Remove defensive Task[] copy from non-generic Task.WhenAll

[stephentoub]

When calling it with a Task[] or List<Task>, we'll no longer generate a temporary array; the only allocation in those cases will be for the returned Task itself, at least if it completes successfully (if multiple of the constituent tasks fail or are canceled, we'll end up with an additional list allocation beyond what we had before). If you call it with an enumerable that's not an ICollection<Task>, we'll also save a Task[].

Contributes to #77873

Method	Toolchain	Mean	Ratio	Allocated	Alloc Ratio
WhenAll_Array	\main\corerun.exe	155.5 ns	1.00	304 B	1.00
WhenAll_Array	\pr\corerun.exe	149.6 ns	0.96	264 B	0.87
WhenAll_List	\main\corerun.exe	212.3 ns	1.00	376 B	1.00
WhenAll_List	\pr\corerun.exe	178.5 ns	0.87	296 B	0.79
WhenAll_Collection	\main\corerun.exe	210.3 ns	1.00	360 B	1.00
WhenAll_Collection	\pr\corerun.exe	177.6 ns	0.82	328 B	0.91
WhenAll_Enumerable	\main\corerun.exe	241.8 ns	1.00	472 B	1.00
WhenAll_Enumerable	\pr\corerun.exe	239.8 ns	0.99	432 B	0.92

using BenchmarkDotNet.Attributes;
using BenchmarkDotNet.Running;
using System.Collections.Generic;
using System.Collections.ObjectModel;
using System.Runtime.CompilerServices;
using System.Threading.Tasks;

[MemoryDiagnoser(false)]
public partial class Program
{
    static void Main(string[] args) => BenchmarkSwitcher.FromAssembly(typeof(Program).Assembly).Run(args);

    [Benchmark]
    public void WhenAll_Array()
    {
        AsyncTaskMethodBuilder atmb1 = AsyncTaskMethodBuilder.Create();
        AsyncTaskMethodBuilder atmb2 = AsyncTaskMethodBuilder.Create();
        Task whenAll = Task.WhenAll(atmb1.Task, atmb2.Task);
        atmb1.SetResult();
        atmb2.SetResult();
        whenAll.Wait();
    }

    [Benchmark]
    public void WhenAll_List()
    {
        AsyncTaskMethodBuilder atmb1 = AsyncTaskMethodBuilder.Create();
        AsyncTaskMethodBuilder atmb2 = AsyncTaskMethodBuilder.Create();
        Task whenAll = Task.WhenAll(new List<Task>(2) { atmb1.Task, atmb2.Task });
        atmb1.SetResult();
        atmb2.SetResult();
        whenAll.Wait();
    }

    [Benchmark]
    public void WhenAll_Collection()
    {
        AsyncTaskMethodBuilder atmb1 = AsyncTaskMethodBuilder.Create();
        AsyncTaskMethodBuilder atmb2 = AsyncTaskMethodBuilder.Create();
        Task whenAll = Task.WhenAll(new ReadOnlyCollection<Task>(new[] { atmb1.Task, atmb2.Task }));
        atmb1.SetResult();
        atmb2.SetResult();
        whenAll.Wait();
    }

    [Benchmark]
    public void WhenAll_Enumerable()
    {
        AsyncTaskMethodBuilder atmb1 = AsyncTaskMethodBuilder.Create();
        AsyncTaskMethodBuilder atmb2 = AsyncTaskMethodBuilder.Create();
        var q = new Queue<Task>(2);
        q.Enqueue(atmb1.Task);
        q.Enqueue(atmb2.Task);
        Task whenAll = Task.WhenAll(q);
        atmb1.SetResult();
        atmb2.SetResult();
        whenAll.Wait();
    }
}

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Issue Details

---

When calling it with a Task[] or List<Task>, we'll no longer generate a temporary array; the only allocation in those cases will be for the returned Task itself, at least if it completes successfully (if multiple of the constituent tasks fail or are canceled, we'll end up with an additional list allocation beyond what we had before). If you call it with an enumerable that's not an ICollection<Task>, we'll also save a Task[].

Contributes to #77873

Method	Toolchain	Mean	Ratio	Allocated	Alloc Ratio
WhenAll_Array	\main\corerun.exe	155.5 ns	1.00	304 B	1.00
WhenAll_Array	\pr\corerun.exe	149.6 ns	0.96	264 B	0.87
WhenAll_List	\main\corerun.exe	212.3 ns	1.00	376 B	1.00
WhenAll_List	\pr\corerun.exe	178.5 ns	0.87	296 B	0.79
WhenAll_Collection	\main\corerun.exe	210.3 ns	1.00	360 B	1.00
WhenAll_Collection	\pr\corerun.exe	177.6 ns	0.82	328 B	0.91
WhenAll_Enumerable	\main\corerun.exe	241.8 ns	1.00	472 B	1.00
WhenAll_Enumerable	\pr\corerun.exe	239.8 ns	0.99	432 B	0.92

using BenchmarkDotNet.Attributes;
using BenchmarkDotNet.Running;
using System.Collections.Generic;
using System.Collections.ObjectModel;
using System.Runtime.CompilerServices;
using System.Threading.Tasks;

[MemoryDiagnoser(false)]
public partial class Program
{
    static void Main(string[] args) => BenchmarkSwitcher.FromAssembly(typeof(Program).Assembly).Run(args);

    [Benchmark]
    public void WhenAll_Array()
    {
        AsyncTaskMethodBuilder atmb1 = AsyncTaskMethodBuilder.Create();
        AsyncTaskMethodBuilder atmb2 = AsyncTaskMethodBuilder.Create();
        Task whenAll = Task.WhenAll(atmb1.Task, atmb2.Task);
        atmb1.SetResult();
        atmb2.SetResult();
        whenAll.Wait();
    }

    [Benchmark]
    public void WhenAll_List()
    {
        AsyncTaskMethodBuilder atmb1 = AsyncTaskMethodBuilder.Create();
        AsyncTaskMethodBuilder atmb2 = AsyncTaskMethodBuilder.Create();
        Task whenAll = Task.WhenAll(new List<Task>(2) { atmb1.Task, atmb2.Task });
        atmb1.SetResult();
        atmb2.SetResult();
        whenAll.Wait();
    }

    [Benchmark]
    public void WhenAll_Collection()
    {
        AsyncTaskMethodBuilder atmb1 = AsyncTaskMethodBuilder.Create();
        AsyncTaskMethodBuilder atmb2 = AsyncTaskMethodBuilder.Create();
        Task whenAll = Task.WhenAll(new ReadOnlyCollection<Task>(new[] { atmb1.Task, atmb2.Task }));
        atmb1.SetResult();
        atmb2.SetResult();
        whenAll.Wait();
    }

    [Benchmark]
    public void WhenAll_Enumerable()
    {
        AsyncTaskMethodBuilder atmb1 = AsyncTaskMethodBuilder.Create();
        AsyncTaskMethodBuilder atmb2 = AsyncTaskMethodBuilder.Create();
        var q = new Queue<Task>(2);
        q.Enqueue(atmb1.Task);
        q.Enqueue(atmb2.Task);
        Task whenAll = Task.WhenAll(q);
        atmb1.SetResult();
        atmb2.SetResult();
        whenAll.Wait();
    }
}

Author:	stephentoub
Assignees:	-
Labels:	area-System.Threading.Tasks
Milestone:	-

[dakersnar]

LGTM at a high level.