Speed up RuntimeType.GetHashCode

[EgorBo]

Use mixed type-handle hashes in CoreCLR, preserving COM identity hashing (Windows only).

using System;
using System.Collections.Generic;
using BenchmarkDotNet.Attributes;
using BenchmarkDotNet.Running;

BenchmarkSwitcher.FromAssembly(typeof(Bench).Assembly).Run(args);

public class Bench {
    private readonly Dictionary<Type, int> _dictionary = new() { [typeof(string)] = 42 };

    [Benchmark]
    public int Lookup() => _dictionary[typeof(string)];
}

Results

Win-x64:

Method	Toolchain	Mean	Error	Ratio
Lookup	main	5.961 ns	0.0108 ns	1.38
Lookup	PR #134783	4.323 ns	0.0078 ns	1.00

osx-arm64:

Method	Toolchain	Mean	Error	Ratio
Lookup	main	3.647 ns	0.0717 ns	1.27
Lookup	PR #134783	2.881 ns	0.0009 ns	1.00

[azure-pipelines]

Azure Pipelines:
Successfully started running 3 pipeline(s).
13 pipeline(s) were filtered out due to trigger conditions.
There may be pipelines that require an authorized user to comment /azp run to run.

[dotnet-policy-service]

Tagging subscribers to this area: @dotnet/area-system-runtime
See info in area-owners.md if you want to be subscribed.

[EgorBo]

@EgorBot -windows_intel -linux_amd -osx_arm -linux_arm

using System;
using System.Collections.Generic;
using BenchmarkDotNet.Attributes;
using BenchmarkDotNet.Running;

BenchmarkSwitcher.FromAssembly(typeof(Bench).Assembly).Run(args);

public class Bench {
    private readonly Dictionary<Type, int> _dictionary = new() { [typeof(string)] = 42 };

    [Benchmark]
    public int Lookup() => _dictionary[typeof(string)];
}

Note

Benchmark generated with GitHub Copilot.

[Copilot]

Copilot review overview

🟡 Changes recommended

The performance claim lacks measurements, and the new hash paths lack targeted tests.

Review effort: Balanced
Findings: 1 · 1

Open (2)

• Add tests for CoreCLR hashing and the CLSID exception · New
• Add benchmarks validating the claimed performance improvement · New

What changed in this PR

Optimizes CoreCLR RuntimeType.GetHashCode while retaining existing behavior for Mono and COM identity types.

Changes:

• Uses CoreCLR type handles for hashing.
• Preserves identity hashing for CLSID-backed COM types.
• Moves Mono’s existing identity hash implementation to its runtime-specific file.

File	Description
System/​RuntimeType.Mono.cs	Retains Mono identity hashing.
System/​RuntimeType.cs	Removes the shared implementation.
System/​RuntimeType.CoreCLR.cs	Adds handle-based hashing with a COM exception.

[EgorBo]

@EgorBot -windows_intel -linux_amd -osx_arm -linux_arm -profiler

using System;
using System.Collections.Generic;
using BenchmarkDotNet.Attributes;
using BenchmarkDotNet.Running;

BenchmarkSwitcher.FromAssembly(typeof(Bench).Assembly).Run(args);

public class Bench {
    private readonly Dictionary<Type, int> _dictionary = new() { [typeof(string)] = 42 };

    [Benchmark]
    public int Lookup() => _dictionary[typeof(string)];
}

Note

Benchmark generated with GitHub Copilot.

[EgorBo]

PTAL @jkotas, it seems to speed up _dictionary[typeof(string)] by 20-25% EgorBot/Benchmarks#623

Mainly because JIT constant folds the entire thing into 32bit constant (e.g. mov r14d, 0xd1e70ebc). Had to do some massage for obj?.GetHashCode() ?? 0 (we have a known issue for the root cause behind it against JIT).

[EgorBo]

This kind of dictionaries seem popular