Async Profiler: Reuse V1 dispatcher across a method's suspensions.

[lateralusX]

Summary

Optimizes the V1 (StateMachine) async profiler so a single AsyncStateMachineDispatcher spans all of a method's suspensions, instead of allocating a fresh dispatcher on every await/yield.

Motivation

In the previous model, each time a box's continuation was scheduled (i.e. each yield), CreateDispatcher allocated a new dispatcher wrapping the same box. A method that suspends N times produced N dispatchers and N Create events within the same tree. Since the box identity is stable across its suspensions, that per-yield allocation is avoidable: when the active dispatcher is already dispatching this exact box, we can reuse it. This matches asyncv2's behavior.

What changed

AsyncStateMachineDispatcher / CreateDispatcher:

• Reuse guard: if the active dispatcher's inner box is reference-equal to the box being scheduled, return the existing dispatcher instead of allocating a new one.
• MoveNext now resets LastContinuation / ReachedLastContinuation on re-entry, since a reused dispatcher would otherwise carry stale callstack-walk state from its previous resume cycle.

Preserving the late-frame append on reuse (AsyncProfiler.CreateAsyncContext.Append):

• Allocating a new dispatcher previously ran CreateAsyncContext.Create, whose append hook flushes callstack frames that registered after the last resume walk (e.g. a parent continuation that attached late). The reuse path skips Create, so it now performs just that append-flush (no Create event emitted), gated on IsEnabled.ResumeAsyncContext(flags). This keeps AppendStateMachineAsyncCallstack firing before the subsequent Suspend, exactly as before.

Tests

Updated the shared balance assertions to the reuse invariant:

• AssertCreateBalancesSuspendAndCompleteInChain — now checks creates == completes and resumes == suspends + completes.
• StateMachineAsync_WaitThenYield_BalancesResumeAndComplete — same split (createCount == completeCount, plus the model-agnostic resumeCount == completeCount + suspendCount).
• StateMachineAsync_PoolingValueTask_InlineReentrantCompletion — the mis-attribution assertion now verifies the reused

Outer dispatcher re-suspends before it completes (suspendIdx < completeIdx) rather than the old per-yield "re-suspending frame emits no Complete".

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[Copilot]

Pull request overview

This PR changes async-profiler (asyncv1 / state-machine) dispatcher behavior so the same AsyncStateMachineDispatcher instance can be reused across multiple suspensions of the same boxed state machine, avoiding per-await dispatcher allocation and repeated Create events. It also preserves the “late-frame append” behavior on reuse and updates test invariants to match the new model.

Changes:

• Reuse the currently-active AsyncStateMachineDispatcher when scheduling the same inner box, and reset per-resume callstack-walk state on re-entry.
• Add an AsyncProfiler.CreateAsyncContext.Append helper to flush late-frame async callstack append on the reuse path.
• Update async-profiler tests to assert the new create/complete vs. resume/suspend balancing invariants.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
src/libraries/System.Runtime/tests/System.Threading.Tasks.Tests/System.Runtime.CompilerServices/AsyncProfilerV1Tests.cs	Updates v1 tests’ balancing and ordering assertions for dispatcher reuse.
src/libraries/System.Runtime/tests/System.Threading.Tasks.Tests/System.Runtime.CompilerServices/AsyncProfilerTests.cs	Updates shared assertion helper to reflect reuse semantics (creates == completes; resumes == completes + suspends).
src/libraries/System.Private.CoreLib/src/System/Runtime/CompilerServices/AsyncStateMachineDispatcher.cs	Adds reuse guard in CreateDispatcher and resets LastContinuation/ReachedLastContinuation per MoveNext entry; exposes InnerBox for reuse comparison.
src/libraries/System.Private.CoreLib/src/System/Runtime/CompilerServices/AsyncProfiler.cs	Adds CreateAsyncContext.Append to preserve late-frame append behavior when dispatcher allocation is skipped.

[Copilot]

Pull request overview

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.