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C语言7 switch语句为什么高效
switch语句的定义语法switch(表达式) { case 常亮表达式1: 语句; ...
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2017/12

C语言7 switch语句为什么高效

switch语句的定义

语法

switch(表达式)
{
    case 常亮表达式1:
        语句;
        break;
    case 常亮表达式:
        语句;
        break;
    case 常亮表达式:
        语句;
        break;
     ......
    default:
        语句;
        break;
}

需要注意的点

  1. 表达式结束不能使浮点数
  2. case后的值不能一样
  3. case后的值必须是常量
  • break:
  • break非常重要,当执行到一个分支后 如果没有break就会继续向下执行,遇到break才跳出switch语句
  • default 语句与位置无关,但是当default写在其他条件的前面时。如果没有break;,就会向下继续匹配

Switch语句与 if..else语句的区别:

  1. switch语句只能进行等值判断,而if..else可以进行区间判断
  2. switch语句的执行效率远远高于if..else 在分支条件比较多的情况下,这种趋势愈发明显

观察switch语句的反汇编,看看switch语句为啥效率高

为啥switch比if..else效率高

### 游戏中的switch语句

switch(x)
{
   case 1:
       printf("A \n");
       break;
   case 2:
       printf("B \n");
       break;
   case 3:
       printf("C \n");
       break;
   default:
       printf("default \n");
       break;
}

编译后反汇编代码

11:        switch(x)
12:    {
00401038 8B 45 08             mov         eax,dword ptr [ebp+8]
0040103B 89 45 FC             mov         dword ptr [ebp-4],eax
0040103E 83 7D FC 01          cmp         dword ptr [ebp-4],1
00401042 74 0E                je          MyPrint+32h (00401052)
00401044 83 7D FC 02          cmp         dword ptr [ebp-4],2
00401048 74 17                je          MyPrint+41h (00401061)
0040104A 83 7D FC 03          cmp         dword ptr [ebp-4],3
0040104E 74 20                je          MyPrint+50h (00401070)
00401050 EB 2D                jmp         MyPrint+5Fh (0040107f)
13:       case 1:
14:           printf("A \n");
00401052 68 30 00 42 00       push        offset string "A \n" (00420030)
00401057 E8 B4 00 00 00       call        printf (00401110)
0040105C 83 C4 04             add         esp,4
15:           break;
0040105F EB 2B                jmp         MyPrint+6Ch (0040108c)
16:       case 2:
17:           printf("B \n");
00401061 68 2C 00 42 00       push        offset string "B \n" (0042002c)
00401066 E8 A5 00 00 00       call        printf (00401110)
0040106B 83 C4 04             add         esp,4
18:           break;
0040106E EB 1C                jmp         MyPrint+6Ch (0040108c)
19:       case 3:
20:           printf("C \n");
00401070 68 28 00 42 00       push        offset string "C \n" (00420028)
00401075 E8 96 00 00 00       call        printf (00401110)
0040107A 83 C4 04             add         esp,4
21:           break;
0040107D EB 0D                jmp         MyPrint+6Ch (0040108c)
22:       default:
23:           printf("default \n");
0040107F 68 1C 00 42 00       push        offset string "default \n" (0042001c)
00401084 E8 87 00 00 00       call        printf (00401110)
00401089 83 C4 04             add         esp,4
24:           break;
25:    }
26:   }

### 区别

当调教比较少的时候 没啥区别 。但是当条件比较多的时候:


11:        switch(x)
12:    {
0040B818 8B 45 08             mov         eax,dword ptr [ebp+8]
0040B81B 89 45 FC             mov         dword ptr [ebp-4],eax
0040B81E 8B 4D FC             mov         ecx,dword ptr [ebp-4]
0040B821 83 E9 01             sub         ecx,1
0040B824 89 4D FC             mov         dword ptr [ebp-4],ecx
0040B827 83 7D FC 03          cmp         dword ptr [ebp-4],3
0040B82B 77 46                ja          $L539+0Fh (0040b873)
0040B82D 8B 55 FC             mov         edx,dword ptr [ebp-4]
0040B830 FF 24 95 91 B8 40 00 jmp         dword ptr [edx*4+40B891h]
13:       case 1:
14:           printf("A \n");
0040B837 68 34 0F 42 00       push        offset string "A \n" (00420f34)
0040B83C E8 CF 58 FF FF       call        printf (00401110)
0040B841 83 C4 04             add         esp,4
15:           break;
0040B844 EB 3A                jmp         $L539+1Ch (0040b880)
16:       case 2:
17:           printf("B \n");
0040B846 68 30 00 42 00       push        offset string "B \n" (00420030)
0040B84B E8 C0 58 FF FF       call        printf (00401110)
0040B850 83 C4 04             add         esp,4
18:           break;
0040B853 EB 2B                jmp         $L539+1Ch (0040b880)
19:       case 3:
20:           printf("C \n");
0040B855 68 2C 00 42 00       push        offset string "C \n" (0042002c)
0040B85A E8 B1 58 FF FF       call        printf (00401110)
0040B85F 83 C4 04             add         esp,4
21:           break;
0040B862 EB 1C                jmp         $L539+1Ch (0040b880)
22:       case 4:
23:           printf("D \n");
0040B864 68 28 00 42 00       push        offset string "D \n" (00420028)
0040B869 E8 A2 58 FF FF       call        printf (00401110)
0040B86E 83 C4 04             add         esp,4
24:           break;
0040B871 EB 0D                jmp         $L539+1Ch (0040b880)
25:       default:
26:           printf("default \n");
0040B873 68 1C 00 42 00       push        offset string "default \n" (0042001c)
0040B878 E8 93 58 FF FF       call        printf (00401110)
0040B87D 83 C4 04             add         esp,4
27:           break;
28:    }
29:   }

我们发现 生成一张内存表。
switch通过算法,直接一步算出地址 从表中找到需要跳转的地址

大表和小表

当生成的函数跳转地址表 每个成员有4个字节的时候 我们称之为 大表

总结:

  1. 即没有大表也没有小表:

    • case 项小于等于3项
    • case最大值和最小值差值>=255
  2. 有大表没小表:

    • case项大于3项并且大表项空隙小于等于6个
  3. 有大表有小表

    • case项大于3项并且大表项空隙大于6个并且case最大最小差值<255
Last modification:January 10th, 2018 at 10:11 am
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