Fix unprepared delegate target signatures under GCStress by davidwrighton · Pull Request #136864 · dotnet/runtime

davidwrighton · 2026-11-02T19:33:30Z

Fixes a GCStress assertion (clsload.cpp, type load on the GC thread) hit when a GC occurs in MethodDesc::DoPrestub for a delegate target whose signature contains value types that were never loaded. #135805 removed caller-side signature loading, and delegate targets published via getFunctionFixedEntryPoint were not prepared, so the prestub had no GCRefMap and fell back to signature walking.

Changes:

getFunctionFixedEntryPoint: call PrepareForUseAsACallTargetOfANativeImage() for delegate targets (covers JIT-constructed delegates).
crossgen2: emit a precomputed GCRefMap for delegate constructor fixups in a new DelegateCtorImports section (only for non-generic, non-virtual, non-unboxing targets, not Wasm). The runtime's DynamicHelperFixup uses it, with a debug assert that it matches the computed map.
Test harness: don't pass --verify-type-and-field-layout to crossgen2 when GCStress is enabled, since it can hide early type loads.
Regression test GitHub_136814 (R2R caller + JIT target); it relies on the outerloop GCStress runs and does not force GCStress itself.

Validation (Checked CoreCLR, x64 Linux): the test asserts under GCStress=0xC / JitStress=1 / tiering off without the fix and passes with it; passes normally; crossgen2smoke and coldsignaturedispatch pass under GCStress=0xC. Not run: NativeLibrary tests, the full R2R suite, Windows or Wasm paths.

Resolves #136814

Note

This PR description was AI/Copilot-generated.

…Stress An R2R caller invoking a JIT-compiled delegate target whose signature contains an unloaded value type asserts in the class loader when a GC stress GC walks the prestub frame. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Eagerly prepare delegate target signatures when publishing a fixed entry point, and have crossgen2 emit a precomputed GCRefMap for delegate constructor fixups so the prestub need not load types on the GC thread. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

azure-pipelines · 2026-11-02T19:34:16Z

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dotnet-policy-service · 2026-11-02T19:34:59Z

Tagging subscribers to this area: @dotnet/crossgen-contrib
See info in area-owners.md if you want to be subscribed.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Copilot

🟡 Changes recommended

Delegate fixups can cache stale GC maps for struct layouts that change across versions, causing missed GC roots, while regression tests do not cover the new mapped DelegateCtor path.

2 open issues

What changed in this PR

Addresses a delegate-target signature preparation gap exposed by GCStress after caller-side signature loading was removed. The changes span CoreCLR target preparation, crossgen2 import generation, and regression testing.

Changes:

Prepare JIT-created delegate targets before publication and reuse precomputed GCRefMaps for eligible ReadyToRun delegate constructors.
Share GCRefMap signature support and add a dedicated delegate-constructor import section.
Disable early layout verification under GCStress and add an R2R-caller/JIT-target regression test.

File	Description
`src/tests/Regressions/coreclr/GitHub_136814/test136814.csproj`	Configures isolation, crossgen2, and a JIT-only target assembly.
`src/tests/Regressions/coreclr/GitHub_136814/test136814.cs`	Adds the delegate target and regression entry point.
`src/tests/Regressions/coreclr/GitHub_136814/delegatecaller.csproj`	Defines the helper library project.
`src/tests/Regressions/coreclr/GitHub_136814/delegatecaller.cs`	Provides the non-inlined R2R delegate caller.
`src/tests/Common/CLRTest.CrossGen.targets`	Skips layout verification when GCStress is enabled.
`src/coreclr/vm/prestub.cpp`	Consumes delegate-constructor GCRefMaps during target preparation.
`src/coreclr/vm/jitinterface.cpp`	Prepares delegate targets before exposing entry points.
`src/coreclr/tools/aot/ILCompiler.ReadyToRun/Compiler/DependencyAnalysis/ReadyToRunSymbolNodeFactory.cs`	Routes eligible delegate constructors to mapped imports.
`src/coreclr/tools/aot/ILCompiler.ReadyToRun/Compiler/DependencyAnalysis/ReadyToRunCodegenNodeFactory.cs`	Creates and roots the delegate-constructor import section.
`src/coreclr/tools/aot/ILCompiler.ReadyToRun/Compiler/DependencyAnalysis/ReadyToRun/SignatureEmbeddedPointerIndirectionNode.cs`	Uses shared GCRefMap support for signature offsets.
`src/coreclr/tools/aot/ILCompiler.ReadyToRun/Compiler/DependencyAnalysis/ReadyToRun/Signature.cs`	Introduces the shared GCRefMap signature base class.
`src/coreclr/tools/aot/ILCompiler.ReadyToRun/Compiler/DependencyAnalysis/ReadyToRun/MethodFixupSignature.cs`	Moves existing map storage into the shared base class.
`src/coreclr/tools/aot/ILCompiler.ReadyToRun/Compiler/DependencyAnalysis/ReadyToRun/ImportSectionNode.cs`	Enables map emission through the shared signature abstraction.
`src/coreclr/tools/aot/ILCompiler.ReadyToRun/Compiler/DependencyAnalysis/ReadyToRun/DelegateCtorSignature.cs`	Defines eligibility and emits delegate-target maps.
`src/coreclr/tools/aot/ILCompiler.ReadyToRun.Tests/TestCases/R2RTestSuites.cs`	Allows GCRefMaps on additional executable-pointer import sections.

_{Review effort: Balanced}

+            return !method.Unboxing
+                && !target.HasInstantiation
+                && !target.OwningType.HasInstantiation


+                bool hasMaps = (section.Flags & ReadyToRunImportSectionFlags.GCRefMap) != 0;
+                if (section.Type == ReadyToRunImportSectionType.StubDispatch)
+                    Assert.True(hasMaps);
+                else if ((section.Flags & ReadyToRunImportSectionFlags.PCode) == 0)
+                    Assert.False(hasMaps);


jkoritzinsky · 2026-11-03T22:25:56Z

+    <Optimize>true</Optimize>
+    <!-- R2R code in the helper assembly calls a JIT-compiled delegate target. -->
+    <AlwaysUseCrossGen2>true</AlwaysUseCrossGen2>
+    <CLRTestTargetUnsupported Condition="'$(TargetArchitecture)' == 'wasm'">true</CLRTestTargetUnsupported>


Is this necessary?

jkoritzinsky · 2026-11-03T22:26:30Z

+    <RequiresProcessIsolation>true</RequiresProcessIsolation>
+    <CLRTestPriority>1</CLRTestPriority>


Can we use the [OuterLoop] attribute instead so we can reduce the number of processes?

jkoritzinsky · 2026-11-03T22:26:56Z

+    <!-- Keep the delegate target JIT-compiled so it has no precompiled GC ref map. -->
+    <CLRTestEnvironmentVariable Include="DOTNET_ReadyToRunExcludeList" Value="$(MSBuildProjectName)" />


Replace this with BypassReadyToRunAttribute on the target?

davidwrighton and others added 3 commits November 2, 2026 18:50

Omit crossgen2 layout verification under GCStress

7226ca2

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

github-actions Bot added the area-ReadyToRun label Nov 2, 2026

dotnet-policy-service Bot assigned davidwrighton Nov 2, 2026

This was referenced Nov 2, 2026

Checkout failure: "Git fetch failed with exit code 128" dotnet/arcade#9009

Open

Test failure: IdsConversionsWithRegionTest America/Vancouver America/Los_Angeles mismatch #136758

Open

Allow GCRefMaps in PCode import sections in R2R import test

a2b8542

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

davidwrighton requested a balanced review from Copilot November 3, 2026 19:29

Copilot started reviewing on behalf of davidwrighton November 3, 2026 19:30

View session

davidwrighton requested a review from jkoritzinsky November 3, 2026 19:30

davidwrighton marked this pull request as ready for review November 3, 2026 19:30

Copilot AI reviewed Nov 3, 2026

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jkoritzinsky reviewed Nov 3, 2026

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jkoritzinsky mentioned this pull request Nov 4, 2026

Remove iOS and tvOS simulator x64 support for .NET 12 dotnet/dotnet#10245

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Fix unprepared delegate target signatures under GCStress - #136864

Fix unprepared delegate target signatures under GCStress#136864
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davidwrighton:regression/delegate-signature-gcstress-136814

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		<RequiresProcessIsolation>true</RequiresProcessIsolation>
		<CLRTestPriority>1</CLRTestPriority>

		<!-- Keep the delegate target JIT-compiled so it has no precompiled GC ref map. -->
		<CLRTestEnvironmentVariable Include="DOTNET_ReadyToRunExcludeList" Value="$(MSBuildProjectName)" />

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🟡 Changes recommended

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