C++反汇编->多分支if与switch分析

先敲一段代码：

#include <iostream> using namespace std;  void main() { 	cout<<"hello world"<<endl;     int i=1; 	if (i<0) 	{ 		cout<<"i < 0"<<endl; 	}  	else if(i>0&&i<2) 	{ 		cout<<"i>0 and i<2"<<endl; 	} 	else 	{ 		cout<<"i>=2"<<endl; 	}        int j=2; 	switch (j) 	{ 	case 0: 		cout<<"j ==0"; 		break; 	case 1: 		cout<<"j ==1"; 		break; 	case 2: 		cout<<"j ==2"; 		break; 	default: 		cout<<"j have no choice"; 		 	} 	system("pause");  }

switch 语句与if else语句的比较分析我们看一下内容：
DEBUG版：

00401178     .  68 28104000   PUSH test4.00401028 0040117D     .  68 70404300   PUSH OFFSET test4.??_C@_0M@BNI@hello?5world?$AA@    ;  ASCII "hello world" 00401182     .  68 50B54300   PUSH OFFSET test4.std::cout 00401187     .  E8 14FFFFFF   CALL test4.004010A0 0040118C     .  83C4 08       ADD ESP,8 0040118F     .  8BC8          MOV ECX,EAX 00401191     .  E8 DDFEFFFF   CALL test4.00401073 00401196     .  C745 FC 01000>MOV DWORD PTR SS:[EBP-4],1                          ;  i=1  IF语句开始 0040119D     .  837D FC 00    CMP DWORD PTR SS:[EBP-4],0                          ;  i与0比较大小 004011A1     .  7D 20         JGE SHORT test4.004011C3                            ;  大于等于转移 跳转到 该地址 004011A3     .  68 28104000   PUSH test4.00401028 004011A8     .  68 68404300   PUSH OFFSET test4.??_C@_05JDCB@i?5?$DM?50?$AA@      ;  小于则输出 i<0 004011AD     .  68 50B54300   PUSH OFFSET test4.std::cout 004011B2     .  E8 E9FEFFFF   CALL test4.004010A0 004011B7     .  83C4 08       ADD ESP,8 004011BA     .  8BC8          MOV ECX,EAX 004011BC     .  E8 B2FEFFFF   CALL test4.00401073 004011C1     .  EB 4A         JMP SHORT test4.0040120D 004011C3     >  837D FC 00    CMP DWORD PTR SS:[EBP-4],0                          ;  else if(i>0&&i<2)  开始执行 004011C7     .  7E 26         JLE SHORT test4.004011EF                            ;  小于等于转移 跳转该地址 004011C9     .  837D FC 02    CMP DWORD PTR SS:[EBP-4],2 004011CD     .  7D 20         JGE SHORT test4.004011EF                            ;  大于等于转移 跳转到 该地址 >2则跳转到输出  i>=2语句 004011CF     .  68 28104000   PUSH test4.00401028 004011D4     .  68 58404300   PUSH OFFSET test4.??_C@_0M@PHJF@i?$DO0?5and?5i?$DM2>;  ASCII "i>0 and i<2" 004011D9     .  68 50B54300   PUSH OFFSET test4.std::cout 004011DE     .  E8 BDFEFFFF   CALL test4.004010A0 004011E3     .  83C4 08       ADD ESP,8 004011E6     .  8BC8          MOV ECX,EAX 004011E8     .  E8 86FEFFFF   CALL test4.00401073 004011ED     .  EB 1E         JMP SHORT test4.0040120D 004011EF     >  68 28104000   PUSH test4.00401028                                 ;  else{i>2}执行 004011F4     .  68 50404300   PUSH OFFSET test4.??_C@_04GGJ@i?$DO?$DN2?$AA@       ;  ASCII "i>=2" 004011F9     .  68 50B54300   PUSH OFFSET test4.std::cout 004011FE     .  E8 9DFEFFFF   CALL test4.004010A0 00401203     .  83C4 08       ADD ESP,8 00401206     .  8BC8          MOV ECX,EAX 00401208     .  E8 66FEFFFF   CALL test4.00401073 0040120D     >  C745 F8 02000>MOV DWORD PTR SS:[EBP-8],2                          ;  b=2    SWITCH语句开始 00401214     .  8B45 F8       MOV EAX,DWORD PTR SS:[EBP-8] 00401217     .  8945 F4       MOV DWORD PTR SS:[EBP-C],EAX 0040121A     .  837D F4 00    CMP DWORD PTR SS:[EBP-C],0                          ;  b与0比较 0040121E     .  74 0E         JE SHORT test4.0040122E                             ;  跳转到0040122E位置 00401220     .  837D F4 01    CMP DWORD PTR SS:[EBP-C],1                          ;  与1比较 00401224     .  74 1C         JE SHORT test4.00401242                             ;  跳转到00401242位置 00401226     .  837D F4 02    CMP DWORD PTR SS:[EBP-C],2                          ;  与2比较 0040122A     .  74 2A         JE SHORT test4.00401256                             ;  跳转到00401256位置 0040122C     .  EB 3C         JMP SHORT test4.0040126A                            ;  都没匹配则跳转0040126A位置 0040122E     >  68 48404300   PUSH OFFSET test4.??_C@_05LCKF@j?5?$DN?$DN0?$AA@    ;  ASCII "j ==0" 00401233     .  68 50B54300   PUSH OFFSET test4.std::cout 00401238     .  E8 63FEFFFF   CALL test4.004010A0 0040123D     .  83C4 08       ADD ESP,8 00401240     .  EB 3A         JMP SHORT test4.0040127C 00401242     >  68 40404300   PUSH OFFSET test4.??_C@_05BIDC@j?5?$DN?$DN1?$AA@    ;  ASCII "j ==1" 00401247     .  68 50B54300   PUSH OFFSET test4.std::cout 0040124C     .  E8 4FFEFFFF   CALL test4.004010A0 00401251     .  83C4 08       ADD ESP,8 00401254     .  EB 26         JMP SHORT test4.0040127C 00401256     >  68 38404300   PUSH OFFSET test4.??_C@_05OHIL@j?5?$DN?$DN2?$AA@    ;  ASCII "j ==2" 0040125B     .  68 50B54300   PUSH OFFSET test4.std::cout 00401260     .  E8 3BFEFFFF   CALL test4.004010A0 00401265     .  83C4 08       ADD ESP,8 00401268     .  EB 12         JMP SHORT test4.0040127C 0040126A     >  68 24404300   PUSH OFFSET test4.??_C@_0BB@ICIL@j?5have?5no?5choic>;  ASCII "j have no choice" 0040126F     .  68 50B54300   PUSH OFFSET test4.std::cout 00401274     .  E8 27FEFFFF   CALL test4.004010A0 00401279     .  83C4 08       ADD ESP,8 0040127C     >  68 1C404300   PUSH OFFSET test4.??_C@_05PBCN@pause?$AA@           ; /command = "pause" 00401281     .  E8 9A7E0000   CALL test4.system                                   ; \system

小结：1.switch语句在这个里面是有序线性的，而if语句条件跳转紧跟语句块。
2.当判断分支多时，采用switch语句效率比if语句强。