text: //===- 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/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/OrderedBasicBlock.h"
  #include "llvm/Analysis/TargetLibraryInfo.h"
  #include "llvm/Analysis/ValueTracking.h"
  #include "llvm/IR/Argument.h"
  #include "llvm/IR/BasicBlock.h"
  #include "llvm/IR/CallSite.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/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/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/ErrorHandling.h"
  #include "llvm/Support/MathExtras.h"
  #include "llvm/Support/raw_ostream.h"
  #include "llvm/Transforms/Scalar.h"
  #include "llvm/Transforms/Utils/Local.h"
  #include <algorithm>
  #include <cassert>
  #include <cstddef>
  #include <cstdint>
  #include <iterator>
  #include <map>
  #include <utility>
  
  using namespace llvm;
  
  #define DEBUG_TYPE "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");
  
  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"));
  
  //===----------------------------------------------------------------------===//
  // 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, OrderedBasicBlock &OBB,
                        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);
  
      // 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);
      OBB.eraseInstruction(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 CS = CallSite(I)) {
      if (Function *F = CS.getCalledFunction()) {
        LibFunc LF;
        if (TLI.getLibFunc(*F, 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 CS = CallSite(Inst))
      // All the supported TLI functions so far happen to have dest as their
      // first argument.
      return MemoryLocation(CS.getArgument(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 CS = CallSite(I))
      return CS.getInstruction()->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) {
    SmallVector<BasicBlock *, 16> WorkList;
    SmallPtrSet<BasicBlock *, 8> Visited;
    BasicBlock::iterator FirstBBI(FirstI);
    ++FirstBBI;
    BasicBlock::iterator SecondBBI(SecondI);
    BasicBlock *FirstBB = FirstI->getParent();
    BasicBlock *SecondBB = SecondI->getParent();
    MemoryLocation MemLoc = MemoryLocation::get(SecondI);
  
    // Start checking the SecondBB.
    WorkList.push_back(SecondBB);
    bool isFirstBlock = true;
  
    // Check all blocks going backward until we reach the FirstBB.
    while (!WorkList.empty()) {
      BasicBlock *B = WorkList.pop_back_val();
  
      // 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)))
            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) {
          if (!Visited.insert(*PredI).second)
            continue;
          WorkList.push_back(*PredI);
        }
      }
    }
    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, OrderedBasicBlock &OBB,
                         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, OBB,
                              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, OrderedBasicBlock &OBB,
                             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.hasByValOrInAllocaAttr())
        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, OBB, 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, OBB, 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,
                                 OrderedBasicBlock &OBB,
                                 MapVector<Instruction *, bool> &ThrowableInst) {
    // 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)) {
  
        LLVM_DEBUG(
            dbgs() << "DSE: Remove Store Of Load from same pointer:\n  LOAD: "
                   << *DepLoad << "\n  STORE: " << *SI << '\n');
  
        deleteDeadInstruction(SI, &BBI, *MD, *TLI, IOL, OBB, 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)) {
        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, OBB, ThrowableInst);
        ++NumRedundantStores;
        return true;
      }
    }
    return false;
  }
  
  static bool eliminateDeadStores(BasicBlock &BB, AliasAnalysis *AA,
                                  MemoryDependenceResults *MD, DominatorTree *DT,
                                  const TargetLibraryInfo *TLI) {
    const DataLayout &DL = BB.getModule()->getDataLayout();
    bool MadeChange = false;
  
    OrderedBasicBlock OBB(&BB);
    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, OBB, 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, OBB,
                             ThrowableInst)) {
        MadeChange = true;
        continue;
      }
  
      // If we find something that writes memory, get its memory dependence.
      MemDepResult InstDep = MD->getDependency(Inst, &OBB);
  
      // 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 && OBB.dominates(DepWrite, 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, OBB,
                                  ThrowableInst);
            ++NumFastStores;
            MadeChange = true;
  
            // We erased DepWrite; start over.
            InstDep = MD->getDependency(Inst, &OBB);
            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 (Earlier && isa<ConstantInt>(Earlier->getValueOperand()) &&
                DL.typeSizeEqualsStoreSize(
                    Earlier->getValueOperand()->getType()) &&
                Later && isa<ConstantInt>(Later->getValueOperand()) &&
                DL.typeSizeEqualsStoreSize(
                    Later->getValueOperand()->getType()) &&
                memoryIsNotModifiedBetween(Earlier, Later, AA)) {
              // 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: " << *DepWrite
                                << "\n  Later: " << *Inst
                                << "\n  Merged Value: " << Merged << '\n');
  
              auto *SI = new StoreInst(
                  ConstantInt::get(Earlier->getValueOperand()->getType(), Merged),
                  Earlier->getPointerOperand(), false,
                  MaybeAlign(Earlier->getAlignment()), 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;
  
              // Remove earlier, wider, store
              OBB.replaceInstruction(DepWrite, SI);
  
              // Delete the old stores and now-dead instructions that feed them.
              deleteDeadInstruction(Inst, &BBI, *MD, *TLI, IOL, OBB,
                                    ThrowableInst);
              deleteDeadInstruction(DepWrite, &BBI, *MD, *TLI, IOL, OBB,
                                    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, OBB, 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;
  }
  
  //===----------------------------------------------------------------------===//
  // DSE Pass
  //===----------------------------------------------------------------------===//
  PreservedAnalyses DSEPass::run(Function &F, FunctionAnalysisManager &AM) {
    AliasAnalysis *AA = &AM.getResult<AAManager>(F);
    DominatorTree *DT = &AM.getResult<DominatorTreeAnalysis>(F);
    MemoryDependenceResults *MD = &AM.getResult<MemoryDependenceAnalysis>(F);
    const TargetLibraryInfo *TLI = &AM.getResult<TargetLibraryAnalysis>(F);
  
    if (!eliminateDeadStores(F, AA, MD, DT, TLI))
      return PreservedAnalyses::all();
  
    PreservedAnalyses PA;
    PA.preserveSet<CFGAnalyses>();
    PA.preserve<GlobalsAA>();
    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;
  
      DominatorTree *DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
      AliasAnalysis *AA = &getAnalysis<AAResultsWrapperPass>().getAAResults();
      MemoryDependenceResults *MD =
          &getAnalysis<MemoryDependenceWrapperPass>().getMemDep();
      const TargetLibraryInfo *TLI =
          &getAnalysis<TargetLibraryInfoWrapperPass>().getTLI(F);
  
      return eliminateDeadStores(F, AA, MD, DT, TLI);
    }
  
    void getAnalysisUsage(AnalysisUsage &AU) const override {
      AU.setPreservesCFG();
      AU.addRequired<DominatorTreeWrapperPass>();
      AU.addRequired<AAResultsWrapperPass>();
      AU.addRequired<MemoryDependenceWrapperPass>();
      AU.addRequired<TargetLibraryInfoWrapperPass>();
      AU.addPreserved<DominatorTreeWrapperPass>();
      AU.addPreserved<GlobalsAAWrapperPass>();
      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(AAResultsWrapperPass)
  INITIALIZE_PASS_DEPENDENCY(GlobalsAAWrapperPass)
  INITIALIZE_PASS_DEPENDENCY(MemoryDependenceWrapperPass)
  INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfoWrapperPass)
  INITIALIZE_PASS_END(DSELegacyPass, "dse", "Dead Store Elimination", false,
                      false)
  
  FunctionPass *llvm::createDeadStoreEliminationPass() {
    return new DSELegacyPass();
  }
  