switch为何高效率
在用switch的同时，为何能高效，又如何避免编译退化？
arm-linux-objdump的一个基础实验，你懂得。
int main(void)
{
834c: e24dd008 sub sp, sp, #8 ; 0x8
int a = 4;
8350: e3a03004 mov r3, #4 ; 0x4
8354: e58d3004 str r3, [sp, #4]
switch(a)
8358: e59d3004 ldr r3, [sp, #4]
835c: e3530005 cmp r3, #5 ; 0x5
8360: 908ff103 addls pc, pc, r3, lsl #2
8364: ea00001c b 83dc <main+0x90>
8368: ea000004 b 8380 <main+0x34>
836c: ea000007 b 8390 <main+0x44>
8370: ea00000a b 83a0 <main+0x54>
8374: ea00000d b 83b0 <main+0x64>
8378: ea000010 b 83c0 <main+0x74> //通过跳转的方式，空间换时间
837c: ea000013 b 83d0 <main+0x84>
{
case 0:
a++; break;
8380: e59d3004 ldr r3, [sp, #4]
8384: e2833001 add r3, r3, #1 ; 0x1
8388: e58d3004 str r3, [sp, #4]
838c: ea000012 b 83dc <main+0x90>
case 1:
a++; break;
8390: e59d3004 ldr r3, [sp, #4]
8394: e2833001 add r3, r3, #1 ; 0x1
8398: e58d3004 str r3, [sp, #4]
839c: ea00000e b 83dc <main+0x90>
case 2:
a++; break;
83a0: e59d3004 ldr r3, [sp, #4]
83a4: e2833001 add r3, r3, #1 ; 0x1
83a8: e58d3004 str r3, [sp, #4]
83ac: ea00000a b 83dc <main+0x90>
case 3:
a++; break;
83b0: e59d3004 ldr r3, [sp, #4]
83b4: e2833001 add r3, r3, #1 ; 0x1
83b8: e58d3004 str r3, [sp, #4]
83bc: ea000006 b 83dc <main+0x90>
case 4:
a++; break;
83c0: e59d3004 ldr r3, [sp, #4]
83c4: e2833001 add r3, r3, #1 ; 0x1
83c8: e58d3004 str r3, [sp, #4]
83cc: ea000002 b 83dc <main+0x90>
case 5:
a++; break;
83d0: e59d3004 ldr r3, [sp, #4]
83d4: e2833001 add r3, r3, #1 ; 0x1
83d8: e58d3004 str r3, [sp, #4]
default:
break;
}
return 0;
83dc: e3a03000 mov r3, #0 ; 0x0
}
再来试一些不规则的case。
int main(void)
{
834c: e24dd008 sub sp, sp, #8 ; 0x8
int a = 9;
8350: e3a03009 mov r3, #9 ; 0x9
8354: e58d3004 str r3, [sp, #4]
switch(a)
8358: e59d3004 ldr r3, [sp, #4]
835c: e3530009 cmp r3, #9 ; 0x9
8360: 908ff103 addls pc, pc, r3, lsl #2
8364: ea000020 b 83ec <main+0xa0>
8368: ea000008 b 8390 <main+0x44>
836c: ea00000b b 83a0 <main+0x54>
8370: ea00000e b 83b0 <main+0x64>
8374: ea00001c b 83ec <main+0xa0>
8378: ea000018 b 83e0 <main+0x94>
837c: ea00001a b 83ec <main+0xa0>
8380: ea000012 b 83d0 <main+0x84>
8384: ea000018 b 83ec <main+0xa0>
8388: ea000017 b 83ec <main+0xa0>
838c: ea00000b b 83c0 <main+0x74> //发生了看似不规则的变化，跳转未按地址顺序排列
{
case 0:
a++; break;
8390: e59d3004 ldr r3, [sp, #4]
8394: e2833001 add r3, r3, #1 ; 0x1
8398: e58d3004 str r3, [sp, #4]
839c: ea000012 b 83ec <main+0xa0>
case 1:
a++; break;
83a0: e59d3004 ldr r3, [sp, #4]
83a4: e2833001 add r3, r3, #1 ; 0x1
83a8: e58d3004 str r3, [sp, #4]
83ac: ea00000e b 83ec <main+0xa0>
case 2:
a++; break;
83b0: e59d3004 ldr r3, [sp, #4]
83b4: e2833001 add r3, r3, #1 ; 0x1
83b8: e58d3004 str r3, [sp, #4]
83bc: ea00000a b 83ec <main+0xa0>
case 9:
a++; break;
83c0: e59d3004 ldr r3, [sp, #4]
83c4: e2833001 add r3, r3, #1 ; 0x1
83c8: e58d3004 str r3, [sp, #4]
83cc: ea000006 b 83ec <main+0xa0>
case 6:
a++; break;
83d0: e59d3004 ldr r3, [sp, #4]
83d4: e2833001 add r3, r3, #1 ; 0x1
83d8: e58d3004 str r3, [sp, #4]
83dc: ea000002 b 83ec <main+0xa0>
case 4:
a++; break;
83e0: e59d3004 ldr r3, [sp, #4]
83e4: e2833001 add r3, r3, #1 ; 0x1
83e8: e58d3004 str r3, [sp, #4]
default:
break;
}
return 0;
83ec: e3a03000 mov r3, #0 ; 0x0
}
事物总不是绝对的，switch也是如此。
int main(void)
{
834c: e24dd010 sub sp, sp, #16 ; 0x10
int a = 90;
8350: e3a0305a mov r3, #90 ; 0x5a
8354: e58d300c str r3, [sp, #12]
switch(a)
8358: e59d300c ldr r3, [sp, #12]
835c: e58d3004 str r3, [sp, #4]
8360: e59d3004 ldr r3, [sp, #4]
8364: e3530002 cmp r3, #2 ; 0x2
8368: 0a00001b beq 83dc <main+0x90> //退化成了 if ！
836c: e59d3004 ldr r3, [sp, #4]
8370: e3530002 cmp r3, #2 ; 0x2
8374: ca000006 bgt 8394 <main+0x48>
8378: e59d3004 ldr r3, [sp, #4]
837c: e3530000 cmp r3, #0 ; 0x0
8380: 0a00000d beq 83bc <main+0x70>
8384: e59d3004 ldr r3, [sp, #4]
8388: e3530001 cmp r3, #1 ; 0x1
838c: 0a00000e beq 83cc <main+0x80>
8390: ea000020 b 8418 <main+0xcc>
8394: e59d3004 ldr r3, [sp, #4]
8398: e3530006 cmp r3, #6 ; 0x6
839c: 0a000016 beq 83fc <main+0xb0>
83a0: e59d3004 ldr r3, [sp, #4]
83a4: e353005a cmp r3, #90 ; 0x5a
83a8: 0a00000f beq 83ec <main+0xa0>
83ac: e59d3004 ldr r3, [sp, #4]
83b0: e3530004 cmp r3, #4 ; 0x4
83b4: 0a000014 beq 840c <main+0xc0>
83b8: ea000016 b 8418 <main+0xcc>
{
case 0:
a++; break;
83bc: e59d300c ldr r3, [sp, #12]
83c0: e2833001 add r3, r3, #1 ; 0x1
83c4: e58d300c str r3, [sp, #12]
83c8: ea000012 b 8418 <main+0xcc>
case 1:
a++; break;
83cc: e59d300c ldr r3, [sp, #12]
83d0: e2833001 add r3, r3, #1 ; 0x1
83d4: e58d300c str r3, [sp, #12]
83d8: ea00000e b 8418 <main+0xcc>
case 2:
a++; break;
83dc: e59d300c ldr r3, [sp, #12]
83e0: e2833001 add r3, r3, #1 ; 0x1
83e4: e58d300c str r3, [sp, #12]
83e8: ea00000a b 8418 <main+0xcc>
case 90:
a++; break;
83ec: e59d300c ldr r3, [sp, #12]
83f0: e2833001 add r3, r3, #1 ; 0x1
83f4: e58d300c str r3, [sp, #12]
83f8: ea000006 b 8418 <main+0xcc>
case 6:
a++; break;
83fc: e59d300c ldr r3, [sp, #12]
8400: e2833001 add r3, r3, #1 ; 0x1
8404: e58d300c str r3, [sp, #12]
8408: ea000002 b 8418 <main+0xcc>
case 4:
a++; break;
840c: e59d300c ldr r3, [sp, #12]
8410: e2833001 add r3, r3, #1 ; 0x1
8414: e58d300c str r3, [sp, #12]
default:
break;
}
return 0;
8418: e3a03000 mov r3, #0 ; 0x0
}
秘密原来就在于此……
0000834c <func>:
int func(int a)
{
834c: e24dd008 sub sp, sp, #8 ; 0x8
8350: e58d0004 str r0, [sp, #4]
switch(a)
8354: e59d3004 ldr r3, [sp, #4]
8358: e3530009 cmp r3, #9 ; 0x9
835c: 908ff103 addls pc, pc, r3, lsl #2 ///看了下面的排列方式，也就清楚了这条指令的内涵
8360: ea000020 b 83e8 <func+0x9c>
8364: ea000008 b 838c <func+0x40> //0
8368: ea00000b b 839c <func+0x50> //1
836c: ea00000e b 83ac <func+0x60> //2
8370: ea00001c b 83e8 <func+0x9c>
8374: ea000018 b 83dc <func+0x90> //4
8378: ea00001a b 83e8 <func+0x9c>
837c: ea000012 b 83cc <func+0x80> //6
8380: ea000018 b 83e8 <func+0x9c>
8384: ea000017 b 83e8 <func+0x9c>
8388: ea00000b b 83bc <func+0x70> //9
{
case 0:
a++; break;
838c: e59d3004 ldr r3, [sp, #4]
8390: e2833001 add r3, r3, #1 ; 0x1
8394: e58d3004 str r3, [sp, #4]
8398: ea000012 b 83e8 <func+0x9c>
case 1:
a++; break;
839c: e59d3004 ldr r3, [sp, #4]
83a0: e2833001 add r3, r3, #1 ; 0x1
83a4: e58d3004 str r3, [sp, #4]
83a8: ea00000e b 83e8 <func+0x9c>
case 2:
a++; break;
83ac: e59d3004 ldr r3, [sp, #4]
83b0: e2833001 add r3, r3, #1 ; 0x1
83b4: e58d3004 str r3, [sp, #4]
83b8: ea00000a b 83e8 <func+0x9c>
case 9:
a++; break;
83bc: e59d3004 ldr r3, [sp, #4]
83c0: e2833001 add r3, r3, #1 ; 0x1
83c4: e58d3004 str r3, [sp, #4]
83c8: ea000006 b 83e8 <func+0x9c>
case 6:
a++; break;
83cc: e59d3004 ldr r3, [sp, #4]
83d0: e2833001 add r3, r3, #1 ; 0x1
83d4: e58d3004 str r3, [sp, #4]
83d8: ea000002 b 83e8 <func+0x9c>
case 4:
a++; break;
83dc: e59d3004 ldr r3, [sp, #4]
83e0: e2833001 add r3, r3, #1 ; 0x1
83e4: e58d3004 str r3, [sp, #4]
default:
break;
}
return a;
83e8: e59d3004 ldr r3, [sp, #4]
}
最后再来一个极端的实验：
0000834c <func>:
int func(int a)
{
834c: e24dd008 sub sp, sp, #8 ; 0x8
8350: e58d0004 str r0, [sp, #4]
switch(a)
8354: e59d3004 ldr r3, [sp, #4]
8358: e353003c cmp r3, #60 ; 0x3c //这里60是上限，至少要六个case，而且要有 case[0-2]，否则编译退化为if
835c: 908ff103 addls pc, pc, r3, lsl #2
8360: ea000053 b 84b4 <func+0x168>
8364: ea00003b b 8458 <func+0x10c> //0
8368: ea00003e b 8468 <func+0x11c> //1
836c: ea000041 b 8478 <func+0x12c> //2
8370: ea00004f b 84b4 <func+0x168>
8374: ea00004b b 84a8 <func+0x15c>
8378: ea00004d b 84b4 <func+0x168>
837c: ea000045 b 8498 <func+0x14c> //6
8380: ea00004b b 84b4 <func+0x168>
8384: ea00004a b 84b4 <func+0x168>
8388: ea000049 b 84b4 <func+0x168>
838c: ea000048 b 84b4 <func+0x168>
8390: ea000047 b 84b4 <func+0x168>
8394: ea000046 b 84b4 <func+0x168>
8398: ea000045 b 84b4 <func+0x168>
839c: ea000044 b 84b4 <func+0x168>
83a0: ea000043 b 84b4 <func+0x168>
83a4: ea000042 b 84b4 <func+0x168>
83a8: ea000041 b 84b4 <func+0x168>
83ac: ea000040 b 84b4 <func+0x168>
83b0: ea00003f b 84b4 <func+0x168>
83b4: ea00003e b 84b4 <func+0x168>
83b8: ea00003d b 84b4 <func+0x168>
83bc: ea00003c b 84b4 <func+0x168>
83c0: ea00003b b 84b4 <func+0x168>
83c4: ea00003a b 84b4 <func+0x168>
83c8: ea000039 b 84b4 <func+0x168>
83cc: ea000038 b 84b4 <func+0x168>
83d0: ea000037 b 84b4 <func+0x168>
83d4: ea000036 b 84b4 <func+0x168>
83d8: ea000035 b 84b4 <func+0x168>
83dc: ea000034 b 84b4 <func+0x168>
83e0: ea000033 b 84b4 <func+0x168>
83e4: ea000032 b 84b4 <func+0x168>
83e8: ea000031 b 84b4 <func+0x168>
83ec: ea000030 b 84b4 <func+0x168>
83f0: ea00002f b 84b4 <func+0x168>
83f4: ea00002e b 84b4 <func+0x168>
83f8: ea00002d b 84b4 <func+0x168>
83fc: ea00002c b 84b4 <func+0x168>
8400: ea00002b b 84b4 <func+0x168>
8404: ea00002a b 84b4 <func+0x168>
8408: ea000029 b 84b4 <func+0x168>
840c: ea000028 b 84b4 <func+0x168>
8410: ea000027 b 84b4 <func+0x168>
8414: ea000026 b 84b4 <func+0x168>
8418: ea000025 b 84b4 <func+0x168>
841c: ea000024 b 84b4 <func+0x168>
8420: ea000023 b 84b4 <func+0x168>
8424: ea000022 b 84b4 <func+0x168>
8428: ea000021 b 84b4 <func+0x168>
842c: ea000020 b 84b4 <func+0x168>
8430: ea00001f b 84b4 <func+0x168>
8434: ea00001e b 84b4 <func+0x168>
8438: ea00001d b 84b4 <func+0x168>
843c: ea00001c b 84b4 <func+0x168>
8440: ea00001b b 84b4 <func+0x168>
8444: ea00001a b 84b4 <func+0x168>
8448: ea000019 b 84b4 <func+0x168>
844c: ea000018 b 84b4 <func+0x168>
8450: ea000017 b 84b4 <func+0x168>
8454: ea00000b b 8488 <func+0x13c>
{
case 0:
a++; break;
8458: e59d3004 ldr r3, [sp, #4]
845c: e2833001 add r3, r3, #1 ; 0x1
8460: e58d3004 str r3, [sp, #4]
8464: ea000012 b 84b4 <func+0x168>
case 1:
a++; break;
8468: e59d3004 ldr r3, [sp, #4]
846c: e2833001 add r3, r3, #1 ; 0x1
8470: e58d3004 str r3, [sp, #4]
8474: ea00000e b 84b4 <func+0x168>
case 2:
a++; break;
8478: e59d3004 ldr r3, [sp, #4]
847c: e2833001 add r3, r3, #1 ; 0x1
8480: e58d3004 str r3, [sp, #4]
8484: ea00000a b 84b4 <func+0x168>
case 60:
a++; break;
8488: e59d3004 ldr r3, [sp, #4]
848c: e2833001 add r3, r3, #1 ; 0x1
8490: e58d3004 str r3, [sp, #4]
8494: ea000006 b 84b4 <func+0x168>
case 6:
a++; break;
8498: e59d3004 ldr r3, [sp, #4]
849c: e2833001 add r3, r3, #1 ; 0x1
84a0: e58d3004 str r3, [sp, #4]
84a4: ea000002 b 84b4 <func+0x168>
case 4:
a++; break;
84a8: e59d3004 ldr r3, [sp, #4]
84ac: e2833001 add r3, r3, #1 ; 0x1
84b0: e58d3004 str r3, [sp, #4]
default:
break;
}
return a;
84b4: e59d3004 ldr r3, [sp, #4]
}
84b8: e1a00003 mov r0, r3
84bc: e28dd008 add sp, sp, #8 ; 0x8
84c0: e12fff1e bx lr
上面的case最大值取了60，若再增加，编译退化。
而对于字符 case 'a' ，switch的退化条件有些区别，本质还是一样，不妨自己动手一试。
只是简单的贴了些代码，重在能说明问题的关键，点到为止。
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