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Tony Kan
97bd985467
`adjust_for_rust_abi` was casting small aggregates to an integer register without propagating `noundef`, causing a performance regression (#123183) — LLVM could no longer assume the bits were fully defined. Add `layout_is_noundef` (conservative) + `fields_are_noundef` helper, then use `cast_to_with_attrs` to forward `NoUndef` when proven: Scalar → `!is_uninit_valid()` ScalarPair → both scalars valid + `s1.size + s2.size == layout.size` (size equality rejects layouts with inter-scalar padding) Array → recurse into element; empty arrays unconditionally noundef Arbitrary → `Variants::Single` required; walk fields in offset order — any gap, non-noundef field, or trailing pad returns false Union / Primitive / Simd → false (conservative) Bless `pass-indirectly-attr.stderr` and `debuginfo-dse.rs` for the new attribute on Cast args. Add `tests/codegen-llvm/abi-noundef-cast.rs` covering positive Cast cases (arrays, plain structs, single-variant enum) and negative Cast cases (MaybeUninit, multi-variant enum, field/pair gap, trailing padding). Fixes #123183. Co-authored-by: Ralf Jung <post@ralfj.de>