clang-misalignment-example.c

#include <stdio.h>

void func(void)
{
    int a[100];
    int n, m = 0;
    scanf("%d", &n);
    for (int i = 0; i < n; ++i)
    {
        scanf("%d", a + i);
        if (a[i] > m) m = a[i];
    }

    printf("max is %d\n", m);
}

/** How to compile
$ clang-5.0 -c -O2 a.c -mstack-alignment=16

 tested on Ubuntu 16.04
*/

/**
From disassemble result below:

   0:   55                      push   rbp
   1:   41 56                   push   r14
   3:   53                      push   rbx
   4:   48 81 ec a0 01 00 00    sub    rsp,0x1a0

Because first 3 `push`es shall push 8x3 = 24 bytes to the stack,
and `sub` will subtract 16-aligned value from the stack pointer,
then the stack pointer shall become to be mis-aligned.
*/

/** All disassemble result

0000000000000000 <func>:
   0:   55                      push   rbp
   1:   41 56                   push   r14
   3:   53                      push   rbx
   4:   48 81 ec a0 01 00 00    sub    rsp,0x1a0
   b:   45 31 f6                xor    r14d,r14d
   e:   48 8d 74 24 0c          lea    rsi,[rsp+0xc]
  13:   bf 00 00 00 00          mov    edi,0x0
  18:   31 c0                   xor    eax,eax
  1a:   e8 00 00 00 00          call   1f <func+0x1f>
  1f:   83 7c 24 0c 00          cmp    DWORD PTR [rsp+0xc],0x0
  24:   7e 33                   jle    59 <func+0x59>
  26:   48 8d 5c 24 10          lea    rbx,[rsp+0x10]
  2b:   31 ed                   xor    ebp,ebp
  2d:   45 31 f6                xor    r14d,r14d
  30:   bf 00 00 00 00          mov    edi,0x0
  35:   31 c0                   xor    eax,eax
  37:   48 89 de                mov    rsi,rbx
  3a:   e8 00 00 00 00          call   3f <func+0x3f>
  3f:   8b 03                   mov    eax,DWORD PTR [rbx]
  41:   44 39 f0                cmp    eax,r14d
  44:   44 0f 4d f0             cmovge r14d,eax
  48:   48 ff c5                inc    rbp
  4b:   48 63 44 24 0c          movsxd rax,DWORD PTR [rsp+0xc]
  50:   48 83 c3 04             add    rbx,0x4
  54:   48 39 c5                cmp    rbp,rax
  57:   7c d7                   jl     30 <func+0x30>
  59:   bf 00 00 00 00          mov    edi,0x0
  5e:   31 c0                   xor    eax,eax
  60:   44 89 f6                mov    esi,r14d
  63:   e8 00 00 00 00          call   68 <func+0x68>
  68:   48 81 c4 a0 01 00 00    add    rsp,0x1a0
  6f:   5b                      pop    rbx
  70:   41 5e                   pop    r14
  72:   5d                      pop    rbp
  73:   c3                      ret    
*/