Visual C++中内嵌汇编的问题

如下,是一个利用内嵌汇编实现的两整数交换的程序。输出结果是2,1;2,1;1,2;2,1.可以看到Swap2这个函数行不通,在函数内两个变量确实交换了,但是调用后a和b没有交换,仍然是2,1。就像是传值一样,而没有传址,令我很困惑。

 #include <cstdio>
void Swap1(int &_int1, int &_int2);
void Swap2(int &_int1, int &_int2);
int main() {
    int a = 1, b = 2;
    Swap1(a, b);
    printf("%d,%d;", a, b);
    Swap2(a, b);
    printf("%d,%d.", a, b);
    return 0;
}
void Swap1(int &_int1, int &_int2) {
    unsigned c = _int1, d = _int2;
    __asm {
        mov ebx, [c];
        xchg ebx, [d];
        mov [c], ebx;
    }
    _int1 = c;
    _int2 = d;
    printf("%d,%d;", _int1, _int2);
}
void Swap2(int &_int1, int &_int2) {
    __asm {
        mov ebx, [_int1];
        xchg ebx, [_int2];
        mov [_int1], ebx;
    }
    printf("%d,%d;", _int1, _int2);
}

请各位朋友帮忙看看为什么会这样。

 21:       Swap2(a, b);
0040107B   lea         ecx,[ebp-8]
0040107E   push        ecx
0040107F   lea         edx,[ebp-4]
00401082   push        edx
00401083   call        @ILT+0(Swap2) (00401005)
00401088   add         esp,8
22:       printf("\n-%d,%d.", a, b);
0040108B   mov         eax,dword ptr [ebp-8]
0040108E   push        eax
0040108F   mov         ecx,dword ptr [ebp-4]
00401092   push        ecx
00401093   push        offset string "\n-%d,%d." (0042201c)
00401098   call        printf (004011d0)
0040109D   add         esp,0Ch

根据如上代码,printf用的是本函数堆栈上的int1 int2,缺少一个写回的动作

 36:   void Swap2(int &_int1, int &_int2) {
00401170   push        ebp
00401171   mov         ebp,esp
00401173   sub         esp,40h
00401176   push        ebx
00401177   push        esi
00401178   push        edi
00401179   lea         edi,[ebp-40h]
0040117C   mov         ecx,10h
00401181   mov         eax,0CCCCCCCCh
00401186   rep stos    dword ptr [edi]
37:       __asm {
38:           mov ebx, [_int1];
00401188   mov         ebx,dword ptr [ebp+8]
39:           xchg ebx, [_int2];
0040118B   xchg        ebx,dword ptr [ebp+0Ch]
40:           mov [_int1], ebx;
0040118E   mov         dword ptr [ebp+8],ebx
41:       }
42:       printf("\n2-%d,%d;", _int1, _int2);
00401191   mov         eax,dword ptr [ebp+0Ch]
00401194   mov         ecx,dword ptr [eax]
00401196   push        ecx
00401197   mov         edx,dword ptr [ebp+8]
0040119A   mov         eax,dword ptr [edx]
0040119C   push        eax
0040119D   push        offset string "\n2-%d,%d;" (00422040)
004011A2   call        printf (004011d0)
004011A7   add         esp,0Ch
43:   }
 25:   void Swap1(int &_int1, int &_int2) {
004010E0   push        ebp
004010E1   mov         ebp,esp
004010E3   sub         esp,48h
004010E6   push        ebx
004010E7   push        esi
004010E8   push        edi
004010E9   lea         edi,[ebp-48h]
004010EC   mov         ecx,12h
004010F1   mov         eax,0CCCCCCCCh
004010F6   rep stos    dword ptr [edi]
26:       unsigned c = _int1, d = _int2;
004010F8   mov         eax,dword ptr [ebp+8]
004010FB   mov         ecx,dword ptr [eax]
004010FD   mov         dword ptr [ebp-4],ecx
00401100   mov         edx,dword ptr [ebp+0Ch]
00401103   mov         eax,dword ptr [edx]
00401105   mov         dword ptr [ebp-8],eax
27:       __asm {
28:           mov ebx, [c];
00401108   mov         ebx,dword ptr [ebp-4]
29:           xchg ebx, [d];
0040110B   xchg        ebx,dword ptr [ebp-8]
30:           mov [c], ebx;
0040110E   mov         dword ptr [ebp-4],ebx
31:       }
32:       _int1 = c;
00401111   mov         ecx,dword ptr [ebp+8]
00401114   mov         edx,dword ptr [ebp-4]
00401117   mov         dword ptr [ecx],edx
33:       _int2 = d;
00401119   mov         eax,dword ptr [ebp+0Ch]
0040111C   mov         ecx,dword ptr [ebp-8]
0040111F   mov         dword ptr [eax],ecx
34:       printf("\n1-%d,%d;", _int1, _int2);
00401121   mov         edx,dword ptr [ebp+0Ch]
00401124   mov         eax,dword ptr [edx]
00401126   push        eax
00401127   mov         ecx,dword ptr [ebp+8]
0040112A   mov         edx,dword ptr [ecx]
0040112C   push        edx
0040112D   push        offset string "\n1-%d,%d;" (00422034)
00401132   call        printf (004011d0)
00401137   add         esp,0Ch
35:   }

手动写个 swap函数
然后查看反汇编,以及自动生成德汇编代码

所谓授人以鱼不如渔,VISUAL C++ 有个好用的工具叫Listing File,好多人都不用,但是和ASM打交道的同学们是必需的工具:Listing Files

通过设置 Listing Files的不同类型,可以产生一个和源代码相应的COD文件,它可以包含汇编代码,机器码和源代码。通过这些内容,可以清晰直观地看到VC编译器对源代码做了些什么改动。

VC工程设置

以当前的ChildView.cpp为例,编译后会产生 ChildView.cod 文件,打开它,这就是你想要的:
COD

应该这样调用:

Swap1(&a, &b);