基本能看懂的C编译器，只有361行！
Fabrice Bellard is a French computer programmer known for writing FFmpeg, QEMU, and the Tiny C Compiler 和许许多多别的。
我修改了下他写的otccelfn.c，增加/删除了一点功能，提高了可读性。它能编译像下面这样的C程序：
gcd(long a, long b) { long i; i = a - b; if (i == 0) return a; else if (i > 0) return gcd(i, b); else return gcd(a, -i); } tonum(long s) { long n, i; for (n = 0; ;s++) { if ((i = *(char*)s) == 0) break; n = n * 10 + (i - '0'); } return n; } main() { long i, j; i = tonum("20"); j = tonum("24"); printf("%ld\n", gcd(i, j)); return 0; }
说明：
- 不支持注释和宏；忽略#include【删除功能以使程序变短】
- 【增加】函数参数类型为long. x64下int是32位，long和void*都是64位，x86下它们都是32位，因此解除了只能-m32的限制。
- 原版：没有类型就是int，64位下char*当int传就错了。
- 数据类型只能用long，但编译器里还叫T_INT.
- 原版和修改版都不支持 op= 如 i += 3; 而且都不支持数组，do和switch等等——太多了列不完。
- Tiny C Compiler功能很完备。wc -l *.c *h 43555行。如果把arm-gen, arm64-asm, i386*, x86_64*等都去掉，改成bytecode和虚拟机，估计3万多行。configure贼快，config.h只有5行，就定义了DIR和VERSION.
- gcd是Greatest Common Divisor (最大公约数)的缩写。
编译器的全部源码：
// Adapted from otccelfn.c (written by Fabrice Bellard) #include <assert.h> #include <stdio.h> #include <ctype.h> #pragma warning(disable:4786) #include <vector> #include <string> #include <map> using namespace std; typedef unsigned char byte; typedef unsigned uint; ///////////////////////// lexer //////////////////////////// FILE* srcfp; enum { T_NUM=1, T_DOUBLE, T_INT='i', T_IF, T_ELSE, T_WHILE, T_FOR, T_BREAK, T_RETURN, T_SYMBOL=128 }; struct opr { uint code; int ws; // weird stuff char ch, ch2; } oprs[] = { // https://en.cppreference.com/w/c/language/operator_precedence { 0x1, 11, '+', '+' }, { 0xff, 11, '-', '-' }, { 0x1, 10, '|', '|' }, { 0x0, 9, '&', '&' }, { 0xc809, 8, '|' }, { 0xc831, 7, '^' }, { 0xc821, 6, '&' }, { 0x4, 5, '=', '=' }, { 0x5, 5, '!', '=' }, { 0xe, 4, '<', '=' }, { 0xd, 4, '>', '=' }, { 0xc, 4, '<' }, { 0xf, 4, '>' }, { 0xe0d391, 3, '<', '<' }, { 0xd391, 3, '>', '>' }, { 0xc801, 2, '+' }, { 0xd8f7c829, 2, '-' }, // f7 d8 is neg %eax; 29 c8 is sub %ecx,%eax { 0xd0f7, 2, '~' }, { 0x4, 2, '!' }, { 0xc1af0f, 1, '*' }, // 0f af c1 imul %ecx, %eax { 0xf9f79991, 1, '/' }, { 0xf9f79991, 1, '%' }, { 0x0, 0, '*' }, // typecast }; typedef map<string, int> Str2N; Str2N keywords; struct Symbols { vector<int> addr; // address = [id] Str2N str2n; // id = [name] int add(const string& name) { int id; Str2N::const_iterator p = str2n.find(name); if (p == str2n.end()) { str2n[name] = id = addr.size(); addr.push_back(0); } else id = p->second; return id; } int& operator[](int id) { return addr[id]; } int& operator[](const string& name) { return addr[str2n[name]]; } } symbols; char _ch; int tk, tkval, tkws; string tkstr; inline bool is_alnum(int c) { return c == '_' || isalnum(c); } inline int read_char() { if ((_ch = getc(srcfp)) == '#') do _ch = getc(srcfp); while (_ch != '\n'); return _ch; } int read_char_unesc() { if (_ch != '\\') return _ch; if (read_char() == 'n') return _ch = '\n'; else assert(!"correct escape sequence"); return 0; } void read_token() { // _ch should already be read tk = tkval = tkws = 0; tkstr = ""; // clear token while (isspace(_ch)) read_char(); // skip white spaces if (is_alnum(_ch)) { do tkstr += _ch; while (is_alnum(read_char())); if (isdigit(tkstr[0])) { tk = T_NUM; tkval = atoi(tkstr.c_str()); return; } // NUMBER Str2N::iterator p = keywords.find(tkstr); if (p != keywords.end()) { tk = p->second; return; } // KEYWORD tk = T_SYMBOL; tkval = symbols.add(tkstr); return; // SYMBOL } char c = _ch; read_char(); // check two chars: c, _ch if (c == '\'') { // '0' ' ' etc tk = T_NUM; // they are also numbers, e.g ' ' is 32 tkstr = tkval = read_char_unesc(); if (read_char() != '\'') assert(!" is '\''"); read_char(); return; // NUMBER } tkstr = tk = c; for (const opr* p = oprs; p < &oprs[sizeof(oprs) / sizeof(oprs[0])]; ++p) { if (p->ch == c && (p->ch2 == _ch || !p->ch2)) { tkval = p->code; tkws = p->ws; if (p->ch2 == _ch) { tk = T_DOUBLE; tkstr += _ch; read_char(); return; } // ++ -- == etc break; } } // ( ) + * and so on. parser handles strings } int next_tk() { read_token(); return tk; if (tk == EOF) printf("EOF\n"); else if (tk == T_NUM) printf("%d\n", tkval); else if (tk == T_DOUBLE) printf("%s\n", tkstr.c_str()); else if (tk >= T_SYMBOL) printf(".%d `%s'\n", tkval, tkstr.c_str()); else if (tk >= T_INT) printf("%s\n", tkstr.c_str()); else printf("%c\n", tk); return tk; } #define curr_must_be(c) (assert(tk == c), next_tk()) #define next_must_be(c) (next_tk(), assert(tk == c), next_tk()) ///////////////////////// yacc/yuck? :-) //////////////////////////// #define STARTUP_CODE_SIZE 7 // startup code calls main and then exit #define FIRST_GLOBAL_VAR_ADDR 4 // don't use 0 -- what about NULL? #define DATA_SECTION_START 0x6800 // loader puts the data section here byte code[4096], data[4096]; uint code_end = STARTUP_CODE_SIZE, data_end = FIRST_GLOBAL_VAR_ADDR; void modify32(uint i, int n) { // code[i] = n; little endian byte* p = code + i; *p = n; p[1] = n >> 8; p[2] = n >> 16; p[3] = n >> 24; } void out_4(uint n) { byte* p = code + code_end; *p = n; p[1] = n >> 8; p[2] = n >> 16; p[3] = n >> 24; code_end += 4; } void out_vl(uint n) { for (;n;n >>= 8) code[code_end++] = n; } // variable length uint gen(uint a, uint n) { out_vl(a); uint r = code_end; out_4(n); return r; } void gen_load_imm(uint x) { gen(0xb8, x); } // b8 ?? ?? ?? ?? mov $0x0,%eax inline uint gen_jmp(uint to) { return gen(0xe9, to); } void patch_jmp(uint i) { while (i) { // it's a chain! const byte* p = code + i; uint next = *p | (p[1] << 8) | (p[2] << 16) | (p[3] << 24); modify32(i, code_end - i - 4); i = next; } } uint gen_je_or_jne(int i, int to) { // i: 0=je, 1=jne out_vl(0x0fc085); // 85 c0 test %eax, %eax; 0f 84 ?? ?? ?? ?? je return gen(0x84 + i, to); } void gen_cmp(int i) { out_vl(0xc139); // 39 c1 cmp %eax,%ecx out_vl(0xc031); // 31 c0 xor %eax, %eax out_vl(0x0f); // 0f 94 c0 sete %al ; set_if_equal out_vl(0x90 + i); // 0f 9f c0 setg %al ; set_if_greater out_vl(0xc0); // 85 c0 test %eax,%eax } // 83 85 08 00 00 00 01 addl $0x1,0x8(%ebp) // 89 85 fc ff ff ff mov %eax,-0x4(%ebp // 8b 85 08 00 00 00 mov 0x8(%ebp),%eax // 8d 85 ?? ?? ?? ?? https://pdos.csail.mit.edu/6.828/2006/readings/i386/LEA.htm void gen_add_mov_lea(int i, int addr) { out_vl(i + 0x83); gen(0x85, addr); } int ret_point, local; void _expr(int); #define do_expr() _expr(11) // 11 is ++ and -- // deal with i= a- b; i== 0) i> gcd( i, b) a, -i and so on void do_2_tokens(bool do_assign) { if (tk == '\"') { // string gen_load_imm(DATA_SECTION_START + data_end); while (_ch != '\"') { read_char_unesc(); data[data_end++] = _ch; read_char(); } data[data_end] = 0; data_end = (data_end + 4) & ~3; // align heap -4=~3 read_char(), next_tk(); return; } // save useful information of current token before calling next_tk int saddr = (tk == T_SYMBOL) ? symbols[tkval] : 0; int tt = tk; // token ype int tkv = tkval; // token value int ws = tkws; string tks = tkstr; next_tk(); //printf("2 tokens: %s%s\n", tks.c_str(), tkstr.c_str()); if (tt == T_NUM) gen_load_imm(tkv); else if (ws == 2) { // + - ! ~ do_2_tokens(false); gen(0xb9, 0); // movl $0, %ecx if (tt == '!') // e.g. !i, to know !i is to know whether i equals 0 gen_cmp(tkv); // that is to compare i and 0, and that's substraction else out_vl(tkv); } else if (tt == '(') { do_expr(); curr_must_be(')'); } else if (tt == '*') { // typecast e.g. *(char*) curr_must_be('('), next_tk(), next_tk(); curr_must_be(')'); do_2_tokens(false); if (tk == '=') { next_tk(); out_vl(0x50); // push %eax do_expr(); out_vl(0x59); // pop %ecx out_vl(0x0188 + (tt == T_SYMBOL)); // movl %eax/%al, (%ecx) } else if (tt) { if (tt == T_INT) out_vl(0x8b); // mov (%eax), %eax else { out_vl(0xbe0f); code[code_end++] = 0; } // movsbl (%eax),%eax } } else if (tt == '&') { // lea imm(%ebp) %eax gen_add_mov_lea(10, saddr); next_tk(); } else if (tk == '(') { // function call int subesp = gen(0xec81, 0); // 81 ec 00 00 00 00 sub $0, %esp (to be modified) next_tk(); int i = 0; for (;tk != ')'; i += 4) { do_expr(); gen(0x248489, i); // 89 84 24 ?? ?? ?? ?? movl %eax, xxx(%esp) if (tk == ',') next_tk(); } modify32(subesp, i); // modify jmp sub $xxx, %esp curr_must_be(')'); gen(0xe8, saddr - code_end - 5); // call if (i) gen(0xc481, i); // add $xxx, %esp } else if (tk == '=' && do_assign) { next_tk(); do_expr(); gen_add_mov_lea(6, saddr); // mov %eax, effective_address } else if (tk != '(') { // variable gen_add_mov_lea(8, saddr); /* mov EA, %eax */ if (tkws == 11) { // ++ -- tkval is instruction gen_add_mov_lea(0, saddr); out_vl(tkval); next_tk(); } } } void _expr(int what) { if (what-- == 1) do_2_tokens(true); // * / % else { _expr(what); uint pos = 0; int tkv = tkval; while (what == tkws) { int tt = tk; tkv = tkval; next_tk(); if (what > 8) { // && || -- ++ pos = gen_je_or_jne(tkv, pos); _expr(what); } else { out_vl(0x50); _expr(what); out_vl(0x59); // push %eax; eval; pop %ecx if (what == 4 || what == 5) gen_cmp(tkv); // > < >= <= != == else { // << >> + - ~ out_vl(tkv); // _expr(2) - _expr(1) - do_2_tokens(true), then here if (tt == '%') out_vl(0x92); // xchg %edx, %eax } } } if (pos && what > 8) { // && || pos = gen_je_or_jne(tkv, pos); gen_load_imm(tkv ^ 1); gen_jmp(5); // jmp $ + 5 patch_jmp(pos); gen_load_imm(tkv); } } } uint do_test_expr() { do_expr(); return gen_je_or_jne(0, 0); } // je 00 00 00 00 void do_block(uint* plevel) { uint pos, p2, p3, tt; if (tk == T_IF) { next_must_be('('); pos = do_test_expr(); curr_must_be(')'); do_block(plevel); if (tk == T_ELSE) { next_tk(); p2 = gen_jmp(0); patch_jmp(pos); do_block(plevel); patch_jmp(p2); } else patch_jmp(pos); } else if (tk == T_WHILE || tk == T_FOR) { tt = tk; next_must_be('('); if (tt == T_WHILE) { p2 = code_end; pos = do_test_expr(); } else { // for (i=0; i<9; i++) {} if (tk != ';') do_expr(); // for (;i<9 ... curr_must_be(';'); p2 = code_end; pos = 0; if (tk != ';') pos = do_test_expr(); // for(;;i++ ... curr_must_be(';'); if (tk != ')') { // for(;;i++) p3 = gen_jmp(0); do_expr(); gen_jmp(p2 - code_end - 5); patch_jmp(p3); p2 = p3 + 4; } } curr_must_be(')'); do_block(&pos); gen_jmp(p2 - code_end - 5); patch_jmp(pos); } else if (tk == '{') { // declaration for (next_tk(); tk == T_INT;) { // int i; int j, k; for (next_tk(); tk == T_SYMBOL;) { symbols[tkval] = -(local += 4); if (next_tk() == ',') next_tk(); } curr_must_be(';'); // TODO: `int ;' } while(tk != '}') do_block(plevel); next_tk(); } else { if (tk == T_RETURN) { if (next_tk() != ';') do_expr(); ret_point = gen_jmp(ret_point); } else if (tk == T_BREAK) { next_tk(); *plevel = gen_jmp(*plevel); } else if (tk != ';') do_expr(); curr_must_be(';'); } } void do_func() { symbols[tkval] = code_end; // put function address int n = 8; // stack: LOW ... ebp ret_addr params HIGH next_must_be('('); while (tk != ')') { // read param name and compute offset curr_must_be(T_INT); assert(tk == T_SYMBOL); symbols[tkval] = n; n += 4; if (next_tk() == ',') next_tk(); } next_tk(); out_vl(0xe58955); // 55 push %ebp; 89 e5 mov %esp, %ebp uint pos = gen(0xec81, 0); // sub imm, %esp ret_point = local = 0; do_block(0); patch_jmp(ret_point); out_vl(0xc3c9); // c9 leave; c3 ret modify32(pos, local); // save local variables } void save_bin_files() { FILE* fp; fp = fopen("data.bin", "wb"); fwrite((void*)data, 1, data_end, fp); fclose(fp); int csize = code_end; code_end = 0; // add the startup code out_vl(0xe8); out_4(symbols["main"] - 5); // e8 ?? ?? ?? ?? call out_vl(0x80cd); // int $0x80 fp = fopen("code.bin", "wb"); fwrite((void*)code, 1, csize, fp); fclose(fp); } int main() { srcfp = fopen("gcd.c", "rt"); assert(srcfp); Str2N& kw = keywords; kw["long"]=T_INT; kw["if"]=T_IF; kw["else"]=T_ELSE; kw["while"]=T_WHILE; kw["for"]=T_FOR; kw["break"]=T_BREAK; kw["return"]=T_RETURN; read_char(), next_tk(); // parser wants a token; lexer wants a char while (tk != EOF) do_func(); fclose(srcfp), save_bin_files(); return 0; }
编译器里：srcfp = fopen("gcd.c", "rt"); assert(srcfp); 这当然很好改。
编译器的输出是code.bin和data.bin，也写死了，也好改。code.bin里是x86 32位机器码，在Linux下，可用objdump反汇编查看：
objdump -b binary -m i386 -D code.bin
如：
0: e8 18 00 00 00 call 0x1d 5: cd 80 int $0x80 7: 55 push %ebp 8: 89 e5 mov %esp,%ebp a: 81 ec 00 00 00 00 sub $0x0,%esp 10: b8 03 00 00 00 mov $0x3,%eax 15: 89 85 08 00 00 00 mov %eax,0x8(%ebp) 1b: c9 leave 1c: c3 ret 1d: 55 push %ebp 1e: 89 e5 mov %esp,%ebp 20: 81 ec 04 00 00 00 sub $0x4,%esp 26: b8 04 00 00 00 mov $0x4,%eax 2b: 89 85 fc ff ff ff mov %eax,-0x4(%ebp) 31: 81 ec 04 00 00 00 sub $0x4,%esp 37: 8b 85 fc ff ff ff mov -0x4(%ebp),%eax 3d: 89 84 24 00 00 00 00 mov %eax,0x0(%esp) 44: e8 be ff ff ff call 0x7 49: 81 c4 04 00 00 00 add $0x4,%esp 4f: c9 leave 50: c3 ret
下面是个玩具虚拟机的全部代码(97行)：
#include <stdio.h> #include <stdlib.h> typedef unsigned char byte; byte mem[65536]; void load(int i) { FILE* fp = fopen(i ? "data.bin" : "code.bin", "rb"); fseek(fp, 0, SEEK_END); int n = ftell(fp); fseek(fp, 0, SEEK_SET); fread(mem + i, 1, n, fp); fclose(fp); } int r32(byte* p) { return *p | (p[1] << 8) | (p[2] << 16) | (p[3] << 24); } // little endian int r32(int a) { return r32(mem + a); } void w32(int a, int v) { *((int*)&mem[a]) = v; } int reg[16]; #define eax reg[0] #define ecx reg[2] #define edx reg[3] #define res reg[10] #define eip reg[11] #define ebp reg[14] #define esp reg[15] void set_al(int c) { eax = (eax & 0xffffff00) | (c ? 1 : 0); } void print() { printf("%3x: %2x | ax %8x cx %8x dx %4x bp %4x sp %4x | -4: ", eip, mem[eip], eax, ecx, edx, ebp, esp); for (int j = esp - 4; j < esp + 12; j++) printf(mem[j] ? "%02x " : " ", mem[j]); puts("+c"); //getchar(); } void quit() { printf("Press Enter to quit."); getchar(); exit(0); } #define _(i) break; int main() { load(0), load(0x6800); for (esp = 65532;;) { int oip = eip, i, j; byte op = mem[eip], *p = &mem[eip]; print(); switch (op) { case 0x01: eax += ecx; eip += 2; _(add %ecx %eax) case 0x0f: if (p[1] == 0x84) { eip += 6; if (res == 0) eip += r32(p+2); } else if (p[1] == 0x9f) { set_al(res > 0); eip += 3; } else if (p[1] == 0x94) { set_al(res == 0); eip += 3; } else if (p[1] == 0xaf) { eax *= ecx; eip += 3; } else if (p[1] == 0xbe) { eax = char(mem[eax]); eip += 3; } break; case 0x29: eax -= ecx; eip += 2; _(sub %ecx %eax) case 0x31: eax ^= eax; eip += 2; _(xor %eax %eax) case 0x39: res = ecx - eax; eip += 2; _(cmp %eax %ecx) case 0x50: w32(esp -= 4, eax); ++eip; _(push %eax) case 0x55: w32(esp -= 4, ebp); ++eip; _(push %ebp) case 0x59: ecx = r32(esp); esp += 4; ++eip; _(pop %ecx) case 0x81: if (p[1] == 0xec) { esp -= r32(p+2); eip += 6; } else if (p[1] == 0xc4) { esp += r32(p+2); eip += 6; } break; case 0x83: i = r32(p+2)+ebp; w32(i, r32(i)+p[6]); eip += 7; _(addl imm imm(%ebp)) case 0x85: res = eax & eax; eip += 2; _(test %eax %eax) case 0x89: if (p[1] == 0xe5) { ebp = esp; eip += 2; } // mov %esp,%ebp else if (p[1] == 0x84) { w32(esp + r32(p+3), eax); eip += 7; } // mov %eax,imm(%esp) else if (p[1] == 0x85) { w32(ebp + r32(p+2), eax); eip += 6; } // mov %eax,imm(%ebp) break; case 0x8b: if (p[1] == 0x85) { eax = r32(ebp + r32(p+2)); eip += 6; } // mov imm(%ebp),%eax break; case 0x8d: eax = r32(p+2) + ebp; eip += 6; _(lea imm(%ebp) %eax) case 0x91: i = eax; eax = ecx; ecx = i; ++eip; _(xchg %eax %ecx) case 0x92: i = eax; eax = edx; edx = i; ++eip; _(xchg %eax %edx) // cltd converts the signed long in EAX to a signed double long in EDX:EAX case 0x99: edx = (eax < 0) ? -1 : 0; ++eip; _(cltd) case 0xb8: eax = r32(p+1); eip += 5; _(mov imm %eax) case 0xb9: ecx = r32(p+1); eip += 5; _(mov imm %ecx) case 0xc3: { eip = r32(esp); esp += 4; } _(ret) case 0xc9: { ebp = r32(esp = ebp); esp += 4; ++eip;} _(leave) case 0xcd: quit(); case 0xe8: eip += 5; // return to next instruction j = eip + (i = r32(p + 1)); // callee: stack: LOW local_vars ebp ret_addr param1 param2 HIGH // caller: sub $8, %esp; movl %eax, 0(%esp); movl %eax, 4(%esp); add $8, %esp // cdecl: caller cleans stack; fastcall, pascal, WINAPI if (!j) puts(""), printf((char*)&mem[r32(esp)], r32(esp + 4)), puts(""); else { w32(esp -= 4, eip); eip += i; } break; case 0xe9: eip += 5 + r32(p + 1); _(jmp) case 0xf7: if (p[1] == 0xd8) eax = -eax; // neg %eax else if (p[1] == 0xf9 && ecx) { edx = eax % ecx; eax /= ecx; } // idiv %ecx eip += 2; break; } if (oip == eip) quit(); } return 0; }
它读入code.bin和data.bin并解释执行，输出像这样：
205: e8 | ax 4 cx 0 dx 0 bp fff4 sp ffe4 | -4: 18 0c 68 04 18 +c 4 20a: 81 | ax 4 cx 0 dx 0 bp fff4 sp ffe4 | -4: 18 0c 68 04 18 +c 210: b8 | ax 4 cx 0 dx 0 bp fff4 sp ffec | -4: 04 18 14 +c 215: e9 | ax 0 cx 0 dx 0 bp fff4 sp ffec | -4: 04 18 14 +c 21a: c9 | ax 0 cx 0 dx 0 bp fff4 sp ffec | -4: 04 18 14 +c 21b: c3 | ax 0 cx 0 dx 0 bp 0 sp fff8 | -4: 05 +c 5: cd | ax 0 cx 0 dx 0 bp 0 sp fffc | -4: 05 +c
- 以上代码都既可以用Visual C++ 6，又可以用相当新的gcc/g++ (如version 10.2.1 20210110)编译。
- 有个bug: printf("%ld\n", 2+3-9); 输出结果不对，正在排查。
- (2+3)-9是对的。如果我是老师就好了，可以当作业布置给学生：你说你看懂了？fix the bug to prove it. :-)
- 2+3*4, 2*3+4, 2*(3+4)都是对的。原版是如何处理运算符优先级的，我没看明白。大胆地怀疑下原版就有处理同优先级运算符不对这个bug.
- 实验表明原版没错，是我改坏了，已改正。
- _expr里while (what == tkws)改成if (what == tkws)是我改坏的，结果好像是数不对局部变量的个数，已改正。
- 如何改成用bison？生成语法树先？总不能YYSTYPE里有个vector放已生成的代码吧？gawk用的是链表。C++里vector赋值，可以swap而不是复制一遍。还可以用vector<byte>*或realloc？
- 有些非科班生鄙视科班生，说他们只会写九九乘法表，不会用C#写私服；有些科班生鄙视非科班生，说他们基础不牢。
I challenge both to: 把do_2_tokens和_expr改成书上那种标准的自顶向下 :-)
就像Brian W. Kernighan在The UNIX Programming Environment里写的那样。 - cpp自顶向下+bison自底向上混合型实现(不支持VC6)
