Vendor import of llvm trunk r256633:

https://llvm.org/svn/llvm-project/llvm/trunk@256633

1 files changed, 647 insertions, 700 deletions

diff --git a/lib/Transforms/Scalar/LoopIdiomRecognize.cpp b/lib/Transforms/Scalar/LoopIdiomRecognize.cpp
index a21ca2417ca1..2d577de7c2b8 100644
--- a/lib/Transforms/Scalar/LoopIdiomRecognize.cpp
+++ b/lib/Transforms/Scalar/LoopIdiomRecognize.cpp
@@ -31,11 +31,6 @@
 //   void foo(_Complex float *P)
 //     for (i) { __real__(*P) = 0;  __imag__(*P) = 0; }
 //
-// We should enhance this to handle negative strides through memory.
-// Alternatively (and perhaps better) we could rely on an earlier pass to force
-// forward iteration through memory, which is generally better for cache
-// behavior.  Negative strides *do* happen for memset/memcpy loops.
-//
 // This could recognize common matrix multiplies and dot product idioms and
 // replace them with calls to BLAS (if linked in??).
 //
@@ -44,7 +39,10 @@
 #include "llvm/Transforms/Scalar.h"
 #include "llvm/ADT/Statistic.h"
 #include "llvm/Analysis/AliasAnalysis.h"
+#include "llvm/Analysis/BasicAliasAnalysis.h"
+#include "llvm/Analysis/GlobalsModRef.h"
 #include "llvm/Analysis/LoopPass.h"
+#include "llvm/Analysis/ScalarEvolutionAliasAnalysis.h"
 #include "llvm/Analysis/ScalarEvolutionExpander.h"
 #include "llvm/Analysis/ScalarEvolutionExpressions.h"
 #include "llvm/Analysis/TargetLibraryInfo.h"
@@ -67,149 +65,85 @@ STATISTIC(NumMemCpy, "Number of memcpy's formed from loop load+stores");
 
 namespace {
 
-  class LoopIdiomRecognize;
+class LoopIdiomRecognize : public LoopPass {
+  Loop *CurLoop;
+  AliasAnalysis *AA;
+  DominatorTree *DT;
+  LoopInfo *LI;
+  ScalarEvolution *SE;
+  TargetLibraryInfo *TLI;
+  const TargetTransformInfo *TTI;
+  const DataLayout *DL;
+
+public:
+  static char ID;
+  explicit LoopIdiomRecognize() : LoopPass(ID) {
+    initializeLoopIdiomRecognizePass(*PassRegistry::getPassRegistry());
+  }
 
-  /// This class defines some utility functions for loop idiom recognization.
-  class LIRUtil {
-  public:
-    /// Return true iff the block contains nothing but an uncondition branch
-    /// (aka goto instruction).
-    static bool isAlmostEmpty(BasicBlock *);
+  bool runOnLoop(Loop *L, LPPassManager &LPM) override;
+
+  /// This transformation requires natural loop information & requires that
+  /// loop preheaders be inserted into the CFG.
+  ///
+  void getAnalysisUsage(AnalysisUsage &AU) const override {
+    AU.addRequired<LoopInfoWrapperPass>();
+    AU.addPreserved<LoopInfoWrapperPass>();
+    AU.addRequiredID(LoopSimplifyID);
+    AU.addPreservedID(LoopSimplifyID);
+    AU.addRequiredID(LCSSAID);
+    AU.addPreservedID(LCSSAID);
+    AU.addRequired<AAResultsWrapperPass>();
+    AU.addPreserved<AAResultsWrapperPass>();
+    AU.addRequired<ScalarEvolutionWrapperPass>();
+    AU.addPreserved<ScalarEvolutionWrapperPass>();
+    AU.addPreserved<SCEVAAWrapperPass>();
+    AU.addRequired<DominatorTreeWrapperPass>();
+    AU.addPreserved<DominatorTreeWrapperPass>();
+    AU.addRequired<TargetLibraryInfoWrapperPass>();
+    AU.addRequired<TargetTransformInfoWrapperPass>();
+    AU.addPreserved<BasicAAWrapperPass>();
+    AU.addPreserved<GlobalsAAWrapperPass>();
+  }
 
-    static BranchInst *getBranch(BasicBlock *BB) {
-      return dyn_cast<BranchInst>(BB->getTerminator());
-    }
+private:
+  typedef SmallVector<StoreInst *, 8> StoreList;
+  StoreList StoreRefs;
 
-    /// Derive the precondition block (i.e the block that guards the loop
-    /// preheader) from the given preheader.
-    static BasicBlock *getPrecondBb(BasicBlock *PreHead);
-  };
-
-  /// This class is to recoginize idioms of population-count conducted in
-  /// a noncountable loop. Currently it only recognizes this pattern:
-  /// \code
-  ///   while(x) {cnt++; ...; x &= x - 1; ...}
-  /// \endcode
-  class NclPopcountRecognize {
-    LoopIdiomRecognize &LIR;
-    Loop *CurLoop;
-    BasicBlock *PreCondBB;
-
-    typedef IRBuilder<> IRBuilderTy;
-
-  public:
-    explicit NclPopcountRecognize(LoopIdiomRecognize &TheLIR);
-    bool recognize();
-
-  private:
-    /// Take a glimpse of the loop to see if we need to go ahead recoginizing
-    /// the idiom.
-    bool preliminaryScreen();
-
-    /// Check if the given conditional branch is based on the comparison
-    /// between a variable and zero, and if the variable is non-zero, the
-    /// control yields to the loop entry. If the branch matches the behavior,
-    /// the variable involved in the comparion is returned. This function will
-    /// be called to see if the precondition and postcondition of the loop
-    /// are in desirable form.
-    Value *matchCondition(BranchInst *Br, BasicBlock *NonZeroTarget) const;
-
-    /// Return true iff the idiom is detected in the loop. and 1) \p CntInst
-    /// is set to the instruction counting the population bit. 2) \p CntPhi
-    /// is set to the corresponding phi node. 3) \p Var is set to the value
-    /// whose population bits are being counted.
-    bool detectIdiom
-      (Instruction *&CntInst, PHINode *&CntPhi, Value *&Var) const;
-
-    /// Insert ctpop intrinsic function and some obviously dead instructions.
-    void transform(Instruction *CntInst, PHINode *CntPhi, Value *Var);
-
-    /// Create llvm.ctpop.* intrinsic function.
-    CallInst *createPopcntIntrinsic(IRBuilderTy &IRB, Value *Val, DebugLoc DL);
-  };
-
-  class LoopIdiomRecognize : public LoopPass {
-    Loop *CurLoop;
-    DominatorTree *DT;
-    ScalarEvolution *SE;
-    TargetLibraryInfo *TLI;
-    const TargetTransformInfo *TTI;
-  public:
-    static char ID;
-    explicit LoopIdiomRecognize() : LoopPass(ID) {
-      initializeLoopIdiomRecognizePass(*PassRegistry::getPassRegistry());
-      DT = nullptr;
-      SE = nullptr;
-      TLI = nullptr;
-      TTI = nullptr;
-    }
+  /// \name Countable Loop Idiom Handling
+  /// @{
 
-    bool runOnLoop(Loop *L, LPPassManager &LPM) override;
-    bool runOnLoopBlock(BasicBlock *BB, const SCEV *BECount,
-                        SmallVectorImpl<BasicBlock*> &ExitBlocks);
-
-    bool processLoopStore(StoreInst *SI, const SCEV *BECount);
-    bool processLoopMemSet(MemSetInst *MSI, const SCEV *BECount);
-
-    bool processLoopStridedStore(Value *DestPtr, unsigned StoreSize,
-                                 unsigned StoreAlignment,
-                                 Value *SplatValue, Instruction *TheStore,
-                                 const SCEVAddRecExpr *Ev,
-                                 const SCEV *BECount);
-    bool processLoopStoreOfLoopLoad(StoreInst *SI, unsigned StoreSize,
-                                    const SCEVAddRecExpr *StoreEv,
-                                    const SCEVAddRecExpr *LoadEv,
-                                    const SCEV *BECount);
-
-    /// This transformation requires natural loop information & requires that
-    /// loop preheaders be inserted into the CFG.
-    ///
-    void getAnalysisUsage(AnalysisUsage &AU) const override {
-      AU.addRequired<LoopInfoWrapperPass>();
-      AU.addPreserved<LoopInfoWrapperPass>();
-      AU.addRequiredID(LoopSimplifyID);
-      AU.addPreservedID(LoopSimplifyID);
-      AU.addRequiredID(LCSSAID);
-      AU.addPreservedID(LCSSAID);
-      AU.addRequired<AliasAnalysis>();
-      AU.addPreserved<AliasAnalysis>();
-      AU.addRequired<ScalarEvolution>();
-      AU.addPreserved<ScalarEvolution>();
-      AU.addPreserved<DominatorTreeWrapperPass>();
-      AU.addRequired<DominatorTreeWrapperPass>();
-      AU.addRequired<TargetLibraryInfoWrapperPass>();
-      AU.addRequired<TargetTransformInfoWrapperPass>();
-    }
+  bool runOnCountableLoop();
+  bool runOnLoopBlock(BasicBlock *BB, const SCEV *BECount,
+                      SmallVectorImpl<BasicBlock *> &ExitBlocks);
 
-    DominatorTree *getDominatorTree() {
-      return DT ? DT
-                : (DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree());
-    }
+  void collectStores(BasicBlock *BB);
+  bool isLegalStore(StoreInst *SI);
+  bool processLoopStore(StoreInst *SI, const SCEV *BECount);
+  bool processLoopMemSet(MemSetInst *MSI, const SCEV *BECount);
 
-    ScalarEvolution *getScalarEvolution() {
-      return SE ? SE : (SE = &getAnalysis<ScalarEvolution>());
-    }
+  bool processLoopStridedStore(Value *DestPtr, unsigned StoreSize,
+                               unsigned StoreAlignment, Value *SplatValue,
+                               Instruction *TheStore, const SCEVAddRecExpr *Ev,
+                               const SCEV *BECount, bool NegStride);
+  bool processLoopStoreOfLoopLoad(StoreInst *SI, unsigned StoreSize,
+                                  const SCEVAddRecExpr *StoreEv,
+                                  const SCEV *BECount, bool NegStride);
 
-    TargetLibraryInfo *getTargetLibraryInfo() {
-      if (!TLI)
-        TLI = &getAnalysis<TargetLibraryInfoWrapperPass>().getTLI();
+  /// @}
+  /// \name Noncountable Loop Idiom Handling
+  /// @{
 
-      return TLI;
-    }
+  bool runOnNoncountableLoop();
 
-    const TargetTransformInfo *getTargetTransformInfo() {
-      return TTI ? TTI
-                 : (TTI = &getAnalysis<TargetTransformInfoWrapperPass>().getTTI(
-                        *CurLoop->getHeader()->getParent()));
-    }
+  bool recognizePopcount();
+  void transformLoopToPopcount(BasicBlock *PreCondBB, Instruction *CntInst,
+                               PHINode *CntPhi, Value *Var);
 
-    Loop *getLoop() const { return CurLoop; }
+  /// @}
+};
 
-  private:
-    bool runOnNoncountableLoop();
-    bool runOnCountableLoop();
-  };
-}
+} // End anonymous namespace.
 
 char LoopIdiomRecognize::ID = 0;
 INITIALIZE_PASS_BEGIN(LoopIdiomRecognize, "loop-idiom", "Recognize loop idioms",
@@ -218,9 +152,12 @@ INITIALIZE_PASS_DEPENDENCY(LoopInfoWrapperPass)
 INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
 INITIALIZE_PASS_DEPENDENCY(LoopSimplify)
 INITIALIZE_PASS_DEPENDENCY(LCSSA)
-INITIALIZE_PASS_DEPENDENCY(ScalarEvolution)
+INITIALIZE_PASS_DEPENDENCY(ScalarEvolutionWrapperPass)
 INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfoWrapperPass)
-INITIALIZE_AG_DEPENDENCY(AliasAnalysis)
+INITIALIZE_PASS_DEPENDENCY(BasicAAWrapperPass)
+INITIALIZE_PASS_DEPENDENCY(AAResultsWrapperPass)
+INITIALIZE_PASS_DEPENDENCY(GlobalsAAWrapperPass)
+INITIALIZE_PASS_DEPENDENCY(SCEVAAWrapperPass)
 INITIALIZE_PASS_DEPENDENCY(TargetTransformInfoWrapperPass)
 INITIALIZE_PASS_END(LoopIdiomRecognize, "loop-idiom", "Recognize loop idioms",
                     false, false)
@@ -242,406 +179,64 @@ static void deleteDeadInstruction(Instruction *I,
 
 //===----------------------------------------------------------------------===//
 //
-//          Implementation of LIRUtil
-//
-//===----------------------------------------------------------------------===//
-
-// This function will return true iff the given block contains nothing but goto.
-// A typical usage of this function is to check if the preheader function is
-// "almost" empty such that generated intrinsic functions can be moved across
-// the preheader and be placed at the end of the precondition block without
-// the concern of breaking data dependence.
-bool LIRUtil::isAlmostEmpty(BasicBlock *BB) {
-  if (BranchInst *Br = getBranch(BB)) {
-    return Br->isUnconditional() && Br == BB->begin();
-  }
-  return false;
-}
-
-BasicBlock *LIRUtil::getPrecondBb(BasicBlock *PreHead) {
-  if (BasicBlock *BB = PreHead->getSinglePredecessor()) {
-    BranchInst *Br = getBranch(BB);
-    return Br && Br->isConditional() ? BB : nullptr;
-  }
-  return nullptr;
-}
-
-//===----------------------------------------------------------------------===//
-//
-//          Implementation of NclPopcountRecognize
+//          Implementation of LoopIdiomRecognize
 //
 //===----------------------------------------------------------------------===//
 
-NclPopcountRecognize::NclPopcountRecognize(LoopIdiomRecognize &TheLIR):
-  LIR(TheLIR), CurLoop(TheLIR.getLoop()), PreCondBB(nullptr) {
-}
-
-bool NclPopcountRecognize::preliminaryScreen() {
-  const TargetTransformInfo *TTI = LIR.getTargetTransformInfo();
-  if (TTI->getPopcntSupport(32) != TargetTransformInfo::PSK_FastHardware)
-    return false;
-
-  // Counting population are usually conducted by few arithmetic instructions.
-  // Such instructions can be easilly "absorbed" by vacant slots in a
-  // non-compact loop. Therefore, recognizing popcount idiom only makes sense
-  // in a compact loop.
-
-  // Give up if the loop has multiple blocks or multiple backedges.
-  if (CurLoop->getNumBackEdges() != 1 || CurLoop->getNumBlocks() != 1)
-    return false;
-
-  BasicBlock *LoopBody = *(CurLoop->block_begin());
-  if (LoopBody->size() >= 20) {
-    // The loop is too big, bail out.
-    return false;
-  }
-
-  // It should have a preheader containing nothing but a goto instruction.
-  BasicBlock *PreHead = CurLoop->getLoopPreheader();
-  if (!PreHead || !LIRUtil::isAlmostEmpty(PreHead))
+bool LoopIdiomRecognize::runOnLoop(Loop *L, LPPassManager &LPM) {
+  if (skipOptnoneFunction(L))
     return false;
 
-  // It should have a precondition block where the generated popcount instrinsic
-  // function will be inserted.
-  PreCondBB = LIRUtil::getPrecondBb(PreHead);
-  if (!PreCondBB)
+  CurLoop = L;
+  // If the loop could not be converted to canonical form, it must have an
+  // indirectbr in it, just give up.
+  if (!L->getLoopPreheader())
     return false;
 
-  return true;
-}
-
-Value *NclPopcountRecognize::matchCondition(BranchInst *Br,
-                                            BasicBlock *LoopEntry) const {
-  if (!Br || !Br->isConditional())
-    return nullptr;
-
-  ICmpInst *Cond = dyn_cast<ICmpInst>(Br->getCondition());
-  if (!Cond)
-    return nullptr;
-
-  ConstantInt *CmpZero = dyn_cast<ConstantInt>(Cond->getOperand(1));
-  if (!CmpZero || !CmpZero->isZero())
-    return nullptr;
-
-  ICmpInst::Predicate Pred = Cond->getPredicate();
-  if ((Pred == ICmpInst::ICMP_NE && Br->getSuccessor(0) == LoopEntry) ||
-      (Pred == ICmpInst::ICMP_EQ && Br->getSuccessor(1) == LoopEntry))
-    return Cond->getOperand(0);
-
-  return nullptr;
-}
-
-bool NclPopcountRecognize::detectIdiom(Instruction *&CntInst,
-                                       PHINode *&CntPhi,
-                                       Value *&Var) const {
-  // Following code tries to detect this idiom:
-  //
-  //    if (x0 != 0)
-  //      goto loop-exit // the precondition of the loop
-  //    cnt0 = init-val;
-  //    do {
-  //       x1 = phi (x0, x2);
-  //       cnt1 = phi(cnt0, cnt2);
-  //
-  //       cnt2 = cnt1 + 1;
-  //        ...
-  //       x2 = x1 & (x1 - 1);
-  //        ...
-  //    } while(x != 0);
-  //
-  // loop-exit:
-  //
-
-  // step 1: Check to see if the look-back branch match this pattern:
-  //    "if (a!=0) goto loop-entry".
-  BasicBlock *LoopEntry;
-  Instruction *DefX2, *CountInst;
-  Value *VarX1, *VarX0;
-  PHINode *PhiX, *CountPhi;
-
-  DefX2 = CountInst = nullptr;
-  VarX1 = VarX0 = nullptr;
-  PhiX = CountPhi = nullptr;
-  LoopEntry = *(CurLoop->block_begin());
-
-  // step 1: Check if the loop-back branch is in desirable form.
-  {
-    if (Value *T = matchCondition (LIRUtil::getBranch(LoopEntry), LoopEntry))
-      DefX2 = dyn_cast<Instruction>(T);
-    else
-      return false;
-  }
-
-  // step 2: detect instructions corresponding to "x2 = x1 & (x1 - 1)"
-  {
-    if (!DefX2 || DefX2->getOpcode() != Instruction::And)
-      return false;
-
-    BinaryOperator *SubOneOp;
-
-    if ((SubOneOp = dyn_cast<BinaryOperator>(DefX2->getOperand(0))))
-      VarX1 = DefX2->getOperand(1);
-    else {
-      VarX1 = DefX2->getOperand(0);
-      SubOneOp = dyn_cast<BinaryOperator>(DefX2->getOperand(1));
-    }
-    if (!SubOneOp)
-      return false;
-
-    Instruction *SubInst = cast<Instruction>(SubOneOp);
-    ConstantInt *Dec = dyn_cast<ConstantInt>(SubInst->getOperand(1));
-    if (!Dec ||
-        !((SubInst->getOpcode() == Instruction::Sub && Dec->isOne()) ||
-          (SubInst->getOpcode() == Instruction::Add && Dec->isAllOnesValue()))) {
-      return false;
-    }
-  }
-
-  // step 3: Check the recurrence of variable X
-  {
-    PhiX = dyn_cast<PHINode>(VarX1);
-    if (!PhiX ||
-        (PhiX->getOperand(0) != DefX2 && PhiX->getOperand(1) != DefX2)) {
-      return false;
-    }
-  }
-
-  // step 4: Find the instruction which count the population: cnt2 = cnt1 + 1
-  {
-    CountInst = nullptr;
-    for (BasicBlock::iterator Iter = LoopEntry->getFirstNonPHI(),
-           IterE = LoopEntry->end(); Iter != IterE; Iter++) {
-      Instruction *Inst = Iter;
-      if (Inst->getOpcode() != Instruction::Add)
-        continue;
-
-      ConstantInt *Inc = dyn_cast<ConstantInt>(Inst->getOperand(1));
-      if (!Inc || !Inc->isOne())
-        continue;
-
-      PHINode *Phi = dyn_cast<PHINode>(Inst->getOperand(0));
-      if (!Phi || Phi->getParent() != LoopEntry)
-        continue;
-
-      // Check if the result of the instruction is live of the loop.
-      bool LiveOutLoop = false;
-      for (User *U : Inst->users()) {
-        if ((cast<Instruction>(U))->getParent() != LoopEntry) {
-          LiveOutLoop = true; break;
-        }
-      }
-
-      if (LiveOutLoop) {
-        CountInst = Inst;
-        CountPhi = Phi;
-        break;
-      }
-    }
-
-    if (!CountInst)
-      return false;
-  }
-
-  // step 5: check if the precondition is in this form:
-  //   "if (x != 0) goto loop-head ; else goto somewhere-we-don't-care;"
-  {
-    BranchInst *PreCondBr = LIRUtil::getBranch(PreCondBB);
-    Value *T = matchCondition (PreCondBr, CurLoop->getLoopPreheader());
-    if (T != PhiX->getOperand(0) && T != PhiX->getOperand(1))
-      return false;
-
-    CntInst = CountInst;
-    CntPhi = CountPhi;
-    Var = T;
-  }
-
-  return true;
-}
-
-void NclPopcountRecognize::transform(Instruction *CntInst,
-                                     PHINode *CntPhi, Value *Var) {
-
-  ScalarEvolution *SE = LIR.getScalarEvolution();
-  TargetLibraryInfo *TLI = LIR.getTargetLibraryInfo();
-  BasicBlock *PreHead = CurLoop->getLoopPreheader();
-  BranchInst *PreCondBr = LIRUtil::getBranch(PreCondBB);
-  const DebugLoc DL = CntInst->getDebugLoc();
-
-  // Assuming before transformation, the loop is following:
-  //  if (x) // the precondition
-  //     do { cnt++; x &= x - 1; } while(x);
-
-  // Step 1: Insert the ctpop instruction at the end of the precondition block
-  IRBuilderTy Builder(PreCondBr);
-  Value *PopCnt, *PopCntZext, *NewCount, *TripCnt;
-  {
-    PopCnt = createPopcntIntrinsic(Builder, Var, DL);
-    NewCount = PopCntZext =
-      Builder.CreateZExtOrTrunc(PopCnt, cast<IntegerType>(CntPhi->getType()));
-
-    if (NewCount != PopCnt)
-      (cast<Instruction>(NewCount))->setDebugLoc(DL);
-
-    // TripCnt is exactly the number of iterations the loop has
-    TripCnt = NewCount;
-
-    // If the population counter's initial value is not zero, insert Add Inst.
-    Value *CntInitVal = CntPhi->getIncomingValueForBlock(PreHead);
-    ConstantInt *InitConst = dyn_cast<ConstantInt>(CntInitVal);
-    if (!InitConst || !InitConst->isZero()) {
-      NewCount = Builder.CreateAdd(NewCount, CntInitVal);
-      (cast<Instruction>(NewCount))->setDebugLoc(DL);
-    }
-  }
-
-  // Step 2: Replace the precondition from "if(x == 0) goto loop-exit" to
-  //   "if(NewCount == 0) loop-exit". Withtout this change, the intrinsic
-  //   function would be partial dead code, and downstream passes will drag
-  //   it back from the precondition block to the preheader.
-  {
-    ICmpInst *PreCond = cast<ICmpInst>(PreCondBr->getCondition());
-
-    Value *Opnd0 = PopCntZext;
-    Value *Opnd1 = ConstantInt::get(PopCntZext->getType(), 0);
-    if (PreCond->getOperand(0) != Var)
-      std::swap(Opnd0, Opnd1);
-
-    ICmpInst *NewPreCond =
-      cast<ICmpInst>(Builder.CreateICmp(PreCond->getPredicate(), Opnd0, Opnd1));
-    PreCondBr->setCondition(NewPreCond);
-
-    RecursivelyDeleteTriviallyDeadInstructions(PreCond, TLI);
-  }
-
-  // Step 3: Note that the population count is exactly the trip count of the
-  // loop in question, which enble us to to convert the loop from noncountable
-  // loop into a countable one. The benefit is twofold:
-  //
-  //  - If the loop only counts population, the entire loop become dead after
-  //    the transformation. It is lots easier to prove a countable loop dead
-  //    than to prove a noncountable one. (In some C dialects, a infite loop
-  //    isn't dead even if it computes nothing useful. In general, DCE needs
-  //    to prove a noncountable loop finite before safely delete it.)
-  //
-  //  - If the loop also performs something else, it remains alive.
-  //    Since it is transformed to countable form, it can be aggressively
-  //    optimized by some optimizations which are in general not applicable
-  //    to a noncountable loop.
-  //
-  // After this step, this loop (conceptually) would look like following:
-  //   newcnt = __builtin_ctpop(x);
-  //   t = newcnt;
-  //   if (x)
-  //     do { cnt++; x &= x-1; t--) } while (t > 0);
-  BasicBlock *Body = *(CurLoop->block_begin());
-  {
-    BranchInst *LbBr = LIRUtil::getBranch(Body);
-    ICmpInst *LbCond = cast<ICmpInst>(LbBr->getCondition());
-    Type *Ty = TripCnt->getType();
-
-    PHINode *TcPhi = PHINode::Create(Ty, 2, "tcphi", Body->begin());
-
-    Builder.SetInsertPoint(LbCond);
-    Value *Opnd1 = cast<Value>(TcPhi);
-    Value *Opnd2 = cast<Value>(ConstantInt::get(Ty, 1));
-    Instruction *TcDec =
-      cast<Instruction>(Builder.CreateSub(Opnd1, Opnd2, "tcdec", false, true));
-
-    TcPhi->addIncoming(TripCnt, PreHead);
-    TcPhi->addIncoming(TcDec, Body);
-
-    CmpInst::Predicate Pred = (LbBr->getSuccessor(0) == Body) ?
-      CmpInst::ICMP_UGT : CmpInst::ICMP_SLE;
-    LbCond->setPredicate(Pred);
-    LbCond->setOperand(0, TcDec);
-    LbCond->setOperand(1, cast<Value>(ConstantInt::get(Ty, 0)));
-  }
-
-  // Step 4: All the references to the original population counter outside
-  //  the loop are replaced with the NewCount -- the value returned from
-  //  __builtin_ctpop().
-  CntInst->replaceUsesOutsideBlock(NewCount, Body);
-
-  // step 5: Forget the "non-computable" trip-count SCEV associated with the
-  //   loop. The loop would otherwise not be deleted even if it becomes empty.
-  SE->forgetLoop(CurLoop);
-}
-
-CallInst *NclPopcountRecognize::createPopcntIntrinsic(IRBuilderTy &IRBuilder,
-                                                      Value *Val, DebugLoc DL) {
-  Value *Ops[] = { Val };
-  Type *Tys[] = { Val->getType() };
-
-  Module *M = (*(CurLoop->block_begin()))->getParent()->getParent();
-  Value *Func = Intrinsic::getDeclaration(M, Intrinsic::ctpop, Tys);
-  CallInst *CI = IRBuilder.CreateCall(Func, Ops);
-  CI->setDebugLoc(DL);
-
-  return CI;
-}
-
-/// recognize - detect population count idiom in a non-countable loop. If
-///   detected, transform the relevant code to popcount intrinsic function
-///   call, and return true; otherwise, return false.
-bool NclPopcountRecognize::recognize() {
-
-  if (!LIR.getTargetTransformInfo())
+  // Disable loop idiom recognition if the function's name is a common idiom.
+  StringRef Name = L->getHeader()->getParent()->getName();
+  if (Name == "memset" || Name == "memcpy")
     return false;
 
-  LIR.getScalarEvolution();
-
-  if (!preliminaryScreen())
-    return false;
+  AA = &getAnalysis<AAResultsWrapperPass>().getAAResults();
+  DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
+  LI = &getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
+  SE = &getAnalysis<ScalarEvolutionWrapperPass>().getSE();
+  TLI = &getAnalysis<TargetLibraryInfoWrapperPass>().getTLI();
+  TTI = &getAnalysis<TargetTransformInfoWrapperPass>().getTTI(
+      *CurLoop->getHeader()->getParent());
+  DL = &CurLoop->getHeader()->getModule()->getDataLayout();
 
-  Instruction *CntInst;
-  PHINode *CntPhi;
-  Value *Val;
-  if (!detectIdiom(CntInst, CntPhi, Val))
-    return false;
+  if (SE->hasLoopInvariantBackedgeTakenCount(L))
+    return runOnCountableLoop();
 
-  transform(CntInst, CntPhi, Val);
-  return true;
+  return runOnNoncountableLoop();
 }
 
-//===----------------------------------------------------------------------===//
-//
-//          Implementation of LoopIdiomRecognize
-//
-//===----------------------------------------------------------------------===//
-
 bool LoopIdiomRecognize::runOnCountableLoop() {
   const SCEV *BECount = SE->getBackedgeTakenCount(CurLoop);
   assert(!isa<SCEVCouldNotCompute>(BECount) &&
-    "runOnCountableLoop() called on a loop without a predictable"
-    "backedge-taken count");
+         "runOnCountableLoop() called on a loop without a predictable"
+         "backedge-taken count");
 
   // If this loop executes exactly one time, then it should be peeled, not
   // optimized by this pass.
   if (const SCEVConstant *BECst = dyn_cast<SCEVConstant>(BECount))
-    if (BECst->getValue()->getValue() == 0)
+    if (BECst->getAPInt() == 0)
       return false;
 
-  // set DT
-  (void)getDominatorTree();
-
-  LoopInfo &LI = getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
-  TLI = &getAnalysis<TargetLibraryInfoWrapperPass>().getTLI();
-
-  // set TLI
-  (void)getTargetLibraryInfo();
-
-  SmallVector<BasicBlock*, 8> ExitBlocks;
+  SmallVector<BasicBlock *, 8> ExitBlocks;
   CurLoop->getUniqueExitBlocks(ExitBlocks);
 
   DEBUG(dbgs() << "loop-idiom Scanning: F["
-               << CurLoop->getHeader()->getParent()->getName()
-               << "] Loop %" << CurLoop->getHeader()->getName() << "\n");
+               << CurLoop->getHeader()->getParent()->getName() << "] Loop %"
+               << CurLoop->getHeader()->getName() << "\n");
 
   bool MadeChange = false;
   // Scan all the blocks in the loop that are not in subloops.
   for (auto *BB : CurLoop->getBlocks()) {
     // Ignore blocks in subloops.
-    if (LI.getLoopFor(BB) != CurLoop)
+    if (LI->getLoopFor(BB) != CurLoop)
       continue;
 
     MadeChange |= runOnLoopBlock(BB, BECount, ExitBlocks);
@@ -649,41 +244,109 @@ bool LoopIdiomRecognize::runOnCountableLoop() {
   return MadeChange;
 }
 
-bool LoopIdiomRecognize::runOnNoncountableLoop() {
-  NclPopcountRecognize Popcount(*this);
-  if (Popcount.recognize())
-    return true;
+static unsigned getStoreSizeInBytes(StoreInst *SI, const DataLayout *DL) {
+  uint64_t SizeInBits = DL->getTypeSizeInBits(SI->getValueOperand()->getType());
+  assert(((SizeInBits & 7) || (SizeInBits >> 32) == 0) &&
+         "Don't overflow unsigned.");
+  return (unsigned)SizeInBits >> 3;
+}
 
-  return false;
+static unsigned getStoreStride(const SCEVAddRecExpr *StoreEv) {
+  const SCEVConstant *ConstStride = cast<SCEVConstant>(StoreEv->getOperand(1));
+  return ConstStride->getAPInt().getZExtValue();
 }
 
-bool LoopIdiomRecognize::runOnLoop(Loop *L, LPPassManager &LPM) {
-  if (skipOptnoneFunction(L))
+/// getMemSetPatternValue - If a strided store of the specified value is safe to
+/// turn into a memset_pattern16, return a ConstantArray of 16 bytes that should
+/// be passed in.  Otherwise, return null.
+///
+/// Note that we don't ever attempt to use memset_pattern8 or 4, because these
+/// just replicate their input array and then pass on to memset_pattern16.
+static Constant *getMemSetPatternValue(Value *V, const DataLayout *DL) {
+  // If the value isn't a constant, we can't promote it to being in a constant
+  // array.  We could theoretically do a store to an alloca or something, but
+  // that doesn't seem worthwhile.
+  Constant *C = dyn_cast<Constant>(V);
+  if (!C)
+    return nullptr;
+
+  // Only handle simple values that are a power of two bytes in size.
+  uint64_t Size = DL->getTypeSizeInBits(V->getType());
+  if (Size == 0 || (Size & 7) || (Size & (Size - 1)))
+    return nullptr;
+
+  // Don't care enough about darwin/ppc to implement this.
+  if (DL->isBigEndian())
+    return nullptr;
+
+  // Convert to size in bytes.
+  Size /= 8;
+
+  // TODO: If CI is larger than 16-bytes, we can try slicing it in half to see
+  // if the top and bottom are the same (e.g. for vectors and large integers).
+  if (Size > 16)
+    return nullptr;
+
+  // If the constant is exactly 16 bytes, just use it.
+  if (Size == 16)
+    return C;
+
+  // Otherwise, we'll use an array of the constants.
+  unsigned ArraySize = 16 / Size;
+  ArrayType *AT = ArrayType::get(V->getType(), ArraySize);
+  return ConstantArray::get(AT, std::vector<Constant *>(ArraySize, C));
+}
+
+bool LoopIdiomRecognize::isLegalStore(StoreInst *SI) {
+  // Don't touch volatile stores.
+  if (!SI->isSimple())
     return false;
 
-  CurLoop = L;
+  Value *StoredVal = SI->getValueOperand();
+  Value *StorePtr = SI->getPointerOperand();
 
-  // If the loop could not be converted to canonical form, it must have an
-  // indirectbr in it, just give up.
-  if (!L->getLoopPreheader())
+  // Reject stores that are so large that they overflow an unsigned.
+  uint64_t SizeInBits = DL->getTypeSizeInBits(StoredVal->getType());
+  if ((SizeInBits & 7) || (SizeInBits >> 32) != 0)
     return false;
 
-  // Disable loop idiom recognition if the function's name is a common idiom.
-  StringRef Name = L->getHeader()->getParent()->getName();
-  if (Name == "memset" || Name == "memcpy")
+  // See if the pointer expression is an AddRec like {base,+,1} on the current
+  // loop, which indicates a strided store.  If we have something else, it's a
+  // random store we can't handle.
+  const SCEVAddRecExpr *StoreEv =
+      dyn_cast<SCEVAddRecExpr>(SE->getSCEV(StorePtr));
+  if (!StoreEv || StoreEv->getLoop() != CurLoop || !StoreEv->isAffine())
     return false;
 
-  SE = &getAnalysis<ScalarEvolution>();
-  if (SE->hasLoopInvariantBackedgeTakenCount(L))
-    return runOnCountableLoop();
-  return runOnNoncountableLoop();
+  // Check to see if we have a constant stride.
+  if (!isa<SCEVConstant>(StoreEv->getOperand(1)))
+    return false;
+
+  return true;
+}
+
+void LoopIdiomRecognize::collectStores(BasicBlock *BB) {
+  StoreRefs.clear();
+  for (Instruction &I : *BB) {
+    StoreInst *SI = dyn_cast<StoreInst>(&I);
+    if (!SI)
+      continue;
+
+    // Make sure this is a strided store with a constant stride.
+    if (!isLegalStore(SI))
+      continue;
+
+    // Save the store locations.
+    StoreRefs.push_back(SI);
+  }
 }
 
 /// runOnLoopBlock - Process the specified block, which lives in a counted loop
 /// with the specified backedge count.  This block is known to be in the current
 /// loop and not in any subloops.
-bool LoopIdiomRecognize::runOnLoopBlock(BasicBlock *BB, const SCEV *BECount,
-                                     SmallVectorImpl<BasicBlock*> &ExitBlocks) {
+bool LoopIdiomRecognize::runOnLoopBlock(
+    BasicBlock *BB, const SCEV *BECount,
+    SmallVectorImpl<BasicBlock *> &ExitBlocks) {
   // We can only promote stores in this block if they are unconditionally
   // executed in the loop.  For a block to be unconditionally executed, it has
   // to dominate all the exit blocks of the loop.  Verify this now.
@@ -692,25 +355,18 @@ bool LoopIdiomRecognize::runOnLoopBlock(BasicBlock *BB, const SCEV *BECount,
       return false;
 
   bool MadeChange = false;
-  for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ) {
-    Instruction *Inst = I++;
-    // Look for store instructions, which may be optimized to memset/memcpy.
-    if (StoreInst *SI = dyn_cast<StoreInst>(Inst))  {
-      WeakVH InstPtr(I);
-      if (!processLoopStore(SI, BECount)) continue;
-      MadeChange = true;
-
-      // If processing the store invalidated our iterator, start over from the
-      // top of the block.
-      if (!InstPtr)
-        I = BB->begin();
-      continue;
-    }
+  // Look for store instructions, which may be optimized to memset/memcpy.
+  collectStores(BB);
+  for (auto &SI : StoreRefs)
+    MadeChange |= processLoopStore(SI, BECount);
 
+  for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E;) {
+    Instruction *Inst = &*I++;
     // Look for memset instructions, which may be optimized to a larger memset.
-    if (MemSetInst *MSI = dyn_cast<MemSetInst>(Inst))  {
-      WeakVH InstPtr(I);
-      if (!processLoopMemSet(MSI, BECount)) continue;
+    if (MemSetInst *MSI = dyn_cast<MemSetInst>(Inst)) {
+      WeakVH InstPtr(&*I);
+      if (!processLoopMemSet(MSI, BECount))
+        continue;
       MadeChange = true;
 
       // If processing the memset invalidated our iterator, start over from the
@@ -724,71 +380,38 @@ bool LoopIdiomRecognize::runOnLoopBlock(BasicBlock *BB, const SCEV *BECount,
   return MadeChange;
 }
 
-
 /// processLoopStore - See if this store can be promoted to a memset or memcpy.
 bool LoopIdiomRecognize::processLoopStore(StoreInst *SI, const SCEV *BECount) {
-  if (!SI->isSimple()) return false;
+  assert(SI->isSimple() && "Expected only non-volatile stores.");
 
   Value *StoredVal = SI->getValueOperand();
   Value *StorePtr = SI->getPointerOperand();
 
-  // Reject stores that are so large that they overflow an unsigned.
-  auto &DL = CurLoop->getHeader()->getModule()->getDataLayout();
-  uint64_t SizeInBits = DL.getTypeSizeInBits(StoredVal->getType());
-  if ((SizeInBits & 7) || (SizeInBits >> 32) != 0)
-    return false;
-
-  // See if the pointer expression is an AddRec like {base,+,1} on the current
-  // loop, which indicates a strided store.  If we have something else, it's a
-  // random store we can't handle.
-  const SCEVAddRecExpr *StoreEv =
-    dyn_cast<SCEVAddRecExpr>(SE->getSCEV(StorePtr));
-  if (!StoreEv || StoreEv->getLoop() != CurLoop || !StoreEv->isAffine())
-    return false;
-
   // Check to see if the stride matches the size of the store.  If so, then we
   // know that every byte is touched in the loop.
-  unsigned StoreSize = (unsigned)SizeInBits >> 3;
-  const SCEVConstant *Stride = dyn_cast<SCEVConstant>(StoreEv->getOperand(1));
-
-  if (!Stride || StoreSize != Stride->getValue()->getValue()) {
-    // TODO: Could also handle negative stride here someday, that will require
-    // the validity check in mayLoopAccessLocation to be updated though.
-    // Enable this to print exact negative strides.
-    if (0 && Stride && StoreSize == -Stride->getValue()->getValue()) {
-      dbgs() << "NEGATIVE STRIDE: " << *SI << "\n";
-      dbgs() << "BB: " << *SI->getParent();
-    }
-
+  const SCEVAddRecExpr *StoreEv = cast<SCEVAddRecExpr>(SE->getSCEV(StorePtr));
+  unsigned Stride = getStoreStride(StoreEv);
+  unsigned StoreSize = getStoreSizeInBytes(SI, DL);
+  if (StoreSize != Stride && StoreSize != -Stride)
     return false;
-  }
+
+  bool NegStride = StoreSize == -Stride;
 
   // See if we can optimize just this store in isolation.
   if (processLoopStridedStore(StorePtr, StoreSize, SI->getAlignment(),
-                              StoredVal, SI, StoreEv, BECount))
+                              StoredVal, SI, StoreEv, BECount, NegStride))
     return true;
 
-  // If the stored value is a strided load in the same loop with the same stride
-  // this this may be transformable into a memcpy.  This kicks in for stuff like
-  //   for (i) A[i] = B[i];
-  if (LoadInst *LI = dyn_cast<LoadInst>(StoredVal)) {
-    const SCEVAddRecExpr *LoadEv =
-      dyn_cast<SCEVAddRecExpr>(SE->getSCEV(LI->getOperand(0)));
-    if (LoadEv && LoadEv->getLoop() == CurLoop && LoadEv->isAffine() &&
-        StoreEv->getOperand(1) == LoadEv->getOperand(1) && LI->isSimple())
-      if (processLoopStoreOfLoopLoad(SI, StoreSize, StoreEv, LoadEv, BECount))
-        return true;
-  }
-  //errs() << "UNHANDLED strided store: " << *StoreEv << " - " << *SI << "\n";
-
-  return false;
+  // Optimize the store into a memcpy, if it feeds an similarly strided load.
+  return processLoopStoreOfLoopLoad(SI, StoreSize, StoreEv, BECount, NegStride);
 }
 
 /// processLoopMemSet - See if this memset can be promoted to a large memset.
-bool LoopIdiomRecognize::
-processLoopMemSet(MemSetInst *MSI, const SCEV *BECount) {
+bool LoopIdiomRecognize::processLoopMemSet(MemSetInst *MSI,
+                                           const SCEV *BECount) {
   // We can only handle non-volatile memsets with a constant size.
-  if (MSI->isVolatile() || !isa<ConstantInt>(MSI->getLength())) return false;
+  if (MSI->isVolatile() || !isa<ConstantInt>(MSI->getLength()))
+    return false;
 
   // If we're not allowed to hack on memset, we fail.
   if (!TLI->has(LibFunc::memset))
@@ -818,17 +441,16 @@ processLoopMemSet(MemSetInst *MSI, const SCEV *BECount) {
     return false;
 
   return processLoopStridedStore(Pointer, (unsigned)SizeInBytes,
-                                 MSI->getAlignment(), MSI->getValue(),
-                                 MSI, Ev, BECount);
+                                 MSI->getAlignment(), MSI->getValue(), MSI, Ev,
+                                 BECount, /*NegStride=*/false);
 }
 
-
 /// mayLoopAccessLocation - Return true if the specified loop might access the
 /// specified pointer location, which is a loop-strided access.  The 'Access'
 /// argument specifies what the verboten forms of access are (read or write).
-static bool mayLoopAccessLocation(Value *Ptr,AliasAnalysis::ModRefResult Access,
-                                  Loop *L, const SCEV *BECount,
-                                  unsigned StoreSize, AliasAnalysis &AA,
+static bool mayLoopAccessLocation(Value *Ptr, ModRefInfo Access, Loop *L,
+                                  const SCEV *BECount, unsigned StoreSize,
+                                  AliasAnalysis &AA,
                                   Instruction *IgnoredStore) {
   // Get the location that may be stored across the loop.  Since the access is
   // strided positively through memory, we say that the modified location starts
@@ -838,7 +460,7 @@ static bool mayLoopAccessLocation(Value *Ptr,AliasAnalysis::ModRefResult Access,
   // If the loop iterates a fixed number of times, we can refine the access size
   // to be exactly the size of the memset, which is (BECount+1)*StoreSize
   if (const SCEVConstant *BECst = dyn_cast<SCEVConstant>(BECount))
-    AccessSize = (BECst->getValue()->getZExtValue()+1)*StoreSize;
+    AccessSize = (BECst->getValue()->getZExtValue() + 1) * StoreSize;
 
   // TODO: For this to be really effective, we have to dive into the pointer
   // operand in the store.  Store to &A[i] of 100 will always return may alias
@@ -849,59 +471,31 @@ static bool mayLoopAccessLocation(Value *Ptr,AliasAnalysis::ModRefResult Access,
   for (Loop::block_iterator BI = L->block_begin(), E = L->block_end(); BI != E;
        ++BI)
     for (BasicBlock::iterator I = (*BI)->begin(), E = (*BI)->end(); I != E; ++I)
-      if (&*I != IgnoredStore &&
-          (AA.getModRefInfo(I, StoreLoc) & Access))
+      if (&*I != IgnoredStore && (AA.getModRefInfo(&*I, StoreLoc) & Access))
         return true;
 
   return false;
 }
 
-/// getMemSetPatternValue - If a strided store of the specified value is safe to
-/// turn into a memset_pattern16, return a ConstantArray of 16 bytes that should
-/// be passed in.  Otherwise, return null.
-///
-/// Note that we don't ever attempt to use memset_pattern8 or 4, because these
-/// just replicate their input array and then pass on to memset_pattern16.
-static Constant *getMemSetPatternValue(Value *V, const DataLayout &DL) {
-  // If the value isn't a constant, we can't promote it to being in a constant
-  // array.  We could theoretically do a store to an alloca or something, but
-  // that doesn't seem worthwhile.
-  Constant *C = dyn_cast<Constant>(V);
-  if (!C) return nullptr;
-
-  // Only handle simple values that are a power of two bytes in size.
-  uint64_t Size = DL.getTypeSizeInBits(V->getType());
-  if (Size == 0 || (Size & 7) || (Size & (Size-1)))
-    return nullptr;
-
-  // Don't care enough about darwin/ppc to implement this.
-  if (DL.isBigEndian())
-    return nullptr;
-
-  // Convert to size in bytes.
-  Size /= 8;
-
-  // TODO: If CI is larger than 16-bytes, we can try slicing it in half to see
-  // if the top and bottom are the same (e.g. for vectors and large integers).
-  if (Size > 16) return nullptr;
-
-  // If the constant is exactly 16 bytes, just use it.
-  if (Size == 16) return C;
-
-  // Otherwise, we'll use an array of the constants.
-  unsigned ArraySize = 16/Size;
-  ArrayType *AT = ArrayType::get(V->getType(), ArraySize);
-  return ConstantArray::get(AT, std::vector<Constant*>(ArraySize, C));
+// If we have a negative stride, Start refers to the end of the memory location
+// we're trying to memset.  Therefore, we need to recompute the base pointer,
+// which is just Start - BECount*Size.
+static const SCEV *getStartForNegStride(const SCEV *Start, const SCEV *BECount,
+                                        Type *IntPtr, unsigned StoreSize,
+                                        ScalarEvolution *SE) {
+  const SCEV *Index = SE->getTruncateOrZeroExtend(BECount, IntPtr);
+  if (StoreSize != 1)
+    Index = SE->getMulExpr(Index, SE->getConstant(IntPtr, StoreSize),
+                           SCEV::FlagNUW);
+  return SE->getMinusSCEV(Start, Index);
 }
 
-
 /// processLoopStridedStore - We see a strided store of some value.  If we can
 /// transform this into a memset or memset_pattern in the loop preheader, do so.
-bool LoopIdiomRecognize::
-processLoopStridedStore(Value *DestPtr, unsigned StoreSize,
-                        unsigned StoreAlignment, Value *StoredVal,
-                        Instruction *TheStore, const SCEVAddRecExpr *Ev,
-                        const SCEV *BECount) {
+bool LoopIdiomRecognize::processLoopStridedStore(
+    Value *DestPtr, unsigned StoreSize, unsigned StoreAlignment,
+    Value *StoredVal, Instruction *TheStore, const SCEVAddRecExpr *Ev,
+    const SCEV *BECount, bool NegStride) {
 
   // If the stored value is a byte-wise value (like i32 -1), then it may be
   // turned into a memset of i8 -1, assuming that all the consecutive bytes
@@ -909,7 +503,6 @@ processLoopStridedStore(Value *DestPtr, unsigned StoreSize,
   // but it can be turned into memset_pattern if the target supports it.
   Value *SplatValue = isBytewiseValue(StoredVal);
   Constant *PatternValue = nullptr;
-  auto &DL = CurLoop->getHeader()->getModule()->getDataLayout();
   unsigned DestAS = DestPtr->getType()->getPointerAddressSpace();
 
   // If we're allowed to form a memset, and the stored value would be acceptable
@@ -936,9 +529,15 @@ processLoopStridedStore(Value *DestPtr, unsigned StoreSize,
   // header.  This allows us to insert code for it in the preheader.
   BasicBlock *Preheader = CurLoop->getLoopPreheader();
   IRBuilder<> Builder(Preheader->getTerminator());
-  SCEVExpander Expander(*SE, DL, "loop-idiom");
+  SCEVExpander Expander(*SE, *DL, "loop-idiom");
 
   Type *DestInt8PtrTy = Builder.getInt8PtrTy(DestAS);
+  Type *IntPtr = Builder.getIntPtrTy(*DL, DestAS);
+
+  const SCEV *Start = Ev->getStart();
+  // Handle negative strided loops.
+  if (NegStride)
+    Start = getStartForNegStride(Start, BECount, IntPtr, StoreSize, SE);
 
   // Okay, we have a strided store "p[i]" of a splattable value.  We can turn
   // this into a memset in the loop preheader now if we want.  However, this
@@ -946,12 +545,9 @@ processLoopStridedStore(Value *DestPtr, unsigned StoreSize,
   // or write to the aliased location.  Check for any overlap by generating the
   // base pointer and checking the region.
   Value *BasePtr =
-    Expander.expandCodeFor(Ev->getStart(), DestInt8PtrTy,
-                           Preheader->getTerminator());
-
-  if (mayLoopAccessLocation(BasePtr, AliasAnalysis::ModRef,
-                            CurLoop, BECount,
-                            StoreSize, getAnalysis<AliasAnalysis>(), TheStore)) {
+      Expander.expandCodeFor(Start, DestInt8PtrTy, Preheader->getTerminator());
+  if (mayLoopAccessLocation(BasePtr, MRI_ModRef, CurLoop, BECount, StoreSize,
+                            *AA, TheStore)) {
     Expander.clear();
     // If we generated new code for the base pointer, clean up.
     RecursivelyDeleteTriviallyDeadInstructions(BasePtr, TLI);
@@ -962,36 +558,30 @@ processLoopStridedStore(Value *DestPtr, unsigned StoreSize,
 
   // The # stored bytes is (BECount+1)*Size.  Expand the trip count out to
   // pointer size if it isn't already.
-  Type *IntPtr = Builder.getIntPtrTy(DL, DestAS);
   BECount = SE->getTruncateOrZeroExtend(BECount, IntPtr);
 
-  const SCEV *NumBytesS = SE->getAddExpr(BECount, SE->getConstant(IntPtr, 1),
-                                         SCEV::FlagNUW);
+  const SCEV *NumBytesS =
+      SE->getAddExpr(BECount, SE->getOne(IntPtr), SCEV::FlagNUW);
   if (StoreSize != 1) {
     NumBytesS = SE->getMulExpr(NumBytesS, SE->getConstant(IntPtr, StoreSize),
                                SCEV::FlagNUW);
   }
 
   Value *NumBytes =
-    Expander.expandCodeFor(NumBytesS, IntPtr, Preheader->getTerminator());
+      Expander.expandCodeFor(NumBytesS, IntPtr, Preheader->getTerminator());
 
   CallInst *NewCall;
   if (SplatValue) {
-    NewCall = Builder.CreateMemSet(BasePtr,
-                                   SplatValue,
-                                   NumBytes,
-                                   StoreAlignment);
+    NewCall =
+        Builder.CreateMemSet(BasePtr, SplatValue, NumBytes, StoreAlignment);
   } else {
     // Everything is emitted in default address space
     Type *Int8PtrTy = DestInt8PtrTy;
 
-    Module *M = TheStore->getParent()->getParent()->getParent();
-    Value *MSP = M->getOrInsertFunction("memset_pattern16",
-                                        Builder.getVoidTy(),
-                                        Int8PtrTy,
-                                        Int8PtrTy,
-                                        IntPtr,
-                                        (void*)nullptr);
+    Module *M = TheStore->getModule();
+    Value *MSP =
+        M->getOrInsertFunction("memset_pattern16", Builder.getVoidTy(),
+                               Int8PtrTy, Int8PtrTy, IntPtr, (void *)nullptr);
 
     // Otherwise we should form a memset_pattern16.  PatternValue is known to be
     // an constant array of 16-bytes.  Plop the value into a mergable global.
@@ -1015,26 +605,47 @@ processLoopStridedStore(Value *DestPtr, unsigned StoreSize,
   return true;
 }
 
-/// processLoopStoreOfLoopLoad - We see a strided store whose value is a
-/// same-strided load.
-bool LoopIdiomRecognize::
-processLoopStoreOfLoopLoad(StoreInst *SI, unsigned StoreSize,
-                           const SCEVAddRecExpr *StoreEv,
-                           const SCEVAddRecExpr *LoadEv,
-                           const SCEV *BECount) {
+/// If the stored value is a strided load in the same loop with the same stride
+/// this may be transformable into a memcpy.  This kicks in for stuff like
+///   for (i) A[i] = B[i];
+bool LoopIdiomRecognize::processLoopStoreOfLoopLoad(
+    StoreInst *SI, unsigned StoreSize, const SCEVAddRecExpr *StoreEv,
+    const SCEV *BECount, bool NegStride) {
   // If we're not allowed to form memcpy, we fail.
   if (!TLI->has(LibFunc::memcpy))
     return false;
 
-  LoadInst *LI = cast<LoadInst>(SI->getValueOperand());
+  // The store must be feeding a non-volatile load.
+  LoadInst *LI = dyn_cast<LoadInst>(SI->getValueOperand());
+  if (!LI || !LI->isSimple())
+    return false;
+
+  // See if the pointer expression is an AddRec like {base,+,1} on the current
+  // loop, which indicates a strided load.  If we have something else, it's a
+  // random load we can't handle.
+  const SCEVAddRecExpr *LoadEv =
+      dyn_cast<SCEVAddRecExpr>(SE->getSCEV(LI->getPointerOperand()));
+  if (!LoadEv || LoadEv->getLoop() != CurLoop || !LoadEv->isAffine())
+    return false;
+
+  // The store and load must share the same stride.
+  if (StoreEv->getOperand(1) != LoadEv->getOperand(1))
+    return false;
 
   // The trip count of the loop and the base pointer of the addrec SCEV is
   // guaranteed to be loop invariant, which means that it should dominate the
   // header.  This allows us to insert code for it in the preheader.
   BasicBlock *Preheader = CurLoop->getLoopPreheader();
   IRBuilder<> Builder(Preheader->getTerminator());
-  const DataLayout &DL = Preheader->getModule()->getDataLayout();
-  SCEVExpander Expander(*SE, DL, "loop-idiom");
+  SCEVExpander Expander(*SE, *DL, "loop-idiom");
+
+  const SCEV *StrStart = StoreEv->getStart();
+  unsigned StrAS = SI->getPointerAddressSpace();
+  Type *IntPtrTy = Builder.getIntPtrTy(*DL, StrAS);
+
+  // Handle negative strided loops.
+  if (NegStride)
+    StrStart = getStartForNegStride(StrStart, BECount, IntPtrTy, StoreSize, SE);
 
   // Okay, we have a strided store "p[i]" of a loaded value.  We can turn
   // this into a memcpy in the loop preheader now if we want.  However, this
@@ -1042,29 +653,31 @@ processLoopStoreOfLoopLoad(StoreInst *SI, unsigned StoreSize,
   // or write the memory region we're storing to.  This includes the load that
   // feeds the stores.  Check for an alias by generating the base address and
   // checking everything.
-  Value *StoreBasePtr =
-    Expander.expandCodeFor(StoreEv->getStart(),
-                           Builder.getInt8PtrTy(SI->getPointerAddressSpace()),
-                           Preheader->getTerminator());
-
-  if (mayLoopAccessLocation(StoreBasePtr, AliasAnalysis::ModRef,
-                            CurLoop, BECount, StoreSize,
-                            getAnalysis<AliasAnalysis>(), SI)) {
+  Value *StoreBasePtr = Expander.expandCodeFor(
+      StrStart, Builder.getInt8PtrTy(StrAS), Preheader->getTerminator());
+
+  if (mayLoopAccessLocation(StoreBasePtr, MRI_ModRef, CurLoop, BECount,
+                            StoreSize, *AA, SI)) {
     Expander.clear();
     // If we generated new code for the base pointer, clean up.
     RecursivelyDeleteTriviallyDeadInstructions(StoreBasePtr, TLI);
     return false;
   }
 
+  const SCEV *LdStart = LoadEv->getStart();
+  unsigned LdAS = LI->getPointerAddressSpace();
+
+  // Handle negative strided loops.
+  if (NegStride)
+    LdStart = getStartForNegStride(LdStart, BECount, IntPtrTy, StoreSize, SE);
+
   // For a memcpy, we have to make sure that the input array is not being
   // mutated by the loop.
-  Value *LoadBasePtr =
-    Expander.expandCodeFor(LoadEv->getStart(),
-                           Builder.getInt8PtrTy(LI->getPointerAddressSpace()),
-                           Preheader->getTerminator());
+  Value *LoadBasePtr = Expander.expandCodeFor(
+      LdStart, Builder.getInt8PtrTy(LdAS), Preheader->getTerminator());
 
-  if (mayLoopAccessLocation(LoadBasePtr, AliasAnalysis::Mod, CurLoop, BECount,
-                            StoreSize, getAnalysis<AliasAnalysis>(), SI)) {
+  if (mayLoopAccessLocation(LoadBasePtr, MRI_Mod, CurLoop, BECount, StoreSize,
+                            *AA, SI)) {
     Expander.clear();
     // If we generated new code for the base pointer, clean up.
     RecursivelyDeleteTriviallyDeadInstructions(LoadBasePtr, TLI);
@@ -1074,34 +687,368 @@ processLoopStoreOfLoopLoad(StoreInst *SI, unsigned StoreSize,
 
   // Okay, everything is safe, we can transform this!
 
-
   // The # stored bytes is (BECount+1)*Size.  Expand the trip count out to
   // pointer size if it isn't already.
-  Type *IntPtrTy = Builder.getIntPtrTy(DL, SI->getPointerAddressSpace());
   BECount = SE->getTruncateOrZeroExtend(BECount, IntPtrTy);
 
-  const SCEV *NumBytesS = SE->getAddExpr(BECount, SE->getConstant(IntPtrTy, 1),
-                                         SCEV::FlagNUW);
+  const SCEV *NumBytesS =
+      SE->getAddExpr(BECount, SE->getOne(IntPtrTy), SCEV::FlagNUW);
   if (StoreSize != 1)
     NumBytesS = SE->getMulExpr(NumBytesS, SE->getConstant(IntPtrTy, StoreSize),
                                SCEV::FlagNUW);
 
   Value *NumBytes =
-    Expander.expandCodeFor(NumBytesS, IntPtrTy, Preheader->getTerminator());
+      Expander.expandCodeFor(NumBytesS, IntPtrTy, Preheader->getTerminator());
 
   CallInst *NewCall =
-    Builder.CreateMemCpy(StoreBasePtr, LoadBasePtr, NumBytes,
-                         std::min(SI->getAlignment(), LI->getAlignment()));
+      Builder.CreateMemCpy(StoreBasePtr, LoadBasePtr, NumBytes,
+                           std::min(SI->getAlignment(), LI->getAlignment()));
   NewCall->setDebugLoc(SI->getDebugLoc());
 
   DEBUG(dbgs() << "  Formed memcpy: " << *NewCall << "\n"
                << "    from load ptr=" << *LoadEv << " at: " << *LI << "\n"
                << "    from store ptr=" << *StoreEv << " at: " << *SI << "\n");
 
-
-  // Okay, the memset has been formed.  Zap the original store and anything that
+  // Okay, the memcpy has been formed.  Zap the original store and anything that
   // feeds into it.
   deleteDeadInstruction(SI, TLI);
   ++NumMemCpy;
   return true;
 }
+
+bool LoopIdiomRecognize::runOnNoncountableLoop() {
+  return recognizePopcount();
+}
+
+/// Check if the given conditional branch is based on the comparison between
+/// a variable and zero, and if the variable is non-zero, the control yields to
+/// the loop entry. If the branch matches the behavior, the variable involved
+/// in the comparion is returned. This function will be called to see if the
+/// precondition and postcondition of the loop are in desirable form.
+static Value *matchCondition(BranchInst *BI, BasicBlock *LoopEntry) {
+  if (!BI || !BI->isConditional())
+    return nullptr;
+
+  ICmpInst *Cond = dyn_cast<ICmpInst>(BI->getCondition());
+  if (!Cond)
+    return nullptr;
+
+  ConstantInt *CmpZero = dyn_cast<ConstantInt>(Cond->getOperand(1));
+  if (!CmpZero || !CmpZero->isZero())
+    return nullptr;
+
+  ICmpInst::Predicate Pred = Cond->getPredicate();
+  if ((Pred == ICmpInst::ICMP_NE && BI->getSuccessor(0) == LoopEntry) ||
+      (Pred == ICmpInst::ICMP_EQ && BI->getSuccessor(1) == LoopEntry))
+    return Cond->getOperand(0);
+
+  return nullptr;
+}
+
+/// Return true iff the idiom is detected in the loop.
+///
+/// Additionally:
+/// 1) \p CntInst is set to the instruction counting the population bit.
+/// 2) \p CntPhi is set to the corresponding phi node.
+/// 3) \p Var is set to the value whose population bits are being counted.
+///
+/// The core idiom we are trying to detect is:
+/// \code
+///    if (x0 != 0)
+///      goto loop-exit // the precondition of the loop
+///    cnt0 = init-val;
+///    do {
+///       x1 = phi (x0, x2);
+///       cnt1 = phi(cnt0, cnt2);
+///
+///       cnt2 = cnt1 + 1;
+///        ...
+///       x2 = x1 & (x1 - 1);
+///        ...
+///    } while(x != 0);
+///
+/// loop-exit:
+/// \endcode
+static bool detectPopcountIdiom(Loop *CurLoop, BasicBlock *PreCondBB,
+                                Instruction *&CntInst, PHINode *&CntPhi,
+                                Value *&Var) {
+  // step 1: Check to see if the look-back branch match this pattern:
+  //    "if (a!=0) goto loop-entry".
+  BasicBlock *LoopEntry;
+  Instruction *DefX2, *CountInst;
+  Value *VarX1, *VarX0;
+  PHINode *PhiX, *CountPhi;
+
+  DefX2 = CountInst = nullptr;
+  VarX1 = VarX0 = nullptr;
+  PhiX = CountPhi = nullptr;
+  LoopEntry = *(CurLoop->block_begin());
+
+  // step 1: Check if the loop-back branch is in desirable form.
+  {
+    if (Value *T = matchCondition(
+            dyn_cast<BranchInst>(LoopEntry->getTerminator()), LoopEntry))
+      DefX2 = dyn_cast<Instruction>(T);
+    else
+      return false;
+  }
+
+  // step 2: detect instructions corresponding to "x2 = x1 & (x1 - 1)"
+  {
+    if (!DefX2 || DefX2->getOpcode() != Instruction::And)
+      return false;
+
+    BinaryOperator *SubOneOp;
+
+    if ((SubOneOp = dyn_cast<BinaryOperator>(DefX2->getOperand(0))))
+      VarX1 = DefX2->getOperand(1);
+    else {
+      VarX1 = DefX2->getOperand(0);
+      SubOneOp = dyn_cast<BinaryOperator>(DefX2->getOperand(1));
+    }
+    if (!SubOneOp)
+      return false;
+
+    Instruction *SubInst = cast<Instruction>(SubOneOp);
+    ConstantInt *Dec = dyn_cast<ConstantInt>(SubInst->getOperand(1));
+    if (!Dec ||
+        !((SubInst->getOpcode() == Instruction::Sub && Dec->isOne()) ||
+          (SubInst->getOpcode() == Instruction::Add &&
+           Dec->isAllOnesValue()))) {
+      return false;
+    }
+  }
+
+  // step 3: Check the recurrence of variable X
+  {
+    PhiX = dyn_cast<PHINode>(VarX1);
+    if (!PhiX ||
+        (PhiX->getOperand(0) != DefX2 && PhiX->getOperand(1) != DefX2)) {
+      return false;
+    }
+  }
+
+  // step 4: Find the instruction which count the population: cnt2 = cnt1 + 1
+  {
+    CountInst = nullptr;
+    for (BasicBlock::iterator Iter = LoopEntry->getFirstNonPHI()->getIterator(),
+                              IterE = LoopEntry->end();
+         Iter != IterE; Iter++) {
+      Instruction *Inst = &*Iter;
+      if (Inst->getOpcode() != Instruction::Add)
+        continue;
+
+      ConstantInt *Inc = dyn_cast<ConstantInt>(Inst->getOperand(1));
+      if (!Inc || !Inc->isOne())
+        continue;
+
+      PHINode *Phi = dyn_cast<PHINode>(Inst->getOperand(0));
+      if (!Phi || Phi->getParent() != LoopEntry)
+        continue;
+
+      // Check if the result of the instruction is live of the loop.
+      bool LiveOutLoop = false;
+      for (User *U : Inst->users()) {
+        if ((cast<Instruction>(U))->getParent() != LoopEntry) {
+          LiveOutLoop = true;
+          break;
+        }
+      }
+
+      if (LiveOutLoop) {
+        CountInst = Inst;
+        CountPhi = Phi;
+        break;
+      }
+    }
+
+    if (!CountInst)
+      return false;
+  }
+
+  // step 5: check if the precondition is in this form:
+  //   "if (x != 0) goto loop-head ; else goto somewhere-we-don't-care;"
+  {
+    auto *PreCondBr = dyn_cast<BranchInst>(PreCondBB->getTerminator());
+    Value *T = matchCondition(PreCondBr, CurLoop->getLoopPreheader());
+    if (T != PhiX->getOperand(0) && T != PhiX->getOperand(1))
+      return false;
+
+    CntInst = CountInst;
+    CntPhi = CountPhi;
+    Var = T;
+  }
+
+  return true;
+}
+
+/// Recognizes a population count idiom in a non-countable loop.
+///
+/// If detected, transforms the relevant code to issue the popcount intrinsic
+/// function call, and returns true; otherwise, returns false.
+bool LoopIdiomRecognize::recognizePopcount() {
+  if (TTI->getPopcntSupport(32) != TargetTransformInfo::PSK_FastHardware)
+    return false;
+
+  // Counting population are usually conducted by few arithmetic instructions.
+  // Such instructions can be easily "absorbed" by vacant slots in a
+  // non-compact loop. Therefore, recognizing popcount idiom only makes sense
+  // in a compact loop.
+
+  // Give up if the loop has multiple blocks or multiple backedges.
+  if (CurLoop->getNumBackEdges() != 1 || CurLoop->getNumBlocks() != 1)
+    return false;
+
+  BasicBlock *LoopBody = *(CurLoop->block_begin());
+  if (LoopBody->size() >= 20) {
+    // The loop is too big, bail out.
+    return false;
+  }
+
+  // It should have a preheader containing nothing but an unconditional branch.
+  BasicBlock *PH = CurLoop->getLoopPreheader();
+  if (!PH)
+    return false;
+  if (&PH->front() != PH->getTerminator())
+    return false;
+  auto *EntryBI = dyn_cast<BranchInst>(PH->getTerminator());
+  if (!EntryBI || EntryBI->isConditional())
+    return false;
+
+  // It should have a precondition block where the generated popcount instrinsic
+  // function can be inserted.
+  auto *PreCondBB = PH->getSinglePredecessor();
+  if (!PreCondBB)
+    return false;
+  auto *PreCondBI = dyn_cast<BranchInst>(PreCondBB->getTerminator());
+  if (!PreCondBI || PreCondBI->isUnconditional())
+    return false;
+
+  Instruction *CntInst;
+  PHINode *CntPhi;
+  Value *Val;
+  if (!detectPopcountIdiom(CurLoop, PreCondBB, CntInst, CntPhi, Val))
+    return false;
+
+  transformLoopToPopcount(PreCondBB, CntInst, CntPhi, Val);
+  return true;
+}
+
+static CallInst *createPopcntIntrinsic(IRBuilder<> &IRBuilder, Value *Val,
+                                       DebugLoc DL) {
+  Value *Ops[] = {Val};
+  Type *Tys[] = {Val->getType()};
+
+  Module *M = IRBuilder.GetInsertBlock()->getParent()->getParent();
+  Value *Func = Intrinsic::getDeclaration(M, Intrinsic::ctpop, Tys);
+  CallInst *CI = IRBuilder.CreateCall(Func, Ops);
+  CI->setDebugLoc(DL);
+
+  return CI;
+}
+
+void LoopIdiomRecognize::transformLoopToPopcount(BasicBlock *PreCondBB,
+                                                 Instruction *CntInst,
+                                                 PHINode *CntPhi, Value *Var) {
+  BasicBlock *PreHead = CurLoop->getLoopPreheader();
+  auto *PreCondBr = dyn_cast<BranchInst>(PreCondBB->getTerminator());
+  const DebugLoc DL = CntInst->getDebugLoc();
+
+  // Assuming before transformation, the loop is following:
+  //  if (x) // the precondition
+  //     do { cnt++; x &= x - 1; } while(x);
+
+  // Step 1: Insert the ctpop instruction at the end of the precondition block
+  IRBuilder<> Builder(PreCondBr);
+  Value *PopCnt, *PopCntZext, *NewCount, *TripCnt;
+  {
+    PopCnt = createPopcntIntrinsic(Builder, Var, DL);
+    NewCount = PopCntZext =
+        Builder.CreateZExtOrTrunc(PopCnt, cast<IntegerType>(CntPhi->getType()));
+
+    if (NewCount != PopCnt)
+      (cast<Instruction>(NewCount))->setDebugLoc(DL);
+
+    // TripCnt is exactly the number of iterations the loop has
+    TripCnt = NewCount;
+
+    // If the population counter's initial value is not zero, insert Add Inst.
+    Value *CntInitVal = CntPhi->getIncomingValueForBlock(PreHead);
+    ConstantInt *InitConst = dyn_cast<ConstantInt>(CntInitVal);
+    if (!InitConst || !InitConst->isZero()) {
+      NewCount = Builder.CreateAdd(NewCount, CntInitVal);
+      (cast<Instruction>(NewCount))->setDebugLoc(DL);
+    }
+  }
+
+  // Step 2: Replace the precondition from "if (x == 0) goto loop-exit" to
+  //   "if (NewCount == 0) loop-exit". Without this change, the intrinsic
+  //   function would be partial dead code, and downstream passes will drag
+  //   it back from the precondition block to the preheader.
+  {
+    ICmpInst *PreCond = cast<ICmpInst>(PreCondBr->getCondition());
+
+    Value *Opnd0 = PopCntZext;
+    Value *Opnd1 = ConstantInt::get(PopCntZext->getType(), 0);
+    if (PreCond->getOperand(0) != Var)
+      std::swap(Opnd0, Opnd1);
+
+    ICmpInst *NewPreCond = cast<ICmpInst>(
+        Builder.CreateICmp(PreCond->getPredicate(), Opnd0, Opnd1));
+    PreCondBr->setCondition(NewPreCond);
+
+    RecursivelyDeleteTriviallyDeadInstructions(PreCond, TLI);
+  }
+
+  // Step 3: Note that the population count is exactly the trip count of the
+  // loop in question, which enable us to to convert the loop from noncountable
+  // loop into a countable one. The benefit is twofold:
+  //
+  //  - If the loop only counts population, the entire loop becomes dead after
+  //    the transformation. It is a lot easier to prove a countable loop dead
+  //    than to prove a noncountable one. (In some C dialects, an infinite loop
+  //    isn't dead even if it computes nothing useful. In general, DCE needs
+  //    to prove a noncountable loop finite before safely delete it.)
+  //
+  //  - If the loop also performs something else, it remains alive.
+  //    Since it is transformed to countable form, it can be aggressively
+  //    optimized by some optimizations which are in general not applicable
+  //    to a noncountable loop.
+  //
+  // After this step, this loop (conceptually) would look like following:
+  //   newcnt = __builtin_ctpop(x);
+  //   t = newcnt;
+  //   if (x)
+  //     do { cnt++; x &= x-1; t--) } while (t > 0);
+  BasicBlock *Body = *(CurLoop->block_begin());
+  {
+    auto *LbBr = dyn_cast<BranchInst>(Body->getTerminator());
+    ICmpInst *LbCond = cast<ICmpInst>(LbBr->getCondition());
+    Type *Ty = TripCnt->getType();
+
+    PHINode *TcPhi = PHINode::Create(Ty, 2, "tcphi", &Body->front());
+
+    Builder.SetInsertPoint(LbCond);
+    Instruction *TcDec = cast<Instruction>(
+        Builder.CreateSub(TcPhi, ConstantInt::get(Ty, 1),
+                          "tcdec", false, true));
+
+    TcPhi->addIncoming(TripCnt, PreHead);
+    TcPhi->addIncoming(TcDec, Body);
+
+    CmpInst::Predicate Pred =
+        (LbBr->getSuccessor(0) == Body) ? CmpInst::ICMP_UGT : CmpInst::ICMP_SLE;
+    LbCond->setPredicate(Pred);
+    LbCond->setOperand(0, TcDec);
+    LbCond->setOperand(1, ConstantInt::get(Ty, 0));
+  }
+
+  // Step 4: All the references to the original population counter outside
+  //  the loop are replaced with the NewCount -- the value returned from
+  //  __builtin_ctpop().
+  CntInst->replaceUsesOutsideBlock(NewCount, Body);
+
+  // step 5: Forget the "non-computable" trip-count SCEV associated with the
+  //   loop. The loop would otherwise not be deleted even if it becomes empty.
+  SE->forgetLoop(CurLoop);
+}