Rewrite Enum.HasFlags and Enum.Equals in C#

[jkotas]

No description provided.

[dotnet-issue-labeler]

I couldn't figure out the best area label to add to this PR. If you have write-permissions please help me learn by adding exactly one area label.

[marek-safar]

What is the performance hit on Mono side? I recall we moved this to native due to bad codegen but it was long time ago.

[vargaz]

The mono changes look ok.

[stephentoub]

We have the ToUInt64 generic helper that takes a ulong and produces a TEnum. Would it be worth adding the inverse and using it here to avoid roundtripping through a boxed object here? Or you don't think it's worthwhile since there's already the array allocation?

[jkotas]

The reason for this change is AOT friendliness. The non-generic GetValues method is not AOT friendly since it creates a new array type dynamically.

Yeah, I agree that this can be done in a better way. I am going to undo this change for this PR, and deal with it in next one.

[stephentoub]

Just linking for reference:
#44355 (comment)

[VSadov]

Nice!

[jkotas]

What is the performance hit on Mono side?

It is performance improvement for Mono.

using System;
using System.Diagnostics;

class Test
{
    static Enum e1 = StringSplitOptions.RemoveEmptyEntries;
    static Enum e2 = StringSplitOptions.TrimEntries;

    static void Main()
    {
        Stopwatch sw = new Stopwatch();
        for (;;)
        {
            sw.Restart();
            for (int i = 0; i < 100000000; i++)
                e1.HasFlag(e2);
            Console.WriteLine(sw.ElapsedMilliseconds);
        }
    }
}

Windows x64

Baseline: ~2500ms per iteration
This change: ~1600ms per iteration (35% improvement)

(For reference, CoreCLR is around 530ms per iteration.)

[jkotas]

Same benchmark for Enum.Equals Windows x64 Mono:
Baseline: 2200ms per iteration
This change: 1400ms per iteration

[jkotas]

The failures are #59541