Question

.Net 4（或任何先前版本）是否基于字符串对较长的switch语句执行任何类型的优化？

我正在解决一个潜在的性能瓶颈，因为一些长的switch语句在这些情况下寻找匹配的字符串，而且我一直认为这些是在线性时间内搜索的（或接近线性的，即不快速使用索引）找到匹配的字符串）。但这似乎是.Net可以优化的一个显而易见的领域，所以我想我会检查是否是这种情况。

这是我最近的一个衍生问题：indexed switch statement, or equivalent? .net, C#

Answer 1

编译以下代码。

public static int Main(string[] args)
{
    switch (args[0])
    {
        case "x": return 1;
        case "y": return 2;
        case "z": return 3;
    }
    return 0;
}

现在使用Reflector或ILDASM来检查C＃编译器生成的IL。继续添加案例陈述并反编译并观察结果。

如果case语句的数量很小，则编译器会发出顺序相等比较。
如果case语句的数量很大，那么编译器会发出Dictionary查找。

我使用的是C＃3.0编译器，我观察到策略在7个案例语句中发生了变化。我怀疑你会看到类似C＃4.0和其他人的东西。

<强>更新

我应该指出，您将在IL输出中看到对Dictionary.Add的调用，它正在构建字典供以后使用。不要误以为每次都会发生这种情况。编译器实际上是生成一个单独的静态类并对其进行内联静态初始化。请特别注意L_0026的说明。如果该类已初始化，则分支将跳过Add次调用。

L_0021: ldsfld class [mscorlib]System.Collections.Generic.Dictionary`2<string, int32> <PrivateImplementationDetails>{816396DD-F271-4C12-83D0-CC9C9CD67AD6}::$$method0x6000001-1
L_0026: brtrue.s L_0089
L_0028: ldc.i4.7 
L_0029: newobj instance void [mscorlib]System.Collections.Generic.Dictionary`2<string, int32>::.ctor(int32)
L_002e: dup 
L_002f: ldstr "x"
L_0034: ldc.i4.0 
L_0035: call instance void [mscorlib]System.Collections.Generic.Dictionary`2<string, int32>::Add(!0, !1)
L_003a: dup 
L_003b: ldstr "y"
L_0040: ldc.i4.1 
L_0041: call instance void [mscorlib]System.Collections.Generic.Dictionary`2<string, int32>::Add(!0, !1)
L_0046: dup 
L_0047: ldstr "z"
L_004c: ldc.i4.2 
L_004d: call instance void [mscorlib]System.Collections.Generic.Dictionary`2<string, int32>::Add(!0, !1)

另外，请注意字典实际上包含从原始字符串到整数的映射。该整数用于在IL中形成单独的开关。

L_0089: volatile. 
L_008b: ldsfld class [mscorlib]System.Collections.Generic.Dictionary`2<string, int32> <PrivateImplementationDetails>{816396DD-F271-4C12-83D0-CC9C9CD67AD6}::$$method0x6000001-1
L_0090: ldloc.2 
L_0091: ldloca.s CS$0$0002
L_0093: call instance bool [mscorlib]System.Collections.Generic.Dictionary`2<string, int32>::TryGetValue(!0, !1&)
L_0098: brfalse.s L_00da
L_009a: ldloc.3 
L_009b: switch (L_00be, L_00c2, L_00c6, L_00ca, L_00ce, L_00d2, L_00d6)
L_00bc: br.s L_00da
L_00be: ldc.i4.1 
L_00bf: stloc.1 
L_00c0: br.s L_00de
L_00c2: ldc.i4.2 
L_00c3: stloc.1 
L_00c4: br.s L_00de
L_00c6: ldc.i4.3

更新2：

对于它的价值，VB.NET似乎没有对其Select构造进行相同的优化。

Answer 2

看起来较新的编译器使用// [19 6 - 19 22] IL_0037: ldarg.0 // args IL_0038: ldc.i4.0 IL_0039: ldelem.ref IL_003a: stloc.s V_5 IL_003c: ldloc.s V_5 IL_003e: call unsigned int32 '<PrivateImplementationDetails>'::ComputeStringHash(string) IL_0043: stloc.s V_6 IL_0045: ldloc.s V_6 IL_0047: ldc.i4 -502520314 // 0xe20c2606 IL_004c: bgt.un.s IL_007b IL_004e: ldloc.s V_6 IL_0050: ldc.i4 -536075552 // 0xe00c22e0 IL_0055: beq IL_00f9 IL_005a: br.s IL_005c IL_005c: ldloc.s V_6 IL_005e: ldc.i4 -519297933 // 0xe10c2473 IL_0063: beq IL_00e9 IL_0068: br.s IL_006a IL_006a: ldloc.s V_6 IL_006c: ldc.i4 -502520314 // 0xe20c2606 IL_0071: beq IL_0119 IL_0076: br IL_014c IL_007b: ldloc.s V_6 IL_007d: ldc.i4 -468965076 // 0xe40c292c IL_0082: bgt.un.s IL_009d IL_0084: ldloc.s V_6 IL_0086: ldc.i4 -485742695 // 0xe30c2799 IL_008b: beq.s IL_0109 IL_008d: br.s IL_008f IL_008f: ldloc.s V_6 IL_0091: ldc.i4 -468965076 // 0xe40c292c IL_0096: beq.s IL_00b6 IL_0098: br IL_014c IL_009d: ldloc.s V_6 IL_009f: ldc.i4 -435409838 // 0xe60c2c52 IL_00a4: beq.s IL_00d9 IL_00a6: br.s IL_00a8 IL_00a8: ldloc.s V_6 IL_00aa: ldc.i4 -418632219 // 0xe70c2de5 IL_00af: beq.s IL_00c9 IL_00b1: br IL_014c IL_00b6: ldloc.s V_5 IL_00b8: ldstr "a" IL_00bd: call bool [mscorlib]System.String::op_Equality(string, string) IL_00c2: brtrue.s IL_0129 IL_00c4: br IL_014c IL_00c9: ldloc.s V_5 IL_00cb: ldstr "b" IL_00d0: call bool [mscorlib]System.String::op_Equality(string, string) IL_00d5: brtrue.s IL_012e IL_00d7: br.s IL_014c IL_00d9: ldloc.s V_5 IL_00db: ldstr "c" IL_00e0: call bool [mscorlib]System.String::op_Equality(string, string) IL_00e5: brtrue.s IL_0133 IL_00e7: br.s IL_014c IL_00e9: ldloc.s V_5 IL_00eb: ldstr "d" IL_00f0: call bool [mscorlib]System.String::op_Equality(string, string) IL_00f5: brtrue.s IL_0138 IL_00f7: br.s IL_014c IL_00f9: ldloc.s V_5 IL_00fb: ldstr "e" IL_0100: call bool [mscorlib]System.String::op_Equality(string, string) IL_0105: brtrue.s IL_013d IL_0107: br.s IL_014c IL_0109: ldloc.s V_5 IL_010b: ldstr "f" IL_0110: call bool [mscorlib]System.String::op_Equality(string, string) IL_0115: brtrue.s IL_0142 IL_0117: br.s IL_014c IL_0119: ldloc.s V_5 IL_011b: ldstr "g" IL_0120: call bool [mscorlib]System.String::op_Equality(string, string) IL_0125: brtrue.s IL_0147 IL_0127: br.s IL_014c // [21 17 - 21 26] IL_0129: ldc.i4.0 IL_012a: stloc.s V_7 IL_012c: br.s IL_01ac // [22 17 - 22 26] IL_012e: ldc.i4.1 IL_012f: stloc.s V_7 IL_0131: br.s IL_01ac // [23 17 - 23 26] IL_0133: ldc.i4.2 IL_0134: stloc.s V_7 IL_0136: br.s IL_01ac // [24 17 - 24 26] IL_0138: ldc.i4.3 IL_0139: stloc.s V_7 IL_013b: br.s IL_01ac // [25 17 - 25 26] IL_013d: ldc.i4.4 IL_013e: stloc.s V_7 IL_0140: br.s IL_01ac // [26 17 - 26 26] IL_0142: ldc.i4.5 IL_0143: stloc.s V_7 IL_0145: br.s IL_01ac // [27 17 - 27 26] IL_0147: ldc.i4.6 IL_0148: stloc.s V_7 IL_014a: br.s IL_01ac // [28 16 - 28 26] IL_014c: ldc.i4.m1 IL_014d: stloc.s V_7 IL_014f: br.s IL_01ac然后在哈希命中而不是字典构造上进行字符串比较。

bquote

.Net switch语句是哈希还是编入索引？

2 个答案: