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diff --git a/lib/Transforms/Scalar/NaryReassociate.cpp b/lib/Transforms/Scalar/NaryReassociate.cpp
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+//===- NaryReassociate.cpp - Reassociate n-ary expressions ----------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This pass reassociates n-ary add expressions and eliminates the redundancy
+// exposed by the reassociation.
+//
+// A motivating example:
+//
+//   void foo(int a, int b) {
+//     bar(a + b);
+//     bar((a + 2) + b);
+//   }
+//
+// An ideal compiler should reassociate (a + 2) + b to (a + b) + 2 and simplify
+// the above code to
+//
+//   int t = a + b;
+//   bar(t);
+//   bar(t + 2);
+//
+// However, the Reassociate pass is unable to do that because it processes each
+// instruction individually and believes (a + 2) + b is the best form according
+// to its rank system.
+//
+// To address this limitation, NaryReassociate reassociates an expression in a
+// form that reuses existing instructions. As a result, NaryReassociate can
+// reassociate (a + 2) + b in the example to (a + b) + 2 because it detects that
+// (a + b) is computed before.
+//
+// NaryReassociate works as follows. For every instruction in the form of (a +
+// b) + c, it checks whether a + c or b + c is already computed by a dominating
+// instruction. If so, it then reassociates (a + b) + c into (a + c) + b or (b +
+// c) + a and removes the redundancy accordingly. To efficiently look up whether
+// an expression is computed before, we store each instruction seen and its SCEV
+// into an SCEV-to-instruction map.
+//
+// Although the algorithm pattern-matches only ternary additions, it
+// automatically handles many >3-ary expressions by walking through the function
+// in the depth-first order. For example, given
+//
+//   (a + c) + d
+//   ((a + b) + c) + d
+//
+// NaryReassociate first rewrites (a + b) + c to (a + c) + b, and then rewrites
+// ((a + c) + b) + d into ((a + c) + d) + b.
+//
+// Finally, the above dominator-based algorithm may need to be run multiple
+// iterations before emitting optimal code. One source of this need is that we
+// only split an operand when it is used only once. The above algorithm can
+// eliminate an instruction and decrease the usage count of its operands. As a
+// result, an instruction that previously had multiple uses may become a
+// single-use instruction and thus eligible for split consideration. For
+// example,
+//
+//   ac = a + c
+//   ab = a + b
+//   abc = ab + c
+//   ab2 = ab + b
+//   ab2c = ab2 + c
+//
+// In the first iteration, we cannot reassociate abc to ac+b because ab is used
+// twice. However, we can reassociate ab2c to abc+b in the first iteration. As a
+// result, ab2 becomes dead and ab will be used only once in the second
+// iteration.
+//
+// Limitations and TODO items:
+//
+// 1) We only considers n-ary adds for now. This should be extended and
+// generalized.
+//
+// 2) Besides arithmetic operations, similar reassociation can be applied to
+// GEPs. For example, if
+//   X = &arr[a]
+// dominates
+//   Y = &arr[a + b]
+// we may rewrite Y into X + b.
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/Analysis/ScalarEvolution.h"
+#include "llvm/Analysis/TargetLibraryInfo.h"
+#include "llvm/IR/Dominators.h"
+#include "llvm/IR/Module.h"
+#include "llvm/IR/PatternMatch.h"
+#include "llvm/Transforms/Scalar.h"
+#include "llvm/Transforms/Utils/Local.h"
+using namespace llvm;
+using namespace PatternMatch;
+
+#define DEBUG_TYPE "nary-reassociate"
+
+namespace {
+class NaryReassociate : public FunctionPass {
+public:
+  static char ID;
+
+  NaryReassociate(): FunctionPass(ID) {
+    initializeNaryReassociatePass(*PassRegistry::getPassRegistry());
+  }
+
+  bool runOnFunction(Function &F) override;
+
+  void getAnalysisUsage(AnalysisUsage &AU) const override {
+    AU.addPreserved<DominatorTreeWrapperPass>();
+    AU.addPreserved<ScalarEvolution>();
+    AU.addPreserved<TargetLibraryInfoWrapperPass>();
+    AU.addRequired<DominatorTreeWrapperPass>();
+    AU.addRequired<ScalarEvolution>();
+    AU.addRequired<TargetLibraryInfoWrapperPass>();
+    AU.setPreservesCFG();
+  }
+
+private:
+  // Runs only one iteration of the dominator-based algorithm. See the header
+  // comments for why we need multiple iterations.
+  bool doOneIteration(Function &F);
+  // Reasssociates I to a better form.
+  Instruction *tryReassociateAdd(Instruction *I);
+  // A helper function for tryReassociateAdd. LHS and RHS are explicitly passed.
+  Instruction *tryReassociateAdd(Value *LHS, Value *RHS, Instruction *I);
+  // Rewrites I to LHS + RHS if LHS is computed already.
+  Instruction *tryReassociatedAdd(const SCEV *LHS, Value *RHS, Instruction *I);
+
+  DominatorTree *DT;
+  ScalarEvolution *SE;
+  TargetLibraryInfo *TLI;
+  // A lookup table quickly telling which instructions compute the given SCEV.
+  // Note that there can be multiple instructions at different locations
+  // computing to the same SCEV, so we map a SCEV to an instruction list.  For
+  // example,
+  //
+  //   if (p1)
+  //     foo(a + b);
+  //   if (p2)
+  //     bar(a + b);
+  DenseMap<const SCEV *, SmallVector<Instruction *, 2>> SeenExprs;
+};
+} // anonymous namespace
+
+char NaryReassociate::ID = 0;
+INITIALIZE_PASS_BEGIN(NaryReassociate, "nary-reassociate", "Nary reassociation",
+                      false, false)
+INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
+INITIALIZE_PASS_DEPENDENCY(ScalarEvolution)
+INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfoWrapperPass)
+INITIALIZE_PASS_END(NaryReassociate, "nary-reassociate", "Nary reassociation",
+                    false, false)
+
+FunctionPass *llvm::createNaryReassociatePass() {
+  return new NaryReassociate();
+}
+
+bool NaryReassociate::runOnFunction(Function &F) {
+  if (skipOptnoneFunction(F))
+    return false;
+
+  DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
+  SE = &getAnalysis<ScalarEvolution>();
+  TLI = &getAnalysis<TargetLibraryInfoWrapperPass>().getTLI();
+
+  bool Changed = false, ChangedInThisIteration;
+  do {
+    ChangedInThisIteration = doOneIteration(F);
+    Changed |= ChangedInThisIteration;
+  } while (ChangedInThisIteration);
+  return Changed;
+}
+
+bool NaryReassociate::doOneIteration(Function &F) {
+  bool Changed = false;
+  SeenExprs.clear();
+  // Traverse the dominator tree in the depth-first order. This order makes sure
+  // all bases of a candidate are in Candidates when we process it.
+  for (auto Node = GraphTraits<DominatorTree *>::nodes_begin(DT);
+       Node != GraphTraits<DominatorTree *>::nodes_end(DT); ++Node) {
+    BasicBlock *BB = Node->getBlock();
+    for (auto I = BB->begin(); I != BB->end(); ++I) {
+      if (I->getOpcode() == Instruction::Add) {
+        if (Instruction *NewI = tryReassociateAdd(I)) {
+          Changed = true;
+          SE->forgetValue(I);
+          I->replaceAllUsesWith(NewI);
+          RecursivelyDeleteTriviallyDeadInstructions(I, TLI);
+          I = NewI;
+        }
+        // We should add the rewritten instruction because tryReassociateAdd may
+        // have invalidated the original one.
+        SeenExprs[SE->getSCEV(I)].push_back(I);
+      }
+    }
+  }
+  return Changed;
+}
+
+Instruction *NaryReassociate::tryReassociateAdd(Instruction *I) {
+  Value *LHS = I->getOperand(0), *RHS = I->getOperand(1);
+  if (auto *NewI = tryReassociateAdd(LHS, RHS, I))
+    return NewI;
+  if (auto *NewI = tryReassociateAdd(RHS, LHS, I))
+    return NewI;
+  return nullptr;
+}
+
+Instruction *NaryReassociate::tryReassociateAdd(Value *LHS, Value *RHS,
+                                                Instruction *I) {
+  Value *A = nullptr, *B = nullptr;
+  // To be conservative, we reassociate I only when it is the only user of A+B.
+  if (LHS->hasOneUse() && match(LHS, m_Add(m_Value(A), m_Value(B)))) {
+    // I = (A + B) + RHS
+    //   = (A + RHS) + B or (B + RHS) + A
+    const SCEV *AExpr = SE->getSCEV(A), *BExpr = SE->getSCEV(B);
+    const SCEV *RHSExpr = SE->getSCEV(RHS);
+    if (auto *NewI = tryReassociatedAdd(SE->getAddExpr(AExpr, RHSExpr), B, I))
+      return NewI;
+    if (auto *NewI = tryReassociatedAdd(SE->getAddExpr(BExpr, RHSExpr), A, I))
+      return NewI;
+  }
+  return nullptr;
+}
+
+Instruction *NaryReassociate::tryReassociatedAdd(const SCEV *LHSExpr,
+                                                 Value *RHS, Instruction *I) {
+  auto Pos = SeenExprs.find(LHSExpr);
+  // Bail out if LHSExpr is not previously seen.
+  if (Pos == SeenExprs.end())
+    return nullptr;
+
+  auto &LHSCandidates = Pos->second;
+  // Look for the closest dominator LHS of I that computes LHSExpr, and replace
+  // I with LHS + RHS.
+  //
+  // Because we traverse the dominator tree in the pre-order, a
+  // candidate that doesn't dominate the current instruction won't dominate any
+  // future instruction either. Therefore, we pop it out of the stack. This
+  // optimization makes the algorithm O(n).
+  while (!LHSCandidates.empty()) {
+    Instruction *LHS = LHSCandidates.back();
+    if (DT->dominates(LHS, I)) {
+      Instruction *NewI = BinaryOperator::CreateAdd(LHS, RHS, "", I);
+      NewI->takeName(I);
+      return NewI;
+    }
+    LHSCandidates.pop_back();
+  }
+  return nullptr;
+}