Eliminate boxing when entering critical sections

[roji]

Closes #21229

[smitpatel]

no tests? 😢

[roji]

@smitpatel I think all the existing ones in ConcurrencyDetectorTestBase are valid, no? Or you mean you want explicit testing for ExitCriticalSection?

[AndriySvyryd]

This isn't a struct, so we will still be allocating. What's the purpose of this change?

[roji]

Sorry, that was a typo (it's 3AM here). Changing to readonly struct.

[roji]

(maybe concurrency-related programming at 3AM isn't such a good idea?)

[AndriySvyryd]

I thought it was an intentional bug like what Arthur suggested to include in every PR

[roji]

Ah of course, that's what it was. Congrats for winning the prize @AndriySvyryd!

[smitpatel]

Does this not create a new struct every time? (rather than reusing like how previous did)

[AndriySvyryd]

struct construction is cheap

[roji]

It does, but creating a struct (on the stack) is free - especially as here it's just wrapping a pointer. So it's the same as allocating a long, or a reference on the stack.

[roji]

Just for fun, here's the benchmark to show this:

BenchmarkDotNet=v0.12.0, OS=ubuntu 20.04
Intel Xeon W-2133 CPU 3.60GHz, 1 CPU, 12 logical and 6 physical cores
.NET Core SDK=5.0.100-preview.6.20266.3
  [Host]     : .NET Core 3.1.1 (CoreCLR 4.700.19.60701, CoreFX 4.700.19.60801), X64 RyuJIT
  DefaultJob : .NET Core 3.1.1 (CoreCLR 4.700.19.60701, CoreFX 4.700.19.60801), X64 RyuJIT

Method	Mean	Error	StdDev	Gen 0	Gen 1	Gen 2	Allocated
MyDisposable	0.0226 ns	0.0030 ns	0.0027 ns	-	-	-	-
Disposable	6.4407 ns	0.2355 ns	0.2202 ns	0.0056	-	-	24 B

Benchmark code

[MemoryDiagnoser]
public class Program
{
    [Benchmark]
    public void MyDisposable()
    {
        using var _ = GetMyDisposable();
    }

    [Benchmark]
    public void Disposable()
    {
        using var _ = GetDisposable();
    }

    static IDisposable GetDisposable() => new MyDisposable();

    static MyDisposable GetMyDisposable() => new MyDisposable();

    static void Main(string[] args)
        => BenchmarkRunner.Run<Program>();
}

readonly struct MyDisposable : IDisposable
{
    public void Dispose() {}
}

Now am off to fix the test failures...