[PATCH] D72579: Evaluate __{c11_,}atomic_is_lock_free to 0 (avoid libcall) if larger than MaxAtomicPromoteWidth

[James Y Knight via Phabricator via cfe-commits]

jyknight requested changes to this revision.
jyknight added a comment.
This revision now requires changes to proceed.

This isn't correct.

The atomic interface is designed to be forward-compatible with new CPUs that have wider atomic instructions. That clang has a MaxAtomicPromoteWidth value _at all_ is unfortunate, because it breaks this ability. We've locked ourselves into an unfortunate ABI here -- and what's worse, it's incompatible with GCC's ABI.

But, the MaxAtomicPromoteWidth value impactsaffects C11 _Atomic (and therefore libc++ std::atomic) variables' default alignment. But, the user can in any case specify additional alignment. For the __atomic_is_lock_free function, MaxAtomicPromoteWidth is irrelevant -- it can be used on variables of any size or alignment and needs to return correct answers, even on future CPUs.

It's not good that clang doesn't provide the ability to create a working libatomic, but the right answer is to fix that, not add hacks like this. I've started writing up a proposal on what we need to do there, but need to finish that up.


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D72579/new/

https://reviews.llvm.org/D72579