Stream: git-wasmtime

Topic: wasmtime / PR #11500 Wasmtime/Cranelift: carry "FP to SP ...

Wasmtime GitHub notifications bot (Aug 21 2025 at 21:50):

cfallin opened PR #11500 from cfallin:exception-sp-from-fp-with-offset to bytecodealliance:main:

Currently Wasmtime unwinds stack frames to look for exception handlers by walking frames one-by-one, following the FP chain as usual, and assuming that these frames are contiguous: that is, that the SP in any given frame (bottom of that frame) is immediately above the FP of the next lower frame, plus the FP/return address pair (e.g. 16 bytes). This allows us to get the SP for any given frame in addition to FP. We need SP for two reasons:

• To look up dynamic context, to match Wasm tag instances for handlers against the thrown tag;
• To actually set SP when we resume, if we do resume to a handler in this frame.

This logic almost but not quite worked: I had forgotten that in our tail-call ABI, we need to clean up incoming stack args in the callee (because only the final callee in a parade of tail-calling functions that reuse the same stack frame location knows how many args it has, not the original caller). This implies that there is an "incoming args area" above the FP/return address pair. Thus, frames are not necessarily contiguous by the above definition.

In #11489 we see a case where a function of signature (func) tail-calls one of (func (param i32 i32 i32 i32 i32)), which on x86-64 (with four arg registers left for Wasm) is sufficient to create incoming stack args, which then trips up the unwinder, reading a bogus vmctx and segfaulting.

The most reasonable solution seems to be to embed the SP-to-FP offset in the exception metadata itself, so from only the FP (which is totally robust -- we rely on the FP chain for multiple kinds of stack-walking) we can get the SP, allowing us to read dynamic context and to reset SP during resume.

This PR does just that. Technically, in our ABI, the SP-to-FP offset is constant for an entire function, but it was simpler in the exception metadata to encode this per callsite instead (there is no other notion of "per-function" data, only "per-callsite", so it would be a separate binary search).

Fixes #11489.

prtest:full

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Wasmtime GitHub notifications bot (Aug 21 2025 at 21:50):

cfallin requested alexcrichton for a review on PR #11500.

Wasmtime GitHub notifications bot (Aug 21 2025 at 21:50):

cfallin requested wasmtime-compiler-reviewers for a review on PR #11500.

Wasmtime GitHub notifications bot (Aug 21 2025 at 21:50):

cfallin requested wasmtime-core-reviewers for a review on PR #11500.

Wasmtime GitHub notifications bot (Aug 21 2025 at 21:58):

alexcrichton submitted PR review.

Wasmtime GitHub notifications bot (Aug 21 2025 at 21:58):

alexcrichton created PR review comment:

Could this (and below) expand documentation to explain what None means?

Wasmtime GitHub notifications bot (Aug 21 2025 at 21:58):

alexcrichton created PR review comment:

Would it be worth it to add a disas test for $throw to ensure that it's handling the offset correctly?

Wasmtime GitHub notifications bot (Aug 21 2025 at 21:58):

alexcrichton created PR review comment:

Could this have a comment as to why unwrap_or is ok here? Or otherwise, if None is the same as 0 should this be represented as a u32 instead of a Option<u32>?

Wasmtime GitHub notifications bot (Aug 21 2025 at 22:04):

cfallin updated PR #11500.

Wasmtime GitHub notifications bot (Aug 21 2025 at 22:05):

cfallin updated PR #11500.

Wasmtime GitHub notifications bot (Aug 21 2025 at 22:20):

cfallin updated PR #11500.

Wasmtime GitHub notifications bot (Aug 21 2025 at 22:20):

cfallin submitted PR review.

Wasmtime GitHub notifications bot (Aug 21 2025 at 22:20):

cfallin created PR review comment:

The interesting new bit is actually the offset at the try_table site (callsite in $start), which is the same as the case in exceptions.wat, so I think we're covered already!

Wasmtime GitHub notifications bot (Aug 21 2025 at 22:20):

cfallin submitted PR review.

Wasmtime GitHub notifications bot (Aug 21 2025 at 22:20):

cfallin created PR review comment:

Updated to explicitly .expect() only when there are handlers. The subtlety is that frame_offset will be None if there are no handlers because we don't encode any metadata in that case.

Wasmtime GitHub notifications bot (Aug 21 2025 at 22:20):

cfallin submitted PR review.

Wasmtime GitHub notifications bot (Aug 21 2025 at 22:20):

cfallin created PR review comment:

Updated!

Wasmtime GitHub notifications bot (Aug 21 2025 at 22:40):

alexcrichton has enabled auto merge for PR #11500.

Wasmtime GitHub notifications bot (Aug 21 2025 at 23:10):

alexcrichton merged PR #11500.

Last updated: Dec 06 2025 at 06:05 UTC