[SLP] Fix vectorization for tree with trunc to minimum required bit width.

Summary:
If the vectorized tree has truncate to minimum required bit width and
the vector type of the cast operation after the truncation is the same
as the vector type of the cast operands, count cost of the vector cast
operation as 0, because this cast will be later removed.
Also, if the vectorization tree root operations are integer cast operations, do not consider them as candidates for truncation. It will just create extra number of the same vector/scalar operations, which will be removed by instcombiner.

Reviewers: RKSimon, spatel, mkuper, hfinkel, mssimpso

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D41948

llvm-svn: 322946

1 files changed, 20 insertions, 2 deletions

diff --git a/llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp b/llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp
index 90f84e00b44..f748ba4b31b 100644
--- a/llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp
+++ b/llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp
@@ -2065,7 +2065,10 @@ int BoUpSLP::getEntryCost(TreeEntry *E) {
                                                          VL0->getType(), SrcTy, VL0);
 
       VectorType *SrcVecTy = VectorType::get(SrcTy, VL.size());
-      int VecCost = TTI->getCastInstrCost(VL0->getOpcode(), VecTy, SrcVecTy, VL0);
+      int VecCost = 0;
+      // Check if the values are candidates to demote.
+      if (!MinBWs.count(VL0) || VecTy != SrcVecTy)
+        VecCost = TTI->getCastInstrCost(VL0->getOpcode(), VecTy, SrcVecTy, VL0);
       return VecCost - ScalarCost;
     }
     case Instruction::FCmp:
@@ -4014,9 +4017,24 @@ void BoUpSLP::computeMinimumValueSizes() {
   // additional roots that require investigating in Roots.
   SmallVector<Value *, 32> ToDemote;
   SmallVector<Value *, 4> Roots;
-  for (auto *Root : TreeRoot)
+  for (auto *Root : TreeRoot) {
+    // Do not include top zext/sext/trunc operations to those to be demoted, it
+    // produces noise cast<vect>, trunc <vect>, exctract <vect>, cast <extract>
+    // sequence.
+    if (isa<Constant>(Root))
+      continue;
+    auto *I = dyn_cast<Instruction>(Root);
+    if (!I || !I->hasOneUse() || !Expr.count(I))
+      return;
+    if (isa<ZExtInst>(I) || isa<SExtInst>(I))
+      continue;
+    if (auto *TI = dyn_cast<TruncInst>(I)) {
+      Roots.push_back(TI->getOperand(0));
+      continue;
+    }
     if (!collectValuesToDemote(Root, Expr, ToDemote, Roots))
       return;
+  }
 
   // The maximum bit width required to represent all the values that can be
   // demoted without loss of precision. It would be safe to truncate the roots