 0 is the master; inside, each thread sees itself as the
master. Shared memory array "int __nvptx_uni[]" stores all-zeros or all-ones
bitmasks for each warp, indicating current mode (0 outside of SIMD regions).
Each thread can bitwise-and the bitmask at position "tid.y" with current lane
index to compute the master lane index.
-mgomp
Generate code for use in OpenMP offloading: enables -msoft-stack and
-muniform-simt options, and selects corresponding multilib variant.
PDP-11 Options
These options are defined for the PDP-11:
-mfpu
Use hardware FPP floating point. This is the default. (FIS floating point on
the PDP-11/40 is not supported.)
-msoft-float
Do not use hardware floating point.
-mac0
Return floating-point results in ac0 (fr0 in Unix assembler syntax).
-mno-ac0
Return floating-point results in memory. This is the default.
-m40
Generate code for a PDP-11/40.
-m45
Generate code for a PDP-11/45. This is the default.
-m10
Generate code for a PDP-11/10.
-mbcopy-builtin
Use inline "movmemhi" patterns for copying memory. This is the default.
-mbcopy
Do not use inline "movmemhi" patterns for copying memory.
-mint16
-mno-int32
Use 16-bit "int". This is the default.
-mint32
-mno-int16
Use 32-bit "int".
-mfloat64
-mno-float32
Use 64-bit "float". This is the default.
-mfloat32
-mno-float64
Use 32-bit "float".
-mabshi
Use "abshi2" pattern. This is the default.
-mno-abshi
Do not use "abshi2" pattern.
-mbranch-expensive
Pretend that branches are expensive. This is for experimenting with code
generation only.
-mbranch-cheap
Do not pretend that branches are expensive. This is the default.
-munix-asm
Use Unix assembler syntax. This is the default when configured for
pdp11-*-bsd.
-mdec-asm
Use DEC assembler syntax. This is the default when configured for any PDP-11
target other than pdp11-*-bsd.
picoChip Options
These -m options are defined for picoChip implementations:
-mae=ae_type
Set the instruction set, register set, and instruction scheduling parameters
for array element type ae_type. Supported values for ae_type are ANY, MUL, and
MAC.
-mae=ANY selects a completely generic AE type. Code generated with this option
runs on any of the other AE types. The code is not as efficient as it would be
if compiled for a specific AE type, and some types of operation (e.g.,
multiplication) do not work properly on all types of AE.
-mae=MUL selects a MUL AE type. This is the most useful AE type for compiled
code, and is the default.
-mae=MAC selects a DSP-style MAC AE. Code compiled with this option may suffer
from poor performance of byte (char) manipulation, since the DSP AE does not
provide hardware support for byte load/stores.
-msymbol-as-address
Enable the compiler to directly use a symbol name as an address in a load/store
instruction, without first loading it into a register. Typically, the use of
this option generates larger programs, which run faster than when the option
isn't used. However, the results vary from program to program, so it is left
as a user option, rather than being permanently enabled.
-mno-inefficient-warnings
Disables warnings about the generation of inefficient code. These warnings can
be generated, for example, when compiling code that performs byte-level memory
operations on the MAC AE type. The MAC AE has no hardware support for byte-
level memory operations, so all byte load/stores must be synthesized from word
load/store operations. This is inefficient and a warning is generated to
indicate that you should rewrite the code to avoid byte operations, or to
target an AE type that has the necessary hardware support. This option
disables these warnings.
PowerPC Options
These are listed under
PowerPC SPE Options
These -m options are defined for PowerPC SPE:
-mmfcrf
-mno-mfcrf
-mpopcntb
-mno-popcntb
You use these options to specify which instructions are available on the
processor you are using. The default value of these options is determined when
configuring GCC. Specifying the -mcpu=cpu_type overrides the specification of
these options. We recommend you use the -mcpu=cpu_type option rather than the
options listed above.
The -mmfcrf option allows GCC to generate the move from condition register
field instruction implemented on the POWER4 processor and other processors that
support the PowerPC V2.01 architecture. The -mpopcntb option allows GCC to
generate the popcount and double-precision FP reciprocal estimate instruction
implemented on the POWER5 processor and other processors that support the
PowerPC V2.02 architecture.
-mcpu=cpu_type
Set architecture type, register usage, and instruction scheduling parameters
for machine type cpu_type. Supported values for cpu_type are 8540, 8548, and
native.
-mcpu=powerpc specifies pure 32-bit PowerPC (either endian), with an
appropriate, generic processor model assumed for scheduling purposes.
Specifying native as cpu type detects and selects the architecture option that
corresponds to the host processor of the system performing the compilation.
-mcpu=native has no effect if GCC does not recognize the processor.
The other options specify a specific processor. Code generated under those
options runs best on that processor, and may not run at all on others.
The -mcpu options automatically enable or disable the following options:
-mhard-float -mmfcrf -mmultiple -mpopcntb -mpopcntd -msingle-float
-mdouble-float -mfloat128
The particular options set for any particular CPU varies between compiler
versions, depending on what setting seems to produce optimal code for that CPU;
it doesn't necessarily reflect the actual hardware's capabilities. If you wish
to set an individual option to a particular value, you may specify it after the
-mcpu option, like -mcpu=8548.
-mtune=cpu_type
Set the instruction scheduling parameters for machine type cpu_type, but do not
set the architecture type or register usage, as -mcpu=cpu_type does. The same
values for cpu_type are used for -mtune as for -mcpu. If both are specified,
the code generated uses the architecture and registers set by -mcpu, but the
scheduling parameters set by -mtune.
-msecure-plt
Generate code that allows ld and ld.so to build executables and shared
libraries with non-executable ".plt" and ".got" sections. This is a PowerPC
32-bit SYSV ABI option.
-mbss-plt
Generate code that uses a BSS ".plt" section that ld.so fills in, and requires
".plt" and ".got" sections that are both writable and executable. This is a
PowerPC 32-bit SYSV ABI option.
-misel
-mno-isel
This switch enables or disables the generation of ISEL instructions.
-misel=yes/no
This switch has been deprecated. Use -misel and -mno-isel instead.
-mspe
-mno-spe
This switch enables or disables the generation of SPE simd instructions.
-mspe=yes/no
This option has been deprecated. Use -mspe and -mno-spe instead.
-mfloat128
-mno-float128
Enable/disable the __float128 keyword for IEEE 128-bit floating point and use
either software emulation for IEEE 128-bit floating point or hardware
instructions.
-mfloat-gprs=yes/single/double/no
-mfloat-gprs
This switch enables or disables the generation of floating-point operations on
the general-purpose registers for architectures that support it.
The argument yes or single enables the use of single-precision floating-point
operations.
The argument double enables the use of single and double-precision floating-
point operations.
The argument no disables floating-point operations on the general-purpose
registers.
This option is currently only available on the MPC854x.
-mfull-toc
-mno-fp-in-toc
-mno-sum-in-toc
-mminimal-toc
Modify generation of the TOC (Table Of Contents), which is created for every
executable file. The -mfull-toc option is selected by default. In that case,
GCC allocates at least one TOC entry for each unique non-automatic variable
reference in your program. GCC also places floating-point constants in the
TOC. However, only 16,384 entries are available in the TOC.
If you receive a linker error message that saying you have overflowed the
available TOC space, you can reduce the amount of TOC space used with the
-mno-fp-in-toc and -mno-sum-in-toc options. -mno-fp-in-toc prevents GCC from
putting floating-point constants in the TOC and -mno-sum-in-toc forces GCC to
generate code to calculate the sum of an address and a constant at run time
instead of putting that sum into the TOC. You may specify one or both of these
options. Each causes GCC to produce very slightly slower and larger code at
the expense of conserving TOC space.
If you still run out of space in the TOC even when you specify both of these
options, specify -mminimal-toc instead. This option causes GCC to make only
one TOC entry for every file. When you specify this option, GCC produces code
that is slower and larger but which uses extremely little TOC space. You may
wish to use this option only on files that contain less frequently-executed
code.
-maix32
Disables the 64-bit ABI. GCC defaults to -maix32.
-mxl-compat
-mno-xl-compat
Produce code that conforms more closely to IBM XL compiler semantics when using
AIX-compatible ABI. Pass floating-point arguments to prototyped functions
beyond the register save area (RSA) on the stack in addition to argument FPRs.
Do not assume that most significant double in 128-bit long double value is
properly rounded when comparing values and converting to double. Use XL symbol
names for long double support routines.
The AIX calling convention was extended but not initially documented to handle
an obscure K&R C case of calling a function that takes the address of its
arguments with fewer arguments than declared. IBM XL compilers access
floating-point arguments that do not fit in the RSA from the stack when a
subroutine is compiled without optimization. Because always storing floating-
point arguments on the stack is inefficient and rarely needed, this option is
not enabled by default and only is necessary when calling subroutines compiled
by IBM XL compilers without optimization.
-malign-natural
-malign-power
On AIX, 32-bit Darwin, and 64-bit PowerPC GNU/Linux, the option -malign-natural
overrides the ABI-defined alignment of larger types, such as floating-point
doubles, on their natural size-based boundary. The option -malign-power
instructs GCC to follow the ABI-specified alignment rules. GCC defaults to the
standard alignment defined in the ABI.
On 64-bit Darwin, natural alignment is the default, and -malign-power is not
supported.
-msoft-float
-mhard-float
Generate code that does not use (uses) the floating-point register set.
Software floating-point emulation is provided if you use the -msoft-float
option, and pass the option to GCC when linking.
-msingle-float
-mdouble-float
Generate code for single- or double-precision floating-point operations.
-mdouble-float implies -msingle-float.
-mmultiple
-mno-multiple
Generate code that uses (does not use) the load multiple word instructions and
the store multiple word instructions. These instructions are generated by
default on POWER systems, and not generated on PowerPC systems. Do not use
-mmultiple on little-endian PowerPC systems, since those instructions do not
work when the processor is in little-endian mode. The exceptions are PPC740
and PPC750 which permit these instructions in little-endian mode.
-mupdate
-mno-update
Generate code that uses (does not use) the load or store instructions that
update the base register to the address of the calculated memory location.
These instructions are generated by default. If you use -mno-update, there is
a small window between the time that the stack pointer is updated and the
address of the previous frame is stored, which means code that walks the stack
frame across interrupts or signals may get corrupted data.
-mavoid-indexed-addresses
-mno-avoid-indexed-addresses
Generate code that tries to avoid (not avoid) the use of indexed load or store
instructions. These instructions can incur a performance penalty on Power6
processors in certain situations, such as when stepping through large arrays
that cross a 16M boundary. This option is enabled by default when targeting
Power6 and disabled otherwise.
-mfused-madd
-mno-fused-madd
Generate code that uses (does not use) the floating-point multiply and
accumulate instructions. These instructions are generated by default if
hardware floating point is used. The machine-dependent -mfused-madd option is
now mapped to the machine-independent -ffp-contract=fast option, and
-mno-fused-madd is mapped to -ffp-contract=off.
-mno-strict-align
-mstrict-align
On System V.4 and embedded PowerPC systems do not (do) assume that unaligned
memory references are handled by the system.
-mrelocatable
-mno-relocatable
Generate code that allows (does not allow) a static executable to be relocated
to a different address at run time. A simple embedded PowerPC system loader
should relocate the entire contents of ".got2" and 4-byte locations listed in
the ".fixup" section, a table of 32-bit addresses generated by this option.
For this to work, all objects linked together must be compiled with
-mrelocatable or -mrelocatable-lib. -mrelocatable code aligns the stack to an
8-byte boundary.
-mrelocatable-lib
-mno-relocatable-lib
Like -mrelocatable, -mrelocatable-lib generates a ".fixup" section to allow
static executables to be relocated at run time, but -mrelocatable-lib does not
use the smaller stack alignment of -mrelocatable. Objects compiled with
-mrelocatable-lib may be linked with objects compiled with any combination of
the -mrelocatable options.
-mno-toc
-mtoc
On System V.4 and embedded PowerPC systems do not (do) assume that register 2
contains a pointer to a global area pointing to the addresses used in the
program.
-mlittle
-mlittle-endian
On System V.4 and embedded PowerPC systems compile code for the processor in
little-endian mode. The -mlittle-endian option is the same as -mlittle.
-mbig
-mbig-endian
On System V.4 and embedded PowerPC systems compile code for the processor in
big-endian mode. The -mbig-endian option is the same as -mbig.
-mdynamic-no-pic
On Darwin and Mac OS X systems, compile code so that it is not relocatable, but
that its external references are relocatable. The resulting code is suitable
for applications, but not shared libraries.
-msingle-pic-base
Treat the register used for PIC addressing as read-only, rather than loading it
in the prologue for each function. The runtime system is responsible for
initializing this register with an appropriate value before execution begins.
-mprioritize-restricted-insns=priority
This option controls the priority that is assigned to dispatch-slot restricted
instructions during the second scheduling pass. The argument priority takes
the value 0, 1, or 2 to assign no, highest, or second-highest (respectively)
priority to dispatch-slot restricted instructions.
-msched-costly-dep=dependence_type
This option controls which dependences are considered costly by the target
during instruction scheduling. The argument dependence_type takes one of the
following values:
no No dependence is costly.
all All dependences are costly.
true_store_to_load
A true dependence from store to load is costly.
store_to_load
Any dependence from store to load is costly.
number
Any dependence for which the latency is greater than or equal to number is
costly.
-minsert-sched-nops=scheme
This option controls which NOP insertion scheme is used during the second
scheduling pass. The argument scheme takes one of the following values:
no Don't insert NOPs.
pad Pad with NOPs any dispatch group that has vacant issue slots, according to
the scheduler's grouping.
regroup_exact
Insert NOPs to force costly dependent insns into separate groups. Insert
exactly as many NOPs as needed to force an insn to a new group, according
to the estimated processor grouping.
number
Insert NOPs to force costly dependent insns into separate groups. Insert
number NOPs to force an insn to a new group.
-mcall-sysv
On System V.4 and embedded PowerPC systems compile code using calling
conventions that adhere to the March 1995 draft of the System V Application
Binary Interface, PowerPC processor supplement. This is the default unless you
configured GCC using powerpc-*-eabiaix.
-mcall-sysv-eabi
-mcall-eabi
Specify both -mcall-sysv and -meabi options.
-mcall-sysv-noeabi
Specify both -mcall-sysv and -mno-eabi options.
-mcall-aixdesc
On System V.4 and embedded PowerPC systems compile code for the AIX operating
system.
-mcall-linux
On System V.4 and embedded PowerPC systems compile code for the Linux-based GNU
system.
-mcall-freebsd
On System V.4 and embedded PowerPC systems compile code for the FreeBSD
operating system.
-mcall-netbsd
On System V.4 and embedded PowerPC systems compile code for the NetBSD
operating system.
-mcall-openbsd
On System V.4 and embedded PowerPC systems compile code for the OpenBSD
operating system.
-maix-struct-return
Return all structures in memory (as specified by the AIX ABI).
-msvr4-struct-return
Return structures smaller than 8 bytes in registers (as specified by the SVR4
ABI).
-mabi=abi-type
Extend the current ABI with a particular extension, or remove such extension.
Valid values are altivec, no-altivec, spe, no-spe, ibmlongdouble,
ieeelongdouble, elfv1, elfv2.
-mabi=spe
Extend the current ABI with SPE ABI extensions. This does not change the
default ABI, instead it adds the SPE ABI extensions to the current ABI.
-mabi=no-spe
Disable Book-E SPE ABI extensions for the current ABI.
-mabi=ibmlongdouble
Change the current ABI to use IBM extended-precision long double. This is not
likely to work if your system defaults to using IEEE extended-precision long
double. If you change the long double type from IEEE extended-precision, the
compiler will issue a warning unless you use the -Wno-psabi option. Requires
-mlong-double-128 to be enabled.
-mabi=ieeelongdouble
Change the current ABI to use IEEE extended-precision long double. This is not
likely to work if your system defaults to using IBM extended-precision long
double. If you change the long double type from IBM extended-precision, the
compiler will issue a warning unless you use the -Wno-psabi option. Requires
-mlong-double-128 to be enabled.
-mabi=elfv1
Change the current ABI to use the ELFv1 ABI. This is the default ABI for big-
endian PowerPC 64-bit Linux. Overriding the default ABI requires special
system support and is likely to fail in spectacular ways.
-mabi=elfv2
Change the current ABI to use the ELFv2 ABI. This is the default ABI for
little-endian PowerPC 64-bit Linux. Overriding the default ABI requires
special system support and is likely to fail in spectacular ways.
-mgnu-attribute
-mno-gnu-attribute
Emit .gnu_attribute assembly directives to set tag/value pairs in a
.gnu.attributes section that specify ABI variations in function parameters or
return values.
-mprototype
-mno-prototype
On System V.4 and embedded PowerPC systems assume that all calls to variable
argument functions are properly prototyped. Otherwise, the compiler must
insert an instruction before every non-prototyped call to set or clear bit 6 of
the condition code register ("CR") to indicate whether floating-point values
are passed in the floating-point registers in case the function takes variable
arguments. With -mprototype, only calls to prototyped variable argument
functions set or clear the bit.
-msim
On embedded PowerPC systems, assume that the startup module is called
sim-crt0.o and that the standard C libraries are libsim.a and libc.a. This is
the default for powerpc-*-eabisim configurations.
-mmvme
On embedded PowerPC systems, assume that the startup module is called crt0.o
and the standard C libraries are libmvme.a and libc.a.
-mads
On embedded PowerPC systems, assume that the startup module is called crt0.o
and the standard C libraries are libads.a and libc.a.
-myellowknife
On embedded PowerPC systems, assume that the startup module is called crt0.o
and the standard C libraries are libyk.a and libc.a.
-mvxworks
On System V.4 and embedded PowerPC systems, specify that you are compiling for
a VxWorks system.
-memb
On embedded PowerPC systems, set the "PPC_EMB" bit in the ELF flags header to
indicate that eabi extended relocations are used.
-meabi
-mno-eabi
On System V.4 and embedded PowerPC systems do (do not) adhere to the Embedded
Applications Binary Interface (EABI), which is a set of modifications to the
System V.4 specifications. Selecting -meabi means that the stack is aligned to
an 8-byte boundary, a function "__eabi" is called from "main" to set up the
EABI environment, and the -msdata option can use both "r2" and "r13" to point
to two separate small data areas. Selecting -mno-eabi means that the stack is
aligned to a 16-byte boundary, no EABI initialization function is called from
"main", and the -msdata option only uses "r13" to point to a single small data
area. The -meabi option is on by default if you configured GCC using one of
the powerpc*-*-eabi* options.
-msdata=eabi
On System V.4 and embedded PowerPC systems, put small initialized "const"
global and static data in the ".sdata2" section, which is pointed to by
register "r2". Put small initialized non-"const" global and static data in the
".sdata" section, which is pointed to by register "r13". Put small
uninitialized global and static data in the ".sbss" section, which is adjacent
to the ".sdata" section. The -msdata=eabi option is incompatible with the
-mrelocatable option. The -msdata=eabi option also sets the -memb option.
-msdata=sysv
On System V.4 and embedded PowerPC systems, put small global and static data in
the ".sdata" section, which is pointed to by register "r13". Put small
uninitialized global and static data in the ".sbss" section, which is adjacent
to the ".sdata" section. The -msdata=sysv option is incompatible with the
-mrelocatable option.
-msdata=default
-msdata
On System V.4 and embedded PowerPC systems, if -meabi is used, compile code the
same as -msdata=eabi, otherwise compile code the same as -msdata=sysv.
-msdata=data
On System V.4 and embedded PowerPC systems, put small global data in the
".sdata" section. Put small uninitialized global data in the ".sbss" section.
Do not use register "r13" to address small data however. This is the default
behavior unless other -msdata options are used.
-msdata=none
-mno-sdata
On embedded PowerPC systems, put all initialized global and static data in the
".data" section, and all uninitialized data in the ".bss" section.
-mblock-move-inline-limit=num
Inline all block moves (such as calls to "memcpy" or structure copies) less
than or equal to num bytes. The minimum value for num is 32 bytes on 32-bit
targets and 64 bytes on 64-bit targets. The default value is target-specific.
-G num
On embedded PowerPC systems, put global and static items less than or equal to
num bytes into the small data or BSS sections instead of the normal data or BSS
section. By default, num is 8. The -G num switch is also passed to the
linker. All modules should be compiled with the same -G num value.
-mregnames
-mno-regnames
On System V.4 and embedded PowerPC systems do (do not) emit register names in
the assembly language output using symbolic forms.
-mlongcall
-mno-longcall
By default assume that all calls are far away so that a longer and more
expensive calling sequence is required. This is required for calls farther
than 32 megabytes (33,554,432 bytes) from the current location. A short call
is generated if the compiler knows the call cannot be that far away. This
setting can be overridden by the "shortcall" function attribute, or by "#pragma
longcall(0)".
Some linkers are capable of detecting out-of-range calls and generating glue
code on the fly. On these systems, long calls are unnecessary and generate
slower code. As of this writing, the AIX linker can do this, as can the GNU
linker for PowerPC/64. It is planned to add this feature to the GNU linker for
32-bit PowerPC systems as well.
In the future, GCC may ignore all longcall specifications when the linker is
known to generate glue.
-mtls-markers
-mno-tls-markers
Mark (do not mark) calls to "__tls_get_addr" with a relocation specifying the
function argument. The relocation allows the linker to reliably associate
function call with argument setup instructions for TLS optimization, which in
turn allows GCC to better schedule the sequence.
-mrecip
-mno-recip
This option enables use of the reciprocal estimate and reciprocal square root
estimate instructions with additional Newton-Raphson steps to increase
precision instead of doing a divide or square root and divide for floating-
point arguments. You should use the -ffast-math option when using -mrecip (or
at least -funsafe-math-optimizations, -ffinite-math-only, -freciprocal-math and
-fno-trapping-math). Note that while the throughput of the sequence is
generally higher than the throughput of the non-reciprocal instruction, the
precision of the sequence can be decreased by up to 2 ulp (i.e. the inverse of
1.0 equals 0.99999994) for reciprocal square roots.
-mrecip=opt
This option controls which reciprocal estimate instructions may be used. opt
is a comma-separated list of options, which may be preceded by a "!" to invert
the option:
all Enable all estimate instructions.
default
Enable the default instructions, equivalent to -mrecip.
none
Disable all estimate instructions, equivalent to -mno-recip.
div Enable the reciprocal approximation instructions for both single and double
precision.
divf
Enable the single-precision reciprocal approximation instructions.
divd
Enable the double-precision reciprocal approximation instructions.
rsqrt
Enable the reciprocal square root approximation instructions for both
single and double precision.
rsqrtf
Enable the single-precision reciprocal square root approximation
instructions.
rsqrtd
Enable the double-precision reciprocal square root approximation
instructions.
So, for example, -mrecip=all,!rsqrtd enables all of the reciprocal estimate
instructions, except for the "FRSQRTE", "XSRSQRTEDP", and "XVRSQRTEDP"
instructions which handle the double-precision reciprocal square root
calculations.
-mrecip-precision
-mno-recip-precision
Assume (do not assume) that the reciprocal estimate instructions provide
higher-precision estimates than is mandated by the PowerPC ABI. Selecting
-mcpu=power6, -mcpu=power7 or -mcpu=power8 automatically selects
-mrecip-precision. The double-precision square root estimate instructions are
not generated by default on low-precision machines, since they do not provide
an estimate that converges after three steps.
-mpointers-to-nested-functions
-mno-pointers-to-nested-functions
Generate (do not generate) code to load up the static chain register ("r11")
when calling through a pointer on AIX and 64-bit Linux systems where a function
pointer points to a 3-word descriptor giving the function address, TOC value to
be loaded in register "r2", and static chain value to be loaded in register
"r11". The -mpointers-to-nested-functions is on by default. You cannot call
through pointers to nested functions or pointers to functions compiled in other
languages that use the static chain if you use
-mno-pointers-to-nested-functions.
-msave-toc-indirect
-mno-save-toc-indirect
Generate (do not generate) code to save the TOC value in the reserved stack
location in the function prologue if the function calls through a pointer on
AIX and 64-bit Linux systems. If the TOC value is not saved in the prologue,
it is saved just before the call through the pointer. The
-mno-save-toc-indirect option is the default.
-mcompat-align-parm
-mno-compat-align-parm
Generate (do not generate) code to pass structure parameters with a maximum
alignment of 64 bits, for compatibility with older versions of GCC.
Older versions of GCC (prior to 4.9.0) incorrectly did not align a structure
parameter on a 128-bit boundary when that structure contained a member
requiring 128-bit alignment. This is corrected in more recent versions of GCC.
This option may be used to generate code that is compatible with functions
compiled with older versions of GCC.
The -mno-compat-align-parm option is the default.
-mstack-protector-guard=guard
-mstack-protector-guard-reg=reg
-mstack-protector-guard-offset=offset
-mstack-protector-guard-symbol=symbol
Generate stack protection code using canary at guard. Supported locations are
global for global canary or tls for per-thread canary in the TLS block (the
default with GNU libc version 2.4 or later).
With the latter choice the options -mstack-protector-guard-reg=reg and
-mstack-protector-guard-offset=offset furthermore specify which register to use
as base register for reading the canary, and from what offset from that base
register. The default for those is as specified in the relevant ABI.
-mstack-protector-guard-symbol=symbol overrides the offset with a symbol
reference to a canary in the TLS block.
RISC-V Options
These command-line options are defined for RISC-V targets:
-mbranch-cost=n
Set the cost of branches to roughly n instructions.
-mplt
-mno-plt
When generating PIC code, do or don't allow the use of PLTs. Ignored for non-
PIC. The default is -mplt.
-mabi=ABI-string
Specify integer and floating-point calling convention. ABI-string contains two
parts: the size of integer types and the registers used for floating-point
types. For example -march=rv64ifd -mabi=lp64d means that long and pointers are
64-bit (implicitly defining int to be 32-bit), and that floating-point values
up to 64 bits wide are passed in F registers. Contrast this with
-march=rv64ifd -mabi=lp64f, which still allows the compiler to generate code
that uses the F and D extensions but only allows floating-point values up to 32
bits long to be passed in registers; or -march=rv64ifd -mabi=lp64, in which no
floating-point arguments will be passed in registers.
The default for this argument is system dependent, users who want a specific
calling convention should specify one explicitly. The valid calling
conventions are: ilp32, ilp32f, ilp32d, lp64, lp64f, and lp64d. Some calling
conventions are impossible to implement on some ISAs: for example,
-march=rv32if -mabi=ilp32d is invalid because the ABI requires 64-bit values be
passed in F registers, but F registers are only 32 bits wide.
-mfdiv
-mno-fdiv
Do or don't use hardware floating-point divide and square root instructions.
This requires the F or D extensions for floating-point registers. The default
is to use them if the specified architecture has these instructions.
-mdiv
-mno-div
Do or don't use hardware instructions for integer division. This requires the
M extension. The default is to use them if the specified architecture has
these instructions.
-march=ISA-string
Generate code for given RISC-V ISA (e.g. rv64im). ISA strings must be lower-
case. Examples include rv64i, rv32g, and rv32imaf.
-mtune=processor-string
Optimize the output for the given processor, specified by microarchitecture
name.
-mpreferred-stack-boundary=num
Attempt to keep the stack boundary aligned to a 2 raised to num byte boundary.
If -mpreferred-stack-boundary is not specified, the default is 4 (16 bytes or
128-bits).
Warning: If you use this switch, then you must build all modules with the same
value, including any libraries. This includes the system libraries and startup
modules.
-msmall-data-limit=n
Put global and static data smaller than n bytes into a special section (on some
targets).
-msave-restore
-mno-save-restore
Do or don't use smaller but slower prologue and epilogue code that uses library
function calls. The default is to use fast inline prologues and epilogues.
-mstrict-align
-mno-strict-align
Do not or do generate unaligned memory accesses. The default is set depending
on whether the processor we are optimizing for supports fast unaligned access
or not.
-mcmodel=medlow
Generate code for the medium-low code model. The program and its statically
defined symbols must lie within a single 2 GiB address range and must lie
between absolute addresses -2 GiB and +2 GiB. Programs can be statically or
dynamically linked. This is the default code model.
-mcmodel=medany
Generate code for the medium-any code model. The program and its statically
defined symbols must be within any single 2 GiB address range. Programs can be
statically or dynamically linked.
-mexplicit-relocs
-mno-exlicit-relocs
Use or do not use assembler relocation operators when dealing with symbolic
addresses. The alternative is to use assembler macros instead, which may limit
optimization.
-mrelax
-mno-relax
Take advantage of linker relaxations to reduce the number of instructions
required to materialize symbol addresses. The default is to take advantage of
linker relaxations.
RL78 Options
-msim
Links in additional target libraries to support operation within a simulator.
-mmul=none
-mmul=g10
-mmul=g13
-mmul=g14
-mmul=rl78
Specifies the type of hardware multiplication and division support to be used.
The simplest is "none", which uses software for both multiplication and
division. This is the default. The "g13" value is for the hardware
multiply/divide peripheral found on the RL78/G13 (S2 core) targets. The "g14"
value selects the use of the multiplication and division instructions supported
by the RL78/G14 (S3 core) parts. The value "rl78" is an alias for "g14" and
the value "mg10" is an alias for "none".
In addition a C preprocessor macro is defined, based upon the setting of this
option. Possible values are: "__RL78_MUL_NONE__", "__RL78_MUL_G13__" or
"__RL78_MUL_G14__".
-mcpu=g10
-mcpu=g13
-mcpu=g14
-mcpu=rl78
Specifies the RL78 core to target. The default is the G14 core, also known as
an S3 core or just RL78. The G13 or S2 core does not have multiply or divide
instructions, instead it uses a hardware peripheral for these operations. The
G10 or S1 core does not have register banks, so it uses a different calling
convention.
If this option is set it also selects the type of hardware multiply support to
use, unless this is overridden by an explicit -mmul=none option on the command
line. Thus specifying -mcpu=g13 enables the use of the G13 hardware multiply
peripheral and specifying -mcpu=g10 disables the use of hardware
multiplications altogether.
Note, although the RL78/G14 core is the default target, specifying -mcpu=g14 or
-mcpu=rl78 on the command line does change the behavior of the toolchain since
it also enables G14 hardware multiply support. If these options are not
specified on the command line then software multiplication routines will be
used even though the code targets the RL78 core. This is for backwards
compatibility with older toolchains which did not have hardware multiply and
divide support.
In addition a C preprocessor macro is defined, based upon the setting of this
option. Possible values are: "__RL78_G10__", "__RL78_G13__" or "__RL78_G14__".
-mg10
-mg13
-mg14
-mrl78
These are aliases for the corresponding -mcpu= option. They are provided for
backwards compatibility.
-mallregs
Allow the compiler to use all of the available registers. By default registers
"r24..r31" are reserved for use in interrupt handlers. With this option
enabled these registers can be used in ordinary functions as well.
-m64bit-doubles
-m32bit-doubles
Make the "double" data type be 64 bits (-m64bit-doubles) or 32 bits
(-m32bit-doubles) in size. The default is -m32bit-doubles.
-msave-mduc-in-interrupts
-mno-save-mduc-in-interrupts
Specifies that interrupt handler functions should preserve the MDUC registers.
This is only necessary if normal code might use the MDUC registers, for example
because it performs multiplication and division operations. The default is to
ignore the MDUC registers as this makes the interrupt handlers faster. The
target option -mg13 needs to be passed for this to work as this feature is only
available on the G13 target (S2 core). The MDUC registers will only be saved
if the interrupt handler performs a multiplication or division operation or it
calls another function.
IBM RS/6000 and PowerPC Options
These -m options are defined for the IBM RS/6000 and PowerPC:
-mpowerpc-gpopt
-mno-powerpc-gpopt
-mpowerpc-gfxopt
-mno-powerpc-gfxopt
-mpowerpc64
-mno-powerpc64
-mmfcrf
-mno-mfcrf
-mpopcntb
-mno-popcntb
-mpopcntd
-mno-popcntd
-mfprnd
-mno-fprnd
-mcmpb
-mno-cmpb
-mmfpgpr
-mno-mfpgpr
-mhard-dfp
-mno-hard-dfp
You use these options to specify which instructions are available on the
processor you are using. The default value of these options is determined when
configuring GCC. Specifying the -mcpu=cpu_type overrides the specification of
these options. We recommend you use the -mcpu=cpu_type option rather than the
options listed above.
Specifying -mpowerpc-gpopt allows GCC to use the optional PowerPC architecture
instructions in the General Purpose group, including floating-point square
root. Specifying -mpowerpc-gfxopt allows GCC to use the optional PowerPC
architecture instructions in the Graphics group, including floating-point
select.
The -mmfcrf option allows GCC to generate the move from condition register
field instruction implemented on the POWER4 processor and other processors that
support the PowerPC V2.01 architecture. The -mpopcntb option allows GCC to
generate the popcount and double-precision FP reciprocal estimate instruction
implemented on the POWER5 processor and other processors that support the
PowerPC V2.02 architecture. The -mpopcntd option allows GCC to generate the
popcount instruction implemented on the POWER7 processor and other processors
that support the PowerPC V2.06 architecture. The -mfprnd option allows GCC to
generate the FP round to integer instructions implemented on the POWER5+
processor and other processors that support the PowerPC V2.03 architecture.
The -mcmpb option allows GCC to generate the compare bytes instruction
implemented on the POWER6 processor and other processors that support the
PowerPC V2.05 architecture. The -mmfpgpr option allows GCC to generate the FP
move to/from general-purpose register instructions implemented on the POWER6X
processor and other processors that support the extended PowerPC V2.05
architecture. The -mhard-dfp option allows GCC to generate the decimal
floating-point instructions implemented on some POWER processors.
The -mpowerpc64 option allows GCC to generate the additional 64-bit
instructions that are found in the full PowerPC64 architecture and to treat
GPRs as 64-bit, doubleword quantities. GCC defaults to -mno-powerpc64.
-mcpu=cpu_type
Set architecture type, register usage, and instruction scheduling parameters
for machine type cpu_type. Supported values for cpu_type are 401, 403, 405,
405fp, 440, 440fp, 464, 464fp, 476, 476fp, 505, 601, 602, 603, 603e, 604, 604e,
620, 630, 740, 7400, 7450, 750, 801, 821, 823, 860, 970, 8540, a2, e300c2,
e300c3, e500mc, e500mc64, e5500, e6500, ec603e, G3, G4, G5, titan, power3,
power4, power5, power5+, power6, power6x, power7, power8, power9, powerpc,
powerpc64, powerpc64le, rs64, and native.
-mcpu=powerpc, -mcpu=powerpc64, and -mcpu=powerpc64le specify pure 32-bit
PowerPC (either endian), 64-bit big endian PowerPC and 64-bit little endian
PowerPC architecture machine types, with an appropriate, generic processor
model assumed for scheduling purposes.
Specifying native as cpu type detects and selects the architecture option that
corresponds to the host processor of the system performing the compilation.
-mcpu=native has no effect if GCC does not recognize the processor.
The other options specify a specific processor. Code generated under those
options runs best on that processor, and may not run at all on others.
The -mcpu options automatically enable or disable the following options:
-maltivec -mfprnd -mhard-float -mmfcrf -mmultiple -mpopcntb -mpopcntd
-mpowerpc64 -mpowerpc-gpopt -mpowerpc-gfxopt -msingle-float -mdouble-float
-msimple-fpu -mmulhw -mdlmzb -mmfpgpr -mvsx -mcrypto -mhtm -mpower8-fusion
-mpower8-vector -mquad-memory -mquad-memory-atomic -mfloat128
-mfloat128-hardware
The particular options set for any particular CPU varies between compiler
versions, depending on what setting seems to produce optimal code for that CPU;
it doesn't necessarily reflect the actual hardware's capabilities. If you wish
to set an individual option to a particular value, you may specify it after the
-mcpu option, like -mcpu=970 -mno-altivec.
On AIX, the -maltivec and -mpowerpc64 options are not enabled or disabled by
the -mcpu option at present because AIX does not have full support for these
options. You may still enable or disable them individually if you're sure
it'll work in your environment.
-mtune=cpu_type
Set the instruction scheduling parameters for machine type cpu_type, but do not
set the architecture type or register usage, as -mcpu=cpu_type does. The same
values for cpu_type are used for -mtune as for -mcpu. If both are specified,
the code generated uses the architecture and registers set by -mcpu, but the
scheduling parameters set by -mtune.
-mcmodel=small
Generate PowerPC64 code for the small model: The TOC is limited to 64k.
-mcmodel=medium
Generate PowerPC64 code for the medium model: The TOC and other static data may
be up to a total of 4G in size. This is the default for 64-bit Linux.
-mcmodel=large
Generate PowerPC64 code for the large model: The TOC may be up to 4G in size.
Other data and code is only limited by the 64-bit address space.
-maltivec
-mno-altivec
Generate code that uses (does not use) AltiVec instructions, and also enable
the use of built-in functions that allow more direct access to the AltiVec
instruction set. You may also need to set -mabi=altivec to adjust the current
ABI with AltiVec ABI enhancements.
When -maltivec is used, rather than -maltivec=le or -maltivec=be, the element
order for AltiVec intrinsics such as "vec_splat", "vec_extract", and
"vec_insert" match array element order corresponding to the endianness of the
target. That is, element zero identifies the leftmost element in a vector
register when targeting a big-endian platform, and identifies the rightmost
element in a vector register when targeting a little-endian platform.
-maltivec=be
Generate AltiVec instructions using big-endian element order, regardless of
whether the target is big- or little-endian. This is the default when
targeting a big-endian platform. Using this option is currently deprecated.
Support for this feature will be removed in GCC 9.
The element order is used to interpret element numbers in AltiVec intrinsics
such as "vec_splat", "vec_extract", and "vec_insert". By default, these match
array element order corresponding to the endianness for the target.
-maltivec=le
Generate AltiVec instructions using little-endian element order, regardless of
whether the target is big- or little-endian. This is the default when
targeting a little-endian platform. This option is currently ignored when
targeting a big-endian platform.
The element order is used to interpret element numbers in AltiVec intrinsics
such as "vec_splat", "vec_extract", and "vec_insert". By default, these match
array element order corresponding to the endianness for the target.
-mvrsave
-mno-vrsave
Generate VRSAVE instructions when generating AltiVec code.
-msecure-plt
Generate code that allows ld and ld.so to build executables and shared
libraries with non-executable ".plt" and ".got" sections. This is a PowerPC
32-bit SYSV ABI option.
-mbss-plt
Generate code that uses a BSS ".plt" section that ld.so fills in, and requires
".plt" and ".got" sections that are both writable and executable. This is a
PowerPC 32-bit SYSV ABI option.
-misel
-mno-isel
This switch enables or disables the generation of ISEL instructions.
-misel=yes/no
This switch has been deprecated. Use -misel and -mno-isel instead.
-mpaired
-mno-paired
This switch enables or disables the generation of PAIRED simd instructions.
-mvsx
-mno-vsx
Generate code that uses (does not use) vector/scalar (VSX) instructions, and
also enable the use of built-in functions that allow more direct access to the
VSX instruction set.
-mcrypto
-mno-crypto
Enable the use (disable) of the built-in functions that allow direct access to
the cryptographic instructions that were added in version 2.07 of the PowerPC
ISA.
-mhtm
-mno-htm
Enable (disable) the use of the built-in functions that allow direct access to
the Hardware Transactional Memory (HTM) instructions that were added in version
2.07 of the PowerPC ISA.
-mpower8-fusion
-mno-power8-fusion
Generate code that keeps (does not keeps) some integer operations adjacent so
that the instructions can be fused together on power8 and later processors.
-mpower8-vector
-mno-power8-vector
Generate code that uses (does not use) the vector and scalar instructions that
were added in version 2.07 of the PowerPC ISA. Also enable the use of built-in
functions that allow more direct access to the vector instructions.
-mquad-memory
-mno-quad-memory
Generate code that uses (does not use) the non-atomic quad word memory
instructions. The -mquad-memory option requires use of 64-bit mode.
-mquad-memory-atomic
-mno-quad-memory-atomic
Generate code that uses (does not use) the atomic quad word memory
instructions. The -mquad-memory-atomic option requires use of 64-bit mode.
-mfloat128
-mno-float128
Enable/disable the __float128 keyword for IEEE 128-bit floating point and use
either software emulation for IEEE 128-bit floating point or hardware
instructions.
The VSX instruction set (-mvsx, -mcpu=power7, -mcpu=power8), or -mcpu=power9
must be enabled to use the IEEE 128-bit floating point support. The IEEE
128-bit floating point support only works on PowerPC Linux systems.
The default for -mfloat128 is enabled on PowerPC Linux systems using the VSX
instruction set, and disabled on other systems.
If you use the ISA 3.0 instruction set (-mpower9-vector or -mcpu=power9) on a
64-bit system, the IEEE 128-bit floating point support will also enable the
generation of ISA 3.0 IEEE 128-bit floating point instructions. Otherwise, if
you do not specify to generate ISA 3.0 instructions or you are targeting a
32-bit big endian system, IEEE 128-bit floating point will be done with
software emulation.
-mfloat128-hardware
-mno-float128-hardware
Enable/disable using ISA 3.0 hardware instructions to support the __float128
data type.
The default for -mfloat128-hardware is enabled on PowerPC Linux systems using
the ISA 3.0 instruction set, and disabled on other systems.
-m32
-m64
Generate code for 32-bit or 64-bit environments of Darwin and SVR4 targets
(including GNU/Linux). The 32-bit environment sets int, long and pointer to 32
bits and generates code that runs on any PowerPC variant. The 64-bit
environment sets int to 32 bits and long and pointer to 64 bits, and generates
code for PowerPC64, as for -mpowerpc64.
-mfull-toc
-mno-fp-in-toc
-mno-sum-in-toc
-mminimal-toc
Modify generation of the TOC (Table Of Contents), which is created for every
executable file. The -mfull-toc option is selected by default. In that case,
GCC allocates at least one TOC entry for each unique non-automatic variable
reference in your program. GCC also places floating-point constants in the
TOC. However, only 16,384 entries are available in the TOC.
If you receive a linker error message that saying you have overflowed the
available TOC space, you can reduce the amount of TOC space used with the
-mno-fp-in-toc and -mno-sum-in-toc options. -mno-fp-in-toc prevents GCC from
putting floating-point constants in the TOC and -mno-sum-in-toc forces GCC to
generate code to calculate the sum of an address and a constant at run time
instead of putting that sum into the TOC. You may specify one or both of these
options. Each causes GCC to produce very slightly slower and larger code at
the expense of conserving TOC space.
If you still run out of space in the TOC even when you specify both of these
options, specify -mminimal-toc instead. This option causes GCC to make only
one TOC entry for every file. When you specify this option, GCC produces code
that is slower and larger but which uses extremely little TOC space. You may
wish to use this option only on files that contain less frequently-executed
code.
-maix64
-maix32
Enable 64-bit AIX ABI and calling convention: 64-bit pointers, 64-bit "long"
type, and the infrastructure needed to support them. Specifying -maix64
implies -mpowerpc64, while -maix32 disables the 64-bit ABI and implies
-mno-powerpc64. GCC defaults to -maix32.
-mxl-compat
-mno-xl-compat
Produce code that conforms more closely to IBM XL compiler semantics when using
AIX-compatible ABI. Pass floating-point arguments to prototyped functions
beyond the register save area (RSA) on the stack in addition to argument FPRs.
Do not assume that most significant double in 128-bit long double value is
properly rounded when comparing values and converting to double. Use XL symbol
names for long double support routines.
The AIX calling convention was extended but not initially documented to handle
an obscure K&R C case of calling a function that takes the address of its
arguments with fewer arguments than declared. IBM XL compilers access
floating-point arguments that do not fit in the RSA from the stack when a
subroutine is compiled without optimization. Because always storing floating-
point arguments on the stack is inefficient and rarely needed, this option is
not enabled by default and only is necessary when calling subroutines compiled
by IBM XL compilers without optimization.
-mpe
Support IBM RS/6000 SP Parallel Environment (PE). Link an application written
to use message passing with special startup code to enable the application to
run. The system must have PE installed in the standard location
(/usr/lpp/ppe.poe/), or the specs file must be overridden with the -specs=
option to specify the appropriate directory location. The Parallel Environment
does not support threads, so the -mpe option and the -pthread option are
incompatible.
-malign-natural
-malign-power
On AIX, 32-bit Darwin, and 64-bit PowerPC GNU/Linux, the option -malign-natural
overrides the ABI-defined alignment of larger types, such as floating-point
doubles, on their natural size-based boundary. The option -malign-power
instructs GCC to follow the ABI-specified alignment rules. GCC defaults to the
standard alignment defined in the ABI.
On 64-bit Darwin, natural alignment is the default, and -malign-power is not
supported.
-msoft-float
-mhard-float
Generate code that does not use (uses) the floating-point register set.
Software floating-point emulation is provided if you use the -msoft-float
option, and pass the option to GCC when linking.
-msingle-float
-mdouble-float
Generate code for single- or double-precision floating-point operations.
-mdouble-float implies -msingle-float.
-msimple-fpu
Do not generate "sqrt" and "div" instructions for hardware floating-point unit.
-mfpu=name
Specify type of floating-point unit. Valid values for name are sp_lite
(equivalent to -msingle-float -msimple-fpu), dp_lite (equivalent to
-mdouble-float -msimple-fpu), sp_full (equivalent to -msingle-float), and
dp_full (equivalent to -mdouble-float).
-mxilinx-fpu
Perform optimizations for the floating-point unit on Xilinx PPC 405/440.
-mmultiple
-mno-multiple
Generate code that uses (does not use) the load multiple word instructions and
the store multiple word instructions. These instructions are generated by
default on POWER systems, and not generated on PowerPC systems. Do not use
-mmultiple on little-endian PowerPC systems, since those instructions do not
work when the processor is in little-endian mode. The exceptions are PPC740
and PPC750 which permit these instructions in little-endian mode.
-mupdate
-mno-update
Generate code that uses (does not use) the load or store instructions that
update the base register to the address of the calculated memory location.
These instructions are generated by default. If you use -mno-update, there is
a small window between the time that the stack pointer is updated and the
address of the previous frame is stored, which means code that walks the stack
frame across interrupts or signals may get corrupted data.
-mavoid-indexed-addresses
-mno-avoid-indexed-addresses
Generate code that tries to avoid (not avoid) the use of indexed load or store
instructions. These instructions can incur a performance penalty on Power6
processors in certain situations, such as when stepping through large arrays
that cross a 16M boundary. This option is enabled by default when targeting
Power6 and disabled otherwise.
-mfused-madd
-mno-fused-madd
Generate code that uses (does not use) the floating-point multiply and
accumulate instructions. These instructions are generated by default if
hardware floating point is used. The machine-dependent -mfused-madd option is
now mapped to the machine-independent -ffp-contract=fast option, and
-mno-fused-madd is mapped to -ffp-contract=off.
-mmulhw
-mno-mulhw
Generate code that uses (does not use) the half-word multiply and multiply-
accumulate instructions on the IBM 405, 440, 464 and 476 processors. These
instructions are generated by default when targeting those processors.
-mdlmzb
-mno-dlmzb
Generate code that uses (does not use) the string-search dlmzb instruction on
the IBM 405, 440, 464 and 476 processors. This instruction is generated by
default when targeting those processors.
-mno-bit-align
-mbit-align
On System V.4 and embedded PowerPC systems do not (do) force structures and
unions that contain bit-fields to be aligned to the base type of the bit-field.
For example, by default a structure containing nothing but 8 "unsigned" bit-
fields of length 1 is aligned to a 4-byte boundary and has a size of 4 bytes.
By using -mno-bit-align, the structure is aligned to a 1-byte boundary and is 1
byte in size.
-mno-strict-align
-mstrict-align
On System V.4 and embedded PowerPC systems do not (do) assume that unaligned
memory references are handled by the system.
-mrelocatable
-mno-relocatable
Generate code that allows (does not allow) a static executable to be relocated
to a different address at run time. A simple embedded PowerPC system loader
should relocate the entire contents of ".got2" and 4-byte locations listed in
the ".fixup" section, a table of 32-bit addresses generated by this option.
For this to work, all objects linked together must be compiled with
-mrelocatable or -mrelocatable-lib. -mrelocatable code aligns the stack to an
8-byte boundary.
-mrelocatable-lib
-mno-relocatable-lib
Like -mrelocatable, -mrelocatable-lib generates a ".fixup" section to allow
static executables to be relocated at run time, but -mrelocatable-lib does not
use the smaller stack alignment of -mrelocatable. Objects compiled with
-mrelocatable-lib may be linked with objects compiled with any combination of
the -mrelocatable options.
-mno-toc
-mtoc
On System V.4 and embedded PowerPC systems do not (do) assume that register 2
contains a pointer to a global area pointing to the addresses used in the
program.
-mlittle
-mlittle-endian
On System V.4 and embedded PowerPC systems compile code for the processor in
little-endian mode. The -mlittle-endian option is the same as -mlittle.
-mbig
-mbig-endian
On System V.4 and embedded PowerPC systems compile code for the processor in
big-endian mode. The -mbig-endian option is the same as -mbig.
-mdynamic-no-pic
On Darwin and Mac OS X systems, compile code so that it is not relocatable, but
that its external references are relocatable. The resulting code is suitable
for applications, but not shared libraries.
-msingle-pic-base
Treat the register used for PIC addressing as read-only, rather than loading it
in the prologue for each function. The runtime system is responsible for
initializing this register with an appropriate value before execution begins.
-mprioritize-restricted-insns=priority
This option controls the priority that is assigned to dispatch-slot restricted
instructions during the second scheduling pass. The argument priority takes
the value 0, 1, or 2 to assign no, highest, or second-highest (respectively)
priority to dispatch-slot restricted instructions.
-msched-costly-dep=dependence_type
This option controls which dependences are considered costly by the target
during instruction scheduling. The argument dependence_type takes one of the
following values:
no No dependence is costly.
all All dependences are costly.
true_store_to_load
A true dependence from store to load is costly.
store_to_load
Any dependence from store to load is costly.
number
Any dependence for which the latency is greater than or equal to number is
costly.
-minsert-sched-nops=scheme
This option controls which NOP insertion scheme is used during the second
scheduling pass. The argument scheme takes one of the following values:
no Don't insert NOPs.
pad Pad with NOPs any dispatch group that has vacant issue slots, according to
the scheduler's grouping.
regroup_exact
Insert NOPs to force costly dependent insns into separate groups. Insert
exactly as many NOPs as needed to force an insn to a new group, according
to the estimated processor grouping.
number
Insert NOPs to force costly dependent insns into separate groups. Insert
number NOPs to force an insn to a new group.
-mcall-sysv
On System V.4 and embedded PowerPC systems compile code using calling
conventions that adhere to the March 1995 draft of the System V Application
Binary Interface, PowerPC processor supplement. This is the default unless you
configured GCC using powerpc-*-eabiaix.
-mcall-sysv-eabi
-mcall-eabi
Specify both -mcall-sysv and -meabi options.
-mcall-sysv-noeabi
Specify both -mcall-sysv and -mno-eabi options.
-mcall-aixdesc
On System V.4 and embedded PowerPC systems compile code for the AIX operating
system.
-mcall-linux
On System V.4 and embedded PowerPC systems compile code for the Linux-based GNU
system.
-mcall-freebsd
On System V.4 and embedded PowerPC systems compile code for the FreeBSD
operating system.
-mcall-netbsd
On System V.4 and embedded PowerPC systems compile code for the NetBSD
operating system.
-mcall-openbsd
On System V.4 and embedded PowerPC systems compile code for the OpenBSD
operating system.
-mtraceback=traceback_type
Select the type of traceback table. Valid values for traceback_type are full,
part, and no.
-maix-struct-return
Return all structures in memory (as specified by the AIX ABI).
-msvr4-struct-return
Return structures smaller than 8 bytes in registers (as specified by the SVR4
ABI).
-mabi=abi-type
Extend the current ABI with a particular extension, or remove such extension.
Valid values are altivec, no-altivec, spe, no-spe, ibmlongdouble,
ieeelongdouble, elfv1, elfv2.
-mabi=ibmlongdouble
Change the current ABI to use IBM extended-precision long double. This is not
likely to work if your system defaults to using IEEE extended-precision long
double. If you change the long double type from IEEE extended-precision, the
compiler will issue a warning unless you use the -Wno-psabi option. Requires
-mlong-double-128 to be enabled.
-mabi=ieeelongdouble
Change the current ABI to use IEEE extended-precision long double. This is not
likely to work if your system defaults to using IBM extended-precision long
double. If you change the long double type from IBM extended-precision, the
compiler will issue a warning unless you use the -Wno-psabi option. Requires
-mlong-double-128 to be enabled.
-mabi=elfv1
Change the current ABI to use the ELFv1 ABI. This is the default ABI for big-
endian PowerPC 64-bit Linux. Overriding the default ABI requires special
system support and is likely to fail in spectacular ways.
-mabi=elfv2
Change the current ABI to use the ELFv2 ABI. This is the default ABI for
little-endian PowerPC 64-bit Linux. Overriding the default ABI requires
special system support and is likely to fail in spectacular ways.
-mgnu-attribute
-mno-gnu-attribute
Emit .gnu_attribute assembly directives to set tag/value pairs in a
.gnu.attributes section that specify ABI variations in function parameters or
return values.
-mprototype
-mno-prototype
On System V.4 and embedded PowerPC systems assume that all calls to variable
argument functions are properly prototyped. Otherwise, the compiler must
insert an instruction before every non-prototyped call to set or clear bit 6 of
the condition code register ("CR") to indicate whether floating-point values
are passed in the floating-point registers in case the function takes variable
arguments. With -mprototype, only calls to prototyped variable argument
functions set or clear the bit.
-msim
On embedded PowerPC systems, assume that the startup module is called
sim-crt0.o and that the standard C libraries are libsim.a and libc.a. This is
the default for powerpc-*-eabisim configurations.
-mmvme
On embedded PowerPC systems, assume that the startup module is called crt0.o
and the standard C libraries are libmvme.a and libc.a.
-mads
On embedded PowerPC systems, assume that the startup module is called crt0.o
and the standard C libraries are libads.a and libc.a.
-myellowknife
On embedded PowerPC systems, assume that the startup module is called crt0.o
and the standard C libraries are libyk.a and libc.a.
-mvxworks
On System V.4 and embedded PowerPC systems, specify that you are compiling for
a VxWorks system.
-memb
On embedded PowerPC systems, set the "PPC_EMB" bit in the ELF flags header to
indicate that eabi extended relocations are used.
-meabi
-mno-eabi
On System V.4 and embedded PowerPC systems do (do not) adhere to the Embedded
Applications Binary Interface (EABI), which is a set of modifications to the
System V.4 specifications. Selecting -meabi means that the stack is aligned to
an 8-byte boundary, a function "__eabi" is called from "main" to set up the
EABI environment, and the -msdata option can use both "r2" and "r13" to point
to two separate small data areas. Selecting -mno-eabi means that the stack is
aligned to a 16-byte boundary, no EABI initialization function is called from
"main", and the -msdata option only uses "r13" to point to a single small data
area. The -meabi option is on by default if you configured GCC using one of
the powerpc*-*-eabi* options.
-msdata=eabi
On System V.4 and embedded PowerPC systems, put small initialized "const"
global and static data in the ".sdata2" section, which is pointed to by
register "r2". Put small initialized non-"const" global and static data in the
".sdata" section, which is pointed to by register "r13". Put small
uninitialized global and static data in the ".sbss" section, which is adjacent
to the ".sdata" section. The -msdata=eabi option is incompatible with the
-mrelocatable option. The -msdata=eabi option also sets the -memb option.
-msdata=sysv
On System V.4 and embedded PowerPC systems, put small global and static data in
the ".sdata" section, which is pointed to by register "r13". Put small
uninitialized global and static data in the ".sbss" section, which is adjacent
to the ".sdata" section. The -msdata=sysv option is incompatible with the
-mrelocatable option.
-msdata=default
-msdata
On System V.4 and embedded PowerPC systems, if -meabi is used, compile code the
same as -msdata=eabi, otherwise compile code the same as -msdata=sysv.
-msdata=data
On System V.4 and embedded PowerPC systems, put small global data in the
".sdata" section. Put small uninitialized global data in the ".sbss" section.
Do not use register "r13" to address small data however. This is the default
behavior unless other -msdata options are used.
-msdata=none
-mno-sdata
On embedded PowerPC systems, put all initialized global and static data in the
".data" section, and all uninitialized data in the ".bss" section.
-mreadonly-in-sdata
-mreadonly-in-sdata
Put read-only objects in the ".sdata" section as well. This is the default.
-mblock-move-inline-limit=num
Inline all block moves (such as calls to "memcpy" or structure copies) less
than or equal to num bytes. The minimum value for num is 32 bytes on 32-bit
targets and 64 bytes on 64-bit targets. The default value is target-specific.
-mblock-compare-inline-limit=num
Generate non-looping inline code for all block compares (such as calls to
"memcmp" or structure compares) less than or equal to num bytes. If num is 0,
all inline expansion (non-loop and loop) of block compare is disabled. The
default value is target-specific.
-mblock-compare-inline-loop-limit=num
Generate an inline expansion using loop code for all block compares that are
less than or equal to num bytes, but greater than the limit for non-loop inline
block compare expansion. If the block length is not constant, at most num bytes
will be compared before "memcmp" is called to compare the remainder of the
block. The default value is target-specific.
-mstring-compare-inline-limit=num
Generate at most num pairs of load instructions to compare the string inline.
If the difference or end of string is not found at the end of the inline
compare a call to "strcmp" or "strncmp" will take care of the rest of the
comparison. The default is 8 pairs of loads, which will compare 64 bytes on a
64-bit target and 32 bytes on a 32-bit target.
-G num
On embedded PowerPC systems, put global and static items less than or equal to
num bytes into the small data or BSS sections instead of the normal data or BSS
section. By default, num is 8. The -G num switch is also passed to the
linker. All modules should be compiled with the same -G num value.
-mregnames
-mno-regnames
On System V.4 and embedded PowerPC systems do (do not) emit register names in
the assembly language output using symbolic forms.
-mlongcall
-mno-longcall
By default assume that all calls are far away so that a longer and more
expensive calling sequence is required. This is required for calls farther
than 32 megabytes (33,554,432 bytes) from the current location. A short call
is generated if the compiler knows the call cannot be that far away. This
setting can be overridden by the "shortcall" function attribute, or by "#pragma
longcall(0)".
Some linkers are capable of detecting out-of-range calls and generating glue
code on the fly. On these systems, long calls are unnecessary and generate
slower code. As of this writing, the AIX linker can do this, as can the GNU
linker for PowerPC/64. It is planned to add this feature to the GNU linker for
32-bit PowerPC systems as well.
On Darwin/PPC systems, "#pragma longcall" generates "jbsr callee, L42", plus a
branch island (glue code). The two target addresses represent the callee and
the branch island. The Darwin/PPC linker prefers the first address and
generates a "bl callee" if the PPC "bl" instruction reaches the callee
directly; otherwise, the linker generates "bl L42" to call the branch island.
The branch island is appended to the body of the calling function; it computes
the full 32-bit address of the callee and jumps to it.
On Mach-O (Darwin) systems, this option directs the compiler emit to the glue
for every direct call, and the Darwin linker decides whether to use or discard
it.
In the future, GCC may ignore all longcall specifications when the linker is
known to generate glue.
-mtls-markers
-mno-tls-markers
Mark (do not mark) calls to "__tls_get_addr" with a relocation specifying the
function argument. The relocation allows the linker to reliably associate
function call with argument setup instructions for TLS optimization, which in
turn allows GCC to better schedule the sequence.
-mrecip
-mno-recip
This option enables use of the reciprocal estimate and reciprocal square root
estimate instructions with additional Newton-Raphson steps to increase
precision instead of doing a divide or square root and divide for floating-
point arguments. You should use the -ffast-math option when using -mrecip (or
at least -funsafe-math-optimizations, -ffinite-math-only, -freciprocal-math and
-fno-trapping-math). Note that while the throughput of the sequence is
generally higher than the throughput of the non-reciprocal instruction, the
precision of the sequence can be decreased by up to 2 ulp (i.e. the inverse of
1.0 equals 0.99999994) for reciprocal square roots.
-mrecip=opt
This option controls which reciprocal estimate instructions may be used. opt
is a comma-separated list of options, which may be preceded by a "!" to invert
the option:
all Enable all estimate instructions.
default
Enable the default instructions, equivalent to -mrecip.
none
Disable all estimate instructions, equivalent to -mno-recip.
div Enable the reciprocal approximation instructions for both single and double
precision.
divf
Enable the single-precision reciprocal approximation instructions.
divd
Enable the double-precision reciprocal approximation instructions.
rsqrt
Enable the reciprocal square root approximation instructions for both
single and double precision.
rsqrtf
Enable the single-precision reciprocal square root approximation
instructions.
rsqrtd
Enable the double-precision reciprocal square root approximation
instructions.
So, for example, -mrecip=all,!rsqrtd enables all of the reciprocal estimate
instructions, except for the "FRSQRTE", "XSRSQRTEDP", and "XVRSQRTEDP"
instructions which handle the double-precision reciprocal square root
calculations.
-mrecip-precision
-mno-recip-precision
Assume (do not assume) that the reciprocal estimate instructions provide
higher-precision estimates than is mandated by the PowerPC ABI. Selecting
-mcpu=power6, -mcpu=power7 or -mcpu=power8 automatically selects
-mrecip-precision. The double-precision square root estimate instructions are
not generated by default on low-precision machines, since they do not provide
an estimate that converges after three steps.
-mveclibabi=type
Specifies the ABI type to use for vectorizing intrinsics using an external
library. The only type supported at present is mass, which specifies to use
IBM's Mathematical Acceleration Subsystem (MASS) libraries for vectorizing
intrinsics using external libraries. GCC currently emits calls to "acosd2",
"acosf4", "acoshd2", "acoshf4", "asind2", "asinf4", "asinhd2", "asinhf4",
"atan2d2", "atan2f4", "atand2", "atanf4", "atanhd2", "atanhf4", "cbrtd2",
"cbrtf4", "cosd2", "cosf4", "coshd2", "coshf4", "erfcd2", "erfcf4", "erfd2",
"erff4", "exp2d2", "exp2f4", "expd2", "expf4", "expm1d2", "expm1f4", "hypotd2",
"hypotf4", "lgammad2", "lgammaf4", "log10d2", "log10f4", "log1pd2", "log1pf4",
"log2d2", "log2f4", "logd2", "logf4", "powd2", "powf4", "sind2", "sinf4",
"sinhd2", "sinhf4", "sqrtd2", "sqrtf4", "tand2", "tanf4", "tanhd2", and
"tanhf4" when generating code for power7. Both -ftree-vectorize and
-funsafe-math-optimizations must also be enabled. The MASS libraries must be
specified at link time.
-mfriz
-mno-friz
Generate (do not generate) the "friz" instruction when the
-funsafe-math-optimizations option is used to optimize rounding of floating-
point values to 64-bit integer and back to floating point. The "friz"
instruction does not return the same value if the floating-point number is too
large to fit in an integer.
-mpointers-to-nested-functions
-mno-pointers-to-nested-functions
Generate (do not generate) code to load up the static chain register ("r11")
when calling through a pointer on AIX and 64-bit Linux systems where a function
pointer points to a 3-word descriptor giving the function address, TOC value to
be loaded in register "r2", and static chain value to be loaded in register
"r11". The -mpointers-to-nested-functions is on by default. You cannot call
through pointers to nested functions or pointers to functions compiled in other
languages that use the static chain if you use
-mno-pointers-to-nested-functions.
-msave-toc-indirect
-mno-save-toc-indirect
Generate (do not generate) code to save the TOC value in the reserved stack
location in the function prologue if the function calls through a pointer on
AIX and 64-bit Linux systems. If the TOC value is not saved in the prologue,
it is saved just before the call through the pointer. The
-mno-save-toc-indirect option is the default.
-mcompat-align-parm
-mno-compat-align-parm
Generate (do not generate) code to pass structure parameters with a maximum
alignment of 64 bits, for compatibility with older versions of GCC.
Older versions of GCC (prior to 4.9.0) incorrectly did not align a structure
parameter on a 128-bit boundary when that structure contained a member
requiring 128-bit alignment. This is corrected in more recent versions of GCC.
This option may be used to generate code that is compatible with functions
compiled with older versions of GCC.
The -mno-compat-align-parm option is the default.
-mstack-protector-guard=guard
-mstack-protector-guard-reg=reg
-mstack-protector-guard-offset=offset
-mstack-protector-guard-symbol=symbol
Generate stack protection code using canary at guard. Supported locations are
global for global canary or tls for per-thread canary in the TLS block (the
default with GNU libc version 2.4 or later).
With the latter choice the options -mstack-protector-guard-reg=reg and
-mstack-protector-guard-offset=offset furthermore specify which register to use
as base register for reading the canary, and from what offset from that base
register. The default for those is as specified in the relevant ABI.
-mstack-protector-guard-symbol=symbol overrides the offset with a symbol
reference to a canary in the TLS block.
RX Options
These command-line options are defined for RX targets:
-m64bit-doubles
-m32bit-doubles
Make the "double" data type be 64 bits (-m64bit-doubles) or 32 bits
(-m32bit-doubles) in size. The default is -m32bit-doubles. Note RX floating-
point hardware only works on 32-bit values, which is why the default is
-m32bit-doubles.
-fpu
-nofpu
Enables (-fpu) or disables (-nofpu) the use of RX floating-point hardware. The
default is enabled for the RX600 series and disabled for the RX200 series.
Floating-point instructions are only generated for 32-bit floating-point
values, however, so the FPU hardware is not used for doubles if the
-m64bit-doubles option is used.
Note If the -fpu option is enabled then -funsafe-math-optimizations is also
enabled automatically. This is because the RX FPU instructions are themselves
unsafe.
-mcpu=name
Selects the type of RX CPU to be targeted. Currently three types are
supported, the generic RX600 and RX200 series hardware and the specific RX610
CPU. The default is RX600.
The only difference between RX600 and RX610 is that the RX610 does not support
the "MVTIPL" instruction.
The RX200 series does not have a hardware floating-point unit and so -nofpu is
enabled by default when this type is selected.
-mbig-endian-data
-mlittle-endian-data
Store data (but not code) in the big-endian format. The default is
-mlittle-endian-data, i.e. to store data in the little-endian format.
-msmall-data-limit=N
Specifies the maximum size in bytes of global and static variables which can be
placed into the small data area. Using the small data area can lead to smaller
and faster code, but the size of area is limited and it is up to the programmer
to ensure that the area does not overflow. Also when the small data area is
used one of the RX's registers (usually "r13") is reserved for use pointing to
this area, so it is no longer available for use by the compiler. This could
result in slower and/or larger code if variables are pushed onto the stack
instead of being held in this register.
Note, common variables (variables that have not been initialized) and constants
are not placed into the small data area as they are assigned to other sections
in the output executable.
The default value is zero, which disables this feature. Note, this feature is
not enabled by default with higher optimization levels (-O2 etc) because of the
potentially detrimental effects of reserving a register. It is up to the
programmer to experiment and discover whether this feature is of benefit to
their program. See the description of the -mpid option for a description of
how the actual register to hold the small data area pointer is chosen.
-msim
-mno-sim
Use the simulator runtime. The default is to use the libgloss board-specific
runtime.
-mas100-syntax
-mno-as100-syntax
When generating assembler output use a syntax that is compatible with Renesas's
AS100 assembler. This syntax can also be handled by the GAS assembler, but it
has some restrictions so it is not generated by default.
-mmax-constant-size=N
Specifies the maximum size, in bytes, of a constant that can be used as an
operand in a RX instruction. Although the RX instruction set does allow
constants of up to 4 bytes in length to be used in instructions, a longer value
equates to a longer instruction. Thus in some circumstances it can be
beneficial to restrict the size of constants that are used in instructions.
Constants that are too big are instead placed into a constant pool and
referenced via register indirection.
The value N can be between 0 and 4. A value of 0 (the default) or 4 means that
constants of any size are allowed.
-mrelax
Enable linker relaxation. Linker relaxation is a process whereby the linker
attempts to reduce the size of a program by finding shorter versions of various
instructions. Disabled by default.
-mint-register=N
Specify the number of registers to reserve for fast interrupt handler
functions. The value N can be between 0 and 4. A value of 1 means that
register "r13" is reserved for the exclusive use of fast interrupt handlers. A
value of 2 reserves "r13" and "r12". A value of 3 reserves "r13", "r12" and
"r11", and a value of 4 reserves "r13" through "r10". A value of 0, the
default, does not reserve any registers.
-msave-acc-in-interrupts
Specifies that interrupt handler functions should preserve the accumulator
register. This is only necessary if normal code might use the accumulator
register, for example because it performs 64-bit multiplications. The default
is to ignore the accumulator as this makes the interrupt handlers faster.
-mpid
-mno-pid
Enables the generation of position independent data. When enabled any access
to constant data is done via an offset from a base address held in a register.
This allows the location of constant data to be determined at run time without
requiring the executable to be relocated, which is a benefit to embedded
applications with tight memory constraints. Data that can be modified is not
affected by this option.
Note, using this feature reserves a register, usually "r13", for the constant
data base address. This can result in slower and/or larger code, especially in
complicated functions.
The actual register chosen to hold the constant data base address depends upon
whether the -msmall-data-limit and/or the -mint-register command-line options
are enabled. Starting with register "r13" and proceeding downwards, registers
are allocated first to satisfy the requirements of -mint-register, then -mpid
and finally -msmall-data-limit. Thus it is possible for the small data area
register to be "r8" if both -mint-register=4 and -mpid are specified on the
command line.
By default this feature is not enabled. The default can be restored via the
-mno-pid command-line option.
-mno-warn-multiple-fast-interrupts
-mwarn-multiple-fast-interrupts
Prevents GCC from issuing a warning message if it finds more than one fast
interrupt handler when it is compiling a file. The default is to issue a
warning for each extra fast interrupt handler found, as the RX only supports
one such interrupt.
-mallow-string-insns
-mno-allow-string-insns
Enables or disables the use of the string manipulation instructions "SMOVF",
"SCMPU", "SMOVB", "SMOVU", "SUNTIL" "SWHILE" and also the "RMPA" instruction.
These instructions may prefetch data, which is not safe to do if accessing an
I/O register. (See section 12.2.7 of the RX62N Group User's Manual for more
information).
The default is to allow these instructions, but it is not possible for GCC to
reliably detect all circumstances where a string instruction might be used to
access an I/O register, so their use cannot be disabled automatically. Instead
it is reliant upon the programmer to use the -mno-allow-string-insns option if
their program accesses I/O space.
When the instructions are enabled GCC defines the C preprocessor symbol
"__RX_ALLOW_STRING_INSNS__", otherwise it defines the symbol
"__RX_DISALLOW_STRING_INSNS__".
-mjsr
-mno-jsr
Use only (or not only) "JSR" instructions to access functions. This option can
be used when code size exceeds the range of "BSR" instructions. Note that
-mno-jsr does not mean to not use "JSR" but instead means that any type of
branch may be used.
Note: The generic GCC command-line option -ffixed-reg has special significance to
the RX port when used with the "interrupt" function attribute. This attribute
indicates a function intended to process fast interrupts. GCC ensures that it only
uses the registers "r10", "r11", "r12" and/or "r13" and only provided that the
normal use of the corresponding registers have been restricted via the -ffixed-reg
or -mint-register command-line options.
S/390 and zSeries Options
These are the -m options defined for the S/390 and zSeries architecture.
-mhard-float
-msoft-float
Use (do not use) the hardware floating-point instructions and registers for
floating-point operations. When -msoft-float is specified, functions in
libgcc.a are used to perform floating-point operations. When -mhard-float is
specified, the compiler generates IEEE floating-point instructions. This is
the default.
-mhard-dfp
-mno-hard-dfp
Use (do not use) the hardware decimal-floating-point instructions for decimal-
floating-point operations. When -mno-hard-dfp is specified, functions in
libgcc.a are used to perform decimal-floating-point operations. When
-mhard-dfp is specified, the compiler generates decimal-floating-point hardware
instructions. This is the default for -march=z9-ec or higher.
-mlong-double-64
-mlong-double-128
These switches control the size of "long double" type. A size of 64 bits makes
the "long double" type equivalent to the "double" type. This is the default.
-mbackchain
-mno-backchain
Store (do not store) the address of the caller's frame as backchain pointer
into the callee's stack frame. A backchain may be needed to allow debugging
using tools that do not understand DWARF call frame information. When
-mno-packed-stack is in effect, the backchain pointer is stored at the bottom
of the stack frame; when -mpacked-stack is in effect, the backchain is placed
into the topmost word of the 96/160 byte register save area.
In general, code compiled with -mbackchain is call-compatible with code
compiled with -mmo-backchain; however, use of the backchain for debugging
purposes usually requires that the whole binary is built with -mbackchain.
Note that the combination of -mbackchain, -mpacked-stack and -mhard-float is
not supported. In order to build a linux kernel use -msoft-float.
The default is to not maintain the backchain.
-mpacked-stack
-mno-packed-stack
Use (do not use) the packed stack layout. When -mno-packed-stack is specified,
the compiler uses the all fields of the 96/160 byte register save area only for
their default purpose; unused fields still take up stack space. When
-mpacked-stack is specified, register save slots are densely packed at the top
of the register save area; unused space is reused for other purposes, allowing
for more efficient use of the available stack space. However, when -mbackchain
is also in effect, the topmost word of the save area is always used to store
the backchain, and the return address register is always saved two words below
the backchain.
As long as the stack frame backchain is not used, code generated with
-mpacked-stack is call-compatible with code generated with -mno-packed-stack.
Note that some non-FSF releases of GCC 2.95 for S/390 or zSeries generated code
that uses the stack frame backchain at run time, not just for debugging
purposes. Such code is not call-compatible with code compiled with
-mpacked-stack. Also, note that the combination of -mbackchain, -mpacked-stack
and -mhard-float is not supported. In order to build a linux kernel use
-msoft-float.
The default is to not use the packed stack layout.
-msmall-exec
-mno-small-exec
Generate (or do not generate) code using the "bras" instruction to do
subroutine calls. This only works reliably if the total executable size does
not exceed 64k. The default is to use the "basr" instruction instead, which
does not have this limitation.
-m64
-m31
When -m31 is specified, generate code compliant to the GNU/Linux for S/390 ABI.
When -m64 is specified, generate code compliant to the GNU/Linux for zSeries
ABI. This allows GCC in particular to generate 64-bit instructions. For the
s390 targets, the default is -m31, while the s390x targets default to -m64.
-mzarch
-mesa
When -mzarch is specified, generate code using the instructions available on
z/Architecture. When -mesa is specified, generate code using the instructions
available on ESA/390. Note that -mesa is not possible with -m64. When
generating code compliant to the GNU/Linux for S/390 ABI, the default is -mesa.
When generating code compliant to the GNU/Linux for zSeries ABI, the default is
-mzarch.
-mhtm
-mno-htm
The -mhtm option enables a set of builtins making use of instructions available
with the transactional execution facility introduced with the IBM zEnterprise
EC12 machine generation S/390 System z Built-in Functions. -mhtm is enabled by
default when using -march=zEC12.
-mvx
-mno-vx
When -mvx is specified, generate code using the instructions available with the
vector extension facility introduced with the IBM z13 machine generation. This
option changes the ABI for some vector type values with regard to alignment and
calling conventions. In case vector type values are being used in an ABI-
relevant context a GAS .gnu_attribute command will be added to mark the
resulting binary with the ABI used. -mvx is enabled by default when using
-march=z13.
-mzvector
-mno-zvector
The -mzvector option enables vector language extensions and builtins using
instructions available with the vector extension facility introduced with the
IBM z13 machine generation. This option adds support for vector to be used as
a keyword to define vector type variables and arguments. vector is only
available when GNU extensions are enabled. It will not be expanded when
requesting strict standard compliance e.g. with -std=c99. In addition to the
GCC low-level builtins -mzvector enables a set of builtins added for
compatibility with AltiVec-style implementations like Power and Cell. In order
to make use of these builtins the header file vecintrin.h needs to be included.
-mzvector is disabled by default.
-mmvcle
-mno-mvcle
Generate (or do not generate) code using the "mvcle" instruction to perform
block moves. When -mno-mvcle is specified, use a "mvc" loop instead. This is
the default unless optimizing for size.
-mdebug
-mno-debug
Print (or do not print) additional debug information when compiling. The
default is to not print debug information.
-march=cpu-type
Generate code that runs on cpu-type, which is the name of a system representing
a certain processor type. Possible values for cpu-type are z900/arch5,
z990/arch6, z9-109, z9-ec/arch7, z10/arch8, z196/arch9, zEC12, z13/arch11,
z14/arch12, and native.
The default is -march=z900. g5/arch3 and g6 are deprecated and will be removed
with future releases.
Specifying native as cpu type can be used to select the best architecture
option for the host processor. -march=native has no effect if GCC does not
recognize the processor.
-mtune=cpu-type
Tune to cpu-type everything applicable about the generated code, except for the
ABI and the set of available instructions. The list of cpu-type values is the
same as for -march. The default is the value used for -march.
-mtpf-trace
-mno-tpf-trace
Generate code that adds (does not add) in TPF OS specific branches to trace
routines in the operating system. This option is off by default, even when
compiling for the TPF OS.
-mfused-madd
-mno-fused-madd
Generate code that uses (does not use) the floating-point multiply and
accumulate instructions. These instructions are generated by default if
hardware floating point is used.
-mwarn-framesize=framesize
Emit a warning if the current function exceeds the given frame size. Because
this is a compile-time check it doesn't need to be a real problem when the
program runs. It is intended to identify functions that most probably cause a
stack overflow. It is useful to be used in an environment with limited stack
size e.g. the linux kernel.
-mwarn-dynamicstack
Emit a warning if the function calls "alloca" or uses dynamically-sized arrays.
This is generally a bad idea with a limited stack size.
-mstack-guard=stack-guard
-mstack-size=stack-size
If these options are provided the S/390 back end emits additional instructions
in the function prologue that trigger a trap if the stack size is stack-guard
bytes above the stack-size (remember that the stack on S/390 grows downward).
If the stack-guard option is omitted the smallest power of 2 larger than the
frame size of the compiled function is chosen. These options are intended to
be used to help debugging stack overflow problems. The additionally emitted
code causes only little overhead and hence can also be used in production-like
systems without greater performance degradation. The given values have to be
exact powers of 2 and stack-size has to be greater than stack-guard without
exceeding 64k. In order to be efficient the extra code makes the assumption
that the stack starts at an address aligned to the value given by stack-size.
The stack-guard option can only be used in conjunction with stack-size.
-mhotpatch=pre-halfwords,post-halfwords
If the hotpatch option is enabled, a "hot-patching" function prologue is
generated for all functions in the compilation unit. The funtion label is
prepended with the given number of two-byte NOP instructions (pre-halfwords,
maximum 1000000). After the label, 2 * post-halfwords bytes are appended,
using the largest NOP like instructions the architecture allows (maximum
1000000).
If both arguments are zero, hotpatching is disabled.
This option can be overridden for individual functions with the "hotpatch"
attribute.
Score Options
These options are defined for Score implementations:
-meb
Compile code for big-endian mode. This is the default.
-mel
Compile code for little-endian mode.
-mnhwloop
Disable generation of "bcnz" instructions.
-muls
Enable generation of unaligned load and store instructions.
-mmac
Enable the use of multiply-accumulate instructions. Disabled by default.
-mscore5
Specify the SCORE5 as the target architecture.
-mscore5u
Specify the SCORE5U of the target architecture.
-mscore7
Specify the SCORE7 as the target architecture. This is the default.
-mscore7d
Specify the SCORE7D as the target architecture.
SH Options
These -m options are defined for the SH implementations:
-m1 Generate code for the SH1.
-m2 Generate code for the SH2.
-m2e
Generate code for the SH2e.
-m2a-nofpu
Generate code for the SH2a without FPU, or for a SH2a-FPU in such a way that
the floating-point unit is not used.
-m2a-single-only
Generate code for the SH2a-FPU, in such a way that no double-precision
floating-point operations are used.
-m2a-single
Generate code for the SH2a-FPU assuming the floating-point unit is in single-
precision mode by default.
-m2a
Generate code for the SH2a-FPU assuming the floating-point unit is in double-
precision mode by default.
-m3 Generate code for the SH3.
-m3e
Generate code for the SH3e.
-m4-nofpu
Generate code for the SH4 without a floating-point unit.
-m4-single-only
Generate code for the SH4 with a floating-point unit that only supports single-
precision arithmetic.
-m4-single
Generate code for the SH4 assuming the floating-point unit is in single-
precision mode by default.
-m4 Generate code for the SH4.
-m4-100
Generate code for SH4-100.
-m4-100-nofpu
Generate code for SH4-100 in such a way that the floating-point unit is not
used.
-m4-100-single
Generate code for SH4-100 assuming the floating-point unit is in single-
precision mode by default.
-m4-100-single-only
Generate code for SH4-100 in such a way that no double-precision floating-point
operations are used.
-m4-200
Generate code for SH4-200.
-m4-200-nofpu
Generate code for SH4-200 without in such a way that the floating-point unit is
not used.
-m4-200-single
Generate code for SH4-200 assuming the floating-point unit is in single-
precision mode by default.
-m4-200-single-only
Generate code for SH4-200 in such a way that no double-precision floating-point
operations are used.
-m4-300
Generate code for SH4-300.
-m4-300-nofpu
Generate code for SH4-300 without in such a way that the floating-point unit is
not used.
-m4-300-single
Generate code for SH4-300 in such a way that no double-precision floating-point
operations are used.
-m4-300-single-only
Generate code for SH4-300 in such a way that no double-precision floating-point
operations are used.
-m4-340
Generate code for SH4-340 (no MMU, no FPU).
-m4-500
Generate code for SH4-500 (no FPU). Passes -isa=sh4-nofpu to the assembler.
-m4a-nofpu
Generate code for the SH4al-dsp, or for a SH4a in such a way that the floating-
point unit is not used.
-m4a-single-only
Generate code for the SH4a, in such a way that no double-precision floating-
point operations are used.
-m4a-single
Generate code for the SH4a assuming the floating-point unit is in single-
precision mode by default.
-m4a
Generate code for the SH4a.
-m4al
Same as -m4a-nofpu, except that it implicitly passes -dsp to the assembler.
GCC doesn't generate any DSP instructions at the moment.
-mb Compile code for the processor in big-endian mode.
-ml Compile code for the processor in little-endian mode.
-mdalign
Align doubles at 64-bit boundaries. Note that this changes the calling
conventions, and thus some functions from the standard C library do not work
unless you recompile it first with -mdalign.
-mrelax
Shorten some address references at link time, when possible; uses the linker
option -relax.
-mbigtable
Use 32-bit offsets in "switch" tables. The default is to use 16-bit offsets.
-mbitops
Enable the use of bit manipulation instructions on SH2A.
-mfmovd
Enable the use of the instruction "fmovd". Check -mdalign for alignment
constraints.
-mrenesas
Comply with the calling conventions defined by Renesas.
-mno-renesas
Comply with the calling conventions defined for GCC before the Renesas
conventions were available. This option is the default for all targets of the
SH toolchain.
-mnomacsave
Mark the "MAC" register as call-clobbered, even if -mrenesas is given.
-mieee
-mno-ieee
Control the IEEE compliance of floating-point comparisons, which affects the
handling of cases where the result of a comparison is unordered. By default
-mieee is implicitly enabled. If -ffinite-math-only is enabled -mno-ieee is
implicitly set, which results in faster floating-point greater-equal and less-
equal comparisons. The implicit settings can be overridden by specifying
either -mieee or -mno-ieee.
-minline-ic_invalidate
Inline code to invalidate instruction cache entries after setting up nested
function trampolines. This option has no effect if -musermode is in effect and
the selected code generation option (e.g. -m4) does not allow the use of the
"icbi" instruction. If the selected code generation option does not allow the
use of the "icbi" instruction, and -musermode is not in effect, the inlined
code manipulates the instruction cache address array directly with an
associative write. This not only requires privileged mode at run time, but it
also fails if the cache line had been mapped via the TLB and has become
unmapped.
-misize
Dump instruction size and location in the assembly code.
-mpadstruct
This option is deprecated. It pads structures to multiple of 4 bytes, which is
incompatible with the SH ABI.
-matomic-model=model
Sets the model of atomic operations and additional parameters as a comma
separated list. For details on the atomic built-in functions see __atomic
Builtins. The following models and parameters are supported:
none
Disable compiler generated atomic sequences and emit library calls for
atomic operations. This is the default if the target is not
"sh*-*-linux*".
soft-gusa
Generate GNU/Linux compatible gUSA software atomic sequences for the atomic
built-in functions. The generated atomic sequences require additional
support from the interrupt/exception handling code of the system and are
only suitable for SH3* and SH4* single-core systems. This option is
enabled by default when the target is "sh*-*-linux*" and SH3* or SH4*.
When the target is SH4A, this option also partially utilizes the hardware
atomic instructions "movli.l" and "movco.l" to create more efficient code,
unless strict is specified.
soft-tcb
Generate software atomic sequences that use a variable in the thread
control block. This is a variation of the gUSA sequences which can also be
used on SH1* and SH2* targets. The generated atomic sequences require
additional support from the interrupt/exception handling code of the system
and are only suitable for single-core systems. When using this model, the
gbr-offset= parameter has to be specified as well.
soft-imask
Generate software atomic sequences that temporarily disable interrupts by
setting "SR.IMASK = 1111". This model works only when the program runs in
privileged mode and is only suitable for single-core systems. Additional
support from the interrupt/exception handling code of the system is not
required. This model is enabled by default when the target is
"sh*-*-linux*" and SH1* or SH2*.
hard-llcs
Generate hardware atomic sequences using the "movli.l" and "movco.l"
instructions only. This is only available on SH4A and is suitable for
multi-core systems. Since the hardware instructions support only 32 bit
atomic variables access to 8 or 16 bit variables is emulated with 32 bit
accesses. Code compiled with this option is also compatible with other
software atomic model interrupt/exception handling systems if executed on
an SH4A system. Additional support from the interrupt/exception handling
code of the system is not required for this model.
gbr-offset=
This parameter specifies the offset in bytes of the variable in the thread
control block structure that should be used by the generated atomic
sequences when the soft-tcb model has been selected. For other models this
parameter is ignored. The specified value must be an integer multiple of
four and in the range 0-1020.
strict
This parameter prevents mixed usage of multiple atomic models, even if they
are compatible, and makes the compiler generate atomic sequences of the
specified model only.
-mtas
Generate the "tas.b" opcode for "__atomic_test_and_set". Notice that depending
on the particular hardware and software configuration this can degrade overall
performance due to the operand cache line flushes that are implied by the
"tas.b" instruction. On multi-core SH4A processors the "tas.b" instruction
must be used with caution since it can result in data corruption for certain
cache configurations.
-mprefergot
When generating position-independent code, emit function calls using the Global
Offset Table instead of the Procedure Linkage Table.
-musermode
-mno-usermode
Don't allow (allow) the compiler generating privileged mode code. Specifying
-musermode also implies -mno-inline-ic_invalidate if the inlined code would not
work in user mode. -musermode is the default when the target is
"sh*-*-linux*". If the target is SH1* or SH2* -musermode has no effect, since
there is no user mode.
-multcost=number
Set the cost to assume for a multiply insn.
-mdiv=strategy
Set the division strategy to be used for integer division operations. strategy
can be one of:
call-div1
Calls a library function that uses the single-step division instruction
"div1" to perform the operation. Division by zero calculates an
unspecified result and does not trap. This is the default except for SH4,
SH2A and SHcompact.
call-fp
Calls a library function that performs the operation in double precision
floating point. Division by zero causes a floating-point exception. This
is the default for SHcompact with FPU. Specifying this for targets that do
not have a double precision FPU defaults to "call-div1".
call-table
Calls a library function that uses a lookup table for small divisors and
the "div1" instruction with case distinction for larger divisors. Division
by zero calculates an unspecified result and does not trap. This is the
default for SH4. Specifying this for targets that do not have dynamic
shift instructions defaults to "call-div1".
When a division strategy has not been specified the default strategy is
selected based on the current target. For SH2A the default strategy is to use
the "divs" and "divu" instructions instead of library function calls.
-maccumulate-outgoing-args
Reserve space once for outgoing arguments in the function prologue rather than
around each call. Generally beneficial for performance and size. Also needed
for unwinding to avoid changing the stack frame around conditional code.
-mdivsi3_libfunc=name
Set the name of the library function used for 32-bit signed division to name.
This only affects the name used in the call division strategies, and the
compiler still expects the same sets of input/output/clobbered registers as if
this option were not present.
-mfixed-range=register-range
Generate code treating the given register range as fixed registers. A fixed
register is one that the register allocator can not use. This is useful when
compiling kernel code. A register range is specified as two registers
separated by a dash. Multiple register ranges can be specified separated by a
comma.
-mbranch-cost=num
Assume num to be the cost for a branch instruction. Higher numbers make the
compiler try to generate more branch-free code if possible. If not specified
the value is selected depending on the processor type that is being compiled
for.
-mzdcbranch
-mno-zdcbranch
Assume (do not assume) that zero displacement conditional branch instructions
"bt" and "bf" are fast. If -mzdcbranch is specified, the compiler prefers zero
displacement branch code sequences. This is enabled by default when generating
code for SH4 and SH4A. It can be explicitly disabled by specifying
-mno-zdcbranch.
-mcbranch-force-delay-slot
Force the usage of delay slots for conditional branches, which stuffs the delay
slot with a "nop" if a suitable instruction cannot be found. By default this
option is disabled. It can be enabled to work around hardware bugs as found in
the original SH7055.
-mfused-madd
-mno-fused-madd
Generate code that uses (does not use) the floating-point multiply and
accumulate instructions. These instructions are generated by default if
hardware floating point is used. The machine-dependent -mfused-madd option is
now mapped to the machine-independent -ffp-contract=fast option, and
-mno-fused-madd is mapped to -ffp-contract=off.
-mfsca
-mno-fsca
Allow or disallow the compiler to emit the "fsca" instruction for sine and
cosine approximations. The option -mfsca must be used in combination with
-funsafe-math-optimizations. It is enabled by default when generating code for
SH4A. Using -mno-fsca disables sine and cosine approximations even if
-funsafe-math-optimizations is in effect.
-mfsrra
-mno-fsrra
Allow or disallow the compiler to emit the "fsrra" instruction for reciprocal
square root approximations. The option -mfsrra must be used in combination
with -funsafe-math-optimizations and -ffinite-math-only. It is enabled by
default when generating code for SH4A. Using -mno-fsrra disables reciprocal
square root approximations even if -funsafe-math-optimizations and
-ffinite-math-only are in effect.
-mpretend-cmove
Prefer zero-displacement conditional branches for conditional move instruction
patterns. This can result in faster code on the SH4 processor.
-mfdpic
Generate code using the FDPIC ABI.
Solaris 2 Options
These -m options are supported on Solaris 2:
-mclear-hwcap
-mclear-hwcap tells the compiler to remove the hardware capabilities generated
by the Solaris assembler. This is only necessary when object files use ISA
extensions not supported by the current machine, but check at runtime whether
or not to use them.
-mimpure-text
-mimpure-text, used in addition to -shared, tells the compiler to not pass -z
text to the linker when linking a shared object. Using this option, you can
link position-dependent code into a shared object.
-mimpure-text suppresses the "relocations remain against allocatable but non-
writable sections" linker error message. However, the necessary relocations
trigger copy-on-write, and the shared object is not actually shared across
processes. Instead of using -mimpure-text, you should compile all source code
with -fpic or -fPIC.
These switches are supported in addition to the above on Solaris 2:
-pthreads
This is a synonym for -pthread.
SPARC Options
These -m options are supported on the SPARC:
-mno-app-regs
-mapp-regs
Specify -mapp-regs to generate output using the global registers 2 through 4,
which the SPARC SVR4 ABI reserves for applications. Like the global register
1, each global register 2 through 4 is then treated as an allocable register
that is clobbered by function calls. This is the default.
To be fully SVR4 ABI-compliant at the cost of some performance loss, specify
-mno-app-regs. You should compile libraries and system software with this
option.
-mflat
-mno-flat
With -mflat, the compiler does not generate save/restore instructions and uses
a "flat" or single register window model. This model is compatible with the
regular register window model. The local registers and the input registers
(0--5) are still treated as "call-saved" registers and are saved on the stack
as needed.
With -mno-flat (the default), the compiler generates save/restore instructions
(except for leaf functions). This is the normal operating mode.
-mfpu
-mhard-float
Generate output containing floating-point instructions. This is the default.
-mno-fpu
-msoft-float
Generate output containing library calls for floating point. Warning: the
requisite libraries are not available for all SPARC targets. Normally the
facilities of the machine's usual C compiler are used, but this cannot be done
directly in cross-compilation. You must make your own arrangements to provide
suitable library functions for cross-compilation. The embedded targets
sparc-*-aout and sparclite-*-* do provide software floating-point support.
-msoft-float changes the calling convention in the output file; therefore, it
is only useful if you compile all of a program with this option. In
particular, you need to compile libgcc.a, the library that comes with GCC, with
-msoft-float in order for this to work.
-mhard-quad-float
Generate output containing quad-word (long double) floating-point instructions.
-msoft-quad-float
Generate output containing library calls for quad-word (long double) floating-
point instructions. The functions called are those specified in the SPARC ABI.
This is the default.
As of this writing, there are no SPARC implementations that have hardware
support for the quad-word floating-point instructions. They all invoke a trap
handler for one of these instructions, and then the trap handler emulates the
effect of the instruction. Because of the trap handler overhead, this is much
slower than calling the ABI library routines. Thus the -msoft-quad-float
option is the default.
-mno-unaligned-doubles
-munaligned-doubles
Assume that doubles have 8-byte alignment. This is the default.
With -munaligned-doubles, GCC assumes that doubles have 8-byte alignment only
if they are contained in another type, or if they have an absolute address.
Otherwise, it assumes they have 4-byte alignment. Specifying this option
avoids some rare compatibility problems with code generated by other compilers.
It is not the default because it results in a performance loss, especially for
floating-point code.
-muser-mode
-mno-user-mode
Do not generate code that can only run in supervisor mode. This is relevant
only for the "casa" instruction emitted for the LEON3 processor. This is the
default.
-mfaster-structs
-mno-faster-structs
With -mfaster-structs, the compiler assumes that structures should have 8-byte
alignment. This enables the use of pairs of "ldd" and "std" instructions for
copies in structure assignment, in place of twice as many "ld" and "st" pairs.
However, the use of this changed alignment directly violates the SPARC ABI.
Thus, it's intended only for use on targets where the developer acknowledges
that their resulting code is not directly in line with the rules of the ABI.
-mstd-struct-return
-mno-std-struct-return
With -mstd-struct-return, the compiler generates checking code in functions
returning structures or unions to detect size mismatches between the two sides
of function calls, as per the 32-bit ABI.
The default is -mno-std-struct-return. This option has no effect in 64-bit
mode.
-mlra
-mno-lra
Enable Local Register Allocation. This is the default for SPARC since GCC 7 so
-mno-lra needs to be passed to get old Reload.
-mcpu=cpu_type
Set the instruction set, register set, and instruction scheduling parameters
for machine type cpu_type. Supported values for cpu_type are v7, cypress, v8,
supersparc, hypersparc, leon, leon3, leon3v7, sparclite, f930, f934,
sparclite86x, sparclet, tsc701, v9, ultrasparc, ultrasparc3, niagara, niagara2,
niagara3, niagara4, niagara7 and m8.
Native Solaris and GNU/Linux toolchains also support the value native, which
selects the best architecture option for the host processor. -mcpu=native has
no effect if GCC does not recognize the processor.
Default instruction scheduling parameters are used for values that select an
architecture and not an implementation. These are v7, v8, sparclite, sparclet,
v9.
Here is a list of each supported architecture and their supported
implementations.
v7 cypress, leon3v7
v8 supersparc, hypersparc, leon, leon3
sparclite
f930, f934, sparclite86x
sparclet
tsc701
v9 ultrasparc, ultrasparc3, niagara, niagara2, niagara3, niagara4, niagara7,
m8
By default (unless configured otherwise), GCC generates code for the V7 variant
of the SPARC architecture. With -mcpu=cypress, the compiler additionally
optimizes it for the Cypress CY7C602 chip, as used in the
SPARCStation/SPARCServer 3xx series. This is also appropriate for the older
SPARCStation 1, 2, IPX etc.
With -mcpu=v8, GCC generates code for the V8 variant of the SPARC architecture.
The only difference from V7 code is that the compiler emits the integer
multiply and integer divide instructions which exist in SPARC-V8 but not in
SPARC-V7. With -mcpu=supersparc, the compiler additionally optimizes it for
the SuperSPARC chip, as used in the SPARCStation 10, 1000 and 2000 series.
With -mcpu=sparclite, GCC generates code for the SPARClite variant of the SPARC
architecture. This adds the integer multiply, integer divide step and scan
("ffs") instructions which exist in SPARClite but not in SPARC-V7. With
-mcpu=f930, the compiler additionally optimizes it for the Fujitsu MB86930
chip, which is the original SPARClite, with no FPU. With -mcpu=f934, the
compiler additionally optimizes it for the Fujitsu MB86934 chip, which is the
more recent SPARClite with FPU.
With -mcpu=sparclet, GCC generates code for the SPARClet variant of the SPARC
architecture. This adds the integer multiply, multiply/accumulate, integer
divide step and scan ("ffs") instructions which exist in SPARClet but not in
SPARC-V7. With -mcpu=tsc701, the compiler additionally optimizes it for the
TEMIC SPARClet chip.
With -mcpu=v9, GCC generates code for the V9 variant of the SPARC architecture.
This adds 64-bit integer and floating-point move instructions, 3 additional
floating-point condition code registers and conditional move instructions.
With -mcpu=ultrasparc, the compiler additionally optimizes it for the Sun
UltraSPARC I/II/IIi chips. With -mcpu=ultrasparc3, the compiler additionally
optimizes it for the Sun UltraSPARC III/III+/IIIi/IIIi+/IV/IV+ chips. With
-mcpu=niagara, the compiler additionally optimizes it for Sun UltraSPARC T1
chips. With -mcpu=niagara2, the compiler additionally optimizes it for Sun
UltraSPARC T2 chips. With -mcpu=niagara3, the compiler additionally optimizes
it for Sun UltraSPARC T3 chips. With -mcpu=niagara4, the compiler additionally
optimizes it for Sun UltraSPARC T4 chips. With -mcpu=niagara7, the compiler
additionally optimizes it for Oracle SPARC M7 chips. With -mcpu=m8, the
compiler additionally optimizes it for Oracle M8 chips.
-mtune=cpu_type
Set the instruction scheduling parameters for machine type cpu_type, but do not
set the instruction set or register set that the option -mcpu=cpu_type does.
The same values for -mcpu=cpu_type can be used for -mtune=cpu_type, but the
only useful values are those that select a particular CPU implementation.
Those are cypress, supersparc, hypersparc, leon, leon3, leon3v7, f930, f934,
sparclite86x, tsc701, ultrasparc, ultrasparc3, niagara, niagara2, niagara3,
niagara4, niagara7 and m8. With native Solaris and GNU/Linux toolchains,
native can also be used.
-mv8plus
-mno-v8plus
With -mv8plus, GCC generates code for the SPARC-V8+ ABI. The difference from
the V8 ABI is that the global and out registers are considered 64 bits wide.
This is enabled by default on Solaris in 32-bit mode for all SPARC-V9
processors.
-mvis
-mno-vis
With -mvis, GCC generates code that takes advantage of the UltraSPARC Visual
Instruction Set extensions. The default is -mno-vis.
-mvis2
-mno-vis2
With -mvis2, GCC generates code that takes advantage of version 2.0 of the
UltraSPARC Visual Instruction Set extensions. The default is -mvis2 when
targeting a cpu that supports such instructions, such as UltraSPARC-III and
later. Setting -mvis2 also sets -mvis.
-mvis3
-mno-vis3
With -mvis3, GCC generates code that takes advantage of version 3.0 of the
UltraSPARC Visual Instruction Set extensions. The default is -mvis3 when
targeting a cpu that supports such instructions, such as niagara-3 and later.
Setting -mvis3 also sets -mvis2 and -mvis.
-mvis4
-mno-vis4
With -mvis4, GCC generates code that takes advantage of version 4.0 of the
UltraSPARC Visual Instruction Set extensions. The default is -mvis4 when
targeting a cpu that supports such instructions, such as niagara-7 and later.
Setting -mvis4 also sets -mvis3, -mvis2 and -mvis.
-mvis4b
-mno-vis4b
With -mvis4b, GCC generates code that takes advantage of version 4.0 of the
UltraSPARC Visual Instruction Set extensions, plus the additional VIS
instructions introduced in the Oracle SPARC Architecture 2017. The default is
-mvis4b when targeting a cpu that supports such instructions, such as m8 and
later. Setting -mvis4b also sets -mvis4, -mvis3, -mvis2 and -mvis.
-mcbcond
-mno-cbcond
With -mcbcond, GCC generates code that takes advantage of the UltraSPARC
Compare-and-Branch-on-Condition instructions. The default is -mcbcond when
targeting a CPU that supports such instructions, such as Niagara-4 and later.
-mfmaf
-mno-fmaf
With -mfmaf, GCC generates code that takes advantage of the UltraSPARC Fused
Multiply-Add Floating-point instructions. The default is -mfmaf when targeting
a CPU that supports such instructions, such as Niagara-3 and later.
-mfsmuld
-mno-fsmuld
With -mfsmuld, GCC generates code that takes advantage of the Floating-point
Multiply Single to Double (FsMULd) instruction. The default is -mfsmuld when
targeting a CPU supporting the architecture versions V8 or V9 with FPU except
-mcpu=leon.
-mpopc
-mno-popc
With -mpopc, GCC generates code that takes advantage of the UltraSPARC
Population Count instruction. The default is -mpopc when targeting a CPU that
supports such an instruction, such as Niagara-2 and later.
-msubxc
-mno-subxc
With -msubxc, GCC generates code that takes advantage of the UltraSPARC
Subtract-Extended-with-Carry instruction. The default is -msubxc when
targeting a CPU that supports such an instruction, such as Niagara-7 and later.
-mfix-at697f
Enable the documented workaround for the single erratum of the Atmel AT697F
processor (which corresponds to erratum #13 of the AT697E processor).
-mfix-ut699
Enable the documented workarounds for the floating-point errata and the data
cache nullify errata of the UT699 processor.
-mfix-ut700
Enable the documented workaround for the back-to-back store errata of the
UT699E/UT700 processor.
-mfix-gr712rc
Enable the documented workaround for the back-to-back store errata of the
GR712RC processor.
These -m options are supported in addition to the above on SPARC-V9 processors in
64-bit environments:
-m32
-m64
Generate code for a 32-bit or 64-bit environment. The 32-bit environment sets
int, long and pointer to 32 bits. The 64-bit environment sets int to 32 bits
and long and pointer to 64 bits.
-mcmodel=which
Set the code model to one of
medlow
The Medium/Low code model: 64-bit addresses, programs must be linked in the
low 32 bits of memory. Programs can be statically or dynamically linked.
medmid
The Medium/Middle code model: 64-bit addresses, programs must be linked in
the low 44 bits of memory, the text and data segments must be less than 2GB
in size and the data segment must be located within 2GB of the text
segment.
medany
The Medium/Anywhere code model: 64-bit addresses, programs may be linked
anywhere in memory, the text and data segments must be less than 2GB in
size and the data segment must be located within 2GB of the text segment.
embmedany
The Medium/Anywhere code model for embedded systems: 64-bit addresses, the
text and data segments must be less than 2GB in size, both starting
anywhere in memory (determined at link time). The global register %g4
points to the base of the data segment. Programs are statically linked and
PIC is not supported.
-mmemory-model=mem-model
Set the memory model in force on the processor to one of
default
The default memory model for the processor and operating system.
rmo Relaxed Memory Order
pso Partial Store Order
tso Total Store Order
sc Sequential Consistency
These memory models are formally defined in Appendix D of the SPARC-V9
architecture manual, as set in the processor's "PSTATE.MM" field.
-mstack-bias
-mno-stack-bias
With -mstack-bias, GCC assumes that the stack pointer, and frame pointer if
present, are offset by -2047 which must be added back when making stack frame
references. This is the default in 64-bit mode. Otherwise, assume no such
offset is present.
SPU Options
These -m options are supported on the SPU:
-mwarn-reloc
-merror-reloc
The loader for SPU does not handle dynamic relocations. By default, GCC gives
an error when it generates code that requires a dynamic relocation.
-mno-error-reloc disables the error, -mwarn-reloc generates a warning instead.
-msafe-dma
-munsafe-dma
Instructions that initiate or test completion of DMA must not be reordered with
respect to loads and stores of the memory that is being accessed. With
-munsafe-dma you must use the "volatile" keyword to protect memory accesses,
but that can lead to inefficient code in places where the memory is known to
not change. Rather than mark the memory as volatile, you can use -msafe-dma to
tell the compiler to treat the DMA instructions as potentially affecting all
memory.
-mbranch-hints
By default, GCC generates a branch hint instruction to avoid pipeline stalls
for always-taken or probably-taken branches. A hint is not generated closer
than 8 instructions away from its branch. There is little reason to disable
them, except for debugging purposes, or to make an object a little bit smaller.
-msmall-mem
-mlarge-mem
By default, GCC generates code assuming that addresses are never larger than 18
bits. With -mlarge-mem code is generated that assumes a full 32-bit address.
-mstdmain
By default, GCC links against startup code that assumes the SPU-style main
function interface (which has an unconventional parameter list). With
-mstdmain, GCC links your program against startup code that assumes a C99-style
interface to "main", including a local copy of "argv" strings.
-mfixed-range=register-range
Generate code treating the given register range as fixed registers. A fixed
register is one that the register allocator cannot use. This is useful when
compiling kernel code. A register range is specified as two registers
separated by a dash. Multiple register ranges can be specified separated by a
comma.
-mea32
-mea64
Compile code assuming that pointers to the PPU address space accessed via the
"__ea" named address space qualifier are either 32 or 64 bits wide. The
default is 32 bits. As this is an ABI-changing option, all object code in an
executable must be compiled with the same setting.
-maddress-space-conversion
-mno-address-space-conversion
Allow/disallow treating the "__ea" address space as superset of the generic
address space. This enables explicit type casts between "__ea" and generic
pointer as well as implicit conversions of generic pointers to "__ea" pointers.
The default is to allow address space pointer conversions.
-mcache-size=cache-size
This option controls the version of libgcc that the compiler links to an
executable and selects a software-managed cache for accessing variables in the
"__ea" address space with a particular cache size. Possible options for cache-
size are 8, 16, 32, 64 and 128. The default cache size is 64KB.
-matomic-updates
-mno-atomic-updates
This option controls the version of libgcc that the compiler links to an
executable and selects whether atomic updates to the software-managed cache of
PPU-side variables are used. If you use atomic updates, changes to a PPU
variable from SPU code using the "__ea" named address space qualifier do not
interfere with changes to other PPU variables residing in the same cache line
from PPU code. If you do not use atomic updates, such interference may occur;
however, writing back cache lines is more efficient. The default behavior is
to use atomic updates.
-mdual-nops
-mdual-nops=n
By default, GCC inserts NOPs to increase dual issue when it expects it to
increase performance. n can be a value from 0 to 10. A smaller n inserts
fewer NOPs. 10 is the default, 0 is the same as -mno-dual-nops. Disabled with
-Os.
-mhint-max-nops=n
Maximum number of NOPs to insert for a branch hint. A branch hint must be at
least 8 instructions away from the branch it is affecting. GCC inserts up to n
NOPs to enforce this, otherwise it does not generate the branch hint.
-mhint-max-distance=n
The encoding of the branch hint instruction limits the hint to be within 256
instructions of the branch it is affecting. By default, GCC makes sure it is
within 125.
-msafe-hints
Work around a hardware bug that causes the SPU to stall indefinitely. By
default, GCC inserts the "hbrp" instruction to make sure this stall won't
happen.
Options for System V
These additional options are available on System V Release 4 for compatibility with
other compilers on those systems:
-G Create a shared object. It is recommended that -symbolic or -shared be used
instead.
-Qy Identify the versions of each tool used by the compiler, in a ".ident"
assembler directive in the output.
-Qn Refrain from adding ".ident" directives to the output file (this is the
default).
-YP,dirs
Search the directories dirs, and no others, for libraries specified with -l.
-Ym,dir
Look in the directory dir to find the M4 preprocessor. The assembler uses this
option.
TILE-Gx Options
These -m options are supported on the TILE-Gx:
-mcmodel=small
Generate code for the small model. The distance for direct calls is limited to
500M in either direction. PC-relative addresses are 32 bits. Absolute
addresses support the full address range.
-mcmodel=large
Generate code for the large model. There is no limitation on call distance,
pc-relative addresses, or absolute addresses.
-mcpu=name
Selects the type of CPU to be targeted. Currently the only supported type is
tilegx.
-m32
-m64
Generate code for a 32-bit or 64-bit environment. The 32-bit environment sets
int, long, and pointer to 32 bits. The 64-bit environment sets int to 32 bits
and long and pointer to 64 bits.
-mbig-endian
-mlittle-endian
Generate code in big/little endian mode, respectively.
TILEPro Options
These -m options are supported on the TILEPro:
-mcpu=name
Selects the type of CPU to be targeted. Currently the only supported type is
tilepro.
-m32
Generate code for a 32-bit environment, which sets int, long, and pointer to 32
bits. This is the only supported behavior so the flag is essentially ignored.
V850 Options
These -m options are defined for V850 implementations:
-mlong-calls
-mno-long-calls
Treat all calls as being far away (near). If calls are assumed to be far away,
the compiler always loads the function's address into a register, and calls
indirect through the pointer.
-mno-ep
-mep
Do not optimize (do optimize) basic blocks that use the same index pointer 4 or
more times to copy pointer into the "ep" register, and use the shorter "sld"
and "sst" instructions. The -mep option is on by default if you optimize.
-mno-prolog-function
-mprolog-function
Do not use (do use) external functions to save and restore registers at the
prologue and epilogue of a function. The external functions are slower, but
use less code space if more than one function saves the same number of
registers. The -mprolog-function option is on by default if you optimize.
-mspace
Try to make the code as small as possible. At present, this just turns on the
-mep and -mprolog-function options.
-mtda=n
Put static or global variables whose size is n bytes or less into the tiny data
area that register "ep" points to. The tiny data area can hold up to 256 bytes
in total (128 bytes for byte references).
-msda=n
Put static or global variables whose size is n bytes or less into the small
data area that register "gp" points to. The small data area can hold up to 64
kilobytes.
-mzda=n
Put static or global variables whose size is n bytes or less into the first 32
kilobytes of memory.
-mv850
Specify that the target processor is the V850.
-mv850e3v5
Specify that the target processor is the V850E3V5. The preprocessor constant
"__v850e3v5__" is defined if this option is used.
-mv850e2v4
Specify that the target processor is the V850E3V5. This is an alias for the
-mv850e3v5 option.
-mv850e2v3
Specify that the target processor is the V850E2V3. The preprocessor constant
"__v850e2v3__" is defined if this option is used.
-mv850e2
Specify that the target processor is the V850E2. The preprocessor constant
"__v850e2__" is defined if this option is used.
-mv850e1
Specify that the target processor is the V850E1. The preprocessor constants
"__v850e1__" and "__v850e__" are defined if this option is used.
-mv850es
Specify that the target processor is the V850ES. This is an alias for the
-mv850e1 option.
-mv850e
Specify that the target processor is the V850E. The preprocessor constant
"__v850e__" is defined if this option is used.
If neither -mv850 nor -mv850e nor -mv850e1 nor -mv850e2 nor -mv850e2v3 nor
-mv850e3v5 are defined then a default target processor is chosen and the
relevant __v850*__ preprocessor constant is defined.
The preprocessor constants "__v850" and "__v851__" are always defined,
regardless of which processor variant is the target.
-mdisable-callt
-mno-disable-callt
This option suppresses generation of the "CALLT" instruction for the v850e,
v850e1, v850e2, v850e2v3 and v850e3v5 flavors of the v850 architecture.
This option is enabled by default when the RH850 ABI is in use (see
-mrh850-abi), and disabled by default when the GCC ABI is in use. If "CALLT"
instructions are being generated then the C preprocessor symbol
"__V850_CALLT__" is defined.
-mrelax
-mno-relax
Pass on (or do not pass on) the -mrelax command-line option to the assembler.
-mlong-jumps
-mno-long-jumps
Disable (or re-enable) the generation of PC-relative jump instructions.
-msoft-float
-mhard-float
Disable (or re-enable) the generation of hardware floating point instructions.
This option is only significant when the target architecture is V850E2V3 or
higher. If hardware floating point instructions are being generated then the C
preprocessor symbol "__FPU_OK__" is defined, otherwise the symbol "__NO_FPU__"
is defined.
-mloop
Enables the use of the e3v5 LOOP instruction. The use of this instruction is
not enabled by default when the e3v5 architecture is selected because its use
is still experimental.
-mrh850-abi
-mghs
Enables support for the RH850 version of the V850 ABI. This is the default.
With this version of the ABI the following rules apply:
* Integer sized structures and unions are returned via a memory pointer
rather than a register.
* Large structures and unions (more than 8 bytes in size) are passed by
value.
* Functions are aligned to 16-bit boundaries.
* The -m8byte-align command-line option is supported.
* The -mdisable-callt command-line option is enabled by default. The
-mno-disable-callt command-line option is not supported.
When this version of the ABI is enabled the C preprocessor symbol
"__V850_RH850_ABI__" is defined.
-mgcc-abi
Enables support for the old GCC version of the V850 ABI. With this version of
the ABI the following rules apply:
* Integer sized structures and unions are returned in register "r10".
* Large structures and unions (more than 8 bytes in size) are passed by
reference.
* Functions are aligned to 32-bit boundaries, unless optimizing for size.
* The -m8byte-align command-line option is not supported.
* The -mdisable-callt command-line option is supported but not enabled by
default.
When this version of the ABI is enabled the C preprocessor symbol
"__V850_GCC_ABI__" is defined.
-m8byte-align
-mno-8byte-align
Enables support for "double" and "long long" types to be aligned on 8-byte
boundaries. The default is to restrict the alignment of all objects to at most
4-bytes. When -m8byte-align is in effect the C preprocessor symbol
"__V850_8BYTE_ALIGN__" is defined.
-mbig-switch
Generate code suitable for big switch tables. Use this option only if the
assembler/linker complain about out of range branches within a switch table.
-mapp-regs
This option causes r2 and r5 to be used in the code generated by the compiler.
This setting is the default.
-mno-app-regs
This option causes r2 and r5 to be treated as fixed registers.
VAX Options
These -m options are defined for the VAX:
-munix
Do not output certain jump instructions ("aobleq" and so on) that the Unix
assembler for the VAX cannot handle across long ranges.
-mgnu
Do output those jump instructions, on the assumption that the GNU assembler is
being used.
-mg Output code for G-format floating-point numbers instead of D-format.
Visium Options
-mdebug
A program which performs file I/O and is destined to run on an MCM target
should be linked with this option. It causes the libraries libc.a and
libdebug.a to be linked. The program should be run on the target under the
control of the GDB remote debugging stub.
-msim
A program which performs file I/O and is destined to run on the simulator
should be linked with option. This causes libraries libc.a and libsim.a to be
linked.
-mfpu
-mhard-float
Generate code containing floating-point instructions. This is the default.
-mno-fpu
-msoft-float
Generate code containing library calls for floating-point.
-msoft-float changes the calling convention in the output file; therefore, it
is only useful if you compile all of a program with this option. In
particular, you need to compile libgcc.a, the library that comes with GCC, with
-msoft-float in order for this to work.
-mcpu=cpu_type
Set the instruction set, register set, and instruction scheduling parameters
for machine type cpu_type. Supported values for cpu_type are mcm, gr5 and gr6.
mcm is a synonym of gr5 present for backward compatibility.
By default (unless configured otherwise), GCC generates code for the GR5
variant of the Visium architecture.
With -mcpu=gr6, GCC generates code for the GR6 variant of the Visium
architecture. The only difference from GR5 code is that the compiler will
generate block move instructions.
-mtune=cpu_type
Set the instruction scheduling parameters for machine type cpu_type, but do not
set the instruction set or register set that the option -mcpu=cpu_type would.
-msv-mode
Generate code for the supervisor mode, where there are no restrictions on the
access to general registers. This is the default.
-muser-mode
Generate code for the user mode, where the access to some general registers is
forbidden: on the GR5, registers r24 to r31 cannot be accessed in this mode; on
the GR6, only registers r29 to r31 are affected.
VMS Options
These -m options are defined for the VMS implementations:
-mvms-return-codes
Return VMS condition codes from "main". The default is to return POSIX-style
condition (e.g. error) codes.
-mdebug-main=prefix
Flag the first routine whose name starts with prefix as the main routine for
the debugger.
-mmalloc64
Default to 64-bit memory allocation routines.
-mpointer-size=size
Set the default size of pointers. Possible options for size are 32 or short for
32 bit pointers, 64 or long for 64 bit pointers, and no for supporting only 32
bit pointers. The later option disables "pragma pointer_size".
VxWorks Options
The options in this section are defined for all VxWorks targets. Options specific
to the target hardware are listed with the other options for that target.
-mrtp
GCC can generate code for both VxWorks kernels and real time processes (RTPs).
This option switches from the former to the latter. It also defines the
preprocessor macro "__RTP__".
-non-static
Link an RTP executable against shared libraries rather than static libraries.
The options -static and -shared can also be used for RTPs; -static is the
default.
-Bstatic
-Bdynamic
These options are passed down to the linker. They are defined for
compatibility with Diab.
-Xbind-lazy
Enable lazy binding of function calls. This option is equivalent to -Wl,-z,now
and is defined for compatibility with Diab.
-Xbind-now
Disable lazy binding of function calls. This option is the default and is
defined for compatibility with Diab.
x86 Options
These -m options are defined for the x86 family of computers.
-march=cpu-type
Generate instructions for the machine type cpu-type. In contrast to
-mtune=cpu-type, which merely tunes the generated code for the specified cpu-
type, -march=cpu-type allows GCC to generate code that may not run at all on
processors other than the one indicated. Specifying -march=cpu-type implies
-mtune=cpu-type.
The choices for cpu-type are:
native
This selects the CPU to generate code for at compilation time by
determining the processor type of the compiling machine. Using
-march=native enables all instruction subsets supported by the local
machine (hence the result might not run on different machines). Using
-mtune=native produces code optimized for the local machine under the
constraints of the selected instruction set.
x86-64
A generic CPU with 64-bit extensions.
i386
Original Intel i386 CPU.
i486
Intel i486 CPU. (No scheduling is implemented for this chip.)
i586
pentium
Intel Pentium CPU with no MMX support.
lakemont
Intel Lakemont MCU, based on Intel Pentium CPU.
pentium-mmx
Intel Pentium MMX CPU, based on Pentium core with MMX instruction set
support.
pentiumpro
Intel Pentium Pro CPU.
i686
When used with -march, the Pentium Pro instruction set is used, so the code
runs on all i686 family chips. When used with -mtune, it has the same
meaning as generic.
pentium2
Intel Pentium II CPU, based on Pentium Pro core with MMX instruction set
support.
pentium3
pentium3m
Intel Pentium III CPU, based on Pentium Pro core with MMX and SSE
instruction set support.
pentium-m
Intel Pentium M; low-power version of Intel Pentium III CPU with MMX, SSE
and SSE2 instruction set support. Used by Centrino notebooks.
pentium4
pentium4m
Intel Pentium 4 CPU with MMX, SSE and SSE2 instruction set support.
prescott
Improved version of Intel Pentium 4 CPU with MMX, SSE, SSE2 and SSE3
instruction set support.
nocona
Improved version of Intel Pentium 4 CPU with 64-bit extensions, MMX, SSE,
SSE2 and SSE3 instruction set support.
core2
Intel Core 2 CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3 and SSSE3
instruction set support.
nehalem
Intel Nehalem CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3,
SSE4.1, SSE4.2 and POPCNT instruction set support.
westmere
Intel Westmere CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3,
SSE4.1, SSE4.2, POPCNT, AES and PCLMUL instruction set support.
sandybridge
Intel Sandy Bridge CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3,
SSE4.1, SSE4.2, POPCNT, AVX, AES and PCLMUL instruction set support.
ivybridge
Intel Ivy Bridge CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3,
SSE4.1, SSE4.2, POPCNT, AVX, AES, PCLMUL, FSGSBASE, RDRND and F16C
instruction set support.
haswell
Intel Haswell CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3,
SSSE3, SSE4.1, SSE4.2, POPCNT, AVX, AVX2, AES, PCLMUL, FSGSBASE, RDRND,
FMA, BMI, BMI2 and F16C instruction set support.
broadwell
Intel Broadwell CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3,
SSSE3, SSE4.1, SSE4.2, POPCNT, AVX, AVX2, AES, PCLMUL, FSGSBASE, RDRND,
FMA, BMI, BMI2, F16C, RDSEED, ADCX and PREFETCHW instruction set support.
skylake
Intel Skylake CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3,
SSSE3, SSE4.1, SSE4.2, POPCNT, AVX, AVX2, AES, PCLMUL, FSGSBASE, RDRND,
FMA, BMI, BMI2, F16C, RDSEED, ADCX, PREFETCHW, CLFLUSHOPT, XSAVEC and
XSAVES instruction set support.
bonnell
Intel Bonnell CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3 and
SSSE3 instruction set support.
silvermont
Intel Silvermont CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3,
SSSE3, SSE4.1, SSE4.2, POPCNT, AES, PCLMUL and RDRND instruction set
support.
knl Intel Knight's Landing CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2,
SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AVX, AVX2, AES, PCLMUL, FSGSBASE,
RDRND, FMA, BMI, BMI2, F16C, RDSEED, ADCX, PREFETCHW, AVX512F, AVX512PF,
AVX512ER and AVX512CD instruction set support.
knm Intel Knights Mill CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3,
SSSE3, SSE4.1, SSE4.2, POPCNT, AVX, AVX2, AES, PCLMUL, FSGSBASE, RDRND,
FMA, BMI, BMI2, F16C, RDSEED, ADCX, PREFETCHW, AVX512F, AVX512PF, AVX512ER,
AVX512CD, AVX5124VNNIW, AVX5124FMAPS and AVX512VPOPCNTDQ instruction set
support.
skylake-avx512
Intel Skylake Server CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2,
SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, PKU, AVX, AVX2, AES, PCLMUL, FSGSBASE,
RDRND, FMA, BMI, BMI2, F16C, RDSEED, ADCX, PREFETCHW, CLFLUSHOPT, XSAVEC,
XSAVES, AVX512F, CLWB, AVX512VL, AVX512BW, AVX512DQ and AVX512CD
instruction set support.
cannonlake
Intel Cannonlake Server CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2,
SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, PKU, AVX, AVX2, AES, PCLMUL, FSGSBASE,
RDRND, FMA, BMI, BMI2, F16C, RDSEED, ADCX, PREFETCHW, CLFLUSHOPT, XSAVEC,
XSAVES, AVX512F, AVX512VL, AVX512BW, AVX512DQ, AVX512CD, AVX512VBMI,
AVX512IFMA, SHA and UMIP instruction set support.
icelake-client
Intel Icelake Client CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2,
SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, PKU, AVX, AVX2, AES, PCLMUL, FSGSBASE,
RDRND, FMA, BMI, BMI2, F16C, RDSEED, ADCX, PREFETCHW, CLFLUSHOPT, XSAVEC,
XSAVES, AVX512F, AVX512VL, AVX512BW, AVX512DQ, AVX512CD, AVX512VBMI,
AVX512IFMA, SHA, CLWB, UMIP, RDPID, GFNI, AVX512VBMI2, AVX512VPOPCNTDQ,
AVX512BITALG, AVX512VNNI, VPCLMULQDQ, VAES instruction set support.
icelake-server
Intel Icelake Server CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2,
SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, PKU, AVX, AVX2, AES, PCLMUL, FSGSBASE,
RDRND, FMA, BMI, BMI2, F16C, RDSEED, ADCX, PREFETCHW, CLFLUSHOPT, XSAVEC,
XSAVES, AVX512F, AVX512VL, AVX512BW, AVX512DQ, AVX512CD, AVX512VBMI,
AVX512IFMA, SHA, CLWB, UMIP, RDPID, GFNI, AVX512VBMI2, AVX512VPOPCNTDQ,
AVX512BITALG, AVX512VNNI, VPCLMULQDQ, VAES, PCONFIG and WBNOINVD
instruction set support.
k6 AMD K6 CPU with MMX instruction set support.
k6-2
k6-3
Improved versions of AMD K6 CPU with MMX and 3DNow! instruction set
support.
athlon
athlon-tbird
AMD Athlon CPU with MMX, 3dNOW!, enhanced 3DNow! and SSE prefetch
instructions support.
athlon-4
athlon-xp
athlon-mp
Improved AMD Athlon CPU with MMX, 3DNow!, enhanced 3DNow! and full SSE
instruction set support.
k8
opteron
athlon64
athlon-fx
Processors based on the AMD K8 core with x86-64 instruction set support,
including the AMD Opteron, Athlon 64, and Athlon 64 FX processors. (This
supersets MMX, SSE, SSE2, 3DNow!, enhanced 3DNow! and 64-bit instruction
set extensions.)
k8-sse3
opteron-sse3
athlon64-sse3
Improved versions of AMD K8 cores with SSE3 instruction set support.
amdfam10
barcelona
CPUs based on AMD Family 10h cores with x86-64 instruction set support.
(This supersets MMX, SSE, SSE2, SSE3, SSE4A, 3DNow!, enhanced 3DNow!, ABM
and 64-bit instruction set extensions.)
bdver1
CPUs based on AMD Family 15h cores with x86-64 instruction set support.
(This supersets FMA4, AVX, XOP, LWP, AES, PCL_MUL, CX16, MMX, SSE, SSE2,
SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM and 64-bit instruction set
extensions.)
bdver2
AMD Family 15h core based CPUs with x86-64 instruction set support. (This
supersets BMI, TBM, F16C, FMA, FMA4, AVX, XOP, LWP, AES, PCL_MUL, CX16,
MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM and 64-bit
instruction set extensions.)
bdver3
AMD Family 15h core based CPUs with x86-64 instruction set support. (This
supersets BMI, TBM, F16C, FMA, FMA4, FSGSBASE, AVX, XOP, LWP, AES, PCL_MUL,
CX16, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM and 64-bit
instruction set extensions.
bdver4
AMD Family 15h core based CPUs with x86-64 instruction set support. (This
supersets BMI, BMI2, TBM, F16C, FMA, FMA4, FSGSBASE, AVX, AVX2, XOP, LWP,
AES, PCL_MUL, CX16, MOVBE, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1,
SSE4.2, ABM and 64-bit instruction set extensions.
znver1
AMD Family 17h core based CPUs with x86-64 instruction set support. (This
supersets BMI, BMI2, F16C, FMA, FSGSBASE, AVX, AVX2, ADCX, RDSEED, MWAITX,
SHA, CLZERO, AES, PCL_MUL, CX16, MOVBE, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3,
SSE4.1, SSE4.2, ABM, XSAVEC, XSAVES, CLFLUSHOPT, POPCNT, and 64-bit
instruction set extensions.
btver1
CPUs based on AMD Family 14h cores with x86-64 instruction set support.
(This supersets MMX, SSE, SSE2, SSE3, SSSE3, SSE4A, CX16, ABM and 64-bit
instruction set extensions.)
btver2
CPUs based on AMD Family 16h cores with x86-64 instruction set support.
This includes MOVBE, F16C, BMI, AVX, PCL_MUL, AES, SSE4.2, SSE4.1, CX16,
ABM, SSE4A, SSSE3, SSE3, SSE2, SSE, MMX and 64-bit instruction set
extensions.
winchip-c6
IDT WinChip C6 CPU, dealt in same way as i486 with additional MMX
instruction set support.
winchip2
IDT WinChip 2 CPU, dealt in same way as i486 with additional MMX and 3DNow!
instruction set support.
c3 VIA C3 CPU with MMX and 3DNow! instruction set support. (No scheduling is
implemented for this chip.)
c3-2
VIA C3-2 (Nehemiah/C5XL) CPU with MMX and SSE instruction set support. (No
scheduling is implemented for this chip.)
c7 VIA C7 (Esther) CPU with MMX, SSE, SSE2 and SSE3 instruction set support.
(No scheduling is implemented for this chip.)
samuel-2
VIA Eden Samuel 2 CPU with MMX and 3DNow! instruction set support. (No
scheduling is implemented for this chip.)
nehemiah
VIA Eden Nehemiah CPU with MMX and SSE instruction set support. (No
scheduling is implemented for this chip.)
esther
VIA Eden Esther CPU with MMX, SSE, SSE2 and SSE3 instruction set support.
(No scheduling is implemented for this chip.)
eden-x2
VIA Eden X2 CPU with x86-64, MMX, SSE, SSE2 and SSE3 instruction set
support. (No scheduling is implemented for this chip.)
eden-x4
VIA Eden X4 CPU with x86-64, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2,
AVX and AVX2 instruction set support. (No scheduling is implemented for
this chip.)
nano
Generic VIA Nano CPU with x86-64, MMX, SSE, SSE2, SSE3 and SSSE3
instruction set support. (No scheduling is implemented for this chip.)
nano-1000
VIA Nano 1xxx CPU with x86-64, MMX, SSE, SSE2, SSE3 and SSSE3 instruction
set support. (No scheduling is implemented for this chip.)
nano-2000
VIA Nano 2xxx CPU with x86-64, MMX, SSE, SSE2, SSE3 and SSSE3 instruction
set support. (No scheduling is implemented for this chip.)
nano-3000
VIA Nano 3xxx CPU with x86-64, MMX, SSE, SSE2, SSE3, SSSE3 and SSE4.1
instruction set support. (No scheduling is implemented for this chip.)
nano-x2
VIA Nano Dual Core CPU with x86-64, MMX, SSE, SSE2, SSE3, SSSE3 and SSE4.1
instruction set support. (No scheduling is implemented for this chip.)
nano-x4
VIA Nano Quad Core CPU with x86-64, MMX, SSE, SSE2, SSE3, SSSE3 and SSE4.1
instruction set support. (No scheduling is implemented for this chip.)
geode
AMD Geode embedded processor with MMX and 3DNow! instruction set support.
-mtune=cpu-type
Tune to cpu-type everything applicable about the generated code, except for the
ABI and the set of available instructions. While picking a specific cpu-type
schedules things appropriately for that particular chip, the compiler does not
generate any code that cannot run on the default machine type unless you use a
-march=cpu-type option. For example, if GCC is configured for
i686-pc-linux-gnu then -mtune=pentium4 generates code that is tuned for Pentium
4 but still runs on i686 machines.
The choices for cpu-type are the same as for -march. In addition, -mtune
supports 2 extra choices for cpu-type:
generic
Produce code optimized for the most common IA32/AMD64/EM64T processors. If
you know the CPU on which your code will run, then you should use the
corresponding -mtune or -march option instead of -mtune=generic. But, if
you do not know exactly what CPU users of your application will have, then
you should use this option.
As new processors are deployed in the marketplace, the behavior of this
option will change. Therefore, if you upgrade to a newer version of GCC,
code generation controlled by this option will change to reflect the
processors that are most common at the time that version of GCC is
released.
There is no -march=generic option because -march indicates the instruction
set the compiler can use, and there is no generic instruction set
applicable to all processors. In contrast, -mtune indicates the processor
(or, in this case, collection of processors) for which the code is
optimized.
intel
Produce code optimized for the most current Intel processors, which are
Haswell and Silvermont for this version of GCC. If you know the CPU on
which your code will run, then you should use the corresponding -mtune or
-march option instead of -mtune=intel. But, if you want your application
performs better on both Haswell and Silvermont, then you should use this
option.
As new Intel processors are deployed in the marketplace, the behavior of
this option will change. Therefore, if you upgrade to a newer version of
GCC, code generation controlled by this option will change to reflect the
most current Intel processors at the time that version of GCC is released.
There is no -march=intel option because -march indicates the instruction
set the compiler can use, and there is no common instruction set applicable
to all processors. In contrast, -mtune indicates the processor (or, in
this case, collection of processors) for which the code is optimized.
-mcpu=cpu-type
A deprecated synonym for -mtune.
-mfpmath=unit
Generate floating-point arithmetic for selected unit unit. The choices for
unit are:
387 Use the standard 387 floating-point coprocessor present on the majority of
chips and emulated otherwise. Code compiled with this option runs almost
everywhere. The temporary results are computed in 80-bit precision instead
of the precision specified by the type, resulting in slightly different
results compared to most of other chips. See -ffloat-store for more
detailed description.
This is the default choice for non-Darwin x86-32 targets.
sse Use scalar floating-point instructions present in the SSE instruction set.
This instruction set is supported by Pentium III and newer chips, and in
the AMD line by Athlon-4, Athlon XP and Athlon MP chips. The earlier
version of the SSE instruction set supports only single-precision
arithmetic, thus the double and extended-precision arithmetic are still
done using 387. A later version, present only in Pentium 4 and AMD x86-64
chips, supports double-precision arithmetic too.
For the x86-32 compiler, you must use -march=cpu-type, -msse or -msse2
switches to enable SSE extensions and make this option effective. For the
x86-64 compiler, these extensions are enabled by default.
The resulting code should be considerably faster in the majority of cases
and avoid the numerical instability problems of 387 code, but may break
some existing code that expects temporaries to be 80 bits.
This is the default choice for the x86-64 compiler, Darwin x86-32 targets,
and the default choice for x86-32 targets with the SSE2 instruction set
when -ffast-math is enabled.
sse,387
sse+387
both
Attempt to utilize both instruction sets at once. This effectively doubles
the amount of available registers, and on chips with separate execution
units for 387 and SSE the execution resources too. Use this option with
care, as it is still experimental, because the GCC register allocator does
not model separate functional units well, resulting in unstable
performance.
-masm=dialect
Output assembly instructions using selected dialect. Also affects which
dialect is used for basic "asm" and extended "asm". Supported choices (in
dialect order) are att or intel. The default is att. Darwin does not support
intel.
-mieee-fp
-mno-ieee-fp
Control whether or not the compiler uses IEEE floating-point comparisons.
These correctly handle the case where the result of a comparison is unordered.
-m80387
-mhard-float
Generate output containing 80387 instructions for floating point.
-mno-80387
-msoft-float
Generate output containing library calls for floating point.
Warning: the requisite libraries are not part of GCC. Normally the facilities
of the machine's usual C compiler are used, but this cannot be done directly in
cross-compilation. You must make your own arrangements to provide suitable
library functions for cross-compilation.
On machines where a function returns floating-point results in the 80387
register stack, some floating-point opcodes may be emitted even if -msoft-float
is used.
-mno-fp-ret-in-387
Do not use the FPU registers for return values of functions.
The usual calling convention has functions return values of types "float" and
"double" in an FPU register, even if there is no FPU. The idea is that the
operating system should emulate an FPU.
The option -mno-fp-ret-in-387 causes such values to be returned in ordinary CPU
registers instead.
-mno-fancy-math-387
Some 387 emulators do not support the "sin", "cos" and "sqrt" instructions for
the 387. Specify this option to avoid generating those instructions. This
option is the default on OpenBSD and NetBSD. This option is overridden when
-march indicates that the target CPU always has an FPU and so the instruction
does not need emulation. These instructions are not generated unless you also
use the -funsafe-math-optimizations switch.
-malign-double
-mno-align-double
Control whether GCC aligns "double", "long double", and "long long" variables
on a two-word boundary or a one-word boundary. Aligning "double" variables on
a two-word boundary produces code that runs somewhat faster on a Pentium at the
expense of more memory.
On x86-64, -malign-double is enabled by default.
Warning: if you use the -malign-double switch, structures containing the above
types are aligned differently than the published application binary interface
specifications for the x86-32 and are not binary compatible with structures in
code compiled without that switch.
-m96bit-long-double
-m128bit-long-double
These switches control the size of "long double" type. The x86-32 application
binary interface specifies the size to be 96 bits, so -m96bit-long-double is
the default in 32-bit mode.
Modern architectures (Pentium and newer) prefer "long double" to be aligned to
an 8- or 16-byte boundary. In arrays or structures conforming to the ABI, this
is not possible. So specifying -m128bit-long-double aligns "long double" to a
16-byte boundary by padding the "long double" with an additional 32-bit zero.
In the x86-64 compiler, -m128bit-long-double is the default choice as its ABI
specifies that "long double" is aligned on 16-byte boundary.
Notice that neither of these options enable any extra precision over the x87
standard of 80 bits for a "long double".
Warning: if you override the default value for your target ABI, this changes
the size of structures and arrays containing "long double" variables, as well
as modifying the function calling convention for functions taking "long
double". Hence they are not binary-compatible with code compiled without that
switch.
-mlong-double-64
-mlong-double-80
-mlong-double-128
These switches control the size of "long double" type. A size of 64 bits makes
the "long double" type equivalent to the "double" type. This is the default for
32-bit Bionic C library. A size of 128 bits makes the "long double" type
equivalent to the "__float128" type. This is the default for 64-bit Bionic C
library.
Warning: if you override the default value for your target ABI, this changes
the size of structures and arrays containing "long double" variables, as well
as modifying the function calling convention for functions taking "long
double". Hence they are not binary-compatible with code compiled without that
switch.
-malign-data=type
Control how GCC aligns variables. Supported values for type are compat uses
increased alignment value compatible uses GCC 4.8 and earlier, abi uses
alignment value as specified by the psABI, and cacheline uses increased
alignment value to match the cache line size. compat is the default.
-mlarge-data-threshold=threshold
When -mcmodel=medium is specified, data objects larger than threshold are
placed in the large data section. This value must be the same across all
objects linked into the binary, and defaults to 65535.
-mrtd
Use a different function-calling convention, in which functions that take a
fixed number of arguments return with the "ret num" instruction, which pops
their arguments while returning. This saves one instruction in the caller
since there is no need to pop the arguments there.
You can specify that an individual function is called with this calling
sequence with the function attribute "stdcall". You can also override the
-mrtd option by using the function attribute "cdecl".
Warning: this calling convention is incompatible with the one normally used on
Unix, so you cannot use it if you need to call libraries compiled with the Unix
compiler.
Also, you must provide function prototypes for all functions that take variable
numbers of arguments (including "printf"); otherwise incorrect code is
generated for calls to those functions.
In addition, seriously incorrect code results if you call a function with too
many arguments. (Normally, extra arguments are harmlessly ignored.)
-mregparm=num
Control how many registers are used to pass integer arguments. By default, no
registers are used to pass arguments, and at most 3 registers can be used. You
can control this behavior for a specific function by using the function
attribute "regparm".
Warning: if you use this switch, and num is nonzero, then you must build all
modules with the same value, including any libraries. This includes the system
libraries and startup modules.
-msseregparm
Use SSE register passing conventions for float and double arguments and return
values. You can control this behavior for a specific function by using the
function attribute "sseregparm".
Warning: if you use this switch then you must build all modules with the same
value, including any libraries. This includes the system libraries and startup
modules.
-mvect8-ret-in-mem
Return 8-byte vectors in memory instead of MMX registers. This is the default
on Solaris 8 and 9 and VxWorks to match the ABI of the Sun Studio compilers
until version 12. Later compiler versions (starting with Studio 12 Update 1)
follow the ABI used by other x86 targets, which is the default on Solaris 10
and later. Only use this option if you need to remain compatible with existing
code produced by those previous compiler versions or older versions of GCC.
-mpc32
-mpc64
-mpc80
Set 80387 floating-point precision to 32, 64 or 80 bits. When -mpc32 is
specified, the significands of results of floating-point operations are rounded
to 24 bits (single precision); -mpc64 rounds the significands of results of
floating-point operations to 53 bits (double precision) and -mpc80 rounds the
significands of results of floating-point operations to 64 bits (extended
double precision), which is the default. When this option is used, floating-
point operations in higher precisions are not available to the programmer
without setting the FPU control word explicitly.
Setting the rounding of floating-point operations to less than the default 80
bits can speed some programs by 2% or more. Note that some mathematical
libraries assume that extended-precision (80-bit) floating-point operations are
enabled by default; routines in such libraries could suffer significant loss of
accuracy, typically through so-called "catastrophic cancellation", when this
option is used to set the precision to less than extended precision.
-mstackrealign
Realign the stack at entry. On the x86, the -mstackrealign option generates an
alternate prologue and epilogue that realigns the run-time stack if necessary.
This supports mixing legacy codes that keep 4-byte stack alignment with modern
codes that keep 16-byte stack alignment for SSE compatibility. See also the
attribute "force_align_arg_pointer", applicable to individual functions.
-mpreferred-stack-boundary=num
Attempt to keep the stack boundary aligned to a 2 raised to num byte boundary.
If -mpreferred-stack-boundary is not specified, the default is 4 (16 bytes or
128 bits).
Warning: When generating code for the x86-64 architecture with SSE extensions
disabled, -mpreferred-stack-boundary=3 can be used to keep the stack boundary
aligned to 8 byte boundary. Since x86-64 ABI require 16 byte stack alignment,
this is ABI incompatible and intended to be used in controlled environment
where stack space is important limitation. This option leads to wrong code
when functions compiled with 16 byte stack alignment (such as functions from a
standard library) are called with misaligned stack. In this case, SSE
instructions may lead to misaligned memory access traps. In addition, variable
arguments are handled incorrectly for 16 byte aligned objects (including x87
long double and __int128), leading to wrong results. You must build all
modules with -mpreferred-stack-boundary=3, including any libraries. This
includes the system libraries and startup modules.
-mincoming-stack-boundary=num
Assume the incoming stack is aligned to a 2 raised to num byte boundary. If
-mincoming-stack-boundary is not specified, the one specified by
-mpreferred-stack-boundary is used.
On Pentium and Pentium Pro, "double" and "long double" values should be aligned
to an 8-byte boundary (see -malign-double) or suffer significant run time
performance penalties. On Pentium III, the Streaming SIMD Extension (SSE) data
type "__m128" may not work properly if it is not 16-byte aligned.
To ensure proper alignment of this values on the stack, the stack boundary must
be as aligned as that required by any value stored on the stack. Further,
every function must be generated such that it keeps the stack aligned. Thus
calling a function compiled with a higher preferred stack boundary from a
function compiled with a lower preferred stack boundary most likely misaligns
the stack. It is recommended that libraries that use callbacks always use the
default setting.
This extra alignment does consume extra stack space, and generally increases
code size. Code that is sensitive to stack space usage, such as embedded
systems and operating system kernels, may want to reduce the preferred
alignment to -mpreferred-stack-boundary=2.
-mmmx
-msse
-msse2
-msse3
-mssse3
-msse4
-msse4a
-msse4.1
-msse4.2
-mavx
-mavx2
-mavx512f
-mavx512pf
-mavx512er
-mavx512cd
-mavx512vl
-mavx512bw
-mavx512dq
-mavx512ifma
-mavx512vbmi
-msha
-maes
-mpclmul
-mclflushopt
-mclwb
-mfsgsbase
-mrdrnd
-mf16c
-mfma
-mpconfig
-mwbnoinvd
-mfma4
-mprfchw
-mrdpid
-mprefetchwt1
-mrdseed
-msgx
-mxop
-mlwp
-m3dnow
-m3dnowa
-mpopcnt
-mabm
-madx
-mbmi
-mbmi2
-mlzcnt
-mfxsr
-mxsave
-mxsaveopt
-mxsavec
-mxsaves
-mrtm
-mhle
-mtbm
-mmpx
-mmwaitx
-mclzero
-mpku
-mavx512vbmi2
-mgfni
-mvaes
-mvpclmulqdq
-mavx512bitalg
-mmovdiri
-mmovdir64b
-mavx512vpopcntdq
-mavx5124fmaps
-mavx512vnni
-mavx5124vnniw
These switches enable the use of instructions in the MMX, SSE, SSE2, SSE3,
SSSE3, SSE4, SSE4A, SSE4.1, SSE4.2, AVX, AVX2, AVX512F, AVX512PF, AVX512ER,
AVX512CD, AVX512VL, AVX512BW, AVX512DQ, AVX512IFMA, AVX512VBMI, SHA, AES,
PCLMUL, CLFLUSHOPT, CLWB, FSGSBASE, RDRND, F16C, FMA, PCONFIG, WBNOINVD, FMA4,
PREFETCHW, RDPID, PREFETCHWT1, RDSEED, SGX, XOP, LWP, 3DNow!, enhanced 3DNow!,
POPCNT, ABM, ADX, BMI, BMI2, LZCNT, FXSR, XSAVE, XSAVEOPT, XSAVEC, XSAVES, RTM,
HLE, TBM, MPX, MWAITX, CLZERO, PKU, AVX512VBMI2, GFNI, VAES, VPCLMULQDQ,
AVX512BITALG, MOVDIRI, MOVDIR64B, AVX512VPOPCNTDQ, AVX5124FMAPS, AVX512VNNI, or
AVX5124VNNIW extended instruction sets. Each has a corresponding -mno- option
to disable use of these instructions.
These extensions are also available as built-in functions: see x86 Built-in
Functions, for details of the functions enabled and disabled by these switches.
To generate SSE/SSE2 instructions automatically from floating-point code (as
opposed to 387 instructions), see -mfpmath=sse.
GCC depresses SSEx instructions when -mavx is used. Instead, it generates new
AVX instructions or AVX equivalence for all SSEx instructions when needed.
These options enable GCC to use these extended instructions in generated code,
even without -mfpmath=sse. Applications that perform run-time CPU detection
must compile separate files for each supported architecture, using the
appropriate flags. In particular, the file containing the CPU detection code
should be compiled without these options.
-mdump-tune-features
This option instructs GCC to dump the names of the x86 performance tuning
features and default settings. The names can be used in -mtune-ctrl=feature-
list.
-mtune-ctrl=feature-list
This option is used to do fine grain control of x86 code generation features.
feature-list is a comma separated list of feature names. See also
-mdump-tune-features. When specified, the feature is turned on if it is not
preceded with ^, otherwise, it is turned off. -mtune-ctrl=feature-list is
intended to be used by GCC developers. Using it may lead to code paths not
covered by testing and can potentially result in compiler ICEs or runtime
errors.
-mno-default
This option instructs GCC to turn off all tunable features. See also
-mtune-ctrl=feature-list and -mdump-tune-features.
-mcld
This option instructs GCC to emit a "cld" instruction in the prologue of
functions that use string instructions. String instructions depend on the DF
flag to select between autoincrement or autodecrement mode. While the ABI
specifies the DF flag to be cleared on function entry, some operating systems
violate this specification by not clearing the DF flag in their exception
dispatchers. The exception handler can be invoked with the DF flag set, which
leads to wrong direction mode when string instructions are used. This option
can be enabled by default on 32-bit x86 targets by configuring GCC with the
--enable-cld configure option. Generation of "cld" instructions can be
suppressed with the -mno-cld compiler option in this case.
-mvzeroupper
This option instructs GCC to emit a "vzeroupper" instruction before a transfer
of control flow out of the function to minimize the AVX to SSE transition
penalty as well as remove unnecessary "zeroupper" intrinsics.
-mprefer-avx128
This option instructs GCC to use 128-bit AVX instructions instead of 256-bit
AVX instructions in the auto-vectorizer.
-mprefer-vector-width=opt
This option instructs GCC to use opt-bit vector width in instructions instead
of default on the selected platform.
none
No extra limitations applied to GCC other than defined by the selected
platform.
128 Prefer 128-bit vector width for instructions.
256 Prefer 256-bit vector width for instructions.
512 Prefer 512-bit vector width for instructions.
-mcx16
This option enables GCC to generate "CMPXCHG16B" instructions in 64-bit code to
implement compare-and-exchange operations on 16-byte aligned 128-bit objects.
This is useful for atomic updates of data structures exceeding one machine word
in size. The compiler uses this instruction to implement __sync Builtins.
However, for __atomic Builtins operating on 128-bit integers, a library call is
always used.
-msahf
This option enables generation of "SAHF" instructions in 64-bit code. Early
Intel Pentium 4 CPUs with Intel 64 support, prior to the introduction of
Pentium 4 G1 step in December 2005, lacked the "LAHF" and "SAHF" instructions
which are supported by AMD64. These are load and store instructions,
respectively, for certain status flags. In 64-bit mode, the "SAHF" instruction
is used to optimize "fmod", "drem", and "remainder" built-in functions; see
Other Builtins for details.
-mmovbe
This option enables use of the "movbe" instruction to implement
"__builtin_bswap32" and "__builtin_bswap64".
-mshstk
The -mshstk option enables shadow stack built-in functions from x86 Control-
flow Enforcement Technology (CET).
-mcrc32
This option enables built-in functions "__builtin_ia32_crc32qi",
"__builtin_ia32_crc32hi", "__builtin_ia32_crc32si" and "__builtin_ia32_crc32di"
to generate the "crc32" machine instruction.
-mrecip
This option enables use of "RCPSS" and "RSQRTSS" instructions (and their
vectorized variants "RCPPS" and "RSQRTPS") with an additional Newton-Raphson
step to increase precision instead of "DIVSS" and "SQRTSS" (and their
vectorized variants) for single-precision floating-point arguments. These
instructions are generated only when -funsafe-math-optimizations is enabled
together with -ffinite-math-only and -fno-trapping-math. Note that while the
throughput of the sequence is higher than the throughput of the non-reciprocal
instruction, the precision of the sequence can be decreased by up to 2 ulp
(i.e. the inverse of 1.0 equals 0.99999994).
Note that GCC implements "1.0f/sqrtf(x)" in terms of "RSQRTSS" (or "RSQRTPS")
already with -ffast-math (or the above option combination), and doesn't need
-mrecip.
Also note that GCC emits the above sequence with additional Newton-Raphson step
for vectorized single-float division and vectorized "sqrtf(x)" already with
-ffast-math (or the above option combination), and doesn't need -mrecip.
-mrecip=opt
This option controls which reciprocal estimate instructions may be used. opt
is a comma-separated list of options, which may be preceded by a ! to invert
the option:
all Enable all estimate instructions.
default
Enable the default instructions, equivalent to -mrecip.
none
Disable all estimate instructions, equivalent to -mno-recip.
div Enable the approximation for scalar division.
vec-div
Enable the approximation for vectorized division.
sqrt
Enable the approximation for scalar square root.
vec-sqrt
Enable the approximation for vectorized square root.
So, for example, -mrecip=all,!sqrt enables all of the reciprocal
approximations, except for square root.
-mveclibabi=type
Specifies the ABI type to use for vectorizing intrinsics using an external
library. Supported values for type are svml for the Intel short vector math
library and acml for the AMD math core library. To use this option, both
-ftree-vectorize and -funsafe-math-optimizations have to be enabled, and an
SVML or ACML ABI-compatible library must be specified at link time.
GCC currently emits calls to "vmldExp2", "vmldLn2", "vmldLog102", "vmldPow2",
"vmldTanh2", "vmldTan2", "vmldAtan2", "vmldAtanh2", "vmldCbrt2", "vmldSinh2",
"vmldSin2", "vmldAsinh2", "vmldAsin2", "vmldCosh2", "vmldCos2", "vmldAcosh2",
"vmldAcos2", "vmlsExp4", "vmlsLn4", "vmlsLog104", "vmlsPow4", "vmlsTanh4",
"vmlsTan4", "vmlsAtan4", "vmlsAtanh4", "vmlsCbrt4", "vmlsSinh4", "vmlsSin4",
"vmlsAsinh4", "vmlsAsin4", "vmlsCosh4", "vmlsCos4", "vmlsAcosh4" and
"vmlsAcos4" for corresponding function type when -mveclibabi=svml is used, and
"__vrd2_sin", "__vrd2_cos", "__vrd2_exp", "__vrd2_log", "__vrd2_log2",
"__vrd2_log10", "__vrs4_sinf", "__vrs4_cosf", "__vrs4_expf", "__vrs4_logf",
"__vrs4_log2f", "__vrs4_log10f" and "__vrs4_powf" for the corresponding
function type when -mveclibabi=acml is used.
-mabi=name
Generate code for the specified calling convention. Permissible values are
sysv for the ABI used on GNU/Linux and other systems, and ms for the Microsoft
ABI. The default is to use the Microsoft ABI when targeting Microsoft Windows
and the SysV ABI on all other systems. You can control this behavior for
specific functions by using the function attributes "ms_abi" and "sysv_abi".
-mforce-indirect-call
Force all calls to functions to be indirect. This is useful when using Intel
Processor Trace where it generates more precise timing information for function
calls.
-mcall-ms2sysv-xlogues
Due to differences in 64-bit ABIs, any Microsoft ABI function that calls a
System V ABI function must consider RSI, RDI and XMM6-15 as clobbered. By
default, the code for saving and restoring these registers is emitted inline,
resulting in fairly lengthy prologues and epilogues. Using
-mcall-ms2sysv-xlogues emits prologues and epilogues that use stubs in the
static portion of libgcc to perform these saves and restores, thus reducing
function size at the cost of a few extra instructions.
-mtls-dialect=type
Generate code to access thread-local storage using the gnu or gnu2 conventions.
gnu is the conservative default; gnu2 is more efficient, but it may add
compile- and run-time requirements that cannot be satisfied on all systems.
-mpush-args
-mno-push-args
Use PUSH operations to store outgoing parameters. This method is shorter and
usually equally fast as method using SUB/MOV operations and is enabled by
default. In some cases disabling it may improve performance because of
improved scheduling and reduced dependencies.
-maccumulate-outgoing-args
If enabled, the maximum amount of space required for outgoing arguments is
computed in the function prologue. This is faster on most modern CPUs because
of reduced dependencies, improved scheduling and reduced stack usage when the
preferred stack boundary is not equal to 2. The drawback is a notable increase
in code size. This switch implies -mno-push-args.
-mthreads
Support thread-safe exception handling on MinGW. Programs that rely on thread-
safe exception handling must compile and link all code with the -mthreads
option. When compiling, -mthreads defines -D_MT; when linking, it links in a
special thread helper library -lmingwthrd which cleans up per-thread exception-
handling data.
-mms-bitfields
-mno-ms-bitfields
Enable/disable bit-field layout compatible with the native Microsoft Windows
compiler.
If "packed" is used on a structure, or if bit-fields are used, it may be that
the Microsoft ABI lays out the structure differently than the way GCC normally
does. Particularly when moving packed data between functions compiled with GCC
and the native Microsoft compiler (either via function call or as data in a
file), it may be necessary to access either format.
This option is enabled by default for Microsoft Windows targets. This behavior
can also be controlled locally by use of variable or type attributes. For more
information, see x86 Variable Attributes and x86 Type Attributes.
The Microsoft structure layout algorithm is fairly simple with the exception of
the bit-field packing. The padding and alignment of members of structures and
whether a bit-field can straddle a storage-unit boundary are determine by these
rules:
1. Structure members are stored sequentially in the order in which they are
declared: the first member has the lowest memory address and the last
member the highest.
2. Every data object has an alignment requirement. The alignment requirement
for all data except structures, unions, and arrays is either the size of
the object or the current packing size (specified with either the "aligned"
attribute or the "pack" pragma), whichever is less. For structures,
unions, and arrays, the alignment requirement is the largest alignment
requirement of its members. Every object is allocated an offset so that:
offset % alignment_requirement == 0
3. Adjacent bit-fields are packed into the same 1-, 2-, or 4-byte allocation
unit if the integral types are the same size and if the next bit-field fits
into the current allocation unit without crossing the boundary imposed by
the common alignment requirements of the bit-fields.
MSVC interprets zero-length bit-fields in the following ways:
1. If a zero-length bit-field is inserted between two bit-fields that
are normally coalesced, the bit-fields are not coalesced.
For example:
struct
{
unsigned long bf_1 : 12;
unsigned long : 0;
unsigned long bf_2 : 12;
} t1;
The size of "t1" is 8 bytes with the zero-length bit-field. If the zero-
length bit-field were removed, "t1"'s size would be 4 bytes.
2. If a zero-length bit-field is inserted after a bit-field, "foo", and the
alignment of the zero-length bit-field is greater than the member that
follows it, "bar", "bar" is aligned as the type of the zero-length bit-
field.
For example:
struct
{
char foo : 4;
short : 0;
char bar;
} t2;
struct
{
char foo : 4;
short : 0;
double bar;
} t3;
For "t2", "bar" is placed at offset 2, rather than offset 1. Accordingly,
the size of "t2" is 4. For "t3", the zero-length bit-field does not affect
the alignment of "bar" or, as a result, the size of the structure.
Taking this into account, it is important to note the following:
1. If a zero-length bit-field follows a normal bit-field, the type of the
zero-length bit-field may affect the alignment of the structure as
whole. For example, "t2" has a size of 4 bytes, since the zero-length
bit-field follows a normal bit-field, and is of type short.
2. Even if a zero-length bit-field is not followed by a normal bit-field,
it may
still affect the alignment of the structure:
struct
{
char foo : 6;
long : 0;
} t4;
Here, "t4" takes up 4 bytes.
3. Zero-length bit-fields following non-bit-field members are ignored:
struct
{
char foo;
long : 0;
char bar;
} t5;
Here, "t5" takes up 2 bytes.
-mno-align-stringops
Do not align the destination of inlined string operations. This switch reduces
code size and improves performance in case the destination is already aligned,
but GCC doesn't know about it.
-minline-all-stringops
By default GCC inlines string operations only when the destination is known to
be aligned to least a 4-byte boundary. This enables more inlining and
increases code size, but may improve performance of code that depends on fast
"memcpy", "strlen", and "memset" for short lengths.
-minline-stringops-dynamically
For string operations of unknown size, use run-time checks with inline code for
small blocks and a library call for large blocks.
-mstringop-strategy=alg
Override the internal decision heuristic for the particular algorithm to use
for inlining string operations. The allowed values for alg are:
rep_byte
rep_4byte
rep_8byte
Expand using i386 "rep" prefix of the specified size.
byte_loop
loop
unrolled_loop
Expand into an inline loop.
libcall
Always use a library call.
-mmemcpy-strategy=strategy
Override the internal decision heuristic to decide if "__builtin_memcpy" should
be inlined and what inline algorithm to use when the expected size of the copy
operation is known. strategy is a comma-separated list of
alg:max_size:dest_align triplets. alg is specified in -mstringop-strategy,
max_size specifies the max byte size with which inline algorithm alg is
allowed. For the last triplet, the max_size must be "-1". The max_size of the
triplets in the list must be specified in increasing order. The minimal byte
size for alg is 0 for the first triplet and "max_size + 1" of the preceding
range.
-mmemset-strategy=strategy
The option is similar to -mmemcpy-strategy= except that it is to control
"__builtin_memset" expansion.
-momit-leaf-frame-pointer
Don't keep the frame pointer in a register for leaf functions. This avoids the
instructions to save, set up, and restore frame pointers and makes an extra
register available in leaf functions. The option -fomit-leaf-frame-pointer
removes the frame pointer for leaf functions, which might make debugging
harder.
-mtls-direct-seg-refs
-mno-tls-direct-seg-refs
Controls whether TLS variables may be accessed with offsets from the TLS
segment register (%gs for 32-bit, %fs for 64-bit), or whether the thread base
pointer must be added. Whether or not this is valid depends on the operating
system, and whether it maps the segment to cover the entire TLS area.
For systems that use the GNU C Library, the default is on.
-msse2avx
-mno-sse2avx
Specify that the assembler should encode SSE instructions with VEX prefix. The
option -mavx turns this on by default.
-mfentry
-mno-fentry
If profiling is active (-pg), put the profiling counter call before the
prologue. Note: On x86 architectures the attribute "ms_hook_prologue" isn't
possible at the moment for -mfentry and -pg.
-mrecord-mcount
-mno-record-mcount
If profiling is active (-pg), generate a __mcount_loc section that contains
pointers to each profiling call. This is useful for automatically patching and
out calls.
-mnop-mcount
-mno-nop-mcount
If profiling is active (-pg), generate the calls to the profiling functions as
NOPs. This is useful when they should be patched in later dynamically. This is
likely only useful together with -mrecord-mcount.
-mskip-rax-setup
-mno-skip-rax-setup
When generating code for the x86-64 architecture with SSE extensions disabled,
-mskip-rax-setup can be used to skip setting up RAX register when there are no
variable arguments passed in vector registers.
Warning: Since RAX register is used to avoid unnecessarily saving vector
registers on stack when passing variable arguments, the impacts of this option
are callees may waste some stack space, misbehave or jump to a random location.
GCC 4.4 or newer don't have those issues, regardless the RAX register value.
-m8bit-idiv
-mno-8bit-idiv
On some processors, like Intel Atom, 8-bit unsigned integer divide is much
faster than 32-bit/64-bit integer divide. This option generates a run-time
check. If both dividend and divisor are within range of 0 to 255, 8-bit
unsigned integer divide is used instead of 32-bit/64-bit integer divide.
-mavx256-split-unaligned-load
-mavx256-split-unaligned-store
Split 32-byte AVX unaligned load and store.
-mstack-protector-guard=guard
-mstack-protector-guard-reg=reg
-mstack-protector-guard-offset=offset
Generate stack protection code using canary at guard. Supported locations are
global for global canary or tls for per-thread canary in the TLS block (the
default). This option has effect only when -fstack-protector or
-fstack-protector-all is specified.
With the latter choice the options -mstack-protector-guard-reg=reg and
-mstack-protector-guard-offset=offset furthermore specify which segment
register (%fs or %gs) to use as base register for reading the canary, and from
what offset from that base register. The default for those is as specified in
the relevant ABI.
-mmitigate-rop
Try to avoid generating code sequences that contain unintended return opcodes,
to mitigate against certain forms of attack. At the moment, this option is
limited in what it can do and should not be relied on to provide serious
protection.
-mgeneral-regs-only
Generate code that uses only the general-purpose registers. This prevents the
compiler from using floating-point, vector, mask and bound registers.
-mindirect-branch=choice
Convert indirect call and jump with choice. The default is keep, which keeps
indirect call and jump unmodified. thunk converts indirect call and jump to
call and return thunk. thunk-inline converts indirect call and jump to inlined
call and return thunk. thunk-extern converts indirect call and jump to
external call and return thunk provided in a separate object file. You can
control this behavior for a specific function by using the function attribute
"indirect_branch".
Note that -mcmodel=large is incompatible with -mindirect-branch=thunk and
-mindirect-branch=thunk-extern since the thunk function may not be reachable in
the large code model.
Note that -mindirect-branch=thunk-extern is incompatible with
-fcf-protection=branch and -fcheck-pointer-bounds since the external thunk can
not be modified to disable control-flow check.
-mfunction-return=choice
Convert function return with choice. The default is keep, which keeps function
return unmodified. thunk converts function return to call and return thunk.
thunk-inline converts function return to inlined call and return thunk. thunk-
extern converts function return to external call and return thunk provided in a
separate object file. You can control this behavior for a specific function by
using the function attribute "function_return".
Note that -mcmodel=large is incompatible with -mfunction-return=thunk and
-mfunction-return=thunk-extern since the thunk function may not be reachable in
the large code model.
-mindirect-branch-register
Force indirect call and jump via register.
These -m switches are supported in addition to the above on x86-64 processors in
64-bit environments.
-m32
-m64
-mx32
-m16
-miamcu
Generate code for a 16-bit, 32-bit or 64-bit environment. The -m32 option sets
"int", "long", and pointer types to 32 bits, and generates code that runs on
any i386 system.
The -m64 option sets "int" to 32 bits and "long" and pointer types to 64 bits,
and generates code for the x86-64 architecture. For Darwin only the -m64
option also turns off the -fno-pic and -mdynamic-no-pic options.
The -mx32 option sets "int", "long", and pointer types to 32 bits, and
generates code for the x86-64 architecture.
The -m16 option is the same as -m32, except for that it outputs the
".code16gcc" assembly directive at the beginning of the assembly output so that
the binary can run in 16-bit mode.
The -miamcu option generates code which conforms to Intel MCU psABI. It
requires the -m32 option to be turned on.
-mno-red-zone
Do not use a so-called "red zone" for x86-64 code. The red zone is mandated by
the x86-64 ABI; it is a 128-byte area beyond the location of the stack pointer
that is not modified by signal or interrupt handlers and therefore can be used
for temporary data without adjusting the stack pointer. The flag -mno-red-zone
disables this red zone.
-mcmodel=small
Generate code for the small code model: the program and its symbols must be
linked in the lower 2 GB of the address space. Pointers are 64 bits. Programs
can be statically or dynamically linked. This is the default code model.
-mcmodel=kernel
Generate code for the kernel code model. The kernel runs in the negative 2 GB
of the address space. This model has to be used for Linux kernel code.
-mcmodel=medium
Generate code for the medium model: the program is linked in the lower 2 GB of
the address space. Small symbols are also placed there. Symbols with sizes
larger than -mlarge-data-threshold are put into large data or BSS sections and
can be located above 2GB. Programs can be statically or dynamically linked.
-mcmodel=large
Generate code for the large model. This model makes no assumptions about
addresses and sizes of sections.
-maddress-mode=long
Generate code for long address mode. This is only supported for 64-bit and x32
environments. It is the default address mode for 64-bit environments.
-maddress-mode=short
Generate code for short address mode. This is only supported for 32-bit and
x32 environments. It is the default address mode for 32-bit and x32
environments.
x86 Windows Options
These additional options are available for Microsoft Windows targets:
-mconsole
This option specifies that a console application is to be generated, by
instructing the linker to set the PE header subsystem type required for console
applications. This option is available for Cygwin and MinGW targets and is
enabled by default on those targets.
-mdll
This option is available for Cygwin and MinGW targets. It specifies that a
DLL---a dynamic link library---is to be generated, enabling the selection of
the required runtime startup object and entry point.
-mnop-fun-dllimport
This option is available for Cygwin and MinGW targets. It specifies that the
"dllimport" attribute should be ignored.
-mthread
This option is available for MinGW targets. It specifies that MinGW-specific
thread support is to be used.
-municode
This option is available for MinGW-w64 targets. It causes the "UNICODE"
preprocessor macro to be predefined, and chooses Unicode-capable runtime
startup code.
-mwin32
This option is available for Cygwin and MinGW targets. It specifies that the
typical Microsoft Windows predefined macros are to be set in the pre-processor,
but does not influence the choice of runtime library/startup code.
-mwindows
This option is available for Cygwin and MinGW targets. It specifies that a GUI
application is to be generated by instructing the linker to set the PE header
subsystem type appropriately.
-fno-set-stack-executable
This option is available for MinGW targets. It specifies that the executable
flag for the stack used by nested functions isn't set. This is necessary for
binaries running in kernel mode of Microsoft Windows, as there the User32 API,
which is used to set executable privileges, isn't available.
-fwritable-relocated-rdata
This option is available for MinGW and Cygwin targets. It specifies that
relocated-data in read-only section is put into the ".data" section. This is a
necessary for older runtimes not supporting modification of ".rdata" sections
for pseudo-relocation.
-mpe-aligned-commons
This option is available for Cygwin and MinGW targets. It specifies that the
GNU extension to the PE file format that permits the correct alignment of
COMMON variables should be used when generating code. It is enabled by default
if GCC detects that the target assembler found during configuration supports
the feature.
See also under x86 Options for standard options.
Xstormy16 Options
These options are defined for Xstormy16:
-msim
Choose startup files and linker script suitable for the simulator.
Xtensa Options
These options are supported for Xtensa targets:
-mconst16
-mno-const16
Enable or disable use of "CONST16" instructions for loading constant values.
The "CONST16" instruction is currently not a standard option from Tensilica.
When enabled, "CONST16" instructions are always used in place of the standard
"L32R" instructions. The use of "CONST16" is enabled by default only if the
"L32R" instruction is not available.
-mfused-madd
-mno-fused-madd
Enable or disable use of fused multiply/add and multiply/subtract instructions
in the floating-point option. This has no effect if the floating-point option
is not also enabled. Disabling fused multiply/add and multiply/subtract
instructions forces the compiler to use separate instructions for the multiply
and add/subtract operations. This may be desirable in some cases where strict
IEEE 754-compliant results are required: the fused multiply add/subtract
instructions do not round the intermediate result, thereby producing results
with more bits of precision than specified by the IEEE standard. Disabling
fused multiply add/subtract instructions also ensures that the program output
is not sensitive to the compiler's ability to combine multiply and add/subtract
operations.
-mserialize-volatile
-mno-serialize-volatile
When this option is enabled, GCC inserts "MEMW" instructions before "volatile"
memory references to guarantee sequential consistency. The default is
-mserialize-volatile. Use -mno-serialize-volatile to omit the "MEMW"
instructions.
-mforce-no-pic
For targets, like GNU/Linux, where all user-mode Xtensa code must be position-
independent code (PIC), this option disables PIC for compiling kernel code.
-mtext-section-literals
-mno-text-section-literals
These options control the treatment of literal pools. The default is
-mno-text-section-literals, which places literals in a separate section in the
output file. This allows the literal pool to be placed in a data RAM/ROM, and
it also allows the linker to combine literal pools from separate object files
to remove redundant literals and improve code size. With
-mtext-section-literals, the literals are interspersed in the text section in
order to keep them as close as possible to their references. This may be
necessary for large assembly files. Literals for each function are placed
right before that function.
-mauto-litpools
-mno-auto-litpools
These options control the treatment of literal pools. The default is
-mno-auto-litpools, which places literals in a separate section in the output
file unless -mtext-section-literals is used. With -mauto-litpools the literals
are interspersed in the text section by the assembler. Compiler does not
produce explicit ".literal" directives and loads literals into registers with
"MOVI" instructions instead of "L32R" to let the assembler do relaxation and
place literals as necessary. This option allows assembler to create several
literal pools per function and assemble very big functions, which may not be
possible with -mtext-section-literals.
-mtarget-align
-mno-target-align
When this option is enabled, GCC instructs the assembler to automatically align
instructions to reduce branch penalties at the expense of some code density.
The assembler attempts to widen density instructions to align branch targets
and the instructions following call instructions. If there are not enough
preceding safe density instructions to align a target, no widening is
performed. The default is -mtarget-align. These options do not affect the
treatment of auto-aligned instructions like "LOOP", which the assembler always
aligns, either by widening density instructions or by inserting NOP
instructions.
-mlongcalls
-mno-longcalls
When this option is enabled, GCC instructs the assembler to translate direct
calls to indirect calls unless it can determine that the target of a direct
call is in the range allowed by the call instruction. This translation
typically occurs for calls to functions in other source files. Specifically,
the assembler translates a direct "CALL" instruction into an "L32R" followed by
a "CALLX" instruction. The default is -mno-longcalls. This option should be
used in programs where the call target can potentially be out of range. This
option is implemented in the assembler, not the compiler, so the assembly code
generated by GCC still shows direct call instructions---look at the
disassembled object code to see the actual instructions. Note that the
assembler uses an indirect call for every cross-file call, not just those that
really are out of range.
zSeries Options
These are listed under
ENVIRONMENT
This section describes several environment variables that affect how GCC operates.
Some of them work by specifying directories or prefixes to use when searching for
various kinds of files. Some are used to specify other aspects of the compilation
environment.
Note that you can also specify places to search using options such as -B, -I and
-L. These take precedence over places specified using environment variables, which
in turn take precedence over those specified by the configuration of GCC.
LANG
LC_CTYPE
LC_MESSAGES
LC_ALL
These environment variables control the way that GCC uses localization
information which allows GCC to work with different national conventions. GCC
inspects the locale categories LC_CTYPE and LC_MESSAGES if it has been
configured to do so. These locale categories can be set to any value supported
by your installation. A typical value is en_GB.UTF-8 for English in the United
Kingdom encoded in UTF-8.
The LC_CTYPE environment variable specifies character classification. GCC uses
it to determine the character boundaries in a string; this is needed for some
multibyte encodings that contain quote and escape characters that are otherwise
interpreted as a string end or escape.
The LC_MESSAGES environment variable specifies the language to use in
diagnostic messages.
If the LC_ALL environment variable is set, it overrides the value of LC_CTYPE
and LC_MESSAGES; otherwise, LC_CTYPE and LC_MESSAGES default to the value of
the LANG environment variable. If none of these variables are set, GCC
defaults to traditional C English behavior.
TMPDIR
If TMPDIR is set, it specifies the directory to use for temporary files. GCC
uses temporary files to hold the output of one stage of compilation which is to
be used as input to the next stage: for example, the output of the
preprocessor, which is the input to the compiler proper.
GCC_COMPARE_DEBUG
Setting GCC_COMPARE_DEBUG is nearly equivalent to passing -fcompare-debug to
the compiler driver. See the documentation of this option for more details.
GCC_EXEC_PREFIX
If GCC_EXEC_PREFIX is set, it specifies a prefix to use in the names of the
subprograms executed by the compiler. No slash is added when this prefix is
combined with the name of a subprogram, but you can specify a prefix that ends
with a slash if you wish.
If GCC_EXEC_PREFIX is not set, GCC attempts to figure out an appropriate prefix
to use based on the pathname it is invoked with.
If GCC cannot find the subprogram using the specified prefix, it tries looking
in the usual places for the subprogram.
The default value of GCC_EXEC_PREFIX is prefix/lib/gcc/ where prefix is the
prefix to the installed compiler. In many cases prefix is the value of "prefix"
when you ran the configure script.
Other prefixes specified with -B take precedence over this prefix.
This prefix is also used for finding files such as crt0.o that are used for
linking.
In addition, the prefix is used in an unusual way in finding the directories to
search for header files. For each of the standard directories whose name
normally begins with /usr/local/lib/gcc (more precisely, with the value of
GCC_INCLUDE_DIR), GCC tries replacing that beginning with the specified prefix
to produce an alternate directory name. Thus, with -Bfoo/, GCC searches
foo/bar just before it searches the standard directory /usr/local/lib/bar. If
a standard directory begins with the configured prefix then the value of prefix
is replaced by GCC_EXEC_PREFIX when looking for header files.
COMPILER_PATH
The value of COMPILER_PATH is a colon-separated list of directories, much like
PATH. GCC tries the directories thus specified when searching for subprograms,
if it cannot find the subprograms using GCC_EXEC_PREFIX.
LIBRARY_PATH
The value of LIBRARY_PATH is a colon-separated list of directories, much like
PATH. When configured as a native compiler, GCC tries the directories thus
specified when searching for special linker files, if it cannot find them using
GCC_EXEC_PREFIX. Linking using GCC also uses these directories when searching
for ordinary libraries for the -l option (but directories specified with -L
come first).
LANG
This variable is used to pass locale information to the compiler. One way in
which this information is used is to determine the character set to be used
when character literals, string literals and comments are parsed in C and C++.
When the compiler is configured to allow multibyte characters, the following
values for LANG are recognized:
C-JIS
Recognize JIS characters.
C-SJIS
Recognize SJIS characters.
C-EUCJP
Recognize EUCJP characters.
If LANG is not defined, or if it has some other value, then the compiler uses
"mblen" and "mbtowc" as defined by the default locale to recognize and
translate multibyte characters.
Some additional environment variables affect the behavior of the preprocessor.
CPATH
C_INCLUDE_PATH
CPLUS_INCLUDE_PATH
OBJC_INCLUDE_PATH
Each variable's value is a list of directories separated by a special
character, much like PATH, in which to look for header files. The special
character, "PATH_SEPARATOR", is target-dependent and determined at GCC build
time. For Microsoft Windows-based targets it is a semicolon, and for almost
all other targets it is a colon.
CPATH specifies a list of directories to be searched as if specified with -I,
but after any paths given with -I options on the command line. This
environment variable is used regardless of which language is being
preprocessed.
The remaining environment variables apply only when preprocessing the
particular language indicated. Each specifies a list of directories to be
searched as if specified with -isystem, but after any paths given with -isystem
options on the command line.
In all these variables, an empty element instructs the compiler to search its
current working directory. Empty elements can appear at the beginning or end
of a path. For instance, if the value of CPATH is ":/special/include", that
has the same effect as -I. -I/special/include.
DEPENDENCIES_OUTPUT
If this variable is set, its value specifies how to output dependencies for
Make based on the non-system header files processed by the compiler. System
header files are ignored in the dependency output.
The value of DEPENDENCIES_OUTPUT can be just a file name, in which case the
Make rules are written to that file, guessing the target name from the source
file name. Or the value can have the form file target, in which case the rules
are written to file file using target as the target name.
In other words, this environment variable is equivalent to combining the
options -MM and -MF, with an optional -MT switch too.
SUNPRO_DEPENDENCIES
This variable is the same as DEPENDENCIES_OUTPUT (see above), except that
system header files are not ignored, so it implies -M rather than -MM.
However, the dependence on the main input file is omitted.
SOURCE_DATE_EPOCH
If this variable is set, its value specifies a UNIX timestamp to be used in
replacement of the current date and time in the "__DATE__" and "__TIME__"
macros, so that the embedded timestamps become reproducible.
The value of SOURCE_DATE_EPOCH must be a UNIX timestamp, defined as the number
of seconds (excluding leap seconds) since 01 Jan 1970 00:00:00 represented in
ASCII; identical to the output of @command{date +%s} on GNU/Linux and other
systems that support the %s extension in the "date" command.
The value should be a known timestamp such as the last modification time of the
source or package and it should be set by the build process.
BUGS
For instructions on reporting bugs, see <http://bugzilla.redhat.com/bugzilla>.
FOOTNOTES
1. On some systems, gcc -shared needs to build supplementary stub code for
constructors to work. On multi-libbed systems, gcc -shared must select the
correct support libraries to link against. Failing to supply the correct flags
may lead to subtle defects. Supplying them in cases where they are not
necessary is innocuous.
SEE ALSO
gpl(7), gfdl(7), fsf-funding(7), cpp(1), gcov(1), as(1), ld(1), gdb(1), dbx(1) and
the Info entries for gcc, cpp, as, ld, binutils and gdb.
AUTHOR
See the Info entry for gcc, or
<http://gcc.gnu.org/onlinedocs/gcc/Contributors.html>, for contributors to GCC.
COPYRIGHT
Copyright (c) 1988-2018 Free Software Foundation, Inc.
Permission is granted to copy, distribute and/or modify this document under the
terms of the GNU Free Documentation License, Version 1.3 or any later version
published by the Free Software Foundation; with the Invariant Sections being "GNU
General Public License" and "Funding Free Software", the Front-Cover texts being
(a) (see below), and with the Back-Cover Texts being (b) (see below). A copy of
the license is included in the gfdl(7) man page.
(a) The FSF's Front-Cover Text is:
A GNU Manual
(b) The FSF's Back-Cover Text is:
You have freedom to copy and modify this GNU Manual, like GNU
software. Copies published by the Free Software Foundation raise
funds for GNU development.
gcc-8 2021-05-14 GCC(1)
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