C++反汇编->多分支if与switch分析

C++ if与switch语句对比

最新推荐文章于 2024-10-29 19:57:40 发布

原创最新推荐文章于 2024-10-29 19:57:40 发布 · 978 阅读

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本文通过一段C++代码示例，详细解析了if语句与switch语句的机器码实现方式，并对比了两者在不同场景下的执行效率。

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。