c++ 反汇编 表达式

有符号数溢出：

void BreakFor()
{
     for (int i = 1; i > 0; i++)
     {
         printf("%d 
", i);
    }
}

上面的程序并不是死循环，当有符号数增加到最大整数后，继续加一会进位修改符号位，从而成为负数。

---

自增、自减

98:     int nVarOne = argc;
012665EE 8B 45 08             mov         eax,dword ptr [argc]  
012665F1 89 45 F8             mov         dword ptr [nVarOne],eax  
    99:     int nVarTwo = argc;
012665F4 8B 45 08             mov         eax,dword ptr [argc]  
012665F7 89 45 EC             mov         dword ptr [nVarTwo],eax  
   100:     nVarTwo = 5 + (nVarOne++);
012665FA 8B 45 F8             mov         eax,dword ptr [nVarOne]  
012665FD 83 C0 05             add         eax,5  
01266600 89 45 EC             mov         dword ptr [nVarTwo],eax  
01266603 8B 4D F8             mov         ecx,dword ptr [nVarOne]  
01266606 83 C1 01             add         ecx,1  //后缀自增，在语句结束后加一
01266609 89 4D F8             mov         dword ptr [nVarOne],ecx  
   101:     nVarTwo = 5 + (++nVarOne);
0126660C 8B 45 F8             mov         eax,dword ptr [nVarOne]  
0126660F 83 C0 01             add         eax,1  //前缀自增，先自增  
01266612 89 45 F8             mov         dword ptr [nVarOne],eax  
01266615 8B 4D F8             mov         ecx,dword ptr [nVarOne]  
01266618 83 C1 05             add         ecx,5  
0126661B 89 4D EC             mov         dword ptr [nVarTwo],ecx  
   102: 
   103:     nVarOne = 5 + (nVarTwo--);
0126661E 8B 45 EC             mov         eax,dword ptr [nVarTwo]  
01266621 83 C0 05             add         eax,5  
01266624 89 45 F8             mov         dword ptr [nVarOne],eax  
01266627 8B 4D EC             mov         ecx,dword ptr [nVarTwo]  
0126662A 83 E9 01             sub         ecx,1  
0126662D 89 4D EC             mov         dword ptr [nVarTwo],ecx  
   104:     nVarOne = 5 + (--nVarTwo);
01266630 8B 45 EC             mov         eax,dword ptr [nVarTwo]  
01266633 83 E8 01             sub         eax,1  
01266636 89 45 EC             mov         dword ptr [nVarTwo],eax  
01266639 8B 4D EC             mov         ecx,dword ptr [nVarTwo]  
0126663C 83 C1 05             add         ecx,5  
0126663F 89 4D F8             mov         dword ptr [nVarOne],ecx  
   105: }

---

表达式截断：

124:     (nNumber == 0) || (nNumber += Accumulation(nNumber - 1));
0126650E 83 7D 08 00          cmp         dword ptr [nNumber],0  
01266512 74 15                je          Accumulation+39h (01266529h)  //前面为零，截断表达式，直接跳转
01266514 8B 45 08             mov         eax,dword ptr [nNumber]  
01266517 83 E8 01             sub         eax,1  
0126651A 50                   push        eax  
0126651B E8 8B B4 FE FF       call        Accumulation (012519ABh)  //
01266520 83 C4 04             add         esp,4  
01266523 03 45 08             add         eax,dword ptr [nNumber]  
01266526 89 45 08             mov         dword ptr [nNumber],eax  
   125:     return  nNumber;
01266529 8B 45 08             mov         eax,dword ptr [nNumber]  
   126: }

---

条件表达式：

• 方案1：表达式1为简单比较，而表达式2和表达3两者的差值等于1；

return argc == 5 ? 5 : 6;

00E44BB6      | 8B7D 08            | mov edi,dword ptr ss:[ebp+0x8]  | 
00E44BB9      | 33C0               | xor eax,eax                     |
00E44BBB      | 83FF 05            | cmp edi,0x5                     |
00E44BBE      | 0F95C0             | setne al                        |//差值为一，用到setne，取得ZF值后，取反，再放到AL中，即不等时置AL为1
00E44BC1      | 83C0 05            | add eax,0x5                     |

• 方案2：表达式1为简单比较，而表达式2和表达式3两者的差值大于1；

return argc == 5 ? 4 : 10;

00E44BCF      | BE 0A000000        | mov esi,0xA                     | 
00E44BD4      | 83FF 05            | cmp edi,0x5                     |
00E44BD7      | 8BC6               | mov eax,esi                     | 00E44BD9      | BB 04000000        | mov ebx,0x4                     |
00E44BDE      | 0F44C3             | cmove eax,ebx                   |//相等的时候用ebx-->4代替eax-->10

 

 vc6.0下

• 方案3：表达式2和表达式3两者的差值大于1；

return argc >= 8 ? 4 : 10;

00E44BEC      | 83FF 08            | cmp edi,0x8                     | 
00E44BEF      | 8BC6               | mov eax,esi                     | esi-->1000E44BF1      | 0F4DC3             | cmovge eax,ebx                  |//大于等于时，用ebx-->4代替eax-->10

在vc6.0下

 

• 方案4：表达式2和表达式3有一个为变量，于是无优化。

---

位运算：

141:     unsigned int nVar = argc;
0126668E 8B 45 08             mov         eax,dword ptr [argc]  
01266691 89 45 F8             mov         dword ptr [nVar],eax  
   142:     nVar <<= 3;
01266694 8B 45 F8             mov         eax,dword ptr [nVar]  
01266697 C1 E0 03             shl         eax,3  //无符号数<< 使用shl 逻辑左移
0126669A 89 45 F8             mov         dword ptr [nVar],eax  
   143:     nVar >>= 5;
0126669D 8B 45 F8             mov         eax,dword ptr [nVar]  
012666A0 C1 E8 05             shr         eax,5  //无符号数>>  使用shr 逻辑右移
012666A3 89 45 F8             mov         dword ptr [nVar],eax  
   144: 
   145:     argc = argc << 3;
012666A6 8B 45 08             mov         eax,dword ptr [argc]  
   144: 
   145:     argc = argc << 3;
012666A9 C1 E0 03             shl         eax,3  
012666AC 89 45 08             mov         dword ptr [argc],eax  
   146:     argc = argc >> 5;
012666AF 8B 45 08             mov         eax,dword ptr [argc]  
012666B2 C1 F8 05             sar         eax,5  //有符号数>> 使用sar 算术右移
012666B5 89 45 08             mov         dword ptr [argc],eax  
   147:     argc = argc | 0xFFFF0000;
012666B8 8B 45 08             mov         eax,dword ptr [argc]  
012666BB 0D 00 00 FF FF       or          eax,0FFFF0000h  
012666C0 89 45 08             mov         dword ptr [argc],eax  
   148:     argc = argc & 0x0000FFFF;
012666C3 8B 45 08             mov         eax,dword ptr [argc]  
012666C6 25 FF FF 00 00       and         eax,0FFFFh  
012666CB 89 45 08             mov         dword ptr [argc],eax  
   149:     argc = argc ^ 0xFFFF0000;
012666CE 8B 45 08             mov         eax,dword ptr [argc]  
012666D1 35 00 00 FF FF       xor         eax,0FFFF0000h  
012666D6 89 45 08             mov         dword ptr [argc],eax  
   150:     argc = ~argc;
012666D9 8B 45 08             mov         eax,dword ptr [argc]  
012666DC F7 D0                not         eax  
012666DE 89 45 08             mov         dword ptr [argc],eax  
   151: 
   152:     return argc;
012666E1 8B 45 08             mov         eax,dword ptr [argc]