用 goto + if 模拟 循环

* 循环的效率分析, 用 goto + if 模拟 循环(for, do-while, while)如果模拟费劲(if + goto 的次数统计多)，说明效率低.

备注: for循环的++或--是在循环体的上面, 是低地址. 不能在循环体内更改循环条件.

思路

先用C语言将循环写出来，将汇编代码复制出来备用，将CMP换成if, 将JMP换成goto, 将 JMP的地址换成标号.

这样就可以直接直接将汇编代码按照汇编代码的含义用C代码重写. 就完成了用if + goto 模拟循环的任务

源码

/// @file 2015_1026\exam_1_3\main.c/// @brief/*** 循环的效率分析, 用 goto + if 模拟 循环(for, do-while, while)如果模拟费劲(if + goto 的次数统计多)，说明效率低.备注: for循环的++或--是在循环体的上面, 是低地址. 不能在循环体内更改循环条件.*/#include <stdlib.h>#include <stdio.h>void testLoop(); ///< test real loop(for, do-while, while)void MyLoop_for(); ///< use gogo + if make our loop(for)void MyLoop_do_while(); ///< use gogo + if make our loop(do-while)void MyLoop_while(); ///< use gogo + if make our loop(while)int main(int argc, char** argv){// testLoop();MyLoop_for();MyLoop_do_while();MyLoop_while();getchar();return 0;}void testLoop(){int iIndex = 0;const int iCntMax = 3;printf("===for===\n");for (iIndex = 0; iIndex < iCntMax; iIndex++){printf("iIndex = %d\n", iIndex);}printf("===do-while===\n");iIndex = 0;do {printf("iIndex = %d\n", iIndex);} while (iIndex++ < iCntMax);printf("===while===\n");iIndex = 0;while (iIndex++ < iCntMax){printf("iIndex = %d\n", iIndex);}}void MyLoop_for(){/**29:       for (iIndex = 0; iIndex < iCntMax; iIndex++)00401103   mov         dword ptr [ebp-4],00040110A   jmp         testLoop+45h (00401115)0040110C   mov         eax,dword ptr [ebp-4]0040110F   add         eax,100401112   mov         dword ptr [ebp-4],eax00401115   mov         ecx,dword ptr [ebp-4]00401118   cmp         ecx,dword ptr [ebp-8]0040111B   jge         testLoop+60h (00401130)30:       {31:           printf("iIndex = %d\n", iIndex);0040111D   mov         edx,dword ptr [ebp-4]00401120   push        edx00401121   push        offset string "iIndex = %d\n" (00423040)00401126   call        printf (00401590)0040112B   add         esp,832:       }0040112E   jmp         testLoop+3Ch (0040110c)*//// 模拟 for 循环/// 参考for循环的汇编代码int iIndex = 0;const int iCntMax = 3;printf("===sim for===\n");iIndex = 0;goto label_for_compare;label_for_next:iIndex++;label_for_compare:if (iIndex > (iCntMax - 1))goto label_for_end;printf("iIndex = %d\n", iIndex);goto label_for_next;label_for_end:;}void MyLoop_do_while(){/**37:       iIndex = 0;00401103   mov         dword ptr [ebp-4],038:       do39:       {40:           printf("iIndex = %d\n", iIndex);0040110A   mov         eax,dword ptr [ebp-4]0040110D   push        eax0040110E   push        offset string "iIndex = %d\n" (0042302c)00401113   call        printf (004015c0)00401118   add         esp,841:       } while (iIndex++ < iCntMax);0040111B   mov         ecx,dword ptr [ebp-4]0040111E   mov         edx,dword ptr [ebp-8]00401121   mov         eax,dword ptr [ebp-4]00401124   add         eax,100401127   mov         dword ptr [ebp-4],eax0040112A   cmp         ecx,edx0040112C   jl          testLoop+3Ah (0040110a)*/int iIndex = 0;const int iCntMax = 3;printf("===sim do-while===\n");label_do_while:printf("iIndex = %d\n", iIndex);if (iIndex++ < iCntMax)goto label_do_while;}void MyLoop_while(){/**46:       iIndex = 0;00401113   mov         dword ptr [ebp-4],047:       while (iIndex++ < iCntMax)0040111A   mov         eax,dword ptr [ebp-4]0040111D   mov         ecx,dword ptr [ebp-8]00401120   mov         edx,dword ptr [ebp-4]00401123   add         edx,100401126   mov         dword ptr [ebp-4],edx00401129   cmp         eax,ecx0040112B   jge         testLoop+60h (00401140)48:       {49:           printf("iIndex = %d\n", iIndex);0040112D   mov         eax,dword ptr [ebp-4]00401130   push        eax00401131   push        offset string "iIndex = %d\n" (0042301c)00401136   call        printf (00401630)0040113B   add         esp,850:       }0040113E   jmp         testLoop+3Ah (0040111a)51:   }*/int iIndex = 0;const int iCntMax = 3;iIndex = 0;printf("===sim while===\n");label_while_begin:if (iIndex++ > (iCntMax - 1))goto label_while_end;printf("iIndex = %d\n", iIndex);goto label_while_begin;label_while_end:;}

运行效果

分析

在vsIDE中，在运行到循环入口处，切换到反汇编窗口. 将循环的反汇编代码拷贝出来。将CMP改成if, 将JMP改成goto, 即可用goto + if 完全模拟3种循环(for, do-while, while)

在相同条件下(循环条件相同，循环体代码相同), 经过比较可知 : 效率最高 => do-while => while => for => 效率最低.

 

效率的定义: 汇编代码行数最少.