[adamperlin]

The current fgWasmSpillRefs pass spills all outstanding GC-ref typed temporary values, even if those temps are consumed by the most immediate call. However, for calls it should be the callee's responsibility to maintain GC safety of any incoming GC-ref typed args (either by spilling to a pinned stack home, spilling and reloading, or potentially avoiding spilling at all, see #135543). In the R2R fixup case, where a thunk will fix up the call target on first invocation, we should also be GC-protecting all incoming args by default (See the spilling logic in crossgen2 WasmImportThunk.cs).

This PR removes the need for spilling in cases like:

MyClass x = Allocate();
Use(x);

Note that the spilling STILL needs to happen for nested calls, where a ref has an outstanding use which is NOT its most immediate parent.

MyClass x = Allocate();
M(x, F()) // <-- temporary value of x is an operand of the call to M, and may be live across the other sub-tree call, F where a GC might occur.

Results

SuperPMI

Total bytes of base: 8771651
Total bytes of diff: 8555629
Total bytes of delta: -216,022 (-2.46% of base)

Local Benchmarks

Metric	CoreCLR baseline	CoreCLR after	Change
Mobile cold startup	7,474 ms	6,901 ms	7.7% faster
Desktop walkthrough	5.60 s	6.05 s	8.1% slower
Desktop cold download	10.324 MB	10.215 MB	1.1% smaller
Havit publish	150.81 s	149.21 s	1.1% faster
Cold download and startup improve as expected. I didn't put too much weight on the walkthrough benchmark because it is only run once and is pretty variable.

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🟡 Changes recommended

Explicit call null checks can invoke a GC-triggering throw helper before the callee protects the newly unspilled arguments.

1 open finding

• Preserve temporary ref spilling for NeedsNullCheck calls · New

What changed in this PR

Optimizes Wasm GC-reference handling by avoiding unnecessary spills for arguments passed directly to managed calls.

Changes:

• Extracts operand-use removal into a reusable helper.
• Removes managed-call arguments before spilling other live references.
• Preserves spills for nested, helper, and unmanaged calls.

A blocking GC-safety issue remains for managed calls with explicit null checks.

File	Description
src/​coreclr/​jit/​fgwasm.cpp	Refines GC-reference spill decisions around calls.

🧠 Review effort: Balanced

[adamperlin]

cc @dotnet/wasm-contrib

[EgorBo]

This looks very similar to what async.cpp does, wondering if we can share that