[Wasm RyuJIT] Spill live ref/byref values to pinned stack slots at calls by kg · Pull Request #129059 · dotnet/runtime

kg · 2026-06-05T22:58:34Z

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dotnet-policy-service · 2026-06-05T22:59:47Z

Tagging subscribers to this area: @JulieLeeMSFT, @jakobbotsch
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Copilot

Pull request overview

This PR introduces a Wasm-specific mechanism intended to make ref/byref values GC-visible across call sites by injecting a new IR node (GT_WASM_SPILL_REF) and a new Wasm phase (WasmSpillRefs) that inserts these nodes and allocates pinned stack spill slots used during Wasm codegen.

Changes:

Add GT_WASM_SPILL_REF node kind and operand iteration support.
Add Compiler::WasmSpillRefs phase to insert spill nodes around calls and allocate spill locals.
Extend Wasm codegen/regalloc to track a spill index and (temporarily) force-enregister a scratch “splash zone” local.

Reviewed changes

Copilot reviewed 11 out of 11 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
src/coreclr/jit/regallocwasm.cpp	Forces the “splash zone” local to be treated as a reg candidate.
src/coreclr/jit/gtlist.h	Adds new Wasm node `WASM_SPILL_REF`.
src/coreclr/jit/gentree.cpp	Updates operand-edge iterator to treat `GT_WASM_SPILL_REF` as unary.
src/coreclr/jit/fgwasm.cpp	Implements `Compiler::WasmSpillRefs` to insert spill nodes and allocate spill locals.
src/coreclr/jit/compphases.h	Adds `PHASE_WASM_SPILL_REFS`.
src/coreclr/jit/compmemkind.h	Adds `WasmSpillRefs` memory kind.
src/coreclr/jit/compiler.h	Adds `m_wasmSpillSlots` field and `WasmSpillRefs` declaration.
src/coreclr/jit/compiler.cpp	Wires `WasmSpillRefs` into the Wasm compilation pipeline.
src/coreclr/jit/codegenwasm.cpp	Emits Wasm for `GT_WASM_SPILL_REF` and resets spill index at calls.
src/coreclr/jit/codegenlinear.cpp	Resets spill index at block boundaries on Wasm.
src/coreclr/jit/codegen.h	Adds `wasmSpillRefIndex` state.

AndyAyersMS

Generally looks good.

I'm curious how this intersects/overlaps with LSRA's spill temp mechanism. Not saying we should use that here, but it likely serves a similar purpose.

AndyAyersMS · 2026-06-07T16:31:57Z

+                if (!op->TypeIs(TYP_REF, TYP_BYREF))
+                    return GenTree::VisitResult::Continue;
+
+                for (size_t i = defs.size(); i > 0; i--)


Probably worth commenting what this is doing (removing active defs once we find their use, and keeping the defs collection compact).

…the GC won't move them

kg · 2026-06-08T18:24:45Z

Still working on this, going to try moving to STORE_LCL_VAR/LCL_VAR like Jakob suggested.

Copilot

Pull request overview

Copilot reviewed 11 out of 11 changed files in this pull request and generated 4 comments.

kg · 2026-06-08T19:08:45Z

+        varDsc->lvType = TYP_BYREF;
+        varDsc->lvPinned = true;
+        varDsc->lvImplicitlyReferenced = true;
+        varDsc->lvMustInit = true;
+        lvaSetVarDoNotEnregister(varNum, DoNotEnregisterReason::WasmGCVisibility);


It's true that this could be a problem, but I don't know if we need to do anything about it other than add a TODO-WASM comment for now

kg · 2026-06-08T19:08:06Z

+                        GenTreeUnOp* spill = gtNewOperNode(GT_WASM_SPILL_REF, def->TypeGet(), def);
+                        LIR::Use use;
+                        noway_assert(LIR::AsRange(block).TryGetUse(def, &use));
+                        use.ReplaceWith(spill);
+                        LIR::AsRange(block).InsertAfter(def, spill);
+                        if (def->gtLIRFlags & LIR::Flags::MultiplyUsed)
+                        {
+                            JITDUMP("Transferring multiply-used flag from [%06u] to [%06u] for spill\n", Compiler::dspTreeID(def), Compiler::dspTreeID(spill));
+                            def->gtLIRFlags &= ~LIR::Flags::MultiplyUsed;
+                            spill->gtLIRFlags |= LIR::Flags::MultiplyUsed;
+                        }
+                        anyChanges = true;


Is this the right way to do it? @AndyAyersMS @jakobbotsch

…ill node Remove the splash zone concept

kg · 2026-06-08T19:07:42Z

Updated to use STORE_LCL_VAR and LCL_VAR instead, removing the splash zone var.

Copilot

Pull request overview

Copilot reviewed 6 out of 6 changed files in this pull request and generated 2 comments.

+                        GenTreeLclVar *spill = gtNewStoreLclVarNode(spillSlot, def);
+                        GenTreeLclVar *reload = gtNewLclVarNode(spillSlot);
+                        LIR::Use use;
+                        noway_assert(LIR::AsRange(block).TryGetUse(def, &use));
+                        use.ReplaceWith(reload);
+                        LIR::AsRange(block).InsertAfter(def, spill);
+                        LIR::AsRange(block).InsertAfter(spill, reload);
+                        if (def->gtLIRFlags & LIR::Flags::MultiplyUsed)
+                        {
+                            JITDUMP("Transferring multiply-used flag from [%06u] to [%06u] for spill\n", Compiler::dspTreeID(def), Compiler::dspTreeID(reload));
+                            def->gtLIRFlags &= ~LIR::Flags::MultiplyUsed;
+                            reload->gtLIRFlags |= LIR::Flags::MultiplyUsed;
+                        }


kg · 2026-06-08T19:24:33Z

+                            LclVarDsc* const varDsc = lvaGetDesc(spillSlot);
+                            varDsc->lvType = TYP_BYREF;
+                            varDsc->lvPinned = true;
+                            varDsc->lvImplicitlyReferenced = true;
+                            varDsc->lvMustInit = true;


Isn't it fine to store a REF in a pinned BYREF? Or no?

Storing is fine, it's the reloading that is a bit iffy, we have to trust the thing stored there was really a ref. In your case we know this but the IR doesn't.

I would probably just keep two pools for now like copilot suggests. Possibly reporting refs as refs is a bit cheaper for GC.

Copilot AI review requested due to automatic review settings June 5, 2026 22:58

kg added arch-wasm WebAssembly architecture area-CodeGen-coreclr CLR JIT compiler in src/coreclr/src/jit and related components such as SuperPMI labels Jun 5, 2026

Copilot started reviewing on behalf of kg June 5, 2026 22:58 View session

dotnet-policy-service Bot assigned kg Jun 5, 2026

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Comment thread src/coreclr/jit/fgwasm.cpp

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

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

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

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

This was referenced Jun 6, 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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AndyAyersMS reviewed Jun 7, 2026

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jakobbotsch reviewed Jun 7, 2026

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Comment thread src/coreclr/jit/gtlist.h Outdated

jakobbotsch reviewed Jun 7, 2026

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

jakobbotsch reviewed Jun 7, 2026

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

kg added 3 commits June 7, 2026 10:07

[Wasm] Spill live ref/byref values to pinned stack slots at calls so …

efd6a44

…the GC won't move them

Fix a bounds check

02375bb

Set the spill slots as must-init

9a2f3da

kg force-pushed the wasm-spill-refs branch from b614c8f to 9a2f3da Compare June 7, 2026 17:07

kg added 2 commits June 8, 2026 11:04

Cleanup

1e66891

Improve GetLclVarNameInfo for Wasm

5dd8f82

Copilot AI review requested due to automatic review settings June 8, 2026 18:24

Copilot started reviewing on behalf of kg June 8, 2026 18:24 View session

Copilot AI reviewed Jun 8, 2026

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Use regular local var loads and stores instead of a dedicated wasm sp…

c90720a

…ill node Remove the splash zone concept

kg requested review from AndyAyersMS, Copilot and jakobbotsch June 8, 2026 19:07

Copilot started reviewing on behalf of kg June 8, 2026 19:07 View session

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kg commented Jun 5, 2026

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kg commented Jun 8, 2026

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