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我正在处理一些需要高度优化的 Java 代码，因为它将在我的主程序逻辑中的许多点调用的热函数中运行。此代码的一部分涉及将double变量乘以10任意非负int exponents。获得相乘值的一种快速方法（编辑：但不是最快的，请参见下面的更新 2）是switch在exponent：

double multiplyByPowerOfTen(final double d, final int exponent) {
   switch (exponent) {
      case 0:
         return d;
      case 1:
         return d*10;
      case 2:
         return d*100;
      // ... same pattern
      case 9:
         return d*1000000000;
      case 10:
         return d*10000000000L;
      // ... same pattern with long literals
      case 18:
         return d*1000000000000000000L;
      default:
         throw new ParseException("Unhandled power of ten " + power, 0);
   }
}

上面注释的省略号表示case int常量继续递增 1，因此上面的代码片段中确实有 19 cases。由于我不确定在case语句10thru中是否真的需要 10 的所有幂18，我运行了一些微基准测试，比较了使用此switch语句完成 1000 万次操作的时间switch与仅使用cases 0thru9的时间（exponent限制为 9 或更少避免打破精简switch）。我得到了一个相当令人惊讶的结果（至少对我来说！），越长switch的case语句实际上运行得越快。

在云雀上，我尝试添加更多case的 s ，它只是返回了虚拟值，并发现我可以让开关运行得更快，大约 22-27 个声明case的 s （即使在代码运行时这些虚拟案例实际上从未被命中） ）。（同样，通过将先前常数case增加 s 以连续方式添加 s 。）这些执行时间差异不是很显着：对于 和 之间的随机，虚拟填充语句在 1.49 秒内完成 1000 万次执行，而未填充语句为 1.54 秒版本，每次执行总共节省 5ns。所以，不是那种让人痴迷于填充的东西case1exponent010switchswitch从优化的角度来看，声明值得付出努力。但是我仍然觉得奇怪和违反直觉的是，当添加更多的 s 时，它的 执行switch速度并没有变慢（或者可能充其量保持恒定的O(1)时间） 。case

切换基准测试结果

这些是我通过对随机生成的exponent值进行各种限制而获得的结果。我没有将结果一直计算到1极限exponent，但曲线的总体形状保持不变，在 12-17 的情况下有一个山脊，在 18-28 之间有一个谷。所有测试都在 JUnitBenchmarks 中运行，使用共享容器获取随机值，以确保相同的测试输入。我还按照从最长switch语句到最短语句的顺序运行了测试，反之亦然，以尝试消除与排序相关的测试问题的可能性。如果有人想尝试重现这些结果，我已经将我的测试代码放在了 github repo 上。

那么，这里发生了什么？我的架构或微基准构建的一些变幻莫测？还是 Java在to范围内switch的执行速度真的比up to快一点？1828 case1117

github 测试仓库“切换实验”

更新：我清理了基准测试库，并在 /results 中添加了一个文本文件，其中包含更广泛的可能exponent值的一些输出。我还在测试代码中添加了一个选项，不要抛出Exceptionfrom default，但这似乎不会影响结果。

更新 2：在 2009 年的 xkcd 论坛上发现了一些关于这个问题的很好的讨论：http ://forums.xkcd.com/viewtopic.php?f=11&t=33524 。OP 对 using 的讨论Array.binarySearch()让我想到了上面求幂模式的简单的基于数组的实现。不需要二进制搜索，因为我知道其中的条目是什么array。它的运行速度似乎比 using 快 3 倍左右switch，显然是以牺牲一些提供的控制流为代价的switch。该代码也已添加到 github 存储库中。

4

5 回答 5

236

As pointed out by the other answer, because the case values are contiguous (as opposed to sparse), the generated bytecode for your various tests uses a switch table (bytecode instruction tableswitch).

However, once the JIT starts its job and compiles the bytecode into assembly, the tableswitch instruction does not always result in an array of pointers: sometimes the switch table is transformed into what looks like a lookupswitch (similar to an if/else if structure).

Decompiling the assembly generated by the JIT (hotspot JDK 1.7) shows that it uses a succession of if/else if when there are 17 cases or less, an array of pointers when there are more than 18 (more efficient).

The reason why this magic number of 18 is used seems to come down to the default value of the MinJumpTableSize JVM flag (around line 352 in the code).

I have raised the issue on the hotspot compiler list and it seems to be a legacy of past testing. Note that this default value has been removed in JDK 8 after more benchmarking was performed.

Finally, when the method becomes too long (> 25 cases in my tests), it is in not inlined any longer with the default JVM settings - that is the likeliest cause for the drop in performance at that point.

With 5 cases, the decompiled code looks like this (notice the cmp/je/jg/jmp instructions, the assembly for if/goto):

[Verified Entry Point]
  # {method} 'multiplyByPowerOfTen' '(DI)D' in 'javaapplication4/Test1'
  # parm0:    xmm0:xmm0   = double
  # parm1:    rdx       = int
  #           [sp+0x20]  (sp of caller)
  0x00000000024f0160: mov    DWORD PTR [rsp-0x6000],eax
                                                ;   {no_reloc}
  0x00000000024f0167: push   rbp
  0x00000000024f0168: sub    rsp,0x10           ;*synchronization entry
                                                ; - javaapplication4.Test1::multiplyByPowerOfTen@-1 (line 56)
  0x00000000024f016c: cmp    edx,0x3
  0x00000000024f016f: je     0x00000000024f01c3
  0x00000000024f0171: cmp    edx,0x3
  0x00000000024f0174: jg     0x00000000024f01a5
  0x00000000024f0176: cmp    edx,0x1
  0x00000000024f0179: je     0x00000000024f019b
  0x00000000024f017b: cmp    edx,0x1
  0x00000000024f017e: jg     0x00000000024f0191
  0x00000000024f0180: test   edx,edx
  0x00000000024f0182: je     0x00000000024f01cb
  0x00000000024f0184: mov    ebp,edx
  0x00000000024f0186: mov    edx,0x17
  0x00000000024f018b: call   0x00000000024c90a0  ; OopMap{off=48}
                                                ;*new  ; - javaapplication4.Test1::multiplyByPowerOfTen@72 (line 83)
                                                ;   {runtime_call}
  0x00000000024f0190: int3                      ;*new  ; - javaapplication4.Test1::multiplyByPowerOfTen@72 (line 83)
  0x00000000024f0191: mulsd  xmm0,QWORD PTR [rip+0xffffffffffffffa7]        # 0x00000000024f0140
                                                ;*dmul
                                                ; - javaapplication4.Test1::multiplyByPowerOfTen@52 (line 62)
                                                ;   {section_word}
  0x00000000024f0199: jmp    0x00000000024f01cb
  0x00000000024f019b: mulsd  xmm0,QWORD PTR [rip+0xffffffffffffff8d]        # 0x00000000024f0130
                                                ;*dmul
                                                ; - javaapplication4.Test1::multiplyByPowerOfTen@46 (line 60)
                                                ;   {section_word}
  0x00000000024f01a3: jmp    0x00000000024f01cb
  0x00000000024f01a5: cmp    edx,0x5
  0x00000000024f01a8: je     0x00000000024f01b9
  0x00000000024f01aa: cmp    edx,0x5
  0x00000000024f01ad: jg     0x00000000024f0184  ;*tableswitch
                                                ; - javaapplication4.Test1::multiplyByPowerOfTen@1 (line 56)
  0x00000000024f01af: mulsd  xmm0,QWORD PTR [rip+0xffffffffffffff81]        # 0x00000000024f0138
                                                ;*dmul
                                                ; - javaapplication4.Test1::multiplyByPowerOfTen@64 (line 66)
                                                ;   {section_word}
  0x00000000024f01b7: jmp    0x00000000024f01cb
  0x00000000024f01b9: mulsd  xmm0,QWORD PTR [rip+0xffffffffffffff67]        # 0x00000000024f0128
                                                ;*dmul
                                                ; - javaapplication4.Test1::multiplyByPowerOfTen@70 (line 68)
                                                ;   {section_word}
  0x00000000024f01c1: jmp    0x00000000024f01cb
  0x00000000024f01c3: mulsd  xmm0,QWORD PTR [rip+0xffffffffffffff55]        # 0x00000000024f0120
                                                ;*tableswitch
                                                ; - javaapplication4.Test1::multiplyByPowerOfTen@1 (line 56)
                                                ;   {section_word}
  0x00000000024f01cb: add    rsp,0x10
  0x00000000024f01cf: pop    rbp
  0x00000000024f01d0: test   DWORD PTR [rip+0xfffffffffdf3fe2a],eax        # 0x0000000000430000
                                                ;   {poll_return}
  0x00000000024f01d6: ret

With 18 cases, the assembly looks like this (notice the array of pointers which is used and suppresses the need for all the comparisons: jmp QWORD PTR [r8+r10*1] jumps directly to the right multiplication) - that is the likely reason for the performance improvement:

[Verified Entry Point]
  # {method} 'multiplyByPowerOfTen' '(DI)D' in 'javaapplication4/Test1'
  # parm0:    xmm0:xmm0   = double
  # parm1:    rdx       = int
  #           [sp+0x20]  (sp of caller)
  0x000000000287fe20: mov    DWORD PTR [rsp-0x6000],eax
                                                ;   {no_reloc}
  0x000000000287fe27: push   rbp
  0x000000000287fe28: sub    rsp,0x10           ;*synchronization entry
                                                ; - javaapplication4.Test1::multiplyByPowerOfTen@-1 (line 56)
  0x000000000287fe2c: cmp    edx,0x13
  0x000000000287fe2f: jae    0x000000000287fe46
  0x000000000287fe31: movsxd r10,edx
  0x000000000287fe34: shl    r10,0x3
  0x000000000287fe38: movabs r8,0x287fd70       ;   {section_word}
  0x000000000287fe42: jmp    QWORD PTR [r8+r10*1]  ;*tableswitch
                                                ; - javaapplication4.Test1::multiplyByPowerOfTen@1 (line 56)
  0x000000000287fe46: mov    ebp,edx
  0x000000000287fe48: mov    edx,0x31
  0x000000000287fe4d: xchg   ax,ax
  0x000000000287fe4f: call   0x00000000028590a0  ; OopMap{off=52}
                                                ;*new  ; - javaapplication4.Test1::multiplyByPowerOfTen@202 (line 96)
                                                ;   {runtime_call}
  0x000000000287fe54: int3                      ;*new  ; - javaapplication4.Test1::multiplyByPowerOfTen@202 (line 96)
  0x000000000287fe55: mulsd  xmm0,QWORD PTR [rip+0xfffffffffffffe8b]        # 0x000000000287fce8
                                                ;*dmul
                                                ; - javaapplication4.Test1::multiplyByPowerOfTen@194 (line 92)
                                                ;   {section_word}
  0x000000000287fe5d: jmp    0x000000000287ff16
  0x000000000287fe62: mulsd  xmm0,QWORD PTR [rip+0xfffffffffffffe86]        # 0x000000000287fcf0
                                                ;*dmul
                                                ; - javaapplication4.Test1::multiplyByPowerOfTen@188 (line 90)
                                                ;   {section_word}
  0x000000000287fe6a: jmp    0x000000000287ff16
  0x000000000287fe6f: mulsd  xmm0,QWORD PTR [rip+0xfffffffffffffe81]        # 0x000000000287fcf8
                                                ;*dmul
                                                ; - javaapplication4.Test1::multiplyByPowerOfTen@182 (line 88)
                                                ;   {section_word}
  0x000000000287fe77: jmp    0x000000000287ff16
  0x000000000287fe7c: mulsd  xmm0,QWORD PTR [rip+0xfffffffffffffe7c]        # 0x000000000287fd00
                                                ;*dmul
                                                ; - javaapplication4.Test1::multiplyByPowerOfTen@176 (line 86)
                                                ;   {section_word}
  0x000000000287fe84: jmp    0x000000000287ff16
  0x000000000287fe89: mulsd  xmm0,QWORD PTR [rip+0xfffffffffffffe77]        # 0x000000000287fd08
                                                ;*dmul
                                                ; - javaapplication4.Test1::multiplyByPowerOfTen@170 (line 84)
                                                ;   {section_word}
  0x000000000287fe91: jmp    0x000000000287ff16
  0x000000000287fe96: mulsd  xmm0,QWORD PTR [rip+0xfffffffffffffe72]        # 0x000000000287fd10
                                                ;*dmul
                                                ; - javaapplication4.Test1::multiplyByPowerOfTen@164 (line 82)
                                                ;   {section_word}
  0x000000000287fe9e: jmp    0x000000000287ff16
  0x000000000287fea0: mulsd  xmm0,QWORD PTR [rip+0xfffffffffffffe70]        # 0x000000000287fd18
                                                ;*dmul
                                                ; - javaapplication4.Test1::multiplyByPowerOfTen@158 (line 80)
                                                ;   {section_word}
  0x000000000287fea8: jmp    0x000000000287ff16
  0x000000000287feaa: mulsd  xmm0,QWORD PTR [rip+0xfffffffffffffe6e]        # 0x000000000287fd20
                                                ;*dmul
                                                ; - javaapplication4.Test1::multiplyByPowerOfTen@152 (line 78)
                                                ;   {section_word}
  0x000000000287feb2: jmp    0x000000000287ff16
  0x000000000287feb4: mulsd  xmm0,QWORD PTR [rip+0xfffffffffffffe24]        # 0x000000000287fce0
                                                ;*dmul
                                                ; - javaapplication4.Test1::multiplyByPowerOfTen@146 (line 76)
                                                ;   {section_word}
  0x000000000287febc: jmp    0x000000000287ff16
  0x000000000287febe: mulsd  xmm0,QWORD PTR [rip+0xfffffffffffffe6a]        # 0x000000000287fd30
                                                ;*dmul
                                                ; - javaapplication4.Test1::multiplyByPowerOfTen@140 (line 74)
                                                ;   {section_word}
  0x000000000287fec6: jmp    0x000000000287ff16
  0x000000000287fec8: mulsd  xmm0,QWORD PTR [rip+0xfffffffffffffe68]        # 0x000000000287fd38
                                                ;*dmul
                                                ; - javaapplication4.Test1::multiplyByPowerOfTen@134 (line 72)
                                                ;   {section_word}
  0x000000000287fed0: jmp    0x000000000287ff16
  0x000000000287fed2: mulsd  xmm0,QWORD PTR [rip+0xfffffffffffffe66]        # 0x000000000287fd40
                                                ;*dmul
                                                ; - javaapplication4.Test1::multiplyByPowerOfTen@128 (line 70)
                                                ;   {section_word}
  0x000000000287feda: jmp    0x000000000287ff16
  0x000000000287fedc: mulsd  xmm0,QWORD PTR [rip+0xfffffffffffffe64]        # 0x000000000287fd48
                                                ;*dmul
                                                ; - javaapplication4.Test1::multiplyByPowerOfTen@122 (line 68)
                                                ;   {section_word}
  0x000000000287fee4: jmp    0x000000000287ff16
  0x000000000287fee6: mulsd  xmm0,QWORD PTR [rip+0xfffffffffffffe62]        # 0x000000000287fd50
                                                ;*dmul
                                                ; - javaapplication4.Test1::multiplyByPowerOfTen@116 (line 66)
                                                ;   {section_word}
  0x000000000287feee: jmp    0x000000000287ff16
  0x000000000287fef0: mulsd  xmm0,QWORD PTR [rip+0xfffffffffffffe60]        # 0x000000000287fd58
                                                ;*dmul
                                                ; - javaapplication4.Test1::multiplyByPowerOfTen@110 (line 64)
                                                ;   {section_word}
  0x000000000287fef8: jmp    0x000000000287ff16
  0x000000000287fefa: mulsd  xmm0,QWORD PTR [rip+0xfffffffffffffe5e]        # 0x000000000287fd60
                                                ;*dmul
                                                ; - javaapplication4.Test1::multiplyByPowerOfTen@104 (line 62)
                                                ;   {section_word}
  0x000000000287ff02: jmp    0x000000000287ff16
  0x000000000287ff04: mulsd  xmm0,QWORD PTR [rip+0xfffffffffffffe5c]        # 0x000000000287fd68
                                                ;*dmul
                                                ; - javaapplication4.Test1::multiplyByPowerOfTen@98 (line 60)
                                                ;   {section_word}
  0x000000000287ff0c: jmp    0x000000000287ff16
  0x000000000287ff0e: mulsd  xmm0,QWORD PTR [rip+0xfffffffffffffe12]        # 0x000000000287fd28
                                                ;*tableswitch
                                                ; - javaapplication4.Test1::multiplyByPowerOfTen@1 (line 56)
                                                ;   {section_word}
  0x000000000287ff16: add    rsp,0x10
  0x000000000287ff1a: pop    rbp
  0x000000000287ff1b: test   DWORD PTR [rip+0xfffffffffd9b00df],eax        # 0x0000000000230000
                                                ;   {poll_return}
  0x000000000287ff21: ret

And finally the assembly with 30 cases (below) looks similar to 18 cases, except for the additional movapd xmm0,xmm1 that appears towards the middle of the code, as spotted by @cHao - however the likeliest reason for the drop in performance is that the method is too long to be inlined with the default JVM settings:

[Verified Entry Point]
  # {method} 'multiplyByPowerOfTen' '(DI)D' in 'javaapplication4/Test1'
  # parm0:    xmm0:xmm0   = double
  # parm1:    rdx       = int
  #           [sp+0x20]  (sp of caller)
  0x0000000002524560: mov    DWORD PTR [rsp-0x6000],eax
                                                ;   {no_reloc}
  0x0000000002524567: push   rbp
  0x0000000002524568: sub    rsp,0x10           ;*synchronization entry
                                                ; - javaapplication4.Test1::multiplyByPowerOfTen@-1 (line 56)
  0x000000000252456c: movapd xmm1,xmm0
  0x0000000002524570: cmp    edx,0x1f
  0x0000000002524573: jae    0x0000000002524592  ;*tableswitch
                                                ; - javaapplication4.Test1::multiplyByPowerOfTen@1 (line 56)
  0x0000000002524575: movsxd r10,edx
  0x0000000002524578: shl    r10,0x3
  0x000000000252457c: mulsd  xmm0,QWORD PTR [rip+0xfffffffffffffe3c]        # 0x00000000025243c0
                                                ;*dmul
                                                ; - javaapplication4.Test1::multiplyByPowerOfTen@364 (line 118)
                                                ;   {section_word}
  0x0000000002524584: movabs r8,0x2524450       ;   {section_word}
  0x000000000252458e: jmp    QWORD PTR [r8+r10*1]  ;*tableswitch
                                                ; - javaapplication4.Test1::multiplyByPowerOfTen@1 (line 56)
  0x0000000002524592: mov    ebp,edx
  0x0000000002524594: mov    edx,0x31
  0x0000000002524599: xchg   ax,ax
  0x000000000252459b: call   0x00000000024f90a0  ; OopMap{off=64}
                                                ;*new  ; - javaapplication4.Test1::multiplyByPowerOfTen@370 (line 120)
                                                ;   {runtime_call}
  0x00000000025245a0: int3                      ;*new  ; - javaapplication4.Test1::multiplyByPowerOfTen@370 (line 120)
  0x00000000025245a1: mulsd  xmm0,QWORD PTR [rip+0xfffffffffffffe27]        # 0x00000000025243d0
                                                ;*dmul
                                                ; - javaapplication4.Test1::multiplyByPowerOfTen@358 (line 116)
                                                ;   {section_word}
  0x00000000025245a9: jmp    0x0000000002524744
  0x00000000025245ae: mulsd  xmm0,QWORD PTR [rip+0xfffffffffffffe22]        # 0x00000000025243d8
                                                ;*dmul
                                                ; - javaapplication4.Test1::multiplyByPowerOfTen@348 (line 114)
                                                ;   {section_word}
  0x00000000025245b6: jmp    0x0000000002524744
  0x00000000025245bb: mulsd  xmm0,QWORD PTR [rip+0xfffffffffffffe1d]        # 0x00000000025243e0
                                                ;*dmul
                                                ; - javaapplication4.Test1::multiplyByPowerOfTen@338 (line 112)
                                                ;   {section_word}
  0x00000000025245c3: jmp    0x0000000002524744
  0x00000000025245c8: mulsd  xmm0,QWORD PTR [rip+0xfffffffffffffe18]        # 0x00000000025243e8
                                                ;*dmul
                                                ; - javaapplication4.Test1::multiplyByPowerOfTen@328 (line 110)
                                                ;   {section_word}
  0x00000000025245d0: jmp    0x0000000002524744
  0x00000000025245d5: mulsd  xmm0,QWORD PTR [rip+0xfffffffffffffe13]        # 0x00000000025243f0
                                                ;*dmul
                                                ; - javaapplication4.Test1::multiplyByPowerOfTen@318 (line 108)
                                                ;   {section_word}
  0x00000000025245dd: jmp    0x0000000002524744
  0x00000000025245e2: mulsd  xmm0,QWORD PTR [rip+0xfffffffffffffe0e]        # 0x00000000025243f8
                                                ;*dmul
                                                ; - javaapplication4.Test1::multiplyByPowerOfTen@308 (line 106)
                                                ;   {section_word}
  0x00000000025245ea: jmp    0x0000000002524744
  0x00000000025245ef: mulsd  xmm0,QWORD PTR [rip+0xfffffffffffffe09]        # 0x0000000002524400
                                                ;*dmul
                                                ; - javaapplication4.Test1::multiplyByPowerOfTen@298 (line 104)
                                                ;   {section_word}
  0x00000000025245f7: jmp    0x0000000002524744
  0x00000000025245fc: mulsd  xmm0,QWORD PTR [rip+0xfffffffffffffe04]        # 0x0000000002524408
                                                ;*dmul
                                                ; - javaapplication4.Test1::multiplyByPowerOfTen@288 (line 102)
                                                ;   {section_word}
  0x0000000002524604: jmp    0x0000000002524744
  0x0000000002524609: mulsd  xmm0,QWORD PTR [rip+0xfffffffffffffdff]        # 0x0000000002524410
                                                ;*dmul
                                                ; - javaapplication4.Test1::multiplyByPowerOfTen@278 (line 100)
                                                ;   {section_word}
  0x0000000002524611: jmp    0x0000000002524744
  0x0000000002524616: mulsd  xmm0,QWORD PTR [rip+0xfffffffffffffdfa]        # 0x0000000002524418
                                                ;*dmul
                                                ; - javaapplication4.Test1::multiplyByPowerOfTen@268 (line 98)
                                                ;   {section_word}
  0x000000000252461e: jmp    0x0000000002524744
  0x0000000002524623: mulsd  xmm0,QWORD PTR [rip+0xfffffffffffffd9d]        # 0x00000000025243c8
                                                ;*dmul
                                                ; - javaapplication4.Test1::multiplyByPowerOfTen@258 (line 96)
                                                ;   {section_word}
  0x000000000252462b: jmp    0x0000000002524744
  0x0000000002524630: movapd xmm0,xmm1
  0x0000000002524634: mulsd  xmm0,QWORD PTR [rip+0xfffffffffffffe0c]        # 0x0000000002524448
                                                ;*dmul
                                                ; - javaapplication4.Test1::multiplyByPowerOfTen@242 (line 92)
                                                ;   {section_word}
  0x000000000252463c: jmp    0x0000000002524744
  0x0000000002524641: movapd xmm0,xmm1
  0x0000000002524645: mulsd  xmm0,QWORD PTR [rip+0xfffffffffffffddb]        # 0x0000000002524428
                                                ;*dmul
                                                ; - javaapplication4.Test1::multiplyByPowerOfTen@236 (line 90)
                                                ;   {section_word}
  0x000000000252464d: jmp    0x0000000002524744
  0x0000000002524652: movapd xmm0,xmm1
  0x0000000002524656: mulsd  xmm0,QWORD PTR [rip+0xfffffffffffffdd2]        # 0x0000000002524430
                                                ;*dmul
                                                ; - javaapplication4.Test1::multiplyByPowerOfTen@230 (line 88)
                                                ;   {section_word}
  0x000000000252465e: jmp    0x0000000002524744
  0x0000000002524663: movapd xmm0,xmm1
  0x0000000002524667: mulsd  xmm0,QWORD PTR [rip+0xfffffffffffffdc9]        # 0x0000000002524438
                                                ;*dmul
                                                ; - javaapplication4.Test1::multiplyByPowerOfTen@224 (line 86)
                                                ;   {section_word}

[etc.]

  0x0000000002524744: add    rsp,0x10
  0x0000000002524748: pop    rbp
  0x0000000002524749: test   DWORD PTR [rip+0xfffffffffde1b8b1],eax        # 0x0000000000340000
                                                ;   {poll_return}
  0x000000000252474f: ret
于 2013-03-25T17:59:57.843 回答
46

如果将 case 值放在一个狭窄的范围内，则 Switch - case 会更快。

case 1:
case 2:
case 3:
..
..
case n:

因为，在这种情况下，编译器可以避免对 switch 语句中的每个 case 分支执行比较。编译器制作一个跳转表，其中包含要在不同腿上采取的动作的地址。正在执行切换的值被操作以将其转换为jump table. 在此实现中，switch 语句所用的时间远少于等效 if-else-if 语句级联所用的时间。此外，switch 语句所用的时间与 switch 语句中的 case 分支数无关。

正如维基百科中关于编译部分的switch 语句所述。

如果输入值的范围可以识别为“小”并且只有几个间隙，则某些包含优化器的编译器实际上可能将 switch 语句实现为分支表或索引函数指针数组，而不是冗长的一系列条件指令。这允许 switch 语句立即确定要执行的分支，而无需通过比较列表。

于 2013-03-25T17:34:32.183 回答
31

答案在于字节码：

SwitchTest10.java

public class SwitchTest10 {

    public static void main(String[] args) {
        int n = 0;

        switcher(n);
    }

    public static void switcher(int n) {
        switch(n) {
            case 0: System.out.println(0);
                    break;

            case 1: System.out.println(1);
                    break;

            case 2: System.out.println(2);
                    break;

            case 3: System.out.println(3);
                    break;

            case 4: System.out.println(4);
                    break;

            case 5: System.out.println(5);
                    break;

            case 6: System.out.println(6);
                    break;

            case 7: System.out.println(7);
                    break;

            case 8: System.out.println(8);
                    break;

            case 9: System.out.println(9);
                    break;

            case 10: System.out.println(10);
                    break;

            default: System.out.println("test");
        }
    }       
}

对应的字节码；仅显示相关部分：

public static void switcher(int);
  Code:
   0:   iload_0
   1:   tableswitch{ //0 to 10
        0: 60;
        1: 70;
        2: 80;
        3: 90;
        4: 100;
        5: 110;
        6: 120;
        7: 131;
        8: 142;
        9: 153;
        10: 164;
        default: 175 }

SwitchTest22.java：

public class SwitchTest22 {

    public static void main(String[] args) {
        int n = 0;

        switcher(n);
    }

    public static void switcher(int n) {
        switch(n) {
            case 0: System.out.println(0);
                    break;

            case 1: System.out.println(1);
                    break;

            case 2: System.out.println(2);
                    break;

            case 3: System.out.println(3);
                    break;

            case 4: System.out.println(4);
                    break;

            case 5: System.out.println(5);
                    break;

            case 6: System.out.println(6);
                    break;

            case 7: System.out.println(7);
                    break;

            case 8: System.out.println(8);
                    break;

            case 9: System.out.println(9);
                    break;

            case 100: System.out.println(10);
                    break;

            case 110: System.out.println(10);
                    break;
            case 120: System.out.println(10);
                    break;
            case 130: System.out.println(10);
                    break;
            case 140: System.out.println(10);
                    break;
            case 150: System.out.println(10);
                    break;
            case 160: System.out.println(10);
                    break;
            case 170: System.out.println(10);
                    break;
            case 180: System.out.println(10);
                    break;
            case 190: System.out.println(10);
                    break;
            case 200: System.out.println(10);
                    break;
            case 210: System.out.println(10);
                    break;

            case 220: System.out.println(10);
                    break;

            default: System.out.println("test");
        }
    }       
}

对应的字节码；同样，仅显示相关部分：

public static void switcher(int);
  Code:
   0:   iload_0
   1:   lookupswitch{ //23
        0: 196;
        1: 206;
        2: 216;
        3: 226;
        4: 236;
        5: 246;
        6: 256;
        7: 267;
        8: 278;
        9: 289;
        100: 300;
        110: 311;
        120: 322;
        130: 333;
        140: 344;
        150: 355;
        160: 366;
        170: 377;
        180: 388;
        190: 399;
        200: 410;
        210: 421;
        220: 432;
        default: 443 }

在第一种情况下，范围很窄，编译后的字节码使用tableswitch. 在第二种情况下，编译后的字节码使用lookupswitch.

在tableswitch中，堆栈顶部的整数值用于索引到表中，以找到分支/跳转目标。然后立即执行此跳转/分支。因此，这是一个O(1)操作。

Alookupswitch比较复杂。在这种情况下，需要将整数值与表中的所有键进行比较，直到找到正确的键。找到键后，将使用分支/跳转目标（该键映射到的）进行跳转。中使用的表lookupswitch已排序，并且可以使用二进制搜索算法找到正确的键。性能为二分查找O(log n)，整个过程也是O(log n)，因为跳转还在O(1)。所以在稀疏范围的情况下性能较低的原因是必须首先查找正确的键，因为您不能直接索引到表中。

如果存在稀疏值并且您只能tableswitch使用 a，则 table 基本上将包含指向该default选项的虚拟条目。例如，假设最后一个条目SwitchTest10.java是21而不是10，你会得到：

public static void switcher(int);
  Code:
   0:   iload_0
   1:   tableswitch{ //0 to 21
        0: 104;
        1: 114;
        2: 124;
        3: 134;
        4: 144;
        5: 154;
        6: 164;
        7: 175;
        8: 186;
        9: 197;
        10: 219;
        11: 219;
        12: 219;
        13: 219;
        14: 219;
        15: 219;
        16: 219;
        17: 219;
        18: 219;
        19: 219;
        20: 219;
        21: 208;
        default: 219 }

所以编译器基本上创建了这个巨大的表，其中包含间隙之间的虚拟条目，指向default指令的分支目标。即使没有default，它也会包含指向switch 块之后的指令的条目。我做了一些基本的测试，发现如果最后一个索引和前一个索引（9）之间的差距大于35，它使用 alookupswitch而不是 a tableswitch。

该switch语句的行为在Java 虚拟机规范 (§3.10)中定义：

在 switch 的情况很少的情况下，tableswitch 指令的表表示在空间方面变得低效。可以改为使用 lookupswitch 指令。lookupswitch 指令将 int 键（案例标签的值）与表中的目标偏移量配对。当执行lookupswitch 指令时，将switch 表达式的值与表中的键进行比较。如果其中一个键与表达式的值匹配，则在关联的目标偏移处继续执行。如果没有键匹配，则在默认目标处继续执行。[...]

于 2013-03-25T18:14:55.777 回答
19

由于问题已经得到解答（或多或少），这里有一些提示。利用

private static final double[] mul={1d, 10d...};
static double multiplyByPowerOfTen(final double d, final int exponent) {
      if (exponent<0 || exponent>=mul.length) throw new ParseException();//or just leave the IOOBE be
      return mul[exponent]*d;
}

该代码使用的IC（指令缓存）要少得多，并且始终是内联的。如果代码很热，该数组将位于 L1 数据缓存中。查找表几乎总是胜利。（特别是在微基准上：D）

编辑：如果您希望该方法是热内联的，请考虑非快速路径throw new ParseException()尽可能短或将它们移动到单独的静态方法（因此使它们尽可能短）。这是throw new ParseException("Unhandled power of ten " + power, 0);一个薄弱的想法 b/c 它消耗了大量可以解释的代码的内联预算 - 字符串连接在字节码中非常冗长。更多信息和带有ArrayList 的真实案例

于 2013-03-27T21:58:52.090 回答
0

基于javac 源码，你可以用它使用的方式编写 switch tableswitch。

我们可以使用 javac 源中的计算来计算您的第二个示例的成本。

lo = 0
hi = 220
nlabels = 24

table_space_cost = 4 + hi - lo + 1
table_time_cost = 3
lookup_space_cost = 3 + 2 * nlabels
lookup_time_cost = nlabels

table_cost = table_space_cost + 3 * table_time_cost // 234
lookup_cost = lookup_space_cost + 3 * lookup_time_cos // 123

这里 tableswitch 的成本 (234) 比 lookupswitch (123) 高，因此 lookupswitch 将被选为这个 switch 语句的操作码。

于 2021-04-20T15:56:12.217 回答