[wasm] Publish __stack_pointer before SuppressGCTransition native calls

[lewing]

Summary

On FEATURE_PORTABLE_ENTRYPOINTS (wasm) R2R, generated code keeps its shadow stack pointer in a local and leaves the __stack_pointer global stale. The P/Invoke prolog (JIT_PInvokeBegin) normally publishes it before native code runs, but that prolog is skipped for SuppressGCTransition calls. A native SuppressGCTransition callee (emscripten, which uses __stack_pointer) then allocates its shadow frame from the stale global — the caller's SP, above the R2R frame — and clobbers the caller's address-taken locals.

This manifests in R2R exception handling: FindFirstPassHandler spills its by-ref StackFrameIterator, calls the SuppressGCTransition QCall RhpEHEnumInitFromStackFrameIterator, the callee clobbers the spilled pointer, and the next frameIter.ControlPC read faults with a spurious NullReferenceException. EH dispatch can't GC-transition mid-unwind, which is why its QCalls are SuppressGCTransition and hit this path.

Fixes #130923.

Fix

Publish the shadow SP to the __stack_pointer global just before the call, so the callee allocates its shadow frame below the current frame. The publish is net-zero on the wasm operand stack, so it is safe to emit after the call arguments are pushed. The stackPointer global handle comes from the getWasmWellKnownGlobals JIT-EE API (added in #129717) and is referenced via a WASM_GLOBAL_INDEX_LEB relocation, matching the existing global.get uses in wasm codegen.

Validation

The bug only reproduces with the (not-yet-merged) R2R-on-wasm bring-up stack, so there is no wasm-R2R CI leg on main yet. Verified codegenwasm.cpp compiles clean in the wasm JIT (clr.jit, 0 errors). Behaviorally validated on the R2R-on-wasm prototype: unfixed crashes, fixed passes, plus a byte-identical R2R↔interpreter battery.

Notes for reviewers

• Draft because it cannot be CI-validated on main until the R2R-on-wasm consumption path lands.
• Depends on the getWasmWellKnownGlobals API from [WASM] Use relocs for tableBase / imageBase / stackPointer globals #129717 (fixes [RyuJit Wasm] Emit relocations for WebCIL globals rather than hardcoding global table indices #129712), now on main.
• Related bring-up family: [wasm][R2R] function signature mismatch on the first (cold) R2R call to a method whose native code is not yet on its portable entry point #130634.

Note

This pull request was authored with the assistance of GitHub Copilot.

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[lewing]

cc @dotnet/wasm-contrib

[Copilot]

Pull request overview

This PR updates the WASM JIT call sequence to publish the current shadow stack pointer to the __stack_pointer global immediately before SuppressGCTransition unmanaged calls, ensuring native callees that rely on __stack_pointer observe the correct stack position.

Changes:

• Emit local.get <shadowSP> + global.set <__stack_pointer> just before IsUnmanaged() && IsSuppressGCTransition() calls when the method has a shadow SP local.
• Use the existing eeGetWasmWellKnownGlobals()->stackPointer handle (via WASM_GLOBAL_INDEX_LEB relocation) to reference the well-known global consistently with existing wasm codegen.

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[Copilot]

Pull request overview

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