[LoopVectorize][X86] Clamp interleave factor if we have a known constant trip count that is less than VF*interleave

If we know the trip count, we should make sure the interleave factor won't cause the vectorized loop to exceed it.

Improves one of the cases from PR42674

Differential Revision: https://reviews.llvm.org/D65896

llvm-svn: 368215

1 files changed, 9 insertions, 1 deletions

diff --git a/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp b/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
index dd28b18258f..1f8cbf7f340 100644
--- a/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
+++ b/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
@@ -5121,6 +5121,14 @@ unsigned LoopVectorizationCostModel::selectInterleaveCount(unsigned VF,
       MaxInterleaveCount = ForceTargetMaxVectorInterleaveFactor;
   }
 
+  // If the trip count is constant, limit the interleave count to be less than
+  // the trip count divided by VF.
+  if (TC > 0) {
+    assert(TC >= VF && "VF exceeds trip count?");
+    if ((TC / VF) < MaxInterleaveCount)
+      MaxInterleaveCount = (TC / VF);
+  }
+
   // If we did not calculate the cost for VF (because the user selected the VF)
   // then we calculate the cost of VF here.
   if (LoopCost == 0)
@@ -5129,7 +5137,7 @@ unsigned LoopVectorizationCostModel::selectInterleaveCount(unsigned VF,
   assert(LoopCost && "Non-zero loop cost expected");
 
   // Clamp the calculated IC to be between the 1 and the max interleave count
-  // that the target allows.
+  // that the target and trip count allows.
   if (IC > MaxInterleaveCount)
     IC = MaxInterleaveCount;
   else if (IC < 1)