[PATCH] D140939: Transform AtomicRMW logic operations to BT{R|C|S} if only changing/testing a single bit.

[Noah Goldstein via Phabricator via llvm-commits]

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This is essentially expanding on the optimizations added on: D120199 <https://reviews.llvm.org/D120199>
but applies the optimization to cases where the bit being changed /
tested is not am IMM but is a provable power of 2.

The only case currently added for cases like:
__atomic_fetch_xor(p, 1 << c, __ATOMIC_RELAXED) & (1 << c)

Which instead of using a `cmpxchg` loop can be done with `btcl; setcc; shl`.

There are still a variety of missed cases that could/should be
addressed in the future. This commit documents many of those
cases with Todos.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D140939

Files:
  llvm/include/llvm/IR/IntrinsicsX86.td
  llvm/lib/Target/X86/X86ISelLowering.cpp
  llvm/lib/Target/X86/X86ISelLowering.h
  llvm/lib/Target/X86/X86InstrCompiler.td
  llvm/test/CodeGen/X86/atomic-rm-bit-test.ll

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