[SLP] Fix for PR6246: vectorization for scalar ops on vector elements.

When trying to vectorize trees that start at insertelement instructions
function tryToVectorizeList() uses vectorization factor calculated as
MinVecRegSize/ScalarTypeSize. But sometimes it does not work as tree
cost for this fixed vectorization factor is too high.
Patch tries to improve the situation. It tries different vectorization
factors from max(PowerOf2Floor(NumberOfVectorizedValues),
MinVecRegSize/ScalarTypeSize) to MinVecRegSize/ScalarTypeSize and tries
to choose the best one.

Differential Revision: https://reviews.llvm.org/D27215

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@288412 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Transforms/Vectorize/SLPVectorizer.cpp

@@ -3870,10 +3870,9 @@ bool SLPVectorizerPass::tryToVectorizeList(ArrayRef<Value *> VL, BoUpSLP &R,
 
   unsigned Opcode0 = I0->getOpcode();
 
-  // FIXME: Register size should be a parameter to this function, so we can
-  // try different vectorization factors.
   unsigned Sz = R.getVectorElementSize(I0);
-  unsigned VF = R.getMinVecRegSize() / Sz;
+  unsigned MinVF = R.getMinVecRegSize() / Sz;
+  unsigned MaxVF = std::max<unsigned>(PowerOf2Floor(VL.size()), MinVF);
 
   for (Value *V : VL) {
     Type *Ty = V->getType();
@@ -3889,76 +3888,83 @@ bool SLPVectorizerPass::tryToVectorizeList(ArrayRef<Value *> VL, BoUpSLP &R,
   // Keep track of values that were deleted by vectorizing in the loop below.
   SmallVector<WeakVH, 8> TrackValues(VL.begin(), VL.end());
 
-  for (unsigned i = 0, e = VL.size(); i < e; ++i) {
-    unsigned OpsWidth = 0;
+  unsigned NextInst = 0, MaxInst = VL.size();
+  for (unsigned VF = MaxVF; NextInst + 1 < MaxInst && VF >= MinVF;
+       VF /= 2) {
+    for (unsigned I = NextInst; I < MaxInst; ++I) {
+      unsigned OpsWidth = 0;
 
-    if (i + VF > e)
-      OpsWidth = e - i;
-    else
-      OpsWidth = VF;
-
-    if (!isPowerOf2_32(OpsWidth) || OpsWidth < 2)
-      break;
+      if (I + VF > MaxInst)
+        OpsWidth = MaxInst - I;
+      else
+        OpsWidth = VF;
 
-    // Check that a previous iteration of this loop did not delete the Value.
-    if (hasValueBeenRAUWed(VL, TrackValues, i, OpsWidth))
-      continue;
+      if (!isPowerOf2_32(OpsWidth) || OpsWidth < 2)
+        break;
 
-    DEBUG(dbgs() << "SLP: Analyzing " << OpsWidth << " operations "
-                 << "\n");
-    ArrayRef<Value *> Ops = VL.slice(i, OpsWidth);
-
-    ArrayRef<Value *> BuildVectorSlice;
-    if (!BuildVector.empty())
-      BuildVectorSlice = BuildVector.slice(i, OpsWidth);
-
-    R.buildTree(Ops, BuildVectorSlice);
-    // TODO: check if we can allow reordering for more cases.
-    if (AllowReorder && R.shouldReorder()) {
-      // Conceptually, there is nothing actually preventing us from trying to
-      // reorder a larger list. In fact, we do exactly this when vectorizing
-      // reductions. However, at this point, we only expect to get here from
-      // tryToVectorizePair().
-      assert(Ops.size() == 2);
-      assert(BuildVectorSlice.empty());
-      Value *ReorderedOps[] = { Ops[1], Ops[0] };
-      R.buildTree(ReorderedOps, None);
-    }
-    if (R.isTreeTinyAndNotFullyVectorizable())
-      continue;
+      // Check that a previous iteration of this loop did not delete the Value.
+      if (hasValueBeenRAUWed(VL, TrackValues, I, OpsWidth))
+        continue;
 
-    R.computeMinimumValueSizes();
-    int Cost = R.getTreeCost();
+      DEBUG(dbgs() << "SLP: Analyzing " << OpsWidth << " operations "
+                   << "\n");
+      ArrayRef<Value *> Ops = VL.slice(I, OpsWidth);
+
+      ArrayRef<Value *> BuildVectorSlice;
+      if (!BuildVector.empty())
+        BuildVectorSlice = BuildVector.slice(I, OpsWidth);
+
+      R.buildTree(Ops, BuildVectorSlice);
+      // TODO: check if we can allow reordering for more cases.
+      if (AllowReorder && R.shouldReorder()) {
+        // Conceptually, there is nothing actually preventing us from trying to
+        // reorder a larger list. In fact, we do exactly this when vectorizing
+        // reductions. However, at this point, we only expect to get here from
+        // tryToVectorizePair().
+        assert(Ops.size() == 2);
+        assert(BuildVectorSlice.empty());
+        Value *ReorderedOps[] = {Ops[1], Ops[0]};
+        R.buildTree(ReorderedOps, None);
+      }
+      if (R.isTreeTinyAndNotFullyVectorizable())
+        continue;
 
-    if (Cost < -SLPCostThreshold) {
-      DEBUG(dbgs() << "SLP: Vectorizing list at cost:" << Cost << ".\n");
-      Value *VectorizedRoot = R.vectorizeTree();
-
-      // Reconstruct the build vector by extracting the vectorized root. This
-      // way we handle the case where some elements of the vector are undefined.
-      //  (return (inserelt <4 xi32> (insertelt undef (opd0) 0) (opd1) 2))
-      if (!BuildVectorSlice.empty()) {
-        // The insert point is the last build vector instruction. The vectorized
-        // root will precede it. This guarantees that we get an instruction. The
-        // vectorized tree could have been constant folded.
-        Instruction *InsertAfter = cast<Instruction>(BuildVectorSlice.back());
-        unsigned VecIdx = 0;
-        for (auto &V : BuildVectorSlice) {
-          IRBuilder<NoFolder> Builder(InsertAfter->getParent(),
-                                      ++BasicBlock::iterator(InsertAfter));
-          Instruction *I = cast<Instruction>(V);
-          assert(isa<InsertElementInst>(I) || isa<InsertValueInst>(I));
-          Instruction *Extract = cast<Instruction>(Builder.CreateExtractElement(
-              VectorizedRoot, Builder.getInt32(VecIdx++)));
-          I->setOperand(1, Extract);
-          I->removeFromParent();
-          I->insertAfter(Extract);
-          InsertAfter = I;
+      R.computeMinimumValueSizes();
+      int Cost = R.getTreeCost();
+
+      if (Cost < -SLPCostThreshold) {
+        DEBUG(dbgs() << "SLP: Vectorizing list at cost:" << Cost << ".\n");
+        Value *VectorizedRoot = R.vectorizeTree();
+
+        // Reconstruct the build vector by extracting the vectorized root. This
+        // way we handle the case where some elements of the vector are
+        // undefined.
+        //  (return (inserelt <4 xi32> (insertelt undef (opd0) 0) (opd1) 2))
+        if (!BuildVectorSlice.empty()) {
+          // The insert point is the last build vector instruction. The
+          // vectorized root will precede it. This guarantees that we get an
+          // instruction. The vectorized tree could have been constant folded.
+          Instruction *InsertAfter = cast<Instruction>(BuildVectorSlice.back());
+          unsigned VecIdx = 0;
+          for (auto &V : BuildVectorSlice) {
+            IRBuilder<NoFolder> Builder(InsertAfter->getParent(),
+                                        ++BasicBlock::iterator(InsertAfter));
+            Instruction *I = cast<Instruction>(V);
+            assert(isa<InsertElementInst>(I) || isa<InsertValueInst>(I));
+            Instruction *Extract =
+                cast<Instruction>(Builder.CreateExtractElement(
+                    VectorizedRoot, Builder.getInt32(VecIdx++)));
+            I->setOperand(1, Extract);
+            I->removeFromParent();
+            I->insertAfter(Extract);
+            InsertAfter = I;
+          }
         }
+        // Move to the next bundle.
+        I += VF - 1;
+        NextInst = I + 1;
+        Changed = true;
       }
-      // Move to the next bundle.
-      i += VF - 1;
-      Changed = true;
     }
   }