Accessing out-of-bounds index with and without optimization - c

Question

When I run the following C code, I get different output depending on whether or not the code was run with optimization turned on (gcc -O) or not.
#include <stdio.h>
int main()
{
int b = 55;
int a[2] = {4, 5};
int index;
printf(" index a[index]\n ");
printf("==================\n ");
for(index = 0; index < 6; index++)
{
printf("%2d%12d\n", index, a[index]);
}
return 0;
}
I understand that accessing an index out-of-bounds in C will simply access the stack memory further down from the array (assuming there is enough stack space allocated for that index, otherwise it segfaults) because arrays are just pointers in C. But how does the optimization affect this?

Answer 1

Accessing out-of-bounds is undefined behavior. So the compiler is allowed to do anything it wants and anything is allowed to happen. So there isn't much of a point in trying to "guess" what will happen.
In your case, optimization is probably affecting the ordering and contents of the stack beyond the array. This would give you the varying results.

Answer 2

You're causing undefined behaviour, plain and simple. You can't really say "why does this undefined behaviour cause this result under this circumstance, but a different result under another?"
Use objdump or gdb if you want to see what instructions are causing the stack to be different under optimization.
EDIT: For example, when compiling with the -O flag, there's quite a few differences to the stack at the beginning of main alone before the first printf (compiled as 32-bit for clarity):
Unoptimized:
0x080483c4 <+0>: push ebp
0x080483c5 <+1>: mov ebp,esp
0x080483c7 <+3>: and esp,0xfffffff0
0x080483ca <+6>: sub esp,0x20
0x080483cd <+9>: mov DWORD PTR [esp+0x18],0x37
0x080483d5 <+17>: mov DWORD PTR [esp+0x10],0x4
0x080483dd <+25>: mov DWORD PTR [esp+0x14],0x5
0x080483e5 <+33>: mov eax,0x8048514
0x080483ea <+38>: mov DWORD PTR [esp],eax
0x080483ed <+41>: call 0x80482e0 <printf#plt>
Optimized:
0x080483c4 <+0>: push ebp
0x080483c5 <+1>: mov ebp,esp
0x080483c7 <+3>: push ebx
0x080483c8 <+4>: and esp,0xfffffff0
0x080483cb <+7>: sub esp,0x20
0x080483ce <+10>: mov DWORD PTR [esp+0x18],0x4
0x080483d6 <+18>: mov DWORD PTR [esp+0x1c],0x5
0x080483de <+26>: mov DWORD PTR [esp],0x8048504
0x080483e5 <+33>: call 0x80482e0 <printf#plt>