Optimize Queue<T>.Enumerator

[stephentoub]

Related to #117328

Method	Toolchain	Wraps	Length	Mean	Ratio	Allocated
IntsEnumerable	\main\corerun.exe	False	3	18.187 ns	1.00	-
IntsEnumerable	\pr\corerun.exe	False	3	4.249 ns	0.23	-
IntsQueue	\main\corerun.exe	False	3	9.710 ns	1.00	-
IntsQueue	\pr\corerun.exe	False	3	3.933 ns	0.41	-
StringsEnumerable	\main\corerun.exe	False	3	19.448 ns	1.00	-
StringsEnumerable	\pr\corerun.exe	False	3	4.574 ns	0.24	-
StringsQueue	\main\corerun.exe	False	3	9.564 ns	1.00	-
StringsQueue	\pr\corerun.exe	False	3	3.026 ns	0.32	-
IntsEnumerable	\main\corerun.exe	False	100	212.356 ns	1.00	40 B
IntsEnumerable	\pr\corerun.exe	False	100	119.884 ns	0.56	-
IntsQueue	\main\corerun.exe	False	100	260.983 ns	1.00	-
IntsQueue	\pr\corerun.exe	False	100	76.163 ns	0.29	-
StringsEnumerable	\main\corerun.exe	False	100	363.271 ns	1.00	40 B
StringsEnumerable	\pr\corerun.exe	False	100	126.384 ns	0.35	-
StringsQueue	\main\corerun.exe	False	100	250.484 ns	1.00	-
StringsQueue	\pr\corerun.exe	False	100	76.951 ns	0.31	-
IntsEnumerable	\main\corerun.exe	True	3	16.776 ns	1.00	40 B
IntsEnumerable	\pr\corerun.exe	True	3	5.312 ns	0.32	-
IntsQueue	\main\corerun.exe	True	3	9.567 ns	1.00	-
IntsQueue	\pr\corerun.exe	True	3	2.911 ns	0.30	-
StringsEnumerable	\main\corerun.exe	True	3	20.782 ns	1.00	-
StringsEnumerable	\pr\corerun.exe	True	3	3.919 ns	0.19	-
StringsQueue	\main\corerun.exe	True	3	14.296 ns	1.00	-
StringsQueue	\pr\corerun.exe	True	3	3.835 ns	0.27	-
IntsEnumerable	\main\corerun.exe	True	100	271.858 ns	1.00	40 B
IntsEnumerable	\pr\corerun.exe	True	100	99.918 ns	0.37	-
IntsQueue	\main\corerun.exe	True	100	245.360 ns	1.00	-
IntsQueue	\pr\corerun.exe	True	100	82.029 ns	0.33	-
StringsEnumerable	\main\corerun.exe	True	100	361.511 ns	1.00	40 B
StringsEnumerable	\pr\corerun.exe	True	100	101.535 ns	0.28	-
StringsQueue	\main\corerun.exe	True	100	253.267 ns	1.00	-
StringsQueue	\pr\corerun.exe	True	100	83.072 ns	0.33	-

using BenchmarkDotNet.Attributes;
using BenchmarkDotNet.Running;

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

[MemoryDiagnoser(false)]
public class Bench
{
    private IEnumerable<int> _intsEnumerable;
    private Queue<int> _intsQueue;
    private IEnumerable<string> _stringsEnumerable;
    private Queue<string> _stringsQueue;

    [Params(false, true)]
    public bool Wraps { get; set; }

    [Params(3, 100)]
    public int Length { get; set; }

    [GlobalSetup]
    public void Setup()
    {
        _intsQueue = new Queue<int>(Length);
        _stringsQueue = new Queue<string>(Length);

        for (int i = 0; i < Length; i++)
        {
            _intsQueue.Enqueue(i);
            _stringsQueue.Enqueue(i.ToString());
        }

        if (Wraps)
        {
            for (int i = 0; i < Length - 1; i++)
            {
                _intsQueue.Dequeue();
                _intsQueue.Enqueue(i + Length);

                _stringsQueue.Dequeue();
                _stringsQueue.Enqueue((i + Length).ToString());
            }
        }

        _intsEnumerable = _intsQueue;
        _stringsEnumerable = _stringsQueue;
    }

    [Benchmark]
    public int IntsEnumerable()
    {
        int sum = 0;
        foreach (var i in _intsEnumerable) sum += i;
        return sum;
    }

    [Benchmark]
    public int IntsQueue()
    {
        int sum = 0;
        foreach (var i in _intsQueue) sum += i;
        return sum;
    }

    [Benchmark]
    public int StringsEnumerable()
    {
        int sum = 0;
        foreach (var s in _stringsEnumerable) sum += s.Length;
        return sum;
    }

    [Benchmark]
    public int StringsQueue()
    {
        int sum = 0;
        foreach (var s in _stringsQueue) sum += s.Length;
        return sum;
    }
}

[Copilot]

Pull Request Overview

Optimizes Queue<T>.Enumerator by rewriting internal fields and the MoveNext loop for performance, and updates related test properties to reflect changed behavior.

• Renames internal enumerator fields (_q → _queue, _index → _i) and refactors MoveNext to eliminate expensive modulo operations.
• Updates Current and Reset implementations accordingly.
• Renames test properties in both generic and non-generic Queue tests to include “Empty” in the undefined-operation checks.

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
src/libraries/System.Private.CoreLib/src/System/Collections/Generic/Queue.cs	Refactored enumerator internals, field renames, and loop logic
src/libraries/System.Collections/tests/Generic/Queue/Queue.Tests.cs	Renamed test property for undefined Current operation
src/libraries/System.Collections/tests/Generic/Queue/Queue.Generic.Tests.cs	Renamed generic test property for undefined Current operation

Comments suppressed due to low confidence (2)

src/libraries/System.Collections/tests/Generic/Queue/Queue.Tests.cs:30

• [nitpick] The renamed test property Enumerator_Empty_Current_UndefinedOperation_Throw is inconsistent with its original plural suffix and the naming in base classes. Rename it to Enumerator_Empty_Current_UndefinedOperation_Throws to match the established pattern.

        protected override bool Enumerator_Empty_Current_UndefinedOperation_Throw => true;

src/libraries/System.Collections/tests/Generic/Queue/Queue.Generic.Tests.cs:54

• [nitpick] Ensure the test property naming matches across both generic and non-generic fixtures; if Enumerator_Empty_Current_UndefinedOperation_Throws is intended here, update the non-generic test to use the same name for consistency.

        protected override bool Enumerator_Empty_Current_UndefinedOperation_Throws => true;

[dotnet-policy-service]

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