[PATCH] D34769: [X86] X86::CMOV to Branch heuristic based optimization

[Amjad Aboud via Phabricator via llvm-commits]

aaboud created this revision.
Herald added a subscriber: mgorny.

LLVM compiler recognizes opportunities to transform a branch into IR select instruction(s) - later it will be lowered into X86::CMOV instruction, assuming no other optimization eliminated the SelectInst.
However, it is not always profitable to emit X86::CMOV instruction. For example, branch is preferable over an X86::CMOV instruction when:

- Branch is well predicted
- Condition operand is expensive, compared to True-value and the False-value operands

In CodeGenPrepare pass there is a shallow optimization that tries to convert SelectInst into branch, but it is not enough.
This patch, implements machine optimization pass that converts X86::CMOV instruction(s) into branch, based on a conservative heuristic.


https://reviews.llvm.org/D34769

Files:
  lib/Target/X86/CMakeLists.txt
  lib/Target/X86/X86.h
  lib/Target/X86/X86CmovConversion.cpp
  lib/Target/X86/X86TargetMachine.cpp
  test/CodeGen/X86/2008-01-08-SchedulerCrash.ll
  test/CodeGen/X86/atomic-minmax-i6432.ll
  test/CodeGen/X86/atomic128.ll
  test/CodeGen/X86/x86-cmov-converter.ll

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