JIT: Preserve async continuation arg across EnC remap by tommcdon · Pull Request #128664 · dotnet/runtime

tommcdon · 2026-05-27T20:42:56Z

The async continuation argument was homed in the local frame area which gets zeroed by FixContextForEnC during EnC remap. On platforms with shadow space, the continuation accidentally survived because it was homed above callerSP, outside the zeroed region. On platforms without shadow space, the continuation was destroyed.

Fix: Assign a dedicated IL number (ASYNC_CONTINUATION_ILNUM = -4) so the continuation arg appears in NativeVarInfo and survives EnC remap on all platforms.

This shifts MAX_ILNUM from -4 to -5, changing the NativeVarInfo encoding bias. R2R major version bumped 18→19 accordingly.

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Copilot

Pull request overview

This PR adjusts CoreCLR JIT frame layout for async methods under Edit-and-Continue (EnC) so the async continuation argument’s stack home is placed in the EnC-preserved “frame header” region, and updates GC info preserved-area sizing on x64 and ARM64 accordingly.

Changes:

lclvars.cpp: Skips normal arg/local offset assignment for lvaAsyncContinuationArg under EnC and allocates its stack slot via lvaAllocAsyncContexts.
codegenxarch.cpp: Extends EnC preserved-area size to include the continuation slot on x64.
codegenarmarch.cpp: Extends EnC preserved-area size to include the continuation slot on ARM64.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
src/coreclr/jit/lclvars.cpp	Changes EnC frame-layout allocation/skip logic so the async continuation arg is intended to live in the EnC-preserved header region.
src/coreclr/jit/codegenxarch.cpp	Updates x64 EnC preserved-area size computation to account for the continuation slot.
src/coreclr/jit/codegenarmarch.cpp	Updates ARM64 EnC preserved-area size computation to account for the continuation slot.

jakobbotsch · 2026-05-27T20:52:58Z

How does the generic context get handled? We should align these.

tommcdon · 2026-05-27T21:10:47Z

How does the generic context get handled? We should align these.

Generic context is stored as an explicit parameter so is automatically stored into NativeVarInfo. The continuation arg, however, has UNKNOWN_ILNUM so doesn't automatically get stored.

jakobbotsch · 2026-05-27T21:17:26Z

I wonder if we should give this a number instead. I am not totally sure if things will work out correctly in all cases if we try to allocate an argument together with locals.

tommcdon · 2026-05-28T00:22:04Z

I wonder if we should give this a number instead. I am not totally sure if things will work out correctly in all cases if we try to allocate an argument together with locals.

I think it makes sense. The variable already has a normal parameter home, so we would simply be assigning it a proper IL value, like ASYNC_CONTINUATION_ILNUM = -4. FixContextForEnC will find the variable in NativeVarInfo and copy its value from old frame to the new frame. Unless there are objections, I'll push a commit with the proposed change.

tommcdon · 2026-05-28T16:33:06Z

I wonder if we should give this a number instead. I am not totally sure if things will work out correctly in all cases if we try to allocate an argument together with locals.

I think it makes sense. The variable already has a normal parameter home, so we would simply be assigning it a proper IL value, like ASYNC_CONTINUATION_ILNUM = -4. FixContextForEnC will find the variable in NativeVarInfo and copy its value from old frame to the new frame. Unless there are objections, I'll push a commit with the proposed change.

I believe this approach will require a R2R major version bump, because NativeVarInfo isn't versioned today.

jakobbotsch · 2026-05-28T16:39:20Z

I think that should be ok, we can combine it with the change for #128397.
I think we already took a major version change in .NET 11, so I am not sure if we actually need to take another. We don't guarantee compatibility between previews.

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Pull request overview

Copilot reviewed 7 out of 7 changed files in this pull request and generated 5 comments.

tommcdon · 2026-05-28T19:28:39Z

I think that should be ok, we can combine it with the change for #128397. I think we already took a major version change in .NET 11, so I am not sure if we actually need to take another. We don't guarantee compatibility between previews.

@jakobbotsch you are correct - the R2R major version was bumped on #122704 which is during .NET 11. @jkotas do you see any issues with breaking the R2R format across daily .NET 11 builds and previews?

jkotas · 2026-05-28T19:54:27Z

do you see any issues with breaking the R2R format across daily .NET 11 builds and previews?

I do not see any issues, as you have said we started a new compat tier for .NET 11 already.

jkotas · 2026-05-28T20:31:56Z

I thought that you are asking about whether it is ok to bump the version. We like to bump the version whenever we break the format to save us debugging weird crashes caused by mismatched versions.

tommcdon · 2026-05-28T20:35:56Z

I thought that you are asking about whether it is ok to bump the version. We like to bump the version whenever we break the format to save us debugging weird crashes caused by mismatched versions.

Ahh - I thought there was agreement on no version bump. Bringing it back!

Copilot

Pull request overview

Copilot reviewed 7 out of 7 changed files in this pull request and generated 4 comments.

tommcdon · 2026-05-29T02:23:46Z

+    private const uint MAX_ILNUM = unchecked((uint)-5);



@rcj1 @max-charlamb just checking before dismissing this comment - do you see any concerns with adding ASYNC_CONTINUATION_ILNUM (-4) toNativeVarInfo without adjusting for older builds?

tommcdon · 2026-05-29T19:37:20Z

FYI - it looks like we just took another R2R breaking change in main

<<<<<<< dev/tommcdon/fix-asyncv2-enc-continuation-preserved
// R2R Version 19.0 changes NativeVarInfo encoding to include ASYNC_CONTINUATION_ILNUM
=======
// R2R Version 19 removes the READYTORUN_HELPER_ByRefWriteBarrier helper
>>>>>>> main

jkotas · 2026-05-29T21:15:27Z

it looks like we just took another R2R breaking change in main

Change this one to 20 to resolve the conflict.

When async V2 is enabled and EnC is active, the JIT now assigns ASYNC_CONTINUATION_ILNUM (-4) to the async continuation arg variable instead of UNKNOWN_ILNUM (-4, previously). This allows the debugger to identify and preserve this variable across EnC remaps. Also adds compMapILvarNum reverse mapping case and fixes a typo in cordebuginfo.h. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

The ASYNC_CONTINUATION_ILNUM addition changes MAX_ILNUM from -4 to -5, which alters the NativeVarInfo encoding bias. Bump the R2R major version so old images are rejected rather than silently misinterpreting variable info. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Copilot

Pull request overview

Copilot reviewed 7 out of 7 changed files in this pull request and generated 1 comment.

+#define READYTORUN_MAJOR_VERSION 20
 #define READYTORUN_MINOR_VERSION 0x0000

-#define MINIMUM_READYTORUN_MAJOR_VERSION 19
+#define MINIMUM_READYTORUN_MAJOR_VERSION 20



tommcdon added this to the 11.0.0 milestone May 27, 2026

tommcdon requested review from VSadov and jakobbotsch May 27, 2026 20:42

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

This was referenced May 28, 2026

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

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XHarness package install failure on iOS due to devicectl NSPOSIXErrorDomain error 49 #123796

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Comment thread src/coreclr/jit/lclvars.cpp

Comment thread src/coreclr/jit/lclvars.cpp

Comment thread ...c/Microsoft.Diagnostics.DataContractReader.Contracts/Contracts/DebugInfo/DebugInfoHelpers.cs

Comment thread src/coreclr/inc/readytorun.h Outdated

Comment thread src/coreclr/inc/cordebuginfo.h

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Comment thread src/coreclr/inc/readytorun.h

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build-analysis Bot mentioned this pull request May 29, 2026

slow macOS - "##[error]The job running on agent Azure Pipelines 9 ran longer than the maximum time of 60 minutes." dotnet/dnceng#1883

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tommcdon and others added 2 commits May 29, 2026 18:47

tommcdon force-pushed the dev/tommcdon/fix-asyncv2-enc-continuation-preserved branch from 2e5c78f to 683aa07 Compare May 29, 2026 23:04

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Comment thread src/coreclr/inc/readytorun.h

Comment on lines +22 to 26

#define READYTORUN_MAJOR_VERSION 20

#define READYTORUN_MINOR_VERSION 0x0000

#define MINIMUM_READYTORUN_MAJOR_VERSION 19

#define MINIMUM_READYTORUN_MAJOR_VERSION 20

jakobbotsch mentioned this pull request May 30, 2026

Remove x86 per-source-register write barrier helpers #128797

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