[PATCH] D129947: [X86] Prefer `lock or` over mfence.

[Valentin Churavy via Phabricator via llvm-commits]

vchuravy created this revision.
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LLVM currently emits `mfence` for `__atomic_thread_fence(seq_cst)`. On
modern CPUs `lock or` is more efficient and provides the same sequential
consistency. GCC 11 made this switch as well (see https://gcc.gnu.org/pipermail/gcc-cvs/2020-July/314418.html)
and https://reviews.llvm.org/D61863 and https://reviews.llvm.org/D58632
moved into this direction as well, but didn't touch `fence seq_cst`.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D129947

Files:
  llvm/lib/Target/X86/X86.td
  llvm/lib/Target/X86/X86ISelLowering.cpp
  llvm/lib/Target/X86/X86TargetTransformInfo.h
  llvm/test/CodeGen/X86/atomic-unordered.ll

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