LoopIdiomRecognize.cpp

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//===- LoopIdiomRecognize.cpp - Loop idiom recognition --------------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
//
// This pass implements an idiom recognizer that transforms simple loops into a
// non-loop form.  In cases that this kicks in, it can be a significant
// performance win.
//
// If compiling for code size we avoid idiom recognition if the resulting
// code could be larger than the code for the original loop. One way this could
// happen is if the loop is not removable after idiom recognition due to the
// presence of non-idiom instructions. The initial implementation of the
// heuristics applies to idioms in multi-block loops.
//
//===----------------------------------------------------------------------===//
//
// TODO List:
//
// Future loop memory idioms to recognize: memcmp, etc.
//
// This could recognize common matrix multiplies and dot product idioms and
// replace them with calls to BLAS (if linked in??).
//
//===----------------------------------------------------------------------===//
 
#include "llvm/Transforms/Scalar/LoopIdiomRecognize.h"
#include "ScalarOptions.h"
#include "llvm/ADT/APInt.h"
#include "llvm/ADT/ArrayRef.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/MapVector.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/SetVector.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/ADT/StringRef.h"
#include "llvm/Analysis/AliasAnalysis.h"
#include "llvm/Analysis/CmpInstAnalysis.h"
#include "llvm/Analysis/HashRecognize.h"
#include "llvm/Analysis/LoopAccessAnalysis.h"
#include "llvm/Analysis/LoopInfo.h"
#include "llvm/Analysis/LoopPass.h"
#include "llvm/Analysis/MemoryLocation.h"
#include "llvm/Analysis/MemorySSA.h"
#include "llvm/Analysis/MemorySSAUpdater.h"
#include "llvm/Analysis/MustExecute.h"
#include "llvm/Analysis/OptimizationRemarkEmitter.h"
#include "llvm/Analysis/ScalarEvolution.h"
#include "llvm/Analysis/ScalarEvolutionExpressions.h"
#include "llvm/Analysis/ScalarEvolutionPatternMatch.h"
#include "llvm/Analysis/TargetLibraryInfo.h"
#include "llvm/Analysis/TargetTransformInfo.h"
#include "llvm/Analysis/ValueTracking.h"
#include "llvm/IR/BasicBlock.h"
#include "llvm/IR/Constant.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/DataLayout.h"
#include "llvm/IR/DebugLoc.h"
#include "llvm/IR/DerivedTypes.h"
#include "llvm/IR/Dominators.h"
#include "llvm/IR/GlobalValue.h"
#include "llvm/IR/GlobalVariable.h"
#include "llvm/IR/IRBuilder.h"
#include "llvm/IR/InstrTypes.h"
#include "llvm/IR/Instruction.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/IntrinsicInst.h"
#include "llvm/IR/Intrinsics.h"
#include "llvm/IR/LLVMContext.h"
#include "llvm/IR/Module.h"
#include "llvm/IR/PassManager.h"
#include "llvm/IR/PatternMatch.h"
#include "llvm/IR/ProfDataUtils.h"
#include "llvm/IR/Type.h"
#include "llvm/IR/User.h"
#include "llvm/IR/Value.h"
#include "llvm/IR/ValueHandle.h"
#include "llvm/Support/Casting.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/InstructionCost.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Transforms/Utils/BuildLibCalls.h"
#include "llvm/Transforms/Utils/Local.h"
#include "llvm/Transforms/Utils/LoopUtils.h"
#include "llvm/Transforms/Utils/ScalarEvolutionExpander.h"
#include <algorithm>
#include <cassert>
#include <cstdint>
#include <utility>
 
using namespace llvm;
using namespace SCEVPatternMatch;
 
#define DEBUG_TYPE "loop-idiom"
 
STATISTIC(NumMemSet, "Number of memset's formed from loop stores");
STATISTIC(NumMemCpy, "Number of memcpy's formed from loop load+stores");
STATISTIC(NumMemMove, "Number of memmove's formed from loop load+stores");
STATISTIC(NumStrLen, "Number of strlen's and wcslen's formed from loop loads");
STATISTIC(
    NumShiftUntilBitTest,
    "Number of uncountable loops recognized as 'shift until bitttest' idiom");
STATISTIC(NumShiftUntilZero,
          "Number of uncountable loops recognized as 'shift until zero' idiom");
 
namespace llvm {
bool DisableLIRP::All;
static cl::opt<bool, true>
    DisableLIRPAll("disable-" DEBUG_TYPE "-all",
                   cl::desc("Options to disable Loop Idiom Recognize Pass."),
                   cl::location(DisableLIRP::All), cl::init(false),
                   cl::ReallyHidden);
 
bool DisableLIRP::Memset;
static cl::opt<bool, true>
    DisableLIRPMemset("disable-" DEBUG_TYPE "-memset",
                      cl::desc("Proceed with loop idiom recognize pass, but do "
                               "not convert loop(s) to memset."),
                      cl::location(DisableLIRP::Memset), cl::init(false),
                      cl::ReallyHidden);
 
bool DisableLIRP::Memcpy;
static cl::opt<bool, true>
    DisableLIRPMemcpy("disable-" DEBUG_TYPE "-memcpy",
                      cl::desc("Proceed with loop idiom recognize pass, but do "
                               "not convert loop(s) to memcpy."),
                      cl::location(DisableLIRP::Memcpy), cl::init(false),
                      cl::ReallyHidden);
 
bool DisableLIRP::Strlen;
static cl::opt<bool, true>
    DisableLIRPStrlen("disable-loop-idiom-strlen",
                      cl::desc("Proceed with loop idiom recognize pass, but do "
                               "not convert loop(s) to strlen."),
                      cl::location(DisableLIRP::Strlen), cl::init(false),
                      cl::ReallyHidden);
 
bool DisableLIRP::Wcslen;
static cl::opt<bool, true>
    EnableLIRPWcslen("disable-loop-idiom-wcslen",
                     cl::desc("Proceed with loop idiom recognize pass, "
                              "enable conversion of loop(s) to wcslen."),
                     cl::location(DisableLIRP::Wcslen), cl::init(false),
                     cl::ReallyHidden);
 
bool DisableLIRP::HashRecognize;
static cl::opt<bool, true>
    DisableLIRPHashRecognize("disable-" DEBUG_TYPE "-hashrecognize",
                             cl::desc("Proceed with loop idiom recognize pass, "
                                      "but do not do hash-recognize analysis."),
                             cl::location(DisableLIRP::HashRecognize),
                             cl::init(false), cl::ReallyHidden);
 
} // namespace llvm
 
namespace {
 
class LoopIdiomRecognize {
  const ScalarOptions &Opts;
  Loop *CurLoop = nullptr;
  AliasAnalysis *AA;
  DominatorTree *DT;
  LoopInfo *LI;
  ScalarEvolution *SE;
  TargetLibraryInfo *TLI;
  const TargetTransformInfo *TTI;
  const DataLayout *DL;
  OptimizationRemarkEmitter &ORE;
  bool ApplyCodeSizeHeuristics;
  std::unique_ptr<MemorySSAUpdater> MSSAU;
 
public:
  explicit LoopIdiomRecognize(AliasAnalysis *AA, DominatorTree *DT,
                              LoopInfo *LI, ScalarEvolution *SE,
                              TargetLibraryInfo *TLI,
                              const TargetTransformInfo *TTI, MemorySSA *MSSA,
                              const DataLayout *DL,
                              OptimizationRemarkEmitter &ORE)
      : Opts(ScalarOptions::Global), AA(AA), DT(DT), LI(LI), SE(SE), TLI(TLI),
        TTI(TTI), DL(DL), ORE(ORE) {
    if (MSSA)
      MSSAU = std::make_unique<MemorySSAUpdater>(MSSA);
  }
 
  bool runOnLoop(Loop *L);
 
private:
  using StoreList = SmallVector<StoreInst *, 8>;
  using StoreListMap = MapVector<Value *, StoreList>;
 
  StoreListMap StoreRefsForMemset;
  StoreListMap StoreRefsForMemsetPattern;
  StoreList StoreRefsForMemcpy;
  bool HasMemset;
  bool HasMemsetPattern;
  bool HasMemcpy;
 
  /// Return code for isLegalStore()
  enum LegalStoreKind {
    None = 0,
    Memset,
    MemsetPattern,
    Memcpy,
    UnorderedAtomicMemcpy,
    DontUse // Dummy retval never to be used. Allows catching errors in retval
            // handling.
  };
 
  /// \name Countable Loop Idiom Handling
  /// @{
 
  bool runOnCountableLoop();
  bool runOnLoopBlock(BasicBlock *BB, const SCEV *BECount,
                      SmallVectorImpl<BasicBlock *> &ExitBlocks);
 
  void collectStores(BasicBlock *BB);
  LegalStoreKind isLegalStore(StoreInst *SI);
  enum class ForMemset { No, Yes };
  bool processLoopStores(SmallVectorImpl<StoreInst *> &SL, const SCEV *BECount,
                         ForMemset For);
 
  template <typename MemInst>
  bool processLoopMemIntrinsic(
      BasicBlock *BB,
      bool (LoopIdiomRecognize::*Processor)(MemInst *, const SCEV *),
      const SCEV *BECount);
  bool processLoopMemCpy(MemCpyInst *MCI, const SCEV *BECount);
  bool processLoopMemSet(MemSetInst *MSI, const SCEV *BECount);
 
  bool processLoopStridedStore(Value *DestPtr, const SCEV *StoreSizeSCEV,
                               MaybeAlign StoreAlignment, Value *StoredVal,
                               Instruction *TheStore,
                               SmallPtrSetImpl<Instruction *> &Stores,
                               const SCEVAddRecExpr *Ev, const SCEV *BECount,
                               bool IsNegStride, bool IsLoopMemset = false);
  bool processLoopStoreOfLoopLoad(StoreInst *SI, const SCEV *BECount);
  bool processLoopStoreOfLoopLoad(Value *DestPtr, Value *SourcePtr,
                                  const SCEV *StoreSize, MaybeAlign StoreAlign,
                                  MaybeAlign LoadAlign, Instruction *TheStore,
                                  Instruction *TheLoad,
                                  const SCEVAddRecExpr *StoreEv,
                                  const SCEVAddRecExpr *LoadEv,
                                  const SCEV *BECount);
  bool avoidLIRForMultiBlockLoop(bool IsMemset = false,
                                 bool IsLoopMemset = false);
  bool optimizeCRCLoop(const PolynomialInfo &Info);
  void optimizeCRCLoopUsingClmul(const PolynomialInfo &Info);
  void optimizeCRCLoopUsingTableLookup(const PolynomialInfo &Info);
 
  /// @}
  /// \name Noncountable Loop Idiom Handling
  /// @{
 
  bool runOnNoncountableLoop();
 
  bool recognizePopcount();
  void transformLoopToPopcount(BasicBlock *PreCondBB, Instruction *CntInst,
                               PHINode *CntPhi, Value *Var);
  bool isProfitableToInsertFFS(Intrinsic::ID IntrinID, Value *InitX,
                               bool ZeroCheck, size_t CanonicalSize);
  bool insertFFSIfProfitable(Intrinsic::ID IntrinID, Value *InitX,
                             Instruction *DefX, PHINode *CntPhi,
                             Instruction *CntInst);
  bool recognizeAndInsertFFS();  /// Find First Set: ctlz or cttz
  bool recognizeShiftUntilLessThan();
  void transformLoopToCountable(Intrinsic::ID IntrinID, BasicBlock *PreCondBB,
                                Instruction *CntInst, PHINode *CntPhi,
                                Value *Var, Instruction *DefX,
                                const DebugLoc &DL, bool ZeroCheck,
                                bool IsCntPhiUsedOutsideLoop,
                                bool InsertSub = false);
 
  bool recognizeShiftUntilBitTest();
  bool recognizeShiftUntilZero();
  bool recognizeAndInsertStrLen();
 
  /// @}
};
} // end anonymous namespace
 
PreservedAnalyses LoopIdiomRecognizePass::run(Loop &L, LoopAnalysisManager &AM,
                                              LoopStandardAnalysisResults &AR,
                                              LPMUpdater &) {
  if (DisableLIRP::All)
    return PreservedAnalyses::all();
 
  const auto *DL = &L.getHeader()->getDataLayout();
 
  // For the new PM, we also can't use OptimizationRemarkEmitter as an analysis
  // pass.  Function analyses need to be preserved across loop transformations
  // but ORE cannot be preserved (see comment before the pass definition).
  OptimizationRemarkEmitter ORE(L.getHeader()->getParent());
 
  LoopIdiomRecognize LIR(&AR.AA, &AR.DT, &AR.LI, &AR.SE, &AR.TLI, &AR.TTI,
                         AR.MSSA, DL, ORE);
  if (!LIR.runOnLoop(&L))
    return PreservedAnalyses::all();
 
  auto PA = getLoopPassPreservedAnalyses();
  if (AR.MSSA)
    PA.preserve<MemorySSAAnalysis>();
  return PA;
}
 
static void deleteDeadInstruction(Instruction *I) {
  I->replaceAllUsesWith(PoisonValue::get(I->getType()));
  I->eraseFromParent();
}
 
//===----------------------------------------------------------------------===//
//
//          Implementation of LoopIdiomRecognize
//
//===----------------------------------------------------------------------===//
 
bool LoopIdiomRecognize::runOnLoop(Loop *L) {
  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;
 
  // Disable loop idiom recognition if the function's name is a common idiom.
  StringRef Name = L->getHeader()->getParent()->getName();
  if (Name == "memset" || Name == "memcpy" || Name == "strlen" ||
      Name == "wcslen")
    return false;
 
  // Determine if code size heuristics need to be applied.
  ApplyCodeSizeHeuristics =
      L->getHeader()->getParent()->hasOptSize() && Opts.use_lir_code_size_heurs;
 
  HasMemset = TLI->has(LibFunc_memset);
  // TODO: Unconditionally enable use of the memset pattern intrinsic (or at
  // least, opt-in via target hook) once we are confident it will never result
  // in worse codegen than without. For now, use it only when the target
  // supports memset_pattern16 libcall (or unless this is overridden by
  // command line option).
  HasMemsetPattern = TLI->has(LibFunc_memset_pattern16);
  HasMemcpy = TLI->has(LibFunc_memcpy);
 
  if (HasMemset || HasMemsetPattern ||
      Opts.loop_idiom_force_memset_pattern_intrinsic || HasMemcpy ||
      !DisableLIRP::HashRecognize)
    if (SE->hasLoopInvariantBackedgeTakenCount(L))
      return runOnCountableLoop();
 
  return runOnNoncountableLoop();
}
 
bool LoopIdiomRecognize::runOnCountableLoop() {
  const SCEV *BECount = SE->getBackedgeTakenCount(CurLoop);
  assert(!isa<SCEVCouldNotCompute>(BECount) &&
         "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 (BECount->isZero())
    return false;
 
  SmallVector<BasicBlock *, 8> ExitBlocks;
  CurLoop->getUniqueExitBlocks(ExitBlocks);
 
  LLVM_DEBUG(dbgs() << DEBUG_TYPE " Scanning: F["
                    << CurLoop->getHeader()->getParent()->getName()
                    << "] Countable Loop %" << CurLoop->getHeader()->getName()
                    << "\n");
 
  // The following transforms hoist stores/memsets into the loop pre-header.
  // Give up if the loop has instructions that may throw.
  SimpleLoopSafetyInfo SafetyInfo(CurLoop);
  if (SafetyInfo.anyBlockMayThrow())
    return false;
 
  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)
      continue;
 
    MadeChange |= runOnLoopBlock(BB, BECount, ExitBlocks);
  }
 
  // Attempt to optimize a CRC loop if one is detected by HashRecognize.
  if (!DisableLIRP::HashRecognize &&
      Opts.loop_idiom_crc_strategy != CRCStrategyKind::Disable)
    if (auto Res = HashRecognize(*CurLoop, *SE).getResult())
      MadeChange |= optimizeCRCLoop(*Res);
 
  return MadeChange;
}
 
static APInt getStoreStride(const SCEVAddRecExpr *StoreEv) {
  const SCEVConstant *ConstStride = cast<SCEVConstant>(StoreEv->getOperand(1));
  return ConstStride->getAPInt();
}
 
/// getMemSetPatternValue - If a strided store of the specified value is safe to
/// turn into a memset.patternn intrinsic, return the Constant that should
/// be passed in. Otherwise, return null.
///
/// TODO this function could allow more constants than it does today (e.g.
/// those over 16 bytes) now it has transitioned to being used for the
/// memset.pattern intrinsic rather than directly the memset_pattern16
/// libcall.
static Constant *getMemSetPatternValue(Value *V, const DataLayout *DL) {
  // FIXME: This could check for UndefValue because it can be merged into any
  // other valid pattern.
 
  // 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 || isa<ConstantExpr>(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;
 
  // For now, don't handle types that aren't int, floats, or pointers.
  Type *CTy = C->getType();
  if (!CTy->isIntOrPtrTy() && !CTy->isFloatingPointTy())
    return nullptr;
 
  return C;
}
 
LoopIdiomRecognize::LegalStoreKind
LoopIdiomRecognize::isLegalStore(StoreInst *SI) {
  // Don't touch volatile stores.
  if (SI->isVolatile())
    return LegalStoreKind::None;
  // We only want simple or unordered-atomic stores.
  if (!SI->isUnordered())
    return LegalStoreKind::None;
 
  // Avoid merging nontemporal stores.
  if (SI->getMetadata(LLVMContext::MD_nontemporal))
    return LegalStoreKind::None;
 
  Value *StoredVal = SI->getValueOperand();
  Value *StorePtr = SI->getPointerOperand();
 
  if (DL->hasUnstableRepresentation(StoredVal->getType()))
    return LegalStoreKind::None;
 
  // Transformations could invalidate the external-state pointers
  //   memcpy - LangRef specifies that a valid memcpy must preserve external
  //            state, so no transformations are blocked by it.
  //   memset - We assume that a memset of 0 has an equivalent external state
  //            effect as a null pointer store. This is currently not explicitly
  //            specified, but is true of the one exemplar we have (CHERI
  //            capabilities). All other memset formations are not safe.
  bool MustPreserveExternalState = DL->hasExternalState(StoredVal->getType()) &&
                                   !isa<ConstantPointerNull>(StoredVal);
 
  // Reject stores that are so large that they overflow an unsigned.
  // When storing out scalable vectors we bail out for now, since the code
  // below currently only works for constant strides.
  TypeSize SizeInBits = DL->getTypeSizeInBits(StoredVal->getType());
  if (SizeInBits.isScalable() || (SizeInBits.getFixedValue() & 7) ||
      (SizeInBits.getFixedValue() >> 32) != 0)
    return LegalStoreKind::None;
 
  // 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 SCEV *StoreEv = SE->getSCEV(StorePtr);
  const SCEVConstant *Stride;
  if (!match(StoreEv, m_scev_AffineAddRec(m_SCEV(), m_SCEVConstant(Stride),
                                          m_SpecificLoop(CurLoop))))
    return LegalStoreKind::None;
 
  // See if the store can be turned into a memset.
 
  // 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
  // are stored.  A store of i32 0x01020304 can never be turned into a memset,
  // but it can be turned into memset_pattern if the target supports it.
  Value *SplatValue = isBytewiseValue(StoredVal, *DL);
 
  // Note: memset and memset_pattern on unordered-atomic is yet not supported
  bool UnorderedAtomic = SI->isUnordered() && !SI->isSimple();
 
  // If we're allowed to form a memset, and the stored value would be
  // acceptable for memset, use it.
  if (!MustPreserveExternalState && !UnorderedAtomic && HasMemset &&
      SplatValue && !DisableLIRP::Memset &&
      // Verify that the stored value is loop invariant.  If not, we can't
      // promote the memset.
      CurLoop->isLoopInvariant(SplatValue)) {
    // It looks like we can use SplatValue.
    return LegalStoreKind::Memset;
  }
  if (!MustPreserveExternalState && !UnorderedAtomic &&
      (HasMemsetPattern || Opts.loop_idiom_force_memset_pattern_intrinsic) &&
      !DisableLIRP::Memset &&
      // Don't create memset_pattern16s with address spaces.
      StorePtr->getType()->getPointerAddressSpace() == 0 &&
      getMemSetPatternValue(StoredVal, DL)) {
    // It looks like we can use PatternValue!
    return LegalStoreKind::MemsetPattern;
  }
 
  // Otherwise, see if the store can be turned into a memcpy.
  if (HasMemcpy && !DisableLIRP::Memcpy) {
    // 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 = DL->getTypeStoreSize(SI->getValueOperand()->getType());
    APInt StrideAP = Stride->getAPInt();
    if (StoreSize != StrideAP && StoreSize != -StrideAP)
      return LegalStoreKind::None;
 
    // The store must be feeding a non-volatile load.
    LoadInst *LI = dyn_cast<LoadInst>(SI->getValueOperand());
 
    // Only allow non-volatile loads
    if (!LI || LI->isVolatile())
      return LegalStoreKind::None;
    // Only allow simple or unordered-atomic loads
    if (!LI->isUnordered())
      return LegalStoreKind::None;
 
    // 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 SCEV *LoadEv = SE->getSCEV(LI->getPointerOperand());
 
    // The store and load must share the same stride.
    if (!match(LoadEv, m_scev_AffineAddRec(m_SCEV(), m_scev_Specific(Stride),
                                           m_SpecificLoop(CurLoop))))
      return LegalStoreKind::None;
 
    // Success.  This store can be converted into a memcpy.
    UnorderedAtomic = UnorderedAtomic || LI->isAtomic();
    return UnorderedAtomic ? LegalStoreKind::UnorderedAtomicMemcpy
                           : LegalStoreKind::Memcpy;
  }
  // This store can't be transformed into a memset/memcpy.
  return LegalStoreKind::None;
}
 
void LoopIdiomRecognize::collectStores(BasicBlock *BB) {
  StoreRefsForMemset.clear();
  StoreRefsForMemsetPattern.clear();
  StoreRefsForMemcpy.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.
    switch (isLegalStore(SI)) {
    case LegalStoreKind::None:
      // Nothing to do
      break;
    case LegalStoreKind::Memset: {
      // Find the base pointer.
      Value *Ptr = getUnderlyingObject(SI->getPointerOperand());
      StoreRefsForMemset[Ptr].push_back(SI);
    } break;
    case LegalStoreKind::MemsetPattern: {
      // Find the base pointer.
      Value *Ptr = getUnderlyingObject(SI->getPointerOperand());
      StoreRefsForMemsetPattern[Ptr].push_back(SI);
    } break;
    case LegalStoreKind::Memcpy:
    case LegalStoreKind::UnorderedAtomicMemcpy:
      StoreRefsForMemcpy.push_back(SI);
      break;
    default:
      assert(false && "unhandled return value");
      break;
    }
  }
}
 
/// 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) {
  // 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.
  for (BasicBlock *ExitBlock : ExitBlocks)
    if (!DT->dominates(BB, ExitBlock))
      return false;
 
  bool MadeChange = false;
  // Look for store instructions, which may be optimized to memset/memcpy.
  collectStores(BB);
 
  // Look for a single store or sets of stores with a common base, which can be
  // optimized into a memset (memset_pattern).  The latter most commonly happens
  // with structs and handunrolled loops.
  for (auto &SL : StoreRefsForMemset)
    MadeChange |= processLoopStores(SL.second, BECount, ForMemset::Yes);
 
  for (auto &SL : StoreRefsForMemsetPattern)
    MadeChange |= processLoopStores(SL.second, BECount, ForMemset::No);
 
  // Optimize the store into a memcpy, if it feeds an similarly strided load.
  for (auto &SI : StoreRefsForMemcpy)
    MadeChange |= processLoopStoreOfLoopLoad(SI, BECount);
 
  MadeChange |= processLoopMemIntrinsic<MemCpyInst>(
      BB, &LoopIdiomRecognize::processLoopMemCpy, BECount);
  MadeChange |= processLoopMemIntrinsic<MemSetInst>(
      BB, &LoopIdiomRecognize::processLoopMemSet, BECount);
 
  return MadeChange;
}
 
/// See if this store(s) can be promoted to a memset.
bool LoopIdiomRecognize::processLoopStores(SmallVectorImpl<StoreInst *> &SL,
                                           const SCEV *BECount, ForMemset For) {
  // Try to find consecutive stores that can be transformed into memsets.
  SetVector<StoreInst *> Heads, Tails;
  SmallDenseMap<StoreInst *, StoreInst *> ConsecutiveChain;
 
  // Do a quadratic search on all of the given stores and find
  // all of the pairs of stores that follow each other.
  SmallVector<unsigned, 16> IndexQueue;
  for (unsigned i = 0, e = SL.size(); i < e; ++i) {
    assert(SL[i]->isSimple() && "Expected only non-volatile stores.");
 
    Value *FirstStoredVal = SL[i]->getValueOperand();
    Value *FirstStorePtr = SL[i]->getPointerOperand();
    const SCEVAddRecExpr *FirstStoreEv =
        cast<SCEVAddRecExpr>(SE->getSCEV(FirstStorePtr));
    APInt FirstStride = getStoreStride(FirstStoreEv);
    unsigned FirstStoreSize = DL->getTypeStoreSize(SL[i]->getValueOperand()->getType());
 
    // See if we can optimize just this store in isolation.
    if (FirstStride == FirstStoreSize || -FirstStride == FirstStoreSize) {
      Heads.insert(SL[i]);
      continue;
    }
 
    Value *FirstSplatValue = nullptr;
    Constant *FirstPatternValue = nullptr;
 
    if (For == ForMemset::Yes)
      FirstSplatValue = isBytewiseValue(FirstStoredVal, *DL);
    else
      FirstPatternValue = getMemSetPatternValue(FirstStoredVal, DL);
 
    assert((FirstSplatValue || FirstPatternValue) &&
           "Expected either splat value or pattern value.");
 
    IndexQueue.clear();
    // If a store has multiple consecutive store candidates, search Stores
    // array according to the sequence: from i+1 to e, then from i-1 to 0.
    // This is because usually pairing with immediate succeeding or preceding
    // candidate create the best chance to find memset opportunity.
    unsigned j = 0;
    for (j = i + 1; j < e; ++j)
      IndexQueue.push_back(j);
    for (j = i; j > 0; --j)
      IndexQueue.push_back(j - 1);
 
    for (auto &k : IndexQueue) {
      assert(SL[k]->isSimple() && "Expected only non-volatile stores.");
      Value *SecondStorePtr = SL[k]->getPointerOperand();
      const SCEVAddRecExpr *SecondStoreEv =
          cast<SCEVAddRecExpr>(SE->getSCEV(SecondStorePtr));
      APInt SecondStride = getStoreStride(SecondStoreEv);
 
      if (FirstStride != SecondStride)
        continue;
 
      Value *SecondStoredVal = SL[k]->getValueOperand();
      Value *SecondSplatValue = nullptr;
      Constant *SecondPatternValue = nullptr;
 
      if (For == ForMemset::Yes)
        SecondSplatValue = isBytewiseValue(SecondStoredVal, *DL);
      else
        SecondPatternValue = getMemSetPatternValue(SecondStoredVal, DL);
 
      assert((SecondSplatValue || SecondPatternValue) &&
             "Expected either splat value or pattern value.");
 
      if (isConsecutiveAccess(SL[i], SL[k], *DL, *SE, false)) {
        if (For == ForMemset::Yes) {
          if (isa<UndefValue>(FirstSplatValue))
            FirstSplatValue = SecondSplatValue;
          if (FirstSplatValue != SecondSplatValue)
            continue;
        } else {
          if (isa<UndefValue>(FirstPatternValue))
            FirstPatternValue = SecondPatternValue;
          if (FirstPatternValue != SecondPatternValue)
            continue;
        }
        Tails.insert(SL[k]);
        Heads.insert(SL[i]);
        ConsecutiveChain[SL[i]] = SL[k];
        break;
      }
    }
  }
 
  // We may run into multiple chains that merge into a single chain. We mark the
  // stores that we transformed so that we don't visit the same store twice.
  SmallPtrSet<Value *, 16> TransformedStores;
  bool Changed = false;
 
  // For stores that start but don't end a link in the chain:
  for (StoreInst *I : Heads) {
    if (Tails.count(I))
      continue;
 
    // We found a store instr that starts a chain. Now follow the chain and try
    // to transform it.
    SmallPtrSet<Instruction *, 8> AdjacentStores;
    StoreInst *HeadStore = I;
    unsigned StoreSize = 0;
 
    // Collect the chain into a list.
    while (Tails.count(I) || Heads.count(I)) {
      if (TransformedStores.count(I))
        break;
      AdjacentStores.insert(I);
 
      StoreSize += DL->getTypeStoreSize(I->getValueOperand()->getType());
      // Move to the next value in the chain.
      I = ConsecutiveChain[I];
    }
 
    Value *StoredVal = HeadStore->getValueOperand();
    Value *StorePtr = HeadStore->getPointerOperand();
    const SCEVAddRecExpr *StoreEv = cast<SCEVAddRecExpr>(SE->getSCEV(StorePtr));
    APInt Stride = getStoreStride(StoreEv);
 
    // Check to see if the stride matches the size of the stores.  If so, then
    // we know that every byte is touched in the loop.
    if (StoreSize != Stride && StoreSize != -Stride)
      continue;
 
    bool IsNegStride = StoreSize == -Stride;
 
    Type *IntIdxTy = DL->getIndexType(StorePtr->getType());
    const SCEV *StoreSizeSCEV = SE->getConstant(IntIdxTy, StoreSize);
    if (processLoopStridedStore(StorePtr, StoreSizeSCEV,
                                MaybeAlign(HeadStore->getAlign()), StoredVal,
                                HeadStore, AdjacentStores, StoreEv, BECount,
                                IsNegStride)) {
      TransformedStores.insert_range(AdjacentStores);
      Changed = true;
    }
  }
 
  return Changed;
}
 
/// processLoopMemIntrinsic - Template function for calling different processor
/// functions based on mem intrinsic type.
template <typename MemInst>
bool LoopIdiomRecognize::processLoopMemIntrinsic(
    BasicBlock *BB,
    bool (LoopIdiomRecognize::*Processor)(MemInst *, const SCEV *),
    const SCEV *BECount) {
  bool MadeChange = false;
  for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E;) {
    Instruction *Inst = &*I++;
    // Look for memory instructions, which may be optimized to a larger one.
    if (MemInst *MI = dyn_cast<MemInst>(Inst)) {
      WeakTrackingVH InstPtr(&*I);
      if (!(this->*Processor)(MI, BECount))
        continue;
      MadeChange = true;
 
      // If processing the instruction invalidated our iterator, start over from
      // the top of the block.
      if (!InstPtr)
        I = BB->begin();
    }
  }
  return MadeChange;
}
 
/// processLoopMemCpy - See if this memcpy can be promoted to a large memcpy
bool LoopIdiomRecognize::processLoopMemCpy(MemCpyInst *MCI,
                                           const SCEV *BECount) {
  // We can only handle non-volatile memcpys with a constant size.
  if (MCI->isVolatile() || !isa<ConstantInt>(MCI->getLength()))
    return false;
 
  // If we're not allowed to hack on memcpy, we fail. We don't mess with the
  // inlined version as generating a larger inline mempcy could affect code
  // size.
  if (!HasMemcpy || MCI->isForceInlined() || DisableLIRP::Memcpy)
    return false;
 
  Value *Dest = MCI->getDest();
  Value *Source = MCI->getSource();
  if (!Dest || !Source)
    return false;
 
  // See if the load and store pointer expressions are AddRec like {base,+,1} on
  // the current loop, which indicates a strided load and store.  If we have
  // something else, it's a random load or store we can't handle.
  const SCEV *StoreEv = SE->getSCEV(Dest);
  const SCEV *LoadEv = SE->getSCEV(Source);
  const APInt *StoreStrideValue, *LoadStrideValue;
  if (!match(StoreEv,
             m_scev_AffineAddRec(m_SCEV(), m_scev_APInt(StoreStrideValue),
                                 m_SpecificLoop(CurLoop))) ||
      !match(LoadEv,
             m_scev_AffineAddRec(m_SCEV(), m_scev_APInt(LoadStrideValue),
                                 m_SpecificLoop(CurLoop))))
    return false;
 
  // Reject memcpys that are so large that they overflow an unsigned.
  uint64_t SizeInBytes = cast<ConstantInt>(MCI->getLength())->getZExtValue();
  if ((SizeInBytes >> 32) != 0)
    return false;
 
  // Huge stride value - give up
  if (StoreStrideValue->getBitWidth() > 64 ||
      LoadStrideValue->getBitWidth() > 64)
    return false;
 
  if (SizeInBytes != *StoreStrideValue && SizeInBytes != -*StoreStrideValue) {
    ORE.emit([&]() {
      return OptimizationRemarkMissed(DEBUG_TYPE, "SizeStrideUnequal", MCI)
             << ore::NV("Inst", "memcpy") << " in "
             << ore::NV("Function", MCI->getFunction())
             << " function will not be hoisted: "
             << ore::NV("Reason", "memcpy size is not equal to stride");
    });
    return false;
  }
 
  int64_t StoreStrideInt = StoreStrideValue->getSExtValue();
  int64_t LoadStrideInt = LoadStrideValue->getSExtValue();
  // Check if the load stride matches the store stride.
  if (StoreStrideInt != LoadStrideInt)
    return false;
 
  return processLoopStoreOfLoopLoad(
      Dest, Source, SE->getConstant(Dest->getType(), SizeInBytes),
      MCI->getDestAlign(), MCI->getSourceAlign(), MCI, MCI,
      cast<SCEVAddRecExpr>(StoreEv), cast<SCEVAddRecExpr>(LoadEv), BECount);
}
 
/// processLoopMemSet - See if this memset can be promoted to a large memset.
bool LoopIdiomRecognize::processLoopMemSet(MemSetInst *MSI,
                                           const SCEV *BECount) {
  // We can only handle non-volatile memsets.
  if (MSI->isVolatile())
    return false;
 
  // If we're not allowed to hack on memset, we fail. We don't mess with the
  // inlined version as generating a larger memset could affect code size.
  if (!HasMemset || MSI->isForceInlined() || DisableLIRP::Memset)
    return false;
 
  Value *Pointer = MSI->getDest();
 
  // 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 SCEV *Ev = SE->getSCEV(Pointer);
  const SCEV *PointerStrideSCEV;
  if (!match(Ev, m_scev_AffineAddRec(m_SCEV(), m_SCEV(PointerStrideSCEV),
                                     m_SpecificLoop(CurLoop)))) {
    LLVM_DEBUG(dbgs() << "  Pointer is not affine, abort\n");
    return false;
  }
 
  SCEVUse MemsetSizeSCEV = SE->getSCEV(MSI->getLength());
 
  bool IsNegStride = false;
  const bool IsConstantSize = isa<ConstantInt>(MSI->getLength());
 
  if (IsConstantSize) {
    // Memset size is constant.
    // Check if the pointer stride matches the memset size. If so, then
    // we know that every byte is touched in the loop.
    LLVM_DEBUG(dbgs() << "  memset size is constant\n");
    uint64_t SizeInBytes = cast<ConstantInt>(MSI->getLength())->getZExtValue();
    const APInt *Stride;
    if (!match(PointerStrideSCEV, m_scev_APInt(Stride)))
      return false;
 
    if (SizeInBytes != *Stride && SizeInBytes != -*Stride)
      return false;
 
    IsNegStride = SizeInBytes == -*Stride;
  } else {
    // Memset size is non-constant.
    // Check if the pointer stride matches the memset size.
    // To be conservative, the pass would not promote pointers that aren't in
    // address space zero. Also, the pass only handles memset length and stride
    // that are invariant for the top level loop.
    LLVM_DEBUG(dbgs() << "  memset size is non-constant\n");
    if (Pointer->getType()->getPointerAddressSpace() != 0) {
      LLVM_DEBUG(dbgs() << "  pointer is not in address space zero, "
                        << "abort\n");
      return false;
    }
    if (!SE->isLoopInvariant(MemsetSizeSCEV, CurLoop)) {
      LLVM_DEBUG(dbgs() << "  memset size is not a loop-invariant, "
                        << "abort\n");
      return false;
    }
 
    // Compare positive direction PointerStrideSCEV with MemsetSizeSCEV
    IsNegStride = PointerStrideSCEV->isNonConstantNegative();
    SCEVUse PositiveStrideSCEV =
        IsNegStride ? SCEVUse(SE->getNegativeSCEV(PointerStrideSCEV))
                    : SCEVUse(PointerStrideSCEV);
    LLVM_DEBUG(dbgs() << "  MemsetSizeSCEV: " << *MemsetSizeSCEV << "\n"
                      << "  PositiveStrideSCEV: " << *PositiveStrideSCEV
                      << "\n");
 
    if (PositiveStrideSCEV != MemsetSizeSCEV) {
      // If an expression is covered by the loop guard, compare again and
      // proceed with optimization if equal.
      const SCEV *FoldedPositiveStride =
          SE->applyLoopGuards(PositiveStrideSCEV, CurLoop);
      const SCEV *FoldedMemsetSize =
          SE->applyLoopGuards(MemsetSizeSCEV, CurLoop);
 
      LLVM_DEBUG(dbgs() << "  Try to fold SCEV based on loop guard\n"
                        << "    FoldedMemsetSize: " << *FoldedMemsetSize << "\n"
                        << "    FoldedPositiveStride: " << *FoldedPositiveStride
                        << "\n");
 
      if (FoldedPositiveStride != FoldedMemsetSize) {
        LLVM_DEBUG(dbgs() << "  SCEV don't match, abort\n");
        return false;
      }
    }
  }
 
  // Verify that the memset value is loop invariant.  If not, we can't promote
  // the memset.
  Value *SplatValue = MSI->getValue();
  if (!SplatValue || !CurLoop->isLoopInvariant(SplatValue))
    return false;
 
  SmallPtrSet<Instruction *, 1> MSIs;
  MSIs.insert(MSI);
  return processLoopStridedStore(Pointer, SE->getSCEV(MSI->getLength()),
                                 MSI->getDestAlign(), SplatValue, MSI, MSIs,
                                 cast<SCEVAddRecExpr>(Ev), BECount, IsNegStride,
                                 /*IsLoopMemset=*/true);
}
 
/// Return true if \p I is a (simple, loop-invariant-valued) store of the same
/// bytewise value \p SplatByte.
static bool isSameByteValueStore(Instruction &I, Value *SplatByte, Loop *L,
                                 const DataLayout &DL) {
  assert(SplatByte && "expected a bytewise splat value to match against");
  auto *SI = dyn_cast<StoreInst>(&I);
  if (!SI || !SI->isSimple() || !L->isLoopInvariant(SI->getValueOperand()))
    return false;
  return isBytewiseValue(SI->getValueOperand(), DL) == SplatByte;
}
 
/// 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).
///
/// When the access size cannot be bounded, fall back to allow stores writing
/// the same byte value \p SplatByte.
static bool mayLoopAccessLocation(Value *Ptr, ModRefInfo Access, Loop *L,
                                  const SCEV *BECount,
                                  const SCEV *StoreSizeSCEV, AliasAnalysis &AA,
                                  SmallPtrSetImpl<Instruction *> &IgnoredInsts,
                                  Value *SplatByte = nullptr,
                                  const DataLayout *DL = nullptr) {
  // 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
  // at the pointer and has infinite size.
  LocationSize AccessSize = LocationSize::afterPointer();
 
  // 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
  const APInt *BECst, *ConstSize;
  if (match(BECount, m_scev_APInt(BECst)) &&
      match(StoreSizeSCEV, m_scev_APInt(ConstSize))) {
    std::optional<uint64_t> BEInt = BECst->tryZExtValue();
    std::optional<uint64_t> SizeInt = ConstSize->tryZExtValue();
    // FIXME: Should this check for overflow?
    if (BEInt && SizeInt)
      AccessSize = LocationSize::precise((*BEInt + 1) * *SizeInt);
  }
 
  // 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
  // with store of &A[100], we need to StoreLoc to be "A" with size of 100,
  // which will then no-alias a store to &A[100].
  MemoryLocation StoreLoc(Ptr, AccessSize);
 
  // Only consult the same-byte-value fallback when the access size stayed
  // infinite (non-constant trip count); with a precise size AA is accurate.
  bool TrySameByteValue = !AccessSize.isPrecise() && SplatByte && DL;
 
  for (BasicBlock *B : L->blocks())
    for (Instruction &I : *B)
      if (!IgnoredInsts.contains(&I) &&
          isModOrRefSet(AA.getModRefInfo(&I, StoreLoc) & Access)) {
        if (TrySameByteValue && isSameByteValueStore(I, SplatByte, L, *DL))
          continue;
        return true;
      }
  return false;
}
 
// 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, const SCEV *StoreSizeSCEV,
                                        ScalarEvolution *SE) {
  const SCEV *Index = SE->getTruncateOrZeroExtend(BECount, IntPtr);
  if (!StoreSizeSCEV->isOne()) {
    // index = back edge count * store size
    Index = SE->getMulExpr(Index,
                           SE->getTruncateOrZeroExtend(StoreSizeSCEV, IntPtr),
                           SCEV::FlagNUW);
  }
  // base pointer = start - index * store size
  return SE->getMinusSCEV(Start, Index);
}
 
/// Compute the number of bytes as a SCEV from the backedge taken count.
///
/// This also maps the SCEV into the provided type and tries to handle the
/// computation in a way that will fold cleanly.
static const SCEV *getNumBytes(const SCEV *BECount, Type *IntPtr,
                               const SCEV *StoreSizeSCEV, Loop *CurLoop,
                               const DataLayout *DL, ScalarEvolution *SE) {
  const SCEV *TripCountSCEV =
      SE->getTripCountFromExitCount(BECount, IntPtr, CurLoop);
  return SE->getMulExpr(TripCountSCEV,
                        SE->getTruncateOrZeroExtend(StoreSizeSCEV, IntPtr),
                        SCEV::FlagNUW);
}
 
/// 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, const SCEV *StoreSizeSCEV, MaybeAlign StoreAlignment,
    Value *StoredVal, Instruction *TheStore,
    SmallPtrSetImpl<Instruction *> &Stores, const SCEVAddRecExpr *Ev,
    const SCEV *BECount, bool IsNegStride, bool IsLoopMemset) {
  // We currently don't convert inline intrinsics into larger ones, to avoid
  // code size increase. `processLoopMemSet` checks that the intrinsic is not
  // inline before calling this function.
  assert((isa<StoreInst>(TheStore) ||
          !cast<MemIntrinsic>(TheStore)->isForceInlined()) &&
         "inline mem intrinsics should be filtered out by callers");
 
  Module *M = TheStore->getModule();
 
  // 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.
  unsigned DestAS = DestPtr->getType()->getPointerAddressSpace();
  BasicBlock *Preheader = CurLoop->getLoopPreheader();
  IRBuilder<> Builder(Preheader->getTerminator());
  SCEVExpander Expander(*SE, "loop-idiom");
  SCEVExpanderCleaner ExpCleaner(Expander);
 
  Type *DestInt8PtrTy = Builder.getPtrTy(DestAS);
  Type *IntIdxTy = DL->getIndexType(DestPtr->getType());
 
  bool Changed = false;
  const SCEV *Start = Ev->getStart();
  // Handle negative strided loops.
  if (IsNegStride)
    Start = getStartForNegStride(Start, BECount, IntIdxTy, StoreSizeSCEV, SE);
 
  // TODO: ideally we should still be able to generate memset if SCEV expander
  // is taught to generate the dependencies at the latest point.
  if (!Expander.isSafeToExpand(Start))
    return Changed;
 
  // 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
  // would be unsafe to do if there is anything else in the loop that may read
  // or write to the aliased location.  Check for any overlap by generating the
  // base pointer and checking the region.
  Value *BasePtr =
      Expander.expandCodeFor(Start, DestInt8PtrTy, Preheader->getTerminator());
 
  // From here on out, conservatively report to the pass manager that we've
  // changed the IR, even if we later clean up these added instructions. There
  // may be structural differences e.g. in the order of use lists not accounted
  // for in just a textual dump of the IR. This is written as a variable, even
  // though statically all the places this dominates could be replaced with
  // 'true', with the hope that anyone trying to be clever / "more precise" with
  // the return value will read this comment, and leave them alone.
  Changed = true;
 
  Value *SplatValue = isBytewiseValue(StoredVal, *DL);
  if (mayLoopAccessLocation(BasePtr, ModRefInfo::ModRef, CurLoop, BECount,
                            StoreSizeSCEV, *AA, Stores, SplatValue, DL))
    return Changed;
 
  if (avoidLIRForMultiBlockLoop(/*IsMemset=*/true, IsLoopMemset))
    return Changed;
 
  // Okay, everything looks good, insert the memset.
  Constant *PatternValue = nullptr;
  if (!SplatValue)
    PatternValue = getMemSetPatternValue(StoredVal, DL);
 
  // MemsetArg is the number of bytes for the memset libcall, and the number
  // of pattern repetitions if the memset.pattern intrinsic is being used.
  Value *MemsetArg;
  std::optional<int64_t> BytesWritten;
 
  if (PatternValue &&
      (HasMemsetPattern || Opts.loop_idiom_force_memset_pattern_intrinsic)) {
    const SCEV *TripCountS =
        SE->getTripCountFromExitCount(BECount, IntIdxTy, CurLoop);
    if (!Expander.isSafeToExpand(TripCountS))
      return Changed;
    const SCEVConstant *ConstStoreSize = dyn_cast<SCEVConstant>(StoreSizeSCEV);
    if (!ConstStoreSize)
      return Changed;
    Value *TripCount = Expander.expandCodeFor(TripCountS, IntIdxTy,
                                              Preheader->getTerminator());
    uint64_t PatternRepsPerTrip =
        (ConstStoreSize->getValue()->getZExtValue() * 8) /
        DL->getTypeSizeInBits(PatternValue->getType());
    // If ConstStoreSize is not equal to the width of PatternValue, then
    // MemsetArg is TripCount * (ConstStoreSize/PatternValueWidth). Else
    // MemSetArg is just TripCount.
    MemsetArg =
        PatternRepsPerTrip == 1
            ? TripCount
            : Builder.CreateMul(TripCount,
                                Builder.getIntN(IntIdxTy->getIntegerBitWidth(),
                                                PatternRepsPerTrip));
    if (auto *CI = dyn_cast<ConstantInt>(TripCount))
      BytesWritten =
          CI->getZExtValue() * ConstStoreSize->getValue()->getZExtValue();
 
  } else {
    const SCEV *NumBytesS =
        getNumBytes(BECount, IntIdxTy, StoreSizeSCEV, CurLoop, DL, SE);
 
    // TODO: ideally we should still be able to generate memset if SCEV expander
    // is taught to generate the dependencies at the latest point.
    if (!Expander.isSafeToExpand(NumBytesS))
      return Changed;
    MemsetArg =
        Expander.expandCodeFor(NumBytesS, IntIdxTy, Preheader->getTerminator());
    if (auto *CI = dyn_cast<ConstantInt>(MemsetArg))
      BytesWritten = CI->getZExtValue();
  }
  assert(MemsetArg && "MemsetArg should have been set");
 
  AAMDNodes AATags = TheStore->getAAMetadata();
  for (Instruction *Store : Stores)
    AATags = AATags.merge(Store->getAAMetadata());
  if (BytesWritten)
    AATags = AATags.extendTo(BytesWritten.value());
  else
    AATags = AATags.extendTo(-1);
 
  CallInst *NewCall;
  if (SplatValue) {
    NewCall = Builder.CreateMemSet(BasePtr, SplatValue, MemsetArg,
                                   MaybeAlign(StoreAlignment),
                                   /*isVolatile=*/false, AATags);
  } else if (Opts.loop_idiom_force_memset_pattern_intrinsic ||
             isLibFuncEmittable(M, TLI, LibFunc_memset_pattern16)) {
    assert(isa<SCEVConstant>(StoreSizeSCEV) && "Expected constant store size");
 
    NewCall = Builder.CreateIntrinsicWithoutFolding(
        Intrinsic::experimental_memset_pattern,
        {DestInt8PtrTy, PatternValue->getType(), IntIdxTy},
        {BasePtr, PatternValue, MemsetArg,
         ConstantInt::getFalse(M->getContext())});
    if (StoreAlignment)
      cast<MemSetPatternInst>(NewCall)->setDestAlignment(*StoreAlignment);
    NewCall->setAAMetadata(AATags);
  } else {
    // Neither a memset, nor memset_pattern16
    return Changed;
  }
 
  NewCall->setDebugLoc(TheStore->getDebugLoc());
 
  if (MSSAU) {
    MemoryAccess *NewMemAcc = MSSAU->createMemoryAccessInBB(
        NewCall, nullptr, NewCall->getParent(), MemorySSA::BeforeTerminator);
    MSSAU->insertDef(cast<MemoryDef>(NewMemAcc), true);
  }
 
  LLVM_DEBUG(dbgs() << "  Formed memset: " << *NewCall << "\n"
                    << "    from store to: " << *Ev << " at: " << *TheStore
                    << "\n");
 
  ORE.emit([&]() {
    OptimizationRemark R(DEBUG_TYPE, "ProcessLoopStridedStore",
                         NewCall->getDebugLoc(), Preheader);
    R << "Transformed loop-strided store in "
      << ore::NV("Function", TheStore->getFunction())
      << " function into a call to "
      << ore::NV("NewFunction", NewCall->getCalledFunction())
      << "() intrinsic";
    if (!Stores.empty())
      R << ore::setExtraArgs();
    for (auto *I : Stores) {
      R << ore::NV("FromBlock", I->getParent()->getName())
        << ore::NV("ToBlock", Preheader->getName());
    }
    return R;
  });
 
  // Okay, the memset has been formed.  Zap the original store and anything that
  // feeds into it.
  for (auto *I : Stores) {
    if (MSSAU)
      MSSAU->removeMemoryAccess(I, true);
    deleteDeadInstruction(I);
  }
  if (MSSAU && VerifyMemorySSA)
    MSSAU->getMemorySSA()->verifyMemorySSA();
  ++NumMemSet;
  ExpCleaner.markResultUsed();
  return true;
}
 
/// 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,
                                                    const SCEV *BECount) {
  assert(SI->isUnordered() && "Expected only non-volatile non-ordered stores.");
 
  Value *StorePtr = SI->getPointerOperand();
  const SCEVAddRecExpr *StoreEv = cast<SCEVAddRecExpr>(SE->getSCEV(StorePtr));
  unsigned StoreSize = DL->getTypeStoreSize(SI->getValueOperand()->getType());
 
  // The store must be feeding a non-volatile load.
  LoadInst *LI = cast<LoadInst>(SI->getValueOperand());
  assert(LI->isUnordered() && "Expected only non-volatile non-ordered loads.");
 
  // 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.
  Value *LoadPtr = LI->getPointerOperand();
  const SCEVAddRecExpr *LoadEv = cast<SCEVAddRecExpr>(SE->getSCEV(LoadPtr));
 
  const SCEV *StoreSizeSCEV = SE->getConstant(StorePtr->getType(), StoreSize);
  return processLoopStoreOfLoopLoad(StorePtr, LoadPtr, StoreSizeSCEV,
                                    SI->getAlign(), LI->getAlign(), SI, LI,
                                    StoreEv, LoadEv, BECount);
}
 
namespace {
class MemmoveVerifier {
public:
  explicit MemmoveVerifier(const SCEV &LoadStart, const SCEV &StoreStart,
                           ScalarEvolution &SE)
      : DL(SE.getDataLayout()),
        Off(dyn_cast<SCEVConstant>(SE.getMinusSCEV(&StoreStart, &LoadStart))),
        BasePtr(dyn_cast<SCEVUnknown>(SE.getPointerBase(&StoreStart))),
        IsSameObject(Off != nullptr) {}
 
  bool loadAndStoreMayFormMemmove(unsigned StoreSize, bool IsNegStride,
                                  const Instruction &TheLoad,
                                  bool IsMemCpy) const {
    // The store must be at a constant offset from the load, and there must be
    // an underlying pointer.
    if (!Off || !BasePtr)
      return false;
    const APInt &OffVal = Off->getAPInt();
    // If null is defined then the base pointer can't be null
    auto *NullBase = dyn_cast<ConstantPointerNull>(BasePtr->getValue());
    if (NullBase && NullPointerIsDefined(
                        TheLoad.getParent()->getParent(),
                        NullBase->getPointerType()->getPointerAddressSpace()))
      return false;
    int64_t LoadSize;
    if (IsMemCpy) {
      // memcpy is equivalent to a sequence of byte loads and stores
      LoadSize = 1;
    } else {
      LoadSize = DL.getTypeSizeInBits(TheLoad.getType()).getFixedValue() / 8;
      if (LoadSize != StoreSize)
        return false;
    }
    // Ensure that LoadBasePtr is after StoreBasePtr or before StoreBasePtr
    // for negative stride. LoadBasePtr shouldn't overlap with StoreBasePtr.
    if (IsNegStride ? OffVal.slt(LoadSize) : OffVal.sgt(-LoadSize))
      return false;
    return true;
  }
 
private:
  const DataLayout &DL;
  const SCEVConstant *Off;
  const SCEVUnknown *BasePtr;
 
public:
  const bool IsSameObject;
};
} // namespace
 
bool LoopIdiomRecognize::processLoopStoreOfLoopLoad(
    Value *DestPtr, Value *SourcePtr, const SCEV *StoreSizeSCEV,
    MaybeAlign StoreAlign, MaybeAlign LoadAlign, Instruction *TheStore,
    Instruction *TheLoad, const SCEVAddRecExpr *StoreEv,
    const SCEVAddRecExpr *LoadEv, const SCEV *BECount) {
  // We currently don't convert inline intrinsics into larger ones, to avoid
  // code size increase. `processLoopMemCpy` checks that the intrinsic is not
  // inline before calling this function.
  assert((isa<StoreInst>(TheStore) ||
          !cast<MemIntrinsic>(TheStore)->isForceInlined()) &&
         "inline mem intrinsics should be filtered out by callers");
 
  // 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());
  SCEVExpander Expander(*SE, "loop-idiom");
 
  SCEVExpanderCleaner ExpCleaner(Expander);
 
  bool Changed = false;
  const SCEV *StrStart = StoreEv->getStart();
  unsigned StrAS = DestPtr->getType()->getPointerAddressSpace();
  Type *IntIdxTy = Builder.getIntNTy(DL->getIndexSizeInBits(StrAS));
 
  APInt Stride = getStoreStride(StoreEv);
  const SCEVConstant *ConstStoreSize = dyn_cast<SCEVConstant>(StoreSizeSCEV);
 
  // TODO: Deal with non-constant size; Currently expect constant store size
  assert(ConstStoreSize && "store size is expected to be a constant");
 
  int64_t StoreSize = ConstStoreSize->getValue()->getZExtValue();
  bool IsNegStride = StoreSize == -Stride;
 
  // Handle negative strided loops.
  if (IsNegStride)
    StrStart =
        getStartForNegStride(StrStart, BECount, IntIdxTy, StoreSizeSCEV, 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
  // would be unsafe to do if there is anything else in the loop that may read
  // 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(
      StrStart, Builder.getPtrTy(StrAS), Preheader->getTerminator());
 
  // From here on out, conservatively report to the pass manager that we've
  // changed the IR, even if we later clean up these added instructions. There
  // may be structural differences e.g. in the order of use lists not accounted
  // for in just a textual dump of the IR. This is written as a variable, even
  // though statically all the places this dominates could be replaced with
  // 'true', with the hope that anyone trying to be clever / "more precise" with
  // the return value will read this comment, and leave them alone.
  Changed = true;
 
  SmallPtrSet<Instruction *, 2> IgnoredInsts;
  IgnoredInsts.insert(TheStore);
 
  bool IsMemCpy = isa<MemCpyInst>(TheStore);
  const StringRef InstRemark = IsMemCpy ? "memcpy" : "load and store";
 
  bool LoopAccessStore =
      mayLoopAccessLocation(StoreBasePtr, ModRefInfo::ModRef, CurLoop, BECount,
                            StoreSizeSCEV, *AA, IgnoredInsts);
  if (LoopAccessStore) {
    // For memmove case it's not enough to guarantee that loop doesn't access
    // TheStore and TheLoad. Additionally we need to make sure that TheStore is
    // the only user of TheLoad.
    if (!TheLoad->hasOneUse())
      return Changed;
    IgnoredInsts.insert(TheLoad);
    if (mayLoopAccessLocation(StoreBasePtr, ModRefInfo::ModRef, CurLoop,
                              BECount, StoreSizeSCEV, *AA, IgnoredInsts)) {
      ORE.emit([&]() {
        return OptimizationRemarkMissed(DEBUG_TYPE, "LoopMayAccessStore",
                                        TheStore)
               << ore::NV("Inst", InstRemark) << " in "
               << ore::NV("Function", TheStore->getFunction())
               << " function will not be hoisted: "
               << ore::NV("Reason", "The loop may access store location");
      });
      return Changed;
    }
    IgnoredInsts.erase(TheLoad);
  }
 
  const SCEV *LdStart = LoadEv->getStart();
  unsigned LdAS = SourcePtr->getType()->getPointerAddressSpace();
 
  // Handle negative strided loops.
  if (IsNegStride)
    LdStart =
        getStartForNegStride(LdStart, BECount, IntIdxTy, StoreSizeSCEV, SE);
 
  // For a memcpy, we have to make sure that the input array is not being
  // mutated by the loop.
  Value *LoadBasePtr = Expander.expandCodeFor(LdStart, Builder.getPtrTy(LdAS),
                                              Preheader->getTerminator());
 
  // If the store is a memcpy instruction, we must check if it will write to
  // the load memory locations. So remove it from the ignored stores.
  MemmoveVerifier Verifier(*LdStart, *StrStart, *SE);
  if (IsMemCpy && !Verifier.IsSameObject)
    IgnoredInsts.erase(TheStore);
  if (mayLoopAccessLocation(LoadBasePtr, ModRefInfo::Mod, CurLoop, BECount,
                            StoreSizeSCEV, *AA, IgnoredInsts)) {
    ORE.emit([&]() {
      return OptimizationRemarkMissed(DEBUG_TYPE, "LoopMayAccessLoad", TheLoad)
             << ore::NV("Inst", InstRemark) << " in "
             << ore::NV("Function", TheStore->getFunction())
             << " function will not be hoisted: "
             << ore::NV("Reason", "The loop may access load location");
    });
    return Changed;
  }
 
  bool IsAtomic = TheStore->isAtomic() || TheLoad->isAtomic();
  bool UseMemMove = IsMemCpy ? Verifier.IsSameObject : LoopAccessStore;
 
  if (IsAtomic) {
    // For now don't support unordered atomic memmove.
    if (UseMemMove)
      return Changed;
 
    // We cannot allow unaligned ops for unordered load/store, so reject
    // anything where the alignment isn't at least the element size.
    assert((StoreAlign && LoadAlign) &&
           "Expect unordered load/store to have align.");
    if (*StoreAlign < StoreSize || *LoadAlign < StoreSize)
      return Changed;
 
    // If the element.atomic memcpy is not lowered into explicit
    // loads/stores later, then it will be lowered into an element-size
    // specific lib call. If the lib call doesn't exist for our store size, then
    // we shouldn't generate the memcpy.
    if (StoreSize > TTI->getAtomicMemIntrinsicMaxElementSize())
      return Changed;
  }
 
  if (UseMemMove)
    if (!Verifier.loadAndStoreMayFormMemmove(StoreSize, IsNegStride, *TheLoad,
                                             IsMemCpy))
      return Changed;
 
  if (avoidLIRForMultiBlockLoop())
    return Changed;
 
  // Okay, everything is safe, we can transform this!
 
  const SCEV *NumBytesS =
      getNumBytes(BECount, IntIdxTy, StoreSizeSCEV, CurLoop, DL, SE);
 
  Value *NumBytes =
      Expander.expandCodeFor(NumBytesS, IntIdxTy, Preheader->getTerminator());
 
  AAMDNodes AATags = TheLoad->getAAMetadata();
  AAMDNodes StoreAATags = TheStore->getAAMetadata();
  AATags = AATags.merge(StoreAATags);
  if (auto CI = dyn_cast<ConstantInt>(NumBytes))
    AATags = AATags.extendTo(CI->getZExtValue());
  else
    AATags = AATags.extendTo(-1);
 
  CallInst *NewCall = nullptr;
  // Check whether to generate an unordered atomic memcpy:
  //  If the load or store are atomic, then they must necessarily be unordered
  //  by previous checks.
  if (!IsAtomic) {
    if (UseMemMove)
      NewCall = Builder.CreateMemMove(StoreBasePtr, StoreAlign, LoadBasePtr,
                                      LoadAlign, NumBytes,
                                      /*isVolatile=*/false, AATags);
    else
      NewCall =
          Builder.CreateMemCpy(StoreBasePtr, StoreAlign, LoadBasePtr, LoadAlign,
                               NumBytes, /*isVolatile=*/false, AATags);
  } else {
    // Create the call.
    // Note that unordered atomic loads/stores are *required* by the spec to
    // have an alignment but non-atomic loads/stores may not.
    NewCall = Builder.CreateElementUnorderedAtomicMemCpy(
        StoreBasePtr, *StoreAlign, LoadBasePtr, *LoadAlign, NumBytes, StoreSize,
        AATags);
  }
  NewCall->setDebugLoc(TheStore->getDebugLoc());
 
  if (MSSAU) {
    MemoryAccess *NewMemAcc = MSSAU->createMemoryAccessInBB(
        NewCall, nullptr, NewCall->getParent(), MemorySSA::BeforeTerminator);
    MSSAU->insertDef(cast<MemoryDef>(NewMemAcc), true);
  }
 
  LLVM_DEBUG(dbgs() << "  Formed new call: " << *NewCall << "\n"
                    << "    from load ptr=" << *LoadEv << " at: " << *TheLoad
                    << "\n"
                    << "    from store ptr=" << *StoreEv << " at: " << *TheStore
                    << "\n");
 
  ORE.emit([&]() {
    return OptimizationRemark(DEBUG_TYPE, "ProcessLoopStoreOfLoopLoad",
                              NewCall->getDebugLoc(), Preheader)
           << "Formed a call to "
           << ore::NV("NewFunction", NewCall->getCalledFunction())
           << "() intrinsic from " << ore::NV("Inst", InstRemark)
           << " instruction in " << ore::NV("Function", TheStore->getFunction())
           << " function"
           << ore::setExtraArgs()
           << ore::NV("FromBlock", TheStore->getParent()->getName())
           << ore::NV("ToBlock", Preheader->getName());
  });
 
  // Okay, a new call to memcpy/memmove has been formed.  Zap the original store
  // and anything that feeds into it.
  if (MSSAU)
    MSSAU->removeMemoryAccess(TheStore, true);
  deleteDeadInstruction(TheStore);
  if (MSSAU && VerifyMemorySSA)
    MSSAU->getMemorySSA()->verifyMemorySSA();
  if (UseMemMove)
    ++NumMemMove;
  else
    ++NumMemCpy;
  ExpCleaner.markResultUsed();
  return true;
}
 
// When compiling for codesize we avoid idiom recognition for a multi-block loop
// unless it is a loop_memset idiom or a memset/memcpy idiom in a nested loop.
//
bool LoopIdiomRecognize::avoidLIRForMultiBlockLoop(bool IsMemset,
                                                   bool IsLoopMemset) {
  if (ApplyCodeSizeHeuristics && CurLoop->getNumBlocks() > 1) {
    if (CurLoop->isOutermost() && (!IsMemset || !IsLoopMemset)) {
      LLVM_DEBUG(dbgs() << "  " << CurLoop->getHeader()->getParent()->getName()
                        << " : LIR " << (IsMemset ? "Memset" : "Memcpy")
                        << " avoided: multi-block top-level loop\n");
      return true;
    }
  }
 
  return false;
}
 
bool LoopIdiomRecognize::optimizeCRCLoop(const PolynomialInfo &Info) {
  // FIXME: Hexagon has a special HexagonLoopIdiom that optimizes CRC using
  // carry-less multiplication instructions, which is more efficient than our
  // Sarwate table-lookup optimization. Hence, until we're able to emit
  // target-specific instructions for Hexagon, subsuming HexagonLoopIdiom,
  // disable the optimization for Hexagon.
  Module &M = *CurLoop->getHeader()->getModule();
  Triple TT(M.getTargetTriple());
  if (TT.getArch() == Triple::hexagon)
    return false;
 
  LLVMContext &Ctx = Info.LHS->getContext();
  Type *CRCTy = Info.LHS->getType();
  unsigned CRCBW = CRCTy->getIntegerBitWidth();
 
  // CRC computation is mostly serial, so latency works best for comparison.
  TargetTransformInfo::TargetCostKind CostKind =
      TargetTransformInfo::TCK_Latency;
 
  InstructionCost XorCost =
      TTI->getArithmeticInstrCost(Instruction::Xor, CRCTy, CostKind);
  InstructionCost ShiftCost =
      TTI->getArithmeticInstrCost(Instruction::LShr, CRCTy, CostKind);
  InstructionCost AndCost =
      TTI->getArithmeticInstrCost(Instruction::And, CRCTy, CostKind);
  InstructionCost SelectCost =
      TTI->getCmpSelInstrCost(Instruction::Select, CRCTy, Type::getInt1Ty(Ctx),
                              CmpInst::BAD_ICMP_PREDICATE, CostKind);
  InstructionCost LoadCost =
      TTI->getMemoryOpCost(Instruction::Load, CRCTy, DL->getABITypeAlign(CRCTy),
                           DL->getDefaultGlobalsAddressSpace(), CostKind);
  auto ClmulCost = [&](unsigned BW) {
    auto *Ty = IntegerType::get(Ctx, BW);
    IntrinsicCostAttributes Attrs(Intrinsic::clmul, Ty, {Ty, Ty});
    return TTI->getIntrinsicInstrCost(Attrs, CostKind);
  };
 
  // Estimate the cost of the original, unoptimized loop.
  InstructionCost OrigLoopCost =
      (2 * ShiftCost + 2 * XorCost + AndCost + SelectCost) * Info.TripCount;
 
  // Estimate the cost of the Sarwate lookup table optimization strategy.
  // As mentioned previously, a byte-multiple trip count is required.
  InstructionCost TableStrategyCost =
      Info.TripCount % 8 != 0
          ? InstructionCost::getInvalid()
          : (LoadCost + XorCost + 2 * ShiftCost) * (Info.TripCount / 8);
 
  // Estimate the cost of the carry-less multiplication optimization strategy.
  InstructionCost ClmulStrategyCost = ClmulCost(2 * Info.TripCount) +
                                      ClmulCost(CRCBW + Info.TripCount) +
                                      2 * XorCost + 2 * ShiftCost + AndCost;
 
  ORE.emit([&]() {
    return OptimizationRemarkAnalysis(DEBUG_TYPE, "CRCLoopCosts",
                                      CurLoop->getStartLoc(),
                                      CurLoop->getHeader())
           << "CRC loop costs: original="
           << ore::NV("OrigLoopCost", OrigLoopCost)
           << ", table=" << ore::NV("TableStrategyCost", TableStrategyCost)
           << ", clmul=" << ore::NV("ClmulStrategyCost", ClmulStrategyCost);
  });
 
  auto ReportMissed = [&](StringRef Reason) {
    ORE.emit([&]() {
      return OptimizationRemarkMissed(DEBUG_TYPE, "CRCLoopMissed",
                                      CurLoop->getStartLoc(),
                                      CurLoop->getHeader())
             << "CRC loop not optimized: " << Reason;
    });
  };
  auto ReportOptimized = [&](StringRef Strategy, StringRef Reason) {
    ORE.emit([&]() {
      return OptimizationRemark(DEBUG_TYPE, "CRCLoopOptimized",
                                CurLoop->getStartLoc(), CurLoop->getHeader())
             << "CRC loop optimized using " << ore::NV("Strategy", Strategy)
             << ": " << Reason;
    });
  };
 
  switch (Opts.loop_idiom_crc_strategy) {
  default:
    ReportMissed("disabled by user");
    return false;
  case CRCStrategyKind::Table:
    // The table strategy is not possible in its current form without a byte-
    // multiple trip count.
    if (Info.TripCount % 8 == 0) {
      optimizeCRCLoopUsingTableLookup(Info);
      ReportOptimized("table", "forced by user");
      return true;
    }
    ReportMissed("table strategy forced, but not possible");
    return false;
  case CRCStrategyKind::Clmul:
    optimizeCRCLoopUsingClmul(Info);
    ReportOptimized("clmul", "forced by user");
    return true;
  case CRCStrategyKind::Auto:
    // When using the auto strategy, bail if we are optimizing for size since
    // there's usually not a clear size benefit.
    // TODO: The clmul optimization is around the same size in many cases, so it
    // could be worth it to take advantage of that fact, especially if it would
    // be much faster than the original loop.
    if (ApplyCodeSizeHeuristics) {
      ReportMissed("optimizing for size");
      return false;
    }
 
    // Only apply an optimization if there's a clear benefit to doing so.
    if (std::min(TableStrategyCost, ClmulStrategyCost) >= OrigLoopCost) {
      ReportMissed("no profitable strategy");
      return false;
    }
 
    if (TableStrategyCost <= ClmulStrategyCost) {
      optimizeCRCLoopUsingTableLookup(Info);
      ReportOptimized("table", "most profitable strategy");
    } else {
      optimizeCRCLoopUsingClmul(Info);
      ReportOptimized("clmul", "most profitable strategy");
    }
    return true;
  }
}
 
// The algorithm used in this optimization is a Polynomial (GF(2)) Barrett
// Reduction based on Intel's "Fast CRC Computation for Generic Polynomials
// Using PCLMULQDQ Instruction" white paper (December 2009).
void LoopIdiomRecognize::optimizeCRCLoopUsingClmul(const PolynomialInfo &Info) {
  // TODO: If clmul exists on the target but not for the required width, it
  // might be possible to split into multiple iterations of reduction.
  Type *CRCTy = Info.LHS->getType();
  LLVMContext &Ctx = CRCTy->getContext();
  unsigned CRCBW = CRCTy->getIntegerBitWidth();
  // The loop's TripCount determines how many bits of the data are processed,
  // regardless of whether the actual data bit width matches (if auxiliary data
  // is even used at all).
  unsigned TC = Info.TripCount;
  // Based on the clmul inputs, the first clmul needs 2*TC bits, and the second
  // needs CRCBW+TC bits. However, only the low TC bits of the first clmul are
  // used in little-endian, so a clmul in TC bits suffices in that case.
  IntegerType *ClmulMuTy =
      IntegerType::get(Ctx, Info.IsBigEndian ? 2 * TC : TC);
  IntegerType *ClmulGPTy = IntegerType::get(Ctx, CRCBW + TC);
 
  // First, generate the constants required for GF(2) Barrett reduction.
  auto [Mu, FullGenPoly] = HashRecognize::genBarrettConstants(Info);
  Value *MuConst =
      ConstantInt::get(Ctx, Mu.zextOrTrunc(ClmulMuTy->getBitWidth()));
  Value *GenPolyConst =
      ConstantInt::get(Ctx, FullGenPoly.zext(ClmulGPTy->getBitWidth()));
 
  IRBuilder<> Builder(CurLoop->getLoopPreheader()->getTerminator());
 
  // If a shift needs to occur in the setup for the first clmul with MuConst, it
  // will be by abs(TC - CRCBW). To ensure that the shift can work without
  // losing information or creating poison, give it CRCBW + TC bits.
  bool SetupShiftNeeded = Info.IsBigEndian && TC != CRCBW;
  auto *SetupTy = IntegerType::get(Ctx, SetupShiftNeeded ? CRCBW + TC : TC);
 
  // Based on the Intel white paper, in our case, we have
  // R(x) = (LHS*x^TC) xor (LHSAux ? getTCBits(LHSAux)*x^CRCBW : 0)
  // since the CRC loop multiplies LHS by x each iteration, and the x^CRCBW term
  // of getTCBits(LHSAux) is XORed in for the significant bit check.
  // Rather than compute the full R(x), we can split it in two: a quotient for
  // step 1 (floor(R(x)/x^CRCBW)) and a remainder for step 3 (R(x) mod x^CRCBW).
  //
  // ClmulMuInput is an evolving variable that will eventually become the part
  // used in step 1, which can be simplified to
  // (LHS*x^(TC-CRCBW)) xor (LHSAux ? getTCBits(LHSAux) : 0).
  // Thanks to restrictions imposed by HashRecognize for big-endian CRC loops,
  // getTCBits(LHSAux) = LHSAux*x^(TC-CRCBW), so this can be further simplified
  // to (LHS xor (LHSAux ? LHSAux : 0))*x^(TC-CRCBW).
  Value *ClmulMuInput =
      Builder.CreateZExtOrTrunc(Info.LHS, SetupTy, "crc.cast");
 
  // If auxiliary data is present, XOR it in with the CRC.
  if (Value *Data = Info.LHSAux) {
    // This is usually a zext, but DataBW may exceed CRCBW+TC if both CRCBW and
    // TC are small enough.
    Data = Builder.CreateZExtOrTrunc(Data, SetupTy, "data.cast");
 
    ClmulMuInput = Builder.CreateXor(ClmulMuInput, Data, "xor.crc.data");
  }
 
  // Align the current CRC with TripCount (multiply or divide by x^(TC-CRCBW)).
  if (SetupShiftNeeded) {
    ClmulMuInput =
        TC > CRCBW
            ? Builder.CreateShl(ClmulMuInput, TC - CRCBW, "crc.align.tc")
            : Builder.CreateLShr(ClmulMuInput, CRCBW - TC, "crc.align.tc");
  }
 
  // Zero out any bits above (TC-1) for calculation since the original loop
  // doesn't use them in the significant bit checks.
  if (SetupTy->getBitWidth() > TC) {
    auto *Mask =
        ConstantInt::get(Ctx, APInt::getLowBitsSet(SetupTy->getBitWidth(), TC));
    ClmulMuInput = Builder.CreateAnd(ClmulMuInput, Mask, "crc.tcbits");
  }
 
  // Step 1: T1(x) = floor(R(x)/x^CRCBW) * mu
  // Input is TC bits and mu is TC+1 bits, so result will be 2*TC bits.
  ClmulMuInput =
      Builder.CreateZExtOrTrunc(ClmulMuInput, ClmulMuTy, "tcbits.cast");
  Value *ClmulMu = Builder.CreateBinaryIntrinsic(
      Intrinsic::clmul, ClmulMuInput, MuConst, /*FMFSource=*/{}, "clmul.mu");
 
  // Calculate floor(T1(x)/x^TC) for step 2.
  Value *ClmulGPInput =
      Info.IsBigEndian ? Builder.CreateLShr(ClmulMu, TC, "quot.lshr") : ClmulMu;
 
  // Step 2: T2(x) = floor(T1(x)/x^TC) * P(x)
  // Input is TC bits and P(x) is CRCBW+1 bits, so result will be CRCBW+TC bits.
  ClmulGPInput =
      Builder.CreateZExtOrTrunc(ClmulGPInput, ClmulGPTy, "quot.cast");
  Value *ClmulGP = Builder.CreateBinaryIntrinsic(Intrinsic::clmul, ClmulGPInput,
                                                 GenPolyConst,
                                                 /*FMFSource=*/{}, "clmul.gp");
 
  // Calculate the least significant part of R(x) for step 3 as specified above.
  // R(x) mod x^CRCBW = LHS*x^TC mod x^CRCBW, though the (mod x^CRCBW) is
  // handled later on when truncating back to CRCBW for ComputedValue.
  Value *CRCNext = Builder.CreateZExt(Info.LHS, ClmulGPTy, "crc.recast");
  if (Info.IsBigEndian)
    CRCNext = Builder.CreateShl(CRCNext, TC, "crc.shl");
 
  // Step 3: C(x) = (R(x) xor T2(x)) mod x^CRCBW
  CRCNext = Builder.CreateXor(CRCNext, ClmulGP, "xor.crc.mult");
  if (!Info.IsBigEndian)
    CRCNext = Builder.CreateLShr(CRCNext, TC, "crc.lshr");
 
  // Bring the result back down the the CRC bit width.
  CRCNext = Builder.CreateTrunc(CRCNext, CRCTy, "crc.next");
 
  // Replace the result of the loop with the new computed CRC value.
  Info.ComputedValue->replaceUsesOutsideBlock(CRCNext, CurLoop->getLoopLatch());
 
  // Finally, clean up the loop as much as possible so it can be trivially
  // deleted.
  {
    for (PHINode &PN : make_early_inc_range(CurLoop->getHeader()->phis())) {
      PN.replaceAllUsesWith(PoisonValue::get(PN.getType()));
      RecursivelyDeleteDeadPHINode(&PN);
    }
    // Replace the exit condition with constant true/false to always cause a
    // branch to the exit block.
    deleteDeadInstruction(CurLoop->getLatchCmpInst());
    auto *BrInst = cast<CondBrInst>(CurLoop->getLoopLatch()->getTerminator());
    BrInst->setCondition(ConstantInt::getBool(
        Ctx, BrInst->getSuccessor(0) == CurLoop->getExitBlock()));
    SE->forgetLoop(CurLoop);
  }
}
 
void LoopIdiomRecognize::optimizeCRCLoopUsingTableLookup(
    const PolynomialInfo &Info) {
  assert(Info.TripCount % 8 == 0 && "A byte-multiple trip count is required");
 
  // First, create a new GlobalVariable corresponding to the
  // Sarwate-lookup-table.
  Type *CRCTy = Info.LHS->getType();
  unsigned CRCBW = CRCTy->getIntegerBitWidth();
  std::array<Constant *, 256> CRCConstants;
  transform(HashRecognize::genSarwateTable(Info.RHS, Info.IsBigEndian),
            CRCConstants.begin(),
            [CRCTy](const APInt &E) { return ConstantInt::get(CRCTy, E); });
  Constant *ConstArray =
      ConstantArray::get(ArrayType::get(CRCTy, 256), CRCConstants);
  GlobalVariable *GV = new GlobalVariable(
      *CurLoop->getHeader()->getModule(), ConstArray->getType(), true,
      GlobalValue::PrivateLinkage, ConstArray, ".crctable");
 
  PHINode *IV = CurLoop->getCanonicalInductionVariable();
  SmallVector<PHINode *, 2> Cleanup;
 
  // Next, mark all PHIs for removal except IV.
  {
    for (PHINode &PN : CurLoop->getHeader()->phis()) {
      if (&PN == IV)
        continue;
      PN.replaceAllUsesWith(PoisonValue::get(PN.getType()));
      Cleanup.push_back(&PN);
    }
  }
 
  // Next, fix up the trip count.
  {
    unsigned NewBTC = (Info.TripCount / 8) - 1;
    BasicBlock *LoopBlk = CurLoop->getLoopLatch();
    CondBrInst *BrInst = cast<CondBrInst>(LoopBlk->getTerminator());
    CmpPredicate ExitPred = BrInst->getSuccessor(0) == LoopBlk
                                ? ICmpInst::Predicate::ICMP_NE
                                : ICmpInst::Predicate::ICMP_EQ;
    Instruction *ExitCond = CurLoop->getLatchCmpInst();
    Value *ExitLimit = ConstantInt::get(IV->getType(), NewBTC);
    IRBuilder<> Builder(ExitCond);
    Value *NewExitCond =
        Builder.CreateICmp(ExitPred, IV, ExitLimit, "exit.cond");
    ExitCond->replaceAllUsesWith(NewExitCond);
    deleteDeadInstruction(ExitCond);
  }
 
  // Finally, fill the loop with the Sarwate-table-lookup logic, and replace all
  // uses of ComputedValue.
  //
  // Little-endian:
  //   crc = (crc >> 8) ^ tbl[(iv'th byte of data) ^ (bottom byte of crc)]
  // Big-Endian:
  //   crc = (crc << 8) ^ tbl[(iv'th byte of data) ^ (top byte of crc)]
  {
    auto LoByte = [](IRBuilderBase &Builder, Value *Op, const Twine &Name) {
      return Builder.CreateZExtOrTrunc(
          Op, IntegerType::getInt8Ty(Op->getContext()), Name);
    };
    auto HiIdx = [LoByte, CRCBW](IRBuilderBase &Builder, Value *Op,
                                 const Twine &Name) {
      // Shift the top bits of Op to the bottom byte by using the CRC bitwidth
      // as a reference.
      if (CRCBW != 8) {
        Op = CRCBW > 8 ? Builder.CreateLShr(Op, CRCBW - 8, Name)
                       : Builder.CreateShl(Op, 8 - CRCBW, Name);
      }
      return LoByte(Builder, Op, Name + ".lo.byte");
    };
 
    IRBuilder<> Builder(CurLoop->getHeader()->getFirstNonPHIIt());
 
    // Create the CRC PHI, and initialize its incoming value to the initial
    // value of CRC.
    PHINode *CRCPhi = Builder.CreatePHI(CRCTy, 2, "crc");
    CRCPhi->addIncoming(Info.LHS, CurLoop->getLoopPreheader());
 
    // CRC is now an evolving variable, initialized to the PHI.
    Value *CRC = CRCPhi;
 
    // TableIndexer = ((top|bottom) byte of CRC). It is XOR'ed with (iv'th byte
    // of LHSAux), if LHSAux is non-nullptr.
    Value *Indexer = CRC;
    if (Value *Data = Info.LHSAux) {
      Type *DataTy = Data->getType();
 
      // To index into the (iv'th byte of LHSAux), we multiply iv by 8, and we
      // shift right by that amount, and take the lo-byte (in the little-endian
      // case), or shift left by that amount, and take the hi-idx (in the
      // big-endian case).
      Value *IVBits = Builder.CreateZExtOrTrunc(
          Builder.CreateShl(IV, 3, "iv.bits"), DataTy, "iv.indexer");
      Value *DataIndexer =
          Info.IsBigEndian ? Builder.CreateShl(Data, IVBits, "data.indexer")
                           : Builder.CreateLShr(Data, IVBits, "data.indexer");
      Indexer = Builder.CreateXor(
          DataIndexer,
          Builder.CreateZExtOrTrunc(Indexer, DataTy, "crc.indexer.cast"),
          "crc.data.indexer");
    }
 
    Indexer = Info.IsBigEndian ? HiIdx(Builder, Indexer, "indexer.hi")
                               : LoByte(Builder, Indexer, "indexer.lo");
 
    // Always index into a GEP using the index type.
    Indexer = Builder.CreateZExt(
        Indexer, SE->getDataLayout().getIndexType(GV->getType()),
        "indexer.ext");
 
    // CRCTableLd = CRCTable[(iv'th byte of data) ^ (top|bottom) byte of CRC].
    Value *CRCTableGEP =
        Builder.CreateInBoundsGEP(CRCTy, GV, Indexer, "tbl.ptradd");
    Instruction *CRCTableLd = Builder.CreateLoad(CRCTy, CRCTableGEP, "tbl.ld");
 
    // Update MemorySSA since we just created a new load instruction.
    if (MSSAU) {
      auto *NewMemAcc = MSSAU->createMemoryAccessInBB(
          CRCTableLd, /*Definition=*/nullptr, CRCTableLd->getParent(),
          MemorySSA::Beginning);
      MSSAU->insertUse(cast<MemoryUse>(NewMemAcc), /*RenameUses=*/true);
    }
 
    // CRCNext = (CRC (<<|>>) 8) ^ CRCTableLd, or simply CRCTableLd in case of
    // CRC-8.
    Value *CRCNext = CRCTableLd;
    if (CRCBW > 8) {
      Value *CRCShift = Info.IsBigEndian
                            ? Builder.CreateShl(CRC, 8, "crc.be.shift")
                            : Builder.CreateLShr(CRC, 8, "crc.le.shift");
      CRCNext = Builder.CreateXor(CRCShift, CRCTableLd, "crc.next");
    }
 
    // Connect the back-edge for the loop, and RAUW the ComputedValue.
    CRCPhi->addIncoming(CRCNext, CurLoop->getLoopLatch());
    Info.ComputedValue->replaceUsesOutsideBlock(CRCNext,
                                                CurLoop->getLoopLatch());
  }
 
  // Cleanup.
  {
    for (PHINode *PN : Cleanup)
      RecursivelyDeleteDeadPHINode(PN);
    SE->forgetLoop(CurLoop);
    if (MSSAU && VerifyMemorySSA)
      MSSAU->getMemorySSA()->verifyMemorySSA();
  }
}
 
bool LoopIdiomRecognize::runOnNoncountableLoop() {
  LLVM_DEBUG(dbgs() << DEBUG_TYPE " Scanning: F["
                    << CurLoop->getHeader()->getParent()->getName()
                    << "] Noncountable Loop %"
                    << CurLoop->getHeader()->getName() << "\n");
 
  return recognizePopcount() || recognizeAndInsertFFS() ||
         recognizeShiftUntilBitTest() || recognizeShiftUntilZero() ||
         recognizeShiftUntilLessThan() || recognizeAndInsertStrLen();
}
 
/// Check if the given conditional branch is based on the comparison between
/// a variable and zero, and if the variable is non-zero or zero (JmpOnZero is
/// true), the control yields to the loop entry. If the branch matches the
/// behavior, the variable involved in the comparison is returned. This function
/// will be called to see if the precondition and postcondition of the loop are
/// in desirable form.
static Value *matchCondition(CondBrInst *BI, BasicBlock *LoopEntry,
                             bool JmpOnZero = false) {
  ICmpInst *Cond = dyn_cast<ICmpInst>(BI->getCondition());
  if (!Cond)
    return nullptr;
 
  auto *CmpZero = dyn_cast<ConstantInt>(Cond->getOperand(1));
  if (!CmpZero || !CmpZero->isZero())
    return nullptr;
 
  BasicBlock *TrueSucc = BI->getSuccessor(0);
  BasicBlock *FalseSucc = BI->getSuccessor(1);
  if (JmpOnZero)
    std::swap(TrueSucc, FalseSucc);
 
  ICmpInst::Predicate Pred = Cond->getPredicate();
  if ((Pred == ICmpInst::ICMP_NE && TrueSucc == LoopEntry) ||
      (Pred == ICmpInst::ICMP_EQ && FalseSucc == LoopEntry))
    return Cond->getOperand(0);
 
  return nullptr;
}
 
namespace {
 
class StrlenVerifier {
public:
  explicit StrlenVerifier(const Loop *CurLoop, ScalarEvolution *SE,
                          const TargetLibraryInfo *TLI)
      : CurLoop(CurLoop), SE(SE), TLI(TLI) {}
 
  bool isValidStrlenIdiom() {
    // Give up if the loop has multiple blocks, multiple backedges, or
    // multiple exit blocks
    if (CurLoop->getNumBackEdges() != 1 || CurLoop->getNumBlocks() != 1 ||
        !CurLoop->getUniqueExitBlock())
      return false;
 
    // It should have a preheader and a branch instruction.
    BasicBlock *Preheader = CurLoop->getLoopPreheader();
    if (!Preheader ||
        !isa<UncondBrInst, CondBrInst>(Preheader->getTerminator()))
      return false;
 
    // The loop exit must be conditioned on an icmp with 0 the null terminator.
    // The icmp operand has to be a load on some SSA reg that increments
    // by 1 in the loop.
    BasicBlock *LoopBody = *CurLoop->block_begin();
 
    // Skip if the body is too big as it most likely is not a strlen idiom.
    if (!LoopBody || LoopBody->size() >= 15)
      return false;
 
    CondBrInst *LoopTerm = dyn_cast<CondBrInst>(LoopBody->getTerminator());
    if (!LoopTerm)
      return false;
    Value *LoopCond = matchCondition(LoopTerm, LoopBody);
    if (!LoopCond)
      return false;
 
    LoadInst *LoopLoad = dyn_cast<LoadInst>(LoopCond);
    if (!LoopLoad || LoopLoad->getPointerAddressSpace() != 0)
      return false;
 
    OperandType = LoopLoad->getType();
    if (!OperandType || !OperandType->isIntegerTy())
      return false;
 
    // See if the pointer expression is an AddRec with constant step a of form
    // ({n,+,a}) where a is the width of the char type.
    Value *IncPtr = LoopLoad->getPointerOperand();
    const SCEV *LoadEv = SE->getSCEV(IncPtr);
    const APInt *Step;
    if (!match(LoadEv,
               m_scev_AffineAddRec(m_SCEV(LoadBaseEv), m_scev_APInt(Step))))
      return false;
 
    LLVM_DEBUG(dbgs() << "pointer load scev: " << *LoadEv << "\n");
 
    uint64_t StepSize = Step->getZExtValue();
 
    // Verify that StepSize is consistent with platform char width.
    OpWidth = OperandType->getIntegerBitWidth();
    unsigned WcharSize = TLI->getWCharSize(*LoopLoad->getModule());
    if (OpWidth != 8 && OpWidth != 16 && OpWidth != 32)
      return false;
    if (StepSize != OpWidth / 8)
      return false;
    if (OpWidth >= 16) {
      if (OpWidth != WcharSize * 8)
        return false;
 
      // Don't call `wcslen` if the base pointer isn't known to be aligned.
      const DataLayout &DL = LoopLoad->getDataLayout();
      Align OperandAlign = DL.getABITypeAlign(OperandType);
      if (LoopLoad->getAlign() < OperandAlign &&
          Align::fromLog2(SE->getMinTrailingZeros(LoadBaseEv)) < OperandAlign)
        return false;
    }
 
    // Scan every instruction in the loop to ensure there are no side effects.
    for (Instruction &I : *LoopBody)
      if (I.mayHaveSideEffects())
        return false;
 
    BasicBlock *LoopExitBB = CurLoop->getExitBlock();
    if (!LoopExitBB)
      return false;
 
    for (PHINode &PN : LoopExitBB->phis()) {
      if (!SE->isSCEVable(PN.getType()))
        return false;
 
      const SCEV *Ev = SE->getSCEV(&PN);
      if (!Ev)
        return false;
 
      LLVM_DEBUG(dbgs() << "loop exit phi scev: " << *Ev << "\n");
 
      // Since we verified that the loop trip count will be a valid strlen
      // idiom, we can expand all lcssa phi with {n,+,1} as (n + strlen) and use
      // SCEVExpander materialize the loop output.
      const SCEVAddRecExpr *AddRecEv = dyn_cast<SCEVAddRecExpr>(Ev);
      if (!AddRecEv || !AddRecEv->isAffine())
        return false;
 
      // We only want RecAddExpr with recurrence step that is constant. This
      // is good enough for all the idioms we want to recognize. Later we expand
      // and materialize the recurrence as {base,+,a} -> (base + a * strlen)
      if (!isa<SCEVConstant>(AddRecEv->getStepRecurrence(*SE)))
        return false;
    }
 
    return true;
  }
 
public:
  const Loop *CurLoop;
  ScalarEvolution *SE;
  const TargetLibraryInfo *TLI;
 
  unsigned OpWidth;
  ConstantInt *StepSizeCI;
  const SCEV *LoadBaseEv;
  Type *OperandType;
};
 
} // namespace
 
/// The Strlen Idiom we are trying to detect has the following structure
///
/// preheader:
///   ...
///   br label %body, ...
///
/// body:
///   ... ; %0 is incremented by a gep
///   %1 = load i8, ptr %0, align 1
///   %2 = icmp eq i8 %1, 0
///   br i1 %2, label %exit, label %body
///
/// exit:
///   %lcssa = phi [%0, %body], ...
///
/// We expect the strlen idiom to have a load of a character type that
/// is compared against '\0', and such load pointer operand must have scev
/// expression of the form {%str,+,c} where c is a ConstantInt of the
/// appropiate character width for the idiom, and %str is the base of the string
/// And, that all lcssa phis have the form {...,+,n} where n is a constant,
///
/// When transforming the output of the strlen idiom, the lccsa phi are
/// expanded using SCEVExpander as {base scev,+,a} -> (base scev + a * strlen)
/// and all subsequent uses are replaced. For example,
///
/// \code{.c}
///     const char* base = str;
///     while (*str != '\0')
///         ++str;
///     size_t result = str - base;
/// \endcode
///
/// will be transformed as follows: The idiom will be replaced by a strlen
/// computation to compute the address of the null terminator of the string.
///
/// \code{.c}
///     const char* base = str;
///     const char* end = base + strlen(str);
///     size_t result = end - base;
/// \endcode
///
/// In the case we index by an induction variable, as long as the induction
/// variable has a constant int increment, we can replace all such indvars
/// with the closed form computation of strlen
///
/// \code{.c}
///     size_t i = 0;
///     while (str[i] != '\0')
///         ++i;
///     size_t result = i;
/// \endcode
///
/// Will be replaced by
///
/// \code{.c}
///     size_t i = 0 + strlen(str);
///     size_t result = i;
/// \endcode
///
bool LoopIdiomRecognize::recognizeAndInsertStrLen() {
  if (DisableLIRP::All)
    return false;
 
  StrlenVerifier Verifier(CurLoop, SE, TLI);
 
  if (!Verifier.isValidStrlenIdiom())
    return false;
 
  BasicBlock *Preheader = CurLoop->getLoopPreheader();
  BasicBlock *LoopBody = *CurLoop->block_begin();
  BasicBlock *LoopExitBB = CurLoop->getExitBlock();
  CondBrInst *LoopTerm = cast<CondBrInst>(LoopBody->getTerminator());
  assert(Preheader && LoopBody && LoopExitBB &&
         "Should be verified to be valid by StrlenVerifier");
 
  if (Verifier.OpWidth == 8) {
    if (DisableLIRP::Strlen)
      return false;
    if (!isLibFuncEmittable(Preheader->getModule(), TLI, LibFunc_strlen))
      return false;
  } else {
    if (DisableLIRP::Wcslen)
      return false;
    if (!isLibFuncEmittable(Preheader->getModule(), TLI, LibFunc_wcslen))
      return false;
  }
 
  IRBuilder<> Builder(Preheader->getTerminator());
  Builder.SetCurrentDebugLocation(CurLoop->getStartLoc());
  SCEVExpander Expander(*SE, "strlen_idiom");
  Value *MaterialzedBase = Expander.expandCodeFor(
      Verifier.LoadBaseEv, Verifier.LoadBaseEv->getType(),
      Builder.GetInsertPoint());
 
  Value *StrLenFunc = nullptr;
  if (Verifier.OpWidth == 8) {
    StrLenFunc = emitStrLen(MaterialzedBase, Builder, *DL, TLI);
  } else {
    StrLenFunc = emitWcsLen(MaterialzedBase, Builder, *DL, TLI);
  }
  assert(StrLenFunc && "Failed to emit strlen function.");
 
  const SCEV *StrlenEv = SE->getSCEV(StrLenFunc);
  SmallVector<PHINode *, 4> Cleanup;
  for (PHINode &PN : LoopExitBB->phis()) {
    // We can now materialize the loop output as all phi have scev {base,+,a}.
    // We expand the phi as:
    //   %strlen = call i64 @strlen(%str)
    //   %phi.new = base expression + step * %strlen
    const SCEV *Ev = SE->getSCEV(&PN);
    const SCEVAddRecExpr *AddRecEv = dyn_cast<SCEVAddRecExpr>(Ev);
    const SCEVConstant *Step =
        dyn_cast<SCEVConstant>(AddRecEv->getStepRecurrence(*SE));
    const SCEV *Base = AddRecEv->getStart();
 
    // It is safe to truncate to base since if base is narrower than size_t
    // the equivalent user code will have to truncate anyways.
    const SCEV *NewEv = SE->getAddExpr(
        Base, SE->getMulExpr(Step, SE->getTruncateOrSignExtend(
                                       StrlenEv, Base->getType())));
 
    Value *MaterializedPHI = Expander.expandCodeFor(NewEv, NewEv->getType(),
                                                    Builder.GetInsertPoint());
    Expander.clear();
    PN.replaceAllUsesWith(MaterializedPHI);
    Cleanup.push_back(&PN);
  }
 
  // All LCSSA Loop Phi are dead, the left over dead loop body can be cleaned
  // up by later passes
  for (PHINode *PN : Cleanup)
    RecursivelyDeleteDeadPHINode(PN);
 
  // LoopDeletion only delete invariant loops with known trip-count. We can
  // update the condition so it will reliablely delete the invariant loop
  assert((LoopTerm->getSuccessor(0) == LoopBody ||
          LoopTerm->getSuccessor(1) == LoopBody) &&
         "loop body must have a successor that is it self");
  ConstantInt *NewLoopCond = LoopTerm->getSuccessor(0) == LoopBody
                                 ? Builder.getFalse()
                                 : Builder.getTrue();
  LoopTerm->setCondition(NewLoopCond);
  SE->forgetLoop(CurLoop);
 
  ++NumStrLen;
  LLVM_DEBUG(dbgs() << "  Formed strlen idiom: " << *StrLenFunc << "\n");
  ORE.emit([&]() {
    return OptimizationRemark(DEBUG_TYPE, "recognizeAndInsertStrLen",
                              CurLoop->getStartLoc(), Preheader)
           << "Transformed " << StrLenFunc->getName() << " loop idiom";
  });
 
  return true;
}
 
/// Check if the given conditional branch is based on an unsigned less-than
/// comparison between a variable and a constant, and if the comparison is false
/// the control yields to the loop entry. If the branch matches the behaviour,
/// the variable involved in the comparison is returned.
static Value *matchShiftULTCondition(CondBrInst *BI, BasicBlock *LoopEntry,
                                     APInt &Threshold) {
  ICmpInst *Cond = dyn_cast<ICmpInst>(BI->getCondition());
  if (!Cond)
    return nullptr;
 
  ConstantInt *CmpConst = dyn_cast<ConstantInt>(Cond->getOperand(1));
  if (!CmpConst)
    return nullptr;
 
  BasicBlock *FalseSucc = BI->getSuccessor(1);
  ICmpInst::Predicate Pred = Cond->getPredicate();
 
  if (Pred == ICmpInst::ICMP_ULT && FalseSucc == LoopEntry) {
    Threshold = CmpConst->getValue();
    return Cond->getOperand(0);
  }
 
  return nullptr;
}
 
// Check if the recurrence variable `VarX` is in the right form to create
// the idiom. Returns the value coerced to a PHINode if so.
static PHINode *getRecurrenceVar(Value *VarX, Instruction *DefX,
                                 BasicBlock *LoopEntry) {
  auto *PhiX = dyn_cast<PHINode>(VarX);
  if (PhiX && PhiX->getParent() == LoopEntry &&
      (PhiX->getOperand(0) == DefX || PhiX->getOperand(1) == DefX))
    return PhiX;
  return nullptr;
}
 
/// Return true if the idiom is detected in the loop.
///
/// Additionally:
/// 1) \p CntInst is set to the instruction Counting Leading Zeros (CTLZ)
///       or nullptr if there is no such.
/// 2) \p CntPhi is set to the corresponding phi node
///       or nullptr if there is no such.
/// 3) \p InitX is set to the value whose CTLZ could be used.
/// 4) \p DefX is set to the instruction calculating Loop exit condition.
/// 5) \p Threshold is set to the constant involved in the unsigned less-than
///       comparison.
///
/// The core idiom we are trying to detect is:
/// \code
///    if (x0 < 2)
///      goto loop-exit // the precondition of the loop
///    cnt0 = init-val
///    do {
///      x = phi (x0, x.next);   //PhiX
///      cnt = phi (cnt0, cnt.next)
///
///      cnt.next = cnt + 1;
///       ...
///      x.next = x >> 1;   // DefX
///    } while (x >= 4)
/// loop-exit:
/// \endcode
static bool detectShiftUntilLessThanIdiom(Loop *CurLoop, const DataLayout &DL,
                                          Intrinsic::ID &IntrinID,
                                          Value *&InitX, Instruction *&CntInst,
                                          PHINode *&CntPhi, Instruction *&DefX,
                                          APInt &Threshold) {
  BasicBlock *LoopEntry;
 
  DefX = nullptr;
  CntInst = nullptr;
  CntPhi = nullptr;
  LoopEntry = *(CurLoop->block_begin());
 
  // step 1: Check if the loop-back branch is in desirable form.
  auto *EntryBI = dyn_cast<CondBrInst>(LoopEntry->getTerminator());
  if (!EntryBI)
    return false;
  if (Value *T = matchShiftULTCondition(EntryBI, LoopEntry, Threshold))
    DefX = dyn_cast<Instruction>(T);
  else
    return false;
 
  // step 2: Check the recurrence of variable X
  if (!DefX || !isa<PHINode>(DefX))
    return false;
 
  PHINode *VarPhi = cast<PHINode>(DefX);
  int Idx = VarPhi->getBasicBlockIndex(LoopEntry);
  if (Idx == -1)
    return false;
 
  DefX = dyn_cast<Instruction>(VarPhi->getIncomingValue(Idx));
  if (!DefX || DefX->getNumOperands() == 0 || DefX->getOperand(0) != VarPhi)
    return false;
 
  // step 3: detect instructions corresponding to "x.next = x >> 1"
  if (DefX->getOpcode() != Instruction::LShr)
    return false;
 
  IntrinID = Intrinsic::ctlz;
  ConstantInt *Shft = dyn_cast<ConstantInt>(DefX->getOperand(1));
  if (!Shft || !Shft->isOne())
    return false;
 
  InitX = VarPhi->getIncomingValueForBlock(CurLoop->getLoopPreheader());
 
  // step 4: Find the instruction which count the CTLZ: cnt.next = cnt + 1
  //         or cnt.next = cnt + -1.
  // TODO: We can skip the step. If loop trip count is known (CTLZ),
  //       then all uses of "cnt.next" could be optimized to the trip count
  //       plus "cnt0". Currently it is not optimized.
  //       This step could be used to detect POPCNT instruction:
  //       cnt.next = cnt + (x.next & 1)
  for (Instruction &Inst :
       llvm::make_range(LoopEntry->getFirstNonPHIIt(), LoopEntry->end())) {
    if (Inst.getOpcode() != Instruction::Add)
      continue;
 
    ConstantInt *Inc = dyn_cast<ConstantInt>(Inst.getOperand(1));
    if (!Inc || (!Inc->isOne() && !Inc->isMinusOne()))
      continue;
 
    PHINode *Phi = getRecurrenceVar(Inst.getOperand(0), &Inst, LoopEntry);
    if (!Phi)
      continue;
 
    CntInst = &Inst;
    CntPhi = Phi;
    break;
  }
  if (!CntInst)
    return false;
 
  return true;
}
 
/// 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.
  {
    auto *LoopTerm = dyn_cast<CondBrInst>(LoopEntry->getTerminator());
    if (!LoopTerm)
      return false;
    DefX2 = dyn_cast_or_null<Instruction>(matchCondition(LoopTerm, LoopEntry));
  }
 
  // 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 || SubOneOp->getOperand(0) != VarX1)
      return false;
 
    ConstantInt *Dec = dyn_cast<ConstantInt>(SubOneOp->getOperand(1));
    if (!Dec ||
        !((SubOneOp->getOpcode() == Instruction::Sub && Dec->isOne()) ||
          (SubOneOp->getOpcode() == Instruction::Add &&
           Dec->isMinusOne()))) {
      return false;
    }
  }
 
  // step 3: Check the recurrence of variable X
  PhiX = getRecurrenceVar(VarX1, DefX2, LoopEntry);
  if (!PhiX)
    return false;
 
  // step 4: Find the instruction which count the population: cnt2 = cnt1 + 1
  {
    CountInst = nullptr;
    for (Instruction &Inst :
         llvm::make_range(LoopEntry->getFirstNonPHIIt(), LoopEntry->end())) {
      if (Inst.getOpcode() != Instruction::Add)
        continue;
 
      ConstantInt *Inc = dyn_cast<ConstantInt>(Inst.getOperand(1));
      if (!Inc || !Inc->isOne())
        continue;
 
      PHINode *Phi = getRecurrenceVar(Inst.getOperand(0), &Inst, LoopEntry);
      if (!Phi)
        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<CondBrInst>(PreCondBB->getTerminator());
    if (!PreCondBr)
      return false;
    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;
}
 
/// Return true if the idiom is detected in the loop.
///
/// Additionally:
/// 1) \p CntInst is set to the instruction Counting Leading Zeros (CTLZ)
///       or nullptr if there is no such.
/// 2) \p CntPhi is set to the corresponding phi node
///       or nullptr if there is no such.
/// 3) \p Var is set to the value whose CTLZ could be used.
/// 4) \p DefX is set to the instruction calculating Loop exit condition.
///
/// 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 {
///       x = phi (x0, x.next);   //PhiX
///       cnt = phi(cnt0, cnt.next);
///
///       cnt.next = cnt + 1;
///        ...
///       x.next = x >> 1;   // DefX
///        ...
///    } while(x.next != 0);
///
/// loop-exit:
/// \endcode
static bool detectShiftUntilZeroIdiom(Loop *CurLoop, const DataLayout &DL,
                                      Intrinsic::ID &IntrinID, Value *&InitX,
                                      Instruction *&CntInst, PHINode *&CntPhi,
                                      Instruction *&DefX) {
  BasicBlock *LoopEntry;
  Value *VarX = nullptr;
 
  DefX = nullptr;
  CntInst = nullptr;
  CntPhi = nullptr;
  LoopEntry = *(CurLoop->block_begin());
 
  // step 1: Check if the loop-back branch is in desirable form.
  auto *LoopTerm = dyn_cast<CondBrInst>(LoopEntry->getTerminator());
  if (!LoopTerm)
    return false;
  DefX = dyn_cast_or_null<Instruction>(matchCondition(LoopTerm, LoopEntry));
 
  // step 2: detect instructions corresponding to "x.next = x >> 1 or x << 1"
  if (!DefX || !DefX->isShift())
    return false;
  IntrinID = DefX->getOpcode() == Instruction::Shl ? Intrinsic::cttz :
                                                     Intrinsic::ctlz;
  ConstantInt *Shft = dyn_cast<ConstantInt>(DefX->getOperand(1));
  if (!Shft || !Shft->isOne())
    return false;
  VarX = DefX->getOperand(0);
 
  // step 3: Check the recurrence of variable X
  PHINode *PhiX = getRecurrenceVar(VarX, DefX, LoopEntry);
  if (!PhiX)
    return false;
 
  InitX = PhiX->getIncomingValueForBlock(CurLoop->getLoopPreheader());
 
  // Make sure the initial value can't be negative otherwise the ashr in the
  // loop might never reach zero which would make the loop infinite.
  if (DefX->getOpcode() == Instruction::AShr && !isKnownNonNegative(InitX, DL))
    return false;
 
  // step 4: Find the instruction which count the CTLZ: cnt.next = cnt + 1
  //         or cnt.next = cnt + -1.
  // TODO: We can skip the step. If loop trip count is known (CTLZ),
  //       then all uses of "cnt.next" could be optimized to the trip count
  //       plus "cnt0". Currently it is not optimized.
  //       This step could be used to detect POPCNT instruction:
  //       cnt.next = cnt + (x.next & 1)
  for (Instruction &Inst :
       llvm::make_range(LoopEntry->getFirstNonPHIIt(), LoopEntry->end())) {
    if (Inst.getOpcode() != Instruction::Add)
      continue;
 
    ConstantInt *Inc = dyn_cast<ConstantInt>(Inst.getOperand(1));
    if (!Inc || (!Inc->isOne() && !Inc->isMinusOne()))
      continue;
 
    PHINode *Phi = getRecurrenceVar(Inst.getOperand(0), &Inst, LoopEntry);
    if (!Phi)
      continue;
 
    CntInst = &Inst;
    CntPhi = Phi;
    break;
  }
  if (!CntInst)
    return false;
 
  return true;
}
 
// Check if CTLZ / CTTZ intrinsic is profitable. Assume it is always
// profitable if we delete the loop.
bool LoopIdiomRecognize::isProfitableToInsertFFS(Intrinsic::ID IntrinID,
                                                 Value *InitX, bool ZeroCheck,
                                                 size_t CanonicalSize) {
  const Value *Args[] = {InitX,
                         ConstantInt::getBool(InitX->getContext(), ZeroCheck)};
 
  uint32_t HeaderSize = CurLoop->getHeader()->size();
 
  IntrinsicCostAttributes Attrs(IntrinID, InitX->getType(), Args);
  InstructionCost Cost = TTI->getIntrinsicInstrCost(
      Attrs, TargetTransformInfo::TCK_SizeAndLatency);
  if (HeaderSize != CanonicalSize && Cost > TargetTransformInfo::TCC_Basic)
    return false;
 
  return true;
}
 
/// Convert CTLZ / CTTZ idiom loop into countable loop.
/// If CTLZ / CTTZ inserted as a new trip count returns true; otherwise,
/// returns false.
bool LoopIdiomRecognize::insertFFSIfProfitable(Intrinsic::ID IntrinID,
                                               Value *InitX, Instruction *DefX,
                                               PHINode *CntPhi,
                                               Instruction *CntInst) {
  bool IsCntPhiUsedOutsideLoop = false;
  for (User *U : CntPhi->users())
    if (!CurLoop->contains(cast<Instruction>(U))) {
      IsCntPhiUsedOutsideLoop = true;
      break;
    }
  bool IsCntInstUsedOutsideLoop = false;
  for (User *U : CntInst->users())
    if (!CurLoop->contains(cast<Instruction>(U))) {
      IsCntInstUsedOutsideLoop = true;
      break;
    }
  // If both CntInst and CntPhi are used outside the loop the profitability
  // is questionable.
  if (IsCntInstUsedOutsideLoop && IsCntPhiUsedOutsideLoop)
    return false;
 
  // For some CPUs result of CTLZ(X) intrinsic is undefined
  // when X is 0. If we can not guarantee X != 0, we need to check this
  // when expand.
  bool ZeroCheck = false;
  // It is safe to assume Preheader exist as it was checked in
  // parent function RunOnLoop.
  BasicBlock *PH = CurLoop->getLoopPreheader();
 
  // If we are using the count instruction outside the loop, make sure we
  // have a zero check as a precondition. Without the check the loop would run
  // one iteration for before any check of the input value. This means 0 and 1
  // would have identical behavior in the original loop and thus
  if (!IsCntPhiUsedOutsideLoop) {
    auto *PreCondBB = PH->getSinglePredecessor();
    auto *PreCondBI =
        PreCondBB ? dyn_cast<CondBrInst>(PreCondBB->getTerminator()) : nullptr;
    if (!(PreCondBI && matchCondition(PreCondBI, PH) == InitX) &&
        !isKnownNonZero(
            InitX, SimplifyQuery(*DL, DT, /*AC=*/nullptr, PH->getTerminator())))
      return false;
    ZeroCheck = true;
  }
 
  // FFS idiom loop has only 6 instructions:
  //  %n.addr.0 = phi [ %n, %entry ], [ %shr, %while.cond ]
  //  %i.0 = phi [ %i0, %entry ], [ %inc, %while.cond ]
  //  %shr = ashr %n.addr.0, 1
  //  %tobool = icmp eq %shr, 0
  //  %inc = add nsw %i.0, 1
  //  br i1 %tobool
  size_t IdiomCanonicalSize = 6;
  if (!isProfitableToInsertFFS(IntrinID, InitX, ZeroCheck, IdiomCanonicalSize))
    return false;
 
  transformLoopToCountable(IntrinID, PH, CntInst, CntPhi, InitX, DefX,
                           DefX->getDebugLoc(), ZeroCheck,
                           IsCntPhiUsedOutsideLoop);
  return true;
}
 
/// Recognize CTLZ or CTTZ idiom in a non-countable loop and convert the loop
/// to countable (with CTLZ / CTTZ trip count). If CTLZ / CTTZ inserted as a new
/// trip count returns true; otherwise, returns false.
bool LoopIdiomRecognize::recognizeAndInsertFFS() {
  // Give up if the loop has multiple blocks or multiple backedges.
  if (CurLoop->getNumBackEdges() != 1 || CurLoop->getNumBlocks() != 1)
    return false;
 
  Intrinsic::ID IntrinID;
  Value *InitX;
  Instruction *DefX = nullptr;
  PHINode *CntPhi = nullptr;
  Instruction *CntInst = nullptr;
 
  if (!detectShiftUntilZeroIdiom(CurLoop, *DL, IntrinID, InitX, CntInst, CntPhi,
                                 DefX))
    return false;
 
  return insertFFSIfProfitable(IntrinID, InitX, DefX, CntPhi, CntInst);
}
 
bool LoopIdiomRecognize::recognizeShiftUntilLessThan() {
  // Give up if the loop has multiple blocks or multiple backedges.
  if (CurLoop->getNumBackEdges() != 1 || CurLoop->getNumBlocks() != 1)
    return false;
 
  Intrinsic::ID IntrinID;
  Value *InitX;
  Instruction *DefX = nullptr;
  PHINode *CntPhi = nullptr;
  Instruction *CntInst = nullptr;
 
  APInt LoopThreshold;
  if (!detectShiftUntilLessThanIdiom(CurLoop, *DL, IntrinID, InitX, CntInst,
                                     CntPhi, DefX, LoopThreshold))
    return false;
 
  if (LoopThreshold == 2) {
    // Treat as regular FFS.
    return insertFFSIfProfitable(IntrinID, InitX, DefX, CntPhi, CntInst);
  }
 
  // Look for Floor Log2 Idiom.
  if (LoopThreshold != 4)
    return false;
 
  // Abort if CntPhi is used outside of the loop.
  for (User *U : CntPhi->users())
    if (!CurLoop->contains(cast<Instruction>(U)))
      return false;
 
  // It is safe to assume Preheader exist as it was checked in
  // parent function RunOnLoop.
  BasicBlock *PH = CurLoop->getLoopPreheader();
  auto *PreCondBB = PH->getSinglePredecessor();
  if (!PreCondBB)
    return false;
  auto *PreCondBI = dyn_cast<CondBrInst>(PreCondBB->getTerminator());
  if (!PreCondBI)
    return false;
 
  APInt PreLoopThreshold;
  if (matchShiftULTCondition(PreCondBI, PH, PreLoopThreshold) != InitX ||
      PreLoopThreshold != 2)
    return false;
 
  bool ZeroCheck = true;
 
  // the loop has only 6 instructions:
  //  %n.addr.0 = phi [ %n, %entry ], [ %shr, %while.cond ]
  //  %i.0 = phi [ %i0, %entry ], [ %inc, %while.cond ]
  //  %shr = ashr %n.addr.0, 1
  //  %tobool = icmp ult %n.addr.0, C
  //  %inc = add nsw %i.0, 1
  //  br i1 %tobool
  size_t IdiomCanonicalSize = 6;
  if (!isProfitableToInsertFFS(IntrinID, InitX, ZeroCheck, IdiomCanonicalSize))
    return false;
 
  // log2(x) = w − 1 − clz(x)
  transformLoopToCountable(IntrinID, PH, CntInst, CntPhi, InitX, DefX,
                           DefX->getDebugLoc(), ZeroCheck,
                           /*IsCntPhiUsedOutsideLoop=*/false,
                           /*InsertSub=*/true);
  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 || &PH->front() != PH->getTerminator())
    return false;
  auto *EntryBI = dyn_cast<UncondBrInst>(PH->getTerminator());
  if (!EntryBI)
    return false;
 
  // It should have a precondition block where the generated popcount intrinsic
  // function can be inserted.
  auto *PreCondBB = PH->getSinglePredecessor();
  if (!PreCondBB)
    return false;
  auto *PreCondBI = dyn_cast<CondBrInst>(PreCondBB->getTerminator());
  if (!PreCondBI)
    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 Value *createPopcntIntrinsic(IRBuilder<> &IRBuilder, Value *Val,
                                    const DebugLoc &DL) {
  Value *Ops[] = {Val};
  Type *Tys[] = {Val->getType()};
 
  IRBuilder.SetCurrentDebugLocation(DL);
  return IRBuilder.CreateIntrinsic(Intrinsic::ctpop, Tys, Ops);
}
 
static Value *createFFSIntrinsic(IRBuilder<> &IRBuilder, Value *Val,
                                 const DebugLoc &DL, bool ZeroCheck,
                                 Intrinsic::ID IID) {
  Value *Ops[] = {Val, IRBuilder.getInt1(ZeroCheck)};
  Type *Tys[] = {Val->getType()};
 
  IRBuilder.SetCurrentDebugLocation(DL);
  return IRBuilder.CreateIntrinsic(IID, Tys, Ops);
}
 
/// Transform the following loop (Using CTLZ, CTTZ is similar):
/// loop:
///   CntPhi = PHI [Cnt0, CntInst]
///   PhiX = PHI [InitX, DefX]
///   CntInst = CntPhi + 1
///   DefX = PhiX >> 1
///   LOOP_BODY
///   Br: loop if (DefX != 0)
/// Use(CntPhi) or Use(CntInst)
///
/// Into:
/// If CntPhi used outside the loop:
///   CountPrev = BitWidth(InitX) - CTLZ(InitX >> 1)
///   Count = CountPrev + 1
/// else
///   Count = BitWidth(InitX) - CTLZ(InitX)
/// loop:
///   CntPhi = PHI [Cnt0, CntInst]
///   PhiX = PHI [InitX, DefX]
///   PhiCount = PHI [Count, Dec]
///   CntInst = CntPhi + 1
///   DefX = PhiX >> 1
///   Dec = PhiCount - 1
///   LOOP_BODY
///   Br: loop if (Dec != 0)
/// Use(CountPrev + Cnt0) // Use(CntPhi)
/// or
/// Use(Count + Cnt0) // Use(CntInst)
///
/// If LOOP_BODY is empty the loop will be deleted.
/// If CntInst and DefX are not used in LOOP_BODY they will be removed.
void LoopIdiomRecognize::transformLoopToCountable(
    Intrinsic::ID IntrinID, BasicBlock *Preheader, Instruction *CntInst,
    PHINode *CntPhi, Value *InitX, Instruction *DefX, const DebugLoc &DL,
    bool ZeroCheck, bool IsCntPhiUsedOutsideLoop, bool InsertSub) {
  // Step 1: Insert the CTLZ/CTTZ instruction at the end of the preheader block
  IRBuilder<> Builder(Preheader->getTerminator());
  Builder.SetCurrentDebugLocation(DL);
 
  // If there are no uses of CntPhi crate:
  //   Count = BitWidth - CTLZ(InitX);
  //   NewCount = Count;
  // If there are uses of CntPhi create:
  //   NewCount = BitWidth - CTLZ(InitX >> 1);
  //   Count = NewCount + 1;
  Value *InitXNext;
  if (IsCntPhiUsedOutsideLoop) {
    if (DefX->getOpcode() == Instruction::AShr)
      InitXNext = Builder.CreateAShr(InitX, 1);
    else if (DefX->getOpcode() == Instruction::LShr)
      InitXNext = Builder.CreateLShr(InitX, 1);
    else if (DefX->getOpcode() == Instruction::Shl) // cttz
      InitXNext = Builder.CreateShl(InitX, 1);
    else
      llvm_unreachable("Unexpected opcode!");
  } else
    InitXNext = InitX;
  Value *Count =
      createFFSIntrinsic(Builder, InitXNext, DL, ZeroCheck, IntrinID);
  Type *CountTy = Count->getType();
  Count = Builder.CreateSub(
      ConstantInt::get(CountTy, CountTy->getIntegerBitWidth()), Count);
  if (InsertSub)
    Count = Builder.CreateSub(Count, ConstantInt::get(CountTy, 1));
  Value *NewCount = Count;
  if (IsCntPhiUsedOutsideLoop)
    Count = Builder.CreateAdd(Count, ConstantInt::get(CountTy, 1));
 
  NewCount = Builder.CreateZExtOrTrunc(NewCount, CntInst->getType());
 
  Value *CntInitVal = CntPhi->getIncomingValueForBlock(Preheader);
  if (cast<ConstantInt>(CntInst->getOperand(1))->isOne()) {
    // If the counter was being incremented in the loop, add NewCount to the
    // counter's initial value, but only if the initial value is not zero.
    ConstantInt *InitConst = dyn_cast<ConstantInt>(CntInitVal);
    if (!InitConst || !InitConst->isZero())
      NewCount = Builder.CreateAdd(NewCount, CntInitVal);
  } else {
    // If the count was being decremented in the loop, subtract NewCount from
    // the counter's initial value.
    NewCount = Builder.CreateSub(CntInitVal, NewCount);
  }
 
  // Step 2: Insert new IV and loop condition:
  // loop:
  //   ...
  //   PhiCount = PHI [Count, Dec]
  //   ...
  //   Dec = PhiCount - 1
  //   ...
  //   Br: loop if (Dec != 0)
  BasicBlock *Body = *(CurLoop->block_begin());
  auto *LbBr = cast<CondBrInst>(Body->getTerminator());
  ICmpInst *LbCond = cast<ICmpInst>(LbBr->getCondition());
 
  PHINode *TcPhi = PHINode::Create(CountTy, 2, "tcphi");
  TcPhi->insertBefore(Body->begin());
 
  Builder.SetInsertPoint(LbCond);
  Instruction *TcDec = cast<Instruction>(Builder.CreateSub(
      TcPhi, ConstantInt::get(CountTy, 1), "tcdec", false, true));
 
  TcPhi->addIncoming(Count, Preheader);
  TcPhi->addIncoming(TcDec, Body);
 
  CmpInst::Predicate Pred =
      (LbBr->getSuccessor(0) == Body) ? CmpInst::ICMP_NE : CmpInst::ICMP_EQ;
  LbCond->setPredicate(Pred);
  LbCond->setOperand(0, TcDec);
  LbCond->setOperand(1, ConstantInt::get(CountTy, 0));
 
  // Step 3: All the references to the original counter outside
  //  the loop are replaced with the NewCount
  if (IsCntPhiUsedOutsideLoop)
    CntPhi->replaceUsesOutsideBlock(NewCount, Body);
  else
    CntInst->replaceUsesOutsideBlock(NewCount, Body);
 
  // step 4: 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);
}
 
void LoopIdiomRecognize::transformLoopToPopcount(BasicBlock *PreCondBB,
                                                 Instruction *CntInst,
                                                 PHINode *CntPhi, Value *Var) {
  BasicBlock *PreHead = CurLoop->getLoopPreheader();
  auto *PreCondBr = cast<CondBrInst>(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 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 = cast<CondBrInst>(Body->getTerminator());
    ICmpInst *LbCond = cast<ICmpInst>(LbBr->getCondition());
    Type *Ty = TripCnt->getType();
 
    PHINode *TcPhi = PHINode::Create(Ty, 2, "tcphi");
    TcPhi->insertBefore(Body->begin());
 
    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);
}
 
/// Match loop-invariant value.
template <typename SubPattern_t> struct match_LoopInvariant {
  SubPattern_t SubPattern;
  const Loop *L;
 
  match_LoopInvariant(const SubPattern_t &SP, const Loop *L)
      : SubPattern(SP), L(L) {}
 
  template <typename ITy> bool match(ITy *V) const {
    return L->isLoopInvariant(V) && SubPattern.match(V);
  }
};
 
/// Matches if the value is loop-invariant.
template <typename Ty>
inline match_LoopInvariant<Ty> m_LoopInvariant(const Ty &M, const Loop *L) {
  return match_LoopInvariant<Ty>(M, L);
}
 
/// Return true if the idiom is detected in the loop.
///
/// The core idiom we are trying to detect is:
/// \code
///   entry:
///     <...>
///     %bitmask = shl i32 1, %bitpos
///     br label %loop
///
///   loop:
///     %x.curr = phi i32 [ %x, %entry ], [ %x.next, %loop ]
///     %x.curr.bitmasked = and i32 %x.curr, %bitmask
///     %x.curr.isbitunset = icmp eq i32 %x.curr.bitmasked, 0
///     %x.next = shl i32 %x.curr, 1
///     <...>
///     br i1 %x.curr.isbitunset, label %loop, label %end
///
///   end:
///     %x.curr.res = phi i32 [ %x.curr, %loop ] <...>
///     %x.next.res = phi i32 [ %x.next, %loop ] <...>
///     <...>
/// \endcode
static bool detectShiftUntilBitTestIdiom(Loop *CurLoop, Value *&BaseX,
                                         Value *&BitMask, Value *&BitPos,
                                         Value *&CurrX, Instruction *&NextX) {
  LLVM_DEBUG(dbgs() << DEBUG_TYPE
             " Performing shift-until-bittest idiom detection.\n");
 
  // Give up if the loop has multiple blocks or multiple backedges.
  if (CurLoop->getNumBlocks() != 1 || CurLoop->getNumBackEdges() != 1) {
    LLVM_DEBUG(dbgs() << DEBUG_TYPE " Bad block/backedge count.\n");
    return false;
  }
 
  BasicBlock *LoopHeaderBB = CurLoop->getHeader();
  BasicBlock *LoopPreheaderBB = CurLoop->getLoopPreheader();
  assert(LoopPreheaderBB && "There is always a loop preheader.");
 
  using namespace PatternMatch;
 
  // Step 1: Check if the loop backedge is in desirable form.
 
  CmpPredicate Pred;
  Value *CmpLHS, *CmpRHS;
  BasicBlock *TrueBB, *FalseBB;
  if (!match(LoopHeaderBB->getTerminator(),
             m_Br(m_ICmp(Pred, m_Value(CmpLHS), m_Value(CmpRHS)),
                  m_BasicBlock(TrueBB), m_BasicBlock(FalseBB)))) {
    LLVM_DEBUG(dbgs() << DEBUG_TYPE " Bad backedge structure.\n");
    return false;
  }
 
  // Step 2: Check if the backedge's condition is in desirable form.
 
  auto MatchVariableBitMask = [&]() {
    return ICmpInst::isEquality(Pred) && match(CmpRHS, m_Zero()) &&
           match(CmpLHS,
                 m_c_And(m_Value(CurrX),
                         m_CombineAnd(
                             m_Value(BitMask),
                             m_LoopInvariant(m_Shl(m_One(), m_Value(BitPos)),
                                             CurLoop))));
  };
 
  auto MatchDecomposableConstantBitMask = [&]() {
    auto Res = llvm::decomposeBitTestICmp(
        CmpLHS, CmpRHS, Pred, /*LookThroughTrunc=*/true,
        /*AllowNonZeroC=*/false, /*DecomposeAnd=*/true);
    if (Res && Res->Mask.isPowerOf2()) {
      assert(ICmpInst::isEquality(Res->Pred));
      Pred = Res->Pred;
      CurrX = Res->X;
      BitMask = ConstantInt::get(CurrX->getType(), Res->Mask);
      BitPos = ConstantInt::get(CurrX->getType(), Res->Mask.logBase2());
      return true;
    }
    return false;
  };
 
  if (!MatchVariableBitMask() && !MatchDecomposableConstantBitMask()) {
    LLVM_DEBUG(dbgs() << DEBUG_TYPE " Bad backedge comparison.\n");
    return false;
  }
 
  // Step 3: Check if the recurrence is in desirable form.
  auto *CurrXPN = dyn_cast<PHINode>(CurrX);
  if (!CurrXPN || CurrXPN->getParent() != LoopHeaderBB) {
    LLVM_DEBUG(dbgs() << DEBUG_TYPE " Not an expected PHI node.\n");
    return false;
  }
 
  BaseX = CurrXPN->getIncomingValueForBlock(LoopPreheaderBB);
  NextX =
      dyn_cast<Instruction>(CurrXPN->getIncomingValueForBlock(LoopHeaderBB));
 
  assert(CurLoop->isLoopInvariant(BaseX) &&
         "Expected BaseX to be available in the preheader!");
 
  if (!NextX || (!match(NextX, m_Shl(m_Specific(CurrX), m_One())) &&
                 !match(NextX, m_LShr(m_Specific(CurrX), m_One())))) {
    LLVM_DEBUG(dbgs() << DEBUG_TYPE " Bad recurrence.\n");
    return false;
  }
 
  // Step 4: Check if the backedge's destinations are in desirable form.
 
  assert(ICmpInst::isEquality(Pred) &&
         "Should only get equality predicates here.");
 
  // cmp-br is commutative, so canonicalize to a single variant.
  if (Pred != ICmpInst::Predicate::ICMP_EQ) {
    Pred = ICmpInst::getInversePredicate(Pred);
    std::swap(TrueBB, FalseBB);
  }
 
  // We expect to exit loop when comparison yields false,
  // so when it yields true we should branch back to loop header.
  if (TrueBB != LoopHeaderBB) {
    LLVM_DEBUG(dbgs() << DEBUG_TYPE " Bad backedge flow.\n");
    return false;
  }
 
  // Okay, idiom checks out.
  return true;
}
 
static bool isRightShiftBitTestKnownToTerminate(Value *X, Value *BitPos,
                                                BasicBlock *Preheader,
                                                const DataLayout &DL,
                                                DominatorTree *DT) {
  using namespace PatternMatch;
 
  auto *Pos = dyn_cast<ConstantInt>(BitPos);
  unsigned Bitwidth = X->getType()->getScalarSizeInBits();
  if (!Pos || Bitwidth == 1 || Pos->getValue().uge(Bitwidth))
    return false;
 
  SimplifyQuery SQ(DL, DT, /*AC=*/nullptr, Preheader->getTerminator());
  if (!isKnownNonZero(X, SQ))
    return false;
  if (Pos->isZero())
    return true;
 
  // The guard must rule out set bits below BitPos; those bits can never
  // reach the tested position when X shifts right.
  BasicBlock *GuardBB = Preheader->getSinglePredecessor();
  if (!GuardBB)
    return false;
  auto *Guard = dyn_cast<CondBrInst>(GuardBB->getTerminator());
  if (!Guard)
    return false;
 
  CmpPredicate Pred;
  Value *LHS;
  if (!match(Guard->getCondition(), m_ICmp(Pred, m_Value(LHS), m_Zero())) ||
      !ICmpInst::isEquality(Pred))
    return false;
 
  const APInt *Mask;
  if (!match(LHS, m_And(m_Specific(X), m_APInt(Mask))))
    return false;
 
  BasicBlock *ZeroSuccessor =
      Guard->getSuccessor(Pred == ICmpInst::ICMP_EQ ? 0 : 1);
  if (ZeroSuccessor != Preheader)
    return false;
 
  return Mask->countr_one() >= Pos->getZExtValue();
}
 
/// Look for the following loop:
/// \code
///   entry:
///     <...>
///     %bitmask = shl i32 1, %bitpos
///     br label %loop
///
///   loop:
///     %x.curr = phi i32 [ %x, %entry ], [ %x.next, %loop ]
///     %x.curr.bitmasked = and i32 %x.curr, %bitmask
///     %x.curr.isbitunset = icmp eq i32 %x.curr.bitmasked, 0
///     %x.next = shl i32 %x.curr, 1
///     <...>
///     br i1 %x.curr.isbitunset, label %loop, label %end
///
///   end:
///     %x.curr.res = phi i32 [ %x.curr, %loop ] <...>
///     %x.next.res = phi i32 [ %x.next, %loop ] <...>
///     <...>
/// \endcode
///
/// And transform it into:
/// \code
///   entry:
///     %bitmask = shl i32 1, %bitpos
///     %lowbitmask = add i32 %bitmask, -1
///     %mask = or i32 %lowbitmask, %bitmask
///     %x.masked = and i32 %x, %mask
///     %x.masked.numleadingzeros = call i32 @llvm.ctlz.i32(i32 %x.masked,
///                                                         i1 true)
///     %x.masked.numactivebits = sub i32 32, %x.masked.numleadingzeros
///     %x.masked.leadingonepos = add i32 %x.masked.numactivebits, -1
///     %backedgetakencount = sub i32 %bitpos, %x.masked.leadingonepos
///     %tripcount = add i32 %backedgetakencount, 1
///     %x.curr = shl i32 %x, %backedgetakencount
///     %x.next = shl i32 %x, %tripcount
///     br label %loop
///
///   loop:
///     %loop.iv = phi i32 [ 0, %entry ], [ %loop.iv.next, %loop ]
///     %loop.iv.next = add nuw i32 %loop.iv, 1
///     %loop.ivcheck = icmp eq i32 %loop.iv.next, %tripcount
///     <...>
///     br i1 %loop.ivcheck, label %end, label %loop
///
///   end:
///     %x.curr.res = phi i32 [ %x.curr, %loop ] <...>
///     %x.next.res = phi i32 [ %x.next, %loop ] <...>
///     <...>
/// \endcode
/// A logical right-shift recurrence uses cttz of X shifted by BitPos instead,
/// provided the preheader proves that the loop will reach a set bit.
bool LoopIdiomRecognize::recognizeShiftUntilBitTest() {
  bool MadeChange = false;
 
  Value *X, *BitMask, *BitPos, *XCurr;
  Instruction *XNext;
  if (!detectShiftUntilBitTestIdiom(CurLoop, X, BitMask, BitPos, XCurr,
                                    XNext)) {
    LLVM_DEBUG(dbgs() << DEBUG_TYPE
               " shift-until-bittest idiom detection failed.\n");
    return MadeChange;
  }
  LLVM_DEBUG(dbgs() << DEBUG_TYPE " shift-until-bittest idiom detected!\n");
 
  // Ok, it is the idiom we were looking for, we *could* transform this loop,
  // but is it profitable to transform?
 
  BasicBlock *LoopHeaderBB = CurLoop->getHeader();
  BasicBlock *LoopPreheaderBB = CurLoop->getLoopPreheader();
  assert(LoopPreheaderBB && "There is always a loop preheader.");
 
  BasicBlock *SuccessorBB = CurLoop->getExitBlock();
  assert(SuccessorBB && "There is only a single successor.");
 
  IRBuilder<> Builder(LoopPreheaderBB->getTerminator());
  Builder.SetCurrentDebugLocation(cast<Instruction>(XCurr)->getDebugLoc());
 
  bool IsRightShift = XNext->getOpcode() == Instruction::LShr;
  Intrinsic::ID IntrID = IsRightShift ? Intrinsic::cttz : Intrinsic::ctlz;
  Type *Ty = X->getType();
  unsigned Bitwidth = Ty->getScalarSizeInBits();
 
  // A right shift must be guaranteed to reach a set bit at BitPos.
  if (IsRightShift && !isRightShiftBitTestKnownToTerminate(
                          X, BitPos, LoopPreheaderBB, *DL, DT)) {
    return false;
  }
 
  TargetTransformInfo::TargetCostKind CostKind =
      TargetTransformInfo::TCK_SizeAndLatency;
 
  // The rewrite is considered to be unprofitable iff and only iff the
  // intrinsic/shift we'll use are not cheap. Note that we are okay with *just*
  // making the loop countable, even if nothing else changes.
  IntrinsicCostAttributes Attrs(
      IntrID, Ty, {PoisonValue::get(Ty), /*is_zero_poison=*/Builder.getTrue()});
  InstructionCost Cost = TTI->getIntrinsicInstrCost(Attrs, CostKind);
  if (Cost > TargetTransformInfo::TCC_Basic) {
    LLVM_DEBUG(dbgs() << DEBUG_TYPE
               " Intrinsic is too costly, not beneficial\n");
    return MadeChange;
  }
  if (TTI->getArithmeticInstrCost(XNext->getOpcode(), Ty, CostKind) >
      TargetTransformInfo::TCC_Basic) {
    LLVM_DEBUG(dbgs() << DEBUG_TYPE " Shift is too costly, not beneficial\n");
    return MadeChange;
  }
 
  // Ok, transform appears worthwhile.
  MadeChange = true;
 
  if (!isGuaranteedNotToBeUndefOrPoison(BitPos)) {
    // BitMask may be computed from BitPos, Freeze BitPos so we can increase
    // it's use count.
    std::optional<BasicBlock::iterator> InsertPt = std::nullopt;
    if (auto *BitPosI = dyn_cast<Instruction>(BitPos))
      InsertPt = BitPosI->getInsertionPointAfterDef();
    else
      InsertPt = DT->getRoot()->getFirstNonPHIOrDbgOrAlloca();
    if (!InsertPt)
      return false;
    FreezeInst *BitPosFrozen =
        new FreezeInst(BitPos, BitPos->getName() + ".fr", *InsertPt);
    BitPos->replaceUsesWithIf(BitPosFrozen, [BitPosFrozen](Use &U) {
      return U.getUser() != BitPosFrozen;
    });
    BitPos = BitPosFrozen;
  }
 
  // Step 1: Compute the loop trip count.
 
  Value *LoopBackedgeTakenCount;
  if (IsRightShift) {
    Value *ShiftedX = Builder.CreateLShr(X, BitPos);
    LoopBackedgeTakenCount = Builder.CreateIntrinsic(
        IntrID, Ty, {ShiftedX, /*is_zero_poison=*/Builder.getTrue()});
  } else {
    Value *LowBitMask =
        Builder.CreateAdd(BitMask, Constant::getAllOnesValue(Ty),
                          BitPos->getName() + ".lowbitmask");
    Value *Mask =
        Builder.CreateOr(LowBitMask, BitMask, BitPos->getName() + ".mask");
    Value *XMasked = Builder.CreateAnd(X, Mask, X->getName() + ".masked");
    Value *XMaskedNumLeadingZeros = Builder.CreateIntrinsic(
        IntrID, Ty, {XMasked, /*is_zero_poison=*/Builder.getTrue()},
        /*FMFSource=*/nullptr, XMasked->getName() + ".numleadingzeros");
    Value *XMaskedNumActiveBits = Builder.CreateSub(
        ConstantInt::get(Ty, Ty->getScalarSizeInBits()), XMaskedNumLeadingZeros,
        XMasked->getName() + ".numactivebits",
        /*HasNUW=*/true, /*HasNSW=*/Bitwidth != 2);
    Value *XMaskedLeadingOnePos = Builder.CreateAdd(
        XMaskedNumActiveBits, Constant::getAllOnesValue(Ty),
        XMasked->getName() + ".leadingonepos", /*HasNUW=*/false,
        /*HasNSW=*/Bitwidth > 2);
 
    LoopBackedgeTakenCount = Builder.CreateSub(
        BitPos, XMaskedLeadingOnePos,
        CurLoop->getName() + ".backedgetakencount", /*HasNUW=*/true,
        /*HasNSW=*/true);
  }
  // We know loop's backedge-taken count, but what's loop's trip count?
  // Note that while NUW is always safe, while NSW is only for bitwidths != 2.
  Value *LoopTripCount =
      Builder.CreateAdd(LoopBackedgeTakenCount, ConstantInt::get(Ty, 1),
                        CurLoop->getName() + ".tripcount", /*HasNUW=*/true,
                        /*HasNSW=*/Bitwidth != 2);
 
  // Step 2: Compute the recurrence's final value without a loop.
 
  // NewX is always safe to compute, because `LoopBackedgeTakenCount`
  // will always be smaller than `bitwidth(X)`, i.e. we never get poison.
  Value *NewX = IsRightShift ? Builder.CreateLShr(X, LoopBackedgeTakenCount)
                             : Builder.CreateShl(X, LoopBackedgeTakenCount);
  NewX->takeName(XCurr);
  if (auto *I = dyn_cast<Instruction>(NewX))
    I->copyIRFlags(XNext, /*IncludeWrapFlags=*/true);
 
  Value *NewXNext;
  // Shifting X by LoopTripCount is poison if the count equals Bitwidth.
  // For right shifts, shift NewX by one instead. For left shifts, this can
  // happen when BitPos is the highest bit and X is one, so use a single
  // additional shift unless the larger shift is known to be safe.
  if (IsRightShift) {
    NewXNext = Builder.CreateLShr(NewX, ConstantInt::get(Ty, 1));
  } else if (XNext->hasNoSignedWrap() || XNext->hasNoUnsignedWrap() ||
             PatternMatch::match(BitPos,
                                 PatternMatch::m_SpecificInt_ICMP(
                                     ICmpInst::ICMP_NE,
                                     APInt(Ty->getScalarSizeInBits(),
                                           Ty->getScalarSizeInBits() - 1)))) {
    NewXNext = Builder.CreateShl(X, LoopTripCount);
  } else {
    // Otherwise, just additionally shift by one. It's the smallest solution,
    // alternatively, we could check that NewX is INT_MIN (or BitPos is )
    // and select 0 instead.
    NewXNext = Builder.CreateShl(NewX, ConstantInt::get(Ty, 1));
  }
 
  NewXNext->takeName(XNext);
  if (auto *I = dyn_cast<Instruction>(NewXNext))
    I->copyIRFlags(XNext, /*IncludeWrapFlags=*/true);
 
  // Step 3: Adjust the successor basic block to receive the computed
  //         recurrence's final value instead of the recurrence itself.
 
  XCurr->replaceUsesOutsideBlock(NewX, LoopHeaderBB);
  XNext->replaceUsesOutsideBlock(NewXNext, LoopHeaderBB);
 
  // Step 4: Rewrite the loop into a countable form, with canonical IV.
 
  // The new canonical induction variable.
  Builder.SetInsertPoint(LoopHeaderBB->begin());
  auto *IV = Builder.CreatePHI(Ty, 2, CurLoop->getName() + ".iv");
 
  // The induction itself.
  // Note that while NUW is always safe, while NSW is only for bitwidths != 2.
  Builder.SetInsertPoint(LoopHeaderBB->getTerminator());
  auto *IVNext =
      Builder.CreateAdd(IV, ConstantInt::get(Ty, 1), IV->getName() + ".next",
                        /*HasNUW=*/true, /*HasNSW=*/Bitwidth != 2);
 
  // The loop trip count check.
  auto *IVCheck = Builder.CreateICmpEQ(IVNext, LoopTripCount,
                                       CurLoop->getName() + ".ivcheck");
  SmallVector<uint32_t> BranchWeights;
  const bool HasBranchWeights =
      extractBranchWeights(*LoopHeaderBB->getTerminator(), BranchWeights);
 
  auto *BI = Builder.CreateCondBr(IVCheck, SuccessorBB, LoopHeaderBB);
  if (HasBranchWeights) {
    if (SuccessorBB == LoopHeaderBB->getTerminator()->getSuccessor(1))
      std::swap(BranchWeights[0], BranchWeights[1]);
    // We're not changing the loop profile, so we can reuse the original loop's
    // profile.
    setBranchWeights(*BI, BranchWeights,
                     /*IsExpected=*/false);
  }
 
  LoopHeaderBB->getTerminator()->eraseFromParent();
 
  // Populate the IV PHI.
  IV->addIncoming(ConstantInt::get(Ty, 0), LoopPreheaderBB);
  IV->addIncoming(IVNext, LoopHeaderBB);
 
  // 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);
 
  // Other passes will take care of actually deleting the loop if possible.
 
  LLVM_DEBUG(dbgs() << DEBUG_TYPE " shift-until-bittest idiom optimized!\n");
 
  ++NumShiftUntilBitTest;
  return MadeChange;
}
 
/// Return true if the idiom is detected in the loop.
///
/// The core idiom we are trying to detect is:
/// \code
///   entry:
///     <...>
///     %start = <...>
///     %extraoffset = <...>
///     <...>
///     br label %for.cond
///
///   loop:
///     %iv = phi i8 [ %start, %entry ], [ %iv.next, %for.cond ]
///     %nbits = add nsw i8 %iv, %extraoffset
///     %val.shifted = {{l,a}shr,shl} i8 %val, %nbits
///     %val.shifted.iszero = icmp eq i8 %val.shifted, 0
///     %iv.next = add i8 %iv, 1
///     <...>
///     br i1 %val.shifted.iszero, label %end, label %loop
///
///   end:
///     %iv.res = phi i8 [ %iv, %loop ] <...>
///     %nbits.res = phi i8 [ %nbits, %loop ] <...>
///     %val.shifted.res = phi i8 [ %val.shifted, %loop ] <...>
///     %val.shifted.iszero.res = phi i1 [ %val.shifted.iszero, %loop ] <...>
///     %iv.next.res = phi i8 [ %iv.next, %loop ] <...>
///     <...>
/// \endcode
static bool detectShiftUntilZeroIdiom(Loop *CurLoop, ScalarEvolution *SE,
                                      Instruction *&ValShiftedIsZero,
                                      Intrinsic::ID &IntrinID, Instruction *&IV,
                                      Value *&Start, Value *&Val,
                                      const SCEV *&ExtraOffsetExpr,
                                      bool &InvertedCond) {
  LLVM_DEBUG(dbgs() << DEBUG_TYPE
             " Performing shift-until-zero idiom detection.\n");
 
  // Give up if the loop has multiple blocks or multiple backedges.
  if (CurLoop->getNumBlocks() != 1 || CurLoop->getNumBackEdges() != 1) {
    LLVM_DEBUG(dbgs() << DEBUG_TYPE " Bad block/backedge count.\n");
    return false;
  }
 
  Instruction *ValShifted, *NBits, *IVNext;
  Value *ExtraOffset;
 
  BasicBlock *LoopHeaderBB = CurLoop->getHeader();
  BasicBlock *LoopPreheaderBB = CurLoop->getLoopPreheader();
  assert(LoopPreheaderBB && "There is always a loop preheader.");
 
  using namespace PatternMatch;
 
  // Step 1: Check if the loop backedge, condition is in desirable form.
 
  CmpPredicate Pred;
  BasicBlock *TrueBB, *FalseBB;
  if (!match(LoopHeaderBB->getTerminator(),
             m_Br(m_Instruction(ValShiftedIsZero), m_BasicBlock(TrueBB),
                  m_BasicBlock(FalseBB))) ||
      !match(ValShiftedIsZero,
             m_ICmp(Pred, m_Instruction(ValShifted), m_Zero())) ||
      !ICmpInst::isEquality(Pred)) {
    LLVM_DEBUG(dbgs() << DEBUG_TYPE " Bad backedge structure.\n");
    return false;
  }
 
  // Step 2: Check if the comparison's operand is in desirable form.
  // FIXME: Val could be a one-input PHI node, which we should look past.
  if (!match(ValShifted, m_Shift(m_LoopInvariant(m_Value(Val), CurLoop),
                                 m_Instruction(NBits)))) {
    LLVM_DEBUG(dbgs() << DEBUG_TYPE " Bad comparisons value computation.\n");
    return false;
  }
  IntrinID = ValShifted->getOpcode() == Instruction::Shl ? Intrinsic::cttz
                                                         : Intrinsic::ctlz;
 
  // Step 3: Check if the shift amount is in desirable form.
 
  if (match(NBits, m_c_Add(m_Instruction(IV),
                           m_LoopInvariant(m_Value(ExtraOffset), CurLoop))) &&
      (NBits->hasNoSignedWrap() || NBits->hasNoUnsignedWrap()))
    ExtraOffsetExpr = SE->getNegativeSCEV(SE->getSCEV(ExtraOffset));
  else if (match(NBits,
                 m_Sub(m_Instruction(IV),
                       m_LoopInvariant(m_Value(ExtraOffset), CurLoop))) &&
           NBits->hasNoSignedWrap())
    ExtraOffsetExpr = SE->getSCEV(ExtraOffset);
  else {
    IV = NBits;
    ExtraOffsetExpr = SE->getZero(NBits->getType());
  }
 
  // Step 4: Check if the recurrence is in desirable form.
  auto *IVPN = dyn_cast<PHINode>(IV);
  if (!IVPN || IVPN->getParent() != LoopHeaderBB) {
    LLVM_DEBUG(dbgs() << DEBUG_TYPE " Not an expected PHI node.\n");
    return false;
  }
 
  Start = IVPN->getIncomingValueForBlock(LoopPreheaderBB);
  IVNext = dyn_cast<Instruction>(IVPN->getIncomingValueForBlock(LoopHeaderBB));
 
  if (!IVNext || !match(IVNext, m_Add(m_Specific(IVPN), m_One()))) {
    LLVM_DEBUG(dbgs() << DEBUG_TYPE " Bad recurrence.\n");
    return false;
  }
 
  // Step 4: Check if the backedge's destinations are in desirable form.
 
  assert(ICmpInst::isEquality(Pred) &&
         "Should only get equality predicates here.");
 
  // cmp-br is commutative, so canonicalize to a single variant.
  InvertedCond = Pred != ICmpInst::Predicate::ICMP_EQ;
  if (InvertedCond) {
    Pred = ICmpInst::getInversePredicate(Pred);
    std::swap(TrueBB, FalseBB);
  }
 
  // We expect to exit loop when comparison yields true,
  // so when it yields false we should branch back to loop header.
  if (FalseBB != LoopHeaderBB) {
    LLVM_DEBUG(dbgs() << DEBUG_TYPE " Bad backedge flow.\n");
    return false;
  }
 
  // The new, countable, loop will certainly only run a known number of
  // iterations, It won't be infinite. But the old loop might be infinite
  // under certain conditions. For logical shifts, the value will become zero
  // after at most bitwidth(%Val) loop iterations. However, for arithmetic
  // right-shift, iff the sign bit was set, the value will never become zero,
  // and the loop may never finish.
  if (ValShifted->getOpcode() == Instruction::AShr &&
      !isMustProgress(CurLoop) && !SE->isKnownNonNegative(SE->getSCEV(Val))) {
    LLVM_DEBUG(dbgs() << DEBUG_TYPE " Can not prove the loop is finite.\n");
    return false;
  }
 
  // Okay, idiom checks out.
  return true;
}
 
/// Look for the following loop:
/// \code
///   entry:
///     <...>
///     %start = <...>
///     %extraoffset = <...>
///     <...>
///     br label %loop
///
///   loop:
///     %iv = phi i8 [ %start, %entry ], [ %iv.next, %loop ]
///     %nbits = add nsw i8 %iv, %extraoffset
///     %val.shifted = {{l,a}shr,shl} i8 %val, %nbits
///     %val.shifted.iszero = icmp eq i8 %val.shifted, 0
///     %iv.next = add i8 %iv, 1
///     <...>
///     br i1 %val.shifted.iszero, label %end, label %loop
///
///   end:
///     %iv.res = phi i8 [ %iv, %loop ] <...>
///     %nbits.res = phi i8 [ %nbits, %loop ] <...>
///     %val.shifted.res = phi i8 [ %val.shifted, %loop ] <...>
///     %val.shifted.iszero.res = phi i1 [ %val.shifted.iszero, %loop ] <...>
///     %iv.next.res = phi i8 [ %iv.next, %loop ] <...>
///     <...>
/// \endcode
///
/// And transform it into:
/// \code
///   entry:
///     <...>
///     %start = <...>
///     %extraoffset = <...>
///     <...>
///     %val.numleadingzeros = call i8 @llvm.ct{l,t}z.i8(i8 %val, i1 0)
///     %val.numactivebits = sub i8 8, %val.numleadingzeros
///     %extraoffset.neg = sub i8 0, %extraoffset
///     %tmp = add i8 %val.numactivebits, %extraoffset.neg
///     %iv.final = call i8 @llvm.smax.i8(i8 %tmp, i8 %start)
///     %loop.tripcount = sub i8 %iv.final, %start
///     br label %loop
///
///   loop:
///     %loop.iv = phi i8 [ 0, %entry ], [ %loop.iv.next, %loop ]
///     %loop.iv.next = add i8 %loop.iv, 1
///     %loop.ivcheck = icmp eq i8 %loop.iv.next, %loop.tripcount
///     %iv = add i8 %loop.iv, %start
///     <...>
///     br i1 %loop.ivcheck, label %end, label %loop
///
///   end:
///     %iv.res = phi i8 [ %iv.final, %loop ] <...>
///     <...>
/// \endcode
bool LoopIdiomRecognize::recognizeShiftUntilZero() {
  bool MadeChange = false;
 
  Instruction *ValShiftedIsZero;
  Intrinsic::ID IntrID;
  Instruction *IV;
  Value *Start, *Val;
  const SCEV *ExtraOffsetExpr;
  bool InvertedCond;
  if (!detectShiftUntilZeroIdiom(CurLoop, SE, ValShiftedIsZero, IntrID, IV,
                                 Start, Val, ExtraOffsetExpr, InvertedCond)) {
    LLVM_DEBUG(dbgs() << DEBUG_TYPE
               " shift-until-zero idiom detection failed.\n");
    return MadeChange;
  }
  LLVM_DEBUG(dbgs() << DEBUG_TYPE " shift-until-zero idiom detected!\n");
 
  // Ok, it is the idiom we were looking for, we *could* transform this loop,
  // but is it profitable to transform?
 
  BasicBlock *LoopHeaderBB = CurLoop->getHeader();
  BasicBlock *LoopPreheaderBB = CurLoop->getLoopPreheader();
  assert(LoopPreheaderBB && "There is always a loop preheader.");
 
  BasicBlock *SuccessorBB = CurLoop->getExitBlock();
  assert(SuccessorBB && "There is only a single successor.");
 
  IRBuilder<> Builder(LoopPreheaderBB->getTerminator());
  Builder.SetCurrentDebugLocation(IV->getDebugLoc());
 
  Type *Ty = Val->getType();
  unsigned Bitwidth = Ty->getScalarSizeInBits();
 
  TargetTransformInfo::TargetCostKind CostKind =
      TargetTransformInfo::TCK_SizeAndLatency;
 
  // The rewrite is considered to be unprofitable iff and only iff the
  // intrinsic we'll use are not cheap. Note that we are okay with *just*
  // making the loop countable, even if nothing else changes.
  IntrinsicCostAttributes Attrs(
      IntrID, Ty, {PoisonValue::get(Ty), /*is_zero_poison=*/Builder.getFalse()});
  InstructionCost Cost = TTI->getIntrinsicInstrCost(Attrs, CostKind);
  if (Cost > TargetTransformInfo::TCC_Basic) {
    LLVM_DEBUG(dbgs() << DEBUG_TYPE
               " Intrinsic is too costly, not beneficial\n");
    return MadeChange;
  }
 
  // Ok, transform appears worthwhile.
  MadeChange = true;
 
  bool OffsetIsZero = ExtraOffsetExpr->isZero();
 
  // Step 1: Compute the loop's final IV value / trip count.
 
  Value *ValNumLeadingZeros = Builder.CreateIntrinsic(
      IntrID, Ty, {Val, /*is_zero_poison=*/Builder.getFalse()},
      /*FMFSource=*/nullptr, Val->getName() + ".numleadingzeros");
  Value *ValNumActiveBits = Builder.CreateSub(
      ConstantInt::get(Ty, Ty->getScalarSizeInBits()), ValNumLeadingZeros,
      Val->getName() + ".numactivebits", /*HasNUW=*/true,
      /*HasNSW=*/Bitwidth != 2);
 
  SCEVExpander Expander(*SE, "loop-idiom");
  Expander.setInsertPoint(&*Builder.GetInsertPoint());
  Value *ExtraOffset = Expander.expandCodeFor(ExtraOffsetExpr);
 
  Value *ValNumActiveBitsOffset = Builder.CreateAdd(
      ValNumActiveBits, ExtraOffset, ValNumActiveBits->getName() + ".offset",
      /*HasNUW=*/OffsetIsZero, /*HasNSW=*/true);
  Value *IVFinal = Builder.CreateIntrinsic(Intrinsic::smax, {Ty},
                                           {ValNumActiveBitsOffset, Start},
                                           /*FMFSource=*/nullptr, "iv.final");
 
  auto *LoopBackedgeTakenCount = cast<Instruction>(Builder.CreateSub(
      IVFinal, Start, CurLoop->getName() + ".backedgetakencount",
      /*HasNUW=*/OffsetIsZero, /*HasNSW=*/true));
  // FIXME: or when the offset was `add nuw`
 
  // We know loop's backedge-taken count, but what's loop's trip count?
  Value *LoopTripCount =
      Builder.CreateAdd(LoopBackedgeTakenCount, ConstantInt::get(Ty, 1),
                        CurLoop->getName() + ".tripcount", /*HasNUW=*/true,
                        /*HasNSW=*/Bitwidth != 2);
 
  // Step 2: Adjust the successor basic block to receive the original
  //         induction variable's final value instead of the orig. IV itself.
 
  IV->replaceUsesOutsideBlock(IVFinal, LoopHeaderBB);
 
  // Step 3: Rewrite the loop into a countable form, with canonical IV.
 
  // The new canonical induction variable.
  Builder.SetInsertPoint(LoopHeaderBB->begin());
  auto *CIV = Builder.CreatePHI(Ty, 2, CurLoop->getName() + ".iv");
 
  // The induction itself.
  Builder.SetInsertPoint(LoopHeaderBB->getFirstNonPHIIt());
  auto *CIVNext =
      Builder.CreateAdd(CIV, ConstantInt::get(Ty, 1), CIV->getName() + ".next",
                        /*HasNUW=*/true, /*HasNSW=*/Bitwidth != 2);
 
  // The loop trip count check.
  auto *CIVCheck = Builder.CreateICmpEQ(CIVNext, LoopTripCount,
                                        CurLoop->getName() + ".ivcheck");
  auto *NewIVCheck = CIVCheck;
  if (InvertedCond) {
    NewIVCheck = Builder.CreateNot(CIVCheck);
    NewIVCheck->takeName(ValShiftedIsZero);
  }
 
  // The original IV, but rebased to be an offset to the CIV.
  auto *IVDePHId = Builder.CreateAdd(CIV, Start, "", /*HasNUW=*/false,
                                     /*HasNSW=*/true); // FIXME: what about NUW?
  IVDePHId->takeName(IV);
 
  // The loop terminator.
  Builder.SetInsertPoint(LoopHeaderBB->getTerminator());
  SmallVector<uint32_t> BranchWeights;
  const bool HasBranchWeights =
      extractBranchWeights(*LoopHeaderBB->getTerminator(), BranchWeights);
 
  auto *BI = Builder.CreateCondBr(CIVCheck, SuccessorBB, LoopHeaderBB);
  if (HasBranchWeights) {
    if (InvertedCond)
      std::swap(BranchWeights[0], BranchWeights[1]);
    // We're not changing the loop profile, so we can reuse the original loop's
    // profile.
    setBranchWeights(*BI, BranchWeights, /*IsExpected=*/false);
  }
  LoopHeaderBB->getTerminator()->eraseFromParent();
 
  // Populate the IV PHI.
  CIV->addIncoming(ConstantInt::get(Ty, 0), LoopPreheaderBB);
  CIV->addIncoming(CIVNext, LoopHeaderBB);
 
  // Step 4: 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);
 
  // Step 5: Try to cleanup the loop's body somewhat.
  IV->replaceAllUsesWith(IVDePHId);
  IV->eraseFromParent();
 
  ValShiftedIsZero->replaceAllUsesWith(NewIVCheck);
  ValShiftedIsZero->eraseFromParent();
 
  // Other passes will take care of actually deleting the loop if possible.
 
  LLVM_DEBUG(dbgs() << DEBUG_TYPE " shift-until-zero idiom optimized!\n");
 
  ++NumShiftUntilZero;
  return MadeChange;
}