[LV] Model if-converted phi node costs

Phi nodes in non-header blocks are converted to select instructions after
if-conversion. This patch updates the cost model to account for the selects.

Differential Revision: https://reviews.llvm.org/D31906

llvm-svn: 300980

1 files changed, 10 insertions, 2 deletions

diff --git a/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp b/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
index 7eb8fabe0b2..bc7c54dc4a3 100644
--- a/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
+++ b/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
@@ -7324,8 +7324,16 @@ unsigned LoopVectorizationCostModel::getInstructionCost(Instruction *I,
       return TTI.getShuffleCost(TargetTransformInfo::SK_ExtractSubvector,
                                 VectorTy, VF - 1, VectorTy);
 
-    // TODO: IF-converted IFs become selects.
-    return 0;
+    // Phi nodes in non-header blocks (not inductions, reductions, etc.) are
+    // converted into select instructions. We require N - 1 selects per phi
+    // node, where N is the number of incoming values.
+    if (VF > 1 && Phi->getParent() != TheLoop->getHeader())
+      return (Phi->getNumIncomingValues() - 1) *
+             TTI.getCmpSelInstrCost(
+                 Instruction::Select, ToVectorTy(Phi->getType(), VF),
+                 ToVectorTy(Type::getInt1Ty(Phi->getContext()), VF));
+
+    return TTI.getCFInstrCost(Instruction::PHI);
   }
   case Instruction::UDiv:
   case Instruction::SDiv: