[SLP]Do not reorder top nodes if they do not require reordering.

[ABataev]

Summary

No need to reorder the top nodes, if they are not stores or
insertelement instructions and each node should be analized only
once, when the bottom-to-top analysis is performed.
We still endup with extractelements for the top node scalars and
the final shuffle just adds an extra cost and currently
crashes the compiler for PHI nodes.

[RKSimon]

llvm/test/Transforms/SLPVectorizer/X86/reordered-top-scalars.ll

3

Can this be pre-commited to show the diff?

[ABataev]

llvm/test/Transforms/SLPVectorizer/X86/reordered-top-scalars.ll

3

No, currently it crashes the compiler.

[RKSimon]

OK - LGTM

[RKSimon]

@ABataev Any blockers to this being committed?

llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp

for (const auto &Data : Users) {
                  });
         continue;
       }
       // All operands are reordered and used only in this node - propagate the
       // most used order to the user node.
       MapVector<OrdersType, unsigned,
                 DenseMap<OrdersType, unsigned, OrdersTypeDenseMapInfo>>
           OrdersUses;
+      // Do the analysis for each tree entry only once, otherwise the order of
+      // the same node my be considered several times, though might be not
+      // profitable.
       SmallPtrSet<const TreeEntry *, 4> VisitedOps;
       for (const auto &Op : Data.second) {
         TreeEntry *OpTE = Op.second;
+        if (!VisitedOps.insert(OpTE).second)
+          continue;
         if (!OpTE->ReuseShuffleIndices.empty() ||
             (IgnoreReorder && OpTE == VectorizableTree.front().get()))
           continue;
         const auto &Order = [OpTE, &GathersToOrders]() -> const OrdersType & {
           if (OpTE->State == TreeEntry::NeedToGather)
             return GathersToOrders.find(OpTE)->second;
           return OpTE->ReorderIndices;
         }();
         // Stores actually store the mask, not the order, need to invert.
         if (OpTE->State == TreeEntry::Vectorize && !OpTE->isAltShuffle() &&
             OpTE->getOpcode() == Instruction::Store && !Order.empty()) {
           SmallVector<int> Mask;
           inversePermutation(Order, Mask);
           unsigned E = Order.size();
           OrdersType CurrentOrder(E, E);
           transform(Mask, CurrentOrder.begin(), [E](int Idx) {
             return Idx == UndefMaskElem ? E : static_cast<unsigned>(Idx);
           });
           fixupOrderingIndices(CurrentOrder);
           ++OrdersUses.insert(std::make_pair(CurrentOrder, 0)).first->second;
         } else {
           ++OrdersUses.insert(std::make_pair(Order, 0)).first->second;
         }
-        if (VisitedOps.insert(OpTE).second)
-          OrdersUses.insert(std::make_pair(OrdersType(), 0)).first->second +=
-              OpTE->UserTreeIndices.size();
         assert(OrdersUses[{}] > 0 && "Counter cannot be less than 0.");
         --OrdersUses[{}];
       }
       // If no orders - skip current nodes and jump to the next one, if any.
       if (OrdersUses.empty()) {
         for_each(Data.second,
                  [&OrderedEntries](const std::pair<unsigned, TreeEntry *> &Op) {
                    OrderedEntries.remove(Op.second);
for (unsigned I = NextInst; I < MaxInst; ++I) {
 
       LLVM_DEBUG(dbgs() << "SLP: Analyzing " << OpsWidth << " operations "
                         << "\n");
 
       R.buildTree(Ops);
       if (R.isTreeTinyAndNotFullyVectorizable())
         continue;
       R.reorderTopToBottom();
-      R.reorderBottomToTop();
+      R.reorderBottomToTop(!isa<InsertElementInst>(Ops.front()));
       R.buildExternalUses();
 
       R.computeMinimumValueSizes();
       InstructionCost Cost = R.getTreeCost();
       CandidateFound = true;
       MinCost = std::min(MinCost, Cost);
 
       if (Cost < -SLPCostThreshold) {

llvm/test/Transforms/SLPVectorizer/X86/reordered-top-scalars.ll

+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt -S --slp-vectorizer -mtriple=x86_64-unknown %s -slp-threshold=-5 | FileCheck %s
+
+define i32 @test(i32* %isec) {
+; CHECK-LABEL: @test(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[ARRAYIDX10:%.*]] = getelementptr inbounds i32, i32* [[ISEC:%.*]], i32 0
+; CHECK-NEXT:    [[TMP0:%.*]] = bitcast i32* [[ARRAYIDX10]] to <2 x i32>*
+; CHECK-NEXT:    [[TMP1:%.*]] = load <2 x i32>, <2 x i32>* [[TMP0]], align 8
+; CHECK-NEXT:    [[SHUFFLE:%.*]] = shufflevector <2 x i32> [[TMP1]], <2 x i32> poison, <2 x i32> <i32 1, i32 0>
+; CHECK-NEXT:    [[TMP2:%.*]] = extractelement <2 x i32> [[TMP1]], i32 0
+; CHECK-NEXT:    [[TMP3:%.*]] = insertelement <2 x i32> poison, i32 [[TMP2]], i32 0
+; CHECK-NEXT:    [[TMP4:%.*]] = extractelement <2 x i32> [[TMP1]], i32 1
+; CHECK-NEXT:    [[TMP5:%.*]] = insertelement <2 x i32> [[TMP3]], i32 [[TMP4]], i32 1
+; CHECK-NEXT:    br i1 false, label [[BLOCK1:%.*]], label [[BLOCK3:%.*]]
+; CHECK:       block1:
+; CHECK-NEXT:    br i1 false, label [[BLOCK2:%.*]], label [[BLOCK3]]
+; CHECK:       block2:
+; CHECK-NEXT:    br label [[BLOCK3]]
+; CHECK:       block3:
+; CHECK-NEXT:    [[TMP6:%.*]] = phi <2 x i32> [ [[SHUFFLE]], [[BLOCK1]] ], [ [[SHUFFLE]], [[BLOCK2]] ], [ [[TMP5]], [[ENTRY:%.*]] ]
+; CHECK-NEXT:    [[TMP7:%.*]] = extractelement <2 x i32> [[TMP6]], i32 0
+; CHECK-NEXT:    [[TMP8:%.*]] = extractelement <2 x i32> [[TMP6]], i32 1
+; CHECK-NEXT:    [[TMP9:%.*]] = mul i32 [[TMP7]], [[TMP8]]
+; CHECK-NEXT:    ret i32 [[TMP9]]
+;
+entry:
+  %arrayidx10 = getelementptr inbounds i32, i32* %isec, i32 0
+  %0 = bitcast i32* %arrayidx10 to <2 x i32>*
+  %1 = load <2 x i32>, <2 x i32>* %0, align 8
+  %2 = extractelement <2 x i32> %1, i32 1
+  %3 = extractelement <2 x i32> %1, i32 0
+  br i1 false, label %block1, label %block3
+block1:
+  br i1 false, label %block2, label %block3
+block2:
+  br label %block3
+block3:
+  %4 = phi i32 [ %2, %block1 ], [ %2, %block2 ], [ %3, %entry ]
+  %5 = phi i32 [ %3, %block1 ], [ %3, %block2 ], [ %2, %entry ]
+  %6 = mul i32 %4, %5
+  ret i32 %6
+}