[LoopVectorizer] Improve handling of branches during cost estimation.

The cost for a branch after vectorization is very different depending on if
the vectorizer will if-convert the block (branch is eliminated), or if
scalarized and predicated blocks will be produced (branch duplicated before
each block). There is also the case of remaining scalar branches, such as the
back-edge branch.

This patch handles these cases differently with TTI based cost estimates.

Review: Matthew Simpson
https://reviews.llvm.org/D31175

llvm-svn: 300058

1 files changed, 32 insertions, 1 deletions

diff --git a/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp b/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
index 26bcbcb5f18..b0ced809d94 100644
--- a/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
+++ b/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
@@ -2088,6 +2088,10 @@ private:
   /// pairs.
   typedef DenseMap<Instruction *, unsigned> ScalarCostsTy;
 
+  /// A set containing all BasicBlocks that are known to present after
+  /// vectorization as a predicated block.
+  SmallPtrSet<BasicBlock *, 4> PredicatedBBsAfterVectorization;
+
   /// A map holding scalar costs for different vectorization factors. The
   /// presence of a cost for an instruction in the mapping indicates that the
   /// instruction will be scalarized when vectorizing with the associated
@@ -6826,6 +6830,9 @@ void LoopVectorizationCostModel::collectInstsToScalarize(unsigned VF) {
         ScalarCostsTy ScalarCosts;
         if (computePredInstDiscount(&I, ScalarCosts, VF) >= 0)
           ScalarCostsVF.insert(ScalarCosts.begin(), ScalarCosts.end());
+
+        // Remember that BB will remain after vectorization.
+        PredicatedBBsAfterVectorization.insert(BB);
       }
   }
 }
@@ -7272,7 +7279,31 @@ unsigned LoopVectorizationCostModel::getInstructionCost(Instruction *I,
     // instruction cost.
     return 0;
   case Instruction::Br: {
-    return TTI.getCFInstrCost(I->getOpcode());
+    // In cases of scalarized and predicated instructions, there will be VF
+    // predicated blocks in the vectorized loop. Each branch around these
+    // blocks requires also an extract of its vector compare i1 element.
+    bool ScalarPredicatedBB = false;
+    BranchInst *BI = cast<BranchInst>(I);
+    if (VF > 1 && BI->isConditional() &&
+        (PredicatedBBsAfterVectorization.count(BI->getSuccessor(0)) ||
+         PredicatedBBsAfterVectorization.count(BI->getSuccessor(1))))
+      ScalarPredicatedBB = true;
+
+    if (ScalarPredicatedBB) {
+      // Return cost for branches around scalarized and predicated blocks.
+      Type *Vec_i1Ty =
+          VectorType::get(IntegerType::getInt1Ty(RetTy->getContext()), VF);
+      return (TTI.getScalarizationOverhead(Vec_i1Ty, false, true) +
+              (TTI.getCFInstrCost(Instruction::Br) * VF));
+    } else if (I->getParent() == TheLoop->getLoopLatch() || VF == 1)
+      // The back-edge branch will remain, as will all scalar branches.
+      return TTI.getCFInstrCost(Instruction::Br);
+    else
+      // This branch will be eliminated by if-conversion.
+      return 0;
+    // Note: We currently assume zero cost for an unconditional branch inside
+    // a predicated block since it will become a fall-through, although we
+    // may decide in the future to call TTI for all branches.
   }
   case Instruction::PHI: {
     auto *Phi = cast<PHINode>(I);