[Polly] Data flow reduction detection to cover more cases

polly/include/polly/ScopBuilder.h

@@ -663,19 +663,6 @@ class ScopBuilder final {
   ///         nullptr if it cannot be hoisted at all.
   isl::set getNonHoistableCtx(MemoryAccess *Access, isl::union_map Writes);
 
-  /// Collect loads which might form a reduction chain with @p StoreMA.
-  ///
-  /// Check if the stored value for @p StoreMA is a binary operator with one or
-  /// two loads as operands. If the binary operand is commutative & associative,
-  /// used only once (by @p StoreMA) and its load operands are also used only
-  /// once, we have found a possible reduction chain. It starts at an operand
-  /// load and includes the binary operator and @p StoreMA.
-  ///
-  /// Note: We allow only one use to ensure the load and binary operator cannot
-  ///       escape this block or into any other store except @p StoreMA.
-  void collectCandidateReductionLoads(MemoryAccess *StoreMA,
-                                      SmallVectorImpl<MemoryAccess *> &Loads);
-
   /// Build the access relation of all memory accesses of @p Stmt.
   void buildAccessRelations(ScopStmt &Stmt);
 

polly/include/polly/ScopInfo.h

@@ -470,6 +470,8 @@ class MemoryAccess final {
     RT_BOR,  ///< Bitwise Or
     RT_BXOR, ///< Bitwise XOr
     RT_BAND, ///< Bitwise And
+
+    RT_BOTTOM, ///< Pseudo type for the data flow analysis
   };
 
   using SubscriptsTy = SmallVector<const SCEV *, 4>;
@@ -1139,6 +1141,7 @@ class ScopStmt final {
   friend class ScopBuilder;
 
 public:
+  using MemoryAccessVec = llvm::SmallVector<MemoryAccess *, 8>;
   /// Create the ScopStmt from a BasicBlock.
   ScopStmt(Scop &parent, BasicBlock &bb, StringRef Name, Loop *SurroundingLoop,
            std::vector<Instruction *> Instructions);
@@ -1206,7 +1209,6 @@ class ScopStmt final {
   /// The memory accesses of this statement.
   ///
   /// The only side effects of a statement are its memory accesses.
-  using MemoryAccessVec = llvm::SmallVector<MemoryAccess *, 8>;
   MemoryAccessVec MemAccs;
 
   /// Mapping from instructions to (scalar) memory accesses.

polly/lib/Analysis/ScopBuilder.cpp

@@ -2481,8 +2481,8 @@ void ScopBuilder::collectSurroundingLoops(ScopStmt &Stmt) {
 }
 
 /// Return the reduction type for a given binary operator.
-static MemoryAccess::ReductionType getReductionType(const BinaryOperator *BinOp,
-                                                    const Instruction *Load) {
+static MemoryAccess::ReductionType
+getReductionType(const BinaryOperator *BinOp) {
   if (!BinOp)
     return MemoryAccess::RT_NONE;
   switch (BinOp->getOpcode()) {
@@ -2511,6 +2511,17 @@ static MemoryAccess::ReductionType getReductionType(const BinaryOperator *BinOp,
   }
 }
 
+/// @brief Combine two reduction types
+static MemoryAccess::ReductionType
+combineReductionType(MemoryAccess::ReductionType RT0,
+                     MemoryAccess::ReductionType RT1) {
+  if (RT0 == MemoryAccess::RT_BOTTOM)
+    return RT1;
+  if (RT0 == RT1)
+    return RT1;
+  return MemoryAccess::RT_NONE;
+}
+
 ///  True if @p AllAccs intersects with @p MemAccs execpt @p LoadMA and @p
 ///  StoreMA
 bool hasIntersectingAccesses(isl::set AllAccs, MemoryAccess *LoadMA,
@@ -2571,47 +2582,206 @@ bool checkCandidatePairAccesses(MemoryAccess *LoadMA, MemoryAccess *StoreMA,
     AllAccsRel = AllAccsRel.intersect_domain(Domain);
     isl::set AllAccs = AllAccsRel.range();
     Valid = !hasIntersectingAccesses(AllAccs, LoadMA, StoreMA, Domain, MemAccs);
-
     POLLY_DEBUG(dbgs() << " == The accessed memory is " << (Valid ? "not " : "")
                        << "accessed by other instructions!\n");
   }
+
   return Valid;
 }
 
 void ScopBuilder::checkForReductions(ScopStmt &Stmt) {
-  SmallVector<MemoryAccess *, 2> Loads;
-  SmallVector<std::pair<MemoryAccess *, MemoryAccess *>, 4> Candidates;
+  // Perform a data flow analysis on the current scop statement to propagate the
+  // uses of loaded values. Then check and mark the memory accesses which are
+  // part of reduction like chains.
+  // During the data flow analysis we use the State variable to keep track of
+  // the used "load-instructions" for each instruction in the scop statement.
+  // This includes the LLVM-IR of the load and the "number of uses" (or the
+  // number of paths in the operand tree which end in this load).
+  using StatePairTy = std::pair<unsigned, MemoryAccess::ReductionType>;
+  using FlowInSetTy = MapVector<const LoadInst *, StatePairTy>;
+  using StateTy = MapVector<const Instruction *, FlowInSetTy>;
+  StateTy State;
+
+  // Invalid loads are loads which have uses we can't track properly in the
+  // state map. This includes loads which:
+  //   o do not form a reduction when they flow into a memory location:
+  //     (e.g., A[i] = B[i] * 3 and  A[i] = A[i] * A[i] + A[i])
+  //   o are used by a non binary operator or one which is not commutative
+  //     and associative (e.g., A[i] = A[i] % 3)
+  //   o might change the control flow            (e.g., if (A[i]))
+  //   o are used in indirect memory accesses     (e.g., A[B[i]])
+  //   o are used outside the current scop statement
+  SmallPtrSet<const Instruction *, 8> InvalidLoads;
+  SmallVector<BasicBlock *, 8> ScopBlocks;
+  BasicBlock *BB = Stmt.getBasicBlock();
+  if (BB)
+    ScopBlocks.push_back(BB);
+  else
+    for (BasicBlock *Block : Stmt.getRegion()->blocks())
+      ScopBlocks.push_back(Block);
+  // Run the data flow analysis for all values in the scop statement
+  for (BasicBlock *Block : ScopBlocks) {
+    for (Instruction &Inst : *Block) {
+      if ((Stmt.getParent())->getStmtFor(&Inst) != &Stmt)
+        continue;
+      bool UsedOutsideStmt = any_of(Inst.users(), [&Stmt](User *U) {
+        return (Stmt.getParent())->getStmtFor(cast<Instruction>(U)) != &Stmt;
+      });
+      //  Treat loads and stores special
+      if (auto *Load = dyn_cast<LoadInst>(&Inst)) {
+        // Invalidate all loads used which feed into the address of this load.
+        if (auto *Ptr = dyn_cast<Instruction>(Load->getPointerOperand())) {
+          const auto &It = State.find(Ptr);
+          if (It != State.end())
+            for (const auto &FlowInSetElem : It->second)
+              InvalidLoads.insert(FlowInSetElem.first);
+        }
 
-  // First collect candidate load-store reduction chains by iterating over all
-  // stores and collecting possible reduction loads.
-  for (MemoryAccess *StoreMA : Stmt) {
-    if (StoreMA->isRead())
-      continue;
+        // If this load is used outside this stmt, invalidate it.
+        if (UsedOutsideStmt)
+          InvalidLoads.insert(Load);
+
+        // And indicate that this load uses itself once but without specifying
+        // any reduction operator.
+        State[Load].insert(
+            std::make_pair(Load, std::make_pair(1, MemoryAccess::RT_BOTTOM)));
+        continue;
+      }
+
+      if (auto *Store = dyn_cast<StoreInst>(&Inst)) {
+        // Invalidate all loads which feed into the address of this store.
+        if (const Instruction *Ptr =
+                dyn_cast<Instruction>(Store->getPointerOperand())) {
+          const auto &It = State.find(Ptr);
+          if (It != State.end())
+            for (const auto &FlowInSetElem : It->second)
+              InvalidLoads.insert(FlowInSetElem.first);
+        }
+
+        // Propagate the uses of the value operand to the store
+        if (auto *ValueInst = dyn_cast<Instruction>(Store->getValueOperand()))
+          State.insert(std::make_pair(Store, State[ValueInst]));
+        continue;
+      }
+
+      // Non load and store instructions are either binary operators or they
+      // will invalidate all used loads.
+      auto *BinOp = dyn_cast<BinaryOperator>(&Inst);
+      MemoryAccess::ReductionType CurRedType = getReductionType(BinOp);
+      POLLY_DEBUG(dbgs() << "CurInst: " << Inst << " RT: " << CurRedType
+                         << "\n");
+
+      // Iterate over all operands and propagate their input loads to
+      // instruction.
+      FlowInSetTy &InstInFlowSet = State[&Inst];
+      for (Use &Op : Inst.operands()) {
+        auto *OpInst = dyn_cast<Instruction>(Op);
+        if (!OpInst)
+          continue;
+
+        POLLY_DEBUG(dbgs().indent(4) << "Op Inst: " << *OpInst << "\n");
+        const StateTy::iterator &OpInFlowSetIt = State.find(OpInst);
+        if (OpInFlowSetIt == State.end())
+          continue;
+
+        // Iterate over all the input loads of the operand and combine them
+        // with the input loads of current instruction.
+        FlowInSetTy &OpInFlowSet = OpInFlowSetIt->second;
+        for (auto &OpInFlowPair : OpInFlowSet) {
+          unsigned OpFlowIn = OpInFlowPair.second.first;
+          unsigned InstFlowIn = InstInFlowSet[OpInFlowPair.first].first;
+
+          MemoryAccess::ReductionType OpRedType = OpInFlowPair.second.second;
+          MemoryAccess::ReductionType InstRedType =
+              InstInFlowSet[OpInFlowPair.first].second;
+
+          MemoryAccess::ReductionType NewRedType =
+              combineReductionType(OpRedType, CurRedType);
+          if (InstFlowIn)
+            NewRedType = combineReductionType(NewRedType, InstRedType);
+
+          POLLY_DEBUG(dbgs().indent(8) << "OpRedType: " << OpRedType << "\n");
+          POLLY_DEBUG(dbgs().indent(8) << "NewRedType: " << NewRedType << "\n");
+          InstInFlowSet[OpInFlowPair.first] =
+              std::make_pair(OpFlowIn + InstFlowIn, NewRedType);
+        }
+      }
 
-    Loads.clear();
-    collectCandidateReductionLoads(StoreMA, Loads);
-    for (MemoryAccess *LoadMA : Loads)
-      Candidates.push_back(std::make_pair(LoadMA, StoreMA));
+      // If this operation is used outside the stmt, invalidate all the loads
+      // which feed into it.
+      if (UsedOutsideStmt)
+        for (const auto &FlowInSetElem : InstInFlowSet)
+          InvalidLoads.insert(FlowInSetElem.first);
+    }
   }
 
-  // Then check each possible candidate pair.
-  for (const auto &CandidatePair : Candidates) {
-    MemoryAccess *LoadMA = CandidatePair.first;
-    MemoryAccess *StoreMA = CandidatePair.second;
-    bool Valid = checkCandidatePairAccesses(LoadMA, StoreMA, Stmt.getDomain(),
-                                            Stmt.MemAccs);
-    if (!Valid)
+  // All used loads are propagated through the whole basic block; now try to
+  // find valid reduction-like candidate pairs. These load-store pairs fulfill
+  // all reduction like properties with regards to only this load-store chain.
+  // We later have to check if the loaded value was invalidated by an
+  // instruction not in that chain.
+  using MemAccPair = std::pair<MemoryAccess *, MemoryAccess *>;
+  DenseMap<MemAccPair, MemoryAccess::ReductionType> ValidCandidates;
+  DominatorTree *DT = Stmt.getParent()->getDT();
+
+  // Iterate over all write memory accesses and check the loads flowing into
+  // it for reduction candidate pairs.
+  for (MemoryAccess *WriteMA : Stmt.MemAccs) {
+    if (WriteMA->isRead())
+      continue;
+    StoreInst *St = dyn_cast<StoreInst>(WriteMA->getAccessInstruction());
+    if (!St)
       continue;
+    assert(!St->isVolatile());
+
+    FlowInSetTy &MaInFlowSet = State[WriteMA->getAccessInstruction()];
+    for (auto &MaInFlowSetElem : MaInFlowSet) {
+      MemoryAccess *ReadMA = &Stmt.getArrayAccessFor(MaInFlowSetElem.first);
+      assert(ReadMA && "Couldn't find memory access for incoming load!");
 
-    const LoadInst *Load =
-        dyn_cast<const LoadInst>(CandidatePair.first->getAccessInstruction());
-    MemoryAccess::ReductionType RT =
-        getReductionType(dyn_cast<BinaryOperator>(Load->user_back()), Load);
+      POLLY_DEBUG(dbgs() << "'" << *ReadMA->getAccessInstruction()
+                         << "'\n\tflows into\n'"
+                         << *WriteMA->getAccessInstruction() << "'\n\t #"
+                         << MaInFlowSetElem.second.first << " times & RT: "
+                         << MaInFlowSetElem.second.second << "\n");
 
-    // If no overlapping access was found we mark the load and store as
-    // reduction like.
-    LoadMA->markAsReductionLike(RT);
-    StoreMA->markAsReductionLike(RT);
+      MemoryAccess::ReductionType RT = MaInFlowSetElem.second.second;
+      unsigned NumAllowableInFlow = 1;
+
+      // We allow the load to flow in exactly once for binary reductions
+      bool Valid = (MaInFlowSetElem.second.first == NumAllowableInFlow);
+
+      // Check if we saw a valid chain of binary operators.
+      Valid = Valid && RT != MemoryAccess::RT_BOTTOM;
+      Valid = Valid && RT != MemoryAccess::RT_NONE;
+
+      // Then check if the memory accesses allow a reduction.
+      Valid = Valid && checkCandidatePairAccesses(
+                           ReadMA, WriteMA, Stmt.getDomain(), Stmt.MemAccs);
+
+      // Finally, mark the pair as a candidate or the load as a invalid one.
+      if (Valid)
+        ValidCandidates[std::make_pair(ReadMA, WriteMA)] = RT;
+      else
+        InvalidLoads.insert(ReadMA->getAccessInstruction());
+    }
+  }
+
+  // In the last step mark the memory accesses of candidate pairs as reduction
+  // like if the load wasn't marked invalid in the previous step.
+  for (auto &CandidatePair : ValidCandidates) {
+    MemoryAccess *LoadMA = CandidatePair.first.first;
+    if (InvalidLoads.count(LoadMA->getAccessInstruction()))
+      continue;
+    POLLY_DEBUG(
+        dbgs() << " Load :: "
+               << *((CandidatePair.first.first)->getAccessInstruction())
+               << "\n Store :: "
+               << *((CandidatePair.first.second)->getAccessInstruction())
+               << "\n are marked as reduction like\n");
+    MemoryAccess::ReductionType RT = CandidatePair.second;
+    CandidatePair.first.first->markAsReductionLike(RT);
+    CandidatePair.first.second->markAsReductionLike(RT);
   }
 }
 
@@ -2965,52 +3135,6 @@ void ScopBuilder::addInvariantLoads(ScopStmt &Stmt,
   }
 }
 
-void ScopBuilder::collectCandidateReductionLoads(
-    MemoryAccess *StoreMA, SmallVectorImpl<MemoryAccess *> &Loads) {
-  ScopStmt *Stmt = StoreMA->getStatement();
-
-  auto *Store = dyn_cast<StoreInst>(StoreMA->getAccessInstruction());
-  if (!Store)
-    return;
-
-  // Skip if there is not one binary operator between the load and the store
-  auto *BinOp = dyn_cast<BinaryOperator>(Store->getValueOperand());
-  if (!BinOp)
-    return;
-
-  // Skip if the binary operators has multiple uses
-  if (BinOp->getNumUses() != 1)
-    return;
-
-  // Skip if the opcode of the binary operator is not commutative/associative
-  if (!BinOp->isCommutative() || !BinOp->isAssociative())
-    return;
-
-  // Skip if the binary operator is outside the current SCoP
-  if (BinOp->getParent() != Store->getParent())
-    return;
-
-  // Skip if it is a multiplicative reduction and we disabled them
-  if (DisableMultiplicativeReductions &&
-      (BinOp->getOpcode() == Instruction::Mul ||
-       BinOp->getOpcode() == Instruction::FMul))
-    return;
-
-  // Check the binary operator operands for a candidate load
-  auto *PossibleLoad0 = dyn_cast<LoadInst>(BinOp->getOperand(0));
-  auto *PossibleLoad1 = dyn_cast<LoadInst>(BinOp->getOperand(1));
-  if (!PossibleLoad0 && !PossibleLoad1)
-    return;
-
-  // A load is only a candidate if it cannot escape (thus has only this use)
-  if (PossibleLoad0 && PossibleLoad0->getNumUses() == 1)
-    if (PossibleLoad0->getParent() == Store->getParent())
-      Loads.push_back(&Stmt->getArrayAccessFor(PossibleLoad0));
-  if (PossibleLoad1 && PossibleLoad1->getNumUses() == 1)
-    if (PossibleLoad1->getParent() == Store->getParent())
-      Loads.push_back(&Stmt->getArrayAccessFor(PossibleLoad1));
-}
-
 /// Find the canonical scop array info object for a set of invariant load
 /// hoisted loads. The canonical array is the one that corresponds to the
 /// first load in the list of accesses which is used as base pointer of a

polly/lib/Analysis/ScopInfo.cpp

@@ -533,6 +533,9 @@ MemoryAccess::getReductionOperatorStr(MemoryAccess::ReductionType RT) {
   case MemoryAccess::RT_NONE:
     llvm_unreachable("Requested a reduction operator string for a memory "
                      "access which isn't a reduction");
+  case MemoryAccess::RT_BOTTOM:
+    llvm_unreachable("Requested a reduction operator string for a internal "
+                     "reduction type!");
   case MemoryAccess::RT_ADD:
     return "+";
   case MemoryAccess::RT_MUL:
@@ -915,10 +918,15 @@ isl::id MemoryAccess::getId() const { return Id; }
 
 raw_ostream &polly::operator<<(raw_ostream &OS,
                                MemoryAccess::ReductionType RT) {
-  if (RT == MemoryAccess::RT_NONE)
+  switch (RT) {
+  case MemoryAccess::RT_NONE:
+  case MemoryAccess::RT_BOTTOM:
     OS << "NONE";
-  else
+    break;
+  default:
     OS << MemoryAccess::getReductionOperatorStr(RT);
+    break;
+  }
   return OS;
 }