· 5 min read
C# 람다 내부 구현 C# Lambda Internals
C# 람다 식을 컴파일러가 클래스와 메서드로 변환하는 과정을 IL 코드와 함께 분석한다. 클로저, 캡처 변수, 성능 영향까지. There is no magic. It's worth understanding how the compiler implements lambdas under the hood.
마법은 없다. Lambda를 컴파일러가 어떤식으로 구현하는지 알아둘 필요가 있다.
public static class LambdaExample
{
public static void Run()
{
int localInt = 123456789;
string localString = "LocalString";
ActionRunner(() =>
{
Console.WriteLine($"{localInt}, {localString}");
});
}
private static void ActionRunner(Action action)
{
action?.Invoke();
}
}Decompiled il source
// Type: CSharpExamples.LambdaExample
// Assembly: CSharpExamples, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null
// MVID: B420F7F8-9269-4798-8719-2A8ECDC7FBCA
// Location: /Users/geunheepark/Projects/private/CSharpExamples/CSharpExamples/bin/Debug/netcoreapp3.1/CSharpExamples.dll
// Sequence point data from /Users/geunheepark/Projects/private/CSharpExamples/CSharpExamples/bin/Debug/netcoreapp3.1/CSharpExamples.pdb
.class public abstract sealed auto ansi beforefieldinit
CSharpExamples.LambdaExample
extends [System.Runtime]System.Object
{
.class nested private sealed auto ansi beforefieldinit
'<>c__DisplayClass0_0'
extends [System.Runtime]System.Object
{
.custom instance void [System.Runtime]System.Runtime.CompilerServices.CompilerGeneratedAttribute::.ctor()
= (01 00 00 00 )
.field public int32 localInt
.field public string localString
.method public hidebysig specialname rtspecialname instance void
.ctor() cil managed
{
.maxstack 8
IL_0000: ldarg.0 // this
IL_0001: call instance void [System.Runtime]System.Object::.ctor()
IL_0006: nop
IL_0007: ret
} // end of method '<>c__DisplayClass0_0'::.ctor
.method assembly hidebysig instance void
'Runb__0'() cil managed
{
.maxstack 8
// [12 13 - 12 14]
IL_0000: nop
// [13 17 - 13 65]
IL_0001: ldstr "{0}, {1}"
IL_0006: ldarg.0 // this
IL_0007: ldfld int32 CSharpExamples.LambdaExample/'<>c__DisplayClass0_0'::localInt
IL_000c: box [System.Runtime]System.Int32
IL_0011: ldarg.0 // this
IL_0012: ldfld string CSharpExamples.LambdaExample/'<>c__DisplayClass0_0'::localString
IL_0017: call string [System.Runtime]System.String::Format(string, object, object)
IL_001c: call void [System.Console]System.Console::WriteLine(string)
IL_0021: nop
// [14 13 - 14 14]
IL_0022: ret
} // end of method '<>c__DisplayClass0_0'::'Runb__0'
} // end of class '<>c__DisplayClass0_0'
.method public hidebysig static void
Run() cil managed
{
.maxstack 2
.locals init (
[0] class CSharpExamples.LambdaExample/'<>c__DisplayClass0_0' 'CS$<>8__locals0'
)
IL_0000: newobj instance void CSharpExamples.LambdaExample/'<>c__DisplayClass0_0'::.ctor()
IL_0005: stloc.0 // 'CS$<>8__locals0'
// [8 9 - 8 10]
IL_0006: nop
// [9 13 - 9 38]
IL_0007: ldloc.0 // 'CS$<>8__locals0'
IL_0008: ldc.i4 123456789 // 0x075bcd15
IL_000d: stfld int32 CSharpExamples.LambdaExample/'<>c__DisplayClass0_0'::localInt
// [10 13 - 10 48]
IL_0012: ldloc.0 // 'CS$<>8__locals0'
IL_0013: ldstr "LocalString"
IL_0018: stfld string CSharpExamples.LambdaExample/'<>c__DisplayClass0_0'::localString
// [11 13 - 14 16]
IL_001d: ldloc.0 // 'CS$<>8__locals0'
IL_001e: ldftn instance void CSharpExamples.LambdaExample/'<>c__DisplayClass0_0'::'Runb__0'()
IL_0024: newobj instance void [System.Runtime]System.Action::.ctor(object, native int)
IL_0029: call void CSharpExamples.LambdaExample::ActionRunner(class [System.Runtime]System.Action)
IL_002e: nop
// [15 9 - 15 10]
IL_002f: ret
} // end of method LambdaExample::Run
.method private hidebysig static void
ActionRunner(
class [System.Runtime]System.Action action
) cil managed
{
.maxstack 8
// [18 9 - 18 10]
IL_0000: nop
// [19 13 - 19 30]
IL_0001: ldarg.0 // action
IL_0002: brtrue.s IL_0006
IL_0004: br.s IL_000d
IL_0006: ldarg.0 // action
IL_0007: callvirt instance void [System.Runtime]System.Action::Invoke()
IL_000c: nop
// [20 9 - 20 10]
IL_000d: ret
} // end of method LambdaExample::ActionRunner
} // end of class CSharpExamples.LambdaExample
위의 코드는 아래와 같이 해석될 수 있다.
public static class LambdaExampleTranslated
{
public class LambdaClass
{
public int localInt;
public string localString;
public void Run()
{
Console.WriteLine($"{localInt}, {localString}");
}
}
public static void Run()
{
int localInt = 123456789;
string localString = "LocalString";
ActionRunner(new LambdaClass
{
localInt = localInt,
localString = localString
}
.Run);
}
private static void ActionRunner(Action action)
{
action?.Invoke();
}
}Lambda의 변수 Capture를 위해 Lambda 객체는 Capture할 변수들을 멤버로 지니게 된다.
→ Capture할 변수가 많으면 많을수록, Lambda 객체 자체의 Size도 커진다.
매 Lambda 식 전달마다, Lambda 클래스 객체가 생성되고 사용후 버려지기 때문에 GC Heap에 부담을 준다.
→ 빈번한 GC 발생은 Program performance 에 악영향을 끼침.
그러므로 performance- critical 한 Code에서는 Lambda 를 대체하여 최적화를 할 필요가 있다.
// Lambda 객체 10000회 생성!
for (int i = 0; i < 10000; ++i)
{
ActionRunner(() =>
{
Console.WriteLine($"{localInt}, {localString}");
});
}
// Lambda 객체 1회 생성!
Action action = () =>
{
Console.WriteLine($"{localInt}, {localString}");
};
for (int i = 0; i < 10000; ++i)
{
ActionRunner(action);
}Method → Delegate 변환 Overhead
Method를 Delegate로 전달시 Action
→ 생성 과정에서 GC Alloc, 사용 이후엔 Garbage가 되는 Overhead가 있기 때문에 조심해야 한다.
public class DelegateConversion
{
public void Run()
{
// Action 10000회 생성.
for (int i = 0; i < 10000; ++i)
ActionRunner(MemberMethod);
// Action 1회 생성.
Action action = MemberMethod;
for (int i = 0; i < 10000; ++i)
ActionRunner(action);
}
private static void ActionRunner(Action action)
{
action?.Invoke();
}
private void MemberMethod() { }
}❇️ Dash/Net/SendQueue.cs 에선 Delegate 캐싱 최적화를 통해 Overhead를 줄였음.
public class SendQueue
{
private readonly IEventLoop _eventLoop;
// 미리 Delegate를 캐싱하여 GC Alloc 회피.
private readonly Action _eventLoopAction = null;
...
public SendQueue()
{
_eventLoopAction = OnEventLoop;
}
public void Send()
{
...
_eventLoop.Execute(_eventLoopAction);
...
}
public void OnEventLoop() { ... }
}❗컴파일러 / CLR 버전 / 실행 환경에 따라 최적화등으로 인해 결과가 상이할 수 있다.
