mono AOT compiler cannot resolve and inline static virtual calls in gshared methods

Testcase:

using System;
using System.Diagnostics;
using System.Numerics;

public class Tests
{
        private interface INegator<T> where T : struct
        {
            static abstract bool NegateIfNeeded(bool equals);
        }

        private readonly struct DontNegate<T> : INegator<T> where T : struct
        {
            public static bool NegateIfNeeded(bool equals) => equals;
        }

        private readonly struct Negate<T> : INegator<T> where T : struct
        {
            public static bool NegateIfNeeded(bool equals) => !equals;
        }

    private static int IndexOfValueType<TValue, TNegator>()
            where TValue : struct, INumber<TValue>
            where TNegator : struct, INegator<TValue>
    {
        for (int j = 0; j < 1000; ++j) {
            for (int i = 0; i < 100000; ++i)
                TNegator.NegateIfNeeded (true);
        }
        return 0;
    }

    public static void Main () {
        var w = Stopwatch.StartNew ();
        IndexOfValueType<byte, DontNegate<byte>> ();
        w.Stop ();
        Console.WriteLine (w.ElapsedMilliseconds);
    }
}

When calling IndexOfValueType<byte, DontNegate<byte>>, the AOT compiler will compile a shared instance for bytes/enums with byte basetype, but inside the shared method it can't resolve the

constrained. !!TNegator
 call       bool class Tests/INegator`1<!!TValue>::NegateIfNeeded(bool)

call to DontNegate so it doesn't get inlined.

This is hit with the recent BCL changes to SpanHelpers.

@BrzVlad

cc @BrzVlad

@vargaz

@vargaz As you know I am working on the 7.0 fix and I wonder whether I could avoid copying the old code back. I have two questions:

Would constraining T help?

- private interface INegator<T> where T : struct
+ private interface INegator<T> where T : struct, INumber<T>

Would using typeof instead help?

- equals = TNegator.NegateIfNeeded(Vector256.Equals(values, Vector256.LoadUnsafe(ref currentSearchSpace)));
+ equals = Vector256.Equals(values, Vector256.LoadUnsafe(ref currentSearchSpace));
+ if (typeof(TNegator) == typeof(Negate)) // assuming I would remove T from INegator
+ {
+     equals = ~equals;
+ }

I think the lowest risk for net 7.0 is to copy the code back and put it inside ifdefs.

@vargaz

I think the lowest risk for net 7.0 is to copy the code back and put it inside ifdefs.

@vargaz I assume that it means that the two workarounds posted above would not help? I have started copying the code and it's a lot of code (hundreds of lines) so it's not minimal risk.

You can add copy of the old implementation files verbatim under a different name, and just do minimal changes on top of it to get things compile.

[mono][aot] Prefer specific instances instead of gshared for methods …

@vargaz

@vargaz Can I revert #75917 now?

This only solves (part of) the AOT slowdowns, the interpreter slowdowns are still there.

So for a simple test case:

        var arr = new int [100000];
        var w = Stopwatch.StartNew ();
        var span = new Span<int> (arr);
        for (int i = 0; i < 100000; ++i)
            span.IndexOf ((int)50);
        w.Stop ();
        Console.WriteLine (w.ElapsedMilliseconds);

Without the span revert: 80s
Without span revert + this PR: 8.4s
With span revert: 8s

So there is still about a 5% slowdown, but it's not that bad anymore.

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mono AOT compiler cannot resolve and inline static virtual calls in gshared methods #75801

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mono AOT compiler cannot resolve and inline static virtual calls in gshared methods #75801

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SamMonoRT commented on Sep 19, 2022

adamsitnik commented on Sep 19, 2022

vargaz commented on Sep 19, 2022

adamsitnik commented on Sep 19, 2022

jkotas commented on Sep 19, 2022

adamsitnik commented on Sep 27, 2022

vargaz commented on Sep 27, 2022

vargaz commented on Sep 28, 2022

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