Xiangling_L added a comment. In D88659#2306403 <https://reviews.llvm.org/D88659#2306403>, @jyknight wrote:
> Hm, to start with, the current state of this confuses me. > > In GCC, the preprocessor macro `__BIGGEST_ALIGNMENT__` was supposed to expose > the alignment used by `__attribute__((aligned))` (no arg specified), as well > the alignment used for alloca. However, this is no longer the case on x86: > `BIGGEST_ALIGNMENT` is 512bits with avx-512 enabled, 256bits with avx > enabled, and otherwise 128bits. Alloca follows this too. But, > `__attribute__((aligned))` was fixed at 128bit alignment, regardless of AVX > being enabled, in order to not break ABI compatibility with structs using > that. On other architectures, the 3 values seem to be always the same. > > In clang, we similarly have (before this patch) both > DefaultAlignForAttributeAligned (used for ``attribute((aligned))`), and > SuitableAlign (used for the predefined `__BIGGEST_ALIGNMENT__` and alignment > for alloca). But these values are different on very many > architectures...which I think is probably wrong. Furthermore, SuitableAlign > isn't being adjusted to be suitable for vectors, like it is in gcc, which > _also_ seems wrong. Looks like there's actually an earlier patch to fix that > which was never merged: https://reviews.llvm.org/D39313 > > So, anyways -- back to this patch: On AIX PPC, you want alloca to align to > 128bits, `__attribute__((aligned))` to align to 128bits (aka 8 bytes), but > `__BIGGEST_ALIGNMENT__` to only be 4? > > That seems pretty weird, and probably wrong? As you mentioned, without this patch, `SuitableAlign` is used for the predefined `__BIGGEST_ALIGNMENT__` and alignment for alloca. But on AIX, the __BIGGEST_ALIGNMENT__ should be 8bytes, alloca and `__attribute__((aligned))` should align to 16bytes considering vector types which have to be aligned to 16bytes, that's why we want to split `SuitableAlign`. Repository: rG LLVM Github Monorepo CHANGES SINCE LAST ACTION https://reviews.llvm.org/D88659/new/ https://reviews.llvm.org/D88659 _______________________________________________ cfe-commits mailing list cfe-commits@lists.llvm.org https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits
