wat.c

#include <stdio.h>

typedef struct {} a;
typedef struct { a a1; a a2; } b;

int main(int argc, char * argv[])
{
  a as[999999];

  printf("%d\n", sizeof(b) == sizeof(a));
  printf("%zu\n", sizeof(as));

  int i;
  for(i = 0; i < sizeof(as) / sizeof(as[0]); i++)
  {
    puts("wat");
  }

  return 0;
}

/* $ gcc wat.c -O0 -o wat_
 * $ ./wat_ 
 * 1
 * 0
 * wat
 */

Oops. Ignore the last one. That's the -O1 output:

Shiny:~ johnvanenk$ otool -vtdI wat_
wat_:
Indirect symbols for (__TEXT,__stubs) 3 entries
address            index name
0x0000000100000f18     8 _exit
0x0000000100000f1e     9 _printf
0x0000000100000f24    10 _puts
Indirect symbols for (__DATA,__nl_symbol_ptr) 2 entries
address            index name
0x0000000100001028    11 dyld_stub_binder
0x0000000100001030 ABSOLUTE
Indirect symbols for (__DATA,__la_symbol_ptr) 3 entries
address            index name
0x0000000100001038     8 _exit
0x0000000100001040     9 _printf
0x0000000100001048    10 _puts
(__TEXT,__text) section
start:
0000000100000e50    pushq   $0x00
0000000100000e52    movq    %rsp,%rbp
0000000100000e55    andq    $0xf0,%rsp
0000000100000e59    movq    0x08(%rbp),%rdi
0000000100000e5d    leaq    0x10(%rbp),%rsi
0000000100000e61    movl    %edi,%edx
0000000100000e63    addl    $0x01,%edx
0000000100000e66    shll    $0x03,%edx
0000000100000e69    addq    %rsi,%rdx
0000000100000e6c    movq    %rdx,%rcx
0000000100000e6f    jmp 0x100000e75
0000000100000e71    addq    $0x08,%rcx
0000000100000e75    cmpq    $0x00,(%rcx)
0000000100000e79    jne 0x100000e71
0000000100000e7b    addq    $0x08,%rcx
0000000100000e7f    callq   0x100000e90
0000000100000e84    movl    %eax,%edi
0000000100000e86    callq   0x100000f18
0000000100000e8b    hlt
0000000100000e8c    nop
0000000100000e8d    nop
0000000100000e8e    nop
0000000100000e8f    nop
_main:
0000000100000e90    pushq   %rbp
0000000100000e91    movq    %rsp,%rbp
0000000100000e94    subq    $0x30,%rsp
0000000100000e98    movq    %rsi,%rax
0000000100000e9b    movl    %edi,%ecx
0000000100000e9d    movl    %ecx,0xfc(%rbp)
0000000100000ea0    movq    %rax,0xf0(%rbp)
0000000100000ea4    movl    $0x00000001,%eax
0000000100000ea9    xorb    %cl,%cl
0000000100000eab    leaq    0x000000a8(%rip),%rdx
0000000100000eb2    movq    %rdx,%rdi
0000000100000eb5    movl    %eax,%esi
0000000100000eb7    movb    %cl,%al
0000000100000eb9    callq   0x100000f1e
0000000100000ebe    xorl    %ecx,%ecx
0000000100000ec0    xorb    %dl,%dl
0000000100000ec2    leaq    0x00000095(%rip),%rsi
0000000100000ec9    movq    %rsi,%rdi
0000000100000ecc    movq    %rcx,%rsi
0000000100000ecf    movb    %dl,%al
0000000100000ed1    callq   0x100000f1e
0000000100000ed6    movl    $0x00000000,0xdc(%rbp)
0000000100000edd    jmp 0x100000ef7
0000000100000edf    leaq    0x0000007d(%rip),%rax
0000000100000ee6    movq    %rax,%rdi
0000000100000ee9    callq   0x100000f24
0000000100000eee    movl    0xdc(%rbp),%eax
0000000100000ef1    addl    $0x01,%eax
0000000100000ef4    movl    %eax,0xdc(%rbp)
0000000100000ef7    movl    0xdc(%rbp),%eax
0000000100000efa    movslq  %eax,%rax
0000000100000efd    cmpq    %rcx,%rax
0000000100000f00    jb  0x100000edf
0000000100000f02    movl    $0x00000000,0xe8(%rbp)
0000000100000f09    movl    0xe8(%rbp),%eax
0000000100000f0c    movl    %eax,0xec(%rbp)
0000000100000f0f    movl    0xec(%rbp),%eax
0000000100000f12    addq    $0x30,%rsp
0000000100000f16    popq    %rbp
0000000100000f17    ret

loop fragment:

{{{

0000000100000edd jmp 0x100000ef7
0000000100000edf leaq 0x0000007d(%rip),%rax
0000000100000ee6 movq %rax,%rdi
0000000100000ee9 callq 0x100000f24
0000000100000eee movl 0xdc(%rbp),%eax
0000000100000ef1 addl $0x01,%eax
0000000100000ef4 movl %eax,0xdc(%rbp)
0000000100000ef7 movl 0xdc(%rbp),%eax
0000000100000efa movslq %eax,%rax
0000000100000efd cmpq %rcx,%rax
0000000100000f00 jb 0x100000edf
}}}

sorry, forgot how to do github

0000000100000edd    jmp 0x100000ef7
0000000100000edf    leaq    0x0000007d(%rip),%rax
0000000100000ee6    movq    %rax,%rdi
0000000100000ee9    callq   0x100000f24
0000000100000eee    movl    0xdc(%rbp),%eax
0000000100000ef1    addl    $0x01,%eax
0000000100000ef4    movl    %eax,0xdc(%rbp)
0000000100000ef7    movl    0xdc(%rbp),%eax
0000000100000efa    movslq  %eax,%rax
0000000100000efd    cmpq    %rcx,%rax
0000000100000f00    jb  0x100000edf

0xdc(%rbp) is the loop counter, temporarily stashed in %rax / %eax
%ecx is the loop limit, set to zero at ..ebe
branch enters at ..ef7 and loop body at ..edf
cmpq at ..efd sets CF 0 and ZF 0, jb at ..f00 should NOT branch. does this particular compilation emit the "wat"?