[stephencanon]

It's actually worse than that; from what I can piece together of the history:

  1. the people at Intel who originally implemented `_BitInt()` made `_BitInt(128)` eight-byte aligned
  2. the x86_64 psABI document was updated to agree with that
  3. it was implemented in clang in such a way that it also made `_BitInt(128)` eight-byte aligned on arm64 ...
  4. ... but the AAPCS says that it's sixteen-byte aligned
  5. ... and maybe the x86_64 psABI is going to change to say that it's sixteen byte aligned after-all.

So--as of a month ago when I was studying this because we're in the middle of enabling `[U]Int128` in Swift--_BitInt(128) didn't agree with __int128[_t] on either platform, and that's definitely a bug on arm64, and, although it behaves as documented on x86_64, that also might change.