Make the reassociation pass more powerful so that it can handle expressions

with arbitrary topologies (previously it would give up when hitting a diamond
in the use graph for example).  The testcase from PR12764 is now reduced from
a pile of additions to the optimal 1617*%x0+208.  In doing this I changed the
previous strategy of dropping all uses for expression leaves to one of dropping
all but one use.  This works out more neatly (but required a bunch of tweaks)
and is also safer: some recently fixed bugs during recursive linearization were
because the linearization code thinks it completely owns a node if it has no uses
outside the expression it is linearizing.  But if the node was also in another
expression that had been linearized (and thus all uses of the node from that
expression dropped) then the conclusion that it is completely owned by the
expression currently being linearized is wrong.  Keeping one use from within each
linearized expression avoids this kind of mistake.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@157467 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 405 insertions, 255 deletions

diff --git a/lib/Transforms/Scalar/Reassociate.cpp b/lib/Transforms/Scalar/Reassociate.cpp
index 6ef0c97d37..91a16c0d63 100644
--- a/lib/Transforms/Scalar/Reassociate.cpp
+++ b/lib/Transforms/Scalar/Reassociate.cpp
@@ -35,14 +35,14 @@
 #include "llvm/Support/Debug.h"
 #include "llvm/Support/ValueHandle.h"
 #include "llvm/Support/raw_ostream.h"
+#include "llvm/ADT/DenseMap.h"
 #include "llvm/ADT/PostOrderIterator.h"
+#include "llvm/ADT/SmallMap.h"
 #include "llvm/ADT/STLExtras.h"
 #include "llvm/ADT/Statistic.h"
-#include "llvm/ADT/DenseMap.h"
 #include <algorithm>
 using namespace llvm;
 
-STATISTIC(NumLinear , "Number of insts linearized");
 STATISTIC(NumChanged, "Number of insts reassociated");
 STATISTIC(NumAnnihil, "Number of expr tree annihilated");
 STATISTIC(NumFactor , "Number of multiplies factored");
@@ -134,8 +134,7 @@ namespace {
     void BuildRankMap(Function &F);
     unsigned getRank(Value *V);
     Value *ReassociateExpression(BinaryOperator *I);
-    void RewriteExprTree(BinaryOperator *I, SmallVectorImpl<ValueEntry> &Ops,
-                         unsigned Idx = 0);
+    void RewriteExprTree(BinaryOperator *I, SmallVectorImpl<ValueEntry> &Ops);
     Value *OptimizeExpression(BinaryOperator *I,
                               SmallVectorImpl<ValueEntry> &Ops);
     Value *OptimizeAdd(Instruction *I, SmallVectorImpl<ValueEntry> &Ops);
@@ -145,7 +144,6 @@ namespace {
                                    SmallVectorImpl<Factor> &Factors);
     Value *OptimizeMul(BinaryOperator *I, SmallVectorImpl<ValueEntry> &Ops);
     void LinearizeExprTree(BinaryOperator *I, SmallVectorImpl<ValueEntry> &Ops);
-    void LinearizeExpr(BinaryOperator *I);
     Value *RemoveFactorFromExpression(Value *V, Value *Factor);
     void ReassociateInst(BasicBlock::iterator &BBI);
 
@@ -160,17 +158,32 @@ INITIALIZE_PASS(Reassociate, "reassociate",
 // Public interface to the Reassociate pass
 FunctionPass *llvm::createReassociatePass() { return new Reassociate(); }
 
+/// isReassociableOp - Return true if V is an instruction of the specified
+/// opcode and if it only has one use.
+static BinaryOperator *isReassociableOp(Value *V, unsigned Opcode) {
+  if (V->hasOneUse() && isa<Instruction>(V) &&
+      cast<Instruction>(V)->getOpcode() == Opcode)
+    return cast<BinaryOperator>(V);
+  return 0;
+}
+
 void Reassociate::RemoveDeadBinaryOp(Value *V) {
-  Instruction *Op = dyn_cast<Instruction>(V);
-  if (!Op || !isa<BinaryOperator>(Op))
+  BinaryOperator *Op = dyn_cast<BinaryOperator>(V);
+  if (!Op)
     return;
 
-  Value *LHS = Op->getOperand(0), *RHS = Op->getOperand(1);
-
   ValueRankMap.erase(Op);
   DeadInsts.push_back(Op);
-  RemoveDeadBinaryOp(LHS);
-  RemoveDeadBinaryOp(RHS);
+
+  BinaryOperator *LHS = isReassociableOp(Op->getOperand(0), Op->getOpcode());
+  BinaryOperator *RHS = isReassociableOp(Op->getOperand(1), Op->getOpcode());
+  Op->setOperand(0, UndefValue::get(Op->getType()));
+  Op->setOperand(1, UndefValue::get(Op->getType()));
+
+  if (LHS)
+    RemoveDeadBinaryOp(LHS);
+  if (RHS)
+    RemoveDeadBinaryOp(RHS);
 }
 
 static bool isUnmovableInstruction(Instruction *I) {
@@ -244,22 +257,14 @@ unsigned Reassociate::getRank(Value *V) {
   return ValueRankMap[I] = Rank;
 }
 
-/// isReassociableOp - Return true if V is an instruction of the specified
-/// opcode and if it only has one use.
-static BinaryOperator *isReassociableOp(Value *V, unsigned Opcode) {
-  if ((V->hasOneUse() || V->use_empty()) && isa<Instruction>(V) &&
-      cast<Instruction>(V)->getOpcode() == Opcode)
-    return cast<BinaryOperator>(V);
-  return 0;
-}
-
 /// LowerNegateToMultiply - Replace 0-X with X*-1.
 ///
-static Instruction *LowerNegateToMultiply(Instruction *Neg,
+static BinaryOperator *LowerNegateToMultiply(Instruction *Neg,
                          DenseMap<AssertingVH<Value>, unsigned> &ValueRankMap) {
   Constant *Cst = Constant::getAllOnesValue(Neg->getType());
 
-  Instruction *Res = BinaryOperator::CreateMul(Neg->getOperand(1), Cst, "",Neg);
+  BinaryOperator *Res =
+    BinaryOperator::CreateMul(Neg->getOperand(1), Cst, "",Neg);
   ValueRankMap.erase(Neg);
   Res->takeName(Neg);
   Neg->replaceAllUsesWith(Res);
@@ -268,174 +273,355 @@ static Instruction *LowerNegateToMultiply(Instruction *Neg,
   return Res;
 }
 
-// Given an expression of the form '(A+B)+(D+C)', turn it into '(((A+B)+C)+D)'.
-// Note that if D is also part of the expression tree that we recurse to
-// linearize it as well.  Besides that case, this does not recurse into A,B, or
-// C.
-void Reassociate::LinearizeExpr(BinaryOperator *I) {
-  BinaryOperator *LHS = isReassociableOp(I->getOperand(0), I->getOpcode());
-  BinaryOperator *RHS = isReassociableOp(I->getOperand(1), I->getOpcode());
-  assert(LHS && RHS && "Not an expression that needs linearization?");
-
-  DEBUG({
-      dbgs() << "Linear:\n";
-      dbgs() << '\t' << *LHS << "\t\n" << *RHS << "\t\n" << *I << '\n';
-    });
-
-  // Move the RHS instruction to live immediately before I, avoiding breaking
-  // dominator properties.
-  RHS->moveBefore(I);
-
-  // Move operands around to do the linearization.
-  I->setOperand(1, RHS->getOperand(0));
-  RHS->setOperand(0, LHS);
-  I->setOperand(0, RHS);
-
-  // Conservatively clear all the optional flags, which may not hold
-  // after the reassociation.
-  I->clearSubclassOptionalData();
-  LHS->clearSubclassOptionalData();
-  RHS->clearSubclassOptionalData();
-
-  ++NumLinear;
-  MadeChange = true;
-  DEBUG(dbgs() << "Linearized: " << *I << '\n');
-
-  // If D is part of this expression tree, tail recurse.
-  if (isReassociableOp(I->getOperand(1), I->getOpcode()))
-    LinearizeExpr(I);
-}
-
-/// LinearizeExprTree - Given an associative binary expression tree, traverse
-/// all of the uses putting it into canonical form.  This forces a left-linear
-/// form of the expression (((a+b)+c)+d), and collects information about the
-/// rank of the non-tree operands.
+/// LinearizeExprTree - Given an associative binary expression, return the leaf
+/// nodes in Ops.  The original expression is the same as Ops[0] op ... Ops[N].
+/// Note that a node may occur multiple times in Ops, but if so all occurrences
+/// are consecutive in the vector.
+///
+/// A leaf node is either not a binary operation of the same kind as the root
+/// node 'I' (i.e. is not a binary operator at all, or is, but with a different
+/// opcode), or is the same kind of binary operator but has a use which either
+/// does not belong to the expression, or does belong to the expression but is
+/// a leaf node.  Every leaf node has at least one use that is a non-leaf node
+/// of the expression, while for non-leaf nodes (except for the root 'I') every
+/// use is a non-leaf node of the expression.
+///
+/// For example:
+///           expression graph        node names
+///
+///                     +        |        I
+///                    / \       |
+///                   +   +      |      A,  B
+///                  / \ / \     |
+///                 *   +   *    |    C,  D,  E
+///                / \ / \ / \   |
+///                   +   *      |      F,  G
+///
+/// The leaf nodes are C, E, F and G.  The Ops array will contain (maybe not in
+/// that order) C, E, F, F, G, G.
+///
+/// The expression is maximal: if some instruction is a binary operator of the
+/// same kind as 'I', and all of its uses are non-leaf nodes of the expression,
+/// then the instruction also belongs to the expression, is not a leaf node of
+/// it, and its operands also belong to the expression (but may be leaf nodes).
 ///
-/// NOTE: These intentionally destroys the expression tree operands (turning
-/// them into undef values) to reduce #uses of the values.  This means that the
+/// NOTE: This routine will set operands of non-leaf non-root nodes to undef in
+/// order to ensure that every non-root node in the expression has *exactly one*
+/// use by a non-leaf node of the expression.  This destruction means that the
 /// caller MUST use something like RewriteExprTree to put the values back in.
 ///
+/// In the above example either the right operand of A or the left operand of B
+/// will be replaced by undef.  If it is B's operand then this gives:
+///
+///                     +        |        I
+///                    / \       |
+///                   +   +      |      A,  B - operand of B replaced with undef
+///                  / \   \     |
+///                 *   +   *    |    C,  D,  E
+///                / \ / \ / \   |
+///                   +   *      |      F,  G
+///
+/// Note that if you visit operands recursively starting from a leaf node then
+/// you will never encounter such an undef operand unless you get back to 'I',
+/// which requires passing through a phi node.
+///
+/// Note that this routine may also mutate binary operators of the wrong type
+/// that have all uses inside the expression (i.e. only used by non-leaf nodes
+/// of the expression) if it can turn them into binary operators of the right
+/// type and thus make the expression bigger.
+
 void Reassociate::LinearizeExprTree(BinaryOperator *I,
                                     SmallVectorImpl<ValueEntry> &Ops) {
-  Value *LHS = I->getOperand(0), *RHS = I->getOperand(1);
+  DEBUG(dbgs() << "LINEARIZE: " << *I << '\n');
+
+  // Visit all operands of the expression, keeping track of their weight (the
+  // number of paths from the expression root to the operand, or if you like
+  // the number of times that operand occurs in the linearized expression).
+  // For example, if I = X + A, where X = A + B, then I, X and B have weight 1
+  // while A has weight two.
+
+  // Worklist of non-leaf nodes (their operands are in the expression too) along
+  // with their weights, representing a certain number of paths to the operator.
+  // If an operator occurs in the worklist multiple times then we found multiple
+  // ways to get to it.
+  SmallVector<std::pair<BinaryOperator*, unsigned>, 8> Worklist; // (Op, Weight)
+  Worklist.push_back(std::make_pair(I, 1));
   unsigned Opcode = I->getOpcode();
 
-  // First step, linearize the expression if it is in ((A+B)+(C+D)) form.
-  BinaryOperator *LHSBO = isReassociableOp(LHS, Opcode);
-  BinaryOperator *RHSBO = isReassociableOp(RHS, Opcode);
-
-  // If this is a multiply expression tree and it contains internal negations,
-  // transform them into multiplies by -1 so they can be reassociated.
-  if (I->getOpcode() == Instruction::Mul) {
-    if (!LHSBO && LHS->hasOneUse() && BinaryOperator::isNeg(LHS)) {
-      LHS = LowerNegateToMultiply(cast<Instruction>(LHS), ValueRankMap);
-      LHSBO = isReassociableOp(LHS, Opcode);
-    }
-    if (!RHSBO && RHS->hasOneUse() && BinaryOperator::isNeg(RHS)) {
-      RHS = LowerNegateToMultiply(cast<Instruction>(RHS), ValueRankMap);
-      RHSBO = isReassociableOp(RHS, Opcode);
-    }
-  }
+  // Leaves of the expression are values that either aren't the right kind of
+  // operation (eg: a constant, or a multiply in an add tree), or are, but have
+  // some uses that are not inside the expression.  For example, in I = X + X,
+  // X = A + B, the value X has two uses (by I) that are in the expression.  If
+  // X has any other uses, for example in a return instruction, then we consider
+  // X to be a leaf, and won't analyze it further.  When we first visit a value,
+  // if it has more than one use then at first we conservatively consider it to
+  // be a leaf.  Later, as the expression is explored, we may discover some more
+  // uses of the value from inside the expression.  If all uses turn out to be
+  // from within the expression (and the value is a binary operator of the right
+  // kind) then the value is no longer considered to be a leaf, and its operands
+  // are explored.
+
+  // Leaves - Keeps track of the set of putative leaves as well as the number of
+  // paths to each leaf seen so far.
+  typedef SmallMap<Value*, unsigned, 8> LeafMap;
+  LeafMap Leaves; // Leaf -> Total weight so far.
+  SmallVector<Value*, 8> LeafOrder; // Ensure deterministic leaf output order.
 
-  if (!LHSBO) {
-    if (!RHSBO) {
-      // Neither the LHS or RHS as part of the tree, thus this is a leaf.  As
-      // such, just remember these operands and their rank.
-      Ops.push_back(ValueEntry(getRank(LHS), LHS));
-      Ops.push_back(ValueEntry(getRank(RHS), RHS));
+#ifndef NDEBUG
+  SmallPtrSet<Value*, 8> Visited; // For sanity checking the iteration scheme.
+#endif
+  while (!Worklist.empty()) {
+    std::pair<BinaryOperator*, unsigned> P = Worklist.pop_back_val();
+    I = P.first; // We examine the operands of this binary operator.
+    assert(P.second >= 1 && "No paths to here, so how did we get here?!");
+
+    for (unsigned OpIdx = 0; OpIdx < 2; ++OpIdx) { // Visit operands.
+      Value *Op = I->getOperand(OpIdx);
+      unsigned Weight = P.second; // Number of paths to this operand.
+      DEBUG(dbgs() << "OPERAND: " << *Op << " (" << Weight << ")\n");
+      assert(!Op->use_empty() && "No uses, so how did we get to it?!");
+
+      // If this is a binary operation of the right kind with only one use then
+      // add its operands to the expression.
+      if (BinaryOperator *BO = isReassociableOp(Op, Opcode)) {
+        assert(Visited.insert(Op) && "Not first visit!");
+        DEBUG(dbgs() << "DIRECT ADD: " << *Op << " (" << Weight << ")\n");
+        Worklist.push_back(std::make_pair(BO, Weight));
+        continue;
+      }
 
-      // Clear the leaves out.
-      I->setOperand(0, UndefValue::get(I->getType()));
-      I->setOperand(1, UndefValue::get(I->getType()));
-      return;
-    }
+      // Appears to be a leaf.  Is the operand already in the set of leaves?
+      LeafMap::iterator It = Leaves.find(Op);
+      if (It == Leaves.end()) {
+        // Not in the leaf map.  Must be the first time we saw this operand.
+        assert(Visited.insert(Op) && "Not first visit!");
+        if (!Op->hasOneUse()) {
+          // This value has uses not accounted for by the expression, so it is
+          // not safe to modify.  Mark it as being a leaf.
+          DEBUG(dbgs() << "ADD USES LEAF: " << *Op << " (" << Weight << ")\n");
+          LeafOrder.push_back(Op);
+          Leaves[Op] = Weight;
+          continue;
+        }
+        // No uses outside the expression, try morphing it.
+      } else if (It != Leaves.end()) {
+        // Already in the leaf map.
+        assert(Visited.count(Op) && "In leaf map but not visited!");
+
+        // Update the number of paths to the leaf.
+        It->second += Weight;
+
+        // The leaf already has one use from inside the expression.  As we want
+        // exactly one such use, drop this new use of the leaf.
+        assert(!Op->hasOneUse() && "Only one use, but we got here twice!");
+        I->setOperand(OpIdx, UndefValue::get(I->getType()));
+        MadeChange = true;
 
-    // Turn X+(Y+Z) -> (Y+Z)+X
-    std::swap(LHSBO, RHSBO);
-    std::swap(LHS, RHS);
-    bool Success = !I->swapOperands();
-    assert(Success && "swapOperands failed");
-    (void)Success;
-    MadeChange = true;
-  } else if (RHSBO) {
-    // Turn (A+B)+(C+D) -> (((A+B)+C)+D).  This guarantees the RHS is not
-    // part of the expression tree.
-    LinearizeExpr(I);
-    LHS = LHSBO = cast<BinaryOperator>(I->getOperand(0));
-    RHS = I->getOperand(1);
-    RHSBO = 0;
-  }
+        // If the leaf is a binary operation of the right kind and we now see
+        // that its multiple original uses were in fact all by nodes belonging
+        // to the expression, then no longer consider it to be a leaf and add
+        // its operands to the expression.
+        if (BinaryOperator *BO = isReassociableOp(Op, Opcode)) {
+          DEBUG(dbgs() << "UNLEAF: " << *Op << " (" << It->second << ")\n");
+          Worklist.push_back(std::make_pair(BO, It->second));
+          Leaves.erase(It);
+          continue;
+        }
 
-  // Okay, now we know that the LHS is a nested expression and that the RHS is
-  // not.  Perform reassociation.
-  assert(!isReassociableOp(RHS, Opcode) && "LinearizeExpr failed!");
+        // If we still have uses that are not accounted for by the expression
+        // then it is not safe to modify the value.
+        if (!Op->hasOneUse())
+          continue;
 
-  // Move LHS right before I to make sure that the tree expression dominates all
-  // values.
-  LHSBO->moveBefore(I);
+        // No uses outside the expression, try morphing it.
+        Weight = It->second;
+        Leaves.erase(It); // Since the value may be morphed below.
+      }
 
-  // Linearize the expression tree on the LHS.
-  LinearizeExprTree(LHSBO, Ops);
+      // At this point we have a value which, first of all, is not a binary
+      // expression of the right kind, and secondly, is only used inside the
+      // expression.  This means that it can safely be modified.  See if we
+      // can usefully morph it into an expression of the right kind.
+      assert((!isa<Instruction>(Op) ||
+              cast<Instruction>(Op)->getOpcode() != Opcode) &&
+             "Should have been handled above!");
+      assert(Op->hasOneUse() && "Has uses outside the expression tree!");
+
+      // If this is a multiply expression, turn any internal negations into
+      // multiplies by -1 so they can be reassociated.
+      BinaryOperator *BO = dyn_cast<BinaryOperator>(Op);
+      if (Opcode == Instruction::Mul && BO && BinaryOperator::isNeg(BO)) {
+        DEBUG(dbgs() << "MORPH LEAF: " << *Op << " (" << Weight << ") TO ");
+        BO = LowerNegateToMultiply(BO, ValueRankMap);
+        DEBUG(dbgs() << *BO << 'n');
+        Worklist.push_back(std::make_pair(BO, Weight));
+        MadeChange = true;
+        continue;
+      }
 
-  // Remember the RHS operand and its rank.
-  Ops.push_back(ValueEntry(getRank(RHS), RHS));
+      // Failed to morph into an expression of the right type.  This really is
+      // a leaf.
+      DEBUG(dbgs() << "ADD LEAF: " << *Op << " (" << Weight << ")\n");
+      assert(!isReassociableOp(Op, Opcode) && "Value was morphed?");
+      LeafOrder.push_back(Op);
+      Leaves[Op] = Weight;
+    }
+  }
 
-  // Clear the RHS leaf out.
-  I->setOperand(1, UndefValue::get(I->getType()));
+  // The leaves, repeated according to their weights, represent the linearized
+  // form of the expression.
+  for (unsigned i = 0, e = LeafOrder.size(); i != e; ++i) {
+    Value *V = LeafOrder[i];
+    LeafMap::iterator It = Leaves.find(V);
+    if (It == Leaves.end())
+      // Leaf already output, or node initially thought to be a leaf wasn't.
+      continue;
+    assert(!isReassociableOp(V, Opcode) && "Shouldn't be a leaf!");
+    unsigned Weight = It->second;
+    assert(Weight > 0 && "No paths to this value!");
+    // FIXME: Rather than repeating values Weight times, use a vector of
+    // (ValueEntry, multiplicity) pairs.
+    Ops.append(Weight, ValueEntry(getRank(V), V));
+    // Ensure the leaf is only output once.
+    Leaves.erase(It);
+  }
 }
 
 // RewriteExprTree - Now that the operands for this expression tree are
-// linearized and optimized, emit them in-order.  This function is written to be
-// tail recursive.
+// linearized and optimized, emit them in-order.
 void Reassociate::RewriteExprTree(BinaryOperator *I,
-                                  SmallVectorImpl<ValueEntry> &Ops,
-                                  unsigned i) {
-  if (i+2 == Ops.size()) {
-    if (I->getOperand(0) != Ops[i].Op ||
-        I->getOperand(1) != Ops[i+1].Op) {
-      Value *OldLHS = I->getOperand(0);
-      DEBUG(dbgs() << "RA: " << *I << '\n');
-      I->setOperand(0, Ops[i].Op);
-      I->setOperand(1, Ops[i+1].Op);
-
-      // Clear all the optional flags, which may not hold after the
-      // reassociation if the expression involved more than just this operation.
-      if (Ops.size() != 2)
-        I->clearSubclassOptionalData();
-
-      DEBUG(dbgs() << "TO: " << *I << '\n');
+                                  SmallVectorImpl<ValueEntry> &Ops) {
+  assert(Ops.size() > 1 && "Single values should be used directly!");
+
+  // Since our optimizations never increase the number of operations, the new
+  // expression can always be written by reusing the existing binary operators
+  // from the original expression tree, without creating any new instructions,
+  // though the rewritten expression may have a completely different topology.
+  // We take care to not change anything if the new expression will be the same
+  // as the original.  If more than trivial changes (like commuting operands)
+  // were made then we are obliged to clear out any optional subclass data like
+  // nsw flags.
+
+  /// NodesToRewrite - Nodes from the original expression available for writing
+  /// the new expression into.
+  SmallVector<BinaryOperator*, 8> NodesToRewrite;
+  unsigned Opcode = I->getOpcode();
+  NodesToRewrite.push_back(I);
+
+  // ExpressionChanged - Whether the rewritten expression differs non-trivially
+  // from the original, requiring the clearing of all optional flags.
+  bool ExpressionChanged = false;
+  BinaryOperator *Previous;
+  BinaryOperator *Op = 0;
+  for (unsigned i = 0, e = Ops.size(); i != e; ++i) {
+    assert(!NodesToRewrite.empty() &&
+           "Optimized expressions has more nodes than original!");
+    Previous = Op; Op = NodesToRewrite.pop_back_val();
+    // Compactify the tree instructions together with each other to guarantee
+    // that the expression tree is dominated by all of Ops.
+    if (Previous)
+      Op->moveBefore(Previous);
+
+    // The last operation (which comes earliest in the IR) is special as both
+    // operands will come from Ops, rather than just one with the other being
+    // a subexpression.
+    if (i+2 == Ops.size()) {
+      Value *NewLHS = Ops[i].Op;
+      Value *NewRHS = Ops[i+1].Op;
+      Value *OldLHS = Op->getOperand(0);
+      Value *OldRHS = Op->getOperand(1);
+
+      if (NewLHS == OldLHS && NewRHS == OldRHS)
+        // Nothing changed, leave it alone.
+        break;
+
+      if (NewLHS == OldRHS && NewRHS == OldLHS) {
+        // The order of the operands was reversed.  Swap them.
+        DEBUG(dbgs() << "RA: " << *Op << '\n');
+        Op->swapOperands();
+        DEBUG(dbgs() << "TO: " << *Op << '\n');
+        MadeChange = true;
+        ++NumChanged;
+        break;
+      }
+
+      // The new operation differs non-trivially from the original. Overwrite
+      // the old operands with the new ones.
+      DEBUG(dbgs() << "RA: " << *Op << '\n');
+      if (NewLHS != OldLHS) {
+        if (BinaryOperator *BO = isReassociableOp(OldLHS, Opcode))
+          NodesToRewrite.push_back(BO);
+        Op->setOperand(0, NewLHS);
+      }
+      if (NewRHS != OldRHS) {
+        if (BinaryOperator *BO = isReassociableOp(OldRHS, Opcode))
+          NodesToRewrite.push_back(BO);
+        Op->setOperand(1, NewRHS);
+      }
+      DEBUG(dbgs() << "TO: " << *Op << '\n');
+
+      ExpressionChanged = true;
       MadeChange = true;
       ++NumChanged;
 
-      // If we reassociated a tree to fewer operands (e.g. (1+a+2) -> (a+3)
-      // delete the extra, now dead, nodes.
-      RemoveDeadBinaryOp(OldLHS);
+      break;
     }
-    return;
-  }
-  assert(i+2 < Ops.size() && "Ops index out of range!");
 
-  if (I->getOperand(1) != Ops[i].Op) {
-    DEBUG(dbgs() << "RA: " << *I << '\n');
-    I->setOperand(1, Ops[i].Op);
+    // Not the last operation.  The left-hand side will be a sub-expression
+    // while the right-hand side will be the current element of Ops.
+    Value *NewRHS = Ops[i].Op;
+    if (NewRHS != Op->getOperand(1)) {
+      DEBUG(dbgs() << "RA: " << *Op << '\n');
+      if (NewRHS == Op->getOperand(0)) {
+        // The new right-hand side was already present as the left operand.  If
+        // we are lucky then swapping the operands will sort out both of them.
+        Op->swapOperands();
+      } else {
+        // Overwrite with the new right-hand side.
+        if (BinaryOperator *BO = isReassociableOp(Op->getOperand(1), Opcode))
+          NodesToRewrite.push_back(BO);
+        Op->setOperand(1, NewRHS);
+        ExpressionChanged = true;
+      }
+      DEBUG(dbgs() << "TO: " << *Op << '\n');
+      MadeChange = true;
+      ++NumChanged;
+    }
 
-    // Conservatively clear all the optional flags, which may not hold
-    // after the reassociation.
-    I->clearSubclassOptionalData();
+    // Now deal with the left-hand side.  If this is already an operation node
+    // from the original expression then just rewrite the rest of the expression
+    // into it.
+    if (BinaryOperator *BO = isReassociableOp(Op->getOperand(0), Opcode)) {
+      NodesToRewrite.push_back(BO);
+      continue;
+    }
 
-    DEBUG(dbgs() << "TO: " << *I << '\n');
+    // Otherwise, grab a spare node from the original expression and use that as
+    // the left-hand side.
+    assert(!NodesToRewrite.empty() &&
+           "Optimized expressions has more nodes than original!");
+    DEBUG(dbgs() << "RA: " << *Op << '\n');
+    Op->setOperand(0, NodesToRewrite.back());
+    DEBUG(dbgs() << "TO: " << *Op << '\n');
+    ExpressionChanged = true;
     MadeChange = true;
     ++NumChanged;
   }
 
-  BinaryOperator *LHS = cast<BinaryOperator>(I->getOperand(0));
-  assert(LHS->getOpcode() == I->getOpcode() &&
-         "Improper expression tree!");
+  // If the expression changed non-trivially then clear out all subclass data in
+  // the entire rewritten expression.
+  if (ExpressionChanged) {
+    do {
+      Op->clearSubclassOptionalData();
+      if (Op == I)
+        break;
+      Op = cast<BinaryOperator>(*Op->use_begin());
+    } while (1);
+  }
 
-  // Compactify the tree instructions together with each other to guarantee
-  // that the expression tree is dominated by all of Ops.
-  LHS->moveBefore(I);
-  RewriteExprTree(LHS, Ops, i+1);
+  // Throw away any left over nodes from the original expression.
+  for (unsigned i = 0, e = NodesToRewrite.size(); i != e; ++i)
+    RemoveDeadBinaryOp(NodesToRewrite[i]);
 }
 
 /// NegateValue - Insert instructions before the instruction pointed to by BI,
@@ -455,21 +641,20 @@ static Value *NegateValue(Value *V, Instruction *BI) {
   // the constants.  We assume that instcombine will clean up the mess later if
   // we introduce tons of unnecessary negation instructions.
   //
-  if (Instruction *I = dyn_cast<Instruction>(V))
-    if (I->getOpcode() == Instruction::Add && I->hasOneUse()) {
-      // Push the negates through the add.
-      I->setOperand(0, NegateValue(I->getOperand(0), BI));
-      I->setOperand(1, NegateValue(I->getOperand(1), BI));
-
-      // We must move the add instruction here, because the neg instructions do
-      // not dominate the old add instruction in general.  By moving it, we are
-      // assured that the neg instructions we just inserted dominate the
-      // instruction we are about to insert after them.
-      //
-      I->moveBefore(BI);
-      I->setName(I->getName()+".neg");
-      return I;
-    }
+  if (BinaryOperator *I = isReassociableOp(V, Instruction::Add)) {
+    // Push the negates through the add.
+    I->setOperand(0, NegateValue(I->getOperand(0), BI));
+    I->setOperand(1, NegateValue(I->getOperand(1), BI));
+
+    // We must move the add instruction here, because the neg instructions do
+    // not dominate the old add instruction in general.  By moving it, we are
+    // assured that the neg instructions we just inserted dominate the
+    // instruction we are about to insert after them.
+    //
+    I->moveBefore(BI);
+    I->setName(I->getName()+".neg");
+    return I;
+  }
 
   // Okay, we need to materialize a negated version of V with an instruction.
   // Scan the use lists of V to see if we have one already.
@@ -653,8 +838,7 @@ Value *Reassociate::RemoveFactorFromExpression(Value *V, Value *Factor) {
   // If this was just a single multiply, remove the multiply and return the only
   // remaining operand.
   if (Factors.size() == 1) {
-    ValueRankMap.erase(BO);
-    DeadInsts.push_back(BO);
+    RemoveDeadBinaryOp(BO);
     V = Factors[0].Op;
   } else {
     RewriteExprTree(BO, Factors);
@@ -673,31 +857,16 @@ Value *Reassociate::RemoveFactorFromExpression(Value *V, Value *Factor) {
 /// Ops is the top-level list of add operands we're trying to factor.
 static void FindSingleUseMultiplyFactors(Value *V,
                                          SmallVectorImpl<Value*> &Factors,
-                                       const SmallVectorImpl<ValueEntry> &Ops,
-                                         bool IsRoot) {
-  BinaryOperator *BO;
-  if (!(V->hasOneUse() || V->use_empty()) || // More than one use.
-      !(BO = dyn_cast<BinaryOperator>(V)) ||
-      BO->getOpcode() != Instruction::Mul) {
+                                       const SmallVectorImpl<ValueEntry> &Ops) {
+  BinaryOperator *BO = isReassociableOp(V, Instruction::Mul);
+  if (!BO) {
     Factors.push_back(V);
     return;
   }
 
-  // If this value has a single use because it is another input to the add
-  // tree we're reassociating and we dropped its use, it actually has two
-  // uses and we can't factor it.
-  if (!IsRoot) {
-    for (unsigned i = 0, e = Ops.size(); i != e; ++i)
-      if (Ops[i].Op == V) {
-        Factors.push_back(V);
-        return;
-      }
-  }
-
-
   // Otherwise, add the LHS and RHS to the list of factors.
-  FindSingleUseMultiplyFactors(BO->getOperand(1), Factors, Ops, false);
-  FindSingleUseMultiplyFactors(BO->getOperand(0), Factors, Ops, false);
+  FindSingleUseMultiplyFactors(BO->getOperand(1), Factors, Ops);
+  FindSingleUseMultiplyFactors(BO->getOperand(0), Factors, Ops);
 }
 
 /// OptimizeAndOrXor - Optimize a series of operands to an 'and', 'or', or 'xor'
@@ -835,13 +1004,13 @@ Value *Reassociate::OptimizeAdd(Instruction *I,
   unsigned MaxOcc = 0;
   Value *MaxOccVal = 0;
   for (unsigned i = 0, e = Ops.size(); i != e; ++i) {
-    BinaryOperator *BOp = dyn_cast<BinaryOperator>(Ops[i].Op);
-    if (BOp == 0 || BOp->getOpcode() != Instruction::Mul || !BOp->use_empty())
+    BinaryOperator *BOp = isReassociableOp(Ops[i].Op, Instruction::Mul);
+    if (!BOp)
       continue;
 
     // Compute all of the factors of this added value.
     SmallVector<Value*, 8> Factors;
-    FindSingleUseMultiplyFactors(BOp, Factors, Ops, true);
+    FindSingleUseMultiplyFactors(BOp, Factors, Ops);
     assert(Factors.size() > 1 && "Bad linearize!");
 
     // Add one to FactorOccurrences for each unique factor in this op.
@@ -881,8 +1050,8 @@ Value *Reassociate::OptimizeAdd(Instruction *I,
     SmallVector<WeakVH, 4> NewMulOps;
     for (unsigned i = 0; i != Ops.size(); ++i) {
       // Only try to remove factors from expressions we're allowed to.
-      BinaryOperator *BOp = dyn_cast<BinaryOperator>(Ops[i].Op);
-      if (BOp == 0 || BOp->getOpcode() != Instruction::Mul || !BOp->use_empty())
+      BinaryOperator *BOp = isReassociableOp(Ops[i].Op, Instruction::Mul);
+      if (!BOp)
         continue;
 
       if (Value *V = RemoveFactorFromExpression(Ops[i].Op, MaxOccVal)) {
@@ -959,34 +1128,21 @@ namespace {
 /// \returns Whether any factors have a power greater than one.
 bool Reassociate::collectMultiplyFactors(SmallVectorImpl<ValueEntry> &Ops,
                                          SmallVectorImpl<Factor> &Factors) {
+  // FIXME: Have Ops be (ValueEntry, Multiplicity) pairs, simplifying this.
+  // Compute the sum of powers of simplifiable factors.
   unsigned FactorPowerSum = 0;
-  DenseMap<Value *, unsigned> FactorCounts;
-  for (unsigned LastIdx = 0, Idx = 0, Size = Ops.size(); Idx < Size; ++Idx) {
-    // Note that 'use_empty' uses means the only use is in the linearized tree
-    // represented by Ops -- we remove the values from the actual operations to
-    // reduce their use count.
-    if (!Ops[Idx].Op->use_empty()) {
-      if (LastIdx == Idx)
-        ++LastIdx;
-      continue;
-    }
-    if (LastIdx == Idx || Ops[LastIdx].Op != Ops[Idx].Op) {
-      LastIdx = Idx;
-      continue;
-    }
+  for (unsigned Idx = 1, Size = Ops.size(); Idx < Size; ++Idx) {
+    Value *Op = Ops[Idx-1].Op;
+
+    // Count the number of occurrences of this value.
+    unsigned Count = 1;
+    for (; Idx < Size && Ops[Idx].Op == Op; ++Idx)
+      ++Count;
     // Track for simplification all factors which occur 2 or more times.
-    DenseMap<Value *, unsigned>::iterator CountIt;
-    bool Inserted;
-    llvm::tie(CountIt, Inserted)
-      = FactorCounts.insert(std::make_pair(Ops[Idx].Op, 2));
-    if (Inserted) {
-      FactorPowerSum += 2;
-      Factors.push_back(Factor(Ops[Idx].Op, 2));
-    } else {
-      ++CountIt->second;
-      ++FactorPowerSum;
-    }
+    if (Count > 1)
+      FactorPowerSum += Count;
   }
+
   // We can only simplify factors if the sum of the powers of our simplifiable
   // factors is 4 or higher. When that is the case, we will *always* have
   // a simplification. This is an important invariant to prevent cyclicly
@@ -994,35 +1150,29 @@ bool Reassociate::collectMultiplyFactors(SmallVectorImpl<ValueEntry> &Ops,
   if (FactorPowerSum < 4)
     return false;
 
-  // Remove all the operands which are in the map.
-  Ops.erase(std::remove_if(Ops.begin(), Ops.end(), IsValueInMap(FactorCounts)),
-            Ops.end());
+  // Now gather the simplifiable factors, removing them from Ops.
+  FactorPowerSum = 0;
+  for (unsigned Idx = 1; Idx < Ops.size(); ++Idx) {
+    Value *Op = Ops[Idx-1].Op;
 
-  // Record the adjusted power for the simplification factors. We add back into
-  // the Ops list any values with an odd power, and make the power even. This
-  // allows the outer-most multiplication tree to remain in tact during
-  // simplification.
-  unsigned OldOpsSize = Ops.size();
-  for (unsigned Idx = 0, Size = Factors.size(); Idx != Size; ++Idx) {
-    Factors[Idx].Power = FactorCounts[Factors[Idx].Base];
-    if (Factors[Idx].Power & 1) {
-      Ops.push_back(ValueEntry(getRank(Factors[Idx].Base), Factors[Idx].Base));
-      --Factors[Idx].Power;
-      --FactorPowerSum;
-    }
+    // Count the number of occurrences of this value.
+    unsigned Count = 1;
+    for (; Idx < Ops.size() && Ops[Idx].Op == Op; ++Idx)
+      ++Count;
+    if (Count == 1)
+      continue;
+    // Move an even number of occurences to Factors.
+    Count &= ~1U;
+    Idx -= Count;
+    FactorPowerSum += Count;
+    Factors.push_back(Factor(Op, Count));
+    Ops.erase(Ops.begin()+Idx, Ops.begin()+Idx+Count);
   }
+
   // None of the adjustments above should have reduced the sum of factor powers
   // below our mininum of '4'.
   assert(FactorPowerSum >= 4);
 
-  // Patch up the sort of the ops vector by sorting the factors we added back
-  // onto the back, and merging the two sequences.
-  if (OldOpsSize != Ops.size()) {
-    SmallVectorImpl<ValueEntry>::iterator MiddleIt = Ops.begin() + OldOpsSize;
-    std::sort(MiddleIt, Ops.end());
-    std::inplace_merge(Ops.begin(), MiddleIt, Ops.end());
-  }
-
   std::sort(Factors.begin(), Factors.end(), Factor::PowerDescendingSorter());
   return true;
 }
@@ -1098,7 +1248,6 @@ Value *Reassociate::buildMinimalMultiplyDAG(IRBuilder<> &Builder,
     return OuterProduct.front();
 
   Value *V = buildMultiplyTree(Builder, OuterProduct);
-  RedoInsts.push_back(V);
   return V;
 }
 
@@ -1297,8 +1446,6 @@ Value *Reassociate::ReassociateExpression(BinaryOperator *I) {
   SmallVector<ValueEntry, 8> Ops;
   LinearizeExprTree(I, Ops);
 
-  DEBUG(dbgs() << "RAIn:\t"; PrintOps(I, Ops); dbgs() << '\n');
-
   // Now that we have linearized the tree to a list and have gathered all of
   // the operands and their ranks, sort the operands by their rank.  Use a
   // stable_sort so that values with equal ranks will have their relative
@@ -1307,6 +1454,8 @@ Value *Reassociate::ReassociateExpression(BinaryOperator *I) {
   // the vector.
   std::stable_sort(Ops.begin(), Ops.end());
 
+  DEBUG(dbgs() << "RAIn:\t"; PrintOps(I, Ops); dbgs() << '\n');
+
   // OptimizeExpression - Now that we have the expression tree in a convenient
   // sorted form, optimize it globally if possible.
   if (Value *V = OptimizeExpression(I, Ops)) {
@@ -1368,13 +1517,14 @@ bool Reassociate::runOnFunction(Function &F) {
       ReassociateInst(BBI);
     }
 
+  // We are done with the rank map.
+  RankMap.clear();
+  ValueRankMap.clear();
+
   // Now that we're done, delete any instructions which are no longer used.
   while (!DeadInsts.empty())
     if (Value *V = DeadInsts.pop_back_val())
       RecursivelyDeleteTriviallyDeadInstructions(V);
 
-  // We are done with the rank map.
-  RankMap.clear();
-  ValueRankMap.clear();
   return MadeChange;
 }