Enable runtime async via compiler intrinsics by majocha · Pull Request #20235 · dotnet/fsharp

majocha · 2026-08-08T07:15:38Z

Add preview F# compiler support for .NET runtime-async methods. Compiler-recognized __runtimeAsyncReturn intrinsics mark Task/ValueTask methods and lambdas with MethodImplOptions.Async, while AsyncHelpers.Await* calls become runtime suspension points.

The optimizer preserves and specializes inline suspension fragments, rewrites suspending exception handlers and finally compensations, and reports unsupported byref or suspension patterns. The feature is gated by langversion:preview and target-runtime metadata support; FSharp.Core exposes the intrinsics only for net10.0, with builders remaining application-defined.

Consider an inline "sync" CE builder. applying __runtimeAsyncReturn to the inlined code in its Run method compiles the computation expression into a single runtime async method:

member inline _.Run([<InlineIfLambda>] code) = __runtimeAsyncReturn(code())

Resumption is handled by the runtime, calling AsyncHelpers.Await in Bind is all that is needed to make the resulting CE async:

member inline _.Bind(task, [<InlineIfLambda>] continuation) = AsyncHelpers.Await task |> continuation

runtime spec :
Runtime-async specification

interesting docs and links:

github-actions · 2026-08-08T07:15:49Z

❗ Release notes required

You can open this PR in browser to add release notes: open in github.dev

✅ Found changes and release notes in following paths:

Change path Release notes path Description

`src/FSharp.Core` docs/release-notes/.FSharp.Core/11.0.100.md

`src/Compiler` docs/release-notes/.FSharp.Compiler.Service/11.0.100.md

`src/Compiler/Facilities/LanguageFeatures.fsi` docs/release-notes/.Language/preview.md

…c; add Language preview release notes The features dictionary lost ImplicitDIMCoverage, MethodOverloadsCache, ErrorOnMissingSignatureAttribute, DirectDelegateConstruction, AccessProtectedBaseFieldFromClosure and RecordSpreads entries, causing 54 CI test failures ('Unable to find feature' internal errors and preview features not enabled). Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

majocha · 2026-08-13T17:22:20Z

Another thing to think through is inlining. Currently there are no checks at all for use of suspending AsyncHelpers members outside of an async method. According to spec, this is illegal but the idea is that any AsyncHelpers.Await calls should be contained by or inlined into the resulting async method (see the sample runtimeTask builder in the tests here). It seems to get an efficient single method from a CE the builder needs to declare every method inline and make use of InlineIfLambda.

Currently it is up to the "expert" user to not misuse AsyncHelpers. Ideally the compiler should check for any such illegal calls only after inlining.

This is still a sketch, but it successfully compiles runtimeTask builder. The builder passes ported Tasks.fs tests, which is promising.

T-Gro · 2026-08-14T10:48:53Z

Ideally the compiler should check for any such illegal calls only after inlining.

We could have a notion of PostIlxGen checks.
Agree it must run after all optimizations.

Roslyn-async2-inspired edge cases for the runtime-async intrinsic, driven through the test-only runtimeTask CE (treated as a hypothetical library): * execution fixture (RuntimeAsync/RuntimeAsyncEdgeCases.fs): locals/loops across suspension, non-ref struct across suspension, ValueTask operand, exception propagation, IAsyncDisposable with genuinely-async DisposeAsync. * facts (RuntimeAsyncEdgeCaseTests.fs): Await overload selection per operand type, no compiler state machine (direct + CE), the C1 forbidden `tail.` prefix, and a parametrized set of currently-undiagnosed contract-forbidden patterns (await-in-finally/catch, ref-struct- and byref-across-suspension). Every asserted IL substring and runtime symptom was captured empirically on the pinned net11 preview; the forbidden patterns match docs/runtime-async.md. Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com> Copilot-Session: 55dd72c8-46d3-4959-9677-c52d41779596

The sequence-points baseline (and ildasm) cannot render MethodImplOptions.Async (0x2000), so the lifted __runtimeAsync body shows up as a plain outer@<line> closure. Factor the metadata flag check into assertAsyncFlagOnLiftedClosureOnly and chain it onto the sequence-points fact so the exact program that emits the .bsl also proves the async marker lands only on the lifted closure, never on the user's outer/helper methods. Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com> Copilot-Session: 55dd72c8-46d3-4959-9677-c52d41779596

majocha · 2026-08-20T07:43:30Z

Looks like the runtime feature is quickly evolving, see #19056 (comment)

runtime-async-tiering-and-tail-await-optimizations

The previous run was SIGKILL'd by the OOM-killer mid-suite (0 real test failures; 229 tests never ran). Empty commit to re-run the pipeline. Same exit-137 flake also hit unrelated PRs #20274 and #20235 at the same time. Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

majocha · 2026-08-20T21:36:58Z

I wonder how to support the other allowed return types.

__runtimeAsyncReturn<'T>       : 'T -> Task<'T>
__runtimeAsyncReturnValueTask<'T> : 'T -> ValueTask<'T>
__runtimeAsyncReturnUnit       : unit -> Task
__runtimeAsyncReturnValueTaskUnit : unit -> ValueTask

and it quickly becomes a whole zoo. Do we need the non-generic versions at all? Only for potential C# interop, I guess. The upside is that the current type check is all we need to keep it correct, without any extra handling.

The other alternative it to have a unconstrained __runtimeAsyncReturn: 'T -> 'Carrier and do extra checks that the 'Carrier type is supported.

…sharp into runtime-async-intrinsic

majocha · 2026-09-07T14:12:25Z

I ordered the AI to take the sample runtime async asyncSeq and make a full FSharp.Control.TaskSeq reimplementation:

https://github.com/majocha/FSharp.Control.TaskSeq/tree/runtime-async

Remarkably it passes the whole test suite. (in Release).

It also reveals some more debug configuration bugs in this PR.

The coreclr_release job was canceled after the 120-minute limit due to a flaky infrastructure timeout (memory pressure hanging an unrelated test assembly). The same job timed out on unrelated PRs #20235 and #20393 in the last 10 days. ComponentTests (all DIM/interface tests affected by this PR) passed fully, so the merge resolution is correct. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

T-Gro

🤖🕵️ AI-assisted review. Every finding reproduced on a local build of dac78834: captured output, attribution controls, --optimize+ and --optimize-.

T-Gro · 2026-09-09T13:21:21Z

            // application of local type functions with type parameters = measure types and body = local value - inline the body
            GenExpr cenv cgbuf eenv v sequel

+        | Expr.App _ when TryGetRuntimeAsyncReturn g expr |> Option.isSome -> GenRuntimeAsyncReturnAsStartedTask cenv cgbuf eenv expr sequel


🤖🕵️ byref captured by the closure synthesized after PostInferenceChecks, so FS0406 has already run. The flag lands on f@8::Invoke, not f.

[<NoCompilerInlining>] let f (x: byref<int>) : Task<int> = let y = x __runtimeAsyncReturn (x + y)

now: compiles clean; ILVerify StackByRef @0x06; run → NRE; both opt modes

expected: rejected, or lowered without a capturing closure

no leading let, or y unused → flag on M::f, verifies clean, returns 2

fix: reject byref/byref-like/pinning free values before the lambda is built

github-actions · 2026-09-09T23:33:22Z

🔍 Tooling Safety Check — Affects-Bootstrap, Affects-Build-Infra, Affects-Compiler-Output, Affects-Test-Tooling
Affects-Bootstrap: Compiler sources alter the self-hosting compiler chain.
Affects-Build-Infra: Compiler project changes participate in builds.
Affects-Compiler-Output: Code generation changes alter emitted binaries.
Affects-Test-Tooling: Component-test project infrastructure is modified.

Generated by PR Tooling Safety Check · gpt56 2.6M · ◷

majocha · 2026-09-10T07:44:24Z

Unfortunatelly to make async iterators like TaskSeq implementable efficiently, we still need resumable state machine. In fact we need to extend it to generate a runtime-async MoveNext. It would allow to do runtime-async AsyncHelpers.Await directly in a state machine step.

See:

https://github.com/dotnet/roslyn/blob/features/runtime-async-streams/docs/compilers/CSharp/Runtime%20Async-Streams%20Design.md

majocha · 2026-09-10T10:14:54Z

AI thoughts on runtime async state machines implementation:
https://gist.github.com/majocha/785b8cf30de031eec340cc4227e93061

majocha added 5 commits August 8, 2026 08:34

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majocha and others added 5 commits August 8, 2026 09:20

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Merge branch 'main' into runtime-async-intrinsic

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restrict to net10, rename to __runtimeAsyncReturn

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Merge branch 'runtime-async-edge-tests' of https://github.com/T-Gro/f…

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majocha added 3 commits September 3, 2026 07:15

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majocha added 7 commits September 8, 2026 07:26

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