汇编代码看过程调用

源代码如下：

#include <stdio.h>

int add(int x,int y)
{
    int t = x + y;
    return t;
}

int main(void)
{
    int x=4, y=1;
    return add(x,y);
}

经过 gcc -S hello.c

产生汇编代码如下：

add:
    pushl   %ebp
    movl    %esp, %ebp
    subl    $16, %esp
    movl    12(%ebp), %eax
    movl    8(%ebp), %edx
    leal    (%edx,%eax), %eax
    movl    %eax, -4(%ebp)
    movl    -4(%ebp), %eax
    leave
    ret
main:
    pushl   %ebp  //其实有两步，subl $4 %esp，mov %ebp (%esp)，保存当前的栈低
    movl    %esp, %ebp  //得到新栈低，将当前栈顶赋予栈低
    subl    $24, %esp
    movl    $4, -4(%ebp)  //x入栈
    movl    $1, -8(%ebp)  //y入栈
    movl    -8(%ebp), %eax  
    movl    %eax, 4(%esp)  //实参x入栈
    movl    -4(%ebp), %eax 
    movl    %eax, (%esp)   //实参y入栈
    call    add
    leave
    ret

若采用优化 gcc -O2  -S hello.c

产生汇编代码如下：

add:
    pushl   %ebp
    movl    %esp, %ebp
    movl    12(%ebp), %eax
    addl    8(%ebp), %eax
    popl    %ebp
    ret
main:
    pushl   %ebp  //保存当前栈低
    movl    $5, %eax
    movl    %esp, %ebp  //得到新栈低
    popl    %ebp  //过程调用结束，恢复旧栈低
    ret