[SimplifyCFG] Don't do if-conversion if there is a long dependence chain

If after if-conversion, most of the instructions in this new BB construct a long and slow dependence chain, it may be slower than cmp/branch, even if the branch has a high miss rate, because the control dependence is transformed into data dependence, and control dependence can be speculated, and thus, the second part can execute in parallel with the first part on modern OOO processor.

This patch checks for the long dependence chain, and give up if-conversion if find one.

Differential Revision: https://reviews.llvm.org/D39352

llvm-svn: 321377

1 files changed, 4 insertions, 0 deletions

diff --git a/llvm/lib/Analysis/TargetTransformInfo.cpp b/llvm/lib/Analysis/TargetTransformInfo.cpp
index b744cae51ed..c9e9c6d1a41 100644
--- a/llvm/lib/Analysis/TargetTransformInfo.cpp
+++ b/llvm/lib/Analysis/TargetTransformInfo.cpp
@@ -314,6 +314,10 @@ int TargetTransformInfo::getIntImmCost(Intrinsic::ID IID, unsigned Idx,
   return Cost;
 }
 
+bool TargetTransformInfo::isOutOfOrder() const {
+  return TTIImpl->isOutOfOrder();
+}
+
 unsigned TargetTransformInfo::getNumberOfRegisters(bool Vector) const {
   return TTIImpl->getNumberOfRegisters(Vector);
 }