[PATCH] D46042: Cap vector alignment at 16 for all Darwin platforms

[Reid Kleckner via Phabricator via cfe-commits]

rnk added a comment.

It's the typedef alignment changes that are causing problems for us, not the MaxVectorAlign changes. That makes more sense. The new alignment attribute breaks our implementation of `_mm256_loadu_ps`, because the packed struct ends up with a 32-byte alignment. Here's the implementation:

  static __inline __m256 __DEFAULT_FN_ATTRS
  _mm256_loadu_ps(float const *__p)
  {
    struct __loadu_ps {
      __m256 __v;
    } __attribute__((__packed__, __may_alias__));
    return ((struct __loadu_ps*)__p)->__v;
  }

And clang's -fdump-record-layouts says:

  *** Dumping AST Record Layout
           0 | struct __loadu_ps
           0 |   __m256 __v
             | [sizeof=32, align=32]

I think the problem is that `__attribute__((aligned(N)))` beats `__attribute__((packed))` on Windows to match MSVC's behavior with `__declspec(align(N))`.

I think we should revert this for now. Adding the alignment attribute to all Intel vector typedefs is a bigger change than it seems.


Repository:
  rC Clang

https://reviews.llvm.org/D46042