What about this one ? Three PUSHes + sub 0x10,%esp. That makes 28 bytes.
Stack is not aligned on 16-byte boundary in this case.
(gdb) list 8
3 int main(int argc, char **argv) {
4 int a, b, c;
5
6 a = 1;
7 b = 2;
8 c = a + b;
9
10 return 0;
11 }
12
(gdb) disassemble main
Dump of assembler code for function main:
0x08048344 <main+0>: lea 0x4(%esp),%ecx
0x08048348 <main+4>: and $0xfffffff0,%esp
0x0804834b <main+7>: pushl 0xfffffffc(%ecx)
0x0804834e <main+10>: push %ebp
0x0804834f <main+11>: mov %esp,%ebp
0x08048351 <main+13>: push %ecx
0x08048352 <main+14>: sub $0x10,%esp
0x08048355 <main+17>: movl $0x1,0xfffffff0(%ebp)
0x0804835c <main+24>: movl $0x2,0xfffffff4(%ebp)
0x08048363 <main+31>: mov 0xfffffff4(%ebp),%eax
0x08048366 <main+34>: add 0xfffffff0(%ebp),%eax
0x08048369 <main+37>: mov %eax,0xfffffff8(%ebp)
0x0804836c <main+40>: mov $0x0,%eax
0x08048371 <main+45>: add $0x10,%esp
0x08048374 <main+48>: pop %ecx
0x08048375 <main+49>: pop %ebp
0x08048376 <main+50>: lea 0xfffffffc(%ecx),%esp
0x08048379 <main+53>: ret
End of assembler dump.
On 7/20/07, Andrew Haley <aph-gcc@xxxxxxxxxxxxxxxxxxx> wrote:
Pankaj Kohli writes:
> Yeah, that was me who asked it on kerneltrap :)
> If it is trying to align ESP on a 16-byte boundary, that seems fine
> for a single integer variable or anything less than 16 bytes, but why
> is it allocating 116 bytes for 100 byte buffer ? That doesn't fit on a
> 16-byte boundary.
Where's the mystery?
> Dump of assembler code for function main:
> 0x080483a4 : lea 0x4(%esp),%ecx
> 0x080483a8 : and $0xfffffff0,%esp
sp = sp & -16 // sp is 16-aligned
> 0x080483ab : pushl 0xfffffffc(%ecx)
sp -= 4 // sp is 4-aligned
> 0x080483ae : push %ebp
sp -= 4 // sp is 8-aligned
> 0x080483af : mov %esp,%ebp
> 0x080483b1 : push %ecx
sp -= 4 // sp is 4-aligned
> 0x080483b2 : sub $0x74,%esp
sp -= 116 // sp is 16-aligned
Andrew.
--
- Pankaj
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