why gcc 4.x default reserve 8 bytes for stack on linux when calling a method?

Question

as a beginner of asm, I am checking gcc -S generated asm code to learn.

why gcc 4.x default reserve 8 bytes for stack when calling a method?

func18 is the e

Answer 1

As richard mentioned above, it's all because of optimization, showing below. but still I got no idea why 8 bytes reserved is something optimized?!

original c

void func18() {}
int main() {return 0;}

compile without optimization flag specified

    .text                                                                                   
.globl _func18
_func18:
    pushl   %ebp
    movl    %esp, %ebp
    subl    $8, %esp
    leave
    ret
.globl _main
_main:                                                                                      
    pushl   %ebp
    movl    %esp, %ebp
    subl    $8, %esp
    movl    $0, %eax
    leave
    ret
    .subsections_via_symbols

with -Os optimization flag, no more stack reserve

    .text
.globl _func18
_func18:
    pushl   %ebp
    movl    %esp, %ebp
    leave
    ret
.globl _main
_main:
    pushl   %ebp
    xorl    %eax, %eax
    movl    %esp, %ebp
    leave
    ret
    .subsections_via_symbols