EqualityComparer<TEnum?>.Default allocates

[ltrzesniewski]

Description

EqualityComparer<TEnum?>.Default (when T is a nullable enum) is resolved to ObjectEqualityComparer<TEnum?>, which boxes the enum when performing equality comparison. This causes an unnecessary allocation.

Here is a reproduction:

using System;
using System.Collections.Generic;
using System.Runtime.InteropServices;

namespace ConsoleApp;

public static class Program
{
    public static void Main()
    {
        _ = EqualityComparer<DayOfWeek?>.Default.Equals(DayOfWeek.Friday, DayOfWeek.Friday); // Warm-up

        var before = GC.GetAllocatedBytesForCurrentThread();
        _ = EqualityComparer<DayOfWeek?>.Default.Equals(DayOfWeek.Friday, DayOfWeek.Friday);
        var after = GC.GetAllocatedBytesForCurrentThread();

        Console.WriteLine(RuntimeInformation.FrameworkDescription);
        Console.WriteLine(after > before ? "Allocates" : "Allocation-free");
    }
}

This results in:

.NET 7.0.0-preview.2.22152.2
Allocates

If you replace DayOfWeek? with DayOfWeek, or if you use a type such as int? instead, the code will print Allocation-free.

Configuration

I reproduced this on .NET 6 and on .NET 7 preview 2 on Windows x64, but this should be independent of the OS and architecture.

Regression?

This is not a regression. It also reproduces on .NET Framework 4.8.

Analysis

This is roughly due to the fact that ComparerHelpers.CreateDefaultEqualityComparer special-cases both Nullable<T> and enums, but does not special-case nullable enums. This code path ends up using ObjectEqualityComparer<T>.

I'm willing to provide a PR to fix this if you'd like.

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Issue Details

---

Description

EqualityComparer<TEnum?>.Default (when T is a nullable enum) is resolved to ObjectEqualityComparer<TEnum?>, which boxes the enum when performing equality comparison. This causes an unnecessary allocation.

Here is a reproduction:

using System;
using System.Collections.Generic;
using System.Runtime.InteropServices;

namespace ConsoleApp;

public static class Program
{
    public static void Main()
    {
        _ = EqualityComparer<DayOfWeek?>.Default.Equals(DayOfWeek.Friday, DayOfWeek.Friday); // Warm-up

        var before = GC.GetAllocatedBytesForCurrentThread();
        _ = EqualityComparer<DayOfWeek?>.Default.Equals(DayOfWeek.Friday, DayOfWeek.Friday);
        var after = GC.GetAllocatedBytesForCurrentThread();

        Console.WriteLine(RuntimeInformation.FrameworkDescription);
        Console.WriteLine(after > before ? "Allocates" : "Allocation-free");
    }
}

This results in:

.NET 7.0.0-preview.2.22152.2
Allocates

If you replace DayOfWeek? with DayOfWeek, or if you use a type such as int? instead, the code will print Allocation-free.

Configuration

I reproduced this on .NET 6 and on .NET 7 preview 2 on Windows x64, but this should be independent of the OS and architecture.

Regression?

This is not a regression. It also reproduces on .NET Framework 4.8.

Analysis

This is roughly due to the fact that ComparerHelpers.CreateDefaultEqualityComparer special-cases both Nullable<T> and enums, but does not special-case nullable enums. This code path ends up using ObjectEqualityComparer<T>.

I'm willing to provide a PR to fix this if you'd like.

Author:	ltrzesniewski
Assignees:	-
Labels:	area-System.Collections, tenet-performance, untriaged
Milestone:	-

[EgorBo]

I'll take a look, the problem that getDefaultComparerClassHelper currently only devirtualizes Nullable<> for T : IEquatable<T>

runtime/src/coreclr/vm/jitinterface.cpp

Lines 8822 to 8827 in 7bfc61b

	TypeHandle iequatable = TypeHandle(CoreLibBinder::GetClass(CLASS__IEQUATABLEGENERIC)).Instantiate(nullableInst);
	if (nullableInst[0].CanCastTo(iequatable))
	{
	TypeHandle resultTh = ((TypeHandle)CoreLibBinder::GetClass(CLASS__NULLABLE_COMPARER)).Instantiate(nullableInst);
	return CORINFO_CLASS_HANDLE(resultTh.GetMethodTable());
	}

so we might want to introduce a NullableEnumComparer

[ghost]

Tagging subscribers to this area: @JulieLeeMSFT
See info in area-owners.md if you want to be subscribed.

Issue Details

---

Description

EqualityComparer<TEnum?>.Default (when T is a nullable enum) is resolved to ObjectEqualityComparer<TEnum?>, which boxes the enum when performing equality comparison. This causes an unnecessary allocation.

Here is a reproduction:

using System;
using System.Collections.Generic;
using System.Runtime.InteropServices;

namespace ConsoleApp;

public static class Program
{
    public static void Main()
    {
        _ = EqualityComparer<DayOfWeek?>.Default.Equals(DayOfWeek.Friday, DayOfWeek.Friday); // Warm-up

        var before = GC.GetAllocatedBytesForCurrentThread();
        _ = EqualityComparer<DayOfWeek?>.Default.Equals(DayOfWeek.Friday, DayOfWeek.Friday);
        var after = GC.GetAllocatedBytesForCurrentThread();

        Console.WriteLine(RuntimeInformation.FrameworkDescription);
        Console.WriteLine(after > before ? "Allocates" : "Allocation-free");
    }
}

This results in:

.NET 7.0.0-preview.2.22152.2
Allocates

If you replace DayOfWeek? with DayOfWeek, or if you use a type such as int? instead, the code will print Allocation-free.

Configuration

I reproduced this on .NET 6 and on .NET 7 preview 2 on Windows x64, but this should be independent of the OS and architecture.

Regression?

This is not a regression. It also reproduces on .NET Framework 4.8.

Analysis

This is roughly due to the fact that ComparerHelpers.CreateDefaultEqualityComparer special-cases both Nullable<T> and enums, but does not special-case nullable enums. This code path ends up using ObjectEqualityComparer<T>.

I'm willing to provide a PR to fix this if you'd like.

Author:	ltrzesniewski
Assignees:	-
Labels:	tenet-performance, area-CodeGen-coreclr
Milestone:	-

[ltrzesniewski]

Thanks @EgorBo! 🙂