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Fix JIT assertion for AsyncContinuation in GDV compatibility check #123664

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Fix JIT assertion for AsyncContinuation in GDV compatibility check #123664

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Description

JIT hits assertion '!"Unexpected well known arg to method GDV candidate"' when Guarded Devirtualization encounters methods with AsyncContinuation arguments under tiered PGO with delegate/vtable profiling enabled.

Changes

Added WellKnownArg::AsyncContinuation to the signature compatibility check in isCompatibleMethodGDV:

case WellKnownArg::RetBuffer:
case WellKnownArg::ThisPointer:
case WellKnownArg::AsyncContinuation:  // Added
    // Not part of signature but we still expect to see it here
    continue;

Like RetBuffer and ThisPointer, async continuations are compiler-inserted arguments not reflected in the IL signature. The compatibility check must skip them when matching call arguments against the target method signature.

Consistent with existing handling in fginline.cpp and importer.cpp.

Original prompt

This section details on the original issue you should resolve

<issue_title>Assertion failed '!"Unexpected well known arg to method GDV candidate"</issue_title>
<issue_description>https://dev.azure.com/dnceng-public/public/_build/results?buildId=1266373&view=ms.vss-test-web.build-test-results-tab&runId=35337768&resultId=209882&paneView=dotnet-dnceng.dnceng-build-release-tasks.helix-test-information-tab

windows x64 fullpgo method profiling

set DOTNET_TieredCompilation=1
set DOTNET_ReadyToRun=0
set DOTNET_TC_QuickJitForLoops=1
set DOTNET_TieredPGO=1
set DOTNET_JitDelegateProfiling=1
set DOTNET_JitVTableProfiling=1

  Discovering: System.Text.Json.Tests (method display = ClassAndMethod, method display options = None)
  Discovered:  System.Text.Json.Tests (found 10198 of 10271 test cases)
  Starting:    System.Text.Json.Tests (parallel test collections = on [4 threads], stop on fail = off)

Assert failure(PID 44644 [0x0000ae64], Thread: 22492 [0x57dc]): Assertion failed '!"Unexpected well known arg to method GDV candidate"' in 'JsonSerializerApiValidation:<SerializeNullException>b__9_1():this' during 'Importation' (IL size 25; hash 0xed53ad00; Tier1)

    File: D:\a\_work\1\s\src\coreclr\jit\importercalls.cpp:7446
    Image: C:\h\w\AB6F09F9\p\dotnet.exe

```</issue_description>

<agent_instructions>Add `WellKnownArg::AsyncContinuation` to the switch in `isCompatibleMethodGDV`</agent_instructions>

## Comments on the Issue (you are @copilot in this section)

<comments>
<comment_new><author>@AndyAyersMS</author><body>
Also several failures in net11.0-windows-Release-x64-fullpgo_random_gdv_methodprofiling_only-Windows.10.Amd64.Open

DOTNET_JitClassProfiling=0
DOTNET_JitDelegateProfiling=1
DOTNET_JitRandomGuardedDevirtualization=1
DOTNET_JitRandomlyCollect64BitCounts=1
DOTNET_JitVTableProfiling=1
DOTNET_ReadyToRun=0
DOTNET_TC_QuickJitForLoops=1
DOTNET_TieredCompilation=1
DOTNET_TieredPGO=1

Discovering: System.IO.Compression.Brotli.Tests (method display = ClassAndMethod, method display options = None)
Discovered: System.IO.Compression.Brotli.Tests (found 109 of 120 test cases)
Starting: System.IO.Compression.Brotli.Tests (parallel test collections = on [4 threads], stop on fail = off)

Assert failure(PID 16680 [0x00004128], Thread: 14112 [0x3720]): Assertion failed '!"Unexpected well known arg to method GDV candidate"' in 'System.IO.Compression.CompressionStreamUnitTestBase:RoundTripWithZLibCompressionOptions(System.String,System.IO.Compression.ZLibCompressionOptions):this' during 'Importation' (IL size 176; hash 0x03f009b8; Tier1-OSR)

File: D:\a\_work\1\s\src\coreclr\jit\importercalls.cpp:7446


Maybe related to dotnet/runtime#123434? -- fyi @hez2010 </body></comment_new>
</comments>

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@jakobbotsch
Add WellKnownArg::AsyncContinuation to isCompatibleMethodGDV switch
f726890 
Co-authored-by: jakobbotsch <7887810+jakobbotsch@users.noreply.github.com>
Copilot AI changed the title [WIP] Fix assertion failure for GDV candidate Fix JIT assertion for AsyncContinuation in GDV compatibility check Jan 27, 2026
Copilot AI requested a review from jakobbotsch January 27, 2026 11:50
@jakobbotsch jakobbotsch marked this pull request as ready for review January 27, 2026 12:10
Copilot AI review requested due to automatic review settings January 27, 2026 12:10
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jakobbotsch commented Jan 27, 2026

PTAL @dotnet/jit-contrib

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Pull request overview

This PR updates the JIT’s Guarded Devirtualization (GDV) compatibility check to correctly handle compiler-inserted async continuation arguments, preventing an assertion under tiered PGO with delegate/vtable profiling.

Changes:

  • Extend Compiler::isCompatibleMethodGDV to treat WellKnownArg::AsyncContinuation like RetBuffer and ThisPointer as a non-IL-signature argument that should be skipped during compatibility checks.
  • Align GDV compatibility handling with existing async continuation handling already present in fginline.cpp and importer.cpp, removing the "Unexpected well known arg to method GDV candidate" assertion in the reported configurations.

@jkotas jkotas added area-CodeGen-coreclr CLR JIT compiler in src/coreclr/src/jit and related components such as SuperPMI runtime-async labels Jan 27, 2026
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dotnet-policy-service bot commented Jan 27, 2026

Tagging subscribers to this area: @JulieLeeMSFT, @dotnet/jit-contrib
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This was referenced Jan 27, 2026
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@EgorBo EgorBo EgorBo approved these changes

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@jakobbotsch jakobbotsch jakobbotsch approved these changes

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Assertion failed '!"Unexpected well known arg to method GDV candidate"

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