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.

dotnet-policy-service · 2026-05-07T19:17:28Z

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jakobbotsch · 2026-05-07T19:22:13Z

            case DoNotEnregisterReason::PinningRef:
                m_PinningRef++;
                break;


JIT32 GC encoder does not support enregistering pinned locals, while the other GC encoder does. However, we never track pinned locals and hence they are never register candidates anyway. To simplify things I removed the ifdef and started always marking these DNER.

jakobbotsch · 2026-05-07T19:24:50Z

            break;
        case DoNotEnregisterReason::LiveInOutOfHandler:
            JITDUMP("live in/out of a handler\n");
-            varDsc->lvLiveInOutOfHndlr = 1;


(This is under #ifdef DEBUG, so a bit scary to be mutating things...)

jakobbotsch · 2026-05-07T19:26:32Z

-        compiler->lvaSetVarDoNotEnregister(lclNum DEBUGARG(DoNotEnregisterReason::LiveInOutOfHandler));
-        return false;
-    }
-


This was moved into checkForDNER.

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)

Copilot

Pull request overview

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

+    else
+    {
+        // By this point tracked locals live into EH clauses are _very_ likely to not be enregistered.
+        // DNER this for the same reason above, so that we can contain these locals.
+        m_compiler->lvSetEHVarsDoNotEnreg();
+    }


                m_longParamField++;
                break;
 #endif
-#ifdef JIT32_GCENCODER
            case DoNotEnregisterReason::PinningRef:
                m_PinningRef++;
                break;


+        }
+
+        // Fields of dependently promoted structs may be tracked. We shouldn't set lvMustInit on them since
+        // the whole parent struct will be initialized; however, lvLiveInOutOfHndlr should be set on them


Copilot

Pull request overview

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

JIT: Decouple DNER from liveness

8697463

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

Copilot started reviewing on behalf of jakobbotsch May 7, 2026 19:16 View session

dotnet-policy-service Bot assigned jakobbotsch May 7, 2026

jakobbotsch commented May 7, 2026

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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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Copilot AI review requested due to automatic review settings May 7, 2026 19:42

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jakobbotsch added 2 commits May 7, 2026 22:00

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Fix 32 bit

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Copilot AI review requested due to automatic review settings May 8, 2026 08:19

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Comment thread src/coreclr/jit/gschecks.cpp

Set DNER when creating LCL_FLD for split arg

f963186

This was referenced May 8, 2026

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JIT: Decouple "DoNotEnregister" from liveness#127932

JIT: Decouple "DoNotEnregister" from liveness#127932
jakobbotsch wants to merge 7 commits intodotnet:mainfrom
jakobbotsch:decouple-liveness-dner

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