asm source code note 1.7_switch case分支的实现

源代码

// test1020.cpp : Defines the entry point for the console application.
//#include "stdafx.h"
#include <windows.h>
#include <stdlib.h>
#include <time.h>
#include <math.h>enum eNumberRange{eNumberRange_0_100 = 1,eNumberRange_100_200,eNumberRange_200_300,eNumberRange_Unknown
};/// 判断一个输入数的范围
eNumberRange NumJudge(int iNumber);int _tmain(int argc, _TCHAR* argv[])
{int     iDigIn  =   0;srand( (unsigned)time( NULL ) );iDigIn  =   rand();_tprintf(L"NumJudge(%d) = %d\r\n", iDigIn, NumJudge(iDigIn));getwchar();return 0;
}/// 如果iNumber在源代码中给定值, Release版编译后， 会优化掉
eNumberRange NumJudge(int iNumber)
{eNumberRange eType = eNumberRange_Unknown;int iRange = (iNumber - iNumber % 100) / 100 + eNumberRange_0_100;switch (iRange){case eNumberRange_0_100:eType = eNumberRange_0_100;break;case eNumberRange_100_200:eType = eNumberRange_100_200;break;case eNumberRange_200_300:eType = eNumberRange_200_300;break;default:eType = eNumberRange_Unknown;break;}return eType;
}

OD反汇编代码

002E1000 >/$  56            push    esi
002E1001  |.  6A 00         push    0
002E1003  |.  FF15 B0202E00 call    dword ptr [<&MSVCR90._time64>]   ;  MSVCR90._time64
002E1009  |.  50            push    eax                              ; /seed
002E100A  |.  FF15 AC202E00 call    dword ptr [<&MSVCR90.srand>]     ; \srand
002E1010  |.  83C4 08       add     esp, 8
002E1013  |.  FF15 A8202E00 call    dword ptr [<&MSVCR90.rand>]      ; [rand
002E1019  |.  8BF0          mov     esi, eax                         ;  通过修改eax, 判断常量除法的除数 eax = 0x1234(4660)
002E101B  |.  B8 1F85EB51   mov     eax, 51EB851F                    ;  >> 除法1
002E1020  |.  F7EE          imul    esi
002E1022  |.  C1FA 05       sar     edx, 5
002E1025  |.  8BC2          mov     eax, edx
002E1027  |.  C1E8 1F       shr     eax, 1F
002E102A  |.  03C2          add     eax, edx
002E102C  |.  8BC8          mov     ecx, eax                         ;  eax = 0x2e(46), 貌似被除数的百位数, (4660 - 60) % 100
002E102E  |.  6BC9 64       imul    ecx, ecx, 64                     ;  执行后: ecx = 0x11f8(4600), 貌似 (4660 - (4660 % 100))
002E1031  |.  B8 1F85EB51   mov     eax, 51EB851F                    ;  >> 除法2
002E1036  |.  F7E9          imul    ecx
002E1038  |.  C1FA 05       sar     edx, 5
002E103B  |.  8BCA          mov     ecx, edx
002E103D  |.  C1E9 1F       shr     ecx, 1F
002E1040  |.  8D440A 01     lea     eax, dword ptr [edx+ecx+1]       ;  ///< eax = 0x2f(47), 貌似入参的百位数 + 1
002E1044  |.  83E8 01       sub     eax, 1                           ;  ///< 看来0～400是有效输入; Switch (cases 1..3)
002E1047  |.  74 1F         je      short 002E1068                   ;  ///< 百位数减到第几次为0了， 就match了
002E1049  |.  83E8 01       sub     eax, 1
002E104C  |.  74 13         je      short 002E1061
002E104E  |.  83E8 01       sub     eax, 1
002E1051  |.  74 07         je      short 002E105A
002E1053  |.  B8 04000000   mov     eax, 4                           ;  Default case of switch 002E1044
002E1058  |.  EB 13         jmp     short 002E106D
002E105A  |>  B8 03000000   mov     eax, 3                           ;  Case 3 of switch 002E1044
002E105F  |.  EB 0C         jmp     short 002E106D
002E1061  |>  B8 02000000   mov     eax, 2                           ;  Case 2 of switch 002E1044
002E1066  |.  EB 05         jmp     short 002E106D
002E1068  |>  B8 01000000   mov     eax, 1                           ;  Case 1 of switch 002E1044
002E106D  |>  50            push    eax                              ; |<%d>
002E106E  |.  56            push    esi                              ; |<Nu>
002E106F  |.  68 04212E00   push    002E2104                         ; |format = "NumJudge(%d) = %d.."
002E1074  |.  FF15 A4202E00 call    dword ptr [<&MSVCR90.wprintf>]   ; \wprintf
002E107A  |.  83C4 0C       add     esp, 0C
002E107D  |.  FF15 A0202E00 call    dword ptr [<&MSVCR90.getwchar>]  ; [getwchar
002E1083  |.  33C0          xor     eax, eax
002E1085  |.  5E            pop     esi
002E1086  \.  C3            retn

常量除数的除法，比较难判断被除数是多少。暂时只能猜~, 如果靠常用除数常量表判断，也不通用。

通过修改常量除数除法的入参，从除法结果来判断除法到底除的是常量几，还算好使。

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