[NativeAOT] Missed devirtualization opportunities due to Unsafe.As

[MichalStrehovsky]

In the below program we miss a devirtualization opportunity in Method due to the Unsafe.As.

IFace has a single implementor but RyuJIT doesn't even ask for it.

using System;
using System.Runtime.CompilerServices;

interface IFace
{
    void Frob();
}

class Program : IFace
{
    public static IFace s_inst = new Program();

    public object inst;

    public Program() => inst = s_inst;

    public void Frob() { }

    [MethodImpl(MethodImplOptions.NoInlining)]
    public void Method() => Unsafe.As<IFace>(inst).Frob();

    static void Main()
    {
        new Program().Method();
    }
}

I noticed this in the pattern from here as I was looking why interface dispatch is so hot (2.7% of samples spent in RhpInterfaceDispatch1 alone) in BasicMinimalApis:

runtime/src/libraries/System.Private.CoreLib/src/System/Threading/Tasks/ValueTask.cs

Line 719 in 05a3d36

return Unsafe.As<IValueTaskSource<TResult>>(obj).GetStatus(_token) != ValueTaskSourceStatus.Pending;

Cc @EgorBo

[ghost]

Tagging subscribers to this area: @JulieLeeMSFT, @jakobbotsch
See info in area-owners.md if you want to be subscribed.

Issue Details

---

In the below program we miss a devirtualization opportunity in Method due to the Unsafe.As.

IFace has a single implementor but RyuJIT doesn't even ask for it.

using System;
using System.Runtime.CompilerServices;

interface IFace
{
    void Frob();
}

class Program : IFace
{
    public static IFace s_inst = new Program();

    public object inst;

    public Program() => inst = s_inst;

    public void Frob() { }

    [MethodImpl(MethodImplOptions.NoInlining)]
    public void Method() => Unsafe.As<IFace>(inst).Frob();

    static void Main()
    {
        new Program().Method();
    }
}

I noticed this in the pattern from here as I was looking why interface dispatch is so hot (2.7% of samples spent in RhpInterfaceDispatch1 alone) in BasicMinimalApis:

runtime/src/libraries/System.Private.CoreLib/src/System/Threading/Tasks/ValueTask.cs

Line 719 in 05a3d36

return Unsafe.As<IValueTaskSource<TResult>>(obj).GetStatus(_token) != ValueTaskSourceStatus.Pending;

Cc @EgorBo

Author:	MichalStrehovsky
Assignees:	-
Labels:	area-CodeGen-coreclr
Milestone:	-

[EgorBo]

Dup of #84377

Currently Unsafe.As is just a no-op for JIT, I'll take a look today/tomorrow

[EgorBo]

Will keep it open since it also involves single-impl devirtualization