llvm/lib/Transforms/InstCombine/InstCombineLoadStoreAlloca.cpp

//===- InstCombineLoadStoreAlloca.cpp -------------------------------------===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file implements the visit functions for load, store and alloca.
//
//===----------------------------------------------------------------------===//

#include "InstCombineInternal.h"
#include "llvm/ADT/SmallString.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Analysis/Loads.h"
#include "llvm/IR/DataLayout.h"
#include "llvm/IR/LLVMContext.h"
#include "llvm/IR/IntrinsicInst.h"
#include "llvm/IR/MDBuilder.h"
#include "llvm/Transforms/Utils/BasicBlockUtils.h"
#include "llvm/Transforms/Utils/Local.h"
using namespace llvm;

#define DEBUG_TYPE "instcombine"

STATISTIC(NumDeadStore,    "Number of dead stores eliminated");
STATISTIC(NumGlobalCopies, "Number of allocas copied from constant global");

/// pointsToConstantGlobal - Return true if V (possibly indirectly) points to
/// some part of a constant global variable.  This intentionally only accepts
/// constant expressions because we can't rewrite arbitrary instructions.
static bool pointsToConstantGlobal(Value *V) {
  if (GlobalVariable *GV = dyn_cast<GlobalVariable>(V))
    return GV->isConstant();

  if (ConstantExpr *CE = dyn_cast<ConstantExpr>(V)) {
    if (CE->getOpcode() == Instruction::BitCast ||
        CE->getOpcode() == Instruction::AddrSpaceCast ||
        CE->getOpcode() == Instruction::GetElementPtr)
      return pointsToConstantGlobal(CE->getOperand(0));
  }
  return false;
}

/// isOnlyCopiedFromConstantGlobal - Recursively walk the uses of a (derived)
/// pointer to an alloca.  Ignore any reads of the pointer, return false if we
/// see any stores or other unknown uses.  If we see pointer arithmetic, keep
/// track of whether it moves the pointer (with IsOffset) but otherwise traverse
/// the uses.  If we see a memcpy/memmove that targets an unoffseted pointer to
/// the alloca, and if the source pointer is a pointer to a constant global, we
/// can optimize this.
static bool
isOnlyCopiedFromConstantGlobal(Value *V, MemTransferInst *&TheCopy,
                               SmallVectorImpl<Instruction *> &ToDelete) {
  // We track lifetime intrinsics as we encounter them.  If we decide to go
  // ahead and replace the value with the global, this lets the caller quickly
  // eliminate the markers.

  SmallVector<std::pair<Value *, bool>, 35> ValuesToInspect;
  ValuesToInspect.emplace_back(V, false);
  while (!ValuesToInspect.empty()) {
    auto ValuePair = ValuesToInspect.pop_back_val();
    const bool IsOffset = ValuePair.second;
    for (auto &U : ValuePair.first->uses()) {
      auto *I = cast<Instruction>(U.getUser());

      if (auto *LI = dyn_cast<LoadInst>(I)) {
        // Ignore non-volatile loads, they are always ok.
        if (!LI->isSimple()) return false;
        continue;
      }

      if (isa<BitCastInst>(I) || isa<AddrSpaceCastInst>(I)) {
        // If uses of the bitcast are ok, we are ok.
        ValuesToInspect.emplace_back(I, IsOffset);
        continue;
      }
      if (auto *GEP = dyn_cast<GetElementPtrInst>(I)) {
        // If the GEP has all zero indices, it doesn't offset the pointer. If it
        // doesn't, it does.
        ValuesToInspect.emplace_back(I, IsOffset || !GEP->hasAllZeroIndices());
        continue;
      }

      if (auto CS = CallSite(I)) {
        // If this is the function being called then we treat it like a load and
        // ignore it.
        if (CS.isCallee(&U))
          continue;

        unsigned DataOpNo = CS.getDataOperandNo(&U);
        bool IsArgOperand = CS.isArgOperand(&U);

        // Inalloca arguments are clobbered by the call.
        if (IsArgOperand && CS.isInAllocaArgument(DataOpNo))
          return false;

        // If this is a readonly/readnone call site, then we know it is just a
        // load (but one that potentially returns the value itself), so we can
        // ignore it if we know that the value isn't captured.
        if (CS.onlyReadsMemory() &&
            (CS.getInstruction()->use_empty() || CS.doesNotCapture(DataOpNo)))
          continue;

        // If this is being passed as a byval argument, the caller is making a
        // copy, so it is only a read of the alloca.
        if (IsArgOperand && CS.isByValArgument(DataOpNo))
          continue;
      }

      // Lifetime intrinsics can be handled by the caller.
      if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(I)) {
        if (II->getIntrinsicID() == Intrinsic::lifetime_start ||
            II->getIntrinsicID() == Intrinsic::lifetime_end) {
          assert(II->use_empty() && "Lifetime markers have no result to use!");
          ToDelete.push_back(II);
          continue;
        }
      }

      // If this is isn't our memcpy/memmove, reject it as something we can't
      // handle.
      MemTransferInst *MI = dyn_cast<MemTransferInst>(I);
      if (!MI)
        return false;

      // If the transfer is using the alloca as a source of the transfer, then
      // ignore it since it is a load (unless the transfer is volatile).
      if (U.getOperandNo() == 1) {
        if (MI->isVolatile()) return false;
        continue;
      }

      // If we already have seen a copy, reject the second one.
      if (TheCopy) return false;

      // If the pointer has been offset from the start of the alloca, we can't
      // safely handle this.
      if (IsOffset) return false;

      // If the memintrinsic isn't using the alloca as the dest, reject it.
      if (U.getOperandNo() != 0) return false;

      // If the source of the memcpy/move is not a constant global, reject it.
      if (!pointsToConstantGlobal(MI->getSource()))
        return false;

      // Otherwise, the transform is safe.  Remember the copy instruction.
      TheCopy = MI;
    }
  }
  return true;
}

/// isOnlyCopiedFromConstantGlobal - Return true if the specified alloca is only
/// modified by a copy from a constant global.  If we can prove this, we can
/// replace any uses of the alloca with uses of the global directly.
static MemTransferInst *
isOnlyCopiedFromConstantGlobal(AllocaInst *AI,
                               SmallVectorImpl<Instruction *> &ToDelete) {
  MemTransferInst *TheCopy = nullptr;
  if (isOnlyCopiedFromConstantGlobal(AI, TheCopy, ToDelete))
    return TheCopy;
  return nullptr;
}

static Instruction *simplifyAllocaArraySize(InstCombiner &IC, AllocaInst &AI) {
  // Check for array size of 1 (scalar allocation).
  if (!AI.isArrayAllocation()) {
    // i32 1 is the canonical array size for scalar allocations.
    if (AI.getArraySize()->getType()->isIntegerTy(32))
      return nullptr;

    // Canonicalize it.
    Value *V = IC.Builder->getInt32(1);
    AI.setOperand(0, V);
    return &AI;
  }

  // Convert: alloca Ty, C - where C is a constant != 1 into: alloca [C x Ty], 1
  if (const ConstantInt *C = dyn_cast<ConstantInt>(AI.getArraySize())) {
    Type *NewTy = ArrayType::get(AI.getAllocatedType(), C->getZExtValue());
    AllocaInst *New = IC.Builder->CreateAlloca(NewTy, nullptr, AI.getName());
    New->setAlignment(AI.getAlignment());

    // Scan to the end of the allocation instructions, to skip over a block of
    // allocas if possible...also skip interleaved debug info
    //
    BasicBlock::iterator It(New);
    while (isa<AllocaInst>(*It) || isa<DbgInfoIntrinsic>(*It))
      ++It;

    // Now that I is pointing to the first non-allocation-inst in the block,
    // insert our getelementptr instruction...
    //
    Type *IdxTy = IC.getDataLayout().getIntPtrType(AI.getType());
    Value *NullIdx = Constant::getNullValue(IdxTy);
    Value *Idx[2] = {NullIdx, NullIdx};
    Instruction *GEP =
        GetElementPtrInst::CreateInBounds(New, Idx, New->getName() + ".sub");
    IC.InsertNewInstBefore(GEP, *It);

    // Now make everything use the getelementptr instead of the original
    // allocation.
    return IC.replaceInstUsesWith(AI, GEP);
  }

  if (isa<UndefValue>(AI.getArraySize()))
    return IC.replaceInstUsesWith(AI, Constant::getNullValue(AI.getType()));

  // Ensure that the alloca array size argument has type intptr_t, so that
  // any casting is exposed early.
  Type *IntPtrTy = IC.getDataLayout().getIntPtrType(AI.getType());
  if (AI.getArraySize()->getType() != IntPtrTy) {
    Value *V = IC.Builder->CreateIntCast(AI.getArraySize(), IntPtrTy, false);
    AI.setOperand(0, V);
    return &AI;
  }

  return nullptr;
}

Instruction *InstCombiner::visitAllocaInst(AllocaInst &AI) {
  if (auto *I = simplifyAllocaArraySize(*this, AI))
    return I;

  if (AI.getAllocatedType()->isSized()) {
    // If the alignment is 0 (unspecified), assign it the preferred alignment.
    if (AI.getAlignment() == 0)
      AI.setAlignment(DL.getPrefTypeAlignment(AI.getAllocatedType()));

    // Move all alloca's of zero byte objects to the entry block and merge them
    // together.  Note that we only do this for alloca's, because malloc should
    // allocate and return a unique pointer, even for a zero byte allocation.
    if (DL.getTypeAllocSize(AI.getAllocatedType()) == 0) {
      // For a zero sized alloca there is no point in doing an array allocation.
      // This is helpful if the array size is a complicated expression not used
      // elsewhere.
      if (AI.isArrayAllocation()) {
        AI.setOperand(0, ConstantInt::get(AI.getArraySize()->getType(), 1));
        return &AI;
      }

      // Get the first instruction in the entry block.
      BasicBlock &EntryBlock = AI.getParent()->getParent()->getEntryBlock();
      Instruction *FirstInst = EntryBlock.getFirstNonPHIOrDbg();
      if (FirstInst != &AI) {
        // If the entry block doesn't start with a zero-size alloca then move
        // this one to the start of the entry block.  There is no problem with
        // dominance as the array size was forced to a constant earlier already.
        AllocaInst *EntryAI = dyn_cast<AllocaInst>(FirstInst);
        if (!EntryAI || !EntryAI->getAllocatedType()->isSized() ||
            DL.getTypeAllocSize(EntryAI->getAllocatedType()) != 0) {
          AI.moveBefore(FirstInst);
          return &AI;
        }

        // If the alignment of the entry block alloca is 0 (unspecified),
        // assign it the preferred alignment.
        if (EntryAI->getAlignment() == 0)
          EntryAI->setAlignment(
              DL.getPrefTypeAlignment(EntryAI->getAllocatedType()));
        // Replace this zero-sized alloca with the one at the start of the entry
        // block after ensuring that the address will be aligned enough for both
        // types.
        unsigned MaxAlign = std::max(EntryAI->getAlignment(),
                                     AI.getAlignment());
        EntryAI->setAlignment(MaxAlign);
        if (AI.getType() != EntryAI->getType())
          return new BitCastInst(EntryAI, AI.getType());
        return replaceInstUsesWith(AI, EntryAI);
      }
    }
  }

  if (AI.getAlignment()) {
    // Check to see if this allocation is only modified by a memcpy/memmove from
    // a constant global whose alignment is equal to or exceeds that of the
    // allocation.  If this is the case, we can change all users to use
    // the constant global instead.  This is commonly produced by the CFE by
    // constructs like "void foo() { int A[] = {1,2,3,4,5,6,7,8,9...}; }" if 'A'
    // is only subsequently read.
    SmallVector<Instruction *, 4> ToDelete;
    if (MemTransferInst *Copy = isOnlyCopiedFromConstantGlobal(&AI, ToDelete)) {
      unsigned SourceAlign = getOrEnforceKnownAlignment(
          Copy->getSource(), AI.getAlignment(), DL, &AI, &AC, &DT);
      if (AI.getAlignment() <= SourceAlign) {
        DEBUG(dbgs() << "Found alloca equal to global: " << AI << '\n');
        DEBUG(dbgs() << "  memcpy = " << *Copy << '\n');
        for (unsigned i = 0, e = ToDelete.size(); i != e; ++i)
          eraseInstFromFunction(*ToDelete[i]);
        Constant *TheSrc = cast<Constant>(Copy->getSource());
        Constant *Cast
          = ConstantExpr::getPointerBitCastOrAddrSpaceCast(TheSrc, AI.getType());
        Instruction *NewI = replaceInstUsesWith(AI, Cast);
        eraseInstFromFunction(*Copy);
        ++NumGlobalCopies;
        return NewI;
      }
    }
  }

  // At last, use the generic allocation site handler to aggressively remove
  // unused allocas.
  return visitAllocSite(AI);
}

/// \brief Helper to combine a load to a new type.
///
/// This just does the work of combining a load to a new type. It handles
/// metadata, etc., and returns the new instruction. The \c NewTy should be the
/// loaded *value* type. This will convert it to a pointer, cast the operand to
/// that pointer type, load it, etc.
///
/// Note that this will create all of the instructions with whatever insert
/// point the \c InstCombiner currently is using.
static LoadInst *combineLoadToNewType(InstCombiner &IC, LoadInst &LI, Type *NewTy,
                                      const Twine &Suffix = "") {
  Value *Ptr = LI.getPointerOperand();
  unsigned AS = LI.getPointerAddressSpace();
  SmallVector<std::pair<unsigned, MDNode *>, 8> MD;
  LI.getAllMetadata(MD);

  LoadInst *NewLoad = IC.Builder->CreateAlignedLoad(
      IC.Builder->CreateBitCast(Ptr, NewTy->getPointerTo(AS)),
      LI.getAlignment(), LI.isVolatile(), LI.getName() + Suffix);
  NewLoad->setAtomic(LI.getOrdering(), LI.getSynchScope());
  MDBuilder MDB(NewLoad->getContext());
  for (const auto &MDPair : MD) {
    unsigned ID = MDPair.first;
    MDNode *N = MDPair.second;
    // Note, essentially every kind of metadata should be preserved here! This
    // routine is supposed to clone a load instruction changing *only its type*.
    // The only metadata it makes sense to drop is metadata which is invalidated
    // when the pointer type changes. This should essentially never be the case
    // in LLVM, but we explicitly switch over only known metadata to be
    // conservatively correct. If you are adding metadata to LLVM which pertains
    // to loads, you almost certainly want to add it here.
    switch (ID) {
    case LLVMContext::MD_dbg:
    case LLVMContext::MD_tbaa:
    case LLVMContext::MD_prof:
    case LLVMContext::MD_fpmath:
    case LLVMContext::MD_tbaa_struct:
    case LLVMContext::MD_invariant_load:
    case LLVMContext::MD_alias_scope:
    case LLVMContext::MD_noalias:
    case LLVMContext::MD_nontemporal:
    case LLVMContext::MD_mem_parallel_loop_access:
      // All of these directly apply.
      NewLoad->setMetadata(ID, N);
      break;

    case LLVMContext::MD_nonnull:
      // This only directly applies if the new type is also a pointer.
      if (NewTy->isPointerTy()) {
        NewLoad->setMetadata(ID, N);
        break;
      }
      // If it's integral now, translate it to !range metadata.
      if (NewTy->isIntegerTy()) {
        auto *ITy = cast<IntegerType>(NewTy);
        auto *NullInt = ConstantExpr::getPtrToInt(
            ConstantPointerNull::get(cast<PointerType>(Ptr->getType())), ITy);
        auto *NonNullInt =
            ConstantExpr::getAdd(NullInt, ConstantInt::get(ITy, 1));
        NewLoad->setMetadata(LLVMContext::MD_range,
                             MDB.createRange(NonNullInt, NullInt));
      }
      break;
    case LLVMContext::MD_align:
    case LLVMContext::MD_dereferenceable:
    case LLVMContext::MD_dereferenceable_or_null:
      // These only directly apply if the new type is also a pointer.
      if (NewTy->isPointerTy())
        NewLoad->setMetadata(ID, N);
      break;
    case LLVMContext::MD_range:
      // FIXME: It would be nice to propagate this in some way, but the type
      // conversions make it hard. If the new type is a pointer, we could
      // translate it to !nonnull metadata.
      break;
    }
  }
  return NewLoad;
}

/// \brief Combine a store to a new type.
///
/// Returns the newly created store instruction.
static StoreInst *combineStoreToNewValue(InstCombiner &IC, StoreInst &SI, Value *V) {
  Value *Ptr = SI.getPointerOperand();
  unsigned AS = SI.getPointerAddressSpace();
  SmallVector<std::pair<unsigned, MDNode *>, 8> MD;
  SI.getAllMetadata(MD);

  StoreInst *NewStore = IC.Builder->CreateAlignedStore(
      V, IC.Builder->CreateBitCast(Ptr, V->getType()->getPointerTo(AS)),
      SI.getAlignment(), SI.isVolatile());
  NewStore->setAtomic(SI.getOrdering(), SI.getSynchScope());
  for (const auto &MDPair : MD) {
    unsigned ID = MDPair.first;
    MDNode *N = MDPair.second;
    // Note, essentially every kind of metadata should be preserved here! This
    // routine is supposed to clone a store instruction changing *only its
    // type*. The only metadata it makes sense to drop is metadata which is
    // invalidated when the pointer type changes. This should essentially
    // never be the case in LLVM, but we explicitly switch over only known
    // metadata to be conservatively correct. If you are adding metadata to
    // LLVM which pertains to stores, you almost certainly want to add it
    // here.
    switch (ID) {
    case LLVMContext::MD_dbg:
    case LLVMContext::MD_tbaa:
    case LLVMContext::MD_prof:
    case LLVMContext::MD_fpmath:
    case LLVMContext::MD_tbaa_struct:
    case LLVMContext::MD_alias_scope:
    case LLVMContext::MD_noalias:
    case LLVMContext::MD_nontemporal:
    case LLVMContext::MD_mem_parallel_loop_access:
      // All of these directly apply.
      NewStore->setMetadata(ID, N);
      break;

    case LLVMContext::MD_invariant_load:
    case LLVMContext::MD_nonnull:
    case LLVMContext::MD_range:
    case LLVMContext::MD_align:
    case LLVMContext::MD_dereferenceable:
    case LLVMContext::MD_dereferenceable_or_null:
      // These don't apply for stores.
      break;
    }
  }

  return NewStore;
}

/// \brief Combine loads to match the type of their uses' value after looking
/// through intervening bitcasts.
///
/// The core idea here is that if the result of a load is used in an operation,
/// we should load the type most conducive to that operation. For example, when
/// loading an integer and converting that immediately to a pointer, we should
/// instead directly load a pointer.
///
/// However, this routine must never change the width of a load or the number of
/// loads as that would introduce a semantic change. This combine is expected to
/// be a semantic no-op which just allows loads to more closely model the types
/// of their consuming operations.
///
/// Currently, we also refuse to change the precise type used for an atomic load
/// or a volatile load. This is debatable, and might be reasonable to change
/// later. However, it is risky in case some backend or other part of LLVM is
/// relying on the exact type loaded to select appropriate atomic operations.
static Instruction *combineLoadToOperationType(InstCombiner &IC, LoadInst &LI) {
  // FIXME: We could probably with some care handle both volatile and ordered
  // atomic loads here but it isn't clear that this is important.
  if (!LI.isUnordered())
    return nullptr;

  if (LI.use_empty())
    return nullptr;

  Type *Ty = LI.getType();
  const DataLayout &DL = IC.getDataLayout();

  // Try to canonicalize loads which are only ever stored to operate over
  // integers instead of any other type. We only do this when the loaded type
  // is sized and has a size exactly the same as its store size and the store
  // size is a legal integer type.
  if (!Ty->isIntegerTy() && Ty->isSized() &&
      DL.isLegalInteger(DL.getTypeStoreSizeInBits(Ty)) &&
      DL.getTypeStoreSizeInBits(Ty) == DL.getTypeSizeInBits(Ty) &&
      !DL.isNonIntegralPointerType(Ty)) {
    if (all_of(LI.users(), [&LI](User *U) {
          auto *SI = dyn_cast<StoreInst>(U);
          return SI && SI->getPointerOperand() != &LI;
        })) {
      LoadInst *NewLoad = combineLoadToNewType(
          IC, LI,
          Type::getIntNTy(LI.getContext(), DL.getTypeStoreSizeInBits(Ty)));
      // Replace all the stores with stores of the newly loaded value.
      for (auto UI = LI.user_begin(), UE = LI.user_end(); UI != UE;) {
        auto *SI = cast<StoreInst>(*UI++);
        IC.Builder->SetInsertPoint(SI);
        combineStoreToNewValue(IC, *SI, NewLoad);
        IC.eraseInstFromFunction(*SI);
      }
      assert(LI.use_empty() && "Failed to remove all users of the load!");
      // Return the old load so the combiner can delete it safely.
      return &LI;
    }
  }

  // Fold away bit casts of the loaded value by loading the desired type.
  // We can do this for BitCastInsts as well as casts from and to pointer types,
  // as long as those are noops (i.e., the source or dest type have the same
  // bitwidth as the target's pointers).
  if (LI.hasOneUse())
    if (auto* CI = dyn_cast<CastInst>(LI.user_back())) {
      if (CI->isNoopCast(DL)) {
        LoadInst *NewLoad = combineLoadToNewType(IC, LI, CI->getDestTy());
        CI->replaceAllUsesWith(NewLoad);
        IC.eraseInstFromFunction(*CI);
        return &LI;
      }
    }

  // FIXME: We should also canonicalize loads of vectors when their elements are
  // cast to other types.
  return nullptr;
}

static Instruction *unpackLoadToAggregate(InstCombiner &IC, LoadInst &LI) {
  // FIXME: We could probably with some care handle both volatile and atomic
  // stores here but it isn't clear that this is important.
  if (!LI.isSimple())
    return nullptr;

  Type *T = LI.getType();
  if (!T->isAggregateType())
    return nullptr;

  StringRef Name = LI.getName();
  assert(LI.getAlignment() && "Alignment must be set at this point");

  if (auto *ST = dyn_cast<StructType>(T)) {
    // If the struct only have one element, we unpack.
    auto NumElements = ST->getNumElements();
    if (NumElements == 1) {
      LoadInst *NewLoad = combineLoadToNewType(IC, LI, ST->getTypeAtIndex(0U),
                                               ".unpack");
      return IC.replaceInstUsesWith(LI, IC.Builder->CreateInsertValue(
        UndefValue::get(T), NewLoad, 0, Name));
    }

    // We don't want to break loads with padding here as we'd loose
    // the knowledge that padding exists for the rest of the pipeline.
    const DataLayout &DL = IC.getDataLayout();
    auto *SL = DL.getStructLayout(ST);
    if (SL->hasPadding())
      return nullptr;

    auto Align = LI.getAlignment();
    if (!Align)
      Align = DL.getABITypeAlignment(ST);

    auto *Addr = LI.getPointerOperand();
    auto *IdxType = Type::getInt32Ty(T->getContext());
    auto *Zero = ConstantInt::get(IdxType, 0);

    Value *V = UndefValue::get(T);
    for (unsigned i = 0; i < NumElements; i++) {
      Value *Indices[2] = {
        Zero,
        ConstantInt::get(IdxType, i),
      };
      auto *Ptr = IC.Builder->CreateInBoundsGEP(ST, Addr, makeArrayRef(Indices),
                                                Name + ".elt");
      auto EltAlign = MinAlign(Align, SL->getElementOffset(i));
      auto *L = IC.Builder->CreateAlignedLoad(Ptr, EltAlign, Name + ".unpack");
      V = IC.Builder->CreateInsertValue(V, L, i);
    }

    V->setName(Name);
    return IC.replaceInstUsesWith(LI, V);
  }

  if (auto *AT = dyn_cast<ArrayType>(T)) {
    auto *ET = AT->getElementType();
    auto NumElements = AT->getNumElements();
    if (NumElements == 1) {
      LoadInst *NewLoad = combineLoadToNewType(IC, LI, ET, ".unpack");
      return IC.replaceInstUsesWith(LI, IC.Builder->CreateInsertValue(
        UndefValue::get(T), NewLoad, 0, Name));
    }

    const DataLayout &DL = IC.getDataLayout();
    auto EltSize = DL.getTypeAllocSize(ET);
    auto Align = LI.getAlignment();
    if (!Align)
      Align = DL.getABITypeAlignment(T);

    auto *Addr = LI.getPointerOperand();
    auto *IdxType = Type::getInt64Ty(T->getContext());
    auto *Zero = ConstantInt::get(IdxType, 0);

    Value *V = UndefValue::get(T);
    uint64_t Offset = 0;
    for (uint64_t i = 0; i < NumElements; i++) {
      Value *Indices[2] = {
        Zero,
        ConstantInt::get(IdxType, i),
      };
      auto *Ptr = IC.Builder->CreateInBoundsGEP(AT, Addr, makeArrayRef(Indices),
                                                Name + ".elt");
      auto *L = IC.Builder->CreateAlignedLoad(Ptr, MinAlign(Align, Offset),
                                              Name + ".unpack");
      V = IC.Builder->CreateInsertValue(V, L, i);
      Offset += EltSize;
    }

    V->setName(Name);
    return IC.replaceInstUsesWith(LI, V);
  }

  return nullptr;
}

// If we can determine that all possible objects pointed to by the provided
// pointer value are, not only dereferenceable, but also definitively less than
// or equal to the provided maximum size, then return true. Otherwise, return
// false (constant global values and allocas fall into this category).
//
// FIXME: This should probably live in ValueTracking (or similar).
static bool isObjectSizeLessThanOrEq(Value *V, uint64_t MaxSize,
                                     const DataLayout &DL) {
  SmallPtrSet<Value *, 4> Visited;
  SmallVector<Value *, 4> Worklist(1, V);

  do {
    Value *P = Worklist.pop_back_val();
    P = P->stripPointerCasts();

    if (!Visited.insert(P).second)
      continue;

    if (SelectInst *SI = dyn_cast<SelectInst>(P)) {
      Worklist.push_back(SI->getTrueValue());
      Worklist.push_back(SI->getFalseValue());
      continue;
    }

    if (PHINode *PN = dyn_cast<PHINode>(P)) {
      for (Value *IncValue : PN->incoming_values())
        Worklist.push_back(IncValue);
      continue;
    }

    if (GlobalAlias *GA = dyn_cast<GlobalAlias>(P)) {
      if (GA->isInterposable())
        return false;
      Worklist.push_back(GA->getAliasee());
      continue;
    }

    // If we know how big this object is, and it is less than MaxSize, continue
    // searching. Otherwise, return false.
    if (AllocaInst *AI = dyn_cast<AllocaInst>(P)) {
      if (!AI->getAllocatedType()->isSized())
        return false;

      ConstantInt *CS = dyn_cast<ConstantInt>(AI->getArraySize());
      if (!CS)
        return false;

      uint64_t TypeSize = DL.getTypeAllocSize(AI->getAllocatedType());
      // Make sure that, even if the multiplication below would wrap as an
      // uint64_t, we still do the right thing.
      if ((CS->getValue().zextOrSelf(128)*APInt(128, TypeSize)).ugt(MaxSize))
        return false;
      continue;
    }

    if (GlobalVariable *GV = dyn_cast<GlobalVariable>(P)) {
      if (!GV->hasDefinitiveInitializer() || !GV->isConstant())
        return false;

      uint64_t InitSize = DL.getTypeAllocSize(GV->getValueType());
      if (InitSize > MaxSize)
        return false;
      continue;
    }

    return false;
  } while (!Worklist.empty());

  return true;
}

// If we're indexing into an object of a known size, and the outer index is
// not a constant, but having any value but zero would lead to undefined
// behavior, replace it with zero.
//
// For example, if we have:
// @f.a = private unnamed_addr constant [1 x i32] [i32 12], align 4
// ...
// %arrayidx = getelementptr inbounds [1 x i32]* @f.a, i64 0, i64 %x
// ... = load i32* %arrayidx, align 4
// Then we know that we can replace %x in the GEP with i64 0.
//
// FIXME: We could fold any GEP index to zero that would cause UB if it were
// not zero. Currently, we only handle the first such index. Also, we could
// also search through non-zero constant indices if we kept track of the
// offsets those indices implied.
static bool canReplaceGEPIdxWithZero(InstCombiner &IC, GetElementPtrInst *GEPI,
                                     Instruction *MemI, unsigned &Idx) {
  if (GEPI->getNumOperands() < 2)
    return false;

  // Find the first non-zero index of a GEP. If all indices are zero, return
  // one past the last index.
  auto FirstNZIdx = [](const GetElementPtrInst *GEPI) {
    unsigned I = 1;
    for (unsigned IE = GEPI->getNumOperands(); I != IE; ++I) {
      Value *V = GEPI->getOperand(I);
      if (const ConstantInt *CI = dyn_cast<ConstantInt>(V))
        if (CI->isZero())
          continue;

      break;
    }

    return I;
  };

  // Skip through initial 'zero' indices, and find the corresponding pointer
  // type. See if the next index is not a constant.
  Idx = FirstNZIdx(GEPI);
  if (Idx == GEPI->getNumOperands())
    return false;
  if (isa<Constant>(GEPI->getOperand(Idx)))
    return false;

  SmallVector<Value *, 4> Ops(GEPI->idx_begin(), GEPI->idx_begin() + Idx);
  Type *AllocTy =
    GetElementPtrInst::getIndexedType(GEPI->getSourceElementType(), Ops);
  if (!AllocTy || !AllocTy->isSized())
    return false;
  const DataLayout &DL = IC.getDataLayout();
  uint64_t TyAllocSize = DL.getTypeAllocSize(AllocTy);

  // If there are more indices after the one we might replace with a zero, make
  // sure they're all non-negative. If any of them are negative, the overall
  // address being computed might be before the base address determined by the
  // first non-zero index.
  auto IsAllNonNegative = [&]() {
    for (unsigned i = Idx+1, e = GEPI->getNumOperands(); i != e; ++i) {
      bool KnownNonNegative, KnownNegative;
      IC.ComputeSignBit(GEPI->getOperand(i), KnownNonNegative,
                        KnownNegative, 0, MemI);
      if (KnownNonNegative)
        continue;
      return false;
    }

    return true;
  };

  // FIXME: If the GEP is not inbounds, and there are extra indices after the
  // one we'll replace, those could cause the address computation to wrap
  // (rendering the IsAllNonNegative() check below insufficient). We can do
  // better, ignoring zero indices (and other indices we can prove small
  // enough not to wrap).
  if (Idx+1 != GEPI->getNumOperands() && !GEPI->isInBounds())
    return false;

  // Note that isObjectSizeLessThanOrEq will return true only if the pointer is
  // also known to be dereferenceable.
  return isObjectSizeLessThanOrEq(GEPI->getOperand(0), TyAllocSize, DL) &&
         IsAllNonNegative();
}

// If we're indexing into an object with a variable index for the memory
// access, but the object has only one element, we can assume that the index
// will always be zero. If we replace the GEP, return it.
template <typename T>
static Instruction *replaceGEPIdxWithZero(InstCombiner &IC, Value *Ptr,
                                          T &MemI) {
  if (GetElementPtrInst *GEPI = dyn_cast<GetElementPtrInst>(Ptr)) {
    unsigned Idx;
    if (canReplaceGEPIdxWithZero(IC, GEPI, &MemI, Idx)) {
      Instruction *NewGEPI = GEPI->clone();
      NewGEPI->setOperand(Idx,
        ConstantInt::get(GEPI->getOperand(Idx)->getType(), 0));
      NewGEPI->insertBefore(GEPI);
      MemI.setOperand(MemI.getPointerOperandIndex(), NewGEPI);
      return NewGEPI;
    }
  }

  return nullptr;
}

Instruction *InstCombiner::visitLoadInst(LoadInst &LI) {
  Value *Op = LI.getOperand(0);

  // Try to canonicalize the loaded type.
  if (Instruction *Res = combineLoadToOperationType(*this, LI))
    return Res;

  // Attempt to improve the alignment.
  unsigned KnownAlign = getOrEnforceKnownAlignment(
      Op, DL.getPrefTypeAlignment(LI.getType()), DL, &LI, &AC, &DT);
  unsigned LoadAlign = LI.getAlignment();
  unsigned EffectiveLoadAlign =
      LoadAlign != 0 ? LoadAlign : DL.getABITypeAlignment(LI.getType());

  if (KnownAlign > EffectiveLoadAlign)
    LI.setAlignment(KnownAlign);
  else if (LoadAlign == 0)
    LI.setAlignment(EffectiveLoadAlign);

  // Replace GEP indices if possible.
  if (Instruction *NewGEPI = replaceGEPIdxWithZero(*this, Op, LI)) {
      Worklist.Add(NewGEPI);
      return &LI;
  }

  if (Instruction *Res = unpackLoadToAggregate(*this, LI))
    return Res;

  // Do really simple store-to-load forwarding and load CSE, to catch cases
  // where there are several consecutive memory accesses to the same location,
  // separated by a few arithmetic operations.
  BasicBlock::iterator BBI(LI);
  bool IsLoadCSE = false;
  if (Value *AvailableVal =
      FindAvailableLoadedValue(&LI, LI.getParent(), BBI,
                               DefMaxInstsToScan, AA, &IsLoadCSE)) {
    if (IsLoadCSE) {
      LoadInst *NLI = cast<LoadInst>(AvailableVal);
      unsigned KnownIDs[] = {
          LLVMContext::MD_tbaa,            LLVMContext::MD_alias_scope,
          LLVMContext::MD_noalias,         LLVMContext::MD_range,
          LLVMContext::MD_invariant_load,  LLVMContext::MD_nonnull,
          LLVMContext::MD_invariant_group, LLVMContext::MD_align,
          LLVMContext::MD_dereferenceable,
          LLVMContext::MD_dereferenceable_or_null};
      combineMetadata(NLI, &LI, KnownIDs);
    };

    return replaceInstUsesWith(
        LI, Builder->CreateBitOrPointerCast(AvailableVal, LI.getType(),
                                            LI.getName() + ".cast"));
  }

  // None of the following transforms are legal for volatile/ordered atomic
  // loads.  Most of them do apply for unordered atomics.
  if (!LI.isUnordered()) return nullptr;

  // load(gep null, ...) -> unreachable
  if (GetElementPtrInst *GEPI = dyn_cast<GetElementPtrInst>(Op)) {
    const Value *GEPI0 = GEPI->getOperand(0);
    // TODO: Consider a target hook for valid address spaces for this xform.
    if (isa<ConstantPointerNull>(GEPI0) && GEPI->getPointerAddressSpace() == 0){
      // Insert a new store to null instruction before the load to indicate
      // that this code is not reachable.  We do this instead of inserting
      // an unreachable instruction directly because we cannot modify the
      // CFG.
      new StoreInst(UndefValue::get(LI.getType()),
                    Constant::getNullValue(Op->getType()), &LI);
      return replaceInstUsesWith(LI, UndefValue::get(LI.getType()));
    }
  }

  // load null/undef -> unreachable
  // TODO: Consider a target hook for valid address spaces for this xform.
  if (isa<UndefValue>(Op) ||
      (isa<ConstantPointerNull>(Op) && LI.getPointerAddressSpace() == 0)) {
    // Insert a new store to null instruction before the load to indicate that
    // this code is not reachable.  We do this instead of inserting an
    // unreachable instruction directly because we cannot modify the CFG.
    new StoreInst(UndefValue::get(LI.getType()),
                  Constant::getNullValue(Op->getType()), &LI);
    return replaceInstUsesWith(LI, UndefValue::get(LI.getType()));
  }

  if (Op->hasOneUse()) {
    // Change select and PHI nodes to select values instead of addresses: this
    // helps alias analysis out a lot, allows many others simplifications, and
    // exposes redundancy in the code.
    //
    // Note that we cannot do the transformation unless we know that the
    // introduced loads cannot trap!  Something like this is valid as long as
    // the condition is always false: load (select bool %C, int* null, int* %G),
    // but it would not be valid if we transformed it to load from null
    // unconditionally.
    //
    if (SelectInst *SI = dyn_cast<SelectInst>(Op)) {
      // load (select (Cond, &V1, &V2))  --> select(Cond, load &V1, load &V2).
      unsigned Align = LI.getAlignment();
      if (isSafeToLoadUnconditionally(SI->getOperand(1), Align, DL, SI) &&
          isSafeToLoadUnconditionally(SI->getOperand(2), Align, DL, SI)) {
        LoadInst *V1 = Builder->CreateLoad(SI->getOperand(1),
                                           SI->getOperand(1)->getName()+".val");
        LoadInst *V2 = Builder->CreateLoad(SI->getOperand(2),
                                           SI->getOperand(2)->getName()+".val");
        assert(LI.isUnordered() && "implied by above");
        V1->setAlignment(Align);
        V1->setAtomic(LI.getOrdering(), LI.getSynchScope());
        V2->setAlignment(Align);
        V2->setAtomic(LI.getOrdering(), LI.getSynchScope());
        return SelectInst::Create(SI->getCondition(), V1, V2);
      }

      // load (select (cond, null, P)) -> load P
      if (isa<ConstantPointerNull>(SI->getOperand(1)) &&
          LI.getPointerAddressSpace() == 0) {
        LI.setOperand(0, SI->getOperand(2));
        return &LI;
      }

      // load (select (cond, P, null)) -> load P
      if (isa<ConstantPointerNull>(SI->getOperand(2)) &&
          LI.getPointerAddressSpace() == 0) {
        LI.setOperand(0, SI->getOperand(1));
        return &LI;
      }
    }
  }
  return nullptr;
}

/// \brief Look for extractelement/insertvalue sequence that acts like a bitcast.
///
/// \returns underlying value that was "cast", or nullptr otherwise.
///
/// For example, if we have:
///
///     %E0 = extractelement <2 x double> %U, i32 0
///     %V0 = insertvalue [2 x double] undef, double %E0, 0
///     %E1 = extractelement <2 x double> %U, i32 1
///     %V1 = insertvalue [2 x double] %V0, double %E1, 1
///
/// and the layout of a <2 x double> is isomorphic to a [2 x double],
/// then %V1 can be safely approximated by a conceptual "bitcast" of %U.
/// Note that %U may contain non-undef values where %V1 has undef.
static Value *likeBitCastFromVector(InstCombiner &IC, Value *V) {
  Value *U = nullptr;
  while (auto *IV = dyn_cast<InsertValueInst>(V)) {
    auto *E = dyn_cast<ExtractElementInst>(IV->getInsertedValueOperand());
    if (!E)
      return nullptr;
    auto *W = E->getVectorOperand();
    if (!U)
      U = W;
    else if (U != W)
      return nullptr;
    auto *CI = dyn_cast<ConstantInt>(E->getIndexOperand());
    if (!CI || IV->getNumIndices() != 1 || CI->getZExtValue() != *IV->idx_begin())
      return nullptr;
    V = IV->getAggregateOperand();
  }
  if (!isa<UndefValue>(V) ||!U)
    return nullptr;

  auto *UT = cast<VectorType>(U->getType());
  auto *VT = V->getType();
  // Check that types UT and VT are bitwise isomorphic.
  const auto &DL = IC.getDataLayout();
  if (DL.getTypeStoreSizeInBits(UT) != DL.getTypeStoreSizeInBits(VT)) {
    return nullptr;
  }
  if (auto *AT = dyn_cast<ArrayType>(VT)) {
    if (AT->getNumElements() != UT->getNumElements())
      return nullptr;
  } else {
    auto *ST = cast<StructType>(VT);
    if (ST->getNumElements() != UT->getNumElements())
      return nullptr;
    for (const auto *EltT : ST->elements()) {
      if (EltT != UT->getElementType())
        return nullptr;
    }
  }
  return U;
}

/// \brief Combine stores to match the type of value being stored.
///
/// The core idea here is that the memory does not have any intrinsic type and
/// where we can we should match the type of a store to the type of value being
/// stored.
///
/// However, this routine must never change the width of a store or the number of
/// stores as that would introduce a semantic change. This combine is expected to
/// be a semantic no-op which just allows stores to more closely model the types
/// of their incoming values.
///
/// Currently, we also refuse to change the precise type used for an atomic or
/// volatile store. This is debatable, and might be reasonable to change later.
/// However, it is risky in case some backend or other part of LLVM is relying
/// on the exact type stored to select appropriate atomic operations.
///
/// \returns true if the store was successfully combined away. This indicates
/// the caller must erase the store instruction. We have to let the caller erase
/// the store instruction as otherwise there is no way to signal whether it was
/// combined or not: IC.EraseInstFromFunction returns a null pointer.
static bool combineStoreToValueType(InstCombiner &IC, StoreInst &SI) {
  // FIXME: We could probably with some care handle both volatile and ordered
  // atomic stores here but it isn't clear that this is important.
  if (!SI.isUnordered())
    return false;

  Value *V = SI.getValueOperand();

  // Fold away bit casts of the stored value by storing the original type.
  if (auto *BC = dyn_cast<BitCastInst>(V)) {
    V = BC->getOperand(0);
    combineStoreToNewValue(IC, SI, V);
    return true;
  }

  if (Value *U = likeBitCastFromVector(IC, V)) {
    combineStoreToNewValue(IC, SI, U);
    return true;
  }

  // FIXME: We should also canonicalize stores of vectors when their elements
  // are cast to other types.
  return false;
}

static bool unpackStoreToAggregate(InstCombiner &IC, StoreInst &SI) {
  // FIXME: We could probably with some care handle both volatile and atomic
  // stores here but it isn't clear that this is important.
  if (!SI.isSimple())
    return false;

  Value *V = SI.getValueOperand();
  Type *T = V->getType();

  if (!T->isAggregateType())
    return false;

  if (auto *ST = dyn_cast<StructType>(T)) {
    // If the struct only have one element, we unpack.
    unsigned Count = ST->getNumElements();
    if (Count == 1) {
      V = IC.Builder->CreateExtractValue(V, 0);
      combineStoreToNewValue(IC, SI, V);
      return true;
    }

    // We don't want to break loads with padding here as we'd loose
    // the knowledge that padding exists for the rest of the pipeline.
    const DataLayout &DL = IC.getDataLayout();
    auto *SL = DL.getStructLayout(ST);
    if (SL->hasPadding())
      return false;

    auto Align = SI.getAlignment();
    if (!Align)
      Align = DL.getABITypeAlignment(ST);

    SmallString<16> EltName = V->getName();
    EltName += ".elt";
    auto *Addr = SI.getPointerOperand();
    SmallString<16> AddrName = Addr->getName();
    AddrName += ".repack";

    auto *IdxType = Type::getInt32Ty(ST->getContext());
    auto *Zero = ConstantInt::get(IdxType, 0);
    for (unsigned i = 0; i < Count; i++) {
      Value *Indices[2] = {
        Zero,
        ConstantInt::get(IdxType, i),
      };
      auto *Ptr = IC.Builder->CreateInBoundsGEP(ST, Addr, makeArrayRef(Indices),
                                                AddrName);
      auto *Val = IC.Builder->CreateExtractValue(V, i, EltName);
      auto EltAlign = MinAlign(Align, SL->getElementOffset(i));
      IC.Builder->CreateAlignedStore(Val, Ptr, EltAlign);
    }

    return true;
  }

  if (auto *AT = dyn_cast<ArrayType>(T)) {
    // If the array only have one element, we unpack.
    auto NumElements = AT->getNumElements();
    if (NumElements == 1) {
      V = IC.Builder->CreateExtractValue(V, 0);
      combineStoreToNewValue(IC, SI, V);
      return true;
    }

    const DataLayout &DL = IC.getDataLayout();
    auto EltSize = DL.getTypeAllocSize(AT->getElementType());
    auto Align = SI.getAlignment();
    if (!Align)
      Align = DL.getABITypeAlignment(T);

    SmallString<16> EltName = V->getName();
    EltName += ".elt";
    auto *Addr = SI.getPointerOperand();
    SmallString<16> AddrName = Addr->getName();
    AddrName += ".repack";

    auto *IdxType = Type::getInt64Ty(T->getContext());
    auto *Zero = ConstantInt::get(IdxType, 0);

    uint64_t Offset = 0;
    for (uint64_t i = 0; i < NumElements; i++) {
      Value *Indices[2] = {
        Zero,
        ConstantInt::get(IdxType, i),
      };
      auto *Ptr = IC.Builder->CreateInBoundsGEP(AT, Addr, makeArrayRef(Indices),
                                                AddrName);
      auto *Val = IC.Builder->CreateExtractValue(V, i, EltName);
      auto EltAlign = MinAlign(Align, Offset);
      IC.Builder->CreateAlignedStore(Val, Ptr, EltAlign);
      Offset += EltSize;
    }

    return true;
  }

  return false;
}

/// equivalentAddressValues - Test if A and B will obviously have the same
/// value. This includes recognizing that %t0 and %t1 will have the same
/// value in code like this:
///   %t0 = getelementptr \@a, 0, 3
///   store i32 0, i32* %t0
///   %t1 = getelementptr \@a, 0, 3
///   %t2 = load i32* %t1
///
static bool equivalentAddressValues(Value *A, Value *B) {
  // Test if the values are trivially equivalent.
  if (A == B) return true;

  // Test if the values come form identical arithmetic instructions.
  // This uses isIdenticalToWhenDefined instead of isIdenticalTo because
  // its only used to compare two uses within the same basic block, which
  // means that they'll always either have the same value or one of them
  // will have an undefined value.
  if (isa<BinaryOperator>(A) ||
      isa<CastInst>(A) ||
      isa<PHINode>(A) ||
      isa<GetElementPtrInst>(A))
    if (Instruction *BI = dyn_cast<Instruction>(B))
      if (cast<Instruction>(A)->isIdenticalToWhenDefined(BI))
        return true;

  // Otherwise they may not be equivalent.
  return false;
}

Instruction *InstCombiner::visitStoreInst(StoreInst &SI) {
  Value *Val = SI.getOperand(0);
  Value *Ptr = SI.getOperand(1);

  // Try to canonicalize the stored type.
  if (combineStoreToValueType(*this, SI))
    return eraseInstFromFunction(SI);

  // Attempt to improve the alignment.
  unsigned KnownAlign = getOrEnforceKnownAlignment(
      Ptr, DL.getPrefTypeAlignment(Val->getType()), DL, &SI, &AC, &DT);
  unsigned StoreAlign = SI.getAlignment();
  unsigned EffectiveStoreAlign =
      StoreAlign != 0 ? StoreAlign : DL.getABITypeAlignment(Val->getType());

  if (KnownAlign > EffectiveStoreAlign)
    SI.setAlignment(KnownAlign);
  else if (StoreAlign == 0)
    SI.setAlignment(EffectiveStoreAlign);

  // Try to canonicalize the stored type.
  if (unpackStoreToAggregate(*this, SI))
    return eraseInstFromFunction(SI);

  // Replace GEP indices if possible.
  if (Instruction *NewGEPI = replaceGEPIdxWithZero(*this, Ptr, SI)) {
      Worklist.Add(NewGEPI);
      return &SI;
  }

  // Don't hack volatile/ordered stores.
  // FIXME: Some bits are legal for ordered atomic stores; needs refactoring.
  if (!SI.isUnordered()) return nullptr;

  // If the RHS is an alloca with a single use, zapify the store, making the
  // alloca dead.
  if (Ptr->hasOneUse()) {
    if (isa<AllocaInst>(Ptr))
      return eraseInstFromFunction(SI);
    if (GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(Ptr)) {
      if (isa<AllocaInst>(GEP->getOperand(0))) {
        if (GEP->getOperand(0)->hasOneUse())
          return eraseInstFromFunction(SI);
      }
    }
  }

  // Do really simple DSE, to catch cases where there are several consecutive
  // stores to the same location, separated by a few arithmetic operations. This
  // situation often occurs with bitfield accesses.
  BasicBlock::iterator BBI(SI);
  for (unsigned ScanInsts = 6; BBI != SI.getParent()->begin() && ScanInsts;
       --ScanInsts) {
    --BBI;
    // Don't count debug info directives, lest they affect codegen,
    // and we skip pointer-to-pointer bitcasts, which are NOPs.
    if (isa<DbgInfoIntrinsic>(BBI) ||
        (isa<BitCastInst>(BBI) && BBI->getType()->isPointerTy())) {
      ScanInsts++;
      continue;
    }

    if (StoreInst *PrevSI = dyn_cast<StoreInst>(BBI)) {
      // Prev store isn't volatile, and stores to the same location?
      if (PrevSI->isUnordered() && equivalentAddressValues(PrevSI->getOperand(1),
                                                        SI.getOperand(1))) {
        ++NumDeadStore;
        ++BBI;
        eraseInstFromFunction(*PrevSI);
        continue;
      }
      break;
    }

    // If this is a load, we have to stop.  However, if the loaded value is from
    // the pointer we're loading and is producing the pointer we're storing,
    // then *this* store is dead (X = load P; store X -> P).
    if (LoadInst *LI = dyn_cast<LoadInst>(BBI)) {
      if (LI == Val && equivalentAddressValues(LI->getOperand(0), Ptr)) {
        assert(SI.isUnordered() && "can't eliminate ordering operation");
        return eraseInstFromFunction(SI);
      }

      // Otherwise, this is a load from some other location.  Stores before it
      // may not be dead.
      break;
    }

    // Don't skip over loads or things that can modify memory.
    if (BBI->mayWriteToMemory() || BBI->mayReadFromMemory())
      break;
  }

  // store X, null    -> turns into 'unreachable' in SimplifyCFG
  if (isa<ConstantPointerNull>(Ptr) && SI.getPointerAddressSpace() == 0) {
    if (!isa<UndefValue>(Val)) {
      SI.setOperand(0, UndefValue::get(Val->getType()));
      if (Instruction *U = dyn_cast<Instruction>(Val))
        Worklist.Add(U);  // Dropped a use.
    }
    return nullptr;  // Do not modify these!
  }

  // store undef, Ptr -> noop
  if (isa<UndefValue>(Val))
    return eraseInstFromFunction(SI);

  // If this store is the last instruction in the basic block (possibly
  // excepting debug info instructions), and if the block ends with an
  // unconditional branch, try to move it to the successor block.
  BBI = SI.getIterator();
  do {
    ++BBI;
  } while (isa<DbgInfoIntrinsic>(BBI) ||
           (isa<BitCastInst>(BBI) && BBI->getType()->isPointerTy()));
  if (BranchInst *BI = dyn_cast<BranchInst>(BBI))
    if (BI->isUnconditional())
      if (SimplifyStoreAtEndOfBlock(SI))
        return nullptr;  // xform done!

  return nullptr;
}

/// SimplifyStoreAtEndOfBlock - Turn things like:
///   if () { *P = v1; } else { *P = v2 }
/// into a phi node with a store in the successor.
///
/// Simplify things like:
///   *P = v1; if () { *P = v2; }
/// into a phi node with a store in the successor.
///
bool InstCombiner::SimplifyStoreAtEndOfBlock(StoreInst &SI) {
  assert(SI.isUnordered() &&
         "this code has not been auditted for volatile or ordered store case");

  BasicBlock *StoreBB = SI.getParent();

  // Check to see if the successor block has exactly two incoming edges.  If
  // so, see if the other predecessor contains a store to the same location.
  // if so, insert a PHI node (if needed) and move the stores down.
  BasicBlock *DestBB = StoreBB->getTerminator()->getSuccessor(0);

  // Determine whether Dest has exactly two predecessors and, if so, compute
  // the other predecessor.
  pred_iterator PI = pred_begin(DestBB);
  BasicBlock *P = *PI;
  BasicBlock *OtherBB = nullptr;

  if (P != StoreBB)
    OtherBB = P;

  if (++PI == pred_end(DestBB))
    return false;

  P = *PI;
  if (P != StoreBB) {
    if (OtherBB)
      return false;
    OtherBB = P;
  }
  if (++PI != pred_end(DestBB))
    return false;

  // Bail out if all the relevant blocks aren't distinct (this can happen,
  // for example, if SI is in an infinite loop)
  if (StoreBB == DestBB || OtherBB == DestBB)
    return false;

  // Verify that the other block ends in a branch and is not otherwise empty.
  BasicBlock::iterator BBI(OtherBB->getTerminator());
  BranchInst *OtherBr = dyn_cast<BranchInst>(BBI);
  if (!OtherBr || BBI == OtherBB->begin())
    return false;

  // If the other block ends in an unconditional branch, check for the 'if then
  // else' case.  there is an instruction before the branch.
  StoreInst *OtherStore = nullptr;
  if (OtherBr->isUnconditional()) {
    --BBI;
    // Skip over debugging info.
    while (isa<DbgInfoIntrinsic>(BBI) ||
           (isa<BitCastInst>(BBI) && BBI->getType()->isPointerTy())) {
      if (BBI==OtherBB->begin())
        return false;
      --BBI;
    }
    // If this isn't a store, isn't a store to the same location, or is not the
    // right kind of store, bail out.
    OtherStore = dyn_cast<StoreInst>(BBI);
    if (!OtherStore || OtherStore->getOperand(1) != SI.getOperand(1) ||
        !SI.isSameOperationAs(OtherStore))
      return false;
  } else {
    // Otherwise, the other block ended with a conditional branch. If one of the
    // destinations is StoreBB, then we have the if/then case.
    if (OtherBr->getSuccessor(0) != StoreBB &&
        OtherBr->getSuccessor(1) != StoreBB)
      return false;

    // Okay, we know that OtherBr now goes to Dest and StoreBB, so this is an
    // if/then triangle.  See if there is a store to the same ptr as SI that
    // lives in OtherBB.
    for (;; --BBI) {
      // Check to see if we find the matching store.
      if ((OtherStore = dyn_cast<StoreInst>(BBI))) {
        if (OtherStore->getOperand(1) != SI.getOperand(1) ||
            !SI.isSameOperationAs(OtherStore))
          return false;
        break;
      }
      // If we find something that may be using or overwriting the stored
      // value, or if we run out of instructions, we can't do the xform.
      if (BBI->mayReadFromMemory() || BBI->mayWriteToMemory() ||
          BBI == OtherBB->begin())
        return false;
    }

    // In order to eliminate the store in OtherBr, we have to
    // make sure nothing reads or overwrites the stored value in
    // StoreBB.
    for (BasicBlock::iterator I = StoreBB->begin(); &*I != &SI; ++I) {
      // FIXME: This should really be AA driven.
      if (I->mayReadFromMemory() || I->mayWriteToMemory())
        return false;
    }
  }

  // Insert a PHI node now if we need it.
  Value *MergedVal = OtherStore->getOperand(0);
  if (MergedVal != SI.getOperand(0)) {
    PHINode *PN = PHINode::Create(MergedVal->getType(), 2, "storemerge");
    PN->addIncoming(SI.getOperand(0), SI.getParent());
    PN->addIncoming(OtherStore->getOperand(0), OtherBB);
    MergedVal = InsertNewInstBefore(PN, DestBB->front());
  }

  // Advance to a place where it is safe to insert the new store and
  // insert it.
  BBI = DestBB->getFirstInsertionPt();
  StoreInst *NewSI = new StoreInst(MergedVal, SI.getOperand(1),
                                   SI.isVolatile(),
                                   SI.getAlignment(),
                                   SI.getOrdering(),
                                   SI.getSynchScope());
  InsertNewInstBefore(NewSI, *BBI);
  NewSI->setDebugLoc(OtherStore->getDebugLoc());

  // If the two stores had AA tags, merge them.
  AAMDNodes AATags;
  SI.getAAMetadata(AATags);
  if (AATags) {
    OtherStore->getAAMetadata(AATags, /* Merge = */ true);
    NewSI->setAAMetadata(AATags);
  }

  // Nuke the old stores.
  eraseInstFromFunction(SI);
  eraseInstFromFunction(*OtherStore);
  return true;
}