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[wasm] Regalloc: Allow Non-exposed GC Ref Locals to Live in Wasm Locals - #135543
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…r a method and checks the state of local liveness at each potential GC safepoint to determine which locals are live across a safepoint. It distinguishes between normal GC safepoints triggered by a call or and exception-only GC safepoints, where an operation can only collect on its throwing path. This allows us to effectively discount locals as gc-exposed if they are only exposed to GC on a throwing path but are NOT live into a handler. The set of gc-safe locals is the set of gc exposed locals subracted from the set of all locals. This reduced set of gc-safe locals can be used by Wasm regalloc to make gc refs into wasm-local candidates.
…asses to allow safely placing select GC references in locals
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🟡 Changes recommended
The tests do not automatically verify that eligible references are actually assigned Wasm-local homes.
3 open findings
What changed in this PR
Adds Wasm GC-reference liveness analysis so safe reference locals can use Wasm-local homes.
Changes:
- Identifies tracked GC references not live across safepoints.
- Enables their register allocation and preserves call arguments via spills.
- Adds GC-liveness correctness scenarios.
| File | Description |
|---|---|
src/coreclr/jit/fgwasm.cpp |
Implements candidate analysis and spill handling. |
src/coreclr/jit/regallocwasm.cpp |
Allows identified references into Wasm locals. |
src/coreclr/jit/compiler.cpp |
Runs the new analysis phase. |
src/coreclr/jit/compiler.h |
Declares candidate state and phase method. |
src/coreclr/jit/compphases.h |
Registers the new phase. |
src/coreclr/jit/compmemkind.h |
Adds candidate-set allocation tracking. |
src/tests/JIT/Directed/WasmGCLiveness/WasmGCLiveness.cs |
Adds GC-liveness scenarios. |
src/tests/JIT/Directed/WasmGCLiveness/WasmGCLiveness.csproj |
Configures the isolated priority-one test. |
🧠 Review effort: Balanced
Comment on lines
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| bool isRefCandidate = | ||
| varDsc->lvTracked && | ||
| VarSetOps::IsMember(m_compiler, *m_compiler->m_wasmGCRefCandidates, varDsc->lvVarIndex); |
| VarSetOps::Assign(this, compCurLife, block->bbLiveIn); | ||
| for (GenTree* tree : LIR::AsRange(block)) | ||
| { | ||
| // compCurLife should reflect livness into this tree; |
| // of the test objects. | ||
| // | ||
| // Build this project with src/tests/build.sh -Test | ||
| // JIT/Directed/WasmGcLiveness/WasmGcLiveness.csproj -priority1, adding the target |
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The GC does not have any visibility into references which are on the wasm value stack or in wasm locals. As a result, we currently assign stack homes to all GC typed tracked locals unconditionally for GC safety. However, this is not actually necessary for tracked locals which are never exposed to a GC. We can make this determination by tracking liveness across all GC safepoints in a method. A GC safe point can be hit along a non-throwing code path at any call site except for NoGC marked helpers, as well as at explicit GC-poll points. Additionally, invoking a
finallyfunclet may cause a GC.For throwing code paths, we must account for any LIR node which could possibly throw (this includes all null checks, div-by-zero checks, range checks, etc.) and also track which live locals are actually live on the throwing path. A reference stored in a local is not actually exposed to GC over an exception-only GC safepoint path unless it is also live into a handler for that site.
We can then treat all GC-ref tracked locals which are meaningfully live at a safepoint as before and continue to spill them but allow safe unexposed locals to be Wasm register candidates and thus receive
localhomes.