Transforms/Scalar/DeadStoreElimination.cpp

//===- DeadStoreElimination.cpp - Fast 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
//
//===----------------------------------------------------------------------===//
//
// This file implements a trivial dead store elimination that only considers
// basic-block local redundant stores.
//
// FIXME: This should eventually be extended to be a post-dominator tree
// traversal.  Doing so would be pretty trivial.
//
//===----------------------------------------------------------------------===//

#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/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/CaptureTracking.h"
#include "llvm/Analysis/GlobalsModRef.h"
#include "llvm/Analysis/MemoryBuiltins.h"
#include "llvm/Analysis/MemoryDependenceAnalysis.h"
#include "llvm/Analysis/MemoryLocation.h"
#include "llvm/Analysis/MemorySSA.h"
#include "llvm/Analysis/MemorySSAUpdater.h"
#include "llvm/Analysis/PostDominators.h"
#include "llvm/Analysis/TargetLibraryInfo.h"
#include "llvm/Analysis/ValueTracking.h"
#include "llvm/IR/Argument.h"
#include "llvm/IR/BasicBlock.h"
#include "llvm/IR/Constant.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/DataLayout.h"
#include "llvm/IR/Dominators.h"
#include "llvm/IR/Function.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/Intrinsics.h"
#include "llvm/IR/LLVMContext.h"
#include "llvm/IR/Module.h"
#include "llvm/IR/PassManager.h"
#include "llvm/IR/PatternMatch.h"
#include "llvm/IR/Value.h"
#include "llvm/InitializePasses.h"
#include "llvm/Pass.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/MathExtras.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Transforms/Scalar.h"
#include "llvm/Transforms/Utils/AssumeBundleBuilder.h"
#include "llvm/Transforms/Utils/Local.h"
#include <algorithm>
#include <cassert>
#include <cstddef>
#include <cstdint>
#include <iterator>
#include <map>
#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(NumNoopStores, "Number of noop stores deleted");
STATISTIC(NumCFGChecks, "Number of stores modified");
STATISTIC(NumCFGTries, "Number of stores modified");
STATISTIC(NumCFGSuccess, "Number of stores modified");

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<bool>
    EnableMemorySSA("enable-dse-memoryssa", cl::init(false), cl::Hidden,
                    cl::desc("Use the new MemorySSA-backed DSE."));

static cl::opt<unsigned>
    MemorySSAScanLimit("dse-memoryssa-scanlimit", cl::init(100), cl::Hidden,
                       cl::desc("The number of memory instructions to scan for "
                                "dead store elimination (default = 100)"));

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> 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)"));

//===----------------------------------------------------------------------===//
// Helper functions
//===----------------------------------------------------------------------===//
using OverlapIntervalsTy = std::map<int64_t, int64_t>;
using InstOverlapIntervalsTy = DenseMap<Instruction *, OverlapIntervalsTy>;

/// Delete this instruction.  Before we do, go through and zero out all the
/// operands of this instruction.  If any of them become dead, delete them and
/// the computation tree that feeds them.
/// If ValueSet is non-null, remove any deleted instructions from it as well.
static void
deleteDeadInstruction(Instruction *I, BasicBlock::iterator *BBI,
                      MemoryDependenceResults &MD, const TargetLibraryInfo &TLI,
                      InstOverlapIntervalsTy &IOL,
                      MapVector<Instruction *, bool> &ThrowableInst,
                      SmallSetVector<const Value *, 16> *ValueSet = nullptr) {
  SmallVector<Instruction*, 32> NowDeadInsts;

  NowDeadInsts.push_back(I);
  --NumFastOther;

  // Keeping the iterator straight is a pain, so we let this routine tell the
  // caller what the next instruction is after we're done mucking about.
  BasicBlock::iterator NewIter = *BBI;

  // Before we touch this instruction, remove it from memdep!
  do {
    Instruction *DeadInst = NowDeadInsts.pop_back_val();
    // Mark the DeadInst as dead in the list of throwable instructions.
    auto It = ThrowableInst.find(DeadInst);
    if (It != ThrowableInst.end())
      ThrowableInst[It->first] = false;
    ++NumFastOther;

    // Try to preserve debug information attached to the dead instruction.
    salvageDebugInfo(*DeadInst);
    salvageKnowledge(DeadInst);

    // This instruction is dead, zap it, in stages.  Start by removing it from
    // MemDep, which needs to know the operands and needs it to be in the
    // function.
    MD.removeInstruction(DeadInst);

    for (unsigned op = 0, e = DeadInst->getNumOperands(); op != e; ++op) {
      Value *Op = DeadInst->getOperand(op);
      DeadInst->setOperand(op, nullptr);

      // If this operand just became dead, add it to the NowDeadInsts list.
      if (!Op->use_empty()) continue;

      if (Instruction *OpI = dyn_cast<Instruction>(Op))
        if (isInstructionTriviallyDead(OpI, &TLI))
          NowDeadInsts.push_back(OpI);
    }

    if (ValueSet) ValueSet->remove(DeadInst);
    IOL.erase(DeadInst);

    if (NewIter == DeadInst->getIterator())
      NewIter = DeadInst->eraseFromParent();
    else
      DeadInst->eraseFromParent();
  } while (!NowDeadInsts.empty());
  *BBI = NewIter;
  // Pop dead entries from back of ThrowableInst till we find an alive entry.
  while (!ThrowableInst.empty() && !ThrowableInst.back().second)
    ThrowableInst.pop_back();
}

/// Does this instruction write some memory?  This only returns true for things
/// that we can analyze with other helpers below.
static bool hasAnalyzableMemoryWrite(Instruction *I,
                                     const TargetLibraryInfo &TLI) {
  if (isa<StoreInst>(I))
    return true;
  if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(I)) {
    switch (II->getIntrinsicID()) {
    default:
      return false;
    case Intrinsic::memset:
    case Intrinsic::memmove:
    case Intrinsic::memcpy:
    case Intrinsic::memcpy_element_unordered_atomic:
    case Intrinsic::memmove_element_unordered_atomic:
    case Intrinsic::memset_element_unordered_atomic:
    case Intrinsic::init_trampoline:
    case Intrinsic::lifetime_end:
      return true;
    }
  }
  if (auto *CB = dyn_cast<CallBase>(I)) {
    LibFunc LF;
    if (TLI.getLibFunc(*CB, LF) && TLI.has(LF)) {
      switch (LF) {
      case LibFunc_strcpy:
      case LibFunc_strncpy:
      case LibFunc_strcat:
      case LibFunc_strncat:
        return true;
      default:
        return false;
      }
    }
  }
  return false;
}

/// Return a Location stored to by the specified instruction. If isRemovable
/// returns true, this function and getLocForRead completely describe the memory
/// operations for this instruction.
static MemoryLocation getLocForWrite(Instruction *Inst) {

  if (StoreInst *SI = dyn_cast<StoreInst>(Inst))
    return MemoryLocation::get(SI);

  if (auto *MI = dyn_cast<AnyMemIntrinsic>(Inst)) {
    // memcpy/memmove/memset.
    MemoryLocation Loc = MemoryLocation::getForDest(MI);
    return Loc;
  }

  if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(Inst)) {
    switch (II->getIntrinsicID()) {
    default:
      return MemoryLocation(); // Unhandled intrinsic.
    case Intrinsic::init_trampoline:
      return MemoryLocation(II->getArgOperand(0));
    case Intrinsic::lifetime_end: {
      uint64_t Len = cast<ConstantInt>(II->getArgOperand(0))->getZExtValue();
      return MemoryLocation(II->getArgOperand(1), Len);
    }
    }
  }
  if (auto *CB = dyn_cast<CallBase>(Inst))
    // All the supported TLI functions so far happen to have dest as their
    // first argument.
    return MemoryLocation(CB->getArgOperand(0));
  return MemoryLocation();
}

/// Return the location read by the specified "hasAnalyzableMemoryWrite"
/// instruction if any.
static MemoryLocation getLocForRead(Instruction *Inst,
                                    const TargetLibraryInfo &TLI) {
  assert(hasAnalyzableMemoryWrite(Inst, TLI) && "Unknown instruction case");

  // The only instructions that both read and write are the mem transfer
  // instructions (memcpy/memmove).
  if (auto *MTI = dyn_cast<AnyMemTransferInst>(Inst))
    return MemoryLocation::getForSource(MTI);
  return MemoryLocation();
}

/// If the value of this instruction and the memory it writes to is unused, may
/// we delete this instruction?
static bool isRemovable(Instruction *I) {
  // Don't remove volatile/atomic stores.
  if (StoreInst *SI = dyn_cast<StoreInst>(I))
    return SI->isUnordered();

  if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(I)) {
    switch (II->getIntrinsicID()) {
    default: llvm_unreachable("doesn't pass 'hasAnalyzableMemoryWrite' predicate");
    case Intrinsic::lifetime_end:
      // Never remove dead lifetime_end's, e.g. because it is followed by a
      // free.
      return false;
    case Intrinsic::init_trampoline:
      // Always safe to remove init_trampoline.
      return true;
    case Intrinsic::memset:
    case Intrinsic::memmove:
    case Intrinsic::memcpy:
      // Don't remove volatile memory intrinsics.
      return !cast<MemIntrinsic>(II)->isVolatile();
    case Intrinsic::memcpy_element_unordered_atomic:
    case Intrinsic::memmove_element_unordered_atomic:
    case Intrinsic::memset_element_unordered_atomic:
      return true;
    }
  }

  // note: only get here for calls with analyzable writes - i.e. libcalls
  if (auto *CB = dyn_cast<CallBase>(I))
    return CB->use_empty();

  return false;
}

/// 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);
}

/// Return the pointer that is being written to.
static Value *getStoredPointerOperand(Instruction *I) {
  //TODO: factor this to reuse getLocForWrite
  MemoryLocation Loc = getLocForWrite(I);
  assert(Loc.Ptr &&
         "unable to find pointer written for analyzable instruction?");
  // TODO: most APIs don't expect const Value *
  return const_cast<Value*>(Loc.Ptr);
}

static uint64_t 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 Size;
  return MemoryLocation::UnknownSize;
}

namespace {

enum OverwriteResult {
  OW_Begin,
  OW_Complete,
  OW_End,
  OW_PartialEarlierWithFullLater,
  OW_Unknown
};

} // end anonymous namespace

/// Return 'OW_Complete' if a store to the 'Later' location completely
/// overwrites a store to the 'Earlier' location, 'OW_End' if the end of the
/// 'Earlier' location is completely overwritten by 'Later', 'OW_Begin' if the
/// beginning of the 'Earlier' location is overwritten by 'Later'.
/// 'OW_PartialEarlierWithFullLater' means that an earlier (big) store was
/// overwritten by a latter (smaller) store which doesn't write outside the big
/// store's memory locations. Returns 'OW_Unknown' if nothing can be determined.
static OverwriteResult isOverwrite(const MemoryLocation &Later,
                                   const MemoryLocation &Earlier,
                                   const DataLayout &DL,
                                   const TargetLibraryInfo &TLI,
                                   int64_t &EarlierOff, int64_t &LaterOff,
                                   Instruction *DepWrite,
                                   InstOverlapIntervalsTy &IOL,
                                   AliasAnalysis &AA,
                                   const Function *F) {
  // FIXME: Vet that this works for size upper-bounds. Seems unlikely that we'll
  // get imprecise values here, though (except for unknown sizes).
  if (!Later.Size.isPrecise() || !Earlier.Size.isPrecise())
    return OW_Unknown;

  const uint64_t LaterSize = Later.Size.getValue();
  const uint64_t EarlierSize = Earlier.Size.getValue();

  const Value *P1 = Earlier.Ptr->stripPointerCasts();
  const Value *P2 = Later.Ptr->stripPointerCasts();

  // If the start pointers are the same, we just have to compare sizes to see if
  // the later store was larger than the earlier store.
  if (P1 == P2 || AA.isMustAlias(P1, P2)) {
    // Make sure that the Later size is >= the Earlier size.
    if (LaterSize >= EarlierSize)
      return OW_Complete;
  }

  // Check to see if the later store is to the entire object (either a global,
  // an alloca, or a byval/inalloca argument).  If so, then it clearly
  // overwrites any other store to the same object.
  const Value *UO1 = GetUnderlyingObject(P1, DL),
              *UO2 = GetUnderlyingObject(P2, DL);

  // If we can't resolve the same pointers to the same object, then we can't
  // analyze them at all.
  if (UO1 != UO2)
    return OW_Unknown;

  // If the "Later" store is to a recognizable object, get its size.
  uint64_t ObjectSize = getPointerSize(UO2, DL, TLI, F);
  if (ObjectSize != MemoryLocation::UnknownSize)
    if (ObjectSize == LaterSize && ObjectSize >= EarlierSize)
      return OW_Complete;

  // 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.
  EarlierOff = 0;
  LaterOff = 0;
  const Value *BP1 = GetPointerBaseWithConstantOffset(P1, EarlierOff, DL);
  const Value *BP2 = GetPointerBaseWithConstantOffset(P2, LaterOff, DL);

  // If the base pointers still differ, we have two completely different stores.
  if (BP1 != BP2)
    return OW_Unknown;

  // The later store completely overlaps the earlier store if:
  //
  // 1. Both start at the same offset and the later one's size is greater than
  //    or equal to the earlier one's, or
  //
  //      |--earlier--|
  //      |--   later   --|
  //
  // 2. The earlier store has an offset greater than the later offset, but which
  //    still lies completely within the later store.
  //
  //        |--earlier--|
  //    |-----  later  ------|
  //
  // We have to be careful here as *Off is signed while *.Size is unsigned.
  if (EarlierOff >= LaterOff &&
      LaterSize >= EarlierSize &&
      uint64_t(EarlierOff - LaterOff) + EarlierSize <= LaterSize)
    return OW_Complete;

  // We may now overlap, although the overlap is not complete. There might also
  // be other incomplete overlaps, and together, they might cover the complete
  // earlier write.
  // 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 &&
      LaterOff < int64_t(EarlierOff + EarlierSize) &&
      int64_t(LaterOff + LaterSize) >= EarlierOff) {

    // Insert our part of the overlap into the map.
    auto &IM = IOL[DepWrite];
    LLVM_DEBUG(dbgs() << "DSE: Partial overwrite: Earlier [" << EarlierOff
                      << ", " << int64_t(EarlierOff + EarlierSize)
                      << ") Later [" << LaterOff << ", "
                      << int64_t(LaterOff + LaterSize) << ")\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 LaterIntStart = LaterOff, LaterIntEnd = LaterOff + LaterSize;

    // Find any intervals ending at, or after, LaterIntStart which start
    // before LaterIntEnd.
    auto ILI = IM.lower_bound(LaterIntStart);
    if (ILI != IM.end() && ILI->second <= LaterIntEnd) {
      // This existing interval is overlapped with the current store somewhere
      // in [LaterIntStart, LaterIntEnd]. Merge them by erasing the existing
      // intervals and adjusting our start and end.
      LaterIntStart = std::min(LaterIntStart, ILI->second);
      LaterIntEnd = std::max(LaterIntEnd, ILI->first);
      ILI = IM.erase(ILI);

      // Continue erasing and adjusting our end in case other previous
      // intervals are also overlapped with the current store.
      //
      // |--- ealier 1 ---|  |--- ealier 2 ---|
      //     |------- later---------|
      //
      while (ILI != IM.end() && ILI->second <= LaterIntEnd) {
        assert(ILI->second > LaterIntStart && "Unexpected interval");
        LaterIntEnd = std::max(LaterIntEnd, ILI->first);
        ILI = IM.erase(ILI);
      }
    }

    IM[LaterIntEnd] = LaterIntStart;

    ILI = IM.begin();
    if (ILI->second <= EarlierOff &&
        ILI->first >= int64_t(EarlierOff + EarlierSize)) {
      LLVM_DEBUG(dbgs() << "DSE: Full overwrite from partials: Earlier ["
                        << EarlierOff << ", "
                        << int64_t(EarlierOff + EarlierSize)
                        << ") Composite Later [" << ILI->second << ", "
                        << ILI->first << ")\n");
      ++NumCompletePartials;
      return OW_Complete;
    }
  }

  // Check for an earlier store which writes to all the memory locations that
  // the later store writes to.
  if (EnablePartialStoreMerging && LaterOff >= EarlierOff &&
      int64_t(EarlierOff + EarlierSize) > LaterOff &&
      uint64_t(LaterOff - EarlierOff) + LaterSize <= EarlierSize) {
    LLVM_DEBUG(dbgs() << "DSE: Partial overwrite an earlier load ["
                      << EarlierOff << ", "
                      << int64_t(EarlierOff + EarlierSize)
                      << ") by a later store [" << LaterOff << ", "
                      << int64_t(LaterOff + LaterSize) << ")\n");
    // TODO: Maybe come up with a better name?
    return OW_PartialEarlierWithFullLater;
  }

  // Another interesting case is if the later store overwrites the end of the
  // earlier store.
  //
  //      |--earlier--|
  //                |--   later   --|
  //
  // In this case we may want to trim the size of earlier to avoid generating
  // writes to addresses which will definitely be overwritten later
  if (!EnablePartialOverwriteTracking &&
      (LaterOff > EarlierOff && LaterOff < int64_t(EarlierOff + EarlierSize) &&
       int64_t(LaterOff + LaterSize) >= int64_t(EarlierOff + EarlierSize)))
    return OW_End;

  // Finally, we also need to check if the later store overwrites the beginning
  // of the earlier store.
  //
  //                |--earlier--|
  //      |--   later   --|
  //
  // In this case we may want to move the destination address and trim the size
  // of earlier to avoid generating writes to addresses which will definitely
  // be overwritten later.
  if (!EnablePartialOverwriteTracking &&
      (LaterOff <= EarlierOff && int64_t(LaterOff + LaterSize) > EarlierOff)) {
    assert(int64_t(LaterOff + LaterSize) < int64_t(EarlierOff + EarlierSize) &&
           "Expect to be handled as OW_Complete");
    return OW_Begin;
  }
  // Otherwise, they don't completely overlap.
  return OW_Unknown;
}

/// If 'Inst' might be a self read (i.e. a noop copy of a
/// memory region into an identical pointer) then it doesn't actually make its
/// input dead in the traditional sense.  Consider this case:
///
///   memmove(A <- B)
///   memmove(A <- A)
///
/// In this case, the second store to A does not make the first store to A dead.
/// The usual situation isn't an explicit A<-A store like this (which can be
/// trivially removed) but a case where two pointers may alias.
///
/// This function detects when it is unsafe to remove a dependent instruction
/// because the DSE inducing instruction may be a self-read.
static bool isPossibleSelfRead(Instruction *Inst,
                               const MemoryLocation &InstStoreLoc,
                               Instruction *DepWrite,
                               const TargetLibraryInfo &TLI,
                               AliasAnalysis &AA) {
  // Self reads can only happen for instructions that read memory.  Get the
  // location read.
  MemoryLocation InstReadLoc = getLocForRead(Inst, TLI);
  if (!InstReadLoc.Ptr)
    return false; // Not a reading instruction.

  // If the read and written loc obviously don't alias, it isn't a read.
  if (AA.isNoAlias(InstReadLoc, InstStoreLoc))
    return false;

  if (isa<AnyMemCpyInst>(Inst)) {
    // LLVM's memcpy overlap semantics are not fully fleshed out (see PR11763)
    // but in practice memcpy(A <- B) either means that A and B are disjoint or
    // are equal (i.e. there are not partial overlaps).  Given that, if we have:
    //
    //   memcpy/memmove(A <- B)  // DepWrite
    //   memcpy(A <- B)  // Inst
    //
    // with Inst reading/writing a >= size than DepWrite, we can reason as
    // follows:
    //
    //   - If A == B then both the copies are no-ops, so the DepWrite can be
    //     removed.
    //   - If A != B then A and B are disjoint locations in Inst.  Since
    //     Inst.size >= DepWrite.size A and B are disjoint in DepWrite too.
    //     Therefore DepWrite can be removed.
    MemoryLocation DepReadLoc = getLocForRead(DepWrite, TLI);

    if (DepReadLoc.Ptr && AA.isMustAlias(InstReadLoc.Ptr, DepReadLoc.Ptr))
      return false;
  }

  // If DepWrite doesn't read memory or if we can't prove it is a must alias,
  // then it can't be considered dead.
  return true;
}

/// 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,
                                       AliasAnalysis *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 = 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 (auto PredI = pred_begin(B), PE = pred_end(B); PredI != PE; ++PredI) {
        PHITransAddr PredAddr = Addr;
        if (PredAddr.NeedsPHITranslationFromBlock(B)) {
          if (!PredAddr.IsPotentiallyPHITranslatable())
            return false;
          if (PredAddr.PHITranslateValue(B, *PredI, DT, false))
            return false;
        }
        Value *TranslatedPtr = PredAddr.getAddr();
        auto Inserted = Visited.insert(std::make_pair(*PredI, 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(*PredI, PredAddr));
      }
    }
  }
  return true;
}

/// Find all blocks that will unconditionally lead to the block BB and append
/// them to F.
static void findUnconditionalPreds(SmallVectorImpl<BasicBlock *> &Blocks,
                                   BasicBlock *BB, DominatorTree *DT) {
  for (pred_iterator I = pred_begin(BB), E = pred_end(BB); I != E; ++I) {
    BasicBlock *Pred = *I;
    if (Pred == BB) continue;
    Instruction *PredTI = Pred->getTerminator();
    if (PredTI->getNumSuccessors() != 1)
      continue;

    if (DT->isReachableFromEntry(Pred))
      Blocks.push_back(Pred);
  }
}

/// Handle frees of entire structures whose dependency is a store
/// to a field of that structure.
static bool handleFree(CallInst *F, AliasAnalysis *AA,
                       MemoryDependenceResults *MD, DominatorTree *DT,
                       const TargetLibraryInfo *TLI,
                       InstOverlapIntervalsTy &IOL,
                       MapVector<Instruction *, bool> &ThrowableInst) {
  bool MadeChange = false;

  MemoryLocation Loc = MemoryLocation(F->getOperand(0));
  SmallVector<BasicBlock *, 16> Blocks;
  Blocks.push_back(F->getParent());
  const DataLayout &DL = F->getModule()->getDataLayout();

  while (!Blocks.empty()) {
    BasicBlock *BB = Blocks.pop_back_val();
    Instruction *InstPt = BB->getTerminator();
    if (BB == F->getParent()) InstPt = F;

    MemDepResult Dep =
        MD->getPointerDependencyFrom(Loc, false, InstPt->getIterator(), BB);
    while (Dep.isDef() || Dep.isClobber()) {
      Instruction *Dependency = Dep.getInst();
      if (!hasAnalyzableMemoryWrite(Dependency, *TLI) ||
          !isRemovable(Dependency))
        break;

      Value *DepPointer =
          GetUnderlyingObject(getStoredPointerOperand(Dependency), DL);

      // Check for aliasing.
      if (!AA->isMustAlias(F->getArgOperand(0), DepPointer))
        break;

      LLVM_DEBUG(
          dbgs() << "DSE: Dead Store to soon to be freed memory:\n  DEAD: "
                 << *Dependency << '\n');

      // DCE instructions only used to calculate that store.
      BasicBlock::iterator BBI(Dependency);
      deleteDeadInstruction(Dependency, &BBI, *MD, *TLI, IOL,
                            ThrowableInst);
      ++NumFastStores;
      MadeChange = true;

      // Inst's old Dependency is now deleted. Compute the next dependency,
      // which may also be dead, as in
      //    s[0] = 0;
      //    s[1] = 0; // This has just been deleted.
      //    free(s);
      Dep = MD->getPointerDependencyFrom(Loc, false, BBI, BB);
    }

    if (Dep.isNonLocal())
      findUnconditionalPreds(Blocks, BB, DT);
  }

  return MadeChange;
}

/// Check to see if the specified location may alias any of the stack objects in
/// the DeadStackObjects set. If so, they become live because the location is
/// being loaded.
static void removeAccessedObjects(const MemoryLocation &LoadedLoc,
                                  SmallSetVector<const Value *, 16> &DeadStackObjects,
                                  const DataLayout &DL, AliasAnalysis *AA,
                                  const TargetLibraryInfo *TLI,
                                  const Function *F) {
  const Value *UnderlyingPointer = GetUnderlyingObject(LoadedLoc.Ptr, DL);

  // A constant can't be in the dead pointer set.
  if (isa<Constant>(UnderlyingPointer))
    return;

  // If the kill pointer can be easily reduced to an alloca, don't bother doing
  // extraneous AA queries.
  if (isa<AllocaInst>(UnderlyingPointer) || isa<Argument>(UnderlyingPointer)) {
    DeadStackObjects.remove(UnderlyingPointer);
    return;
  }

  // Remove objects that could alias LoadedLoc.
  DeadStackObjects.remove_if([&](const Value *I) {
    // See if the loaded location could alias the stack location.
    MemoryLocation StackLoc(I, getPointerSize(I, DL, *TLI, F));
    return !AA->isNoAlias(StackLoc, LoadedLoc);
  });
}

/// Remove dead stores to stack-allocated locations in the function end block.
/// Ex:
/// %A = alloca i32
/// ...
/// store i32 1, i32* %A
/// ret void
static bool handleEndBlock(BasicBlock &BB, AliasAnalysis *AA,
                           MemoryDependenceResults *MD,
                           const TargetLibraryInfo *TLI,
                           InstOverlapIntervalsTy &IOL,
                           MapVector<Instruction *, bool> &ThrowableInst) {
  bool MadeChange = false;

  // Keep track of all of the stack objects that are dead at the end of the
  // function.
  SmallSetVector<const Value*, 16> DeadStackObjects;

  // Find all of the alloca'd pointers in the entry block.
  BasicBlock &Entry = BB.getParent()->front();
  for (Instruction &I : Entry) {
    if (isa<AllocaInst>(&I))
      DeadStackObjects.insert(&I);

    // Okay, so these are dead heap objects, but if the pointer never escapes
    // then it's leaked by this function anyways.
    else if (isAllocLikeFn(&I, TLI) && !PointerMayBeCaptured(&I, true, true))
      DeadStackObjects.insert(&I);
  }

  // Treat byval or inalloca arguments the same, stores to them are dead at the
  // end of the function.
  for (Argument &AI : BB.getParent()->args())
    if (AI.hasPassPointeeByValueAttr())
      DeadStackObjects.insert(&AI);

  const DataLayout &DL = BB.getModule()->getDataLayout();

  // Scan the basic block backwards
  for (BasicBlock::iterator BBI = BB.end(); BBI != BB.begin(); ){
    --BBI;

    // If we find a store, check to see if it points into a dead stack value.
    if (hasAnalyzableMemoryWrite(&*BBI, *TLI) && isRemovable(&*BBI)) {
      // See through pointer-to-pointer bitcasts
      SmallVector<const Value *, 4> Pointers;
      GetUnderlyingObjects(getStoredPointerOperand(&*BBI), Pointers, DL);

      // Stores to stack values are valid candidates for removal.
      bool AllDead = true;
      for (const Value *Pointer : Pointers)
        if (!DeadStackObjects.count(Pointer)) {
          AllDead = false;
          break;
        }

      if (AllDead) {
        Instruction *Dead = &*BBI;

        LLVM_DEBUG(dbgs() << "DSE: Dead Store at End of Block:\n  DEAD: "
                          << *Dead << "\n  Objects: ";
                   for (SmallVectorImpl<const Value *>::iterator I =
                            Pointers.begin(),
                        E = Pointers.end();
                        I != E; ++I) {
                     dbgs() << **I;
                     if (std::next(I) != E)
                       dbgs() << ", ";
                   } dbgs()
                   << '\n');

        // DCE instructions only used to calculate that store.
        deleteDeadInstruction(Dead, &BBI, *MD, *TLI, IOL, ThrowableInst,
                              &DeadStackObjects);
        ++NumFastStores;
        MadeChange = true;
        continue;
      }
    }

    // Remove any dead non-memory-mutating instructions.
    if (isInstructionTriviallyDead(&*BBI, TLI)) {
      LLVM_DEBUG(dbgs() << "DSE: Removing trivially dead instruction:\n  DEAD: "
                        << *&*BBI << '\n');
      deleteDeadInstruction(&*BBI, &BBI, *MD, *TLI, IOL, ThrowableInst,
                            &DeadStackObjects);
      ++NumFastOther;
      MadeChange = true;
      continue;
    }

    if (isa<AllocaInst>(BBI)) {
      // Remove allocas from the list of dead stack objects; there can't be
      // any references before the definition.
      DeadStackObjects.remove(&*BBI);
      continue;
    }

    if (auto *Call = dyn_cast<CallBase>(&*BBI)) {
      // Remove allocation function calls from the list of dead stack objects;
      // there can't be any references before the definition.
      if (isAllocLikeFn(&*BBI, TLI))
        DeadStackObjects.remove(&*BBI);

      // If this call does not access memory, it can't be loading any of our
      // pointers.
      if (AA->doesNotAccessMemory(Call))
        continue;

      // If the call might load from any of our allocas, then any store above
      // the call is live.
      DeadStackObjects.remove_if([&](const Value *I) {
        // See if the call site touches the value.
        return isRefSet(AA->getModRefInfo(
            Call, I, getPointerSize(I, DL, *TLI, BB.getParent())));
      });

      // If all of the allocas were clobbered by the call then we're not going
      // to find anything else to process.
      if (DeadStackObjects.empty())
        break;

      continue;
    }

    // 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>(*BBI))
      continue;

    MemoryLocation LoadedLoc;

    // If we encounter a use of the pointer, it is no longer considered dead
    if (LoadInst *L = dyn_cast<LoadInst>(BBI)) {
      if (!L->isUnordered()) // Be conservative with atomic/volatile load
        break;
      LoadedLoc = MemoryLocation::get(L);
    } else if (VAArgInst *V = dyn_cast<VAArgInst>(BBI)) {
      LoadedLoc = MemoryLocation::get(V);
    } else if (!BBI->mayReadFromMemory()) {
      // Instruction doesn't read memory.  Note that stores that weren't removed
      // above will hit this case.
      continue;
    } else {
      // Unknown inst; assume it clobbers everything.
      break;
    }

    // Remove any allocas from the DeadPointer set that are loaded, as this
    // makes any stores above the access live.
    removeAccessedObjects(LoadedLoc, DeadStackObjects, DL, AA, TLI, BB.getParent());

    // If all of the allocas were clobbered by the access then we're not going
    // to find anything else to process.
    if (DeadStackObjects.empty())
      break;
  }

  return MadeChange;
}

static bool tryToShorten(Instruction *EarlierWrite, int64_t &EarlierOffset,
                         int64_t &EarlierSize, int64_t LaterOffset,
                         int64_t LaterSize, bool IsOverwriteEnd) {
  // TODO: base this on the target vector size so that if the earlier
  // store was too small to get vector writes anyway then its likely
  // a good idea to shorten it
  // Power of 2 vector writes are probably always a bad idea to optimize
  // as any store/memset/memcpy is likely using vector instructions so
  // shortening it to not vector size is likely to be slower
  auto *EarlierIntrinsic = cast<AnyMemIntrinsic>(EarlierWrite);
  unsigned EarlierWriteAlign = EarlierIntrinsic->getDestAlignment();
  if (!IsOverwriteEnd)
    LaterOffset = int64_t(LaterOffset + LaterSize);

  if (!(isPowerOf2_64(LaterOffset) && EarlierWriteAlign <= LaterOffset) &&
      !((EarlierWriteAlign != 0) && LaterOffset % EarlierWriteAlign == 0))
    return false;

  int64_t NewLength = IsOverwriteEnd
                          ? LaterOffset - EarlierOffset
                          : EarlierSize - (LaterOffset - EarlierOffset);

  if (auto *AMI = dyn_cast<AtomicMemIntrinsic>(EarlierWrite)) {
    // When shortening an atomic memory intrinsic, the newly shortened
    // length must remain an integer multiple of the element size.
    const uint32_t ElementSize = AMI->getElementSizeInBytes();
    if (0 != NewLength % ElementSize)
      return false;
  }

  LLVM_DEBUG(dbgs() << "DSE: Remove Dead Store:\n  OW "
                    << (IsOverwriteEnd ? "END" : "BEGIN") << ": "
                    << *EarlierWrite << "\n  KILLER (offset " << LaterOffset
                    << ", " << EarlierSize << ")\n");

  Value *EarlierWriteLength = EarlierIntrinsic->getLength();
  Value *TrimmedLength =
      ConstantInt::get(EarlierWriteLength->getType(), NewLength);
  EarlierIntrinsic->setLength(TrimmedLength);

  EarlierSize = NewLength;
  if (!IsOverwriteEnd) {
    int64_t OffsetMoved = (LaterOffset - EarlierOffset);
    Value *Indices[1] = {
        ConstantInt::get(EarlierWriteLength->getType(), OffsetMoved)};
    GetElementPtrInst *NewDestGEP = GetElementPtrInst::CreateInBounds(
        EarlierIntrinsic->getRawDest()->getType()->getPointerElementType(),
        EarlierIntrinsic->getRawDest(), Indices, "", EarlierWrite);
    NewDestGEP->setDebugLoc(EarlierIntrinsic->getDebugLoc());
    EarlierIntrinsic->setDest(NewDestGEP);
    EarlierOffset = EarlierOffset + OffsetMoved;
  }
  return true;
}

static bool tryToShortenEnd(Instruction *EarlierWrite,
                            OverlapIntervalsTy &IntervalMap,
                            int64_t &EarlierStart, int64_t &EarlierSize) {
  if (IntervalMap.empty() || !isShortenableAtTheEnd(EarlierWrite))
    return false;

  OverlapIntervalsTy::iterator OII = --IntervalMap.end();
  int64_t LaterStart = OII->second;
  int64_t LaterSize = OII->first - LaterStart;

  if (LaterStart > EarlierStart && LaterStart < EarlierStart + EarlierSize &&
      LaterStart + LaterSize >= EarlierStart + EarlierSize) {
    if (tryToShorten(EarlierWrite, EarlierStart, EarlierSize, LaterStart,
                     LaterSize, true)) {
      IntervalMap.erase(OII);
      return true;
    }
  }
  return false;
}

static bool tryToShortenBegin(Instruction *EarlierWrite,
                              OverlapIntervalsTy &IntervalMap,
                              int64_t &EarlierStart, int64_t &EarlierSize) {
  if (IntervalMap.empty() || !isShortenableAtTheBeginning(EarlierWrite))
    return false;

  OverlapIntervalsTy::iterator OII = IntervalMap.begin();
  int64_t LaterStart = OII->second;
  int64_t LaterSize = OII->first - LaterStart;

  if (LaterStart <= EarlierStart && LaterStart + LaterSize > EarlierStart) {
    assert(LaterStart + LaterSize < EarlierStart + EarlierSize &&
           "Should have been handled as OW_Complete");
    if (tryToShorten(EarlierWrite, EarlierStart, EarlierSize, LaterStart,
                     LaterSize, false)) {
      IntervalMap.erase(OII);
      return true;
    }
  }
  return false;
}

static bool removePartiallyOverlappedStores(AliasAnalysis *AA,
                                            const DataLayout &DL,
                                            InstOverlapIntervalsTy &IOL) {
  bool Changed = false;
  for (auto OI : IOL) {
    Instruction *EarlierWrite = OI.first;
    MemoryLocation Loc = getLocForWrite(EarlierWrite);
    assert(isRemovable(EarlierWrite) && "Expect only removable instruction");

    const Value *Ptr = Loc.Ptr->stripPointerCasts();
    int64_t EarlierStart = 0;
    int64_t EarlierSize = int64_t(Loc.Size.getValue());
    GetPointerBaseWithConstantOffset(Ptr, EarlierStart, DL);
    OverlapIntervalsTy &IntervalMap = OI.second;
    Changed |=
        tryToShortenEnd(EarlierWrite, IntervalMap, EarlierStart, EarlierSize);
    if (IntervalMap.empty())
      continue;
    Changed |=
        tryToShortenBegin(EarlierWrite, IntervalMap, EarlierStart, EarlierSize);
  }
  return Changed;
}

static bool eliminateNoopStore(Instruction *Inst, BasicBlock::iterator &BBI,
                               AliasAnalysis *AA, MemoryDependenceResults *MD,
                               const DataLayout &DL,
                               const TargetLibraryInfo *TLI,
                               InstOverlapIntervalsTy &IOL,
                               MapVector<Instruction *, bool> &ThrowableInst,
                               DominatorTree *DT) {
  // Must be a store instruction.
  StoreInst *SI = dyn_cast<StoreInst>(Inst);
  if (!SI)
    return false;

  // If we're storing the same value back to a pointer that we just loaded from,
  // then the store can be removed.
  if (LoadInst *DepLoad = dyn_cast<LoadInst>(SI->getValueOperand())) {
    if (SI->getPointerOperand() == DepLoad->getPointerOperand() &&
        isRemovable(SI) &&
        memoryIsNotModifiedBetween(DepLoad, SI, AA, DL, DT)) {

      LLVM_DEBUG(
          dbgs() << "DSE: Remove Store Of Load from same pointer:\n  LOAD: "
                 << *DepLoad << "\n  STORE: " << *SI << '\n');

      deleteDeadInstruction(SI, &BBI, *MD, *TLI, IOL, ThrowableInst);
      ++NumRedundantStores;
      return true;
    }
  }

  // Remove null stores into the calloc'ed objects
  Constant *StoredConstant = dyn_cast<Constant>(SI->getValueOperand());
  if (StoredConstant && StoredConstant->isNullValue() && isRemovable(SI)) {
    Instruction *UnderlyingPointer =
        dyn_cast<Instruction>(GetUnderlyingObject(SI->getPointerOperand(), DL));

    if (UnderlyingPointer && isCallocLikeFn(UnderlyingPointer, TLI) &&
        memoryIsNotModifiedBetween(UnderlyingPointer, SI, AA, DL, DT)) {
      LLVM_DEBUG(
          dbgs() << "DSE: Remove null store to the calloc'ed object:\n  DEAD: "
                 << *Inst << "\n  OBJECT: " << *UnderlyingPointer << '\n');

      deleteDeadInstruction(SI, &BBI, *MD, *TLI, IOL, ThrowableInst);
      ++NumRedundantStores;
      return true;
    }
  }
  return false;
}

static Constant *
tryToMergePartialOverlappingStores(StoreInst *Earlier, StoreInst *Later,
                                   int64_t InstWriteOffset,
                                   int64_t DepWriteOffset, const DataLayout &DL,
                                   AliasAnalysis *AA, DominatorTree *DT) {

  if (Earlier && isa<ConstantInt>(Earlier->getValueOperand()) &&
      DL.typeSizeEqualsStoreSize(Earlier->getValueOperand()->getType()) &&
      Later && isa<ConstantInt>(Later->getValueOperand()) &&
      DL.typeSizeEqualsStoreSize(Later->getValueOperand()->getType()) &&
      memoryIsNotModifiedBetween(Earlier, Later, AA, DL, DT)) {
    // If the store we find is:
    //   a) partially overwritten by the store to 'Loc'
    //   b) the later store is fully contained in the earlier one and
    //   c) they both have a constant value
    //   d) none of the two stores need padding
    // Merge the two stores, replacing the earlier store's value with a
    // merge of both values.
    // TODO: Deal with other constant types (vectors, etc), and probably
    // some mem intrinsics (if needed)

    APInt EarlierValue =
        cast<ConstantInt>(Earlier->getValueOperand())->getValue();
    APInt LaterValue = cast<ConstantInt>(Later->getValueOperand())->getValue();
    unsigned LaterBits = LaterValue.getBitWidth();
    assert(EarlierValue.getBitWidth() > LaterValue.getBitWidth());
    LaterValue = LaterValue.zext(EarlierValue.getBitWidth());

    // Offset of the smaller store inside the larger store
    unsigned BitOffsetDiff = (InstWriteOffset - DepWriteOffset) * 8;
    unsigned LShiftAmount = DL.isBigEndian() ? EarlierValue.getBitWidth() -
                                                   BitOffsetDiff - LaterBits
                                             : BitOffsetDiff;
    APInt Mask = APInt::getBitsSet(EarlierValue.getBitWidth(), LShiftAmount,
                                   LShiftAmount + LaterBits);
    // Clear the bits we'll be replacing, then OR with the smaller
    // store, shifted appropriately.
    APInt Merged = (EarlierValue & ~Mask) | (LaterValue << LShiftAmount);
    LLVM_DEBUG(dbgs() << "DSE: Merge Stores:\n  Earlier: " << *Earlier
                      << "\n  Later: " << *Later
                      << "\n  Merged Value: " << Merged << '\n');
    return ConstantInt::get(Earlier->getValueOperand()->getType(), Merged);
  }
  return nullptr;
}

static bool eliminateDeadStores(BasicBlock &BB, AliasAnalysis *AA,
                                MemoryDependenceResults *MD, DominatorTree *DT,
                                const TargetLibraryInfo *TLI) {
  const DataLayout &DL = BB.getModule()->getDataLayout();
  bool MadeChange = false;

  MapVector<Instruction *, bool> ThrowableInst;

  // A map of interval maps representing partially-overwritten value parts.
  InstOverlapIntervalsTy IOL;

  // Do a top-down walk on the BB.
  for (BasicBlock::iterator BBI = BB.begin(), BBE = BB.end(); BBI != BBE; ) {
    // Handle 'free' calls specially.
    if (CallInst *F = isFreeCall(&*BBI, TLI)) {
      MadeChange |= handleFree(F, AA, MD, DT, TLI, IOL, ThrowableInst);
      // Increment BBI after handleFree has potentially deleted instructions.
      // This ensures we maintain a valid iterator.
      ++BBI;
      continue;
    }

    Instruction *Inst = &*BBI++;

    if (Inst->mayThrow()) {
      ThrowableInst[Inst] = true;
      continue;
    }

    // Check to see if Inst writes to memory.  If not, continue.
    if (!hasAnalyzableMemoryWrite(Inst, *TLI))
      continue;

    // eliminateNoopStore will update in iterator, if necessary.
    if (eliminateNoopStore(Inst, BBI, AA, MD, DL, TLI, IOL,
                           ThrowableInst, DT)) {
      MadeChange = true;
      continue;
    }

    // If we find something that writes memory, get its memory dependence.
    MemDepResult InstDep = MD->getDependency(Inst);

    // Ignore any store where we can't find a local dependence.
    // FIXME: cross-block DSE would be fun. :)
    if (!InstDep.isDef() && !InstDep.isClobber())
      continue;

    // Figure out what location is being stored to.
    MemoryLocation Loc = getLocForWrite(Inst);

    // If we didn't get a useful location, fail.
    if (!Loc.Ptr)
      continue;

    // Loop until we find a store we can eliminate or a load that
    // invalidates the analysis. Without an upper bound on the number of
    // instructions examined, this analysis can become very time-consuming.
    // However, the potential gain diminishes as we process more instructions
    // without eliminating any of them. Therefore, we limit the number of
    // instructions we look at.
    auto Limit = MD->getDefaultBlockScanLimit();
    while (InstDep.isDef() || InstDep.isClobber()) {
      // Get the memory clobbered by the instruction we depend on.  MemDep will
      // skip any instructions that 'Loc' clearly doesn't interact with.  If we
      // end up depending on a may- or must-aliased load, then we can't optimize
      // away the store and we bail out.  However, if we depend on something
      // that overwrites the memory location we *can* potentially optimize it.
      //
      // Find out what memory location the dependent instruction stores.
      Instruction *DepWrite = InstDep.getInst();
      if (!hasAnalyzableMemoryWrite(DepWrite, *TLI))
        break;
      MemoryLocation DepLoc = getLocForWrite(DepWrite);
      // If we didn't get a useful location, or if it isn't a size, bail out.
      if (!DepLoc.Ptr)
        break;

      // Find the last throwable instruction not removed by call to
      // deleteDeadInstruction.
      Instruction *LastThrowing = nullptr;
      if (!ThrowableInst.empty())
        LastThrowing = ThrowableInst.back().first;

      // Make sure we don't look past a call which might throw. This is an
      // issue because MemoryDependenceAnalysis works in the wrong direction:
      // it finds instructions which dominate the current instruction, rather than
      // instructions which are post-dominated by the current instruction.
      //
      // If the underlying object is a non-escaping memory allocation, any store
      // to it is dead along the unwind edge. Otherwise, we need to preserve
      // the store.
      if (LastThrowing && DepWrite->comesBefore(LastThrowing)) {
        const Value* Underlying = GetUnderlyingObject(DepLoc.Ptr, DL);
        bool IsStoreDeadOnUnwind = isa<AllocaInst>(Underlying);
        if (!IsStoreDeadOnUnwind) {
            // We're looking for a call to an allocation function
            // where the allocation doesn't escape before the last
            // throwing instruction; PointerMayBeCaptured
            // reasonably fast approximation.
            IsStoreDeadOnUnwind = isAllocLikeFn(Underlying, TLI) &&
                !PointerMayBeCaptured(Underlying, false, true);
        }
        if (!IsStoreDeadOnUnwind)
          break;
      }

      // If we find a write that is a) removable (i.e., non-volatile), b) is
      // completely obliterated by the store to 'Loc', and c) which we know that
      // 'Inst' doesn't load from, then we can remove it.
      // Also try to merge two stores if a later one only touches memory written
      // to by the earlier one.
      if (isRemovable(DepWrite) &&
          !isPossibleSelfRead(Inst, Loc, DepWrite, *TLI, *AA)) {
        int64_t InstWriteOffset, DepWriteOffset;
        OverwriteResult OR = isOverwrite(Loc, DepLoc, DL, *TLI, DepWriteOffset,
                                         InstWriteOffset, DepWrite, IOL, *AA,
                                         BB.getParent());
        if (OR == OW_Complete) {
          LLVM_DEBUG(dbgs() << "DSE: Remove Dead Store:\n  DEAD: " << *DepWrite
                            << "\n  KILLER: " << *Inst << '\n');

          // Delete the store and now-dead instructions that feed it.
          deleteDeadInstruction(DepWrite, &BBI, *MD, *TLI, IOL,
                                ThrowableInst);
          ++NumFastStores;
          MadeChange = true;

          // We erased DepWrite; start over.
          InstDep = MD->getDependency(Inst);
          continue;
        } else if ((OR == OW_End && isShortenableAtTheEnd(DepWrite)) ||
                   ((OR == OW_Begin &&
                     isShortenableAtTheBeginning(DepWrite)))) {
          assert(!EnablePartialOverwriteTracking && "Do not expect to perform "
                                                    "when partial-overwrite "
                                                    "tracking is enabled");
          // The overwrite result is known, so these must be known, too.
          int64_t EarlierSize = DepLoc.Size.getValue();
          int64_t LaterSize = Loc.Size.getValue();
          bool IsOverwriteEnd = (OR == OW_End);
          MadeChange |= tryToShorten(DepWrite, DepWriteOffset, EarlierSize,
                                    InstWriteOffset, LaterSize, IsOverwriteEnd);
        } else if (EnablePartialStoreMerging &&
                   OR == OW_PartialEarlierWithFullLater) {
          auto *Earlier = dyn_cast<StoreInst>(DepWrite);
          auto *Later = dyn_cast<StoreInst>(Inst);
          if (Constant *C = tryToMergePartialOverlappingStores(
                  Earlier, Later, InstWriteOffset, DepWriteOffset, DL, AA,
                  DT)) {
            auto *SI = new StoreInst(
                C, Earlier->getPointerOperand(), false, Earlier->getAlign(),
                Earlier->getOrdering(), Earlier->getSyncScopeID(), DepWrite);

            unsigned MDToKeep[] = {LLVMContext::MD_dbg, LLVMContext::MD_tbaa,
                                   LLVMContext::MD_alias_scope,
                                   LLVMContext::MD_noalias,
                                   LLVMContext::MD_nontemporal};
            SI->copyMetadata(*DepWrite, MDToKeep);
            ++NumModifiedStores;

            // Delete the old stores and now-dead instructions that feed them.
            deleteDeadInstruction(Inst, &BBI, *MD, *TLI, IOL,
                                  ThrowableInst);
            deleteDeadInstruction(DepWrite, &BBI, *MD, *TLI, IOL,
                                  ThrowableInst);
            MadeChange = true;

            // We erased DepWrite and Inst (Loc); start over.
            break;
          }
        }
      }

      // If this is a may-aliased store that is clobbering the store value, we
      // can keep searching past it for another must-aliased pointer that stores
      // to the same location.  For example, in:
      //   store -> P
      //   store -> Q
      //   store -> P
      // we can remove the first store to P even though we don't know if P and Q
      // alias.
      if (DepWrite == &BB.front()) break;

      // Can't look past this instruction if it might read 'Loc'.
      if (isRefSet(AA->getModRefInfo(DepWrite, Loc)))
        break;

      InstDep = MD->getPointerDependencyFrom(Loc, /*isLoad=*/ false,
                                             DepWrite->getIterator(), &BB,
                                             /*QueryInst=*/ nullptr, &Limit);
    }
  }

  if (EnablePartialOverwriteTracking)
    MadeChange |= removePartiallyOverlappedStores(AA, DL, IOL);

  // If this block ends in a return, unwind, or unreachable, all allocas are
  // dead at its end, which means stores to them are also dead.
  if (BB.getTerminator()->getNumSuccessors() == 0)
    MadeChange |= handleEndBlock(BB, AA, MD, TLI, IOL, ThrowableInst);

  return MadeChange;
}

static bool eliminateDeadStores(Function &F, AliasAnalysis *AA,
                                MemoryDependenceResults *MD, DominatorTree *DT,
                                const TargetLibraryInfo *TLI) {
  bool MadeChange = false;
  for (BasicBlock &BB : F)
    // Only check non-dead blocks.  Dead blocks may have strange pointer
    // cycles that will confuse alias analysis.
    if (DT->isReachableFromEntry(&BB))
      MadeChange |= eliminateDeadStores(BB, AA, MD, DT, TLI);

  return MadeChange;
}

namespace {
//=============================================================================
// MemorySSA backed dead store elimination.
//
// 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 (DomAccess) by walking
//    upwards.
// 2. Check that there are no reads between DomAccess and the StartDef by
//    checking all uses starting at DomAccess and walking until we see StartDef.
// 3. For each found DomDef, check that:
//   1. There are no barrier instructions between DomDef and StartDef (like
//       throws or stores with ordering constraints).
//   2. StartDef is executed whenever DomDef is executed.
//   3. StartDef completely overwrites DomDef.
// 4. Erase DomDef from the function and MemorySSA.

// Returns true if \p M is an intrisnic that does not read or write memory.
bool isNoopIntrinsic(MemoryUseOrDef *M) {
  if (const IntrinsicInst *II = dyn_cast<IntrinsicInst>(M->getMemoryInst())) {
    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_addr:
    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.
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(D))
    return true;

  return false;
}

struct DSEState {
  Function &F;
  AliasAnalysis &AA;
  MemorySSA &MSSA;
  DominatorTree &DT;
  PostDominatorTree &PDT;
  const TargetLibraryInfo &TLI;

  // 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 of all of the objects that are invisible to the caller before
  // the function returns.
  SmallPtrSet<const Value *, 16> InvisibleToCallerBeforeRet;
  // Keep track of all of the objects that are invisible to the caller after
  // the function returns.
  SmallPtrSet<const Value *, 16> InvisibleToCallerAfterRet;
  // 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.
  DenseMap<BasicBlock *, InstOverlapIntervalsTy> IOLs;

  DSEState(Function &F, AliasAnalysis &AA, MemorySSA &MSSA, DominatorTree &DT,
           PostDominatorTree &PDT, const TargetLibraryInfo &TLI)
      : F(F), AA(AA), MSSA(MSSA), DT(DT), PDT(PDT), TLI(TLI) {}

  static DSEState get(Function &F, AliasAnalysis &AA, MemorySSA &MSSA,
                      DominatorTree &DT, PostDominatorTree &PDT,
                      const TargetLibraryInfo &TLI) {
    DSEState State(F, AA, MSSA, DT, PDT, TLI);
    // Collect blocks with throwing instructions not modeled in MemorySSA and
    // alloc-like objects.
    unsigned PO = 0;
    for (BasicBlock *BB : post_order(&F)) {
      State.PostOrderNumbers[BB] = PO++;
      for (Instruction &I : *BB) {
        MemoryAccess *MA = MSSA.getMemoryAccess(&I);
        if (I.mayThrow() && !MA)
          State.ThrowingBlocks.insert(I.getParent());

        auto *MD = dyn_cast_or_null<MemoryDef>(MA);
        if (MD && State.MemDefs.size() < MemorySSADefsPerBlockLimit &&
            (State.getLocForWriteEx(&I) || State.isMemTerminatorInst(&I)))
          State.MemDefs.push_back(MD);

        // Track whether alloca and alloca-like objects are visible in the
        // caller before and after the function returns. Alloca objects are
        // invalid in the caller, so they are neither visible before or after
        // the function returns.
        if (isa<AllocaInst>(&I)) {
          State.InvisibleToCallerBeforeRet.insert(&I);
          State.InvisibleToCallerAfterRet.insert(&I);
        }

        // For alloca-like objects we need to check if they are captured before
        // the function returns and if the return might capture the object.
        if (isAllocLikeFn(&I, &TLI)) {
          bool CapturesBeforeRet = PointerMayBeCaptured(&I, false, true);
          if (!CapturesBeforeRet) {
            State.InvisibleToCallerBeforeRet.insert(&I);
            if (!PointerMayBeCaptured(&I, true, false))
              State.InvisibleToCallerAfterRet.insert(&I);
          }
        }
      }
    }

    // Treat byval or inalloca arguments the same as Allocas, stores to them are
    // dead at the end of the function.
    for (Argument &AI : F.args())
      if (AI.hasPassPointeeByValueAttr()) {
        // For byval, the caller doesn't know the address of the allocation.
        if (AI.hasByValAttr())
          State.InvisibleToCallerBeforeRet.insert(&AI);
        State.InvisibleToCallerAfterRet.insert(&AI);
      }

    return State;
  }

  Optional<MemoryLocation> getLocForWriteEx(Instruction *I) const {
    if (!I->mayWriteToMemory())
      return None;

    if (auto *MTI = dyn_cast<AnyMemIntrinsic>(I))
      return {MemoryLocation::getForDest(MTI)};

    if (auto *CB = dyn_cast<CallBase>(I)) {
      LibFunc LF;
      if (TLI.getLibFunc(*CB, LF) && TLI.has(LF)) {
        switch (LF) {
        case LibFunc_strcpy:
        case LibFunc_strncpy:
        case LibFunc_strcat:
        case LibFunc_strncat:
          return {MemoryLocation(CB->getArgOperand(0))};
        default:
          break;
        }
      }
      return None;
    }

    return MemoryLocation::getOrNone(I);
  }

  /// Returns true if \p Use completely overwrites \p DefLoc.
  bool isCompleteOverwrite(MemoryLocation DefLoc, Instruction *UseInst) const {
    // 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;
    auto CC = getLocForWriteEx(UseInst);
    InstOverlapIntervalsTy IOL;

    const DataLayout &DL = F.getParent()->getDataLayout();

    return CC &&
           isOverwrite(*CC, DefLoc, DL, TLI, DepWriteOffset, InstWriteOffset,
                       UseInst, IOL, AA, &F) == OW_Complete;
  }

  /// Returns true if \p Def is not read before returning from the function.
  bool isWriteAtEndOfFunction(MemoryDef *Def) {
    LLVM_DEBUG(dbgs() << "  Check if def " << *Def << " ("
                      << *Def->getMemoryInst()
                      << ") is at the end the function \n");

    auto MaybeLoc = getLocForWriteEx(Def->getMemoryInst());
    if (!MaybeLoc) {
      LLVM_DEBUG(dbgs() << "  ... could not get location for write.\n");
      return false;
    }

    SmallVector<MemoryAccess *, 4> WorkList;
    SmallPtrSet<MemoryAccess *, 8> Visited;
    auto PushMemUses = [&WorkList, &Visited](MemoryAccess *Acc) {
      if (!Visited.insert(Acc).second)
        return;
      for (Use &U : Acc->uses())
        WorkList.push_back(cast<MemoryAccess>(U.getUser()));
    };
    PushMemUses(Def);
    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)) {
        PushMemUses(UseAccess);
        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(*MaybeLoc, UseInst)) {
        LLVM_DEBUG(dbgs() << "  ... hit read clobber " << *UseInst << ".\n");
        return false;
      }

      if (MemoryDef *UseDef = dyn_cast<MemoryDef>(UseAccess))
        PushMemUses(UseDef);
    }
    return true;
  }

  /// 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.
  Optional<std::pair<MemoryLocation, bool>>
  getLocForTerminator(Instruction *I) const {
    uint64_t Len;
    Value *Ptr;
    if (match(I, m_Intrinsic<Intrinsic::lifetime_end>(m_ConstantInt(Len),
                                                      m_Value(Ptr))))
      return {std::make_pair(MemoryLocation(Ptr, Len), false)};

    if (auto *CB = dyn_cast<CallBase>(I)) {
      if (isFreeCall(I, &TLI))
        return {std::make_pair(MemoryLocation(CB->getArgOperand(0)), true)};
    }

    return None;
  }

  /// Returns true if \p I is a memory terminator instruction like
  /// llvm.lifetime.end or free.
  bool isMemTerminatorInst(Instruction *I) const {
    IntrinsicInst *II = dyn_cast<IntrinsicInst>(I);
    return (II && II->getIntrinsicID() == Intrinsic::lifetime_end) ||
           isFreeCall(I, &TLI);
  }

  /// Returns true if \p MaybeTerm is a memory terminator for the same
  /// underlying object as \p DefLoc.
  bool isMemTerminator(MemoryLocation DefLoc, Instruction *MaybeTerm) const {
    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 (MaybeTermLoc->second) {
      DataLayout DL = MaybeTerm->getParent()->getModule()->getDataLayout();
      DefLoc = MemoryLocation(GetUnderlyingObject(DefLoc.Ptr, DL));
    }
    return AA.isMustAlias(MaybeTermLoc->first, DefLoc);
  }

  // Returns true if \p Use may read from \p DefLoc.
  bool isReadClobber(MemoryLocation DefLoc, Instruction *UseInst) const {
    if (!UseInst->mayReadFromMemory())
      return false;

    if (auto *CB = dyn_cast<CallBase>(UseInst))
      if (CB->onlyAccessesInaccessibleMemory())
        return false;

    ModRefInfo MR = AA.getModRefInfo(UseInst, DefLoc);
    // If necessary, perform additional analysis.
    if (isRefSet(MR))
      MR = AA.callCapturesBefore(UseInst, DefLoc, &DT);
    return isRefSet(MR);
  }

  // Find a MemoryDef writing to \p DefLoc and dominating \p Current, with no
  // read access between them or on any other path to a function exit block if
  // \p DefLoc is not accessible after the function returns. If there is no such
  // MemoryDef, return None. The returned value may not (completely) overwrite
  // \p DefLoc. Currently we bail out when we encounter an aliasing MemoryUse
  // (read).
  Optional<MemoryAccess *>
  getDomMemoryDef(MemoryDef *KillingDef, MemoryAccess *Current,
                  MemoryLocation DefLoc, bool DefVisibleToCallerBeforeRet,
                  bool DefVisibleToCallerAfterRet, int &ScanLimit) const {
    MemoryAccess *DomAccess;
    bool StepAgain;
    LLVM_DEBUG(dbgs() << "  trying to get dominating access for " << *Current
                      << "\n");
    // Find the next clobbering Mod access for DefLoc, starting at Current.
    do {
      StepAgain = false;
      // Reached TOP.
      if (MSSA.isLiveOnEntryDef(Current))
        return None;

      if (isa<MemoryPhi>(Current)) {
        DomAccess = Current;
        break;
      }
      MemoryUseOrDef *CurrentUD = cast<MemoryUseOrDef>(Current);
      // Look for access that clobber DefLoc.
      DomAccess = MSSA.getSkipSelfWalker()->getClobberingMemoryAccess(CurrentUD,
                                                                      DefLoc);
      if (MSSA.isLiveOnEntryDef(DomAccess))
        return None;

      if (isa<MemoryPhi>(DomAccess))
        break;

      // Check if we can skip DomDef for DSE.
      MemoryDef *DomDef = dyn_cast<MemoryDef>(DomAccess);
      if (DomDef && canSkipDef(DomDef, DefVisibleToCallerBeforeRet)) {
        StepAgain = true;
        Current = DomDef->getDefiningAccess();
      }

    } while (StepAgain);

    // Accesses to objects accessible after the function returns can only be
    // eliminated if the access is killed along all paths to the exit. Collect
    // the blocks with killing (=completely overwriting MemoryDefs) and check if
    // they cover all paths from DomAccess to any function exit.
    SmallPtrSet<BasicBlock *, 16> KillingBlocks = {KillingDef->getBlock()};
    LLVM_DEBUG({
      dbgs() << "  Checking for reads of " << *DomAccess;
      if (isa<MemoryDef>(DomAccess))
        dbgs() << " (" << *cast<MemoryDef>(DomAccess)->getMemoryInst() << ")\n";
      else
        dbgs() << ")\n";
    });

    SmallSetVector<MemoryAccess *, 32> WorkList;
    auto PushMemUses = [&WorkList](MemoryAccess *Acc) {
      for (Use &U : Acc->uses())
        WorkList.insert(cast<MemoryAccess>(U.getUser()));
    };
    PushMemUses(DomAccess);

    // Check if DomDef may be read.
    for (unsigned I = 0; I < WorkList.size(); I++) {
      MemoryAccess *UseAccess = WorkList[I];

      LLVM_DEBUG(dbgs() << "   " << *UseAccess);
      if (--ScanLimit == 0) {
        LLVM_DEBUG(dbgs() << "\n    ...  hit scan limit\n");
        return None;
      }

      if (isa<MemoryPhi>(UseAccess)) {
        LLVM_DEBUG(dbgs() << "\n    ... adding PHI uses\n");
        PushMemUses(UseAccess);
        continue;
      }

      Instruction *UseInst = cast<MemoryUseOrDef>(UseAccess)->getMemoryInst();
      LLVM_DEBUG(dbgs() << " (" << *UseInst << ")\n");

      if (isNoopIntrinsic(cast<MemoryUseOrDef>(UseAccess))) {
        LLVM_DEBUG(dbgs() << "    ... adding uses of intrinsic\n");
        PushMemUses(UseAccess);
        continue;
      }

      // A memory terminator kills all preceeding MemoryDefs and all succeeding
      // MemoryAccesses. We do not have to check it's users.
      if (isMemTerminator(DefLoc, UseInst))
        continue;

      // Uses which may read the original MemoryDef mean we cannot eliminate the
      // original MD. Stop walk.
      if (isReadClobber(DefLoc, UseInst)) {
        LLVM_DEBUG(dbgs() << "    ... found read clobber\n");
        return None;
      }

      // For the KillingDef and DomAccess 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 || DomAccess == 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) ; <----- DomDef 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(DefLoc, UseInst)) {
          if (DefVisibleToCallerAfterRet && UseAccess != DomAccess) {
            BasicBlock *MaybeKillingBlock = UseInst->getParent();
            if (PostOrderNumbers.find(MaybeKillingBlock)->second <
                PostOrderNumbers.find(DomAccess->getBlock())->second) {

              LLVM_DEBUG(dbgs() << "    ... found killing block "
                                << MaybeKillingBlock->getName() << "\n");
              KillingBlocks.insert(MaybeKillingBlock);
            }
          }
        } else
          PushMemUses(UseDef);
      }
    }

    // For accesses to locations visible after the function returns, make sure
    // that the location is killed (=overwritten) along all paths from DomAccess
    // to the exit.
    if (DefVisibleToCallerAfterRet) {
      assert(!KillingBlocks.empty() &&
             "Expected at least a single killing block");
      // Find the common post-dominator of all killing blocks.
      BasicBlock *CommonPred = *KillingBlocks.begin();
      for (auto I = std::next(KillingBlocks.begin()), E = KillingBlocks.end();
           I != E; I++) {
        if (!CommonPred)
          break;
        CommonPred = PDT.findNearestCommonDominator(CommonPred, *I);
      }

      // If CommonPred is in the set of killing blocks, just check if it
      // post-dominates DomAccess.
      if (KillingBlocks.count(CommonPred)) {
        if (PDT.dominates(CommonPred, DomAccess->getBlock()))
          return {DomAccess};
        return None;
      }

      // If the common post-dominator does not post-dominate DomAccess, there
      // is a path from DomAccess to an exit not going through a killing block.
      if (PDT.dominates(CommonPred, DomAccess->getBlock())) {
        SetVector<BasicBlock *> WorkList;

        // DomAccess's post-order number provides an upper bound of the blocks
        // on a path starting at DomAccess.
        unsigned UpperBound =
            PostOrderNumbers.find(DomAccess->getBlock())->second;

        // 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())
            WorkList.insert(R);

        NumCFGTries++;
        // Check if all paths starting from an exit node go through one of the
        // killing blocks before reaching DomAccess.
        for (unsigned I = 0; I < WorkList.size(); I++) {
          NumCFGChecks++;
          BasicBlock *Current = WorkList[I];
          if (KillingBlocks.count(Current))
            continue;
          if (Current == DomAccess->getBlock())
            return None;

          // DomAccess is reachable from the entry, so we don't have to explore
          // unreachable blocks further.
          if (!DT.isReachableFromEntry(Current))
            continue;

          unsigned CPO = PostOrderNumbers.find(Current)->second;
          // Current block is not on a path starting at DomAccess.
          if (CPO > UpperBound)
            continue;
          for (BasicBlock *Pred : predecessors(Current))
            WorkList.insert(Pred);

          if (WorkList.size() >= MemorySSAPathCheckLimit)
            return None;
        }
        NumCFGSuccess++;
        return {DomAccess};
      }
      return None;
    }

    // No aliasing MemoryUses of DomAccess found, DomAccess is potentially dead.
    return {DomAccess};
  }

  // Delete dead memory defs
  void deleteDeadInstruction(Instruction *SI) {
    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.
      if (MemoryAccess *MA = MSSA.getMemoryAccess(DeadInst)) {
        if (MemoryDef *MD = dyn_cast<MemoryDef>(MA)) {
          SkipStores.insert(MD);
        }
        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 = nullptr;
          if (isInstructionTriviallyDead(OpI, &TLI))
            NowDeadInsts.push_back(OpI);
        }

      DeadInst->eraseFromParent();
    }
  }

  // Check for any extra throws between SI and NI 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 *SI, Instruction *NI,
                       const Value *SILocUnd) const {
    // First see if we can ignore it by using the fact that SI is an
    // alloca/alloca like object that is not visible to the caller during
    // execution of the function.
    if (SILocUnd && InvisibleToCallerBeforeRet.count(SILocUnd))
      return false;

    if (SI->getParent() == NI->getParent())
      return ThrowingBlocks.count(SI->getParent());
    return !ThrowingBlocks.empty();
  }

  // Check if \p NI acts as a DSE barrier for \p SI. The following instructions
  // act as barriers:
  //  * A memory instruction that may throw and \p SI accesses a non-stack
  //  object.
  //  * Atomic stores stronger that monotonic.
  bool isDSEBarrier(const Value *SILocUnd, Instruction *NI) const {
    // If NI may throw it acts as a barrier, unless we are to an alloca/alloca
    // like object that does not escape.
    if (NI->mayThrow() && !InvisibleToCallerBeforeRet.count(SILocUnd))
      return true;

    // If NI is an atomic load/store stronger than monotonic, do not try to
    // eliminate/reorder it.
    if (NI->isAtomic()) {
      if (auto *LI = dyn_cast<LoadInst>(NI))
        return isStrongerThanMonotonic(LI->getOrdering());
      if (auto *SI = dyn_cast<StoreInst>(NI))
        return isStrongerThanMonotonic(SI->getOrdering());
      llvm_unreachable("other instructions should be skipped in MemorySSA");
    }
    return false;
  }

  /// Eliminate writes to objects that are not visible in the caller and are not
  /// accessed before returning from the function.
  bool eliminateDeadWritesAtEndOfFunction() {
    const DataLayout &DL = F.getParent()->getDataLayout();
    bool MadeChange = false;
    LLVM_DEBUG(
        dbgs()
        << "Trying to eliminate MemoryDefs at the end of the function\n");
    for (int I = MemDefs.size() - 1; I >= 0; I--) {
      MemoryDef *Def = MemDefs[I];
      if (SkipStores.find(Def) != SkipStores.end() ||
          !isRemovable(Def->getMemoryInst()))
        continue;

      // TODO: Consider doing the underlying object check first, if it is
      // beneficial compile-time wise.
      if (isWriteAtEndOfFunction(Def)) {
        Instruction *DefI = Def->getMemoryInst();
        // See through pointer-to-pointer bitcasts
        SmallVector<const Value *, 4> Pointers;
        GetUnderlyingObjects(getLocForWriteEx(DefI)->Ptr, Pointers, DL);

        LLVM_DEBUG(dbgs() << "   ... MemoryDef is not accessed until the end "
                             "of the function\n");
        bool CanKill = true;
        for (const Value *Pointer : Pointers) {
          if (!InvisibleToCallerAfterRet.count(Pointer)) {
            CanKill = false;
            break;
          }
        }

        if (CanKill) {
          deleteDeadInstruction(DefI);
          ++NumFastStores;
          MadeChange = true;
        }
      }
    }
    return MadeChange;
  }

  /// \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, MemoryLocation DefLoc, const Value *DefUO) {
    StoreInst *Store = dyn_cast<StoreInst>(Def->getMemoryInst());
    if (!Store)
      return false;

    if (auto *LoadI = dyn_cast<LoadInst>(Store->getOperand(0))) {
      if (LoadI->getPointerOperand() == Store->getOperand(1)) {
        auto *LoadAccess = MSSA.getMemoryAccess(LoadI)->getDefiningAccess();
        // If both accesses share the same defining access, no instructions
        // between them can modify the memory location.
        return LoadAccess == Def->getDefiningAccess();
      }
    }

    Constant *StoredConstant = dyn_cast<Constant>(Store->getOperand(0));
    if (StoredConstant && StoredConstant->isNullValue()) {
      auto *DefUOInst = dyn_cast<Instruction>(DefUO);
      if (DefUOInst && isCallocLikeFn(DefUOInst, &TLI)) {
        auto *UnderlyingDef = cast<MemoryDef>(MSSA.getMemoryAccess(DefUOInst));
        // If UnderlyingDef is the clobbering access of Def, no instructions
        // between them can modify the memory location.
        auto *ClobberDef =
            MSSA.getSkipSelfWalker()->getClobberingMemoryAccess(Def);
        return UnderlyingDef == ClobberDef;
      }
    }
    return false;
  }
};

bool eliminateDeadStoresMemorySSA(Function &F, AliasAnalysis &AA,
                                  MemorySSA &MSSA, DominatorTree &DT,
                                  PostDominatorTree &PDT,
                                  const TargetLibraryInfo &TLI) {
  const DataLayout &DL = F.getParent()->getDataLayout();
  bool MadeChange = false;

  DSEState State = DSEState::get(F, AA, MSSA, DT, PDT, TLI);
  // For each store:
  for (unsigned I = 0; I < State.MemDefs.size(); I++) {
    MemoryDef *KillingDef = State.MemDefs[I];
    if (State.SkipStores.count(KillingDef))
      continue;
    Instruction *SI = KillingDef->getMemoryInst();

    auto MaybeSILoc = State.getLocForWriteEx(SI);
    if (State.isMemTerminatorInst(SI))
      MaybeSILoc = State.getLocForTerminator(SI).map(
          [](const std::pair<MemoryLocation, bool> &P) { return P.first; });
    else
      MaybeSILoc = State.getLocForWriteEx(SI);

    if (!MaybeSILoc) {
      LLVM_DEBUG(dbgs() << "Failed to find analyzable write location for "
                        << *SI << "\n");
      continue;
    }
    MemoryLocation SILoc = *MaybeSILoc;
    assert(SILoc.Ptr && "SILoc should not be null");
    const Value *SILocUnd = GetUnderlyingObject(SILoc.Ptr, DL);

    // Check if the store is a no-op.
    if (isRemovable(SI) && State.storeIsNoop(KillingDef, SILoc, SILocUnd)) {
      LLVM_DEBUG(dbgs() << "DSE: Remove No-Op Store:\n  DEAD: " << *SI << '\n');
      State.deleteDeadInstruction(SI);
      NumNoopStores++;
      MadeChange = true;
      continue;
    }

    Instruction *DefObj =
        const_cast<Instruction *>(dyn_cast<Instruction>(SILocUnd));
    bool DefVisibleToCallerBeforeRet =
        !State.InvisibleToCallerBeforeRet.count(SILocUnd);
    bool DefVisibleToCallerAfterRet =
        !State.InvisibleToCallerAfterRet.count(SILocUnd);
    if (DefObj && isAllocLikeFn(DefObj, &TLI)) {
      if (DefVisibleToCallerBeforeRet)
        DefVisibleToCallerBeforeRet =
            PointerMayBeCapturedBefore(DefObj, false, true, SI, &DT);
    }

    MemoryAccess *Current = KillingDef;
    LLVM_DEBUG(dbgs() << "Trying to eliminate MemoryDefs killed by "
                      << *KillingDef << " (" << *SI << ")\n");

    int ScanLimit = MemorySSAScanLimit;
    // Worklist of MemoryAccesses that may be killed by KillingDef.
    SetVector<MemoryAccess *> ToCheck;
    ToCheck.insert(KillingDef->getDefiningAccess());

    // Check if MemoryAccesses in the worklist are killed by KillingDef.
    for (unsigned I = 0; I < ToCheck.size(); I++) {
      Current = ToCheck[I];
      if (State.SkipStores.count(Current))
        continue;

      Optional<MemoryAccess *> Next = State.getDomMemoryDef(
          KillingDef, Current, SILoc, DefVisibleToCallerBeforeRet,
          DefVisibleToCallerAfterRet, ScanLimit);

      if (!Next) {
        LLVM_DEBUG(dbgs() << "  finished walk\n");
        continue;
      }

      MemoryAccess *DomAccess = *Next;
      LLVM_DEBUG(dbgs() << " Checking if we can kill " << *DomAccess);
      if (isa<MemoryPhi>(DomAccess)) {
        LLVM_DEBUG(dbgs() << "\n  ... adding incoming values to worklist\n");
        for (Value *V : cast<MemoryPhi>(DomAccess)->incoming_values()) {
          MemoryAccess *IncomingAccess = cast<MemoryAccess>(V);
          BasicBlock *IncomingBlock = IncomingAccess->getBlock();
          BasicBlock *PhiBlock = DomAccess->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 (State.PostOrderNumbers[IncomingBlock] >
              State.PostOrderNumbers[PhiBlock])
            ToCheck.insert(IncomingAccess);
        }
        continue;
      }
      MemoryDef *NextDef = dyn_cast<MemoryDef>(DomAccess);
      Instruction *NI = NextDef->getMemoryInst();
      LLVM_DEBUG(dbgs() << " (" << *NI << ")\n");

      // Before we try to remove anything, check for any extra throwing
      // instructions that block us from DSEing
      if (State.mayThrowBetween(SI, NI, SILocUnd)) {
        LLVM_DEBUG(dbgs() << "  ... skip, may throw!\n");
        break;
      }

      // Check for anything that looks like it will be a barrier to further
      // removal
      if (State.isDSEBarrier(SILocUnd, NI)) {
        LLVM_DEBUG(dbgs() << "  ... skip, barrier\n");
        continue;
      }

      ToCheck.insert(NextDef->getDefiningAccess());

      if (!hasAnalyzableMemoryWrite(NI, TLI)) {
        LLVM_DEBUG(dbgs() << "  ... skip, cannot analyze def\n");
        continue;
      }

      if (!isRemovable(NI)) {
        LLVM_DEBUG(dbgs() << "  ... skip, cannot remove def\n");
        continue;
      }

      if (!DebugCounter::shouldExecute(MemorySSACounter))
        continue;

      MemoryLocation NILoc = *State.getLocForWriteEx(NI);

      if (State.isMemTerminatorInst(SI)) {
        const Value *NIUnd = GetUnderlyingObject(NILoc.Ptr, DL);
        if (!SILocUnd || SILocUnd != NIUnd)
          continue;
        LLVM_DEBUG(dbgs() << "DSE: Remove Dead Store:\n  DEAD: " << *NI
                          << "\n  KILLER: " << *SI << '\n');
        State.deleteDeadInstruction(NI);
        ++NumFastStores;
        MadeChange = true;
      } else {
        // Check if NI overwrites SI.
        int64_t InstWriteOffset, DepWriteOffset;
        auto Iter = State.IOLs.insert(
            std::make_pair<BasicBlock *, InstOverlapIntervalsTy>(
                NI->getParent(), InstOverlapIntervalsTy()));
        auto &IOL = Iter.first->second;
        OverwriteResult OR = isOverwrite(SILoc, NILoc, DL, TLI, DepWriteOffset,
                                         InstWriteOffset, NI, IOL, AA, &F);

        if (EnablePartialStoreMerging && OR == OW_PartialEarlierWithFullLater) {
          auto *Earlier = dyn_cast<StoreInst>(NI);
          auto *Later = dyn_cast<StoreInst>(SI);
          if (Constant *Merged = tryToMergePartialOverlappingStores(
                  Earlier, Later, InstWriteOffset, DepWriteOffset, DL, &AA,
                  &DT)) {

            // Update stored value of earlier store to merged constant.
            Earlier->setOperand(0, Merged);
            ++NumModifiedStores;
            MadeChange = true;

            // Remove later store and remove any outstanding overlap intervals
            // for the updated store.
            State.deleteDeadInstruction(Later);
            auto I = State.IOLs.find(Earlier->getParent());
            if (I != State.IOLs.end())
              I->second.erase(Earlier);
            break;
          }
        }

        if (OR == OW_Complete) {
          LLVM_DEBUG(dbgs() << "DSE: Remove Dead Store:\n  DEAD: " << *NI
                            << "\n  KILLER: " << *SI << '\n');
          State.deleteDeadInstruction(NI);
          ++NumFastStores;
          MadeChange = true;
        }
      }
    }
  }

  if (EnablePartialOverwriteTracking)
    for (auto &KV : State.IOLs)
      MadeChange |= removePartiallyOverlappedStores(&AA, DL, KV.second);

  MadeChange |= State.eliminateDeadWritesAtEndOfFunction();
  return MadeChange;
}
} // end anonymous namespace

//===----------------------------------------------------------------------===//
// 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);

  bool Changed = false;
  if (EnableMemorySSA) {
    MemorySSA &MSSA = AM.getResult<MemorySSAAnalysis>(F).getMSSA();
    PostDominatorTree &PDT = AM.getResult<PostDominatorTreeAnalysis>(F);

    Changed = eliminateDeadStoresMemorySSA(F, AA, MSSA, DT, PDT, TLI);
  } else {
    MemoryDependenceResults &MD = AM.getResult<MemoryDependenceAnalysis>(F);

    Changed = eliminateDeadStores(F, &AA, &MD, &DT, &TLI);
  }

#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<GlobalsAA>();
  if (EnableMemorySSA)
    PA.preserve<MemorySSAAnalysis>();
  else
    PA.preserve<MemoryDependenceAnalysis>();
  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);

    bool Changed = false;
    if (EnableMemorySSA) {
      MemorySSA &MSSA = getAnalysis<MemorySSAWrapperPass>().getMSSA();
      PostDominatorTree &PDT =
          getAnalysis<PostDominatorTreeWrapperPass>().getPostDomTree();

      Changed = eliminateDeadStoresMemorySSA(F, AA, MSSA, DT, PDT, TLI);
    } else {
      MemoryDependenceResults &MD =
          getAnalysis<MemoryDependenceWrapperPass>().getMemDep();

      Changed = eliminateDeadStores(F, &AA, &MD, &DT, &TLI);
    }

#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.addPreserved<DominatorTreeWrapperPass>();

    if (EnableMemorySSA) {
      AU.addRequired<PostDominatorTreeWrapperPass>();
      AU.addRequired<MemorySSAWrapperPass>();
      AU.addPreserved<PostDominatorTreeWrapperPass>();
      AU.addPreserved<MemorySSAWrapperPass>();
    } else {
      AU.addRequired<MemoryDependenceWrapperPass>();
      AU.addPreserved<MemoryDependenceWrapperPass>();
    }
  }
};

} // 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_END(DSELegacyPass, "dse", "Dead Store Elimination", false,
                    false)

FunctionPass *llvm::createDeadStoreEliminationPass() {
  return new DSELegacyPass();
}