Delegate caching behavior changes in Roslyn

Given the following code:

public class C
{
    public void M()
    {
        var x = 5;
        Action<int> action = y => Console.WriteLine(y);
    }
}

Using VS2013, .NET 4.5. When looking at the decompiled code, we can see that the compiler is caching the delegate at the call site:

public class C
{
    [CompilerGenerated]
    private static Action<int> CS$<>9__CachedAnonymousMethodDelegate1;
    public void M()
    {
        if (C.CS$<>9__CachedAnonymousMethodDelegate1 == null)
        {
            C.CS$<>9__CachedAnonymousMethodDelegate1 = new Action<int>(C.<M>b__0);
        }
        Action<int> arg_1D_0 = C.CS$<>9__CachedAnonymousMethodDelegate1;
    }
    [CompilerGenerated]
    private static void <M>b__0(int y)
    {
        Console.WriteLine(y);
    }
}

Looking at the same code decompiled in Roslyn (using TryRoslyn), yields the following output:

public class C
{
    [CompilerGenerated]
    private sealed class <>c__DisplayClass0
    {
        public static readonly C.<>c__DisplayClass0 CS$<>9__inst;
        public static Action<int> CS$<>9__CachedAnonymousMethodDelegate2;
        static <>c__DisplayClass0()
        {
            // Note: this type is marked as 'beforefieldinit'.
            C.<>c__DisplayClass0.CS$<>9__inst = new C.<>c__DisplayClass0();
        }
        internal void <M>b__1(int y)
        {
            Console.WriteLine(y);
        }
    }
    public void M()
    {
        Action<int> arg_22_0;
        if (arg_22_0 = C.<>c__DisplayClass0.CS$<>9__CachedAnonymousMethodDelegate2 == null)
        {
            C.<>c__DisplayClass0.CS$<>9__CachedAnonymousMethodDelegate2 =
                            new Action<int>(C.<>c__DisplayClass0.CS$<>9__inst.<M>b__1);
        }
    }
}

We can now see that the delegate is now lifted into a private class inside C, a similar behavior that we're used to seeing when closing over an instance variable / field (closure).

I know this is an implementation detail which may be subject to change at any given time.

Still I wonder, what are the benefits of lifting the delegate into a new class and caching it there over simply caching it at the call site?

Edit:

This issue talks about the same behavior as asked here.

Yes. The most important part is that the method containing lambda implementation is now an instance method.

You can see a delegate as a middleman receiving an instance call through Invoke and dispatching that call according to the calling convention of the implementing method.

Note that there are platform ABI requirements that specify how arguments are passed, how results are returned, what arguments are passed via registers and in which ones, how "this" is being passed and so on. Violating these rules may have bad impact on tools that rely on stack-walking, such as debuggers.

Now, if the implementing method is an instance method, the only thing that needs to happen inside the delegate is to patch "this", which is the delegate instance at the time of Invoke, to be the enclosed Target object. At that point, since everything else is already where it needs to be, the delegate can jump directly to the implementing method body. In many cases this is noticeably less work than what would need to happen if the implementing method was a static method.

Still I wonder, what are the benefits of lifting the delegate into a new class and caching it there over simply caching it at the call site?

You've missed one other really important detail - it's now an instance method. I believe that's the key here. IIRC, it was found that invoking a delegate which was "backed" by an instance method was faster than invoking a delegate backed by a static method - which is the motivation behind the change.

This is all hearsay, vaguely remembered from spending time with Dustin Campbell and Kevin Pilch-Bisson (both from the Roslyn team) at CodeMash, but it would make sense given the code you've shown.

(I haven't validated the performance difference for myself, and it sounds like it's backwards... but CLR internals can be funny like that...)