OK, I understand. But why compiler doesn't use this space in the stack to store some local variables?
For example:
void dummy(char *);
int test (void)
{
char a[8] = "12345678";
dummy(a);
return 0;
}
i got this:
test:
subq $24, %rsp
movabsq $4050765991979987505, %rax
leaq 8(%rsp), %rdi
movq %rax, 8(%rsp)
call dummy
xorl %eax, %eax
addq $24, %rsp
ret
It reserved 24 bytes on stack instead reserve and use only 8 byte and storing "12345678" in it
12.04.2016, 11:19, "Florian Weimer" <fweimer@xxxxxxxxxx>:
> On 04/12/2016 06:34 AM, sztfg@xxxxxxxxx wrote:
>
>> test:
>> subq $8, %rsp # why?
>> call dummy
>> xorl %eax, %eax
>> addq $8, %rsp # why?
>> ret
>> .size test, .-test
>>
>> Why need to do this subq addq?
>
> The x86_64 ABI requires that the stack pointer is a multiple of 16
> before the CALL instruction is executed. This simplifies code in
> functions which need to store vector register values on the stack.
>
> Florian
