JIT: Decouple "DoNotEnregister" from liveness by jakobbotsch · Pull Request #127932 · dotnet/runtime

jakobbotsch · 2026-05-07T19:15:53Z

Liveness was marking locals as DNER in various places, including when it found locals to be live into EH handlers.
DNER is sticky and cannot be unset, and we call liveness multiple times, so that made things hard to reason about and led to unnecessary DNER.

This showed up for runtime async functions. Even when we were able to optimize out the try-fault clauses inserted around the body we would often still end up with lvaAsyncContinuationArg unnecessarily DNER'd because we had already marked it as DNER during a previous liveness pass, when it was live into the fault handler.

This PR:

Resets lvLiveInOutOfHandler on all locals every time we run liveness
Moves all the DNER setting from liveness to LSRA and to lowering
Encapsulates lvLiveInOutOfHandler into LclVarDsc::IsLiveInOutOfHandler() and adds an assert(lvTracked) to this function, so it cannot be misused
Introduces a few updates to heuristics to use IsLiveInOutOfHandler() in addition to DNER

Diffs aren't all that great, but I think this is a good clean up on its own.

Copilot

Pull request overview

This PR refactors CoreCLR JIT local liveness/EH handling so that EH “live-in/out-of-handler” state is recomputed each liveness run and lvDoNotEnregister (DNER) is no longer set by liveness, instead being set by regalloc / lowering.

Changes:

Recompute and encapsulate EH live-in/out state via LclVarDsc::IsLiveInOutOfHandler() and reset EH/must-init flags each liveness run.
Move DNER-setting decisions out of liveness into regalloc (checkForDNER) and lowering (lvSetEHVarsDoNotEnreg), and update heuristics to consult EH-liveness in addition to DNER.
Add Compiler::IsEHVarARegCandidate to centralize “EH write-thru candidate” logic and use it in multiple optimizations.

Reviewed changes

Copilot reviewed 20 out of 20 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
src/coreclr/jit/sideeffects.h	Use `IsLiveInOutOfHandler()` for EH-related aliasing decisions.
src/coreclr/jit/regallocwasm.h	Declare `checkForDNER` for Wasm regalloc.
src/coreclr/jit/regallocwasm.cpp	Call `checkForDNER` and use encapsulated EH-live query.
src/coreclr/jit/regalloc.cpp	Add `checkForDNER` and remove DNER-setting from `isRegCandidate`.
src/coreclr/jit/optimizer.cpp	Switch EH-live checks to `IsLiveInOutOfHandler()`.
src/coreclr/jit/morph.cpp	Stop copying EH-live flag during implicit-byref arg retyping.
src/coreclr/jit/lsra.h	Declare `checkForDNER` for LSRA.
src/coreclr/jit/lsra.cpp	Call `checkForDNER` and use `IsLiveInOutOfHandler()` in LSRA logic.
src/coreclr/jit/lower.cpp	Add `lvSetEHVarsDoNotEnreg()` call to support containment heuristics in lowering.
src/coreclr/jit/liveness.cpp	Reset `lvMustInit` and EH-live state each liveness run; set EH-live without setting DNER.
src/coreclr/jit/lclvars.cpp	Add `lvSetEHVarsDoNotEnreg`; remove `lvaSetVarLiveInOutOfHandler`; adjust DNER behavior.
src/coreclr/jit/jiteh.cpp	Track when EH clauses are removed post-liveness.
src/coreclr/jit/inductionvariableopts.cpp	Update IV widening heuristics to consider EH-liveness independently of DNER.
src/coreclr/jit/gschecks.cpp	Remove direct clearing of EH-live flag on shadowing locals.
src/coreclr/jit/gentree.cpp	Add `Compiler::IsEHVarARegCandidate` helper; update EH-live checks.
src/coreclr/jit/earlyprop.cpp	Update EH-live checks to `IsLiveInOutOfHandler()`.
src/coreclr/jit/copyprop.cpp	Adjust “profitability” gating to compare expected enregistration status incl. EH-liveness.
src/coreclr/jit/compiler.h	Encapsulate EH-live flag, add helper APIs, and add `optRemovedEHClausesAfterLiveness`.
src/coreclr/jit/compiler.cpp	Update enregister stats accounting to include `PinningRef` unconditionally.
src/coreclr/jit/codegenwasm.cpp	Update EH-live checks to `IsLiveInOutOfHandler()`.
src/coreclr/jit/codegencommon.cpp	Update EH-live checks to `IsLiveInOutOfHandler()`.

Comments suppressed due to low confidence (1)

src/coreclr/jit/compiler.h:5734

optRemovedEHClausesAfterLiveness is currently only set/reset (in fgRemoveEHTableEntry and liveness) but not consumed anywhere else. If it’s meant to guard heuristics that rely on IsLiveInOutOfHandler() (e.g., avoid setting DNER based on stale EH liveness after EH removal), consider wiring it into the relevant decision points; otherwise it should be removed to avoid accumulating dead state.

    bool fgBBVarSetsInited = false;

    // Track how many artificial ref counts we've added to fgEntryBB (for OSR)

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Pull request overview

Copilot reviewed 20 out of 20 changed files in this pull request and generated 3 comments.

Copilot

Pull request overview

Copilot reviewed 20 out of 20 changed files in this pull request and generated 1 comment.

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jakobbotsch · 2026-05-12T13:38:25Z

Sorry for the requests, accidentally rebased across a merge here...

jakobbotsch · 2026-05-13T08:07:02Z

/azp run runtime-coreclr jitstress, runtime-coreclr libraries-jitstress

azure-pipelines · 2026-05-13T08:07:24Z

Azure Pipelines successfully started running 2 pipeline(s).

jakobbotsch · 2026-05-13T12:29:48Z

cc @dotnet/jit-contrib PTAL @AndyAyersMS

Diffs.

This will be more important for future work once we start optimizing out IR that allows us to remove the EH we inserted for async.

Copilot

Pull request overview

Copilot reviewed 20 out of 20 changed files in this pull request and generated 2 comments.

JIT: Decouple DNER from liveness

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Copilot AI review requested due to automatic review settings May 7, 2026 19:15

github-actions Bot added the area-CodeGen-coreclr CLR JIT compiler in src/coreclr/src/jit and related components such as SuperPMI label May 7, 2026

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Comment thread src/coreclr/jit/compiler.cpp

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Comment thread src/coreclr/jit/lclvars.cpp

jakobbotsch commented May 7, 2026

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Comment thread src/coreclr/jit/regalloc.cpp

Clean up

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Comment thread src/coreclr/jit/lclvars.cpp

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

jakobbotsch added 2 commits May 7, 2026 21:41

Feedback

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Fix

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Comment thread src/coreclr/jit/lower.cpp

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jakobbotsch added 2 commits May 7, 2026 22:00

More comments

27cc134

Fix 32 bit

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Comment thread src/coreclr/jit/gschecks.cpp

Set DNER when creating LCL_FLD for split arg

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This was referenced May 8, 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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The Operation will be canceled. The next steps may not contain expected logs. dotnet/dnceng#3008

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dotnet-policy-service Bot added the linkable-framework Issues associated with delivering a linker friendly framework label May 12, 2026

DNER structs that are not of enregisterable size before lowering

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jakobbotsch force-pushed the decouple-liveness-dner branch from d3d7400 to 15f6d4e Compare May 12, 2026 13:38

jakobbotsch removed the linkable-framework Issues associated with delivering a linker friendly framework label May 12, 2026

jakobbotsch marked this pull request as ready for review May 13, 2026 12:28

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Comment thread src/coreclr/jit/lclvars.cpp

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AndyAyersMS approved these changes May 16, 2026

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Merge branch 'main' into decouple-liveness-dner

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jakobbotsch merged commit b540c36 into dotnet:main May 18, 2026
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build-analysis Bot mentioned this pull request May 18, 2026

[wasm] Tests failing with DirectoryNotFoundException trying to load test data #128293

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dotnet-maestro Bot mentioned this pull request May 19, 2026

[main] Source code updates from dotnet/runtime dotnet/dotnet#6681

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jtschuster mentioned this pull request Jun 4, 2026

Test failure: System.Security.Cryptography.X509Certificates.Tests.CollectionTests.X509Certificate2CollectionRemoveRangeArray #128392

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JulieLeeMSFT mentioned this pull request Jun 5, 2026

JIT: Fix for IV opt to preserve updates to a counter that is live into an EH handler #129058

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jakobbotsch mentioned this pull request Jun 11, 2026

[ci-scan] Test failure: NumberHandlingTests_Metadata.Number_AsCollectionElement_RoundTrip on NativeAOT windows-x64 #129010

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