Menu
Does .Net 4 (or any prior version) perform any sort of optimization on longer switch statements based on strings?
I'm working around a potential performance bottleneck due to some long switch statements looking for matching strings in the cases, and I've always assumed these are searched in linear time (or near linear, i.e. not using an index to quickly find the matching string). But this seems like an obvious area that .Net could optimize, so thought I'd check if this is the case or not.
This is a derivative question from my recent one: indexed switch statement, or equivalent? .net, C#
792
If you can switch, switch. It will always be as fast as if not faster than a hashmap. For integer values and enum values, it is transformed into an in memory lookup table, which doesn't have the cost of hashing. For a string, it creates the HashMap and uses that to switch.
Last but not least, because a switch statement requires us to modify a lot of classes, it violates the Open-Closed Principle from the SOLID principles. To conclude, switch statement are bad because they are error-prone and they are not maintainable.
In C#, Switch statement is a multiway branch statement. It provides an efficient way to transfer the execution to different parts of a code based on the value of the expression. The switch expression is of integer type such as int, char, byte, or short, or of an enumeration type, or of string type.
A switch statement is usually more efficient than a set of nested ifs. Deciding whether to use if-then-else statements or a switch statement is based on readability and the expression that the statement is testing.
Compile the following code.
public static int Main(string[] args) { switch (args[0]) { case "x": return 1; case "y": return 2; case "z": return 3; } return 0; } Now use Reflector or ILDASM to examine the IL the C# compiler generates. Keep adding case statements and decompiling and observe the result.
Dictionary lookup.I was using the C# 3.0 compiler and I observed that the strategy changes at 7 case statements. I suspect you will see something similiar with C# 4.0 and others.
Update:
I should point that you will see calls to Dictionary.Add in the IL output where it is building up the dictionary for later use. Do not be fooled into thinking this happens everytime. The compiler is actually generating a separate static class and doing an inline static initialization of it. Pay particular attention to the instruction at L_0026. If the class is already initialized then the branch will skip over the Add calls.
L_0021: ldsfld class [mscorlib]System.Collections.Generic.Dictionary`2<string, int32> <PrivateImplementationDetails>{816396DD-F271-4C12-83D0-CC9C9CD67AD6}::$$method0x6000001-1 L_0026: brtrue.s L_0089 L_0028: ldc.i4.7 L_0029: newobj instance void [mscorlib]System.Collections.Generic.Dictionary`2<string, int32>::.ctor(int32) L_002e: dup L_002f: ldstr "x" L_0034: ldc.i4.0 L_0035: call instance void [mscorlib]System.Collections.Generic.Dictionary`2<string, int32>::Add(!0, !1) L_003a: dup L_003b: ldstr "y" L_0040: ldc.i4.1 L_0041: call instance void [mscorlib]System.Collections.Generic.Dictionary`2<string, int32>::Add(!0, !1) L_0046: dup L_0047: ldstr "z" L_004c: ldc.i4.2 L_004d: call instance void [mscorlib]System.Collections.Generic.Dictionary`2<string, int32>::Add(!0, !1) Also, notice that the dictionary actually contains a map from the original string to an integer. This integer is used to formulate a separate switch in IL.
L_0089: volatile. L_008b: ldsfld class [mscorlib]System.Collections.Generic.Dictionary`2<string, int32> <PrivateImplementationDetails>{816396DD-F271-4C12-83D0-CC9C9CD67AD6}::$$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 Update 2:
For what it is worth VB.NET does not seem to have this same optimization for its Select construct.
50
Looks like the newer compilers use ComputeStringHash() and then string comparison on a hash hit instead of dictionary construction.
// [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 If you love us? You can donate to us via Paypal or buy me a coffee so we can maintain and grow! Thank you!
Donate Us With
