0cc

C compiler

Usage

Build

make

and run...

./0cc '<C code>'

Test

make test

What I did

test1.c

int main() {
    return 42;
}

Compile this to test1 & Try objdump

$ gcc test1.c -o test1
$ objdump -d -M intel ./test1

./test1:     ファイル形式 mach-o-x86-64


セクション .text の逆アセンブル:

0000000100000fa5 <_main>:
   100000fa5:	55                   	push   rbp
   100000fa6:	48 89 e5             	mov    rbp,rsp
   100000fa9:	b8 2a 00 00 00       	mov    eax,0x2a
   100000fae:	5d                   	pop    rbp
   100000faf:	c3                   	ret

test1.c is equal to test2.s(Assembly file)

.intel_syntax noprefix
.global _main
_main:
        mov rax, 42
        ret

Try simple function.

test3.c

int plus(int x, int y) {
    return x + y;
}

int main() {
    return plus(20, 22);
}

Objdump this

$ gcc test3.c -o test3
$ objdump -d -M intel ./test3

./test3:     ファイル形式 mach-o-x86-64


セクション .text の逆アセンブル:

0000000100000f4f <_plus>:
   100000f4f:   55                      push   rbp
   100000f50:   48 89 e5                mov    rbp,rsp
   100000f53:   89 7d fc                mov    DWORD PTR [rbp-0x4],edi
   100000f56:   89 75 f8                mov    DWORD PTR [rbp-0x8],esi
   100000f59:   8b 55 fc                mov    edx,DWORD PTR [rbp-0x4]
   100000f5c:   8b 45 f8                mov    eax,DWORD PTR [rbp-0x8]
   100000f5f:   01 d0                   add    eax,edx
   100000f61:   5d                      pop    rbp
   100000f62:   c3                      ret

0000000100000f63 <_main>:
   100000f63:   55                      push   rbp
   100000f64:   48 89 e5                mov    rbp,rsp
   100000f67:   be 16 00 00 00          mov    esi,0x16
   100000f6c:   bf 14 00 00 00          mov    edi,0x14
   100000f71:   e8 d9 ff ff ff          call   100000f4f <_plus>
   100000f76:   5d                      pop    rbp
   100000f77:   c3                      ret

Write assembly equal to test3.c

.intel_syntax noprefix
.global plus, _main

plus:
        add rsi, rdi
        mov rax, rsi
        ret

_main:
        mov rdi, 20
        mov rsi, 22
        call plus
        ret

Register name

rax, rsi, rdi, rsp, rbp: 64-bit register names

---

If a register name has r prefix, it means 64-bit register. On the other hand, if one has e prefix, it means 32-bit register.

If a register name is like r?x(? filled with a, b, c or d), it is called General Purpose Register.

• rax is Accumlator
• rbx is Base index: used for arrays
• rcx is Counter: used for loops and strings
• rdx is used to extend the precision of the accumlator

Registers named rsi or rdi are called Index Registers:

• rsi is specially called Source Index for string operations
• rdi is specially called Destination Index for string operations

When to compile a C function to an assembly, the first argument should be passed on to rdi, and the second argument to rsi.

• rsp is Stack pointer for the stack top address
• rbp is Stack base pointer to hold the current stack frame

References:

• https://en.wikipedia.org/wiki/X86#64-bit

• https://en.wikipedia.org/wiki/X86#Structure

0cc

0cc.c

#include <stdio.h>
#include <stdlib.h>

int main(int argc, char **argv) {
    if (argc != 2) {
        fprintf(stderr, "Wrong number of arguments.\n");
        return 1;
    }

    printf(".intel_syntax noprefix\n");
    printf(".global _main\n");
    printf("_main:\n");
    printf("    mov rax, %d\n", atoi(argv[1]));
    printf("    ret\n");

    return 0;
}

Compile it & Pass any integer as an argument.

$ gcc 0cc.c -o 0cc
$ ./0cc 42 > tmp.s
$ cat tmp.s
.intel_syntax noprefix
.global _main
_main:
    mov rax, 42
    ret
$ gcc tmp.s -o tmp
$ ./tmp
$ echo $?
42

References

• 低レイヤを知りたい人のためのCコンパイラ作成入門