Recognize Task<->ValueTask adaptions in async versions by jakobbotsch · Pull Request #130081 · dotnet/runtime

jakobbotsch · 2026-07-01T14:05:12Z

Support looking through return new ValueTask(TaskReturningFunction()) and return ValueTaskReturningFunction().AsTask() in async versions. These can be transformed into async calls.

This is a common pattern in the BCL, e.g.:

runtime/src/libraries/System.Net.Sockets/src/System/Net/Sockets/Socket.Tasks.cs

Line 268 in 06362e8

return ReceiveAsync(buffer, socketFlags, fromNetworkStream, default).AsTask();

runtime/src/libraries/System.IO.Pipes/src/System/IO/Pipes/PipeStream.Unix.cs

Line 176 in 5ee89e4

return new ValueTask(WriteAsyncCore(buffer, cancellationToken));

Example:

    [MethodImpl(MethodImplOptions.NoInlining)]
    private static ValueTask TestTaskToValueTask()
    {
        return new ValueTask(TestTask());
    }

    [MethodImpl(MethodImplOptions.NoInlining)]
    private static async Task TestTask()
    {
        await Task.Yield();
    }

Diff:

 G_M37697_IG01:
-       push     rbp
-       sub      rsp, 80
-       lea      rbp, [rsp+0x50]
-       vxorps   xmm4, xmm4, xmm4
-       vmovdqu  ymmword ptr [rbp-0x30], ymm4
-       vmovdqa  xmmword ptr [rbp-0x10], xmm4
-       mov      gword ptr [rbp+0x10], rcx
-						;; size=28 bbWeight=1 PerfScore 5.08
+       sub      rsp, 40
+						;; size=4 bbWeight=1 PerfScore 0.25
 G_M37697_IG02:
-       vxorps   xmm0, xmm0, xmm0
-       vmovdqu  xmmword ptr [rbp-0x10], xmm0
-       call     [AsyncMicro.Program:TestTask():System.Threading.Tasks.Task]
-       mov      gword ptr [rbp-0x18], rax
-       mov      rdx, gword ptr [rbp-0x18]
-       lea      rcx, [rbp-0x10]
-       call     [System.Threading.Tasks.ValueTask:.ctor(System.Threading.Tasks.Task):this]
-						;; size=33 bbWeight=1 PerfScore 9.83
-G_M37697_IG03:
-       vmovdqu  xmm0, xmmword ptr [rbp-0x10]
-       vmovdqu  xmmword ptr [rbp-0x28], xmm0
-						;; size=10 bbWeight=1 PerfScore 2.00
-G_M37697_IG04:
-       lea      rdx, [rbp-0x28]
        xor      rcx, rcx
-       call     [System.Runtime.CompilerServices.AsyncHelpers:TransparentAwaitWithResult(System.Threading.Tasks.ValueTask)]
-       mov      gword ptr [rbp-0x30], rcx
-       cmp      gword ptr [rbp-0x30], 0
-       jne      SHORT G_M37697_IG06
+       call     [AsyncMicro.Program:TestTask()]
+       test     rcx, rcx
+       jne      SHORT G_M37697_IG04
        xor      ecx, ecx
-						;; size=25 bbWeight=1 PerfScore 7.00
-G_M37697_IG05:
-       add      rsp, 80
-       pop      rbp
+						;; size=15 bbWeight=1 PerfScore 4.75
+G_M37697_IG03:
+       add      rsp, 40
        ret      
-						;; size=6 bbWeight=1 PerfScore 1.75
-G_M37697_IG06:
-       mov      rax, gword ptr [rbp-0x30]
-       mov      rcx, rax
-						;; size=7 bbWeight=0 PerfScore 0.00
-G_M37697_IG07:
-       add      rsp, 80
-       pop      rbp
+						;; size=5 bbWeight=1 PerfScore 1.25
+G_M37697_IG04:
+       add      rsp, 40
        ret      
-						;; size=6 bbWeight=0 PerfScore 0.00
+						;; size=5 bbWeight=0 PerfScore 0.00
 
-; Total bytes of code 115, prolog size 24, PerfScore 25.67, instruction count 31, allocated bytes for code 115 (MethodHash=6db76cbe) for method AsyncMicro.Program:TestTaskToValueTask() (Tier0)
+; Total bytes of code 29, prolog size 4, PerfScore 6.25, instruction count 10, allocated bytes for code 29 (MethodHash=6db76cbe) for method AsyncMicro.Program:TestTaskToValueTask() (FullOpts)
 ; ============================================================

Support looking through `return new ValueTask(TaskReturningFunction())` and `return ValueTaskReturningFunction().AsTask()` in async versions. These can be transformed into async calls.

dotnet-policy-service · 2026-07-01T14:06:47Z

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

jakobbotsch · 2026-07-01T14:12:33Z

+        // We cannot inline if the callee returns valueTask.AsTask() in an
+        // async version. We need to preserve the continuation in this case to
+        // be able to mark it with CORINFO_CONTINUATION_VALUETASK_ADAPTED_TO_TASK.
+        if ((prefixFlags & PREFIX_IS_ADAPTED_FROM_VALUETASK) != 0)
+        {
+            compInlineResult->NoteFatal(InlineObservation::CALLEE_AWAIT);
+            return;
+        }


Not very satisfied with this. The main problem is that return valueTask.AsTask() comes with additional ceremony around handling IValueTaskSource because the returned Task does not have the exact same semantics as ValueTask for that case. In particular the Task adapter always passes ValueTaskSourceOnCompletedFlags.None to the IValueTaskSource. We need to teach our IValueTaskSource handling about this special case, and that requires introducing the new flag, but then we end up not able to inline once we see this case.

One solution may be to have the JIT insert an extra Continuation into the chain when this flag is present if we inlined.

In any case I think this optimization even if it means we disallow inlining will still be beneficial.

jakobbotsch · 2026-07-01T14:14:07Z

+        ResultIndexFirstBit = 11,
+        ResultIndexNumBits = 21,


This would require another R2R version bump. I think I will include this flag change in #129894 instead.

Copilot

Pull request overview

This PR extends CoreCLR “runtime-async / async-version” recognition so the JIT can see through common Task↔ValueTask adapters (new ValueTask(task) and valueTask.AsTask()) and treat them as direct async calls, reducing wrapper overhead. It also adds tests intended to cover these adapter patterns.

Changes:

Mark ValueTask / ValueTask<T> Task-based constructors and AsTask() methods as [Intrinsic] so the JIT can pattern-match them.
Teach the JIT importer to recognize async-version tail patterns involving ValueTask adapters and propagate a new “adapted” flag into async-call lowering and continuation flags.
Add/adjust async tests/projects for adapter scenarios and align the IL project’s assembly naming.

Reviewed changes

Copilot reviewed 12 out of 13 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
src/tests/async/async-versions/async-versions.ilproj	Adds an IL SDK project for the IL-authored async-version tests; removes injected C# helper attribute source.
src/tests/async/async-versions/async-versions.il	Renames the IL assembly to match the project name (`async-versions`).
src/tests/async/async-versions-task-adapters/async-versions-task-adapters.csproj	New test project to validate Task↔ValueTask adapter behavior.
src/tests/async/async-versions-task-adapters/async-versions-task-adapters.cs	New xUnit tests covering `new ValueTask(Task)`, `ValueTask.AsTask()`, and `ValueTask<Task<int>>` overload binding behavior.
src/libraries/System.Private.CoreLib/src/System/Threading/Tasks/ValueTask.cs	Adds `[Intrinsic]` to relevant constructors/`AsTask()` overloads to enable JIT recognition.
src/coreclr/System.Private.CoreLib/src/System/Runtime/CompilerServices/AsyncHelpers.CoreCLR.cs	Adds a new continuation flag bit and shifts bitfield layout constants.
src/coreclr/jit/namedintrinsiclist.h	Adds new `NamedIntrinsic` IDs for `ValueTask`/`ValueTask<T>` ctor and `AsTask`.
src/coreclr/jit/importercalls.cpp	Blocks certain tail-await optimizations/inlining for adapted patterns; wires new intrinsic lookups.
src/coreclr/jit/importer.cpp	Factors stloc/ldloca matching; adds async-version tail-call pattern matching for `AsTask()` and `newobj ValueTask(..)`.
src/coreclr/jit/gentree.h	Adds `AsyncCallInfo.IsValueTaskAsTask` marker.
src/coreclr/jit/compiler.h	Adds `PREFIX_IS_ADAPTED_FROM_VALUETASK` prefix flag.
src/coreclr/jit/async.cpp	Sets the new continuation flag based on `IsValueTaskAsTask`.
src/coreclr/inc/corinfo.h	Adds `CORINFO_CONTINUATION_VALUETASK_ADAPTED_TO_TASK` and updates bitfield layout constants.

Copilot

Pull request overview

Copilot reviewed 12 out of 13 changed files in this pull request and generated 1 comment.

+        if (info.compRetType != TYP_VOID)
+        {
+            assert((sig.sigInst.classInstCount == 1) && (sig.sigInst.methInstCount == 0));
+            CORINFO_CLASS_HANDLE paramClass = info.compCompHnd->getArgClass(&sig, sig.args);
+            if (paramClass == sig.sigInst.classInst[0])
+            {
+                // This is "class ValueTask<T> { ValueTask(T value) }" overload
+                // which is not what we are looking for. That one gets folded
+                // by impFoldAwaitedTopOfStack.
+                return false;
+            }
+        }


jakobbotsch added 2 commits July 1, 2026 15:24

Recognize Task<->ValueTask adaptions in async versions

5ee695f

Support looking through `return new ValueTask(TaskReturningFunction())` and `return ValueTaskReturningFunction().AsTask()` in async versions. These can be transformed into async calls.

Add tests

f688392

Copilot AI review requested due to automatic review settings July 1, 2026 14:05

github-actions Bot added the area-CodeGen-coreclr CLR JIT compiler in src/coreclr/src/jit and related components such as SuperPMI label Jul 1, 2026

Copilot started reviewing on behalf of jakobbotsch July 1, 2026 14:05 View session

dotnet-policy-service Bot assigned jakobbotsch Jul 1, 2026

jakobbotsch commented Jul 1, 2026

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jakobbotsch added 3 commits July 1, 2026 16:20

feedback

3de295a

Make usage more clear

69b46f5

More feedback

cacdec0

Copilot AI review requested due to automatic review settings July 1, 2026 14:25

Copilot started reviewing on behalf of jakobbotsch July 1, 2026 14:26 View session

Copilot AI reviewed Jul 1, 2026

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jakobbotsch mentioned this pull request Jul 1, 2026

Skip save/restore of contexts in task-returning thunks; mark thunk infrastructure NonVersionable #129894

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Recognize Task<->ValueTask adaptions in async versions #130081

Recognize Task<->ValueTask adaptions in async versions #130081
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jakobbotsch commented Jul 1, 2026 •

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