JIT: Enable implicit tailcalls for tail-awaits by jakobbotsch · Pull Request #129255 · dotnet/runtime

jakobbotsch · 2026-06-10T18:49:09Z

We disabled tailcalls from async methods when we added the context restore logic, but now that have and use tail-awaits we can reenable implicit tailcalls for those cases.

Example:

    private static Task Foo(bool b)
    {
        return b ? Bar() : Baz();
    }

    [MethodImpl(MethodImplOptions.NoInlining)]
    private static async Task Bar()
    {
    }

    [MethodImpl(MethodImplOptions.NoInlining)]
    private static async Task Baz()
    {
    }

 G_M15357_IG01:
-       sub      rsp, 40
-						;; size=4 bbWeight=1 PerfScore 0.25
+						;; size=0 bbWeight=1 PerfScore 0.00
 G_M15357_IG02:
        test     dl, dl
-       je       SHORT G_M15357_IG04
+       je       SHORT G_M15357_IG05
 						;; size=4 bbWeight=1 PerfScore 1.25
 G_M15357_IG03:
        xor      rcx, rcx
-       call     [Program:Bar()]
-       test     rcx, rcx
-       jne      SHORT G_M15357_IG09
-       jmp      SHORT G_M15357_IG05
-						;; size=15 bbWeight=0.50 PerfScore 3.25
+						;; size=2 bbWeight=0.50 PerfScore 0.12
 G_M15357_IG04:
-       xor      rcx, rcx
-       call     [Program:Baz()]
-       test     rcx, rcx
-       jne      SHORT G_M15357_IG08
-						;; size=13 bbWeight=0.50 PerfScore 2.25
+       tail.jmp [Program:Bar()]
+						;; size=6 bbWeight=0.50 PerfScore 1.00
 G_M15357_IG05:
-       xor      ecx, ecx
-						;; size=2 bbWeight=1 PerfScore 0.25
+       xor      rcx, rcx
+						;; size=2 bbWeight=0.50 PerfScore 0.12
 G_M15357_IG06:
-       add      rsp, 40
-       ret      
-						;; size=5 bbWeight=1 PerfScore 1.25
-G_M15357_IG07:
-       add      rsp, 40
-       ret      
-						;; size=5 bbWeight=0 PerfScore 0.00
-G_M15357_IG08:
-       jmp      SHORT G_M15357_IG07
-						;; size=2 bbWeight=0 PerfScore 0.00
-G_M15357_IG09:
-       jmp      SHORT G_M15357_IG07
-						;; size=2 bbWeight=0 PerfScore 0.00
+       tail.jmp [Program:Baz()]
+						;; size=6 bbWeight=0.50 PerfScore 1.00
 
-; Total bytes of code 52, prolog size 4, PerfScore 8.50, instruction count 19, allocated bytes for code 52 (MethodHash=2415c402) for method Program:Foo(bool) (FullOpts)
+; Total bytes of code 20, prolog size 0, PerfScore 3.50, instruction count 6, allocated bytes for code 20 (MethodHash=2415c402) for method Program:Foo(bool) (FullOpts)
 ; ============================================================

We disabled tailcalls from async methods when we added the context restore, but now that have and use tail-awaits we can reenable implicit tailcalls for those cases. That will also benefit async versions once that PR merges.

Copilot

Pull request overview

This change adjusts JIT tailcall eligibility for async methods: instead of blanket-disabling tailcalls from async methods, it allows implicit tailcalls only when the call is a tail await, while continuing to disqualify all other tailcall candidates from async methods.

Changes:

Removed the early “caller is async method” tailcall disqualification.
Added a late-stage tailcall eligibility check that permits tailcalls from async methods only for calls marked as async + IsTailAwait.

Copilot

Pull request overview

Copilot reviewed 1 out of 1 changed files in this pull request and generated 2 comments.

Copilot

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

Copilot

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

jakobbotsch · 2026-06-29T17:51:15Z

cc @dotnet/jit-contrib PTAL @EgorBo

Diffs. Real diffs should be larger, just that we see a significant number of misses due to canTailCall.

We disabled tailcalls from async methods when we added the context restore logic, but now that have and use tail-awaits we can reenable implicit tailcalls for those cases. Example: ```csharp private static Task Foo(bool b) { return b ? Bar() : Baz(); } [MethodImpl(MethodImplOptions.NoInlining)] private static async Task Bar() { } [MethodImpl(MethodImplOptions.NoInlining)] private static async Task Baz() { } ``` ```diff G_M15357_IG01: - sub rsp, 40 - ;; size=4 bbWeight=1 PerfScore 0.25 + ;; size=0 bbWeight=1 PerfScore 0.00 G_M15357_IG02: test dl, dl - je SHORT G_M15357_IG04 + je SHORT G_M15357_IG05 ;; size=4 bbWeight=1 PerfScore 1.25 G_M15357_IG03: xor rcx, rcx - call [Program:Bar()] - test rcx, rcx - jne SHORT G_M15357_IG09 - jmp SHORT G_M15357_IG05 - ;; size=15 bbWeight=0.50 PerfScore 3.25 + ;; size=2 bbWeight=0.50 PerfScore 0.12 G_M15357_IG04: - xor rcx, rcx - call [Program:Baz()] - test rcx, rcx - jne SHORT G_M15357_IG08 - ;; size=13 bbWeight=0.50 PerfScore 2.25 + tail.jmp [Program:Bar()] + ;; size=6 bbWeight=0.50 PerfScore 1.00 G_M15357_IG05: - xor ecx, ecx - ;; size=2 bbWeight=1 PerfScore 0.25 + xor rcx, rcx + ;; size=2 bbWeight=0.50 PerfScore 0.12 G_M15357_IG06: - add rsp, 40 - ret - ;; size=5 bbWeight=1 PerfScore 1.25 -G_M15357_IG07: - add rsp, 40 - ret - ;; size=5 bbWeight=0 PerfScore 0.00 -G_M15357_IG08: - jmp SHORT G_M15357_IG07 - ;; size=2 bbWeight=0 PerfScore 0.00 -G_M15357_IG09: - jmp SHORT G_M15357_IG07 - ;; size=2 bbWeight=0 PerfScore 0.00 + tail.jmp [Program:Baz()] + ;; size=6 bbWeight=0.50 PerfScore 1.00 -; Total bytes of code 52, prolog size 4, PerfScore 8.50, instruction count 19, allocated bytes for code 52 (MethodHash=2415c402) for method Program:Foo(bool) (FullOpts) +; Total bytes of code 20, prolog size 0, PerfScore 3.50, instruction count 6, allocated bytes for code 20 (MethodHash=2415c402) for method Program:Foo(bool) (FullOpts) ; ============================================================ ```

JIT: Enable implicit tailcalls for tail-awaits

dcda86e

We disabled tailcalls from async methods when we added the context restore, but now that have and use tail-awaits we can reenable implicit tailcalls for those cases. That will also benefit async versions once that PR merges.

Copilot AI review requested due to automatic review settings June 10, 2026 18:49

Copilot started reviewing on behalf of jakobbotsch June 10, 2026 18:49 View session

github-actions Bot added the area-CodeGen-coreclr CLR JIT compiler in src/coreclr/src/jit and related components such as SuperPMI label Jun 10, 2026

Copilot AI reviewed Jun 10, 2026

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Comment thread src/coreclr/jit/importercalls.cpp Outdated

Comment thread src/coreclr/jit/importercalls.cpp

This was referenced Jun 11, 2026

The Operation will be canceled. The next steps may not contain expected logs. dotnet/dnceng#3008

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[browser][coreCLR] Wasm.Console.Node.Sample - undefined symbol: SystemInteropJS_GetManagedStackTrace #129229

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2eb525c

build-analysis Bot mentioned this pull request Jun 11, 2026

CI build failure: error CS8032: An instance of analyzer Microsoft.NetCore.CSharp.Analyzers.... cannot be created #129031

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Merge branch 'main' into real-tailcall-for-tailawaits

6f6428c

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Comment thread src/coreclr/jit/importercalls.cpp

Comment thread src/coreclr/jit/importercalls.cpp

jakobbotsch added 2 commits June 26, 2026 17:42

Fix

d4603a8

Also allow TransparentAwait

4fc43ab

Copilot AI review requested due to automatic review settings June 26, 2026 16:10

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Comment thread src/coreclr/jit/importer.cpp

Reset tailcall prefix flag before importing RET in async version

653e982

am11 added the runtime-async label Jun 28, 2026

Address feedback, remove unnecessary check

0d7296d

Copilot AI review requested due to automatic review settings June 29, 2026 15:13

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Comment thread src/coreclr/jit/importer.cpp

Comment thread src/coreclr/jit/importercalls.cpp

jakobbotsch marked this pull request as ready for review June 29, 2026 17:49

jakobbotsch requested a review from EgorBo June 29, 2026 17:51

EgorBo approved these changes Jun 30, 2026

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jakobbotsch merged commit 334e58b into dotnet:main Jul 1, 2026
137 checks passed

jakobbotsch deleted the real-tailcall-for-tailawaits branch July 1, 2026 11:08

dotnet-maestro Bot mentioned this pull request Jul 2, 2026

[main] Source code updates from dotnet/runtime dotnet/dotnet#7528

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dotnet-milestone-bot Bot added this to the 11.0-preview7 milestone Jul 2, 2026

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JIT: Enable implicit tailcalls for tail-awaits#129255

JIT: Enable implicit tailcalls for tail-awaits#129255
jakobbotsch merged 7 commits into
dotnet:mainfrom
jakobbotsch:real-tailcall-for-tailawaits

jakobbotsch commented Jun 10, 2026 •

edited

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jakobbotsch commented Jun 10, 2026 •

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