Wasmtime GitHub notifications bot (Jan 27 2026 at 16:35): Wasmtime GitHub notifications bot (Jan 27 2026 at 16:35):cfallin opened PR #12447 from cfallin:preserve-all-no-returns to bytecodealliance:main:
In #12399 a fuzzbug uncovered an issue whereby a function with
preserve_alland with many return values cannot be called, becauseemit_retval_loadscannot codegen memory-to-memory moves (from on-stack return value slots directly to spillslots) without a temporary/clobberable register, andpreserve_allimplies no such registers exist.I thought about trying to support this by shuffling registers -- such a case implies many return values, and at least some of them will be in registers, so we might be able to temporarily spill one of those and use it as a scratch to move other values memory-to-memory. But the complexity doesn't seem worthwhile, and this path is not actually needed at the moment.
Thinking more broadly, anyone using
preserve_allfor hooks meant to minimally perturb register state will likely not want to use many return values anyway (that defeats the point). We could allow one return value, with the knowledge that this always fits in a register in our existing ABIs, but that also feels somewhat arbitrary, and I could make the argument that "preserve all" should really mean preserve all. Someone wanting to return a value anyway from such a hook could use indirect means (pass in a pointer to a stackslot, for example). I'm happy to tweak this limit if others have more thoughts, however.Fixes #12399.
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Wasmtime GitHub notifications bot (Jan 27 2026 at 16:35):cfallin requested wasmtime-compiler-reviewers for a review on PR #12447.
Wasmtime GitHub notifications bot (Jan 27 2026 at 16:35):cfallin requested alexcrichton for a review on PR #12447.
Wasmtime GitHub notifications bot (Jan 27 2026 at 17:50):github-actions[bot] added the label cranelift on PR #12447.
Wasmtime GitHub notifications bot (Jan 27 2026 at 17:50):github-actions[bot] added the label cranelift:area:machinst on PR #12447.
Wasmtime GitHub notifications bot (Jan 27 2026 at 18:19):fitzgen submitted PR review:
SGTM!
Wasmtime GitHub notifications bot (Jan 27 2026 at 18:19):fitzgen added PR #12447 Cranelift: preserve_all: disallow return values. to the merge queue
Wasmtime GitHub notifications bot (Jan 27 2026 at 18:48):github-merge-queue[bot] removed PR #12447 Cranelift: preserve_all: disallow return values. from the merge queue
Wasmtime GitHub notifications bot (Jan 27 2026 at 18:50):cfallin commented on PR #12447:
Spurious CI failure in a MinGW package download; retrying.
Wasmtime GitHub notifications bot (Jan 27 2026 at 18:50):cfallin added PR #12447 Cranelift: preserve_all: disallow return values. to the merge queue
Wasmtime GitHub notifications bot (Jan 27 2026 at 19:34):cfallin merged PR #12447.
Wasmtime GitHub notifications bot (Jan 27 2026 at 19:34):cfallin removed PR #12447 Cranelift: preserve_all: disallow return values. from the merge queue
Last updated: Jan 29 2026 at 13:25 UTC