Emit noundef on small aggregate returns in the x86 Win32 ABI - #160368
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Emit noundef on small aggregate returns in the x86 Win32 ABI#160368Vastargazing wants to merge 1 commit into
Vastargazing wants to merge 1 commit into
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The x86 Win32 (MSVC) ABI returns small aggregates in an integer register via `cast_to`, which drops the argument attributes -- so these returns never carried `noundef`, unlike scalars, which get it from `arg_attrs_for_rust_scalar`. Switch those casts to `cast_to_maybe_noundef` so a return whose layout is provably free of uninit bytes gets `noundef`, while padded layouts correctly omit it.
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I'm not sure what is special about win32 that we would only do this there. Anyway evaluating actual ABI adjustments is outside what I can do. |
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Yep, win32 was just an easy first target for the cast_to_maybe_noundef helper, nothing special about it. No strong preference on the rest: small per-arch PRs, or fold them into this one, whatever's easiest to review. No rush, happy to follow your lead 🙏 |
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Follow-up to #160313
Now that
ArgAbi::cast_to_maybe_noundefis in place, this PR switches the x86 Win32 (MSVC) small-aggregate return casts over to it.With this change, returns whose layout is provably free of uninit bytes keep
noundef, while padded layouts correctly omit it.As discussed on Zulip, this is the foreign-ABI follow-up split out from the original PR.
Added
tests/codegen-llvm/abi-noundef-cast-win32.rscovering both positive (unpadded struct ->noundef) and negative (padded struct -> nonoundef) cases oni686-pc-windows-msvc.r? @RalfJung