DeadStoreElimination.cpp

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//===- DeadStoreElimination.cpp - MemorySSA Backed Dead Store Elimination -===//
//
// 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
//
//===----------------------------------------------------------------------===//
//
// The code below implements dead store elimination using MemorySSA. It uses
// the following general approach: given a MemoryDef, walk upwards to find
// clobbering MemoryDefs that may be killed by the starting def. Then check
// that there are no uses that may read the location of the original MemoryDef
// in between both MemoryDefs. A bit more concretely:
//
// For all MemoryDefs StartDef:
// 1. Get the next dominating clobbering MemoryDef (MaybeDeadAccess) by walking
//    upwards.
// 2. Check that there are no reads between MaybeDeadAccess and the StartDef by
//    checking all uses starting at MaybeDeadAccess and walking until we see
//    StartDef.
// 3. For each found CurrentDef, check that:
//   1. There are no barrier instructions between CurrentDef and StartDef (like
//       throws or stores with ordering constraints).
//   2. StartDef is executed whenever CurrentDef is executed.
//   3. StartDef completely overwrites CurrentDef.
// 4. Erase CurrentDef from the function and MemorySSA.
//
//===----------------------------------------------------------------------===//
 
#include "llvm/Transforms/Scalar/DeadStoreElimination.h"
#include "llvm/ADT/APInt.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/MapVector.h"
#include "llvm/ADT/PostOrderIterator.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/ScopedHashTable.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/AssumptionCache.h"
#include "llvm/Analysis/CaptureTracking.h"
#include "llvm/Analysis/CycleAnalysis.h"
#include "llvm/Analysis/GlobalsModRef.h"
#include "llvm/Analysis/Loads.h"
#include "llvm/Analysis/MemoryBuiltins.h"
#include "llvm/Analysis/MemoryLocation.h"
#include "llvm/Analysis/MemorySSA.h"
#include "llvm/Analysis/MemorySSAUpdater.h"
#include "llvm/Analysis/MustExecute.h"
#include "llvm/Analysis/PostDominators.h"
#include "llvm/Analysis/TargetLibraryInfo.h"
#include "llvm/Analysis/ValueTracking.h"
#include "llvm/IR/Argument.h"
#include "llvm/IR/AttributeMask.h"
#include "llvm/IR/BasicBlock.h"
#include "llvm/IR/Constant.h"
#include "llvm/IR/ConstantRangeList.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/DataLayout.h"
#include "llvm/IR/DebugInfo.h"
#include "llvm/IR/Dominators.h"
#include "llvm/IR/Function.h"
#include "llvm/IR/IRBuilder.h"
#include "llvm/IR/InstIterator.h"
#include "llvm/IR/InstrTypes.h"
#include "llvm/IR/Instruction.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/IntrinsicInst.h"
#include "llvm/IR/Module.h"
#include "llvm/IR/PassManager.h"
#include "llvm/IR/PatternMatch.h"
#include "llvm/IR/Value.h"
#include "llvm/InitializePasses.h"
#include "llvm/Support/Casting.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/DebugCounter.h"
#include "llvm/Support/ErrorHandling.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Transforms/Scalar.h"
#include "llvm/Transforms/Utils/AssumeBundleBuilder.h"
#include "llvm/Transforms/Utils/BuildLibCalls.h"
#include "llvm/Transforms/Utils/Local.h"
#include <algorithm>
#include <cassert>
#include <cstdint>
#include <map>
#include <optional>
#include <utility>
 
using namespace llvm;
using namespace PatternMatch;
 
#define DEBUG_TYPE "dse"
 
STATISTIC(NumRemainingStores, "Number of stores remaining after DSE");
STATISTIC(NumRedundantStores, "Number of redundant stores deleted");
STATISTIC(NumFastStores, "Number of stores deleted");
STATISTIC(NumFastOther, "Number of other instrs removed");
STATISTIC(NumCompletePartials, "Number of stores dead by later partials");
STATISTIC(NumModifiedStores, "Number of stores modified");
STATISTIC(NumCFGChecks, "Number of stores modified");
STATISTIC(NumCFGTries, "Number of stores modified");
STATISTIC(NumCFGSuccess, "Number of stores modified");
STATISTIC(NumGetDomMemoryDefPassed,
          "Number of times a valid candidate is returned from getDomMemoryDef");
STATISTIC(NumDomMemDefChecks,
          "Number iterations check for reads in getDomMemoryDef");
 
DEBUG_COUNTER(MemorySSACounter, "dse-memoryssa",
              "Controls which MemoryDefs are eliminated.");
 
static cl::opt<bool>
EnablePartialOverwriteTracking("enable-dse-partial-overwrite-tracking",
  cl::init(true), cl::Hidden,
  cl::desc("Enable partial-overwrite tracking in DSE"));
 
static cl::opt<bool>
EnablePartialStoreMerging("enable-dse-partial-store-merging",
  cl::init(true), cl::Hidden,
  cl::desc("Enable partial store merging in DSE"));
 
static cl::opt<unsigned>
    MemorySSAScanLimit("dse-memoryssa-scanlimit", cl::init(150), cl::Hidden,
                       cl::desc("The number of memory instructions to scan for "
                                "dead store elimination (default = 150)"));
static cl::opt<unsigned> MemorySSAUpwardsStepLimit(
    "dse-memoryssa-walklimit", cl::init(90), cl::Hidden,
    cl::desc("The maximum number of steps while walking upwards to find "
             "MemoryDefs that may be killed (default = 90)"));
 
static cl::opt<unsigned> MemorySSAPartialStoreLimit(
    "dse-memoryssa-partial-store-limit", cl::init(5), cl::Hidden,
    cl::desc("The maximum number candidates that only partially overwrite the "
             "killing MemoryDef to consider"
             " (default = 5)"));
 
static cl::opt<unsigned> MemorySSADefsPerBlockLimit(
    "dse-memoryssa-defs-per-block-limit", cl::init(5000), cl::Hidden,
    cl::desc("The number of MemoryDefs we consider as candidates to eliminated "
             "other stores per basic block (default = 5000)"));
 
static cl::opt<unsigned> MemorySSASameBBStepCost(
    "dse-memoryssa-samebb-cost", cl::init(1), cl::Hidden,
    cl::desc(
        "The cost of a step in the same basic block as the killing MemoryDef"
        "(default = 1)"));
 
static cl::opt<unsigned>
    MemorySSAOtherBBStepCost("dse-memoryssa-otherbb-cost", cl::init(5),
                             cl::Hidden,
                             cl::desc("The cost of a step in a different basic "
                                      "block than the killing MemoryDef"
                                      "(default = 5)"));
 
static cl::opt<unsigned> MemorySSAPathCheckLimit(
    "dse-memoryssa-path-check-limit", cl::init(50), cl::Hidden,
    cl::desc("The maximum number of blocks to check when trying to prove that "
             "all paths to an exit go through a killing block (default = 50)"));
 
// This flags allows or disallows DSE to optimize MemorySSA during its
// traversal. Note that DSE optimizing MemorySSA may impact other passes
// downstream of the DSE invocation and can lead to issues not being
// reproducible in isolation (i.e. when MemorySSA is built from scratch). In
// those cases, the flag can be used to check if DSE's MemorySSA optimizations
// impact follow-up passes.
static cl::opt<bool>
    OptimizeMemorySSA("dse-optimize-memoryssa", cl::init(true), cl::Hidden,
                      cl::desc("Allow DSE to optimize memory accesses."));
 
// TODO: remove this flag.
static cl::opt<bool> EnableInitializesImprovement(
    "enable-dse-initializes-attr-improvement", cl::init(true), cl::Hidden,
    cl::desc("Enable the initializes attr improvement in DSE"));
 
static cl::opt<unsigned> MaxDepthRecursion(
    "dse-max-dom-cond-depth", cl::init(1024), cl::Hidden,
    cl::desc("Max dominator tree recursion depth for eliminating redundant "
             "stores via dominating conditions"));
 
//===----------------------------------------------------------------------===//
// Helper functions
//===----------------------------------------------------------------------===//
using OverlapIntervalsTy = std::map<int64_t, int64_t>;
using InstOverlapIntervalsTy = MapVector<Instruction *, OverlapIntervalsTy>;
 
/// Returns true if the end of this instruction can be safely shortened in
/// length.
static bool isShortenableAtTheEnd(Instruction *I) {
  // Don't shorten stores for now
  if (isa<StoreInst>(I))
    return false;
 
  if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(I)) {
    switch (II->getIntrinsicID()) {
      default: return false;
      case Intrinsic::memset:
      case Intrinsic::memcpy:
      case Intrinsic::memcpy_element_unordered_atomic:
      case Intrinsic::memset_element_unordered_atomic:
        // Do shorten memory intrinsics.
        // FIXME: Add memmove if it's also safe to transform.
        return true;
    }
  }
 
  // Don't shorten libcalls calls for now.
 
  return false;
}
 
/// Returns true if the beginning of this instruction can be safely shortened
/// in length.
static bool isShortenableAtTheBeginning(Instruction *I) {
  // FIXME: Handle only memset for now. Supporting memcpy/memmove should be
  // easily done by offsetting the source address.
  return isa<AnyMemSetInst>(I);
}
 
static std::optional<TypeSize> getPointerSize(const Value *V,
                                              const DataLayout &DL,
                                              const TargetLibraryInfo &TLI,
                                              const Function *F) {
  uint64_t Size;
  ObjectSizeOpts Opts;
  Opts.NullIsUnknownSize = NullPointerIsDefined(F);
 
  if (getObjectSize(V, Size, DL, &TLI, Opts))
    return TypeSize::getFixed(Size);
  return std::nullopt;
}
 
namespace {
 
enum OverwriteResult {
  OW_Begin,
  OW_Complete,
  OW_End,
  OW_PartialEarlierWithFullLater,
  OW_MaybePartial,
  OW_None,
  OW_Unknown
};
 
} // end anonymous namespace
 
/// Check if two instruction are masked stores that completely
/// overwrite one another. More specifically, \p KillingI has to
/// overwrite \p DeadI.
static OverwriteResult isMaskedStoreOverwrite(const Instruction *KillingI,
                                              const Instruction *DeadI,
                                              BatchAAResults &AA) {
  const auto *KillingII = dyn_cast<IntrinsicInst>(KillingI);
  const auto *DeadII = dyn_cast<IntrinsicInst>(DeadI);
  if (KillingII == nullptr || DeadII == nullptr)
    return OW_Unknown;
  if (KillingII->getIntrinsicID() != DeadII->getIntrinsicID())
    return OW_Unknown;
 
  switch (KillingII->getIntrinsicID()) {
  case Intrinsic::masked_store:
  case Intrinsic::vp_store: {
    const DataLayout &DL = KillingII->getDataLayout();
    auto *KillingTy = KillingII->getArgOperand(0)->getType();
    auto *DeadTy = DeadII->getArgOperand(0)->getType();
    if (DL.getTypeSizeInBits(KillingTy) != DL.getTypeSizeInBits(DeadTy))
      return OW_Unknown;
    // Element count.
    if (cast<VectorType>(KillingTy)->getElementCount() !=
        cast<VectorType>(DeadTy)->getElementCount())
      return OW_Unknown;
    // Pointers.
    Value *KillingPtr = KillingII->getArgOperand(1);
    Value *DeadPtr = DeadII->getArgOperand(1);
    if (KillingPtr != DeadPtr && !AA.isMustAlias(KillingPtr, DeadPtr))
      return OW_Unknown;
    if (KillingII->getIntrinsicID() == Intrinsic::masked_store) {
      // Masks.
      // TODO: check that KillingII's mask is a superset of the DeadII's mask.
      if (KillingII->getArgOperand(2) != DeadII->getArgOperand(2))
        return OW_Unknown;
    } else if (KillingII->getIntrinsicID() == Intrinsic::vp_store) {
      // Masks.
      // TODO: check that KillingII's mask is a superset of the DeadII's mask.
      if (KillingII->getArgOperand(2) != DeadII->getArgOperand(2))
        return OW_Unknown;
      // Lengths.
      if (KillingII->getArgOperand(3) != DeadII->getArgOperand(3))
        return OW_Unknown;
    }
    return OW_Complete;
  }
  default:
    return OW_Unknown;
  }
}
 
/// Return 'OW_Complete' if a store to the 'KillingLoc' location completely
/// overwrites a store to the 'DeadLoc' location, 'OW_End' if the end of the
/// 'DeadLoc' location is completely overwritten by 'KillingLoc', 'OW_Begin'
/// if the beginning of the 'DeadLoc' location is overwritten by 'KillingLoc'.
/// 'OW_PartialEarlierWithFullLater' means that a dead (big) store was
/// overwritten by a killing (smaller) store which doesn't write outside the big
/// store's memory locations. Returns 'OW_Unknown' if nothing can be determined.
/// NOTE: This function must only be called if both \p KillingLoc and \p
/// DeadLoc belong to the same underlying object with valid \p KillingOff and
/// \p DeadOff.
static OverwriteResult isPartialOverwrite(const MemoryLocation &KillingLoc,
                                          const MemoryLocation &DeadLoc,
                                          int64_t KillingOff, int64_t DeadOff,
                                          Instruction *DeadI,
                                          InstOverlapIntervalsTy &IOL) {
  const uint64_t KillingSize = KillingLoc.Size.getValue();
  const uint64_t DeadSize = DeadLoc.Size.getValue();
  // We may now overlap, although the overlap is not complete. There might also
  // be other incomplete overlaps, and together, they might cover the complete
  // dead store.
  // Note: The correctness of this logic depends on the fact that this function
  // is not even called providing DepWrite when there are any intervening reads.
  if (EnablePartialOverwriteTracking &&
      KillingOff < int64_t(DeadOff + DeadSize) &&
      int64_t(KillingOff + KillingSize) >= DeadOff) {
 
    // Insert our part of the overlap into the map.
    auto &IM = IOL[DeadI];
    LLVM_DEBUG(dbgs() << "DSE: Partial overwrite: DeadLoc [" << DeadOff << ", "
                      << int64_t(DeadOff + DeadSize) << ") KillingLoc ["
                      << KillingOff << ", " << int64_t(KillingOff + KillingSize)
                      << ")\n");
 
    // Make sure that we only insert non-overlapping intervals and combine
    // adjacent intervals. The intervals are stored in the map with the ending
    // offset as the key (in the half-open sense) and the starting offset as
    // the value.
    int64_t KillingIntStart = KillingOff;
    int64_t KillingIntEnd = KillingOff + KillingSize;
 
    // Find any intervals ending at, or after, KillingIntStart which start
    // before KillingIntEnd.
    auto ILI = IM.lower_bound(KillingIntStart);
    if (ILI != IM.end() && ILI->second <= KillingIntEnd) {
      // This existing interval is overlapped with the current store somewhere
      // in [KillingIntStart, KillingIntEnd]. Merge them by erasing the existing
      // intervals and adjusting our start and end.
      KillingIntStart = std::min(KillingIntStart, ILI->second);
      KillingIntEnd = std::max(KillingIntEnd, ILI->first);
      ILI = IM.erase(ILI);
 
      // Continue erasing and adjusting our end in case other previous
      // intervals are also overlapped with the current store.
      //
      // |--- dead 1 ---|  |--- dead 2 ---|
      //     |------- killing---------|
      //
      while (ILI != IM.end() && ILI->second <= KillingIntEnd) {
        assert(ILI->second > KillingIntStart && "Unexpected interval");
        KillingIntEnd = std::max(KillingIntEnd, ILI->first);
        ILI = IM.erase(ILI);
      }
    }
 
    IM[KillingIntEnd] = KillingIntStart;
 
    ILI = IM.begin();
    if (ILI->second <= DeadOff && ILI->first >= int64_t(DeadOff + DeadSize)) {
      LLVM_DEBUG(dbgs() << "DSE: Full overwrite from partials: DeadLoc ["
                        << DeadOff << ", " << int64_t(DeadOff + DeadSize)
                        << ") Composite KillingLoc [" << ILI->second << ", "
                        << ILI->first << ")\n");
      ++NumCompletePartials;
      return OW_Complete;
    }
  }
 
  // Check for a dead store which writes to all the memory locations that
  // the killing store writes to.
  if (EnablePartialStoreMerging && KillingOff >= DeadOff &&
      int64_t(DeadOff + DeadSize) > KillingOff &&
      uint64_t(KillingOff - DeadOff) + KillingSize <= DeadSize) {
    LLVM_DEBUG(dbgs() << "DSE: Partial overwrite a dead load [" << DeadOff
                      << ", " << int64_t(DeadOff + DeadSize)
                      << ") by a killing store [" << KillingOff << ", "
                      << int64_t(KillingOff + KillingSize) << ")\n");
    // TODO: Maybe come up with a better name?
    return OW_PartialEarlierWithFullLater;
  }
 
  // Another interesting case is if the killing store overwrites the end of the
  // dead store.
  //
  //      |--dead--|
  //                |--   killing   --|
  //
  // In this case we may want to trim the size of dead store to avoid
  // generating stores to addresses which will definitely be overwritten killing
  // store.
  if (!EnablePartialOverwriteTracking &&
      (KillingOff > DeadOff && KillingOff < int64_t(DeadOff + DeadSize) &&
       int64_t(KillingOff + KillingSize) >= int64_t(DeadOff + DeadSize)))
    return OW_End;
 
  // Finally, we also need to check if the killing store overwrites the
  // beginning of the dead store.
  //
  //                |--dead--|
  //      |--  killing  --|
  //
  // In this case we may want to move the destination address and trim the size
  // of dead store to avoid generating stores to addresses which will definitely
  // be overwritten killing store.
  if (!EnablePartialOverwriteTracking &&
      (KillingOff <= DeadOff && int64_t(KillingOff + KillingSize) > DeadOff)) {
    assert(int64_t(KillingOff + KillingSize) < int64_t(DeadOff + DeadSize) &&
           "Expect to be handled as OW_Complete");
    return OW_Begin;
  }
  // Otherwise, they don't completely overlap.
  return OW_Unknown;
}
 
/// Returns true if the memory which is accessed by the second instruction is not
/// modified between the first and the second instruction.
/// Precondition: Second instruction must be dominated by the first
/// instruction.
static bool
memoryIsNotModifiedBetween(Instruction *FirstI, Instruction *SecondI,
                           BatchAAResults &AA, const DataLayout &DL,
                           DominatorTree *DT) {
  // Do a backwards scan through the CFG from SecondI to FirstI. Look for
  // instructions which can modify the memory location accessed by SecondI.
  //
  // While doing the walk keep track of the address to check. It might be
  // different in different basic blocks due to PHI translation.
  using BlockAddressPair = std::pair<BasicBlock *, PHITransAddr>;
  SmallVector<BlockAddressPair, 16> WorkList;
  // Keep track of the address we visited each block with. Bail out if we
  // visit a block with different addresses.
  DenseMap<BasicBlock *, Value *> Visited;
 
  BasicBlock::iterator FirstBBI(FirstI);
  ++FirstBBI;
  BasicBlock::iterator SecondBBI(SecondI);
  BasicBlock *FirstBB = FirstI->getParent();
  BasicBlock *SecondBB = SecondI->getParent();
  MemoryLocation MemLoc;
  if (auto *MemSet = dyn_cast<MemSetInst>(SecondI))
    MemLoc = MemoryLocation::getForDest(MemSet);
  else
    MemLoc = MemoryLocation::get(SecondI);
 
  auto *MemLocPtr = const_cast<Value *>(MemLoc.Ptr);
 
  // Start checking the SecondBB.
  WorkList.push_back(
      std::make_pair(SecondBB, PHITransAddr(MemLocPtr, DL, nullptr)));
  bool isFirstBlock = true;
 
  // Check all blocks going backward until we reach the FirstBB.
  while (!WorkList.empty()) {
    BlockAddressPair Current = WorkList.pop_back_val();
    BasicBlock *B = Current.first;
    PHITransAddr &Addr = Current.second;
    Value *Ptr = Addr.getAddr();
 
    // Ignore instructions before FirstI if this is the FirstBB.
    BasicBlock::iterator BI = (B == FirstBB ? FirstBBI : B->begin());
 
    BasicBlock::iterator EI;
    if (isFirstBlock) {
      // Ignore instructions after SecondI if this is the first visit of SecondBB.
      assert(B == SecondBB && "first block is not the store block");
      EI = SecondBBI;
      isFirstBlock = false;
    } else {
      // It's not SecondBB or (in case of a loop) the second visit of SecondBB.
      // In this case we also have to look at instructions after SecondI.
      EI = B->end();
    }
    for (; BI != EI; ++BI) {
      Instruction *I = &*BI;
      if (I->mayWriteToMemory() && I != SecondI)
        if (isModSet(AA.getModRefInfo(I, MemLoc.getWithNewPtr(Ptr))))
          return false;
    }
    if (B != FirstBB) {
      assert(B != &FirstBB->getParent()->getEntryBlock() &&
          "Should not hit the entry block because SI must be dominated by LI");
      for (BasicBlock *Pred : predecessors(B)) {
        PHITransAddr PredAddr = Addr;
        if (PredAddr.needsPHITranslationFromBlock(B)) {
          if (!PredAddr.isPotentiallyPHITranslatable())
            return false;
          if (!PredAddr.translateValue(B, Pred, DT, false))
            return false;
        }
        Value *TranslatedPtr = PredAddr.getAddr();
        auto Inserted = Visited.insert(std::make_pair(Pred, TranslatedPtr));
        if (!Inserted.second) {
          // We already visited this block before. If it was with a different
          // address - bail out!
          if (TranslatedPtr != Inserted.first->second)
            return false;
          // ... otherwise just skip it.
          continue;
        }
        WorkList.push_back(std::make_pair(Pred, PredAddr));
      }
    }
  }
  return true;
}
 
static void shortenAssignment(Instruction *Inst, Value *OriginalDest,
                              uint64_t OldSizeInBits, uint64_t NewSizeInBits,
                              bool IsOverwriteEnd) {
  const DataLayout &DL = Inst->getDataLayout();
  uint64_t DeadSliceSizeInBits = OldSizeInBits - NewSizeInBits;
  // The dead slice offset is relative to OriginalDest, the slice start we hand
  // calculateFragmentIntersect. Shortening the end keeps the front of the
  // store, so the dead bits start where the new store ends; shortening the
  // beginning kills the bits at OriginalDest itself. Don't add OriginalDest's
  // offset from its base object here. calculateFragmentIntersect already
  // measures that pointer against the marker's address.
  uint64_t DeadSliceOffsetInBits = IsOverwriteEnd ? NewSizeInBits : 0;
  auto SetDeadFragExpr = [](auto *Assign,
                            DIExpression::FragmentInfo DeadFragment) {
    // createFragmentExpression expects an offset relative to the existing
    // fragment offset if there is one.
    uint64_t RelativeOffset = DeadFragment.OffsetInBits -
                              Assign->getExpression()
                                  ->getFragmentInfo()
                                  .value_or(DIExpression::FragmentInfo(0, 0))
                                  .OffsetInBits;
    if (auto NewExpr = DIExpression::createFragmentExpression(
            Assign->getExpression(), RelativeOffset, DeadFragment.SizeInBits)) {
      Assign->setExpression(*NewExpr);
      return;
    }
    // Failed to create a fragment expression for this so discard the value,
    // making this a kill location.
    auto *Expr = *DIExpression::createFragmentExpression(
        DIExpression::get(Assign->getContext(), {}), DeadFragment.OffsetInBits,
        DeadFragment.SizeInBits);
    Assign->setExpression(Expr);
    Assign->setKillLocation();
  };
 
  // A DIAssignID to use so that the inserted dbg.assign intrinsics do not
  // link to any instructions. Created in the loop below (once).
  DIAssignID *LinkToNothing = nullptr;
  LLVMContext &Ctx = Inst->getContext();
  auto GetDeadLink = [&Ctx, &LinkToNothing]() {
    if (!LinkToNothing)
      LinkToNothing = DIAssignID::getDistinct(Ctx);
    return LinkToNothing;
  };
 
  // Insert an unlinked dbg.assign intrinsic for the dead fragment after each
  // overlapping dbg.assign intrinsic.
  for (DbgVariableRecord *Assign : at::getDVRAssignmentMarkers(Inst)) {
    std::optional<DIExpression::FragmentInfo> NewFragment;
    if (!at::calculateFragmentIntersect(DL, OriginalDest, DeadSliceOffsetInBits,
                                        DeadSliceSizeInBits, Assign,
                                        NewFragment) ||
        !NewFragment) {
      // Either the intersection couldn't be worked out, or it covers the
      // entire variable region described by the record. Full coverage leaves
      // NewFragment empty rather than making calculateFragmentIntersect fail,
      // so unlink the whole assignment from the store in both cases.
      Assign->setKillAddress();
      Assign->setAssignId(GetDeadLink());
      continue;
    }
    // No intersect.
    if (NewFragment->SizeInBits == 0)
      continue;
 
    // Fragments overlap: insert a new dbg.assign for this dead part.
    auto *NewAssign = static_cast<decltype(Assign)>(Assign->clone());
    NewAssign->insertAfter(Assign->getIterator());
    NewAssign->setAssignId(GetDeadLink());
    if (NewFragment)
      SetDeadFragExpr(NewAssign, *NewFragment);
    NewAssign->setKillAddress();
  }
}
 
/// Update the attributes given that a memory access is updated (the
/// dereferenced pointer could be moved forward when shortening a
/// mem intrinsic).
static void adjustArgAttributes(AnyMemIntrinsic *Intrinsic, unsigned ArgNo,
                                uint64_t PtrOffset) {
  // Remember old attributes.
  AttributeSet OldAttrs = Intrinsic->getParamAttributes(ArgNo);
 
  // Find attributes that should be kept, and remove the rest.
  AttributeMask AttrsToRemove;
  for (auto &Attr : OldAttrs) {
    if (Attr.hasKindAsEnum()) {
      switch (Attr.getKindAsEnum()) {
      default:
        break;
      case Attribute::Alignment:
        // Only keep alignment if PtrOffset satisfy the alignment.
        if (isAligned(Attr.getAlignment().valueOrOne(), PtrOffset))
          continue;
        break;
      case Attribute::Dereferenceable:
      case Attribute::DereferenceableOrNull:
        // We could reduce the size of these attributes according to
        // PtrOffset. But we simply drop these for now.
        break;
      case Attribute::NonNull:
      case Attribute::NoUndef:
        continue;
      }
    }
    AttrsToRemove.addAttribute(Attr);
  }
 
  // Remove the attributes that should be dropped.
  Intrinsic->removeParamAttrs(ArgNo, AttrsToRemove);
}
 
static bool tryToShorten(Instruction *DeadI, int64_t &DeadStart,
                         uint64_t &DeadSize, int64_t KillingStart,
                         uint64_t KillingSize, bool IsOverwriteEnd) {
  auto *DeadIntrinsic = cast<AnyMemIntrinsic>(DeadI);
  Align PrefAlign = DeadIntrinsic->getDestAlign().valueOrOne();
 
  // We assume that memet/memcpy operates in chunks of the "largest" native
  // type size and aligned on the same value. That means optimal start and size
  // of memset/memcpy should be modulo of preferred alignment of that type. That
  // is it there is no any sense in trying to reduce store size any further
  // since any "extra" stores comes for free anyway.
  // On the other hand, maximum alignment we can achieve is limited by alignment
  // of initial store.
 
  // TODO: Limit maximum alignment by preferred (or abi?) alignment of the
  // "largest" native type.
  // Note: What is the proper way to get that value?
  // Should TargetTransformInfo::getRegisterBitWidth be used or anything else?
  // PrefAlign = std::min(DL.getPrefTypeAlign(LargestType), PrefAlign);
 
  int64_t ToRemoveStart = 0;
  uint64_t ToRemoveSize = 0;
  // Compute start and size of the region to remove. Make sure 'PrefAlign' is
  // maintained on the remaining store.
  if (IsOverwriteEnd) {
    // Calculate required adjustment for 'KillingStart' in order to keep
    // remaining store size aligned on 'PerfAlign'.
    uint64_t Off =
        offsetToAlignment(uint64_t(KillingStart - DeadStart), PrefAlign);
    ToRemoveStart = KillingStart + Off;
    if (DeadSize <= uint64_t(ToRemoveStart - DeadStart))
      return false;
    ToRemoveSize = DeadSize - uint64_t(ToRemoveStart - DeadStart);
  } else {
    ToRemoveStart = DeadStart;
    assert(KillingSize >= uint64_t(DeadStart - KillingStart) &&
           "Not overlapping accesses?");
    ToRemoveSize = KillingSize - uint64_t(DeadStart - KillingStart);
    // Calculate required adjustment for 'ToRemoveSize'in order to keep
    // start of the remaining store aligned on 'PerfAlign'.
    uint64_t Off = offsetToAlignment(ToRemoveSize, PrefAlign);
    if (Off != 0) {
      if (ToRemoveSize <= (PrefAlign.value() - Off))
        return false;
      ToRemoveSize -= PrefAlign.value() - Off;
    }
    assert(isAligned(PrefAlign, ToRemoveSize) &&
           "Should preserve selected alignment");
  }
 
  assert(ToRemoveSize > 0 && "Shouldn't reach here if nothing to remove");
  assert(DeadSize > ToRemoveSize && "Can't remove more than original size");
 
  uint64_t NewSize = DeadSize - ToRemoveSize;
  if (DeadIntrinsic->isAtomic()) {
    // When shortening an atomic memory intrinsic, the newly shortened
    // length must remain an integer multiple of the element size.
    const uint32_t ElementSize = DeadIntrinsic->getElementSizeInBytes();
    if (0 != NewSize % ElementSize)
      return false;
  }
 
  LLVM_DEBUG(dbgs() << "DSE: Remove Dead Store:\n  OW "
                    << (IsOverwriteEnd ? "END" : "BEGIN") << ": " << *DeadI
                    << "\n  KILLER [" << ToRemoveStart << ", "
                    << int64_t(ToRemoveStart + ToRemoveSize) << ")\n");
 
  DeadIntrinsic->setLength(NewSize);
  DeadIntrinsic->setDestAlignment(PrefAlign);
 
  Value *OrigDest = DeadIntrinsic->getRawDest();
  if (!IsOverwriteEnd) {
    Value *Indices[1] = {
        ConstantInt::get(DeadIntrinsic->getLength()->getType(), ToRemoveSize)};
    Instruction *NewDestGEP = GetElementPtrInst::CreateInBounds(
        Type::getInt8Ty(DeadIntrinsic->getContext()), OrigDest, Indices, "",
        DeadI->getIterator());
    NewDestGEP->setDebugLoc(DeadIntrinsic->getDebugLoc());
    DeadIntrinsic->setDest(NewDestGEP);
    adjustArgAttributes(DeadIntrinsic, 0, ToRemoveSize);
  }
 
  // Update attached dbg.assign intrinsics. Assume 8-bit byte.
  shortenAssignment(DeadI, OrigDest, DeadSize * 8, NewSize * 8, IsOverwriteEnd);
 
  // Finally update start and size of dead access.
  if (!IsOverwriteEnd)
    DeadStart += ToRemoveSize;
  DeadSize = NewSize;
 
  return true;
}
 
static bool tryToShortenEnd(Instruction *DeadI, OverlapIntervalsTy &IntervalMap,
                            int64_t &DeadStart, uint64_t &DeadSize) {
  if (IntervalMap.empty() || !isShortenableAtTheEnd(DeadI))
    return false;
 
  OverlapIntervalsTy::iterator OII = --IntervalMap.end();
  int64_t KillingStart = OII->second;
  uint64_t KillingSize = OII->first - KillingStart;
 
  assert(OII->first - KillingStart >= 0 && "Size expected to be positive");
 
  if (KillingStart > DeadStart &&
      // Note: "KillingStart - KillingStart" is known to be positive due to
      // preceding check.
      (uint64_t)(KillingStart - DeadStart) < DeadSize &&
      // Note: "DeadSize - (uint64_t)(KillingStart - DeadStart)" is known to
      // be non negative due to preceding checks.
      KillingSize >= DeadSize - (uint64_t)(KillingStart - DeadStart)) {
    if (tryToShorten(DeadI, DeadStart, DeadSize, KillingStart, KillingSize,
                     true)) {
      IntervalMap.erase(OII);
      return true;
    }
  }
  return false;
}
 
static bool tryToShortenBegin(Instruction *DeadI,
                              OverlapIntervalsTy &IntervalMap,
                              int64_t &DeadStart, uint64_t &DeadSize) {
  if (IntervalMap.empty() || !isShortenableAtTheBeginning(DeadI))
    return false;
 
  OverlapIntervalsTy::iterator OII = IntervalMap.begin();
  int64_t KillingStart = OII->second;
  uint64_t KillingSize = OII->first - KillingStart;
 
  assert(OII->first - KillingStart >= 0 && "Size expected to be positive");
 
  if (KillingStart <= DeadStart &&
      // Note: "DeadStart - KillingStart" is known to be non negative due to
      // preceding check.
      KillingSize > (uint64_t)(DeadStart - KillingStart)) {
    // Note: "KillingSize - (uint64_t)(DeadStart - DeadStart)" is known to
    // be positive due to preceding checks.
    assert(KillingSize - (uint64_t)(DeadStart - KillingStart) < DeadSize &&
           "Should have been handled as OW_Complete");
    if (tryToShorten(DeadI, DeadStart, DeadSize, KillingStart, KillingSize,
                     false)) {
      IntervalMap.erase(OII);
      return true;
    }
  }
  return false;
}
 
static Constant *
tryToMergePartialOverlappingStores(StoreInst *KillingI, StoreInst *DeadI,
                                   int64_t KillingOffset, int64_t DeadOffset,
                                   const DataLayout &DL, BatchAAResults &AA,
                                   DominatorTree *DT) {
  assert(KillingI);
  assert(DeadI);
 
  // If the store we find is:
  //   a) partially overwritten by the store to 'Loc'
  //   b) the killing store is fully contained in the dead one and
  //   c) they both have a constant value
  //   d) none of the two stores need padding
  // Merge the two stores, replacing the dead store's value with a
  // merge of both values.
  //
  // TODO: Deal with other constant types (vectors, etc), and probably
  // some mem intrinsics (if needed)
  if (!isa<ConstantInt>(DeadI->getValueOperand()) ||
      !DL.typeSizeEqualsStoreSize(DeadI->getValueOperand()->getType()) ||
      !isa<ConstantInt>(KillingI->getValueOperand()) ||
      !DL.typeSizeEqualsStoreSize(KillingI->getValueOperand()->getType()) ||
      !memoryIsNotModifiedBetween(DeadI, KillingI, AA, DL, DT))
    return nullptr;
 
  // The merge erases KillingI and writes its bytes via DeadI. For that to be
  // safe:
  //   - KillingI must be deletable (not volatile, ordering at most unordered),
  //   - DeadI must be safe to rewrite, and
  //   - their orderings must match, so the bytes originally written by
  //     KillingI keep the same atomicity after they are folded into DeadI.
  // This allows merging two simple stores or two unordered-atomic stores with
  // matching ordering, while leaving volatile and ordered-atomic stores in
  // place.
  if (!KillingI->isUnordered() || !DeadI->isUnordered() ||
      KillingI->getOrdering() != DeadI->getOrdering())
    return nullptr;
 
  APInt DeadValue = cast<ConstantInt>(DeadI->getValueOperand())->getValue();
  APInt KillingValue =
      cast<ConstantInt>(KillingI->getValueOperand())->getValue();
  unsigned KillingBits = KillingValue.getBitWidth();
  assert(DeadValue.getBitWidth() > KillingValue.getBitWidth());
  KillingValue = KillingValue.zext(DeadValue.getBitWidth());
 
  // Offset of the smaller store inside the larger store
  unsigned BitOffsetDiff = (KillingOffset - DeadOffset) * 8;
  unsigned LShiftAmount =
      DL.isBigEndian() ? DeadValue.getBitWidth() - BitOffsetDiff - KillingBits
                       : BitOffsetDiff;
  APInt Mask = APInt::getBitsSet(DeadValue.getBitWidth(), LShiftAmount,
                                 LShiftAmount + KillingBits);
  // Clear the bits we'll be replacing, then OR with the smaller
  // store, shifted appropriately.
  APInt Merged = (DeadValue & ~Mask) | (KillingValue << LShiftAmount);
  LLVM_DEBUG(dbgs() << "DSE: Merge Stores:\n  Dead: " << *DeadI
                    << "\n  Killing: " << *KillingI
                    << "\n  Merged Value: " << Merged << '\n');
  return ConstantInt::get(DeadI->getValueOperand()->getType(), Merged);
}
 
// Returns true if \p I is an intrinsic that does not read or write memory.
static bool isNoopIntrinsic(Instruction *I) {
  if (const IntrinsicInst *II = dyn_cast<IntrinsicInst>(I)) {
    switch (II->getIntrinsicID()) {
    case Intrinsic::lifetime_start:
    case Intrinsic::lifetime_end:
    case Intrinsic::invariant_end:
    case Intrinsic::launder_invariant_group:
    case Intrinsic::assume:
      return true;
    case Intrinsic::dbg_declare:
    case Intrinsic::dbg_label:
    case Intrinsic::dbg_value:
      llvm_unreachable("Intrinsic should not be modeled in MemorySSA");
    default:
      return false;
    }
  }
  return false;
}
 
// Check if we can ignore \p D for DSE.
static bool canSkipDef(MemoryDef *D, bool DefVisibleToCaller) {
  Instruction *DI = D->getMemoryInst();
  // Calls that only access inaccessible memory cannot read or write any memory
  // locations we consider for elimination.
  if (auto *CB = dyn_cast<CallBase>(DI))
    if (CB->onlyAccessesInaccessibleMemory())
      return true;
 
  // We can eliminate stores to locations not visible to the caller across
  // throwing instructions.
  if (DI->mayThrow() && !DefVisibleToCaller)
    return true;
 
  // We can remove the dead stores, irrespective of the fence and its ordering
  // (release/acquire/seq_cst). Fences only constraints the ordering of
  // already visible stores, it does not make a store visible to other
  // threads. So, skipping over a fence does not change a store from being
  // dead.
  if (isa<FenceInst>(DI))
    return true;
 
  // Skip intrinsics that do not really read or modify memory.
  if (isNoopIntrinsic(DI))
    return true;
 
  return false;
}
 
namespace {
 
// A memory location wrapper that represents a MemoryLocation, `MemLoc`,
// defined by `MemDef`.
struct MemoryLocationWrapper {
  MemoryLocationWrapper(MemoryLocation MemLoc, MemoryDef *MemDef,
                        bool DefByInitializesAttr)
      : MemLoc(MemLoc), MemDef(MemDef),
        DefByInitializesAttr(DefByInitializesAttr) {
    assert(MemLoc.Ptr && "MemLoc should be not null");
    UnderlyingObject = getUnderlyingObject(MemLoc.Ptr);
    DefInst = MemDef->getMemoryInst();
  }
 
  MemoryLocation MemLoc;
  const Value *UnderlyingObject;
  MemoryDef *MemDef;
  Instruction *DefInst;
  bool DefByInitializesAttr = false;
};
 
// A memory def wrapper that represents a MemoryDef and the MemoryLocation(s)
// defined by this MemoryDef.
struct MemoryDefWrapper {
  MemoryDefWrapper(MemoryDef *MemDef,
                   ArrayRef<std::pair<MemoryLocation, bool>> MemLocations) {
    DefInst = MemDef->getMemoryInst();
    for (auto &[MemLoc, DefByInitializesAttr] : MemLocations)
      DefinedLocations.push_back(
          MemoryLocationWrapper(MemLoc, MemDef, DefByInitializesAttr));
  }
  Instruction *DefInst;
  SmallVector<MemoryLocationWrapper, 1> DefinedLocations;
};
 
struct ArgumentInitInfo {
  unsigned Idx;
  bool IsDeadOrInvisibleOnUnwind;
  ConstantRangeList Inits;
};
} // namespace
 
static bool hasInitializesAttr(Instruction *I) {
  CallBase *CB = dyn_cast<CallBase>(I);
  return CB && CB->getArgOperandWithAttribute(Attribute::Initializes);
}
 
// Return the intersected range list of the initializes attributes of "Args".
// "Args" are call arguments that alias to each other.
// If any argument in "Args" doesn't have dead_on_unwind attr and
// "CallHasNoUnwindAttr" is false, return empty.
static ConstantRangeList
getIntersectedInitRangeList(ArrayRef<ArgumentInitInfo> Args,
                            bool CallHasNoUnwindAttr) {
  if (Args.empty())
    return {};
 
  // To address unwind, the function should have nounwind attribute or the
  // arguments have dead or invisible on unwind. Otherwise, return empty.
  for (const auto &Arg : Args) {
    if (!CallHasNoUnwindAttr && !Arg.IsDeadOrInvisibleOnUnwind)
      return {};
    if (Arg.Inits.empty())
      return {};
  }
 
  ConstantRangeList IntersectedIntervals = Args.front().Inits;
  for (auto &Arg : Args.drop_front())
    IntersectedIntervals = IntersectedIntervals.intersectWith(Arg.Inits);
 
  return IntersectedIntervals;
}
 
namespace {
 
struct DSEState {
  Function &F;
  AliasAnalysis &AA;
  EarliestEscapeAnalysis EA;
 
  /// The single BatchAA instance that is used to cache AA queries. It will
  /// not be invalidated over the whole run. This is safe, because:
  /// 1. Only memory writes are removed, so the alias cache for memory
  ///    locations remains valid.
  /// 2. No new instructions are added (only instructions removed), so cached
  ///    information for a deleted value cannot be accessed by a re-used new
  ///    value pointer.
  BatchAAResults BatchAA;
 
  MemorySSA &MSSA;
  DominatorTree &DT;
  PostDominatorTree &PDT;
  const TargetLibraryInfo &TLI;
  const DataLayout &DL;
  const CycleInfo &CI;
 
  // All MemoryDefs that potentially could kill other MemDefs.
  SmallVector<MemoryDef *, 64> MemDefs;
  // Any that should be skipped as they are already deleted
  SmallPtrSet<MemoryAccess *, 4> SkipStores;
  // Keep track whether a given object is captured before return or not.
  DenseMap<const Value *, bool> CapturedBeforeReturn;
  // Keep track of all of the objects that are invisible to the caller after
  // the function returns.
  DenseMap<const Value *, bool> InvisibleToCallerAfterRet;
  DenseMap<const Value *, uint64_t> InvisibleToCallerAfterRetBounded;
  // Keep track of blocks with throwing instructions not modeled in MemorySSA.
  SmallPtrSet<BasicBlock *, 16> ThrowingBlocks;
  // Post-order numbers for each basic block. Used to figure out if memory
  // accesses are executed before another access.
  DenseMap<BasicBlock *, unsigned> PostOrderNumbers;
 
  /// Keep track of instructions (partly) overlapping with killing MemoryDefs per
  /// basic block.
  MapVector<BasicBlock *, InstOverlapIntervalsTy> IOLs;
  // Check if there are root nodes that are terminated by UnreachableInst.
  // Those roots pessimize post-dominance queries. If there are such roots,
  // fall back to CFG scan starting from all non-unreachable roots.
  bool AnyUnreachableExit;
 
  // Whether or not we should iterate on removing dead stores at the end of the
  // function due to removing a store causing a previously captured pointer to
  // no longer be captured.
  bool ShouldIterateEndOfFunctionDSE;
 
  /// Dead instructions to be removed at the end of DSE.
  SmallVector<Instruction *> ToRemove;
 
  // Class contains self-reference, make sure it's not copied/moved.
  DSEState(Function &F, AliasAnalysis &AA, MemorySSA &MSSA, DominatorTree &DT,
           PostDominatorTree &PDT, const TargetLibraryInfo &TLI,
           const CycleInfo &CI);
  DSEState(const DSEState &) = delete;
  DSEState &operator=(const DSEState &) = delete;
 
  LocationSize strengthenLocationSize(const Instruction *I,
                                      LocationSize Size) const;
 
  /// Return 'OW_Complete' if a store to the 'KillingLoc' location (by \p
  /// KillingI instruction) completely overwrites a store to the 'DeadLoc'
  /// location (by \p DeadI instruction).
  /// Return OW_MaybePartial if \p KillingI does not completely overwrite
  /// \p DeadI, but they both write to the same underlying object. In that
  /// case, use isPartialOverwrite to check if \p KillingI partially overwrites
  /// \p DeadI. Returns 'OR_None' if \p KillingI is known to not overwrite the
  /// \p DeadI. Returns 'OW_Unknown' if nothing can be determined.
  OverwriteResult isOverwrite(const Instruction *KillingI,
                              const Instruction *DeadI,
                              const MemoryLocation &KillingLoc,
                              const MemoryLocation &DeadLoc,
                              int64_t &KillingOff, int64_t &DeadOff);
 
  bool isInvisibleToCallerAfterRet(const Value *V, const Value *Ptr,
                                   const LocationSize StoreSize);
 
  bool isInvisibleToCallerOnUnwind(const Value *V);
 
  std::optional<MemoryLocation> getLocForWrite(Instruction *I) const;
 
  // Returns a list of <MemoryLocation, bool> pairs written by I.
  // The bool means whether the write is from Initializes attr.
  SmallVector<std::pair<MemoryLocation, bool>, 1>
  getLocForInst(Instruction *I, bool ConsiderInitializesAttr);
 
  /// Assuming this instruction has a dead analyzable write, can we delete
  /// this instruction?
  bool isRemovable(Instruction *I);
 
  /// Returns true if \p UseInst completely overwrites \p DefLoc
  /// (stored by \p DefInst).
  bool isCompleteOverwrite(const MemoryLocation &DefLoc, Instruction *DefInst,
                           Instruction *UseInst);
 
  /// Returns true if \p Def is not read before returning from the function.
  bool isWriteAtEndOfFunction(MemoryDef *Def, const MemoryLocation &DefLoc);
 
  /// If \p I is a memory  terminator like llvm.lifetime.end or free, return a
  /// pair with the MemoryLocation terminated by \p I and a boolean flag
  /// indicating whether \p I is a free-like call.
  std::optional<std::pair<MemoryLocation, bool>>
  getLocForTerminator(Instruction *I) const;
 
  /// Returns true if \p I is a memory terminator instruction like
  /// llvm.lifetime.end or free.
  bool isMemTerminatorInst(Instruction *I) const;
 
  /// Returns true if \p MaybeTerm is a memory terminator for \p Loc from
  /// instruction \p AccessI.
  bool isMemTerminator(const MemoryLocation &Loc, Instruction *AccessI,
                       Instruction *MaybeTerm);
 
  // Returns true if \p Use may read from \p DefLoc.
  bool isReadClobber(const MemoryLocation &DefLoc, Instruction *UseInst);
 
  /// Returns true if a dependency between \p Current and \p KillingDef is
  /// guaranteed to be loop invariant for the loops that they are in. Either
  /// because they are known to be in the same block, in the same loop level or
  /// by guaranteeing that \p CurrentLoc only references a single MemoryLocation
  /// during execution of the containing function.
  bool isGuaranteedLoopIndependent(const Instruction *Current,
                                   const Instruction *KillingDef,
                                   const MemoryLocation &CurrentLoc);
 
  /// Returns true if \p Ptr is guaranteed to be loop invariant for any possible
  /// loop. In particular, this guarantees that it only references a single
  /// MemoryLocation during execution of the containing function.
  bool isGuaranteedLoopInvariant(const Value *Ptr);
 
  // Find a MemoryDef writing to \p KillingLoc and dominating \p StartAccess,
  // with no read access between them or on any other path to a function exit
  // block if \p KillingLoc is not accessible after the function returns. If
  // there is no such MemoryDef, return std::nullopt. The returned value may not
  // (completely) overwrite \p KillingLoc. Currently we bail out when we
  // encounter an aliasing MemoryUse (read).
  std::optional<MemoryAccess *>
  getDomMemoryDef(MemoryDef *KillingDef, MemoryAccess *StartAccess,
                  const MemoryLocation &KillingLoc, const Value *KillingUndObj,
                  unsigned &ScanLimit, unsigned &WalkerStepLimit,
                  bool IsMemTerm, unsigned &PartialLimit,
                  bool IsInitializesAttrMemLoc);
 
  /// Delete dead memory defs and recursively add their operands to ToRemove if
  /// they became dead.
  void
  deleteDeadInstruction(Instruction *SI,
                        SmallPtrSetImpl<MemoryAccess *> *Deleted = nullptr);
 
  // Check for any extra throws between \p KillingI and \p DeadI that block
  // DSE.  This only checks extra maythrows (those that aren't MemoryDef's).
  // MemoryDef that may throw are handled during the walk from one def to the
  // next.
  bool mayThrowBetween(Instruction *KillingI, Instruction *DeadI,
                       const Value *KillingUndObj);
 
  // Check if \p DeadI acts as a DSE barrier for \p KillingI. The following
  // instructions act as barriers:
  //  * A memory instruction that may throw and \p KillingI accesses a non-stack
  //  object.
  //  * Atomic stores stronger that monotonic.
  bool isDSEBarrier(const Value *KillingUndObj, Instruction *DeadI);
 
  /// Eliminate writes to objects that are not visible in the caller and are not
  /// accessed before returning from the function.
  bool eliminateDeadWritesAtEndOfFunction();
 
  /// If we have a zero initializing memset following a call to malloc,
  /// try folding it into a call to calloc.
  bool tryFoldIntoCalloc(MemoryDef *Def, const Value *DefUO);
 
  /// \returns true if \p Def is a no-op store, either because it
  /// directly stores back a loaded value or stores zero to a calloced object.
  bool storeIsNoop(MemoryDef *Def, const Value *DefUO);
 
  bool removePartiallyOverlappedStores(InstOverlapIntervalsTy &IOL);
 
  /// Eliminates writes to locations where the value that is being written
  /// is already stored at the same location.
  bool eliminateRedundantStoresOfExistingValues();
 
  /// If there is a dominating condition that implies the value being stored in
  /// a pointer, and such a condition appears in a node that dominates the
  /// store, then the store may be redundant if no write occurs in between.
  bool eliminateRedundantStoresViaDominatingConditions();
 
  // Return the locations written by the initializes attribute.
  // Note that this function considers:
  // 1. Unwind edge: use "initializes" attribute only if the callee has
  //    "nounwind" attribute, or the argument has "dead_on_unwind" attribute,
  //    or the argument is invisible to caller on unwind. That is, we don't
  //    perform incorrect DSE on unwind edges in the current function.
  // 2. Argument alias: for aliasing arguments, the "initializes" attribute is
  //    the intersected range list of their "initializes" attributes.
  SmallVector<MemoryLocation, 1> getInitializesArgMemLoc(const Instruction *I);
 
  // Try to eliminate dead defs that access `KillingLocWrapper.MemLoc` and are
  // killed by `KillingLocWrapper.MemDef`. Return whether
  // any changes were made, and whether `KillingLocWrapper.DefInst` was deleted.
  std::pair<bool, bool>
  eliminateDeadDefs(const MemoryLocationWrapper &KillingLocWrapper);
 
  // Try to eliminate dead defs killed by `KillingDefWrapper` and return the
  // change state: whether make any change.
  bool eliminateDeadDefs(const MemoryDefWrapper &KillingDefWrapper);
};
 
} // end anonymous namespace
 
static void pushMemUses(MemoryAccess *Acc,
                        SmallVectorImpl<MemoryAccess *> &WorkList,
                        SmallPtrSetImpl<MemoryAccess *> &Visited) {
  for (Use &U : Acc->uses()) {
    auto *MA = cast<MemoryAccess>(U.getUser());
    if (Visited.insert(MA).second)
      WorkList.push_back(MA);
  }
}
 
// Return true if "Arg" is function local and isn't captured before "CB".
static bool isFuncLocalAndNotCaptured(Value *Arg, const CallBase *CB,
                                      EarliestEscapeAnalysis &EA) {
  const Value *UnderlyingObj = getUnderlyingObject(Arg);
  return isIdentifiedFunctionLocal(UnderlyingObj) &&
         capturesNothing(EA.getCapturesBefore(UnderlyingObj, CB, /*OrAt=*/true,
                                              /*ReturnCaptures=*/false));
}
 
DSEState::DSEState(Function &F, AliasAnalysis &AA, MemorySSA &MSSA,
                   DominatorTree &DT, PostDominatorTree &PDT,
                   const TargetLibraryInfo &TLI, const CycleInfo &CI)
    : F(F), AA(AA), EA(DT, nullptr, &CI), BatchAA(AA, &EA), MSSA(MSSA), DT(DT),
      PDT(PDT), TLI(TLI), DL(F.getDataLayout()), CI(CI) {
  // Collect blocks with throwing instructions not modeled in MemorySSA and
  // alloc-like objects.
  unsigned PO = 0;
  for (BasicBlock *BB : post_order(&F)) {
    PostOrderNumbers[BB] = PO++;
    for (Instruction &I : *BB) {
      MemoryAccess *MA = MSSA.getMemoryAccess(&I);
      if (I.mayThrow() && !MA)
        ThrowingBlocks.insert(I.getParent());
 
      auto *MD = dyn_cast_or_null<MemoryDef>(MA);
      if (MD && MemDefs.size() < MemorySSADefsPerBlockLimit &&
          (getLocForWrite(&I) || isMemTerminatorInst(&I) ||
           (EnableInitializesImprovement && hasInitializesAttr(&I))))
        MemDefs.push_back(MD);
    }
  }
 
  // Treat byval, inalloca or dead on return arguments the same as Allocas,
  // stores to them are dead at the end of the function.
  for (Argument &AI : F.args()) {
    if (AI.hasPassPointeeByValueCopyAttr()) {
      InvisibleToCallerAfterRet.insert({&AI, true});
      continue;
    }
 
    if (!AI.getType()->isPointerTy())
      continue;
 
    const DeadOnReturnInfo &Info = AI.getDeadOnReturnInfo();
    if (Info.coversAllReachableMemory())
      InvisibleToCallerAfterRet.insert({&AI, true});
    else if (uint64_t DeadBytes = Info.getNumberOfDeadBytes())
      InvisibleToCallerAfterRetBounded.insert({&AI, DeadBytes});
  }
 
  AnyUnreachableExit = any_of(PDT.roots(), [](const BasicBlock *E) {
    return isa<UnreachableInst>(E->getTerminator());
  });
}
 
LocationSize DSEState::strengthenLocationSize(const Instruction *I,
                                              LocationSize Size) const {
  if (auto *CB = dyn_cast<CallBase>(I)) {
    LibFunc F = TLI.getLibFunc(*CB);
    if (TLI.has(F) && (F == LibFunc_memset_chk || F == LibFunc_memcpy_chk)) {
      // Use the precise location size specified by the 3rd argument
      // for determining KillingI overwrites DeadLoc if it is a memset_chk
      // instruction. memset_chk will write either the amount specified as 3rd
      // argument or the function will immediately abort and exit the program.
      // NOTE: AA may determine NoAlias if it can prove that the access size
      // is larger than the allocation size due to that being UB. To avoid
      // returning potentially invalid NoAlias results by AA, limit the use of
      // the precise location size to isOverwrite.
      if (const auto *Len = dyn_cast<ConstantInt>(CB->getArgOperand(2)))
        return LocationSize::precise(Len->getZExtValue());
    }
  }
  return Size;
}
 
OverwriteResult DSEState::isOverwrite(const Instruction *KillingI,
                                      const Instruction *DeadI,
                                      const MemoryLocation &KillingLoc,
                                      const MemoryLocation &DeadLoc,
                                      int64_t &KillingOff, int64_t &DeadOff) {
  // AliasAnalysis does not always account for loops. Limit overwrite checks
  // to dependencies for which we can guarantee they are independent of any
  // loops they are in.
  if (!isGuaranteedLoopIndependent(DeadI, KillingI, DeadLoc))
    return OW_Unknown;
 
  LocationSize KillingLocSize =
      strengthenLocationSize(KillingI, KillingLoc.Size);
  const Value *DeadPtr = DeadLoc.Ptr->stripPointerCasts();
  const Value *KillingPtr = KillingLoc.Ptr->stripPointerCasts();
  const Value *DeadUndObj = getUnderlyingObject(DeadPtr);
  const Value *KillingUndObj = getUnderlyingObject(KillingPtr);
 
  // Check whether the killing store overwrites the whole object, in which
  // case the size/offset of the dead store does not matter.
  if (DeadUndObj == KillingUndObj && KillingLocSize.isPrecise() &&
      isIdentifiedObject(KillingUndObj)) {
    std::optional<TypeSize> KillingUndObjSize =
        getPointerSize(KillingUndObj, DL, TLI, &F);
    if (KillingUndObjSize && *KillingUndObjSize == KillingLocSize.getValue())
      return OW_Complete;
  }
 
  // FIXME: Vet that this works for size upper-bounds. Seems unlikely that we'll
  // get imprecise values here, though (except for unknown sizes).
  if (!KillingLocSize.isPrecise() || !DeadLoc.Size.isPrecise()) {
    // In case no constant size is known, try to an IR values for the number
    // of bytes written and check if they match.
    const auto *KillingMemI = dyn_cast<MemIntrinsic>(KillingI);
    const auto *DeadMemI = dyn_cast<MemIntrinsic>(DeadI);
    if (KillingMemI && DeadMemI) {
      const Value *KillingV = KillingMemI->getLength();
      const Value *DeadV = DeadMemI->getLength();
      if (KillingV == DeadV && BatchAA.isMustAlias(DeadLoc, KillingLoc))
        return OW_Complete;
    }
 
    // Masked stores have imprecise locations, but we can reason about them
    // to some extent.
    return isMaskedStoreOverwrite(KillingI, DeadI, BatchAA);
  }
 
  const TypeSize KillingSize = KillingLocSize.getValue();
  const TypeSize DeadSize = DeadLoc.Size.getValue();
  // Bail on doing Size comparison which depends on AA for now
  // TODO: Remove AnyScalable once Alias Analysis deal with scalable vectors
  const bool AnyScalable = DeadSize.isScalable() || KillingLocSize.isScalable();
 
  if (AnyScalable)
    return OW_Unknown;
  // Query the alias information
  AliasResult AAR = BatchAA.alias(KillingLoc, DeadLoc);
 
  // If the start pointers are the same, we just have to compare sizes to see if
  // the killing store was larger than the dead store.
  if (AAR == AliasResult::MustAlias) {
    // Make sure that the KillingSize size is >= the DeadSize size.
    if (KillingSize >= DeadSize)
      return OW_Complete;
  }
 
  // If we hit a partial alias we may have a full overwrite
  if (AAR == AliasResult::PartialAlias && AAR.hasOffset()) {
    int32_t Off = AAR.getOffset();
    if (Off >= 0 && (uint64_t)Off + DeadSize <= KillingSize)
      return OW_Complete;
  }
 
  // If we can't resolve the same pointers to the same object, then we can't
  // analyze them at all.
  if (DeadUndObj != KillingUndObj) {
    // Non aliasing stores to different objects don't overlap. Note that
    // if the killing store is known to overwrite whole object (out of
    // bounds access overwrites whole object as well) then it is assumed to
    // completely overwrite any store to the same object even if they don't
    // actually alias (see next check).
    if (AAR == AliasResult::NoAlias)
      return OW_None;
    return OW_Unknown;
  }
 
  // Okay, we have stores to two completely different pointers.  Try to
  // decompose the pointer into a "base + constant_offset" form.  If the base
  // pointers are equal, then we can reason about the two stores.
  DeadOff = 0;
  KillingOff = 0;
  const Value *DeadBasePtr =
      GetPointerBaseWithConstantOffset(DeadPtr, DeadOff, DL);
  const Value *KillingBasePtr =
      GetPointerBaseWithConstantOffset(KillingPtr, KillingOff, DL);
 
  // If the base pointers still differ, we have two completely different
  // stores.
  if (DeadBasePtr != KillingBasePtr)
    return OW_Unknown;
 
  // The killing access completely overlaps the dead store if and only if
  // both start and end of the dead one is "inside" the killing one:
  //    |<->|--dead--|<->|
  //    |-----killing------|
  // Accesses may overlap if and only if start of one of them is "inside"
  // another one:
  //    |<->|--dead--|<-------->|
  //    |-------killing--------|
  //           OR
  //    |-------dead-------|
  //    |<->|---killing---|<----->|
  //
  // We have to be careful here as *Off is signed while *.Size is unsigned.
 
  // Check if the dead access starts "not before" the killing one.
  if (DeadOff >= KillingOff) {
    // If the dead access ends "not after" the killing access then the
    // dead one is completely overwritten by the killing one.
    if (uint64_t(DeadOff - KillingOff) + DeadSize <= KillingSize)
      return OW_Complete;
    // If start of the dead access is "before" end of the killing access
    // then accesses overlap.
    else if ((uint64_t)(DeadOff - KillingOff) < KillingSize)
      return OW_MaybePartial;
  }
  // If start of the killing access is "before" end of the dead access then
  // accesses overlap.
  else if ((uint64_t)(KillingOff - DeadOff) < DeadSize) {
    return OW_MaybePartial;
  }
 
  // Can reach here only if accesses are known not to overlap.
  return OW_None;
}
 
bool DSEState::isInvisibleToCallerAfterRet(const Value *V, const Value *Ptr,
                                           const LocationSize StoreSize) {
  if (isa<AllocaInst>(V))
    return true;
 
  auto IBounded = InvisibleToCallerAfterRetBounded.find(V);
  if (IBounded != InvisibleToCallerAfterRetBounded.end()) {
    int64_t ValueOffset;
    [[maybe_unused]] const Value *BaseValue =
        GetPointerBaseWithConstantOffset(Ptr, ValueOffset, DL);
    // If we are not able to find a constant offset from the UO, we have to
    // pessimistically assume that the store writes to memory out of the
    // dead_on_return bounds.
    if (BaseValue != V)
      return false;
    // This store is only invisible after return if we are in bounds of the
    // range marked dead.
    if (StoreSize.hasValue() &&
        ValueOffset + StoreSize.getValue() <= IBounded->second &&
        ValueOffset >= 0)
      return true;
  }
  auto I = InvisibleToCallerAfterRet.insert({V, false});
  if (I.second && isInvisibleToCallerOnUnwind(V) && isNoAliasCall(V))
    I.first->second = capturesNothing(
        PointerMayBeCaptured(V, CaptureComponents::Provenance).WithRet);
  return I.first->second;
}
 
bool DSEState::isInvisibleToCallerOnUnwind(const Value *V) {
  bool RequiresNoCaptureBeforeUnwind;
  if (!isNotVisibleOnUnwind(V, RequiresNoCaptureBeforeUnwind))
    return false;
  if (!RequiresNoCaptureBeforeUnwind)
    return true;
 
  auto I = CapturedBeforeReturn.insert({V, true});
  if (I.second)
    // NOTE: This could be made more precise by PointerMayBeCapturedBefore
    // with the killing MemoryDef. But we refrain from doing so for now to
    // limit compile-time and this does not cause any changes to the number
    // of stores removed on a large test set in practice.
    I.first->second = capturesAnything(
        PointerMayBeCaptured(V, CaptureComponents::Provenance).WithoutRet);
  return !I.first->second;
}
 
std::optional<MemoryLocation> DSEState::getLocForWrite(Instruction *I) const {
  if (!I->mayWriteToMemory())
    return std::nullopt;
 
  if (auto *CB = dyn_cast<CallBase>(I))
    return MemoryLocation::getForDest(CB, TLI);
 
  return MemoryLocation::getOrNone(I);
}
 
SmallVector<std::pair<MemoryLocation, bool>, 1>
DSEState::getLocForInst(Instruction *I, bool ConsiderInitializesAttr) {
  SmallVector<std::pair<MemoryLocation, bool>, 1> Locations;
  if (isMemTerminatorInst(I)) {
    if (auto Loc = getLocForTerminator(I))
      Locations.push_back(std::make_pair(Loc->first, false));
    return Locations;
  }
 
  if (auto Loc = getLocForWrite(I))
    Locations.push_back(std::make_pair(*Loc, false));
 
  if (ConsiderInitializesAttr) {
    for (auto &MemLoc : getInitializesArgMemLoc(I)) {
      Locations.push_back(std::make_pair(MemLoc, true));
    }
  }
  return Locations;
}
 
bool DSEState::isRemovable(Instruction *I) {
  assert(getLocForWrite(I) && "Must have analyzable write");
 
  // Don't remove volatile/atomic stores.
  if (StoreInst *SI = dyn_cast<StoreInst>(I))
    return SI->isUnordered();
 
  if (auto *CB = dyn_cast<CallBase>(I)) {
    // Don't remove volatile memory intrinsics.
    if (auto *MI = dyn_cast<MemIntrinsic>(CB))
      return !MI->isVolatile();
 
    // Never remove dead lifetime intrinsics, e.g. because they are followed
    // by a free.
    if (CB->isLifetimeStartOrEnd())
      return false;
 
    return CB->use_empty() && CB->willReturn() && CB->doesNotThrow() &&
           !CB->isTerminator();
  }
 
  return false;
}
 
bool DSEState::isCompleteOverwrite(const MemoryLocation &DefLoc,
                                   Instruction *DefInst, Instruction *UseInst) {
  // UseInst has a MemoryDef associated in MemorySSA. It's possible for a
  // MemoryDef to not write to memory, e.g. a volatile load is modeled as a
  // MemoryDef.
  if (!UseInst->mayWriteToMemory())
    return false;
 
  if (auto *CB = dyn_cast<CallBase>(UseInst))
    if (CB->onlyAccessesInaccessibleMemory())
      return false;
 
  int64_t InstWriteOffset, DepWriteOffset;
  if (auto CC = getLocForWrite(UseInst))
    return isOverwrite(UseInst, DefInst, *CC, DefLoc, InstWriteOffset,
                       DepWriteOffset) == OW_Complete;
  return false;
}
 
bool DSEState::isWriteAtEndOfFunction(MemoryDef *Def,
                                      const MemoryLocation &DefLoc) {
  LLVM_DEBUG(dbgs() << "  Check if def " << *Def << " ("
                    << *Def->getMemoryInst()
                    << ") is at the end the function \n");
  SmallVector<MemoryAccess *, 4> WorkList;
  SmallPtrSet<MemoryAccess *, 8> Visited;
 
  pushMemUses(Def, WorkList, Visited);
  for (unsigned I = 0; I < WorkList.size(); I++) {
    if (WorkList.size() >= MemorySSAScanLimit) {
      LLVM_DEBUG(dbgs() << "  ... hit exploration limit.\n");
      return false;
    }
 
    MemoryAccess *UseAccess = WorkList[I];
    if (isa<MemoryPhi>(UseAccess)) {
      // AliasAnalysis does not account for loops. Limit elimination to
      // candidates for which we can guarantee they always store to the same
      // memory location.
      if (!isGuaranteedLoopInvariant(DefLoc.Ptr))
        return false;
 
      pushMemUses(cast<MemoryPhi>(UseAccess), WorkList, Visited);
      continue;
    }
    // TODO: Checking for aliasing is expensive. Consider reducing the amount
    // of times this is called and/or caching it.
    Instruction *UseInst = cast<MemoryUseOrDef>(UseAccess)->getMemoryInst();
    if (isReadClobber(DefLoc, UseInst)) {
      LLVM_DEBUG(dbgs() << "  ... hit read clobber " << *UseInst << ".\n");
      return false;
    }
 
    if (MemoryDef *UseDef = dyn_cast<MemoryDef>(UseAccess))
      pushMemUses(UseDef, WorkList, Visited);
  }
  return true;
}
 
std::optional<std::pair<MemoryLocation, bool>>
DSEState::getLocForTerminator(Instruction *I) const {
  if (auto *CB = dyn_cast<CallBase>(I)) {
    if (CB->getIntrinsicID() == Intrinsic::lifetime_end)
      return {
          std::make_pair(MemoryLocation::getForArgument(CB, 0, &TLI), false)};
    if (Value *FreedOp = getFreedOperand(CB, &TLI))
      return {std::make_pair(MemoryLocation::getAfter(FreedOp), true)};
  }
 
  return std::nullopt;
}
 
bool DSEState::isMemTerminatorInst(Instruction *I) const {
  auto *CB = dyn_cast<CallBase>(I);
  return CB && (CB->getIntrinsicID() == Intrinsic::lifetime_end ||
                getFreedOperand(CB, &TLI) != nullptr);
}
 
bool DSEState::isMemTerminator(const MemoryLocation &Loc, Instruction *AccessI,
                               Instruction *MaybeTerm) {
  std::optional<std::pair<MemoryLocation, bool>> MaybeTermLoc =
      getLocForTerminator(MaybeTerm);
 
  if (!MaybeTermLoc)
    return false;
 
  // If the terminator is a free-like call, all accesses to the underlying
  // object can be considered terminated.
  if (getUnderlyingObject(Loc.Ptr) !=
      getUnderlyingObject(MaybeTermLoc->first.Ptr))
    return false;
 
  auto TermLoc = MaybeTermLoc->first;
  if (MaybeTermLoc->second) {
    const Value *LocUO = getUnderlyingObject(Loc.Ptr);
    return BatchAA.isMustAlias(TermLoc.Ptr, LocUO);
  }
  int64_t InstWriteOffset = 0;
  int64_t DepWriteOffset = 0;
  return isOverwrite(MaybeTerm, AccessI, TermLoc, Loc, InstWriteOffset,
                     DepWriteOffset) == OW_Complete;
}
 
bool DSEState::isReadClobber(const MemoryLocation &DefLoc,
                             Instruction *UseInst) {
  if (isNoopIntrinsic(UseInst))
    return false;
 
  // Monotonic or weaker atomic stores can be re-ordered and do not need to be
  // treated as read clobber.
  if (auto SI = dyn_cast<StoreInst>(UseInst))
    return isStrongerThan(SI->getOrdering(), AtomicOrdering::Monotonic);
 
  if (!UseInst->mayReadFromMemory())
    return false;
 
  if (auto *CB = dyn_cast<CallBase>(UseInst))
    if (CB->onlyAccessesInaccessibleMemory())
      return false;
 
  return isRefSet(BatchAA.getModRefInfo(UseInst, DefLoc));
}
 
bool DSEState::isGuaranteedLoopIndependent(const Instruction *Current,
                                           const Instruction *KillingDef,
                                           const MemoryLocation &CurrentLoc) {
  // If the dependency is within the same block or loop level (being careful
  // of irreducible loops), we know that AA will return a valid result for the
  // memory dependency. (Both at the function level, outside of any loop,
  // would also be valid but we currently disable that to limit compile time).
  if (Current->getParent() == KillingDef->getParent())
    return true;
  CycleRef CurrentC = CI.getCycle(Current->getParent());
  if (CurrentC && CurrentC == CI.getCycle(KillingDef->getParent()))
    return true;
  // Otherwise check the memory location is invariant to any loops.
  return isGuaranteedLoopInvariant(CurrentLoc.Ptr);
}
 
bool DSEState::isGuaranteedLoopInvariant(const Value *Ptr) {
  Ptr = Ptr->stripPointerCasts();
  if (auto *GEP = dyn_cast<GEPOperator>(Ptr))
    if (GEP->hasAllConstantIndices())
      Ptr = GEP->getPointerOperand()->stripPointerCasts();
 
  if (auto *I = dyn_cast<Instruction>(Ptr)) {
    return I->getParent()->isEntryBlock() || !CI.getCycle(I->getParent());
  }
  return true;
}
 
std::optional<MemoryAccess *> DSEState::getDomMemoryDef(
    MemoryDef *KillingDef, MemoryAccess *StartAccess,
    const MemoryLocation &KillingLoc, const Value *KillingUndObj,
    unsigned &ScanLimit, unsigned &WalkerStepLimit, bool IsMemTerm,
    unsigned &PartialLimit, bool IsInitializesAttrMemLoc) {
  if (ScanLimit == 0 || WalkerStepLimit == 0) {
    LLVM_DEBUG(dbgs() << "\n    ...  hit scan limit\n");
    return std::nullopt;
  }
 
  MemoryAccess *Current = StartAccess;
  Instruction *KillingI = KillingDef->getMemoryInst();
  LLVM_DEBUG(dbgs() << "  trying to get dominating access\n");
 
  // Only optimize defining access of KillingDef when directly starting at its
  // defining access. The defining access also must only access KillingLoc. At
  // the moment we only support instructions with a single write location, so
  // it should be sufficient to disable optimizations for instructions that
  // also read from memory.
  bool CanOptimize = OptimizeMemorySSA &&
                     KillingDef->getDefiningAccess() == StartAccess &&
                     !KillingI->mayReadFromMemory();
 
  // Find the next clobbering Mod access for DefLoc, starting at StartAccess.
  std::optional<MemoryLocation> CurrentLoc;
  for (;; Current = cast<MemoryDef>(Current)->getDefiningAccess()) {
    LLVM_DEBUG({
      dbgs() << "   visiting " << *Current;
      if (!MSSA.isLiveOnEntryDef(Current) && isa<MemoryUseOrDef>(Current))
        dbgs() << " (" << *cast<MemoryUseOrDef>(Current)->getMemoryInst()
               << ")";
      dbgs() << "\n";
    });
 
    // Reached TOP.
    if (MSSA.isLiveOnEntryDef(Current)) {
      LLVM_DEBUG(dbgs() << "   ...  found LiveOnEntryDef\n");
      if (CanOptimize && Current != KillingDef->getDefiningAccess())
        // The first clobbering def is... none.
        KillingDef->setOptimized(Current);
      return std::nullopt;
    }
 
    // Cost of a step. Accesses in the same block are more likely to be valid
    // candidates for elimination, hence consider them cheaper.
    unsigned StepCost = KillingDef->getBlock() == Current->getBlock()
                            ? MemorySSASameBBStepCost
                            : MemorySSAOtherBBStepCost;
    if (WalkerStepLimit <= StepCost) {
      LLVM_DEBUG(dbgs() << "   ...  hit walker step limit\n");
      return std::nullopt;
    }
    WalkerStepLimit -= StepCost;
 
    // Return for MemoryPhis. They cannot be eliminated directly and the
    // caller is responsible for traversing them.
    if (isa<MemoryPhi>(Current)) {
      LLVM_DEBUG(dbgs() << "   ...  found MemoryPhi\n");
      return Current;
    }
 
    // Below, check if CurrentDef is a valid candidate to be eliminated by
    // KillingDef. If it is not, check the next candidate.
    MemoryDef *CurrentDef = cast<MemoryDef>(Current);
    Instruction *CurrentI = CurrentDef->getMemoryInst();
 
    if (canSkipDef(CurrentDef, !isInvisibleToCallerOnUnwind(KillingUndObj))) {
      CanOptimize = false;
      continue;
    }
 
    // Before we try to remove anything, check for any extra throwing
    // instructions that block us from DSEing
    if (mayThrowBetween(KillingI, CurrentI, KillingUndObj)) {
      LLVM_DEBUG(dbgs() << "  ... skip, may throw!\n");
      return std::nullopt;
    }
 
    // Check for anything that looks like it will be a barrier to further
    // removal
    if (isDSEBarrier(KillingUndObj, CurrentI)) {
      LLVM_DEBUG(dbgs() << "  ... skip, barrier\n");
      return std::nullopt;
    }
 
    // If Current is known to be on path that reads DefLoc or is a read
    // clobber, bail out, as the path is not profitable. We skip this check
    // for intrinsic calls, because the code knows how to handle memcpy
    // intrinsics.
    if (!isa<IntrinsicInst>(CurrentI) && isReadClobber(KillingLoc, CurrentI))
      return std::nullopt;
 
    // Quick check if there are direct uses that are read-clobbers.
    if (any_of(Current->uses(), [this, &KillingLoc, StartAccess](Use &U) {
          if (auto *UseOrDef = dyn_cast<MemoryUseOrDef>(U.getUser()))
            return !MSSA.dominates(StartAccess, UseOrDef) &&
                   isReadClobber(KillingLoc, UseOrDef->getMemoryInst());
          return false;
        })) {
      LLVM_DEBUG(dbgs() << "   ...  found a read clobber\n");
      return std::nullopt;
    }
 
    // If Current does not have an analyzable write location or is not
    // removable, skip it.
    CurrentLoc = getLocForWrite(CurrentI);
    if (!CurrentLoc || !isRemovable(CurrentI)) {
      CanOptimize = false;
      continue;
    }
 
    // AliasAnalysis does not account for loops. Limit elimination to
    // candidates for which we can guarantee they always store to the same
    // memory location and not located in different loops.
    if (!isGuaranteedLoopIndependent(CurrentI, KillingI, *CurrentLoc)) {
      LLVM_DEBUG(dbgs() << "  ... not guaranteed loop independent\n");
      CanOptimize = false;
      continue;
    }
 
    if (IsMemTerm) {
      // If the killing def is a memory terminator (e.g. lifetime.end), check
      // the next candidate if the current Current does not write the same
      // underlying object as the terminator.
      if (!isMemTerminator(*CurrentLoc, CurrentI, KillingI)) {
        CanOptimize = false;
        continue;
      }
    } else {
      int64_t KillingOffset = 0;
      int64_t DeadOffset = 0;
      auto OR = isOverwrite(KillingI, CurrentI, KillingLoc, *CurrentLoc,
                            KillingOffset, DeadOffset);
      if (CanOptimize) {
        // CurrentDef is the earliest write clobber of KillingDef. Use it as
        // optimized access. Do not optimize if CurrentDef is already the
        // defining access of KillingDef.
        if (CurrentDef != KillingDef->getDefiningAccess() &&
            (OR == OW_Complete || OR == OW_MaybePartial))
          KillingDef->setOptimized(CurrentDef);
 
        // Once a may-aliasing def is encountered do not set an optimized
        // access.
        if (OR != OW_None)
          CanOptimize = false;
      }
 
      // If Current does not write to the same object as KillingDef, check
      // the next candidate.
      if (OR == OW_Unknown || OR == OW_None)
        continue;
      else if (OR == OW_MaybePartial) {
        // If KillingDef only partially overwrites Current, check the next
        // candidate if the partial step limit is exceeded. This aggressively
        // limits the number of candidates for partial store elimination,
        // which are less likely to be removable in the end.
        if (PartialLimit <= 1) {
          WalkerStepLimit -= 1;
          LLVM_DEBUG(dbgs() << "   ... reached partial limit ... continue with "
                               "next access\n");
          continue;
        }
        PartialLimit -= 1;
      }
    }
    break;
  };
 
  // Accesses to objects accessible after the function returns can only be
  // eliminated if the access is dead along all paths to the exit. Collect
  // the blocks with killing (=completely overwriting MemoryDefs) and check if
  // they cover all paths from MaybeDeadAccess to any function exit.
  SmallPtrSet<Instruction *, 16> KillingDefs;
  KillingDefs.insert(KillingDef->getMemoryInst());
  MemoryAccess *MaybeDeadAccess = Current;
  MemoryLocation MaybeDeadLoc = *CurrentLoc;
  Instruction *MaybeDeadI = cast<MemoryDef>(MaybeDeadAccess)->getMemoryInst();
  LLVM_DEBUG(dbgs() << "  Checking for reads of " << *MaybeDeadAccess << " ("
                    << *MaybeDeadI << ")\n");
 
  SmallVector<MemoryAccess *, 32> WorkList;
  SmallPtrSet<MemoryAccess *, 32> Visited;
  pushMemUses(MaybeDeadAccess, WorkList, Visited);
 
  // Check if DeadDef may be read.
  for (unsigned I = 0; I < WorkList.size(); I++) {
    MemoryAccess *UseAccess = WorkList[I];
 
    LLVM_DEBUG(dbgs() << "   " << *UseAccess);
    // Bail out if the number of accesses to check exceeds the scan limit.
    if (ScanLimit < (WorkList.size() - I)) {
      LLVM_DEBUG(dbgs() << "\n    ...  hit scan limit\n");
      return std::nullopt;
    }
    --ScanLimit;
    NumDomMemDefChecks++;
 
    if (isa<MemoryPhi>(UseAccess)) {
      if (any_of(KillingDefs, [this, UseAccess](Instruction *KI) {
            return DT.properlyDominates(KI->getParent(), UseAccess->getBlock());
          })) {
        LLVM_DEBUG(dbgs() << " ... skipping, dominated by killing block\n");
        continue;
      }
      LLVM_DEBUG(dbgs() << "\n    ... adding PHI uses\n");
      pushMemUses(UseAccess, WorkList, Visited);
      continue;
    }
 
    Instruction *UseInst = cast<MemoryUseOrDef>(UseAccess)->getMemoryInst();
    LLVM_DEBUG(dbgs() << " (" << *UseInst << ")\n");
 
    if (any_of(KillingDefs, [this, UseInst](Instruction *KI) {
          return DT.dominates(KI, UseInst);
        })) {
      LLVM_DEBUG(dbgs() << " ... skipping, dominated by killing def\n");
      continue;
    }
 
    // A memory terminator kills all preceeding MemoryDefs and all succeeding
    // MemoryAccesses. We do not have to check it's users.
    if (isMemTerminator(MaybeDeadLoc, MaybeDeadI, UseInst)) {
      LLVM_DEBUG(
          dbgs()
          << " ... skipping, memterminator invalidates following accesses\n");
      continue;
    }
 
    if (isNoopIntrinsic(cast<MemoryUseOrDef>(UseAccess)->getMemoryInst())) {
      LLVM_DEBUG(dbgs() << "    ... adding uses of intrinsic\n");
      pushMemUses(UseAccess, WorkList, Visited);
      continue;
    }
 
    if (UseInst->mayThrow() && !isInvisibleToCallerOnUnwind(KillingUndObj)) {
      LLVM_DEBUG(dbgs() << "  ... found throwing instruction\n");
      return std::nullopt;
    }
 
    // Uses which may read the original MemoryDef mean we cannot eliminate the
    // original MD. Stop walk.
    // If KillingDef is a CallInst with "initializes" attribute, the reads in
    // the callee would be dominated by initializations, so it should be safe.
    bool IsKillingDefFromInitAttr = false;
    if (IsInitializesAttrMemLoc) {
      if (KillingI == UseInst &&
          KillingUndObj == getUnderlyingObject(MaybeDeadLoc.Ptr))
        IsKillingDefFromInitAttr = true;
    }
 
    if (isReadClobber(MaybeDeadLoc, UseInst) && !IsKillingDefFromInitAttr) {
      LLVM_DEBUG(dbgs() << "    ... found read clobber\n");
      return std::nullopt;
    }
 
    // If this worklist walks back to the original memory access (and the
    // pointer is not guarenteed loop invariant) then we cannot assume that a
    // store kills itself.
    if (MaybeDeadAccess == UseAccess &&
        !isGuaranteedLoopInvariant(MaybeDeadLoc.Ptr)) {
      LLVM_DEBUG(dbgs() << "    ... found not loop invariant self access\n");
      return std::nullopt;
    }
    // Otherwise, for the KillingDef and MaybeDeadAccess we only have to check
    // if it reads the memory location.
    // TODO: It would probably be better to check for self-reads before
    // calling the function.
    if (KillingDef == UseAccess || MaybeDeadAccess == UseAccess) {
      LLVM_DEBUG(dbgs() << "    ... skipping killing def/dom access\n");
      continue;
    }
 
    // Check all uses for MemoryDefs, except for defs completely overwriting
    // the original location. Otherwise we have to check uses of *all*
    // MemoryDefs we discover, including non-aliasing ones. Otherwise we might
    // miss cases like the following
    //   1 = Def(LoE) ; <----- DeadDef stores [0,1]
    //   2 = Def(1)   ; (2, 1) = NoAlias,   stores [2,3]
    //   Use(2)       ; MayAlias 2 *and* 1, loads [0, 3].
    //                  (The Use points to the *first* Def it may alias)
    //   3 = Def(1)   ; <---- Current  (3, 2) = NoAlias, (3,1) = MayAlias,
    //                  stores [0,1]
    if (MemoryDef *UseDef = dyn_cast<MemoryDef>(UseAccess)) {
      if (isCompleteOverwrite(MaybeDeadLoc, MaybeDeadI, UseInst)) {
        BasicBlock *MaybeKillingBlock = UseInst->getParent();
        if (PostOrderNumbers.find(MaybeKillingBlock)->second <
            PostOrderNumbers.find(MaybeDeadAccess->getBlock())->second) {
          if (!isInvisibleToCallerAfterRet(KillingUndObj, KillingLoc.Ptr,
                                           KillingLoc.Size)) {
            LLVM_DEBUG(dbgs()
                       << "    ... found killing def " << *UseInst << "\n");
            KillingDefs.insert(UseInst);
          }
        } else {
          LLVM_DEBUG(dbgs()
                     << "    ... found preceeding def " << *UseInst << "\n");
          return std::nullopt;
        }
      } else
        pushMemUses(UseDef, WorkList, Visited);
    }
  }
 
  // For accesses to locations visible after the function returns, make sure
  // that the location is dead (=overwritten) along all paths from
  // MaybeDeadAccess to the exit.
  if (!isInvisibleToCallerAfterRet(KillingUndObj, KillingLoc.Ptr,
                                   KillingLoc.Size)) {
    SmallPtrSet<BasicBlock *, 16> KillingBlocks;
    for (Instruction *KD : KillingDefs)
      KillingBlocks.insert(KD->getParent());
    assert(!KillingBlocks.empty() &&
           "Expected at least a single killing block");
 
    // Find the common post-dominator of all killing blocks.
    BasicBlock *CommonPred = *KillingBlocks.begin();
    for (BasicBlock *BB : llvm::drop_begin(KillingBlocks)) {
      if (!CommonPred)
        break;
      CommonPred = PDT.findNearestCommonDominator(CommonPred, BB);
    }
 
    // If the common post-dominator does not post-dominate MaybeDeadAccess,
    // there is a path from MaybeDeadAccess to an exit not going through a
    // killing block.
    if (!PDT.dominates(CommonPred, MaybeDeadAccess->getBlock())) {
      if (!AnyUnreachableExit)
        return std::nullopt;
 
      // Fall back to CFG scan starting at all non-unreachable roots if not
      // all paths to the exit go through CommonPred.
      CommonPred = nullptr;
    }
 
    // If CommonPred itself is in the set of killing blocks, we're done.
    if (KillingBlocks.count(CommonPred))
      return {MaybeDeadAccess};
 
    SetVector<BasicBlock *> WorkList;
    // If CommonPred is null, there are multiple exits from the function.
    // They all have to be added to the worklist.
    if (CommonPred)
      WorkList.insert(CommonPred);
    else
      for (BasicBlock *R : PDT.roots()) {
        if (!isa<UnreachableInst>(R->getTerminator()))
          WorkList.insert(R);
      }
 
    NumCFGTries++;
    // Check if all paths starting from an exit node go through one of the
    // killing blocks before reaching MaybeDeadAccess.
    for (unsigned I = 0; I < WorkList.size(); I++) {
      NumCFGChecks++;
      BasicBlock *Current = WorkList[I];
      if (KillingBlocks.count(Current))
        continue;
      if (Current == MaybeDeadAccess->getBlock())
        return std::nullopt;
 
      // MaybeDeadAccess is reachable from the entry, so we don't have to
      // explore unreachable blocks further.
      if (!DT.isReachableFromEntry(Current))
        continue;
 
      WorkList.insert_range(predecessors(Current));
 
      if (WorkList.size() >= MemorySSAPathCheckLimit)
        return std::nullopt;
    }
    NumCFGSuccess++;
  }
 
  // No aliasing MemoryUses of MaybeDeadAccess found, MaybeDeadAccess is
  // potentially dead.
  return {MaybeDeadAccess};
}
 
void DSEState::deleteDeadInstruction(Instruction *SI,
                                     SmallPtrSetImpl<MemoryAccess *> *Deleted) {
  MemorySSAUpdater Updater(&MSSA);
  SmallVector<Instruction *, 32> NowDeadInsts;
  NowDeadInsts.push_back(SI);
  --NumFastOther;
 
  while (!NowDeadInsts.empty()) {
    Instruction *DeadInst = NowDeadInsts.pop_back_val();
    ++NumFastOther;
 
    // Try to preserve debug information attached to the dead instruction.
    salvageDebugInfo(*DeadInst);
    salvageKnowledge(DeadInst);
 
    // Remove the Instruction from MSSA.
    MemoryAccess *MA = MSSA.getMemoryAccess(DeadInst);
    bool IsMemDef = MA && isa<MemoryDef>(MA);
    if (MA) {
      if (IsMemDef) {
        auto *MD = cast<MemoryDef>(MA);
        SkipStores.insert(MD);
        if (Deleted)
          Deleted->insert(MD);
        if (auto *SI = dyn_cast<StoreInst>(MD->getMemoryInst())) {
          if (SI->getValueOperand()->getType()->isPointerTy()) {
            const Value *UO = getUnderlyingObject(SI->getValueOperand());
            if (CapturedBeforeReturn.erase(UO))
              ShouldIterateEndOfFunctionDSE = true;
            InvisibleToCallerAfterRet.erase(UO);
            InvisibleToCallerAfterRetBounded.erase(UO);
          }
        }
      }
 
      Updater.removeMemoryAccess(MA);
    }
 
    auto I = IOLs.find(DeadInst->getParent());
    if (I != IOLs.end())
      I->second.erase(DeadInst);
    // Remove its operands
    for (Use &O : DeadInst->operands())
      if (Instruction *OpI = dyn_cast<Instruction>(O)) {
        O.set(PoisonValue::get(O->getType()));
        if (isInstructionTriviallyDead(OpI, &TLI))
          NowDeadInsts.push_back(OpI);
      }
 
    EA.removeInstruction(DeadInst);
    // Remove memory defs directly if they don't produce results, but only
    // queue other dead instructions for later removal. They may have been
    // used as memory locations that have been cached by BatchAA. Removing
    // them here may lead to newly created instructions to be allocated at the
    // same address, yielding stale cache entries.
    if (IsMemDef && DeadInst->getType()->isVoidTy())
      DeadInst->eraseFromParent();
    else
      ToRemove.push_back(DeadInst);
  }
}
 
bool DSEState::mayThrowBetween(Instruction *KillingI, Instruction *DeadI,
                               const Value *KillingUndObj) {
  // First see if we can ignore it by using the fact that KillingI is an
  // alloca/alloca like object that is not visible to the caller during
  // execution of the function.
  if (KillingUndObj && isInvisibleToCallerOnUnwind(KillingUndObj))
    return false;
 
  if (KillingI->getParent() == DeadI->getParent())
    return ThrowingBlocks.count(KillingI->getParent());
  return !ThrowingBlocks.empty();
}
 
bool DSEState::isDSEBarrier(const Value *KillingUndObj, Instruction *DeadI) {
  // If DeadI may throw it acts as a barrier, unless we are to an
  // alloca/alloca like object that does not escape.
  if (DeadI->mayThrow() && !isInvisibleToCallerOnUnwind(KillingUndObj))
    return true;
 
  // If DeadI is an atomic load/store stronger than monotonic, do not try to
  // eliminate/reorder it.
  if (DeadI->isAtomic()) {
    if (auto *LI = dyn_cast<LoadInst>(DeadI))
      return isStrongerThanMonotonic(LI->getOrdering());
    if (auto *SI = dyn_cast<StoreInst>(DeadI))
      return isStrongerThanMonotonic(SI->getOrdering());
    if (auto *ARMW = dyn_cast<AtomicRMWInst>(DeadI))
      return isStrongerThanMonotonic(ARMW->getOrdering());
    if (auto *CmpXchg = dyn_cast<AtomicCmpXchgInst>(DeadI))
      return isStrongerThanMonotonic(CmpXchg->getSuccessOrdering()) ||
             isStrongerThanMonotonic(CmpXchg->getFailureOrdering());
    llvm_unreachable("other instructions should be skipped in MemorySSA");
  }
  return false;
}
 
bool DSEState::eliminateDeadWritesAtEndOfFunction() {
  bool MadeChange = false;
  LLVM_DEBUG(
      dbgs() << "Trying to eliminate MemoryDefs at the end of the function\n");
  do {
    ShouldIterateEndOfFunctionDSE = false;
    for (MemoryDef *Def : llvm::reverse(MemDefs)) {
      if (SkipStores.contains(Def))
        continue;
 
      Instruction *DefI = Def->getMemoryInst();
      auto DefLoc = getLocForWrite(DefI);
      if (!DefLoc || !isRemovable(DefI)) {
        LLVM_DEBUG(dbgs() << "  ... could not get location for write or "
                             "instruction not removable.\n");
        continue;
      }
 
      // NOTE: Currently eliminating writes at the end of a function is
      // limited to MemoryDefs with a single underlying object, to save
      // compile-time. In practice it appears the case with multiple
      // underlying objects is very uncommon. If it turns out to be important,
      // we can use getUnderlyingObjects here instead.
      const Value *UO = getUnderlyingObject(DefLoc->Ptr);
      if (!isInvisibleToCallerAfterRet(UO, DefLoc->Ptr, DefLoc->Size))
        continue;
 
      if (isWriteAtEndOfFunction(Def, *DefLoc)) {
        // See through pointer-to-pointer bitcasts
        LLVM_DEBUG(dbgs() << "   ... MemoryDef is not accessed until the end "
                             "of the function\n");
        deleteDeadInstruction(DefI);
        ++NumFastStores;
        MadeChange = true;
      }
    }
  } while (ShouldIterateEndOfFunctionDSE);
  return MadeChange;
}
 
bool DSEState::eliminateRedundantStoresViaDominatingConditions() {
  bool MadeChange = false;
  LLVM_DEBUG(dbgs() << "Trying to eliminate MemoryDefs whose value being "
                       "written is implied by a dominating condition\n");
 
  using ConditionInfo = std::pair<Value *, Value *>;
  using ScopedHTType = ScopedHashTable<ConditionInfo, Instruction *>;
 
  // We maintain a scoped hash table of the active dominating conditions for a
  // given node.
  ScopedHTType ActiveConditions;
  auto GetDominatingCondition = [&](BasicBlock *BB)
      -> std::optional<std::tuple<ConditionInfo, Instruction *, BasicBlock *>> {
    auto *BI = dyn_cast<CondBrInst>(BB->getTerminator());
    if (!BI)
      return std::nullopt;
 
    // In case both blocks are the same, it is not possible to determine
    // if optimization is possible. (We would not want to optimize a store
    // in the FalseBB if condition is true and vice versa.)
    if (BI->getSuccessor(0) == BI->getSuccessor(1))
      return std::nullopt;
 
    Instruction *ICmpL;
    CmpPredicate Pred;
    Value *StorePtr, *StoreVal;
    if (!match(BI->getCondition(),
               m_c_ICmp(Pred, m_Instruction(ICmpL, m_Load(m_Value(StorePtr))),
                        m_Value(StoreVal))) ||
        !ICmpInst::isEquality(Pred))
      return std::nullopt;
 
    // Ensure the replacement is allowed when comparing pointers, as
    // the equality compares addresses only, not pointers' provenance.
    if (StoreVal->getType()->isPointerTy() &&
        !canReplacePointersIfEqual(StoreVal, ICmpL, DL))
      return std::nullopt;
 
    unsigned ImpliedSuccIdx = Pred == ICmpInst::ICMP_EQ ? 0 : 1;
    BasicBlock *ImpliedSucc = BI->getSuccessor(ImpliedSuccIdx);
    return {{ConditionInfo(StorePtr, StoreVal), ICmpL, ImpliedSucc}};
  };
 
  auto VisitNode = [&](DomTreeNode *Node, unsigned Depth, auto &Self) -> void {
    if (Depth > MaxDepthRecursion)
      return;
 
    BasicBlock *BB = Node->getBlock();
    // Check for redundant stores against active known conditions.
    if (auto *Accesses = MSSA.getBlockDefs(BB)) {
      for (MemoryDef &Def :
           make_early_inc_range(make_isa_range<MemoryDef>(*Accesses))) {
        auto *SI = dyn_cast<StoreInst>(Def.getMemoryInst());
        if (!SI || !SI->isUnordered())
          continue;
 
        Instruction *LI = ActiveConditions.lookup(
            {SI->getPointerOperand(), SI->getValueOperand()});
        if (!LI)
          continue;
 
        // Found a dominating condition that may imply the value being stored.
        // Make sure there does not exist any clobbering access between the
        // load and the potential redundant store.
        MemoryAccess *LoadAccess = MSSA.getMemoryAccess(LI);
        MemoryAccess *ClobberingAccess =
            MSSA.getSkipSelfWalker()->getClobberingMemoryAccess(&Def, BatchAA);
        if (MSSA.dominates(ClobberingAccess, LoadAccess)) {
          LLVM_DEBUG(dbgs()
                     << "Removing No-Op Store:\n  DEAD: " << *SI << '\n');
          deleteDeadInstruction(SI);
          NumRedundantStores++;
          MadeChange = true;
        }
      }
    }
 
    // See whether this basic block establishes a dominating condition.
    auto MaybeCondition = GetDominatingCondition(BB);
 
    for (DomTreeNode *Child : Node->children()) {
      // RAII scope for the active conditions.
      ScopedHTType::ScopeTy Scope(ActiveConditions);
      if (MaybeCondition) {
        const auto &[Cond, LI, ImpliedSucc] = *MaybeCondition;
        if (DT.dominates(BasicBlockEdge(BB, ImpliedSucc), Child->getBlock())) {
          // Found a condition that holds for this child, dominated by the
          // current node via the equality edge. Propagate the condition to
          // the children by pushing it onto the table.
          ActiveConditions.insert(Cond, LI);
        }
      }
 
      // Recursively visit the children of this node. Upon destruction, the no
      // longer active condition before visiting any sibling nodes is popped
      // from the active scope.
      Self(Child, Depth + 1, Self);
    }
  };
 
  // Do a DFS walk of the dom-tree.
  VisitNode(DT.getRootNode(), 0, VisitNode);
 
  return MadeChange;
}
 
bool DSEState::tryFoldIntoCalloc(MemoryDef *Def, const Value *DefUO) {
  Instruction *DefI = Def->getMemoryInst();
  MemSetInst *MemSet = dyn_cast<MemSetInst>(DefI);
  if (!MemSet)
    // TODO: Could handle zero store to small allocation as well.
    return false;
  Constant *StoredConstant = dyn_cast<Constant>(MemSet->getValue());
  if (!StoredConstant || !StoredConstant->isNullValue())
    return false;
 
  if (!isRemovable(DefI))
    // The memset might be volatile..
    return false;
 
  if (F.hasFnAttribute(Attribute::SanitizeMemory) ||
      F.hasFnAttribute(Attribute::SanitizeAddress) ||
      F.hasFnAttribute(Attribute::SanitizeHWAddress) || F.getName() == "calloc")
    return false;
  auto *Malloc = const_cast<CallInst *>(dyn_cast<CallInst>(DefUO));
  if (!Malloc)
    return false;
  auto *InnerCallee = Malloc->getCalledFunction();
  if (!InnerCallee)
    return false;
  LibFunc Func = TLI.getLibFunc(*InnerCallee);
  StringRef ZeroedVariantName;
  if (Func != LibFunc_malloc || !TLI.has(Func)) {
    Attribute Attr = Malloc->getFnAttr("alloc-variant-zeroed");
    if (!Attr.isValid())
      return false;
    ZeroedVariantName = Attr.getValueAsString();
    if (ZeroedVariantName.empty())
      return false;
  }
 
  // Gracefully handle malloc with unexpected memory attributes.
  auto *MallocDef = dyn_cast_or_null<MemoryDef>(MSSA.getMemoryAccess(Malloc));
  if (!MallocDef)
    return false;
 
  auto shouldCreateCalloc = [](CallInst *Malloc, CallInst *Memset) {
    // Check for br(icmp ptr, null), truebb, falsebb) pattern at the end
    // of malloc block
    auto *MallocBB = Malloc->getParent(), *MemsetBB = Memset->getParent();
    if (MallocBB == MemsetBB)
      return true;
    auto *Ptr = Memset->getArgOperand(0);
    auto *TI = MallocBB->getTerminator();
    BasicBlock *TrueBB, *FalseBB;
    if (!match(TI, m_Br(m_SpecificICmp(ICmpInst::ICMP_EQ, m_Specific(Ptr),
                                       m_Zero()),
                        TrueBB, FalseBB)))
      return false;
    if (MemsetBB != FalseBB)
      return false;
    return true;
  };
 
  if (Malloc->getOperand(0) != MemSet->getLength())
    return false;
  if (!shouldCreateCalloc(Malloc, MemSet) || !DT.dominates(Malloc, MemSet) ||
      !memoryIsNotModifiedBetween(Malloc, MemSet, BatchAA, DL, &DT))
    return false;
  IRBuilder<> IRB(Malloc);
  assert(Func == LibFunc_malloc || !ZeroedVariantName.empty());
  Value *Calloc = nullptr;
  if (!ZeroedVariantName.empty()) {
    LLVMContext &Ctx = Malloc->getContext();
    AttributeList Attrs = InnerCallee->getAttributes();
    AllocFnKind AllocKind =
        Attrs.getFnAttr(Attribute::AllocKind).getAllocKind() |
        AllocFnKind::Zeroed;
    AllocKind &= ~AllocFnKind::Uninitialized;
    Attrs =
        Attrs.addFnAttribute(Ctx, Attribute::getWithAllocKind(Ctx, AllocKind))
            .removeFnAttribute(Ctx, "alloc-variant-zeroed");
    FunctionCallee ZeroedVariant = Malloc->getModule()->getOrInsertFunction(
        ZeroedVariantName, InnerCallee->getFunctionType(), Attrs);
    cast<Function>(ZeroedVariant.getCallee())
        ->setCallingConv(Malloc->getCallingConv());
    SmallVector<Value *, 3> Args;
    Args.append(Malloc->arg_begin(), Malloc->arg_end());
    CallInst *CI = IRB.CreateCall(ZeroedVariant, Args, ZeroedVariantName);
    CI->setCallingConv(Malloc->getCallingConv());
    Calloc = CI;
  } else {
    Type *SizeTTy = Malloc->getArgOperand(0)->getType();
    Calloc = emitCalloc(ConstantInt::get(SizeTTy, 1), Malloc->getArgOperand(0),
                        IRB, TLI, Malloc->getType()->getPointerAddressSpace());
  }
  if (!Calloc)
    return false;
 
  if (MDNode *MD = Malloc->getMetadata(LLVMContext::MD_alloc_token))
    cast<Instruction>(Calloc)->setMetadata(LLVMContext::MD_alloc_token, MD);
 
  MemorySSAUpdater Updater(&MSSA);
  auto *NewAccess = Updater.createMemoryAccessAfter(cast<Instruction>(Calloc),
                                                    nullptr, MallocDef);
  auto *NewAccessMD = cast<MemoryDef>(NewAccess);
  Updater.insertDef(NewAccessMD, /*RenameUses=*/true);
  Malloc->replaceAllUsesWith(Calloc);
  deleteDeadInstruction(Malloc);
  return true;
}
 
bool DSEState::storeIsNoop(MemoryDef *Def, const Value *DefUO) {
  Instruction *DefI = Def->getMemoryInst();
  StoreInst *Store = dyn_cast<StoreInst>(DefI);
  MemSetInst *MemSet = dyn_cast<MemSetInst>(DefI);
  Constant *StoredConstant = nullptr;
  if (Store)
    StoredConstant = dyn_cast<Constant>(Store->getOperand(0));
  else if (MemSet)
    StoredConstant = dyn_cast<Constant>(MemSet->getValue());
  else
    return false;
 
  if (!isRemovable(DefI))
    return false;
 
  if (StoredConstant) {
    Constant *InitC =
        getInitialValueOfAllocation(DefUO, &TLI, StoredConstant->getType());
    // If the clobbering access is LiveOnEntry, no instructions between them
    // can modify the memory location.
    if (InitC && InitC == StoredConstant)
      return MSSA.isLiveOnEntryDef(
          MSSA.getSkipSelfWalker()->getClobberingMemoryAccess(Def, BatchAA));
  }
 
  if (!Store)
    return false;
 
  if (auto *LoadI = dyn_cast<LoadInst>(Store->getOperand(0))) {
    if (LoadI->getPointerOperand() == Store->getOperand(1)) {
      // Get the defining access for the load.
      auto *LoadAccess = MSSA.getMemoryAccess(LoadI)->getDefiningAccess();
      // Fast path: the defining accesses are the same.
      if (LoadAccess == Def->getDefiningAccess())
        return true;
 
      // Look through phi accesses. Recursively scan all phi accesses by
      // adding them to a worklist. Bail when we run into a memory def that
      // does not match LoadAccess.
      SetVector<MemoryAccess *> ToCheck;
      MemoryAccess *Current =
          MSSA.getWalker()->getClobberingMemoryAccess(Def, BatchAA);
      // We don't want to bail when we run into the store memory def. But,
      // the phi access may point to it. So, pretend like we've already
      // checked it.
      ToCheck.insert(Def);
      ToCheck.insert(Current);
      // Start at current (1) to simulate already having checked Def.
      for (unsigned I = 1; I < ToCheck.size(); ++I) {
        Current = ToCheck[I];
        if (auto PhiAccess = dyn_cast<MemoryPhi>(Current)) {
          // Check all the operands.
          for (auto &Use : PhiAccess->incoming_values())
            ToCheck.insert(cast<MemoryAccess>(&Use));
          continue;
        }
 
        // If we found a memory def, bail. This happens when we have an
        // unrelated write in between an otherwise noop store.
        assert(isa<MemoryDef>(Current) && "Only MemoryDefs should reach here.");
        // TODO: Skip no alias MemoryDefs that have no aliasing reads.
        // We are searching for the definition of the store's destination.
        // So, if that is the same definition as the load, then this is a
        // noop. Otherwise, fail.
        if (LoadAccess != Current)
          return false;
      }
      return true;
    }
  }
 
  return false;
}
 
bool DSEState::removePartiallyOverlappedStores(InstOverlapIntervalsTy &IOL) {
  bool Changed = false;
  for (auto OI : IOL) {
    Instruction *DeadI = OI.first;
    MemoryLocation Loc = *getLocForWrite(DeadI);
    assert(isRemovable(DeadI) && "Expect only removable instruction");
 
    const Value *Ptr = Loc.Ptr->stripPointerCasts();
    int64_t DeadStart = 0;
    uint64_t DeadSize = Loc.Size.getValue();
    GetPointerBaseWithConstantOffset(Ptr, DeadStart, DL);
    OverlapIntervalsTy &IntervalMap = OI.second;
    Changed |= tryToShortenEnd(DeadI, IntervalMap, DeadStart, DeadSize);
    if (IntervalMap.empty())
      continue;
    Changed |= tryToShortenBegin(DeadI, IntervalMap, DeadStart, DeadSize);
  }
  return Changed;
}
 
bool DSEState::eliminateRedundantStoresOfExistingValues() {
  bool MadeChange = false;
  LLVM_DEBUG(dbgs() << "Trying to eliminate MemoryDefs that write the "
                       "already existing value\n");
  for (auto *Def : MemDefs) {
    if (SkipStores.contains(Def) || MSSA.isLiveOnEntryDef(Def))
      continue;
 
    Instruction *DefInst = Def->getMemoryInst();
    auto MaybeDefLoc = getLocForWrite(DefInst);
    if (!MaybeDefLoc || !isRemovable(DefInst))
      continue;
 
    MemoryDef *UpperDef;
    // To conserve compile-time, we avoid walking to the next clobbering def.
    // Instead, we just try to get the optimized access, if it exists. DSE
    // will try to optimize defs during the earlier traversal.
    if (Def->isOptimized())
      UpperDef = dyn_cast<MemoryDef>(Def->getOptimized());
    else
      UpperDef = dyn_cast<MemoryDef>(Def->getDefiningAccess());
    if (!UpperDef || MSSA.isLiveOnEntryDef(UpperDef))
      continue;
 
    Instruction *UpperInst = UpperDef->getMemoryInst();
    auto IsRedundantStore = [&]() {
      // We don't care about differences in call attributes here.
      if (DefInst->isIdenticalToWhenDefined(UpperInst,
                                            /*IntersectAttrs=*/true))
        return true;
      if (auto *MemSetI = dyn_cast<MemSetInst>(UpperInst)) {
        if (auto *SI = dyn_cast<StoreInst>(DefInst)) {
          // MemSetInst must have a write location.
          auto UpperLoc = getLocForWrite(UpperInst);
          if (!UpperLoc)
            return false;
          int64_t InstWriteOffset = 0;
          int64_t DepWriteOffset = 0;
          auto OR = isOverwrite(UpperInst, DefInst, *UpperLoc, *MaybeDefLoc,
                                InstWriteOffset, DepWriteOffset);
          Value *StoredByte = isBytewiseValue(SI->getValueOperand(), DL);
          return StoredByte && StoredByte == MemSetI->getOperand(1) &&
                 OR == OW_Complete;
        }
      }
      return false;
    };
 
    if (!IsRedundantStore() || isReadClobber(*MaybeDefLoc, DefInst))
      continue;
    LLVM_DEBUG(dbgs() << "DSE: Remove No-Op Store:\n  DEAD: " << *DefInst
                      << '\n');
    deleteDeadInstruction(DefInst);
    NumRedundantStores++;
    MadeChange = true;
  }
  return MadeChange;
}
 
SmallVector<MemoryLocation, 1>
DSEState::getInitializesArgMemLoc(const Instruction *I) {
  const CallBase *CB = dyn_cast<CallBase>(I);
  if (!CB)
    return {};
 
  // Collect aliasing arguments and their initializes ranges.
  SmallMapVector<Value *, SmallVector<ArgumentInitInfo, 2>, 2> Arguments;
  for (unsigned Idx = 0, Count = CB->arg_size(); Idx < Count; ++Idx) {
    Value *CurArg = CB->getArgOperand(Idx);
    if (!CurArg->getType()->isPointerTy())
      continue;
 
    ConstantRangeList Inits;
    Attribute InitializesAttr = CB->getParamAttr(Idx, Attribute::Initializes);
    // initializes on byval arguments refers to the callee copy, not the
    // original memory the caller passed in.
    if (InitializesAttr.isValid() && !CB->isByValArgument(Idx))
      Inits = InitializesAttr.getValueAsConstantRangeList();
 
    // Check whether "CurArg" could alias with global variables. We require
    // either it's function local and isn't captured before or the "CB" only
    // accesses arg or inaccessible mem.
    if (!Inits.empty() && !CB->onlyAccessesInaccessibleMemOrArgMem() &&
        !isFuncLocalAndNotCaptured(CurArg, CB, EA))
      Inits = ConstantRangeList();
 
    // We don't perform incorrect DSE on unwind edges in the current function,
    // and use the "initializes" attribute to kill dead stores if:
    // - The call does not throw exceptions, "CB->doesNotThrow()".
    // - Or the callee parameter has "dead_on_unwind" attribute.
    // - Or the argument is invisible to caller on unwind, and there are no
    //   unwind edges from this call in the current function (e.g. `CallInst`).
    bool IsDeadOrInvisibleOnUnwind =
        CB->paramHasAttr(Idx, Attribute::DeadOnUnwind) ||
        (isa<CallInst>(CB) && isInvisibleToCallerOnUnwind(CurArg));
    ArgumentInitInfo InitInfo{Idx, IsDeadOrInvisibleOnUnwind, Inits};
    bool FoundAliasing = false;
    for (auto &[Arg, AliasList] : Arguments) {
      auto AAR = BatchAA.alias(MemoryLocation::getBeforeOrAfter(Arg),
                               MemoryLocation::getBeforeOrAfter(CurArg));
      if (AAR == AliasResult::NoAlias) {
        continue;
      } else if (AAR == AliasResult::MustAlias) {
        FoundAliasing = true;
        AliasList.push_back(InitInfo);
      } else {
        // For PartialAlias and MayAlias, there is an offset or may be an
        // unknown offset between the arguments and we insert an empty init
        // range to discard the entire initializes info while intersecting.
        FoundAliasing = true;
        AliasList.push_back(ArgumentInitInfo{Idx, IsDeadOrInvisibleOnUnwind,
                                             ConstantRangeList()});
      }
    }
    if (!FoundAliasing)
      Arguments[CurArg] = {InitInfo};
  }
 
  SmallVector<MemoryLocation, 1> Locations;
  for (const auto &[_, Args] : Arguments) {
    auto IntersectedRanges =
        getIntersectedInitRangeList(Args, CB->doesNotThrow());
    if (IntersectedRanges.empty())
      continue;
 
    for (const auto &Arg : Args) {
      for (const auto &Range : IntersectedRanges) {
        int64_t Start = Range.getLower().getSExtValue();
        int64_t End = Range.getUpper().getSExtValue();
        // For now, we only handle locations starting at offset 0.
        if (Start == 0)
          Locations.push_back(MemoryLocation(CB->getArgOperand(Arg.Idx),
                                             LocationSize::precise(End - Start),
                                             CB->getAAMetadata()));
      }
    }
  }
  return Locations;
}
 
std::pair<bool, bool>
DSEState::eliminateDeadDefs(const MemoryLocationWrapper &KillingLocWrapper) {
  bool Changed = false;
  bool DeletedKillingLoc = false;
  unsigned ScanLimit = MemorySSAScanLimit;
  unsigned WalkerStepLimit = MemorySSAUpwardsStepLimit;
  unsigned PartialLimit = MemorySSAPartialStoreLimit;
  // Worklist of MemoryAccesses that may be killed by
  // "KillingLocWrapper.MemDef".
  SmallSetVector<MemoryAccess *, 8> ToCheck;
  // Track MemoryAccesses that have been deleted in the loop below, so we can
  // skip them. Don't use SkipStores for this, which may contain reused
  // MemoryAccess addresses.
  SmallPtrSet<MemoryAccess *, 8> Deleted;
  [[maybe_unused]] unsigned OrigNumSkipStores = SkipStores.size();
  ToCheck.insert(KillingLocWrapper.MemDef->getDefiningAccess());
 
  // Check if MemoryAccesses in the worklist are killed by
  // "KillingLocWrapper.MemDef".
  for (unsigned I = 0; I < ToCheck.size(); I++) {
    MemoryAccess *Current = ToCheck[I];
    if (Deleted.contains(Current))
      continue;
    std::optional<MemoryAccess *> MaybeDeadAccess = getDomMemoryDef(
        KillingLocWrapper.MemDef, Current, KillingLocWrapper.MemLoc,
        KillingLocWrapper.UnderlyingObject, ScanLimit, WalkerStepLimit,
        isMemTerminatorInst(KillingLocWrapper.DefInst), PartialLimit,
        KillingLocWrapper.DefByInitializesAttr);
 
    if (!MaybeDeadAccess) {
      LLVM_DEBUG(dbgs() << "  finished walk\n");
      continue;
    }
    MemoryAccess *DeadAccess = *MaybeDeadAccess;
    LLVM_DEBUG(dbgs() << " Checking if we can kill " << *DeadAccess);
    if (isa<MemoryPhi>(DeadAccess)) {
      LLVM_DEBUG(dbgs() << "\n  ... adding incoming values to worklist\n");
      for (Value *V : cast<MemoryPhi>(DeadAccess)->incoming_values()) {
        MemoryAccess *IncomingAccess = cast<MemoryAccess>(V);
        BasicBlock *IncomingBlock = IncomingAccess->getBlock();
        BasicBlock *PhiBlock = DeadAccess->getBlock();
 
        // We only consider incoming MemoryAccesses that come before the
        // MemoryPhi. Otherwise we could discover candidates that do not
        // strictly dominate our starting def.
        if (PostOrderNumbers[IncomingBlock] > PostOrderNumbers[PhiBlock])
          ToCheck.insert(IncomingAccess);
      }
      continue;
    }
    // We cannot apply the initializes attribute to DeadAccess/DeadDef.
    // It would incorrectly consider a call instruction as redundant store
    // and remove this call instruction.
    // TODO: this conflates the existence of a MemoryLocation with being able
    // to delete the instruction. Fix isRemovable() to consider calls with
    // side effects that cannot be removed, e.g. calls with the initializes
    // attribute, and remove getLocForInst(ConsiderInitializesAttr = false).
    MemoryDefWrapper DeadDefWrapper(
        cast<MemoryDef>(DeadAccess),
        getLocForInst(cast<MemoryDef>(DeadAccess)->getMemoryInst(),
                      /*ConsiderInitializesAttr=*/false));
    assert(DeadDefWrapper.DefinedLocations.size() == 1);
    MemoryLocationWrapper &DeadLocWrapper =
        DeadDefWrapper.DefinedLocations.front();
    LLVM_DEBUG(dbgs() << " (" << *DeadLocWrapper.DefInst << ")\n");
    ToCheck.insert(DeadLocWrapper.MemDef->getDefiningAccess());
    NumGetDomMemoryDefPassed++;
 
    if (!DebugCounter::shouldExecute(MemorySSACounter))
      continue;
    if (isMemTerminatorInst(KillingLocWrapper.DefInst)) {
      if (KillingLocWrapper.UnderlyingObject != DeadLocWrapper.UnderlyingObject)
        continue;
      LLVM_DEBUG(dbgs() << "DSE: Remove Dead Store:\n  DEAD: "
                        << *DeadLocWrapper.DefInst << "\n  KILLER: "
                        << *KillingLocWrapper.DefInst << '\n');
      deleteDeadInstruction(DeadLocWrapper.DefInst, &Deleted);
      ++NumFastStores;
      Changed = true;
    } else {
      // Check if DeadI overwrites KillingI.
      int64_t KillingOffset = 0;
      int64_t DeadOffset = 0;
      OverwriteResult OR =
          isOverwrite(KillingLocWrapper.DefInst, DeadLocWrapper.DefInst,
                      KillingLocWrapper.MemLoc, DeadLocWrapper.MemLoc,
                      KillingOffset, DeadOffset);
      if (OR == OW_MaybePartial) {
        auto &IOL = IOLs[DeadLocWrapper.DefInst->getParent()];
        OR = isPartialOverwrite(KillingLocWrapper.MemLoc, DeadLocWrapper.MemLoc,
                                KillingOffset, DeadOffset,
                                DeadLocWrapper.DefInst, IOL);
      }
      if (EnablePartialStoreMerging && OR == OW_PartialEarlierWithFullLater) {
        auto *DeadSI = dyn_cast<StoreInst>(DeadLocWrapper.DefInst);
        auto *KillingSI = dyn_cast<StoreInst>(KillingLocWrapper.DefInst);
        // We are re-using tryToMergePartialOverlappingStores, which requires
        // DeadSI to dominate KillingSI.
        // TODO: implement tryToMergeParialOverlappingStores using MemorySSA.
        if (DeadSI && KillingSI && DT.dominates(DeadSI, KillingSI)) {
          if (Constant *Merged = tryToMergePartialOverlappingStores(
                  KillingSI, DeadSI, KillingOffset, DeadOffset, DL, BatchAA,
                  &DT)) {
 
            // Update stored value of earlier store to merged constant.
            DeadSI->setOperand(0, Merged);
            ++NumModifiedStores;
            Changed = true;
            DeletedKillingLoc = true;
 
            // Remove killing store and remove any outstanding overlap
            // intervals for the updated store.
            deleteDeadInstruction(KillingSI, &Deleted);
            auto I = IOLs.find(DeadSI->getParent());
            if (I != IOLs.end())
              I->second.erase(DeadSI);
            break;
          }
        }
      }
      if (OR == OW_Complete) {
        LLVM_DEBUG(dbgs() << "DSE: Remove Dead Store:\n  DEAD: "
                          << *DeadLocWrapper.DefInst << "\n  KILLER: "
                          << *KillingLocWrapper.DefInst << '\n');
        deleteDeadInstruction(DeadLocWrapper.DefInst, &Deleted);
        ++NumFastStores;
        Changed = true;
      }
    }
  }
 
  assert(SkipStores.size() - OrigNumSkipStores == Deleted.size() &&
         "SkipStores and Deleted out of sync?");
 
  return {Changed, DeletedKillingLoc};
}
 
bool DSEState::eliminateDeadDefs(const MemoryDefWrapper &KillingDefWrapper) {
  if (KillingDefWrapper.DefinedLocations.empty()) {
    LLVM_DEBUG(dbgs() << "Failed to find analyzable write location for "
                      << *KillingDefWrapper.DefInst << "\n");
    return false;
  }
 
  bool MadeChange = false;
  for (auto &KillingLocWrapper : KillingDefWrapper.DefinedLocations) {
    LLVM_DEBUG(dbgs() << "Trying to eliminate MemoryDefs killed by "
                      << *KillingLocWrapper.MemDef << " ("
                      << *KillingLocWrapper.DefInst << ")\n");
    auto [Changed, DeletedKillingLoc] = eliminateDeadDefs(KillingLocWrapper);
    MadeChange |= Changed;
 
    // Check if the store is a no-op.
    if (!DeletedKillingLoc && storeIsNoop(KillingLocWrapper.MemDef,
                                          KillingLocWrapper.UnderlyingObject)) {
      LLVM_DEBUG(dbgs() << "DSE: Remove No-Op Store:\n  DEAD: "
                        << *KillingLocWrapper.DefInst << '\n');
      deleteDeadInstruction(KillingLocWrapper.DefInst);
      NumRedundantStores++;
      MadeChange = true;
      continue;
    }
    // Can we form a calloc from a memset/malloc pair?
    if (!DeletedKillingLoc &&
        tryFoldIntoCalloc(KillingLocWrapper.MemDef,
                          KillingLocWrapper.UnderlyingObject)) {
      LLVM_DEBUG(dbgs() << "DSE: Remove memset after forming calloc:\n"
                        << "  DEAD: " << *KillingLocWrapper.DefInst << '\n');
      deleteDeadInstruction(KillingLocWrapper.DefInst);
      MadeChange = true;
      continue;
    }
  }
  return MadeChange;
}
 
static bool eliminateDeadStores(Function &F, AliasAnalysis &AA, MemorySSA &MSSA,
                                DominatorTree &DT, PostDominatorTree &PDT,
                                const TargetLibraryInfo &TLI,
                                const CycleInfo &CI) {
  bool MadeChange = false;
  DSEState State(F, AA, MSSA, DT, PDT, TLI, CI);
  // For each store:
  for (unsigned I = 0; I < State.MemDefs.size(); I++) {
    MemoryDef *KillingDef = State.MemDefs[I];
    if (State.SkipStores.count(KillingDef))
      continue;
 
    MemoryDefWrapper KillingDefWrapper(
        KillingDef, State.getLocForInst(KillingDef->getMemoryInst(),
                                        EnableInitializesImprovement));
    MadeChange |= State.eliminateDeadDefs(KillingDefWrapper);
  }
 
  if (EnablePartialOverwriteTracking)
    for (auto &KV : State.IOLs)
      MadeChange |= State.removePartiallyOverlappedStores(KV.second);
 
  MadeChange |= State.eliminateRedundantStoresOfExistingValues();
  MadeChange |= State.eliminateDeadWritesAtEndOfFunction();
  MadeChange |= State.eliminateRedundantStoresViaDominatingConditions();
 
  while (!State.ToRemove.empty()) {
    Instruction *DeadInst = State.ToRemove.pop_back_val();
    DeadInst->eraseFromParent();
  }
 
  return MadeChange;
}
 
//===----------------------------------------------------------------------===//
// DSE Pass
//===----------------------------------------------------------------------===//
PreservedAnalyses DSEPass::run(Function &F, FunctionAnalysisManager &AM) {
  AliasAnalysis &AA = AM.getResult<AAManager>(F);
  const TargetLibraryInfo &TLI = AM.getResult<TargetLibraryAnalysis>(F);
  DominatorTree &DT = AM.getResult<DominatorTreeAnalysis>(F);
  MemorySSA &MSSA = AM.getResult<MemorySSAAnalysis>(F).getMSSA();
  PostDominatorTree &PDT = AM.getResult<PostDominatorTreeAnalysis>(F);
  CycleInfo &CI = AM.getResult<CycleAnalysis>(F);
 
  bool Changed = eliminateDeadStores(F, AA, MSSA, DT, PDT, TLI, CI);
 
#ifdef LLVM_ENABLE_STATS
  if (AreStatisticsEnabled())
    for (auto &I : instructions(F))
      NumRemainingStores += isa<StoreInst>(&I);
#endif
 
  if (!Changed)
    return PreservedAnalyses::all();
 
  PreservedAnalyses PA;
  PA.preserveSet<CFGAnalyses>();
  PA.preserve<MemorySSAAnalysis>();
  return PA;
}
 
namespace {
 
/// A legacy pass for the legacy pass manager that wraps \c DSEPass.
class DSELegacyPass : public FunctionPass {
public:
  static char ID; // Pass identification, replacement for typeid
 
  DSELegacyPass() : FunctionPass(ID) {
    initializeDSELegacyPassPass(*PassRegistry::getPassRegistry());
  }
 
  bool runOnFunction(Function &F) override {
    if (skipFunction(F))
      return false;
 
    AliasAnalysis &AA = getAnalysis<AAResultsWrapperPass>().getAAResults();
    DominatorTree &DT = getAnalysis<DominatorTreeWrapperPass>().getDomTree();
    const TargetLibraryInfo &TLI =
        getAnalysis<TargetLibraryInfoWrapperPass>().getTLI(F);
    MemorySSA &MSSA = getAnalysis<MemorySSAWrapperPass>().getMSSA();
    PostDominatorTree &PDT =
        getAnalysis<PostDominatorTreeWrapperPass>().getPostDomTree();
    CycleInfo &CI = getAnalysis<CycleInfoWrapperPass>().getResult();
 
    bool Changed = eliminateDeadStores(F, AA, MSSA, DT, PDT, TLI, CI);
 
#ifdef LLVM_ENABLE_STATS
    if (AreStatisticsEnabled())
      for (auto &I : instructions(F))
        NumRemainingStores += isa<StoreInst>(&I);
#endif
 
    return Changed;
  }
 
  void getAnalysisUsage(AnalysisUsage &AU) const override {
    AU.setPreservesCFG();
    AU.addRequired<AAResultsWrapperPass>();
    AU.addRequired<TargetLibraryInfoWrapperPass>();
    AU.addPreserved<GlobalsAAWrapperPass>();
    AU.addRequired<DominatorTreeWrapperPass>();
    AU.addRequired<PostDominatorTreeWrapperPass>();
    AU.addRequired<MemorySSAWrapperPass>();
    AU.addPreserved<MemorySSAWrapperPass>();
    AU.addRequired<CycleInfoWrapperPass>();
    AU.addRequired<AssumptionCacheTracker>();
  }
};
 
} // end anonymous namespace
 
char DSELegacyPass::ID = 0;
 
INITIALIZE_PASS_BEGIN(DSELegacyPass, "dse", "Dead Store Elimination", false,
                      false)
INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
INITIALIZE_PASS_DEPENDENCY(PostDominatorTreeWrapperPass)
INITIALIZE_PASS_DEPENDENCY(AAResultsWrapperPass)
INITIALIZE_PASS_DEPENDENCY(GlobalsAAWrapperPass)
INITIALIZE_PASS_DEPENDENCY(MemorySSAWrapperPass)
INITIALIZE_PASS_DEPENDENCY(MemoryDependenceWrapperPass)
INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfoWrapperPass)
INITIALIZE_PASS_DEPENDENCY(CycleInfoWrapperPass)
INITIALIZE_PASS_DEPENDENCY(AssumptionCacheTracker)
INITIALIZE_PASS_END(DSELegacyPass, "dse", "Dead Store Elimination", false,
                    false)
 
LLVM_ABI FunctionPass *llvm::createDeadStoreEliminationPass() {
  return new DSELegacyPass();
}