[PATCH] D36945: [x86] Handle more cases where we can re-use an atomic operation's flags rather than doing a separate comparison.

[Chandler Carruth via Phabricator via llvm-commits]

chandlerc created this revision.
Herald added subscribers: mcrosier, sanjoy.

This both saves an explicit comparision and avoids the use of `xadd`
which introduces register constraints and other challenges to the
generated code.

The motivating case is from atomic reference counts where `1` is the
sentinel rather than `0` for whatever reason. This can and should be
lowered efficiently on x86 by just using a different flag, however the
x86 code only handled the `0` case.

There remains some further opportunities here that are currently hidden
due to canonicalization. I've included test cases that show these and
FIXMEs. However, I don't at the moment have any production use cases and
they seem substantially harder to address.


https://reviews.llvm.org/D36945

Files:
  lib/Target/X86/X86ISelLowering.cpp
  test/CodeGen/X86/atomic-eflags-reuse.ll

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