[X86] X86::CMOV to Branch heuristic based optimization.

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:
1. Branch is well predicted
2. 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 commit, implements machine optimization pass that converts X86::CMOV instruction(s) into branch, based on a conservative heuristic.

Differential Revision: https://reviews.llvm.org/D34769

1 files changed, 1 insertions, 0 deletions

diff --git a/llvm/lib/Target/X86/X86TargetMachine.cpp b/llvm/lib/Target/X86/X86TargetMachine.cpp
index 8d891c983fa..08c2cdaefe7 100644
--- a/llvm/lib/Target/X86/X86TargetMachine.cpp
+++ b/llvm/lib/Target/X86/X86TargetMachine.cpp
@@ -375,6 +375,7 @@ bool X86PassConfig::addILPOpts() {
   addPass(&EarlyIfConverterID);
   if (EnableMachineCombinerPass)
     addPass(&MachineCombinerID);
+  addPass(createX86CmovConverterPass());
   return true;
 }