llvm-project

13b2ee25 - [clang][AST] Fix LazyGenerationalUpdatePtr NumLowBitsAvailable on 32-bit (#188318)

[clang][AST] Fix LazyGenerationalUpdatePtr NumLowBitsAvailable on 32-bit (#188318) The `PointerLikeTypeTraits` for `LazyGenerationalUpdatePtr` claimed `PointerLikeTypeTraits<T>::NumLowBitsAvailable - 1` spare low bits. This assumed that the inner `PointerUnion<T, LazyData*>` has `T_bits - 1` spare bits, which is only true when `alignof(LazyData) >= alignof(*T)`. On 32-bit systems, `LazyData` (containing pointers and `uint32_t`) has `alignof = 4`, giving `LazyData*` only 2 low bits. With `T = Decl*` (3 bits due to `alignas(8)`), the inner `PointerUnion` has `min(3,2) - 1 = 1` spare bit, but the PLTT claimed `3 - 1 = 2`. Historically, the formula was correct when introduced in 053f6c6c9e4d -- at that time `Decl` had no alignment annotation, so `T_bits == LazyData*_bits` on all platforms. It became outdated when 771721cb35f3 added `LLVM_ALIGNAS(8)` to `Decl`, raising `Decl*` to 3 bits on 32-bit while `LazyData*` stayed at 2. The old `PointerIntPair`-based `PointerUnion::doCast` happened to mask with `minLowBitsAvailable()` (tolerant of overclaims), so this was never exposed until the `PunnedPointer` refactoring changed `doCast` to mask with `To::NumLowBitsAvailable`. Fixes: https://github.com/llvm/llvm-project/issues/188269 Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>