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C#中的无用变量用于捕获委托的循环反汇编?

disassembly closures for-loop c#

themiurge · 技术社区 · 9 年前

this old question ,我发现了一些奇怪的事情。

class ThreadTest
{
    static void Main(string[] args)
    {
        for (int i = 0; i < 10; i++)
            new Thread(() => Console.WriteLine(i)).Start();
    }
}

下面是我在拆卸过程中看到的内容:

internal class ThreadTest
{
    private static void Main(string[] args)
    {
        int i;
        int j;
        for (i = 0; i < 10; i = j + 1)
        {
            new Thread(delegate
            {
                Console.WriteLine(i);
            }).Start();
            j = i;
        }
    }
}

是什么 j

.method private hidebysig static 
    void Main (
        string[] args
    ) cil managed 
{
    // Method begins at RVA 0x2050
    // Code size 64 (0x40)
    .maxstack 2
    .entrypoint
    .locals init (
        [0] class ConsoleApplication2.ThreadTest/'<>c__DisplayClass0_0' 'CS$<>8__locals0',
        [1] int32
    )

    IL_0000: newobj instance void ConsoleApplication2.ThreadTest/'<>c__DisplayClass0_0'::.ctor()
    IL_0005: stloc.0
    IL_0006: ldloc.0
    IL_0007: ldc.i4.0
    IL_0008: stfld int32 ConsoleApplication2.ThreadTest/'<>c__DisplayClass0_0'::i
    IL_000d: br.s IL_0035
    // loop start (head: IL_0035)
        IL_000f: ldloc.0
        IL_0010: ldftn instance void ConsoleApplication2.ThreadTest/'<>c__DisplayClass0_0'::'<Main>b__0'()
        IL_0016: newobj instance void [mscorlib]System.Threading.ThreadStart::.ctor(object, native int)
        IL_001b: newobj instance void [mscorlib]System.Threading.Thread::.ctor(class [mscorlib]System.Threading.ThreadStart)
        IL_0020: call instance void [mscorlib]System.Threading.Thread::Start()
        IL_0025: ldloc.0
        IL_0026: ldfld int32 ConsoleApplication2.ThreadTest/'<>c__DisplayClass0_0'::i
        IL_002b: ldc.i4.1
        IL_002c: add
        IL_002d: stloc.1
        IL_002e: ldloc.0
        IL_002f: ldloc.1
        IL_0030: stfld int32 ConsoleApplication2.ThreadTest/'<>c__DisplayClass0_0'::i

        IL_0035: ldloc.0
        IL_0036: ldfld int32 ConsoleApplication2.ThreadTest/'<>c__DisplayClass0_0'::i
        IL_003b: ldc.i4.s 10
        IL_003d: blt.s IL_000f
    // end loop

    IL_003f: ret
} // end of method ThreadTest::Main

但这是最奇怪的事情。如果我像这样更改原始代码 i++ i = i + 1 :

class ThreadTest
{
    static void Main(string[] args)
    {
        for (int i = 0; i < 10; i = i + 1)
            new Thread(() => Console.WriteLine(i)).Start();
    }
}

我明白了:

internal class ThreadTest
{
    private static void Main(string[] args)
    {
        int i;
        for (i = 0; i < 10; i++)
        {
            new Thread(delegate
            {
                Console.WriteLine(i);
            }).Start();
        }
    }
}

以下是字节码:

.method private hidebysig static 
    void Main (
        string[] args
    ) cil managed 
{
    // Method begins at RVA 0x2050
    // Code size 62 (0x3e)
    .maxstack 3
    .entrypoint
    .locals init (
        [0] class ConsoleApplication2.ThreadTest/'<>c__DisplayClass0_0' 'CS$<>8__locals0'
    )

    IL_0000: newobj instance void ConsoleApplication2.ThreadTest/'<>c__DisplayClass0_0'::.ctor()
    IL_0005: stloc.0
    IL_0006: ldloc.0
    IL_0007: ldc.i4.0
    IL_0008: stfld int32 ConsoleApplication2.ThreadTest/'<>c__DisplayClass0_0'::i
    IL_000d: br.s IL_0033
    // loop start (head: IL_0033)
        IL_000f: ldloc.0
        IL_0010: ldftn instance void ConsoleApplication2.ThreadTest/'<>c__DisplayClass0_0'::'<Main>b__0'()
        IL_0016: newobj instance void [mscorlib]System.Threading.ThreadStart::.ctor(object, native int)
        IL_001b: newobj instance void [mscorlib]System.Threading.Thread::.ctor(class [mscorlib]System.Threading.ThreadStart)
        IL_0020: call instance void [mscorlib]System.Threading.Thread::Start()
        IL_0025: ldloc.0
        IL_0026: ldloc.0
        IL_0027: ldfld int32 ConsoleApplication2.ThreadTest/'<>c__DisplayClass0_0'::i
        IL_002c: ldc.i4.1
        IL_002d: add
        IL_002e: stfld int32 ConsoleApplication2.ThreadTest/'<>c__DisplayClass0_0'::i

        IL_0033: ldloc.0
        IL_0034: ldfld int32 ConsoleApplication2.ThreadTest/'<>c__DisplayClass0_0'::i
        IL_0039: ldc.i4.s 10
        IL_003b: blt.s IL_000f
    // end loop

    IL_003d: ret
} // end of method ThreadTest::Main

编译器为什么添加 j 在第一种情况下?

2 回复 | 直到 9 年前

Peter Duniho 9 年前

因为在语义上,当你写 i++ i 因此,它可以用作表达式的结果值。

编译器通过引入一个新变量来实现这一点,在该变量中,新值可以保留到必要时使用。因此,旧的价值观 一、 j 一、 add 指示 j ,因为实际上没有代码需要该值。但是,暂时的价值仍然是旧的,如果需要的话可以使用。

你可能会争辩:

添加 直接进入而不是将其存储在 j

C编译器不负责优化。它的主要工作是将C代码翻译成IL。事实上,我想说这项工作的一部分是是负责优化。

wake-0 9 年前

i++ 不完全是 i = i + 1

请尝试以下代码:

int i = 1;
int x = 5 + i++;
Console.WriteLine("i:" + i + " x: " + x);
i = 1;
int y = 5 + ++i;
Console.WriteLine("i:" + i + " y: " + y);

输出:

i:2 x: 6
i:2 y: 7

How do Prefix (++x) and Postfix (x++) operations work? ).