[LoopVectorizer] Add support for partial reductions (llvm#92418)

Following on from llvm#94499, this
patch adds support to the Loop Vectorizer to emit the partial reduction
intrinsics where they may be beneficial for the target.

---------

Co-authored-by: Samuel Tebbs <samuel.tebbs@arm.com>

llvm/include/llvm/Analysis/TargetTransformInfo.h

@@ -211,6 +211,12 @@ typedef TargetTransformInfo TTI;
 /// for IR-level transformations.
 class TargetTransformInfo {
 public:
+  enum PartialReductionExtendKind { PR_None, PR_SignExtend, PR_ZeroExtend };
+
+  /// Get the kind of extension that an instruction represents.
+  static PartialReductionExtendKind
+  getPartialReductionExtendKind(Instruction *I);
+
   /// Construct a TTI object using a type implementing the \c Concept
   /// API below.
   ///
@@ -1274,6 +1280,18 @@ class TargetTransformInfo {
   /// \return if target want to issue a prefetch in address space \p AS.
   bool shouldPrefetchAddressSpace(unsigned AS) const;
 
+  /// \return The cost of a partial reduction, which is a reduction from a
+  /// vector to another vector with fewer elements of larger size. They are
+  /// represented by the llvm.experimental.partial.reduce.add intrinsic, which
+  /// takes an accumulator and a binary operation operand that itself is fed by
+  /// two extends. An example of an operation that uses a partial reduction is a
+  /// dot product, which reduces a vector to another of 4 times fewer elements.
+  InstructionCost
+  getPartialReductionCost(unsigned Opcode, Type *InputType, Type *AccumType,
+                          ElementCount VF, PartialReductionExtendKind OpAExtend,
+                          PartialReductionExtendKind OpBExtend,
+                          std::optional<unsigned> BinOp = std::nullopt) const;
+
   /// \return The maximum interleave factor that any transform should try to
   /// perform for this target. This number depends on the level of parallelism
   /// and the number of execution units in the CPU.
@@ -2098,6 +2116,18 @@ class TargetTransformInfo::Concept {
   /// \return if target want to issue a prefetch in address space \p AS.
   virtual bool shouldPrefetchAddressSpace(unsigned AS) const = 0;
 
+  /// \return The cost of a partial reduction, which is a reduction from a
+  /// vector to another vector with fewer elements of larger size. They are
+  /// represented by the llvm.experimental.partial.reduce.add intrinsic, which
+  /// takes an accumulator and a binary operation operand that itself is fed by
+  /// two extends. An example of an operation that uses a partial reduction is a
+  /// dot product, which reduces a vector to another of 4 times fewer elements.
+  virtual InstructionCost
+  getPartialReductionCost(unsigned Opcode, Type *InputType, Type *AccumType,
+                          ElementCount VF, PartialReductionExtendKind OpAExtend,
+                          PartialReductionExtendKind OpBExtend,
+                          std::optional<unsigned> BinOp) const = 0;
+
   virtual unsigned getMaxInterleaveFactor(ElementCount VF) = 0;
   virtual InstructionCost getArithmeticInstrCost(
       unsigned Opcode, Type *Ty, TTI::TargetCostKind CostKind,
@@ -2772,6 +2802,15 @@ class TargetTransformInfo::Model final : public TargetTransformInfo::Concept {
     return Impl.shouldPrefetchAddressSpace(AS);
   }
 
+  InstructionCost getPartialReductionCost(
+      unsigned Opcode, Type *InputType, Type *AccumType, ElementCount VF,
+      PartialReductionExtendKind OpAExtend,
+      PartialReductionExtendKind OpBExtend,
+      std::optional<unsigned> BinOp = std::nullopt) const override {
+    return Impl.getPartialReductionCost(Opcode, InputType, AccumType, VF,
+                                        OpAExtend, OpBExtend, BinOp);
+  }
+
   unsigned getMaxInterleaveFactor(ElementCount VF) override {
     return Impl.getMaxInterleaveFactor(VF);
   }

llvm/include/llvm/Analysis/TargetTransformInfoImpl.h

@@ -580,6 +580,15 @@ class TargetTransformInfoImplBase {
   bool enableWritePrefetching() const { return false; }
   bool shouldPrefetchAddressSpace(unsigned AS) const { return !AS; }
 
+  InstructionCost
+  getPartialReductionCost(unsigned Opcode, Type *InputType, Type *AccumType,
+                          ElementCount VF,
+                          TTI::PartialReductionExtendKind OpAExtend,
+                          TTI::PartialReductionExtendKind OpBExtend,
+                          std::optional<unsigned> BinOp = std::nullopt) const {
+    return InstructionCost::getInvalid();
+  }
+
   unsigned getMaxInterleaveFactor(ElementCount VF) const { return 1; }
 
   InstructionCost getArithmeticInstrCost(

llvm/lib/Analysis/TargetTransformInfo.cpp

@@ -858,6 +858,14 @@ bool TargetTransformInfo::shouldPrefetchAddressSpace(unsigned AS) const {
   return TTIImpl->shouldPrefetchAddressSpace(AS);
 }
 
+InstructionCost TargetTransformInfo::getPartialReductionCost(
+    unsigned Opcode, Type *InputType, Type *AccumType, ElementCount VF,
+    PartialReductionExtendKind OpAExtend, PartialReductionExtendKind OpBExtend,
+    std::optional<unsigned> BinOp) const {
+  return TTIImpl->getPartialReductionCost(Opcode, InputType, AccumType, VF,
+                                          OpAExtend, OpBExtend, BinOp);
+}
+
 unsigned TargetTransformInfo::getMaxInterleaveFactor(ElementCount VF) const {
   return TTIImpl->getMaxInterleaveFactor(VF);
 }
@@ -969,6 +977,15 @@ InstructionCost TargetTransformInfo::getShuffleCost(
   return Cost;
 }
 
+TargetTransformInfo::PartialReductionExtendKind
+TargetTransformInfo::getPartialReductionExtendKind(Instruction *I) {
+  if (isa<SExtInst>(I))
+    return PR_SignExtend;
+  if (isa<ZExtInst>(I))
+    return PR_ZeroExtend;
+  return PR_None;
+}
+
 TTI::CastContextHint
 TargetTransformInfo::getCastContextHint(const Instruction *I) {
   if (!I)

llvm/lib/Target/AArch64/AArch64TargetTransformInfo.h

@@ -23,6 +23,7 @@
 #include "llvm/CodeGen/BasicTTIImpl.h"
 #include "llvm/IR/Function.h"
 #include "llvm/IR/Intrinsics.h"
+#include "llvm/Support/InstructionCost.h"
 #include <cstdint>
 #include <optional>
 
@@ -357,6 +358,61 @@ class AArch64TTIImpl : public BasicTTIImplBase<AArch64TTIImpl> {
     return BaseT::isLegalNTLoad(DataType, Alignment);
   }
 
+  InstructionCost
+  getPartialReductionCost(unsigned Opcode, Type *InputType, Type *AccumType,
+                          ElementCount VF,
+                          TTI::PartialReductionExtendKind OpAExtend,
+                          TTI::PartialReductionExtendKind OpBExtend,
+                          std::optional<unsigned> BinOp) const {
+
+    InstructionCost Invalid = InstructionCost::getInvalid();
+    InstructionCost Cost(TTI::TCC_Basic);
+
+    if (Opcode != Instruction::Add)
+      return Invalid;
+
+    EVT InputEVT = EVT::getEVT(InputType);
+    EVT AccumEVT = EVT::getEVT(AccumType);
+
+    if (VF.isScalable() && !ST->isSVEorStreamingSVEAvailable())
+      return Invalid;
+    if (VF.isFixed() && (!ST->isNeonAvailable() || !ST->hasDotProd()))
+      return Invalid;
+
+    if (InputEVT == MVT::i8) {
+      switch (VF.getKnownMinValue()) {
+      default:
+        return Invalid;
+      case 8:
+        if (AccumEVT == MVT::i32)
+          Cost *= 2;
+        else if (AccumEVT != MVT::i64)
+          return Invalid;
+        break;
+      case 16:
+        if (AccumEVT == MVT::i64)
+          Cost *= 2;
+        else if (AccumEVT != MVT::i32)
+          return Invalid;
+        break;
+      }
+    } else if (InputEVT == MVT::i16) {
+      // FIXME: Allow i32 accumulator but increase cost, as we would extend
+      //        it to i64.
+      if (VF.getKnownMinValue() != 8 || AccumEVT != MVT::i64)
+        return Invalid;
+    } else
+      return Invalid;
+
+    if (OpAExtend == TTI::PR_None || OpBExtend == TTI::PR_None)
+      return Invalid;
+
+    if (!BinOp || (*BinOp) != Instruction::Mul)
+      return Invalid;
+
+    return Cost;
+  }
+
   bool enableOrderedReductions() const { return true; }
 
   InstructionCost getInterleavedMemoryOpCost(

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

@@ -7605,6 +7605,10 @@ static bool planContainsAdditionalSimplifications(VPlan &Plan,
         }
         continue;
       }
+      // The VPlan-based cost model is more accurate for partial reduction and
+      // comparing against the legacy cost isn't desirable.
+      if (isa<VPPartialReductionRecipe>(&R))
+        return true;
       if (Instruction *UI = GetInstructionForCost(&R))
         SeenInstrs.insert(UI);
     }
@@ -8827,6 +8831,103 @@ VPReplicateRecipe *VPRecipeBuilder::handleReplication(Instruction *I,
   return Recipe;
 }
 
+/// Find all possible partial reductions in the loop and track all of those that
+/// are valid so recipes can be formed later.
+void VPRecipeBuilder::collectScaledReductions(VFRange &Range) {
+  // Find all possible partial reductions.
+  SmallVector<std::pair<PartialReductionChain, unsigned>, 1>
+      PartialReductionChains;
+  for (const auto &[Phi, RdxDesc] : Legal->getReductionVars())
+    if (std::optional<std::pair<PartialReductionChain, unsigned>> Pair =
+            getScaledReduction(Phi, RdxDesc, Range))
+      PartialReductionChains.push_back(*Pair);
+
+  // A partial reduction is invalid if any of its extends are used by
+  // something that isn't another partial reduction. This is because the
+  // extends are intended to be lowered along with the reduction itself.
+
+  // Build up a set of partial reduction bin ops for efficient use checking.
+  SmallSet<User *, 4> PartialReductionBinOps;
+  for (const auto &[PartialRdx, _] : PartialReductionChains)
+    PartialReductionBinOps.insert(PartialRdx.BinOp);
+
+  auto ExtendIsOnlyUsedByPartialReductions =
+      [&PartialReductionBinOps](Instruction *Extend) {
+        return all_of(Extend->users(), [&](const User *U) {
+          return PartialReductionBinOps.contains(U);
+        });
+      };
+
+  // Check if each use of a chain's two extends is a partial reduction
+  // and only add those that don't have non-partial reduction users.
+  for (auto Pair : PartialReductionChains) {
+    PartialReductionChain Chain = Pair.first;
+    if (ExtendIsOnlyUsedByPartialReductions(Chain.ExtendA) &&
+        ExtendIsOnlyUsedByPartialReductions(Chain.ExtendB))
+      ScaledReductionExitInstrs.insert(std::make_pair(Chain.Reduction, Pair));
+  }
+}
+
+std::optional<std::pair<PartialReductionChain, unsigned>>
+VPRecipeBuilder::getScaledReduction(PHINode *PHI,
+                                    const RecurrenceDescriptor &Rdx,
+                                    VFRange &Range) {
+  // TODO: Allow scaling reductions when predicating. The select at
+  // the end of the loop chooses between the phi value and most recent
+  // reduction result, both of which have different VFs to the active lane
+  // mask when scaling.
+  if (CM.blockNeedsPredicationForAnyReason(Rdx.getLoopExitInstr()->getParent()))
+    return std::nullopt;
+
+  auto *Update = dyn_cast<BinaryOperator>(Rdx.getLoopExitInstr());
+  if (!Update)
+    return std::nullopt;
+
+  Value *Op = Update->getOperand(0);
+  if (Op == PHI)
+    Op = Update->getOperand(1);
+
+  auto *BinOp = dyn_cast<BinaryOperator>(Op);
+  if (!BinOp || !BinOp->hasOneUse())
+    return std::nullopt;
+
+  using namespace llvm::PatternMatch;
+  Value *A, *B;
+  if (!match(BinOp->getOperand(0), m_ZExtOrSExt(m_Value(A))) ||
+      !match(BinOp->getOperand(1), m_ZExtOrSExt(m_Value(B))))
+    return std::nullopt;
+
+  Instruction *ExtA = cast<Instruction>(BinOp->getOperand(0));
+  Instruction *ExtB = cast<Instruction>(BinOp->getOperand(1));
+
+  // Check that the extends extend from the same type.
+  if (A->getType() != B->getType())
+    return std::nullopt;
+
+  TTI::PartialReductionExtendKind OpAExtend =
+      TargetTransformInfo::getPartialReductionExtendKind(ExtA);
+  TTI::PartialReductionExtendKind OpBExtend =
+      TargetTransformInfo::getPartialReductionExtendKind(ExtB);
+
+  PartialReductionChain Chain(Rdx.getLoopExitInstr(), ExtA, ExtB, BinOp);
+
+  unsigned TargetScaleFactor =
+      PHI->getType()->getPrimitiveSizeInBits().getKnownScalarFactor(
+          A->getType()->getPrimitiveSizeInBits());
+
+  if (LoopVectorizationPlanner::getDecisionAndClampRange(
+          [&](ElementCount VF) {
+            InstructionCost Cost = TTI->getPartialReductionCost(
+                Update->getOpcode(), A->getType(), PHI->getType(), VF,
+                OpAExtend, OpBExtend, std::make_optional(BinOp->getOpcode()));
+            return Cost.isValid();
+          },
+          Range))
+    return std::make_pair(Chain, TargetScaleFactor);
+
+  return std::nullopt;
+}
+
 VPRecipeBase *
 VPRecipeBuilder::tryToCreateWidenRecipe(Instruction *Instr,
                                         ArrayRef<VPValue *> Operands,
@@ -8851,9 +8952,14 @@ VPRecipeBuilder::tryToCreateWidenRecipe(Instruction *Instr,
           Legal->getReductionVars().find(Phi)->second;
       assert(RdxDesc.getRecurrenceStartValue() ==
              Phi->getIncomingValueForBlock(OrigLoop->getLoopPreheader()));
-      PhiRecipe = new VPReductionPHIRecipe(Phi, RdxDesc, *StartV,
-                                           CM.isInLoopReduction(Phi),
-                                           CM.useOrderedReductions(RdxDesc));
+
+      // If the PHI is used by a partial reduction, set the scale factor.
+      std::optional<std::pair<PartialReductionChain, unsigned>> Pair =
+          getScaledReductionForInstr(RdxDesc.getLoopExitInstr());
+      unsigned ScaleFactor = Pair ? Pair->second : 1;
+      PhiRecipe = new VPReductionPHIRecipe(
+          Phi, RdxDesc, *StartV, CM.isInLoopReduction(Phi),
+          CM.useOrderedReductions(RdxDesc), ScaleFactor);
     } else {
       // TODO: Currently fixed-order recurrences are modeled as chains of
       // first-order recurrences. If there are no users of the intermediate
@@ -8885,6 +8991,9 @@ VPRecipeBuilder::tryToCreateWidenRecipe(Instruction *Instr,
   if (isa<LoadInst>(Instr) || isa<StoreInst>(Instr))
     return tryToWidenMemory(Instr, Operands, Range);
 
+  if (getScaledReductionForInstr(Instr))
+    return tryToCreatePartialReduction(Instr, Operands);
+
   if (!shouldWiden(Instr, Range))
     return nullptr;
 
@@ -8905,6 +9014,21 @@ VPRecipeBuilder::tryToCreateWidenRecipe(Instruction *Instr,
   return tryToWiden(Instr, Operands, VPBB);
 }
 
+VPRecipeBase *
+VPRecipeBuilder::tryToCreatePartialReduction(Instruction *Reduction,
+                                             ArrayRef<VPValue *> Operands) {
+  assert(Operands.size() == 2 &&
+         "Unexpected number of operands for partial reduction");
+
+  VPValue *BinOp = Operands[0];
+  VPValue *Phi = Operands[1];
+  if (isa<VPReductionPHIRecipe>(BinOp->getDefiningRecipe()))
+    std::swap(BinOp, Phi);
+
+  return new VPPartialReductionRecipe(Reduction->getOpcode(), BinOp, Phi,
+                                      Reduction);
+}
+
 void LoopVectorizationPlanner::buildVPlansWithVPRecipes(ElementCount MinVF,
                                                         ElementCount MaxVF) {
   assert(OrigLoop->isInnermost() && "Inner loop expected.");
@@ -9222,7 +9346,8 @@ LoopVectorizationPlanner::tryToBuildVPlanWithVPRecipes(VFRange &Range) {
   bool HasNUW = !IVUpdateMayOverflow || Style == TailFoldingStyle::None;
   addCanonicalIVRecipes(*Plan, Legal->getWidestInductionType(), HasNUW, DL);
 
-  VPRecipeBuilder RecipeBuilder(*Plan, OrigLoop, TLI, Legal, CM, PSE, Builder);
+  VPRecipeBuilder RecipeBuilder(*Plan, OrigLoop, TLI, &TTI, Legal, CM, PSE,
+                                Builder);
 
   // ---------------------------------------------------------------------------
   // Pre-construction: record ingredients whose recipes we'll need to further
@@ -9268,6 +9393,9 @@ LoopVectorizationPlanner::tryToBuildVPlanWithVPRecipes(VFRange &Range) {
         bool NeedsBlends = BB != HeaderBB && !BB->phis().empty();
         return Legal->blockNeedsPredication(BB) || NeedsBlends;
       });
+
+  RecipeBuilder.collectScaledReductions(Range);
+
   auto *MiddleVPBB = Plan->getMiddleBlock();
   VPBasicBlock::iterator MBIP = MiddleVPBB->getFirstNonPhi();
   for (BasicBlock *BB : make_range(DFS.beginRPO(), DFS.endRPO())) {