[PATCH] D149689: [X86] Lower used `(atomicrmw xor p, SignBit)` as `(atomicrmw add p, SignBit)`

[Noah Goldstein via Phabricator via llvm-commits]

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`(xor X, SignBit)` == `(add X, SignBit)`. For atomics whose result is
used, the `add` option is preferable because of the `xadd` instruction
which allows us to avoid either a CAS loop or a `btc; setcc; shl`.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D149689

Files:
  llvm/lib/Target/X86/X86ISelLowering.cpp
  llvm/test/CodeGen/X86/atomic-bit-test.ll
  llvm/test/CodeGen/X86/atomic-xor.ll

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