流程控制语句反汇编(1)(Debug版)

// 流程控制语句反汇编
//Author:乾卦  Date:2014-5-8

#include<stdio.h>
int main()
{
    int a=,b=;
    if(a>b)
    {
        a=b;
    }
    a=;
    b=;

    return ;
}
//if语句的反汇编
#include<stdio.h>
int main()
{
000000013F711010  push        rdi
000000013F711012  sub         rsp,10h
000000013F711016  mov         rdi,rsp
000000013F711019  mov         ecx,
000000013F71101E  mov         eax,0CCCCCCCCh
000000013F711023  rep stos    dword ptr [rdi]
    int a=,b=;
000000013F711025  mov         dword ptr [rsp],
000000013F71102C  mov         dword ptr [b],0Ah
    if(a>b)
000000013F711034  mov         eax,dword ptr [b]
000000013F711038  cmp         dword ptr [rsp],eax
                  //如果a<=b 则跳过语句块
000000013F71103B  jle         main+34h (013F711044h)
    {
        a=b;
000000013F71103D  mov         eax,dword ptr [b]
000000013F711041  mov         dword ptr [rsp],eax
    }
    a=;
000000013F711044  mov         dword ptr [rsp],
    b=;
000000013F71104B  mov         dword ptr [b],0Bh  

    return ;
000000013F711053  xor         eax,eax
}
000000013F711055  add         rsp,10h
000000013F711059  pop         rdi
000000013F71105A  ret  

if规定是满足条件则执行if语句块。
而汇编与其相反，是满足条件则跳转，绕过某些代码块，这一点要注意。

if-else

// 流程控制语句反汇编
//Author:乾卦  Date:2014-5-8
#include<stdio.h>
int main()
{
    int a=,b=;
    if(a>b)
    {
        a=b;
    }
    else
    {
        b=a;
    }
    a=;
    b=;

    return ;
}

//if-else语句的反汇编

#include<stdio.h>
int main()
{
000000013FF31010  push        rdi
000000013FF31012  sub         rsp,10h
000000013FF31016  mov         rdi,rsp
000000013FF31019  mov         ecx,
000000013FF3101E  mov         eax,0CCCCCCCCh
000000013FF31023  rep stos    dword ptr [rdi]
    int a=,b=;
000000013FF31025  mov         dword ptr [rsp],
000000013FF3102C  mov         dword ptr [b],0Ah
    if(a>b)
000000013FF31034  mov         eax,dword ptr [b]
000000013FF31038  cmp         dword ptr [rsp],eax
                  //这是执行else的跳转语句 也是else的执行条件
000000013FF3103B  jle         main+36h (013FF31046h)
    {
        a=b;
000000013FF3103D  mov         eax,dword ptr [b]
000000013FF31041  mov         dword ptr [rsp],eax
    }
    else          //这是执行完if语句块跳转到else后的跳转
000000013FF31044  jmp         main+3Dh (013FF3104Dh)
    {
        b=a;
000000013FF31046  mov         eax,dword ptr [rsp]
000000013FF31049  mov         dword ptr [b],eax
    }
    a=;
000000013FF3104D  mov         dword ptr [rsp],
    b=;
000000013FF31054  mov         dword ptr [b],0Bh  

    return ;
000000013FF3105C  xor         eax,eax
}
000000013FF3105E  add         rsp,10h
000000013FF31062  pop         rdi
000000013FF31063  ret  

if的反汇编仍然没变，但是else有个jmp。

if-else语句的逻辑：if成功，else就不执行。

汇编的逻辑：if失败，跳转到else。否则执行if语句块，在else之前跳到if-else语句末尾。

两者有且只有一个执行。下面来自：《c++反汇编与逆向分析技术揭秘》作者： 钱松林

if-else if-else:

// 流程控制语句反汇编
//Author:乾卦  Date:2014-5-8

#include<stdio.h>
int main()
{
    int a=,b=;
    if(a>b)
    {
        a=b;
    }
    else if(a==)
    {
        b=a;
    }
    else if(b<)
    {
        b=;
    }
    else
    {
        a=a+b;
    }
    a=;
    b=;

    return ;
}
//if-else if-else
#include<stdio.h>
int main()
{
000000013FAD31E0  push        rdi
000000013FAD31E2  sub         rsp,10h
000000013FAD31E6  mov         rdi,rsp
000000013FAD31E9  mov         ecx,
000000013FAD31EE  mov         eax,0CCCCCCCCh
000000013FAD31F3  rep stos    dword ptr [rdi]
    int a=,b=;
000000013FAD31F5  mov         dword ptr [rsp],
000000013FAD31FC  mov         dword ptr [b],0Ah
    if(a>b)
000000013FAD3204  mov         eax,dword ptr [b]
000000013FAD3208  cmp         dword ptr [rsp],eax
                  //跳到下一个判断语句
000000013FAD320B  jle         main+36h (013FAD3216h)
    {
        a=b;
000000013FAD320D  mov         eax,dword ptr [b]
000000013FAD3211  mov         dword ptr [rsp],eax
000000013FAD3214  jmp         main+64h (013FAD3244h)
    }
    else if(a==)
000000013FAD3216  cmp         dword ptr [rsp],
000000013FAD321A  jne         main+45h (013FAD3225h)
    {
        b=a;
000000013FAD321C  mov         eax,dword ptr [rsp]
000000013FAD321F  mov         dword ptr [b],eax
000000013FAD3223  jmp         main+64h (013FAD3244h)
    }
    else if(b<)
000000013FAD3225  cmp         dword ptr [b],
000000013FAD322A  jge         main+56h (013FAD3236h)
    {
        b=;
000000013FAD322C  mov         dword ptr [b],1Eh
    }
    else
000000013FAD3234  jmp         main+64h (013FAD3244h)
    {
        a=a+b;
000000013FAD3236  mov         eax,dword ptr [b]
000000013FAD323A  mov         ecx,dword ptr [rsp]
000000013FAD323D  add         ecx,eax
000000013FAD323F  mov         eax,ecx
000000013FAD3241  mov         dword ptr [rsp],eax
    }
    a=;
000000013FAD3244  mov         dword ptr [rsp],
    b=;
000000013FAD324B  mov         dword ptr [b],0Bh  

    return ;
000000013FAD3253  xor         eax,eax
}
000000013FAD3255  add         rsp,10h
000000013FAD3259  pop         rdi
000000013FAD325A  ret  

注：红色是一种跳，蓝色是一种跳。

if与else if都是若条件不满足则跳到下一个判断。最后一个else if则是跳到else语句块。else跟以前一样。