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JIT: Enable implicit tailcalls for tail-awaits - #129255

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jakobbotsch merged 7 commits into
dotnet:mainfrom
jakobbotsch:real-tailcall-for-tailawaits
Jul 1, 2026
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JIT: Enable implicit tailcalls for tail-awaits#129255
jakobbotsch merged 7 commits into
dotnet:mainfrom
jakobbotsch:real-tailcall-for-tailawaits

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@jakobbotschjakobbotsch commented Jun 10, 2026•
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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:

privatestaticTaskFoo(boolb){returnb?Bar():Baz();}[MethodImpl(MethodImplOptions.NoInlining)]privatestaticasyncTaskBar(){}[MethodImpl(MethodImplOptions.NoInlining)]privatestaticasyncTaskBaz(){}
 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)
; ============================================================

@jakobbotsch
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.
CopilotAI review requested due to automatic review settings June 10, 2026 18:49
@github-actionsgithub-actionsBot added the area-CodeGen-coreclr CLR JIT compiler in src/coreclr/src/jit and related components such as SuperPMI label Jun 10, 2026
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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.

Comment threadsrc/coreclr/jit/importercalls.cpp Outdated
Comment threadsrc/coreclr/jit/importercalls.cpp
This was referenced Jun 11, 2026
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Pull request overview

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

Comment threadsrc/coreclr/jit/importercalls.cpp
Comment threadsrc/coreclr/jit/importercalls.cpp
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Pull request overview

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

Comment threadsrc/coreclr/jit/importer.cpp
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Pull request overview

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

Comment threadsrc/coreclr/jit/importer.cpp
Comment threadsrc/coreclr/jit/importercalls.cpp
@jakobbotsch
jakobbotsch marked this pull request as ready for review June 29, 2026 17:49
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jakobbotsch commented Jun 29, 2026

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cc @dotnet/jit-contrib PTAL @EgorBo

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

@jakobbotsch
jakobbotsch requested a review from EgorBoJune 29, 2026 17:51
@jakobbotsch
jakobbotsch merged commit 334e58b into dotnet:mainJul 1, 2026
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jakobbotsch deleted the real-tailcall-for-tailawaits branch July 1, 2026 11:08
@dotnet-maestrodotnet-maestroBot mentioned this pull request Jul 2, 2026
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@dotnet-milestone-botdotnet-milestone-botBot added this to the 11.0-preview7 milestone Jul 2, 2026
eiriktsarpalis pushed a commit that referenced this pull request Jul 15, 2026
@jakobbotsch@eiriktsarpalis
JIT: Enable implicit tailcalls for tail-awaits (#129255)
4fdc18d
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)
; ============================================================
```
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