swap_two_number.c

#include <stdio.h>


void print(int a, int b) {
     printf("a=%d, b=%d\n", a, b);
}

int v1(int a, int b) {
     int c;
     c = a;
     a = b;
     b = c;
     print(a, b);
}

int v2(int a, int b) {
     a = a + b - (b = a);
     print(a, b);
}

int v3(int a, int b) {
     a = a + b;
     b = a - b;
     a = a - b;
     print(a, b);
}

int v4(int a, int b) {
     a = a - b;
     b = b + a;
     a = b - a;
     print(a, b);
}

int v5(int a, int b) {
     a = a ^ b;
     b = b ^ a;
     a = a ^ b;
     print(a, b);
}

int v6(int a, int b) {
     b = b ^ a;
     a = a ^ b;
     b = b ^ a;
     print(a, b);
}
     
int main(int argc, char *argv[]) 
{
    int a = 10, b = 20;
    printf("Origin: a=%d, b=%d\n", a, b);

    v1(a, b);
    v2(a, b);
    v3(a, b);
    v4(a, b);
    v5(a, b);
    v6(a, b);
    
    return 0;
}

反汇编a = a + b - (b = a).s

;; #include <stdio.h>

;; int main(int argc, char *argv[])
;; {
;;      int p = 10, q = 20;
;;      p = p + q - (q = p);
;;      return 0;
;; }

	.file	"hello.c"
	.def	___main;	.scl	2;	.type	32;	.endef
	.text
	.globl	_main
	.def	_main;	.scl	2;	.type	32;	.endef
_main:
LFB6:
  ;; .cfi_startproc is used at the beginning of each function that should have an entry in .eh_frame. It initializes some internal data structures and emits architecture dependent initial CFI instructions. Don't forget to close the function by .cfi_endproc.
  ;; from: http://web.mit.edu/rhel-doc/3/rhel-as-en-3/index.html
	.cfi_startproc                ;加锁
  
	pushl	%ebp                    ;将ebp压栈，pushl = push long 
	.cfi_def_cfa_offset 8
	.cfi_offset 5, -8
	movl	%esp, %ebp              ;esp值赋给ebp，设置main函数的栈基址
	.cfi_def_cfa_register 5
	andl	$-16, %esp              ;内存对齐?
	subl	$16, %esp               ;esp向下移动16字节，存储局部变量
  
	call	___main
  ;; 赋值 p q
	movl	$10, 12(%esp)           ;12(esp) = 10
	movl	$20, 8(%esp)            ;8(esp) = 20
	movl	8(%esp), %eax           ;eax = 20
	movl	12(%esp), %edx          ;edx = 10
	addl	%eax, %edx              ;edx = 10 + 20
	movl	12(%esp), %eax          ;eax = 10
	movl	%eax, 8(%esp)           ;8(%esp) = 10
	movl	%edx, %eax              ;eax = 30
	subl	8(%esp), %eax           ;eax = 30 - 10 (%eax - 8(%esp))
	movl	%eax, 12(%esp)          ;12(%esp) = 20
	movl	$0, %eax
  
	leave
	.cfi_restore 5
	.cfi_def_cfa 4, 4
	ret
	.cfi_endproc                  ;解锁
LFE6:

AT&T Assembly :HelloWorld.s
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