some questions about gcc4

some questions about gcc4

[Alex]

Hello.

I need some hints. I have compiled simple source:
(gdb) list
1	void main (void) {
2	  int i;
3	  i = 1;
4	}

And disassembling this code is:
Dump of assembler code for function main:
0x08048394 <main+0>:	push   ebp
0x08048395 <main+1>:	mov    ebp,esp
0x08048397 <main+3>:	sub    esp,0x10
0x0804839a <main+6>:	mov    DWORD PTR [ebp-0x4],0x1
0x080483a1 <main+13>:	leave  
0x080483a2 <main+14>:	ret    
End of assembler dump.


I have only 1 int variable. My system is 32Bit. But why for 1 int
variable compiler make sub esp,0x10 ?
1 int variable located in 4 bytes. What in another 12 ?

Re: some questions about gcc4

[Cedric Roux]

Alex wrote:
> I have only 1 int variable. My system is 32Bit. But why for 1 int
> variable compiler make sub esp,0x10 ?
> 1 int variable located in 4 bytes. What in another 12 ?

gcc man page says:
       -mpreferred-stack-boundary=num
           Attempt to keep the stack boundary aligned to a 2 raised to num
           byte boundary.  If -mpreferred-stack-boundary is not specified, the
           default is 4 (16 bytes or 128 bits).

so do a gcc -mpreferred-stack-boundary=2 and you're done.

I think they set to 4 by default for performances' reasons, when you use SSE
data that likes to be aligned to 16 bytes. (Or maybe because the AMD64
ABI says the stack is 16 bytes aligned...) But let a gcc programmer answer
that if one has time to. :-) (And by the way I'm not sure of what I write,
so don't take it for the truth.)