[LV] Prevent LV to run cost model twice for VF=2

This is a minor fix for LV cost model, where the cost for VF=2 was
computed twice when the vectorization of the loop was forced without
specifying a VF.

Reviewers: xusx595, hsaito, fhahn, mkuper

Reviewed By: hsaito, xusx595

Differential Revision: https://reviews.llvm.org/D48048

llvm-svn: 334840

1 files changed, 4 insertions, 3 deletions

diff --git a/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp b/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
index a9f1ba3d343..8a0d96a9411 100644
--- a/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
+++ b/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
@@ -5022,10 +5022,11 @@ LoopVectorizationCostModel::selectVectorizationFactor(unsigned MaxVF) {
   LLVM_DEBUG(dbgs() << "LV: Scalar loop costs: " << (int)ScalarCost << ".\n");
 
   bool ForceVectorization = Hints->getForce() == LoopVectorizeHints::FK_Enabled;
-  // Ignore scalar width, because the user explicitly wants vectorization.
   if (ForceVectorization && MaxVF > 1) {
-    Width = 2;
-    Cost = expectedCost(Width).first / (float)Width;
+    // Ignore scalar width, because the user explicitly wants vectorization.
+    // Initialize cost to max so that VF = 2 is, at least, chosen during cost
+    // evaluation.
+    Cost = std::numeric_limits<float>::max();
   }
 
   for (unsigned i = 2; i <= MaxVF; i *= 2) {