OC基础：重温weak
一、ISA结构
struct objc_object {
private:
isa_t isa;
};
union isa_t {
Class cls;
uintptr_t bits;
#if defined(ISA_BITFIELD) // ISA_BITFIELD意为 isa位域
struct {
ISA_BITFIELD; // defined in isa.h
};
#endif
};
1. ISA_BITFIELD意为 isa位域 ，其结构如下，64位大小。
# if __arm64__
# define ISA_MASK 0x0000000ffffffff8ULL
# define ISA_MAGIC_MASK 0x000003f000000001ULL
# define ISA_MAGIC_VALUE 0x000001a000000001ULL
# define ISA_BITFIELD \
uintptr_t nonpointer : 1; \
uintptr_t has_assoc : 1; \
uintptr_t has_cxx_dtor : 1; \
uintptr_t shiftcls : 33; /*MACH_VM_MAX_ADDRESS 0x1000000000*/ \
uintptr_t magic : 6; \
uintptr_t weakly_referenced : 1; \
uintptr_t deallocating : 1; \
uintptr_t has_sidetable_rc : 1; \
uintptr_t extra_rc : 19
# define RC_ONE (1ULL<<45)
# define RC_HALF (1ULL<<18)
# elif __x86_64__
# define ISA_MASK 0x00007ffffffffff8ULL
# define ISA_MAGIC_MASK 0x001f800000000001ULL
# define ISA_MAGIC_VALUE 0x001d800000000001ULL
# define ISA_BITFIELD \
uintptr_t nonpointer : 1; \
uintptr_t has_assoc : 1; \
uintptr_t has_cxx_dtor : 1; \
uintptr_t shiftcls : 44; /*MACH_VM_MAX_ADDRESS 0x7fffffe00000*/ \
uintptr_t magic : 6; \
uintptr_t weakly_referenced : 1; \
uintptr_t deallocating : 1; \
uintptr_t has_sidetable_rc : 1; \
uintptr_t extra_rc : 8
# define RC_ONE (1ULL<<56)
# define RC_HALF (1ULL<<7)
2. ISA_BITFIELD各位段的标识
nonpointer：
标识是否是纯指针，0 表示纯指针 ，1 表示优化后的对象指针，isa指针包含了类属性以及引用计数相关等has_assoc：
标识是否是关联对象（objc_setAssociatedObject,objc_getAssociatedObject）has_cxx_dtor：
标识该对象是否有 C++ 或者 Objc 的析构器,如果有析构函数,则需要做析构逻辑shiftcls：
存储类指针的值。开启指针优化的情况下，在 arm64 架构中有 33 位用来存储类指针magic：
标识对象是否完成初始化weakly_referenced：
标识对象是否被指向或曾经指向一个ARC的弱变量，deallocating：
标识对象是否正在释放has_sidetable_rc：
标识是否有引用计数存储在SideTable表refcnts里。若为1，引用计数retaincount = 1 + bits.extra_rc + sidetable.refcnts.secondextra_rc：
标识当前对象的引用计数值（x86_64架构，extra_rc只有8位，存储的引用计数最大为255，若超过了只能存储在sidetable.refcnts；对于arm64，extra_rc有19位，可以存储足够大的引用计数，省去了去获取sidetable引用计数的性能消耗）
二、Tagged Pointer
引用唐巧大佬的一段介绍：在 2013 年 9 月，苹果推出了 iPhone5s，与此同时，iPhone5s 配备了首个采用 64 位架构的 A7 双核处理器，为了节省内存和提高执行效率，苹果提出了Tagged Pointer的概念。对于 64 位程序，引入 Tagged Pointer 后，相关逻辑能减少一半的内存占用，以及 3 倍的访问速度提升，100 倍的创建、销毁速度提升。
注意：Tagged Pointer对象不具有引用计数的相关管理，它的值存放栈上的指针里，是被底层封装成的伪对象；可以在objc源码里看到多处判断isTaggedPointer，若是则return。
inline bool
objc_object::isTaggedPointer()
{
return _objc_isTaggedPointer(this);
}
static inline bool
_objc_isTaggedPointer(const void * _Nullable ptr)
{
return ((uintptr_t)ptr & _OBJC_TAG_MASK) == _OBJC_TAG_MASK;
}
1.判断一个对象指针是否是TaggedPointer，通过_OBJC_TAG_MASK 掩码来确定。
2.看下_OBJC_TAG_MASK 的定义：若处理器是64位架构将开启TaggedPointer
#if __LP64__
#define OBJC_HAVE_TAGGED_POINTERS 1
#endif
3.若是 64-bit Mac，OBJC_MSB_TAGGED_POINTERS 值为0，即 _OBJC_TAG_MASK掩码在低位；若是64-bit iOS，OBJC_MSB_TAGGED_POINTERS 值为1，即 _OBJC_TAG_MASK掩码在高位；
#if (TARGET_OS_OSX || TARGET_OS_IOSMAC) && __x86_64__
// 64-bit Mac - tag bit is LSB
# define OBJC_MSB_TAGGED_POINTERS 0
#else
// Everything else - tag bit is MSB
# define OBJC_MSB_TAGGED_POINTERS 1
#endif
#define _OBJC_TAG_INDEX_MASK 0x7
// array slot includes the tag bit itself
#define _OBJC_TAG_SLOT_COUNT 16
#define _OBJC_TAG_SLOT_MASK 0xf
#define _OBJC_TAG_EXT_INDEX_MASK 0xff
// array slot has no extra bits
#define _OBJC_TAG_EXT_SLOT_COUNT 256
#define _OBJC_TAG_EXT_SLOT_MASK 0xff
#if OBJC_MSB_TAGGED_POINTERS
# define _OBJC_TAG_MASK (1UL<<63)
# define _OBJC_TAG_INDEX_SHIFT 60
# define _OBJC_TAG_SLOT_SHIFT 60
# define _OBJC_TAG_PAYLOAD_LSHIFT 4
# define _OBJC_TAG_PAYLOAD_RSHIFT 4
# define _OBJC_TAG_EXT_MASK (0xfUL<<60)
# define _OBJC_TAG_EXT_INDEX_SHIFT 52
# define _OBJC_TAG_EXT_SLOT_SHIFT 52
# define _OBJC_TAG_EXT_PAYLOAD_LSHIFT 12
# define _OBJC_TAG_EXT_PAYLOAD_RSHIFT 12
#else
# define _OBJC_TAG_MASK 1UL
# define _OBJC_TAG_INDEX_SHIFT 1
# define _OBJC_TAG_SLOT_SHIFT 0
# define _OBJC_TAG_PAYLOAD_LSHIFT 0
# define _OBJC_TAG_PAYLOAD_RSHIFT 4
# define _OBJC_TAG_EXT_MASK 0xfUL
# define _OBJC_TAG_EXT_INDEX_SHIFT 4
# define _OBJC_TAG_EXT_SLOT_SHIFT 4
# define _OBJC_TAG_EXT_PAYLOAD_LSHIFT 0
# define _OBJC_TAG_EXT_PAYLOAD_RSHIFT 12
#endif
4. 若是60-bit payloads这种情况， _OBJC_TAG_MASK掩码在高位首位（第64位），高位第61-63标识OBJC_TAG_INDEX,3位最多标识8种；OBJC_TAG_NSAtom ～ OBJC_TAG_RESERVED_7 刚好8种类型；
OBJC_TAG_NSString代表字符串类型OBJC_TAG_NSNumber代表Number类型OBJC_TAG_NSIndexPath代表IndexPath类型OBJC_TAG_NSManagedObjectID代表Core Data里每条数据中生成的全局唯一idOBJC_TAG_NSDate代表Date类型OBJC_TAG_RESERVED_7预留
// Tagged pointer layout and usage is subject to change on different OS versions.
// Tag indexes 0..<7 have a 60-bit payload.
// Tag index 7 is reserved.
// Tag indexes 8..<264 have a 52-bit payload.
// Tag index 264 is reserved.
#if __has_feature(objc_fixed_enum) || __cplusplus >= 201103L
enum objc_tag_index_t : uint16_t
#else
typedef uint16_t objc_tag_index_t;
enum
#endif
{
// 60-bit payloads
OBJC_TAG_NSAtom = 0,
OBJC_TAG_1 = 1,
OBJC_TAG_NSString = 2,
OBJC_TAG_NSNumber = 3,
OBJC_TAG_NSIndexPath = 4,
OBJC_TAG_NSManagedObjectID = 5,
OBJC_TAG_NSDate = 6,
// 60-bit reserved
OBJC_TAG_RESERVED_7 = 7,
// 52-bit payloads
OBJC_TAG_Photos_1 = 8,
OBJC_TAG_Photos_2 = 9,
OBJC_TAG_Photos_3 = 10,
OBJC_TAG_Photos_4 = 11,
OBJC_TAG_XPC_1 = 12,
OBJC_TAG_XPC_2 = 13,
OBJC_TAG_XPC_3 = 14,
OBJC_TAG_XPC_4 = 15,
OBJC_TAG_First60BitPayload = 0,
OBJC_TAG_Last60BitPayload = 6,
OBJC_TAG_First52BitPayload = 8,
OBJC_TAG_Last52BitPayload = 263,
OBJC_TAG_RESERVED_264 = 264
};
#if __has_feature(objc_fixed_enum) && !defined(__cplusplus)
typedef enum objc_tag_index_t objc_tag_index_t;
#endif
5.以下两个方法是Tagged Pointer通过位运算set/get对象实际值；
static inline void * _Nonnull
_objc_makeTaggedPointer(objc_tag_index_t tag, uintptr_t value)
{
// PAYLOAD_LSHIFT and PAYLOAD_RSHIFT are the payload extraction shifts.
// They are reversed here for payload insertion.
// assert(_objc_taggedPointersEnabled());
if (tag <= OBJC_TAG_Last60BitPayload) {
// assert(((value << _OBJC_TAG_PAYLOAD_RSHIFT) >> _OBJC_TAG_PAYLOAD_LSHIFT) == value);
uintptr_t result =
(_OBJC_TAG_MASK |
((uintptr_t)tag << _OBJC_TAG_INDEX_SHIFT) |
((value << _OBJC_TAG_PAYLOAD_RSHIFT) >> _OBJC_TAG_PAYLOAD_LSHIFT));
return _objc_encodeTaggedPointer(result);
} else {
// assert(tag >= OBJC_TAG_First52BitPayload);
// assert(tag <= OBJC_TAG_Last52BitPayload);
// assert(((value << _OBJC_TAG_EXT_PAYLOAD_RSHIFT) >> _OBJC_TAG_EXT_PAYLOAD_LSHIFT) == value);
uintptr_t result =
(_OBJC_TAG_EXT_MASK |
((uintptr_t)(tag - OBJC_TAG_First52BitPayload) << _OBJC_TAG_EXT_INDEX_SHIFT) |
((value << _OBJC_TAG_EXT_PAYLOAD_RSHIFT) >> _OBJC_TAG_EXT_PAYLOAD_LSHIFT));
return _objc_encodeTaggedPointer(result);
}
}
static inline uintptr_t
_objc_getTaggedPointerValue(const void * _Nullable ptr)
{
// assert(_objc_isTaggedPointer(ptr));
uintptr_t value = _objc_decodeTaggedPointer(ptr);
uintptr_t basicTag = (value >> _OBJC_TAG_INDEX_SHIFT) & _OBJC_TAG_INDEX_MASK;
if (basicTag == _OBJC_TAG_INDEX_MASK) {
return (value << _OBJC_TAG_EXT_PAYLOAD_LSHIFT) >> _OBJC_TAG_EXT_PAYLOAD_RSHIFT;
} else {
return (value << _OBJC_TAG_PAYLOAD_LSHIFT) >> _OBJC_TAG_PAYLOAD_RSHIFT;
}
}
三、SideTableBuf & StripedMap<SideTable>
1.由于libc调用早于C++初始化，且不想通过一个全局结构体指针这种额外迂回的方式，最终使用了字节对齐的全局静态私有常量SideTableBuf ，即StripedMap<SideTable>。
// We cannot use a C++ static initializer to initialize SideTables because
// libc calls us before our C++ initializers run. We also don't want a global
// pointer to this struct because of the extra indirection.
// Do it the hard way.
alignas(StripedMap<SideTable>) static uint8_t
SideTableBuf[sizeof(StripedMap<SideTable>)];
static void SideTableInit() {
new (SideTableBuf) StripedMap<SideTable>();
}
static StripedMap<SideTable>& SideTables() {
//将SideTableBuf强制转换成StripedMap<SideTable>类型指针
return *reinterpret_cast<StripedMap<SideTable>*>(SideTableBuf);
}
2. 若是非模拟器的iPhone设备上， StripedMap存放8张 SideTable表；非iPhone真机，StripedMap存放64张SideTable表。另外，采用了分片加解锁，以提升效率。通过对对象地址的位运算，可以确定对象引用计数管理信息到底存储在哪张SideTable表里。
enum { CacheLineSize = 64 };
// StripedMap<T> is a map of void* -> T, sized appropriately
// for cache-friendly lock striping.
// For example, this may be used as StripedMap<spinlock_t>
// or as StripedMap<SomeStruct> where SomeStruct stores a spin lock.
template<typename T>
class StripedMap {
#if TARGET_OS_IPHONE && !TARGET_OS_SIMULATOR
enum { StripeCount = 8 };
#else
enum { StripeCount = 64 };
#endif
struct PaddedT {
T value alignas(CacheLineSize);
};
PaddedT array[StripeCount];
//对对象地址做位运算获得数组下标，%StripeCount是防越界保护；其实现了hash作用
static unsigned int indexForPointer(const void *p) {
uintptr_t addr = reinterpret_cast<uintptr_t>(p);
return ((addr >> 4) ^ (addr >> 9)) % StripeCount;
}
public:
//通过对象地址做位运算的下标，获取在array中的值
T& operator[] (const void *p) {
return array[indexForPointer(p)].value;
}
//重载[]，强制转换成StripedMap<T>类型
const T& operator[] (const void *p) const {
return const_cast<StripedMap<T>>(this)[p];
}
// Shortcuts for StripedMaps of locks.对每个SideTable进行分片加锁，提升操作效率
void lockAll() {
for (unsigned int i = 0; i < StripeCount; i++) {
array[i].value.lock();
}
}
//对每个SideTable进行分片解锁
void unlockAll() {
for (unsigned int i = 0; i < StripeCount; i++) {
array[i].value.unlock();
}
}
void forceResetAll() {
for (unsigned int i = 0; i < StripeCount; i++) {
array[i].value.forceReset();
}
}
void defineLockOrder() {
for (unsigned int i = 1; i < StripeCount; i++) {
lockdebug_lock_precedes_lock(&array[i-1].value, &array[i].value);
}
}
void precedeLock(const void *newlock) {
// assumes defineLockOrder is also called
lockdebug_lock_precedes_lock(&array[StripeCount-1].value, newlock);
}
void succeedLock(const void *oldlock) {
// assumes defineLockOrder is also called
lockdebug_lock_precedes_lock(oldlock, &array[0].value);
}
const void *getLock(int i) {
if (i < StripeCount) return &array[i].value;
else return nil;
}
#if DEBUG
StripedMap() {
// Verify alignment expectations.
uintptr_t base = (uintptr_t)&array[0].value;
uintptr_t delta = (uintptr_t)&array[1].value - base;
assert(delta % CacheLineSize == 0);
assert(base % CacheLineSize == 0);
}
#else
constexpr StripedMap() {}
#endif
};
3. SideTable结构比较简单，一个自旋锁，一个引用计数表，一个弱引用表。对于强引用对象引用计数的查询，首先通过位运算下标确定该SideTable，其次，通过对象伪指针作为key查询当前hash map（refcnts）里的引用计数（注意：优化过的ISA，ISA位域里也存储着引用计数）。
// RefcountMap disguises its pointers because we
// don't want the table to act as a root for `leaks`.
//DenseMap<DisguisedPtr<objc_object>,size_t,true>是一个hash map，
//对象伪指针作为key，第一个value为引用计数（注意：优化过的ISA，ISA位域里也存储着引用计数），后一个value是当引用计数为0时否清除该数据。
typedef objc::DenseMap<DisguisedPtr<objc_object>,size_t,true> RefcountMap;
// Template parameters.
enum HaveOld { DontHaveOld = false, DoHaveOld = true };
enum HaveNew { DontHaveNew = false, DoHaveNew = true };
struct SideTable {
//自旋锁
spinlock_t slock;
//引用计数表：对于强引用对象引用计数的查询，首先通过位运算下标确定该SideTable
//其次，通过对象伪指针作为key查询当前hash map（refcnts）里的引用计数。
RefcountMap refcnts;
//弱引用表
weak_table_t weak_table;
SideTable() {
memset(&weak_table, 0, sizeof(weak_table));
}
~SideTable() {
_objc_fatal("Do not delete SideTable.");
}
void lock() { slock.lock(); }
void unlock() { slock.unlock(); }
void forceReset() { slock.forceReset(); }
// Address-ordered lock discipline for a pair of side tables.
template<HaveOld, HaveNew>
static void lockTwo(SideTable *lock1, SideTable *lock2);
template<HaveOld, HaveNew>
static void unlockTwo(SideTable *lock1, SideTable *lock2);
};
四、weak_table & weak_entry
1. weak_table_t是一个全局的弱引用表，单独从结构体的声明看不出来。可以看下插入weak_entry_t的方法。
/**
* The global weak references table. Stores object ids as keys,
* and weak_entry_t structs as their values.
*/
struct weak_table_t {
//hash表
weak_entry_t *weak_entries;
size_t num_entries;
uintptr_t mask;
uintptr_t max_hash_displacement;
};
/**
* Add new_entry to the object's table of weak references.
* Does not check whether the referent is already in the table.
*/
static void weak_entry_insert(weak_table_t *weak_table, weak_entry_t *new_entry)
{
weak_entry_t *weak_entries = weak_table->weak_entries;
assert(weak_entries != nil);
//通过位运算获取初始下标，即hash出下标
size_t begin = hash_pointer(new_entry->referent) & (weak_table->mask);
size_t index = begin;
size_t hash_displacement = 0;
//探测一个不存在weak_entries数组的index作为新插入的下标
while (weak_entries[index].referent != nil) {
index = (index+1) & weak_table->mask;
if (index == begin) bad_weak_table(weak_entries);
hash_displacement++;
}
weak_entries[index] = *new_entry;
weak_table->num_entries++;
if (hash_displacement > weak_table->max_hash_displacement) {
weak_table->max_hash_displacement = hash_displacement;
}
}
2. weak_entry_t结构体里存放着一个弱引用对象本身，另外存放着动态hash数组，一个静态连续数组。数组里存放的是引用该弱引用对象的（伪）指针。若weak_referrer_t不足4个，直接使用静态数组存储，否则使用动态数组存储。
/*
The weak table is a hash table governed by a single spin lock.
An allocated blob of memory, most often an object, but under GC any such
allocation, may have its address stored in a __weak marked storage location
through use of compiler generated write-barriers or hand coded uses of the
register weak primitive. Associated with the registration can be a callback
block for the case when one of the allocated chunks of memory is reclaimed.
The table is hashed on the address of the allocated memory. When __weak
marked memory changes its reference, we count on the fact that we can still
see its previous reference.
So, in the hash table, indexed by the weakly referenced item, is a list of
all locations where this address is currently being stored.
For ARC, we also keep track of whether an arbitrary object is being
deallocated by briefly placing it in the table just prior to invoking
dealloc, and removing it via objc_clear_deallocating just prior to memory
reclamation.
*/
// The address of a __weak variable.
// These pointers are stored disguised so memory analysis tools
// don't see lots of interior pointers from the weak table into objects.
typedef DisguisedPtr<objc_object *> weak_referrer_t;
#if __LP64__
#define PTR_MINUS_2 62
#else
#define PTR_MINUS_2 30
#endif
/**
* The internal structure stored in the weak references table.
* It maintains and stores
* a hash set of weak references pointing to an object.
* If out_of_line_ness != REFERRERS_OUT_OF_LINE then the set
* is instead a small inline array.
*/
#define WEAK_INLINE_COUNT 4
// out_of_line_ness field overlaps with the low two bits of inline_referrers[1].
// inline_referrers[1] is a DisguisedPtr of a pointer-aligned address.
// The low two bits of a pointer-aligned DisguisedPtr will always be 0b00
// (disguised nil or 0x80..00) or 0b11 (any other address).
// Therefore out_of_line_ness == 0b10 is used to mark the out-of-line state.
#define REFERRERS_OUT_OF_LINE 2
struct weak_entry_t {
DisguisedPtr<objc_object> referent;
union {
struct {
//动态hash数组
weak_referrer_t *referrers;
uintptr_t out_of_line_ness : 2;
uintptr_t num_refs : PTR_MINUS_2;
uintptr_t mask;
uintptr_t max_hash_displacement;
};
struct {
// out_of_line_ness field is low bits of inline_referrers[1]
//静态连续数组
weak_referrer_t inline_referrers[WEAK_INLINE_COUNT];
};
};
bool out_of_line() {
return (out_of_line_ness == REFERRERS_OUT_OF_LINE);
}
weak_entry_t& operator=(const weak_entry_t& other) {
memcpy(this, &other, sizeof(other));
return *this;
}
weak_entry_t(objc_object *newReferent, objc_object **newReferrer)
: referent(newReferent)
{
inline_referrers[0] = newReferrer;
for (int i = 1; i < WEAK_INLINE_COUNT; i++) {
inline_referrers[i] = nil;
}
}
};
五、objc_initWeak & storeWeak
1. objc_initWeak调用的是storeWeak。
/**
* Initialize a fresh weak pointer to some object location.
* It would be used for code like:
*
* (The nil case)
* __weak id weakPtr;
* (The non-nil case)
* NSObject *o = ...;
* __weak id weakPtr = o;
*
* This function IS NOT thread-safe with respect to concurrent
* modifications to the weak variable. (Concurrent weak clear is safe.)
*
* @param location Address of __weak ptr.
* @param newObj Object ptr.
*/
id
objc_initWeak(id *location, id newObj)
{
if (!newObj) {
*location = nil;
return nil;
}
//调用storeWeak，传入三个模板值
return storeWeak<DontHaveOld, DoHaveNew, DoCrashIfDeallocating>
(location, (objc_object*)newObj);
}
2. HaveOld表示是否有旧引用，HaveNew表示newObj是否有引用，CrashIfDeallocating表示是否在释放过程中crash。
// Update a weak variable.
// If HaveOld is true, the variable has an existing value
// that needs to be cleaned up. This value might be nil.
// If HaveNew is true, there is a new value that needs to be
// assigned into the variable. This value might be nil.
// If CrashIfDeallocating is true, the process is halted if newObj is
// deallocating or newObj's class does not support weak references.
// If CrashIfDeallocating is false, nil is stored instead.
enum CrashIfDeallocating {
DontCrashIfDeallocating = false, DoCrashIfDeallocating = true
};
//模板声明
template <HaveOld haveOld, HaveNew haveNew,
CrashIfDeallocating crashIfDeallocating>
static id
storeWeak(id *location, objc_object *newObj)
{
//断言判空
assert(haveOld || haveNew);
if (!haveNew) assert(newObj == nil);
Class previouslyInitializedClass = nil;
id oldObj;
SideTable *oldTable;
SideTable *newTable;
// Acquire locks for old and new values.
// Order by lock address to prevent lock ordering problems.
// Retry if the old value changes underneath us.
retry:
//获取旧引用的SideTable
if (haveOld) {
oldObj = *location;
oldTable = &SideTables()[oldObj];
} else {
oldTable = nil;
}
//获取新对象的SideTable
if (haveNew) {
newTable = &SideTables()[newObj];
} else {
newTable = nil;
}
//加锁，以防在释放中
SideTable::lockTwo<haveOld, haveNew>(oldTable, newTable);
if (haveOld && *location != oldObj) {
SideTable::unlockTwo<haveOld, haveNew>(oldTable, newTable);
goto retry;
}
// Prevent a deadlock between the weak reference machinery
// and the +initialize machinery by ensuring that no
// weakly-referenced object has an un-+initialized isa.
if (haveNew && newObj) {
Class cls = newObj->getIsa();
//若还没有完成初始化，则先初始化
if (cls != previouslyInitializedClass &&
!((objc_class *)cls)->isInitialized())
{
SideTable::unlockTwo<haveOld, haveNew>(oldTable, newTable);
class_initialize(cls, (id)newObj);
// If this class is finished with +initialize then we're good.
// If this class is still running +initialize on this thread
// (i.e. +initialize called storeWeak on an instance of itself)
// then we may proceed but it will appear initializing and
// not yet initialized to the check above.
// Instead set previouslyInitializedClass to recognize it on retry.
previouslyInitializedClass = cls;
goto retry;
}
}
// Clean up old value, if any.
//调用weak_unregister_no_lock，清除旧值
if (haveOld) {
weak_unregister_no_lock(&oldTable->weak_table, oldObj, location);
}
// Assign new value, if any.
//向sidetable里插入新值
if (haveNew) {
newObj = (objc_object *)
weak_register_no_lock(&newTable->weak_table, (id)newObj, location,
crashIfDeallocating);
// weak_register_no_lock returns nil if weak store should be rejected
// Set is-weakly-referenced bit in refcount table.
if (newObj && !newObj->isTaggedPointer()) {
newObj->setWeaklyReferenced_nolock();
}
// Do not set *location anywhere else. That would introduce a race.
*location = (id)newObj;
}
else {
// No new value. The storage is not changed.
}
//解锁
SideTable::unlockTwo<haveOld, haveNew>(oldTable, newTable);
return (id)newObj;
}
3. weak_register_no_lock作用为向weak_table_t表注册weak_entry_t，未加锁。
/**
* Registers a new (object, weak pointer) pair. Creates a new weak
* object entry if it does not exist.
*
* @param weak_table The global weak table.
* @param referent The object pointed to by the weak reference.
* @param referrer The weak pointer address.
*/
id
weak_register_no_lock(weak_table_t *weak_table, id referent_id,
id *referrer_id, bool crashIfDeallocating)
{
//对象指针
objc_object *referent = (objc_object *)referent_id;
//weak指针地址
objc_object **referrer = (objc_object **)referrer_id;
//对象指针为空或是伪对象，直接return
if (!referent || referent->isTaggedPointer()) return referent_id;
// ensure that the referenced object is viable
//保证对象是安全可处理的
bool deallocating;
if (!referent->ISA()->hasCustomRR()) {
deallocating = referent->rootIsDeallocating();
}
else {
BOOL (*allowsWeakReference)(objc_object *, SEL) =
(BOOL(*)(objc_object *, SEL))
object_getMethodImplementation((id)referent,
SEL_allowsWeakReference);
if ((IMP)allowsWeakReference == _objc_msgForward) {
return nil;
}
deallocating =
! (*allowsWeakReference)(referent, SEL_allowsWeakReference);
}
//若对象正在释放过程中，crash 或是 return nil根据模板传入的crashIfDeallocating值
if (deallocating) {
if (crashIfDeallocating) {
_objc_fatal("Cannot form weak reference to instance (%p) of "
"class %s. It is possible that this object was "
"over-released, or is in the process of deallocation.",
(void*)referent, object_getClassName((id)referent));
} else {
return nil;
}
}
// now remember it and where it is being stored
weak_entry_t *entry;
//若该对象的weak_entry_t存在于weak_table中，直接append到当前entry
if ((entry = weak_entry_for_referent(weak_table, referent))) {
append_referrer(entry, referrer);
}
else {
//若该对象的weak_entry_t不存在于weak_table中，需要创建一个新weak_entry_t插入到weak_table，并且需要对weak_table做是否扩容的处理
weak_entry_t new_entry(referent, referrer);
weak_grow_maybe(weak_table);
weak_entry_insert(weak_table, &new_entry);
}
// Do not set *referrer. objc_storeWeak() requires that the
// value not change.
return referent_id;
}
4. append_referrer() 给当前weak_entry_t，追加weak指针。
/**
* Add the given referrer to set of weak pointers in this entry.
* Does not perform duplicate checking (b/c weak pointers are never
* added to a set twice).
*
* @param entry The entry holding the set of weak pointers.
* @param new_referrer The new weak pointer to be added.
*/
static void append_referrer(weak_entry_t *entry, objc_object **new_referrer)
{
if (! entry->out_of_line()) {
//尝试向连续数组inline_referrers中插入新对象
// Try to insert inline.
for (size_t i = 0; i < WEAK_INLINE_COUNT; i++) {
if (entry->inline_referrers[i] == nil) {
entry->inline_referrers[i] = new_referrer;
return;
}
}
//若连续数组已满，则将连续数组中的weak_referrer_t放到开辟好的动态数组里
// Couldn't insert inline. Allocate out of line.
weak_referrer_t *new_referrers = (weak_referrer_t *)
calloc(WEAK_INLINE_COUNT, sizeof(weak_referrer_t));
// This constructed table is invalid, but grow_refs_and_insert
// will fix it and rehash it.
for (size_t i = 0; i < WEAK_INLINE_COUNT; i++) {
new_referrers[i] = entry->inline_referrers[i];
}
entry->referrers = new_referrers;
entry->num_refs = WEAK_INLINE_COUNT;
entry->out_of_line_ness = REFERRERS_OUT_OF_LINE;
entry->mask = WEAK_INLINE_COUNT-1;
entry->max_hash_displacement = 0;
}
assert(entry->out_of_line());
//处理扩容逻辑
if (entry->num_refs >= TABLE_SIZE(entry) * 3/4) {
return grow_refs_and_insert(entry, new_referrer);
}
//新追加的weak指针通过hash探测到下标，并插入到当前weak_referrer_t
size_t begin = w_hash_pointer(new_referrer) & (entry->mask);
size_t index = begin;
size_t hash_displacement = 0;
while (entry->referrers[index] != nil) {
hash_displacement++;
index = (index+1) & entry->mask;
if (index == begin) bad_weak_table(entry);
}
if (hash_displacement > entry->max_hash_displacement) {
entry->max_hash_displacement = hash_displacement;
}
weak_referrer_t &ref = entry->referrers[index];
ref = new_referrer;
entry->num_refs++;
}
5. weak_unregister_no_lock()从当前weak_table_t表注销该weak指针。
/**
* Unregister an already-registered weak reference.
* This is used when referrer's storage is about to go away, but referent
* isn't dead yet. (Otherwise, zeroing referrer later would be a
* bad memory access.)
* Does nothing if referent/referrer is not a currently active weak reference.
* Does not zero referrer.
*
* FIXME currently requires old referent value to be passed in (lame)
* FIXME unregistration should be automatic if referrer is collected
*
* @param weak_table The global weak table.
* @param referent The object.
* @param referrer The weak reference.
*/
void
weak_unregister_no_lock(weak_table_t *weak_table, id referent_id,
id *referrer_id)
{
//对象指针
objc_object *referent = (objc_object *)referent_id;
//weak指针地址
objc_object **referrer = (objc_object **)referrer_id;
weak_entry_t *entry;
if (!referent) return;
//从当前weak_table表取出weak_entry_t
if ((entry = weak_entry_for_referent(weak_table, referent))) {
//给当前weak_entry_t中的referrer值置为nil
remove_referrer(entry, referrer);
bool empty = true;
if (entry->out_of_line() && entry->num_refs != 0) {
empty = false;
}
else {
for (size_t i = 0; i < WEAK_INLINE_COUNT; i++) {
if (entry->inline_referrers[i]) {
empty = false;
break;
}
}
}
if (empty) {
//若entry中referrer数量不为0，从当前weak_table移除该entry记录
weak_entry_remove(weak_table, entry);
}
}
// Do not set *referrer = nil. objc_storeWeak() requires that the
// value not change.
}
总结：对于弱引用对象，在底层的存储是一个三级hash表存储结构，其中对象地址作为key，weak指针地址作为value。第一级key为对象地址hash算出的下标，value为SideTable，weak_table_t存储在SideTable中。第二级key为对象地址hash算出的下标，value为weak_entry_t。第三级key为对象地址hash算出的下标（当weak指针地址不在连续数组inline_referrers中），value为weak指针地址。
六、延伸
https://juejin.cn/post/6844903847622606861
http://blog.devtang.com/2014/05/30/understand-tagged-pointer/
https://cloud.tencent.com/developer/article/1620346
http://www.leewong.cn/2020/08/16/sidetablestructure/
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