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Fix Wasm ArgIterator SizeOfArgStack for methods with only hidden args - #127532
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The ForceSigWalk method had two bugs in its Wasm-specific path for accounting unnamed arguments (this, retbuf, generic context, etc.) when no named arguments are present: 1. The check 'maxOffset == 0' could never be true because maxOffset is initialized to OffsetOfArgs (8 on Wasm32). Changed to compare against OffsetOfArgs. 2. The fallback 'maxOffset = _wasmOfsStack' was incorrect because _wasmOfsStack is relative to OffsetOfArgs, but maxOffset is an absolute offset. Changed to 'OffsetOfArgs + _wasmOfsStack'. These bugs caused GCRefMapBuilder to allocate a zero-length fake stack for methods with only unnamed arguments (e.g. parameterless instance methods), leading to IndexOutOfRangeException when writing the 'this' pointer GC ref at ThisOffset. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
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Pull request overview
Fixes Wasm32-specific argument stack sizing in ArgIterator.ForceSigWalk() when a method has no explicit (named) parameters but does have hidden arguments (e.g., this, retbuf, generic context, async continuation). This prevents downstream components (e.g., GC ref map building) from seeing a zero-length “fake stack” and hitting out-of-range writes.
Changes:
- Correct the “no named arguments seen” detection for Wasm32 by comparing
maxOffsetagainstOffsetOfArgs(its actual initialization value on Wasm32) instead of0. - Fix Wasm32 fallback stack extent computation by converting
_wasmOfsStack(relative) into an absolute offset viaOffsetOfArgs + _wasmOfsStack.
This was referenced Apr 29, 2026
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davidwrighton
commented
Apr 30, 2026
MemberAuthor
/ba-g Only unknown failure is a disk space issue |
AndyAyersMS
approved these changes
Apr 30, 2026
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The ForceSigWalk method had two bugs in its Wasm-specific path for accounting for hidden arguments (this, retbuf, generic context, etc.) when no named arguments are present:
The check 'maxOffset == 0' could never be true because maxOffset is initialized to OffsetOfArgs (8 on Wasm32). Changed to compare against OffsetOfArgs.
The fallback 'maxOffset = _wasmOfsStack' was incorrect because _wasmOfsStack is relative to OffsetOfArgs, but maxOffset is an absolute offset. Changed to 'OffsetOfArgs + _wasmOfsStack'.
These bugs caused GCRefMapBuilder to allocate a zero-length fake stack for methods with only unnamed arguments (e.g. parameterless instance methods), leading to IndexOutOfRangeException when writing the 'this' pointer GC ref at ThisOffset.