JIT: remove guarded devirtualization restriction for small struct ret…

[AndyAyersMS]

…urns

Now that we're not retyping small structs, the existing transformation works
for methods returning small structs too.

Fixes #51138.

[AndyAyersMS]

@sandreenko PTAL
cc @dotnet/jit-contrib

Will post diffs when I can. SPMI seems out of sync right now.

[sandreenko]

was it protecting only from small struct returns and not from multi-reg?

[AndyAyersMS]

Good question. I think it was just the small struct returns?

[sandreenko]

I think that all struct calls were rejected here because of the one unsupported scenario(small structs). Now we don't need to do anything special for the previously unsupported case but we need to check that it works as expected for all of them.

I have ran the change locally with assert(false) inside the block and we did not go into if (origCall->TypeGet() == TYP_STRUCT) case on framework libraries crossgen/pmi x64/arm64 windows/unix. Is it expected?

However, I saw that we hit it aspnet.run.windows.x64 spmi collection but could not collect the diffs because when we delete this block we start hitting missing answers for the new questions.

[AndyAyersMS]

This optimization requires PGO data, so not too surprising it doesn't fire much for libraries code.

Also by this point any struct return that uses a hidden buffer has been ntransformed and the return type is no longer TYP_STRUCT.

Here's a simple test case where the case does git hit. Run with COMPlus_TieredPGO=1.

using System;
using System.Threading;
using System.Runtime.CompilerServices;

struct P
{
    public int x;
    public int y;
}

class B
{
    virtual public P get() => new P();
}

class D : B
{
    override public P get() => new P() { x = 67, y = 33 };
}

class X
{
    public static int F(B b) => b.get().x + b.get().y;

    [MethodImpl(MethodImplOptions.NoOptimization)]
    public static int Main()
    {
        B b = new D();
        int r = 0;
        for (int i = 0; i < 100; i++)
        {
            r += F(b);
            Thread.Sleep(15);
        }
        
        r = 0;
        for (int i = 0; i < 10000; i++)
        {
            r += F(b);
        }
        
        r /= 10000;
        
        Console.WriteLine($"r={r}");
        
        return r;
    }
}

I also ran jit-experimental CI below which gives us some coverage, and there were no unexpected failures (save for emptystacktrace\OOMException01 which seems to be failing everywhere).

[sandreenko]

LGTM

[AndyAyersMS]

Did some extra testing on SysV and verified GDV now handles 1 and 2 reg struct returns properly.