SLPVectorizer: Handle more horizontal reductions (disabled)

Match reductions starting at binary operation feeding into a phi. The code
handles trees like

 r += v1 + v2 + v3 ...

and

 r += v1
 r += v2
 ...

and

 r *= v1 + v2 + ...

We currently only handle associative operations (add, fadd fast).

The code can now also handle reductions feeding into stores.

 a[i] = v1 + v2 + v3 + ...

The code is currently disabled behind the flag "-slp-vectorize-hor".  The cost
model for most architectures is not there yet.

I found one opportunity of a horizontal reduction feeding a phi in TSVC
(LoopRerolling-flt) and there are several opportunities where reductions feed
into stores.

radar://14607682

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@191108 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 368 insertions, 8 deletions

diff --git a/lib/Transforms/Vectorize/SLPVectorizer.cpp b/lib/Transforms/Vectorize/SLPVectorizer.cpp
index cd3f723cd3..caedd0986b 100644
--- a/lib/Transforms/Vectorize/SLPVectorizer.cpp
+++ b/lib/Transforms/Vectorize/SLPVectorizer.cpp
@@ -49,6 +49,11 @@ static cl::opt<int>
     SLPCostThreshold("slp-threshold", cl::init(0), cl::Hidden,
                      cl::desc("Only vectorize if you gain more than this "
                               "number "));
+
+static cl::opt<bool>
+ShouldVectorizeHor("slp-vectorize-hor", cl::init(false), cl::Hidden,
+                   cl::desc("Attempt to vectorize horizontal reductions"));
+
 namespace {
 
 static const unsigned MinVecRegSize = 128;
@@ -238,17 +243,21 @@ public:
     }
 
   /// \brief Vectorize the tree that starts with the elements in \p VL.
-  void vectorizeTree();
+  /// Returns the vectorized root and the scalar operations the root was based
+  /// on.
+  std::pair<Value *, ValueList *> vectorizeTree();
 
   /// \returns the vectorization cost of the subtree that starts at \p VL.
   /// A negative number means that this is profitable.
   int getTreeCost();
 
-  /// Construct a vectorizable tree that starts at \p Roots.
-  void buildTree(ArrayRef<Value *> Roots);
+  /// Construct a vectorizable tree that starts at \p Roots and is possibly
+  /// used by a reduction of \p RdxOps.
+  void buildTree(ArrayRef<Value *> Roots, ValueSet *RdxOps = 0);
 
   /// Clear the internal data structures that are created by 'buildTree'.
   void deleteTree() {
+    RdxOps = 0;
     VectorizableTree.clear();
     ScalarToTreeEntry.clear();
     MustGather.clear();
@@ -401,6 +410,9 @@ private:
   /// Numbers instructions in different blocks.
   DenseMap<BasicBlock *, BlockNumbering> BlocksNumbers;
 
+  /// Reduction operators.
+  ValueSet *RdxOps;
+
   // Analysis and block reference.
   Function *F;
   ScalarEvolution *SE;
@@ -413,8 +425,9 @@ private:
   IRBuilder<> Builder;
 };
 
-void BoUpSLP::buildTree(ArrayRef<Value *> Roots) {
+void BoUpSLP::buildTree(ArrayRef<Value *> Roots, ValueSet *Rdx) {
   deleteTree();
+  RdxOps = Rdx;
   if (!getSameType(Roots))
     return;
   buildTree_rec(Roots, 0);
@@ -445,8 +458,12 @@ void BoUpSLP::buildTree(ArrayRef<Value *> Roots) {
           assert(!VectorizableTree[Idx].NeedToGather && "Bad state");
           continue;
         }
+        Instruction *UserInst = dyn_cast<Instruction>(*User);
+        if (!UserInst)
+          continue;
 
-        if (!isa<Instruction>(*User))
+        // Ignore uses that are part of the reduction.
+        if (Rdx && std::find(Rdx->begin(), Rdx->end(), UserInst) != Rdx->end())
           continue;
 
         DEBUG(dbgs() << "SLP: Need to extract:" << **User << " from lane " <<
@@ -578,6 +595,10 @@ void BoUpSLP::buildTree_rec(ArrayRef<Value *> VL, unsigned Depth) {
         continue;
       }
 
+      // This user is part of the reduction.
+      if (RdxOps && RdxOps->count(User))
+        continue;
+
       // Make sure that we can schedule this unknown user.
       BlockNumbering &BN = BlocksNumbers[BB];
       int UserIndex = BN.getIndex(User);
@@ -1372,7 +1393,7 @@ Value *BoUpSLP::vectorizeTree(TreeEntry *E) {
   return 0;
 }
 
-void BoUpSLP::vectorizeTree() {
+std::pair<Value *, BoUpSLP::ValueList *> BoUpSLP::vectorizeTree() {
   Builder.SetInsertPoint(F->getEntryBlock().begin());
   vectorizeTree(&VectorizableTree[0]);
 
@@ -1449,7 +1470,10 @@ void BoUpSLP::vectorizeTree() {
           DEBUG(dbgs() << "SLP: \tvalidating user:" << **User << ".\n");
           assert(!MustGather.count(*User) &&
                  "Replacing gathered value with undef");
-          assert(ScalarToTreeEntry.count(*User) &&
+
+          assert((ScalarToTreeEntry.count(*User) ||
+                  // It is legal to replace the reduction users by undef.
+                  (RdxOps && RdxOps->count(*User))) &&
                  "Replacing out-of-tree value with undef");
         }
         Value *Undef = UndefValue::get(Ty);
@@ -1464,6 +1488,9 @@ void BoUpSLP::vectorizeTree() {
     BlocksNumbers[it].forget();
   }
   Builder.ClearInsertionPoint();
+
+  return std::make_pair(VectorizableTree[0].VectorizedValue,
+                        &VectorizableTree[0].Scalars);
 }
 
 void BoUpSLP::optimizeGatherSequence() {
@@ -1887,6 +1914,310 @@ bool SLPVectorizer::tryToVectorize(BinaryOperator *V, BoUpSLP &R) {
   return 0;
 }
 
+/// \brief Generate a shuffle mask to be used in a reduction tree.
+///
+/// \param VecLen The length of the vector to be reduced.
+/// \param NumEltsToRdx The number of elements that should be reduced in the
+///        vector.
+/// \param IsPairwise Whether the reduction is a pairwise or splitting
+///        reduction. A pairwise reduction will generate a mask of 
+///        <0,2,...> or <1,3,..> while a splitting reduction will generate
+///        <2,3, undef,undef> for a vector of 4 and NumElts = 2.
+/// \param IsLeft True will generate a mask of even elements, odd otherwise.
+static Value *createRdxShuffleMask(unsigned VecLen, unsigned NumEltsToRdx,
+                                   bool IsPairwise, bool IsLeft,
+                                   IRBuilder<> &Builder) {
+  assert((IsPairwise || !IsLeft) && "Don't support a <0,1,undef,...> mask");
+
+  SmallVector<Constant *, 32> ShuffleMask(
+      VecLen, UndefValue::get(Builder.getInt32Ty()));
+
+  if (IsPairwise)
+    // Build a mask of 0, 2, ... (left) or 1, 3, ... (right).
+    for (unsigned i = 0; i != NumEltsToRdx; ++i)
+      ShuffleMask[i] = Builder.getInt32(2 * i + !IsLeft);
+  else
+    // Move the upper half of the vector to the lower half.
+    for (unsigned i = 0; i != NumEltsToRdx; ++i)
+      ShuffleMask[i] = Builder.getInt32(NumEltsToRdx + i);
+
+  return ConstantVector::get(ShuffleMask);
+}
+
+
+/// Model horizontal reductions.
+///
+/// A horizontal reduction is a tree of reduction operations (currently add and
+/// fadd) that has operations that can be put into a vector as its leaf.
+/// For example, this tree:
+///
+/// mul mul mul mul
+///  \  /    \  /
+///   +       +
+///    \     /
+///       +
+/// This tree has "mul" as its reduced values and "+" as its reduction
+/// operations. A reduction might be feeding into a store or a binary operation
+/// feeding a phi.
+///    ...
+///    \  /
+///     +
+///     \
+///  phi +=
+///
+///  Or:
+///    ...
+///    \  /
+///     +
+///     \
+///   *p =
+///
+class HorizontalReduction {
+  SmallPtrSet<Value *, 16> ReductionOps;
+  SmallVector<Value *, 32> ReducedVals;
+
+  BinaryOperator *ReductionRoot;
+  PHINode *ReductionPHI;
+
+  /// The opcode of the reduction.
+  unsigned ReductionOpcode;
+  /// The opcode of the values we perform a reduction on.
+  unsigned ReducedValueOpcode;
+  /// The width of one full horizontal reduction operation.
+  unsigned ReduxWidth;
+  /// Should we model this reduction as a pairwise reduction tree or a tree that
+  /// splits the vector in halves and adds those halves.
+  bool IsPairwiseReduction;
+
+public:
+  HorizontalReduction()
+    : ReductionRoot(0), ReductionPHI(0), ReductionOpcode(0),
+    ReducedValueOpcode(0), ReduxWidth(0), IsPairwiseReduction(false) {}
+
+  /// \brief Try to find a reduction tree.
+  bool matchAssociativeReduction(PHINode *Phi, BinaryOperator *B,
+                                 DataLayout *DL) {
+    assert((!Phi ||
+            std::find(Phi->op_begin(), Phi->op_end(), B) != Phi->op_end()) &&
+           "Thi phi needs to use the binary operator");
+
+    // We could have a initial reductions that is not an add.
+    //  r *= v1 + v2 + v3 + v4
+    // In such a case start looking for a tree rooted in the first '+'.
+    if (Phi) {
+      if (B->getOperand(0) == Phi) {
+        Phi = 0;
+        B = dyn_cast<BinaryOperator>(B->getOperand(1));
+      } else if (B->getOperand(1) == Phi) {
+        Phi = 0;
+        B = dyn_cast<BinaryOperator>(B->getOperand(0));
+      }
+    }
+
+    if (!B)
+      return false;
+
+    Type *Ty = B->getType();
+    if (Ty->isVectorTy())
+      return false;
+
+    ReductionOpcode = B->getOpcode();
+    ReducedValueOpcode = 0;
+    ReduxWidth = MinVecRegSize / DL->getTypeSizeInBits(Ty);
+    ReductionRoot = B;
+    ReductionPHI = Phi;
+
+    if (ReduxWidth < 4)
+      return false;
+
+    // We currently only support adds.
+    if (ReductionOpcode != Instruction::Add &&
+        ReductionOpcode != Instruction::FAdd)
+      return false;
+
+    // Post order traverse the reduction tree starting at B. We only handle true
+    // trees containing only binary operators.
+    SmallVector<std::pair<BinaryOperator *, unsigned>, 32> Stack;
+    Stack.push_back(std::make_pair(B, 0));
+    while (!Stack.empty()) {
+      BinaryOperator *TreeN = Stack.back().first;
+      unsigned EdgeToVist = Stack.back().second++;
+      bool IsReducedValue = TreeN->getOpcode() != ReductionOpcode;
+
+      // Only handle trees in the current basic block.
+      if (TreeN->getParent() != B->getParent())
+        return false;
+
+      // Each tree node needs to have one user except for the ultimate
+      // reduction.
+      if (!TreeN->hasOneUse() && TreeN != B)
+        return false;
+
+      // Postorder vist.
+      if (EdgeToVist == 2 || IsReducedValue) {
+        if (IsReducedValue) {
+          // Make sure that the opcodes of the operations that we are going to
+          // reduce match.
+          if (!ReducedValueOpcode)
+            ReducedValueOpcode = TreeN->getOpcode();
+          else if (ReducedValueOpcode != TreeN->getOpcode())
+            return false;
+          ReducedVals.push_back(TreeN);
+        } else {
+          // We need to be able to reassociate the adds.
+          if (!TreeN->isAssociative())
+            return false;
+          ReductionOps.insert(TreeN);
+        }
+        // Retract.
+        Stack.pop_back();
+        continue;
+      }
+
+      // Visit left or right.
+      Value *NextV = TreeN->getOperand(EdgeToVist);
+      BinaryOperator *Next = dyn_cast<BinaryOperator>(NextV);
+      if (Next)
+        Stack.push_back(std::make_pair(Next, 0));
+      else if (NextV != Phi)
+        return false;
+    }
+    return true;
+  }
+
+  /// \brief Attempt to vectorize the tree found by
+  /// matchAssociativeReduction.
+  bool tryToReduce(BoUpSLP &V, TargetTransformInfo *TTI) {
+    if (ReducedVals.empty())
+      return false;
+
+    unsigned NumReducedVals = ReducedVals.size();
+    if (NumReducedVals < ReduxWidth)
+      return false;
+
+    Value *VectorizedTree = 0;
+    IRBuilder<> Builder(ReductionRoot);
+    FastMathFlags Unsafe;
+    Unsafe.setUnsafeAlgebra();
+    Builder.SetFastMathFlags(Unsafe);
+    unsigned i = 0;
+
+    for (; i < NumReducedVals - ReduxWidth + 1; i += ReduxWidth) {
+      ArrayRef<Value *> ValsToReduce(&ReducedVals[i], ReduxWidth);
+      V.buildTree(ValsToReduce, &ReductionOps);
+
+      // Estimate cost.
+      int Cost = V.getTreeCost() + getReductionCost(TTI, ReducedVals[i]);
+      if (Cost >= -SLPCostThreshold)
+        break;
+
+      DEBUG(dbgs() << "SLP: Vectorizing horizontal reduction at cost:" << Cost
+                   << ". (HorRdx)\n");
+
+      // Vectorize a tree.
+      Value *VectorizedRoot;
+      BoUpSLP::ValueList *Scalars;
+      tie(VectorizedRoot, Scalars) = V.vectorizeTree();
+
+      // Emit a reduction.
+      Value *ReducedSubTree = emitReduction(VectorizedRoot, Builder);
+      if (VectorizedTree) {
+        Builder.SetCurrentDebugLocation(
+          cast<Instruction>((*Scalars)[0])->getDebugLoc());
+        VectorizedTree = createBinOp(Builder, ReductionOpcode, VectorizedTree,
+                                     ReducedSubTree, "bin.rdx");
+      } else
+        VectorizedTree = ReducedSubTree;
+    }
+
+    if (VectorizedTree) {
+      // Finish the reduction.
+      for (; i < NumReducedVals; ++i) {
+        Builder.SetCurrentDebugLocation(
+          cast<Instruction>(ReducedVals[i])->getDebugLoc());
+        VectorizedTree = createBinOp(Builder, ReductionOpcode, VectorizedTree,
+                                     ReducedVals[i]);
+      }
+      // Update users.
+      if (ReductionPHI) {
+        assert(ReductionRoot != NULL && "Need a reduction operation");
+        ReductionRoot->setOperand(0, VectorizedTree);
+        ReductionRoot->setOperand(1, ReductionPHI);
+      } else
+        ReductionRoot->replaceAllUsesWith(VectorizedTree);
+    }
+    return VectorizedTree != 0;
+  }
+
+private:
+
+  /// \brief Calcuate the cost of a reduction.
+  int getReductionCost(TargetTransformInfo *TTI, Value *FirstReducedVal) {
+    Type *ScalarTy = FirstReducedVal->getType();
+    Type *VecTy = VectorType::get(ScalarTy, ReduxWidth);
+
+    int PairwiseRdxCost = TTI->getReductionCost(ReductionOpcode, VecTy, true);
+    int SplittingRdxCost = TTI->getReductionCost(ReductionOpcode, VecTy, false);
+
+    IsPairwiseReduction = PairwiseRdxCost < SplittingRdxCost;
+    int VecReduxCost = IsPairwiseReduction ? PairwiseRdxCost : SplittingRdxCost;
+
+    int ScalarReduxCost =
+        ReduxWidth * TTI->getArithmeticInstrCost(ReductionOpcode, VecTy);
+
+    DEBUG(dbgs() << "SLP: Adding cost " << VecReduxCost - ScalarReduxCost
+                 << " for reduction that starts with " << *FirstReducedVal
+                 << " (It is a "
+                 << (IsPairwiseReduction ? "pairwise" : "splitting")
+                 << " reduction)\n");
+
+    return VecReduxCost - ScalarReduxCost;
+  }
+
+  static Value *createBinOp(IRBuilder<> &Builder, unsigned Opcode, Value *L,
+                            Value *R, const Twine &Name = "") {
+    if (Opcode == Instruction::FAdd)
+      return Builder.CreateFAdd(L, R, Name);
+    return Builder.CreateBinOp((Instruction::BinaryOps)Opcode, L, R, Name);
+  }
+
+  /// \brief Emit a horizontal reduction of the vectorized value.
+  Value *emitReduction(Value *VectorizedValue, IRBuilder<> &Builder) {
+    assert(VectorizedValue && "Need to have a vectorized tree node");
+    Instruction *ValToReduce = dyn_cast<Instruction>(VectorizedValue);
+    assert(isPowerOf2_32(ReduxWidth) &&
+           "We only handle power-of-two reductions for now");
+
+    SmallVector<Constant *, 32> ShuffleMask(ReduxWidth, 0);
+    Value *TmpVec = ValToReduce;
+    for (unsigned i = ReduxWidth / 2; i != 0; i >>= 1) {
+      if (IsPairwiseReduction) {
+        Value *LeftMask =
+          createRdxShuffleMask(ReduxWidth, i, true, true, Builder);
+        Value *RightMask =
+          createRdxShuffleMask(ReduxWidth, i, true, false, Builder);
+
+        Value *LeftShuf = Builder.CreateShuffleVector(
+          TmpVec, UndefValue::get(TmpVec->getType()), LeftMask, "rdx.shuf.l");
+        Value *RightShuf = Builder.CreateShuffleVector(
+          TmpVec, UndefValue::get(TmpVec->getType()), (RightMask),
+          "rdx.shuf.r");
+        TmpVec = createBinOp(Builder, ReductionOpcode, LeftShuf, RightShuf,
+                             "bin.rdx");
+      } else {
+        Value *UpperHalf =
+          createRdxShuffleMask(ReduxWidth, i, false, false, Builder);
+        Value *Shuf = Builder.CreateShuffleVector(
+          TmpVec, UndefValue::get(TmpVec->getType()), UpperHalf, "rdx.shuf");
+        TmpVec = createBinOp(Builder, ReductionOpcode, TmpVec, Shuf, "bin.rdx");
+      }
+    }
+
+    // The result is in the first element of the vector.
+    return Builder.CreateExtractElement(TmpVec, Builder.getInt32(0));
+  }
+};
+
 /// \brief Recognize construction of vectors like
 ///  %ra = insertelement <4 x float> undef, float %s0, i32 0
 ///  %rb = insertelement <4 x float> %ra, float %s1, i32 1
@@ -1981,7 +2312,18 @@ bool SLPVectorizer::vectorizeChainsInBlock(BasicBlock *BB, BoUpSLP &R) {
       if (!BI)
         continue;
 
-      Value *Inst = BI->getOperand(0);
+      // Try to match and vectorize a horizontal reduction.
+      HorizontalReduction HorRdx;
+      if (ShouldVectorizeHor &&
+          HorRdx.matchAssociativeReduction(P, BI, DL) &&
+          HorRdx.tryToReduce(R, TTI)) {
+        Changed = true;
+        it = BB->begin();
+        e = BB->end();
+        continue;
+      }
+
+     Value *Inst = BI->getOperand(0);
       if (Inst == P)
         Inst = BI->getOperand(1);
 
@@ -1991,10 +2333,28 @@ bool SLPVectorizer::vectorizeChainsInBlock(BasicBlock *BB, BoUpSLP &R) {
         Changed = true;
         it = BB->begin();
         e = BB->end();
+        continue;
       }
+
       continue;
     }
 
+    // Try to vectorize horizontal reductions feeding into a store.
+    if (StoreInst *SI = dyn_cast<StoreInst>(it))
+      if (BinaryOperator *BinOp =
+              dyn_cast<BinaryOperator>(SI->getValueOperand())) {
+        HorizontalReduction HorRdx;
+        if (ShouldVectorizeHor &&
+            ((HorRdx.matchAssociativeReduction(0, BinOp, DL) &&
+              HorRdx.tryToReduce(R, TTI)) ||
+             tryToVectorize(BinOp, R))) {
+          Changed = true;
+          it = BB->begin();
+          e = BB->end();
+          continue;
+        }
+      }
+
     // Try to vectorize trees that start at compare instructions.
     if (CmpInst *CI = dyn_cast<CmpInst>(it)) {
       if (tryToVectorizePair(CI->getOperand(0), CI->getOperand(1), R)) {