Transforms/Scalar/DeadStoreElimination.cpp

//===- DeadStoreElimination.cpp - Fast Dead Store Elimination -------------===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// 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.
//
//===----------------------------------------------------------------------===//

#define DEBUG_TYPE "dse"
#include "llvm/Transforms/Scalar.h"
#include "llvm/Constants.h"
#include "llvm/Function.h"
#include "llvm/GlobalVariable.h"
#include "llvm/Instructions.h"
#include "llvm/IntrinsicInst.h"
#include "llvm/Pass.h"
#include "llvm/Analysis/AliasAnalysis.h"
#include "llvm/Analysis/Dominators.h"
#include "llvm/Analysis/MemoryBuiltins.h"
#include "llvm/Analysis/MemoryDependenceAnalysis.h"
#include "llvm/Analysis/ValueTracking.h"
#include "llvm/Target/TargetData.h"
#include "llvm/Transforms/Utils/Local.h"
#include "llvm/Support/Debug.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/Statistic.h"
using namespace llvm;

STATISTIC(NumFastStores, "Number of stores deleted");
STATISTIC(NumFastOther , "Number of other instrs removed");

namespace {
  struct DSE : public FunctionPass {
    AliasAnalysis *AA;
    MemoryDependenceAnalysis *MD;

    static char ID; // Pass identification, replacement for typeid
    DSE() : FunctionPass(ID), AA(0), MD(0) {
      initializeDSEPass(*PassRegistry::getPassRegistry());
    }

    virtual bool runOnFunction(Function &F) {
      AA = &getAnalysis<AliasAnalysis>();
      MD = &getAnalysis<MemoryDependenceAnalysis>();
      DominatorTree &DT = getAnalysis<DominatorTree>();

      bool Changed = false;
      for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I)
        // Only check non-dead blocks.  Dead blocks may have strange pointer
        // cycles that will confuse alias analysis.
        if (DT.isReachableFromEntry(I))
          Changed |= runOnBasicBlock(*I);

      AA = 0; MD = 0;
      return Changed;
    }

    bool runOnBasicBlock(BasicBlock &BB);
    bool HandleFree(CallInst *F);
    bool handleEndBlock(BasicBlock &BB);
    void RemoveAccessedObjects(const AliasAnalysis::Location &LoadedLoc,
                               SmallPtrSet<Value*, 16> &DeadStackObjects);

    virtual void getAnalysisUsage(AnalysisUsage &AU) const {
      AU.setPreservesCFG();
      AU.addRequired<DominatorTree>();
      AU.addRequired<AliasAnalysis>();
      AU.addRequired<MemoryDependenceAnalysis>();
      AU.addPreserved<AliasAnalysis>();
      AU.addPreserved<DominatorTree>();
      AU.addPreserved<MemoryDependenceAnalysis>();
    }
  };
}

char DSE::ID = 0;
INITIALIZE_PASS_BEGIN(DSE, "dse", "Dead Store Elimination", false, false)
INITIALIZE_PASS_DEPENDENCY(DominatorTree)
INITIALIZE_PASS_DEPENDENCY(MemoryDependenceAnalysis)
INITIALIZE_AG_DEPENDENCY(AliasAnalysis)
INITIALIZE_PASS_END(DSE, "dse", "Dead Store Elimination", false, false)

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

//===----------------------------------------------------------------------===//
// Helper functions
//===----------------------------------------------------------------------===//

/// DeleteDeadInstruction - 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,
                                  MemoryDependenceAnalysis &MD,
                                  SmallPtrSet<Value*, 16> *ValueSet = 0) {
  SmallVector<Instruction*, 32> NowDeadInsts;

  NowDeadInsts.push_back(I);
  --NumFastOther;

  // Before we touch this instruction, remove it from memdep!
  do {
    Instruction *DeadInst = NowDeadInsts.pop_back_val();
    ++NumFastOther;

    // 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, 0);

      // 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))
          NowDeadInsts.push_back(OpI);
    }

    DeadInst->eraseFromParent();

    if (ValueSet) ValueSet->erase(DeadInst);
  } while (!NowDeadInsts.empty());
}


/// hasMemoryWrite - Does this instruction write some memory?  This only returns
/// true for things that we can analyze with other helpers below.
static bool hasMemoryWrite(Instruction *I) {
  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::init_trampoline:
    case Intrinsic::lifetime_end:
      return true;
    }
  }
  return false;
}

/// getLocForWrite - Return a Location stored to by the specified instruction.
static AliasAnalysis::Location
getLocForWrite(Instruction *Inst, AliasAnalysis &AA) {
  if (StoreInst *SI = dyn_cast<StoreInst>(Inst))
    return AA.getLocation(SI);

  if (MemIntrinsic *MI = dyn_cast<MemIntrinsic>(Inst)) {
    // memcpy/memmove/memset.
    AliasAnalysis::Location Loc = AA.getLocationForDest(MI);
    // If we don't have target data around, an unknown size in Location means
    // that we should use the size of the pointee type.  This isn't valid for
    // memset/memcpy, which writes more than an i8.
    if (Loc.Size == AliasAnalysis::UnknownSize && AA.getTargetData() == 0)
      return AliasAnalysis::Location();
    return Loc;
  }

  IntrinsicInst *II = dyn_cast<IntrinsicInst>(Inst);
  if (II == 0) return AliasAnalysis::Location();

  switch (II->getIntrinsicID()) {
  default: return AliasAnalysis::Location(); // Unhandled intrinsic.
  case Intrinsic::init_trampoline:
    // If we don't have target data around, an unknown size in Location means
    // that we should use the size of the pointee type.  This isn't valid for
    // init.trampoline, which writes more than an i8.
    if (AA.getTargetData() == 0) return AliasAnalysis::Location();

    // FIXME: We don't know the size of the trampoline, so we can't really
    // handle it here.
    return AliasAnalysis::Location(II->getArgOperand(0));
  case Intrinsic::lifetime_end: {
    uint64_t Len = cast<ConstantInt>(II->getArgOperand(0))->getZExtValue();
    return AliasAnalysis::Location(II->getArgOperand(1), Len);
  }
  }
}

/// getLocForRead - Return the location read by the specified "hasMemoryWrite"
/// instruction if any.
static AliasAnalysis::Location
getLocForRead(Instruction *Inst, AliasAnalysis &AA) {
  assert(hasMemoryWrite(Inst) && "Unknown instruction case");

  // The only instructions that both read and write are the mem transfer
  // instructions (memcpy/memmove).
  if (MemTransferInst *MTI = dyn_cast<MemTransferInst>(Inst))
    return AA.getLocationForSource(MTI);
  return AliasAnalysis::Location();
}


/// isRemovable - 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 stores.
  if (StoreInst *SI = dyn_cast<StoreInst>(I))
    return !SI->isVolatile();

  IntrinsicInst *II = cast<IntrinsicInst>(I);
  switch (II->getIntrinsicID()) {
  default: assert(0 && "doesn't pass 'hasMemoryWrite' 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();
  }
}

/// getStoredPointerOperand - Return the pointer that is being written to.
static Value *getStoredPointerOperand(Instruction *I) {
  if (StoreInst *SI = dyn_cast<StoreInst>(I))
    return SI->getPointerOperand();
  if (MemIntrinsic *MI = dyn_cast<MemIntrinsic>(I))
    return MI->getDest();

  IntrinsicInst *II = cast<IntrinsicInst>(I);
  switch (II->getIntrinsicID()) {
  default: assert(false && "Unexpected intrinsic!");
  case Intrinsic::init_trampoline:
    return II->getArgOperand(0);
  }
}

static uint64_t getPointerSize(Value *V, AliasAnalysis &AA) {
  const TargetData *TD = AA.getTargetData();
  if (TD == 0)
    return AliasAnalysis::UnknownSize;

  if (AllocaInst *A = dyn_cast<AllocaInst>(V)) {
    // Get size information for the alloca
    if (ConstantInt *C = dyn_cast<ConstantInt>(A->getArraySize()))
      return C->getZExtValue() * TD->getTypeAllocSize(A->getAllocatedType());
    return AliasAnalysis::UnknownSize;
  }

  assert(isa<Argument>(V) && "Expected AllocaInst or Argument!");
  PointerType *PT = cast<PointerType>(V->getType());
  return TD->getTypeAllocSize(PT->getElementType());
}

/// isObjectPointerWithTrustworthySize - Return true if the specified Value* is
/// pointing to an object with a pointer size we can trust.
static bool isObjectPointerWithTrustworthySize(const Value *V) {
  if (const AllocaInst *AI = dyn_cast<AllocaInst>(V))
    return !AI->isArrayAllocation();
  if (const GlobalVariable *GV = dyn_cast<GlobalVariable>(V))
    return !GV->mayBeOverridden();
  if (const Argument *A = dyn_cast<Argument>(V))
    return A->hasByValAttr();
  return false;
}

/// isCompleteOverwrite - Return true if a store to the 'Later' location
/// completely overwrites a store to the 'Earlier' location.
static bool isCompleteOverwrite(const AliasAnalysis::Location &Later,
                                const AliasAnalysis::Location &Earlier,
                                AliasAnalysis &AA) {
  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) {
    // If we don't know the sizes of either access, then we can't do a
    // comparison.
    if (Later.Size == AliasAnalysis::UnknownSize ||
        Earlier.Size == AliasAnalysis::UnknownSize) {
      // If we have no TargetData information around, then the size of the store
      // is inferrable from the pointee type.  If they are the same type, then
      // we know that the store is safe.
      if (AA.getTargetData() == 0)
        return Later.Ptr->getType() == Earlier.Ptr->getType();
      return false;
    }

    // Make sure that the Later size is >= the Earlier size.
    if (Later.Size < Earlier.Size)
      return false;
    return true;
  }

  // Otherwise, we have to have size information, and the later store has to be
  // larger than the earlier one.
  if (Later.Size == AliasAnalysis::UnknownSize ||
      Earlier.Size == AliasAnalysis::UnknownSize ||
      Later.Size <= Earlier.Size || AA.getTargetData() == 0)
    return false;

  // Check to see if the later store is to the entire object (either a global,
  // an alloca, or a byval argument).  If so, then it clearly overwrites any
  // other store to the same object.
  const TargetData &TD = *AA.getTargetData();

  const Value *UO1 = GetUnderlyingObject(P1, &TD),
              *UO2 = GetUnderlyingObject(P2, &TD);

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

  // If the "Later" store is to a recognizable object, get its size.
  if (isObjectPointerWithTrustworthySize(UO2)) {
    uint64_t ObjectSize =
      TD.getTypeAllocSize(cast<PointerType>(UO2->getType())->getElementType());
    if (ObjectSize == Later.Size)
      return true;
  }

  // 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.
  int64_t EarlierOff = 0, LaterOff = 0;
  const Value *BP1 = GetPointerBaseWithConstantOffset(P1, EarlierOff, TD);
  const Value *BP2 = GetPointerBaseWithConstantOffset(P2, LaterOff, TD);

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

  // 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 &&
      uint64_t(EarlierOff - LaterOff) + Earlier.Size <= Later.Size)
    return true;

  // Otherwise, they don't completely overlap.
  return false;
}

/// isPossibleSelfRead - 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:
///
///   memcpy(A <- B)
///   memcpy(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 AliasAnalysis::Location &InstStoreLoc,
                               Instruction *DepWrite, AliasAnalysis &AA) {
  // Self reads can only happen for instructions that read memory.  Get the
  // location read.
  AliasAnalysis::Location InstReadLoc = getLocForRead(Inst, AA);
  if (InstReadLoc.Ptr == 0) 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;

  // Okay, 'Inst' may copy over itself.  However, we can still remove a the
  // DepWrite instruction if we can prove that it reads from the same location
  // as Inst.  This handles useful cases like:
  //   memcpy(A <- B)
  //   memcpy(A <- B)
  // Here we don't know if A/B may alias, but we do know that B/B are must
  // aliases, so removing the first memcpy is safe (assuming it writes <= #
  // bytes as the second one.
  AliasAnalysis::Location DepReadLoc = getLocForRead(DepWrite, AA);

  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;
}


//===----------------------------------------------------------------------===//
// DSE Pass
//===----------------------------------------------------------------------===//

bool DSE::runOnBasicBlock(BasicBlock &BB) {
  bool MadeChange = false;

  // Do a top-down walk on the BB.
  for (BasicBlock::iterator BBI = BB.begin(), BBE = BB.end(); BBI != BBE; ) {
    Instruction *Inst = BBI++;

    // Handle 'free' calls specially.
    if (CallInst *F = isFreeCall(Inst)) {
      MadeChange |= HandleFree(F);
      continue;
    }

    // If we find something that writes memory, get its memory dependence.
    if (!hasMemoryWrite(Inst))
      continue;

    MemDepResult InstDep = MD->getDependency(Inst);

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

    // If we're storing the same value back to a pointer that we just
    // loaded from, then the store can be removed.
    if (StoreInst *SI = dyn_cast<StoreInst>(Inst)) {
      if (LoadInst *DepLoad = dyn_cast<LoadInst>(InstDep.getInst())) {
        if (SI->getPointerOperand() == DepLoad->getPointerOperand() &&
            SI->getOperand(0) == DepLoad && !SI->isVolatile()) {
          DEBUG(dbgs() << "DSE: Remove Store Of Load from same pointer:\n  "
                       << "LOAD: " << *DepLoad << "\n  STORE: " << *SI << '\n');

          // DeleteDeadInstruction can delete the current instruction.  Save BBI
          // in case we need it.
          WeakVH NextInst(BBI);

          DeleteDeadInstruction(SI, *MD);

          if (NextInst == 0)  // Next instruction deleted.
            BBI = BB.begin();
          else if (BBI != BB.begin())  // Revisit this instruction if possible.
            --BBI;
          ++NumFastStores;
          MadeChange = true;
          continue;
        }
      }
    }

    // Figure out what location is being stored to.
    AliasAnalysis::Location Loc = getLocForWrite(Inst, *AA);

    // If we didn't get a useful location, fail.
    if (Loc.Ptr == 0)
      continue;

    while (!InstDep.isNonLocal() && !InstDep.isUnknown()) {
      // 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 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();
      AliasAnalysis::Location DepLoc = getLocForWrite(DepWrite, *AA);
      // If we didn't get a useful location, or if it isn't a size, bail out.
      if (DepLoc.Ptr == 0)
        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.
      if (isRemovable(DepWrite) && isCompleteOverwrite(Loc, DepLoc, *AA) &&
          !isPossibleSelfRead(Inst, Loc, DepWrite, *AA)) {
        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, *MD);
        ++NumFastStores;
        MadeChange = true;

        // DeleteDeadInstruction can delete the current instruction in loop
        // cases, reset BBI.
        BBI = Inst;
        if (BBI != BB.begin())
          --BBI;
        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 (AA->getModRefInfo(DepWrite, Loc) & AliasAnalysis::Ref)
        break;

      InstDep = MD->getPointerDependencyFrom(Loc, false, DepWrite, &BB);
    }
  }

  // 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);

  return MadeChange;
}

/// HandleFree - Handle frees of entire structures whose dependency is a store
/// to a field of that structure.
bool DSE::HandleFree(CallInst *F) {
  bool MadeChange = false;

  MemDepResult Dep = MD->getDependency(F);

  while (!Dep.isNonLocal() && !Dep.isUnknown()) {
    Instruction *Dependency = Dep.getInst();
    if (!hasMemoryWrite(Dependency) || !isRemovable(Dependency))
      return MadeChange;

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

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

    // DCE instructions only used to calculate that store
    DeleteDeadInstruction(Dependency, *MD);
    ++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->getDependency(F);
  };

  return MadeChange;
}

/// handleEndBlock - Remove dead stores to stack-allocated locations in the
/// function end block.  Ex:
/// %A = alloca i32
/// ...
/// store i32 1, i32* %A
/// ret void
bool DSE::handleEndBlock(BasicBlock &BB) {
  bool MadeChange = false;

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

  // Find all of the alloca'd pointers in the entry block.
  BasicBlock *Entry = BB.getParent()->begin();
  for (BasicBlock::iterator I = Entry->begin(), E = Entry->end(); I != E; ++I)
    if (AllocaInst *AI = dyn_cast<AllocaInst>(I))
      DeadStackObjects.insert(AI);

  // Treat byval arguments the same, stores to them are dead at the end of the
  // function.
  for (Function::arg_iterator AI = BB.getParent()->arg_begin(),
       AE = BB.getParent()->arg_end(); AI != AE; ++AI)
    if (AI->hasByValAttr())
      DeadStackObjects.insert(AI);

  // 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 (hasMemoryWrite(BBI) && isRemovable(BBI)) {
      // See through pointer-to-pointer bitcasts
      Value *Pointer = GetUnderlyingObject(getStoredPointerOperand(BBI));

      // Stores to stack values are valid candidates for removal.
      if (DeadStackObjects.count(Pointer)) {
        Instruction *Dead = BBI++;

        DEBUG(dbgs() << "DSE: Dead Store at End of Block:\n  DEAD: "
                     << *Dead << "\n  Object: " << *Pointer << '\n');

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

    // Remove any dead non-memory-mutating instructions.
    if (isInstructionTriviallyDead(BBI)) {
      Instruction *Inst = BBI++;
      DeleteDeadInstruction(Inst, *MD, &DeadStackObjects);
      ++NumFastOther;
      MadeChange = true;
      continue;
    }

    if (AllocaInst *A = dyn_cast<AllocaInst>(BBI)) {
      DeadStackObjects.erase(A);
      continue;
    }

    if (CallSite CS = cast<Value>(BBI)) {
      // If this call does not access memory, it can't be loading any of our
      // pointers.
      if (AA->doesNotAccessMemory(CS))
        continue;

      // If the call might load from any of our allocas, then any store above
      // the call is live.
      SmallVector<Value*, 8> LiveAllocas;
      for (SmallPtrSet<Value*, 16>::iterator I = DeadStackObjects.begin(),
           E = DeadStackObjects.end(); I != E; ++I) {
        // See if the call site touches it.
        AliasAnalysis::ModRefResult A =
          AA->getModRefInfo(CS, *I, getPointerSize(*I, *AA));

        if (A == AliasAnalysis::ModRef || A == AliasAnalysis::Ref)
          LiveAllocas.push_back(*I);
      }

      for (SmallVector<Value*, 8>::iterator I = LiveAllocas.begin(),
           E = LiveAllocas.end(); I != E; ++I)
        DeadStackObjects.erase(*I);

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

      continue;
    }

    AliasAnalysis::Location LoadedLoc;

    // If we encounter a use of the pointer, it is no longer considered dead
    if (LoadInst *L = dyn_cast<LoadInst>(BBI)) {
      LoadedLoc = AA->getLocation(L);
    } else if (VAArgInst *V = dyn_cast<VAArgInst>(BBI)) {
      LoadedLoc = AA->getLocation(V);
    } else if (MemTransferInst *MTI = dyn_cast<MemTransferInst>(BBI)) {
      LoadedLoc = AA->getLocationForSource(MTI);
    } else {
      // Not a loading instruction.
      continue;
    }

    // Remove any allocas from the DeadPointer set that are loaded, as this
    // makes any stores above the access live.
    RemoveAccessedObjects(LoadedLoc, DeadStackObjects);

    // 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;
}

/// RemoveAccessedObjects - 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.
void DSE::RemoveAccessedObjects(const AliasAnalysis::Location &LoadedLoc,
                                SmallPtrSet<Value*, 16> &DeadStackObjects) {
  const Value *UnderlyingPointer = GetUnderlyingObject(LoadedLoc.Ptr);

  // 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.erase(const_cast<Value*>(UnderlyingPointer));
    return;
  }

  SmallVector<Value*, 16> NowLive;
  for (SmallPtrSet<Value*, 16>::iterator I = DeadStackObjects.begin(),
       E = DeadStackObjects.end(); I != E; ++I) {
    // See if the loaded location could alias the stack location.
    AliasAnalysis::Location StackLoc(*I, getPointerSize(*I, *AA));
    if (!AA->isNoAlias(StackLoc, LoadedLoc))
      NowLive.push_back(*I);
  }

  for (SmallVector<Value*, 16>::iterator I = NowLive.begin(), E = NowLive.end();
       I != E; ++I)
    DeadStackObjects.erase(*I);
}