When I compile hello.c with: clang -g -O0 hello.c -o hello, and disassemble it in GDB, I get the contents of hello.s.
The line that confuses me is <main+34>. It seems that when optimizations are off, clang saves unused return values on the stack. <main+34> is saving the return value of printf even though hello.c doesn't do anything with it at all. I assume there's a reason for this. Why does it happen?
| int main() | |
| { | |
| printf("Hello, world!"); | |
| return 0; | |
| } |
| 0x0000000100000f10 <main+0>: push %rbp | |
| 0x0000000100000f11 <main+1>: mov %rsp,%rbp | |
| 0x0000000100000f14 <main+4>: sub $0x10,%rsp | |
| 0x0000000100000f18 <main+8>: lea 0x3f(%rip),%rdi # 0x100000f5e | |
| 0x0000000100000f1f <main+15>: movl $0x0,-0x4(%rbp) | |
| 0x0000000100000f26 <main+22>: mov $0x0,%al | |
| 0x0000000100000f28 <main+24>: callq 0x100000f3e <dyld_stub_printf> | |
| 0x0000000100000f2d <main+29>: mov $0x0,%ecx | |
| 0x0000000100000f32 <main+34>: mov %eax,-0x8(%rbp) | |
| 0x0000000100000f35 <main+37>: mov %ecx,%eax | |
| 0x0000000100000f37 <main+39>: add $0x10,%rsp | |
| 0x0000000100000f3b <main+43>: pop %rbp | |
| 0x0000000100000f3c <main+44>: retq |