lib/Transforms/Scalar/DeadStoreElimination.cpp - llvm - Git at Google

 //===- 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/Instructions.h"
 #include "llvm/IntrinsicInst.h"
 #include "llvm/Pass.h"
 #include "llvm/ADT/SmallPtrSet.h"
 #include "llvm/ADT/Statistic.h"
 #include "llvm/Analysis/AliasAnalysis.h"
 #include "llvm/Analysis/Dominators.h"
 #include "llvm/Analysis/MemoryDependenceAnalysis.h"
 #include "llvm/Target/TargetData.h"
 #include "llvm/Transforms/Utils/Local.h"
 #include "llvm/Support/Compiler.h"
 using namespace llvm;

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

 namespace {
   struct VISIBILITY_HIDDEN DSE : public FunctionPass {
     static char ID; // Pass identification, replacement for typeid
     DSE() : FunctionPass(&ID) {}

     virtual bool runOnFunction(Function &F) {
       bool Changed = false;
       for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I)
         Changed |= runOnBasicBlock(*I);
       return Changed;
     }

     bool runOnBasicBlock(BasicBlock &BB);
     bool handleFreeWithNonTrivialDependency(FreeInst *F, MemDepResult Dep);
     bool handleEndBlock(BasicBlock &BB);
     bool RemoveUndeadPointers(Value* Ptr, uint64_t killPointerSize,
                               BasicBlock::iterator& BBI,
                               SmallPtrSet<Value*, 64>& deadPointers);
     void DeleteDeadInstruction(Instruction *I,
                                SmallPtrSet<Value*, 64> *deadPointers = 0);


     // getAnalysisUsage - We require post dominance frontiers (aka Control
     // Dependence Graph)
     virtual void getAnalysisUsage(AnalysisUsage &AU) const {
       AU.setPreservesCFG();
       AU.addRequired<DominatorTree>();
       AU.addRequired<TargetData>();
       AU.addRequired<AliasAnalysis>();
       AU.addRequired<MemoryDependenceAnalysis>();
       AU.addPreserved<DominatorTree>();
       AU.addPreserved<AliasAnalysis>();
       AU.addPreserved<MemoryDependenceAnalysis>();
     }
   };
 }

 char DSE::ID = 0;
 static RegisterPass<DSE> X("dse", "Dead Store Elimination");

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

 bool DSE::runOnBasicBlock(BasicBlock &BB) {
   MemoryDependenceAnalysis& MD = getAnalysis<MemoryDependenceAnalysis>();
   TargetData &TD = getAnalysis<TargetData>();

   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++;

     // If we find a store or a free, get it's memory dependence.
     if (!isa<StoreInst>(Inst) && !isa<FreeInst>(Inst))
       continue;

     // Don't molest volatile stores or do queries that will return "clobber".
     if (StoreInst *SI = dyn_cast<StoreInst>(Inst))
       if (SI->isVolatile())
         continue;

     MemDepResult InstDep = MD.getDependency(Inst);

     // Ignore non-local stores.
     // FIXME: cross-block DSE would be fun. :)
     if (InstDep.isNonLocal()) continue;

     // Handle frees whose dependencies are non-trivial.
     if (FreeInst *FI = dyn_cast<FreeInst>(Inst)) {
       MadeChange |= handleFreeWithNonTrivialDependency(FI, InstDep);
       continue;
     }

     StoreInst *SI = cast<StoreInst>(Inst);

     // If not a definite must-alias dependency, ignore it.
     if (!InstDep.isDef())
       continue;

     // If this is a store-store dependence, then the previous store is dead so
     // long as this store is at least as big as it.
     if (StoreInst *DepStore = dyn_cast<StoreInst>(InstDep.getInst()))
       if (TD.getTypeStoreSize(DepStore->getOperand(0)->getType()) <=
           TD.getTypeStoreSize(SI->getOperand(0)->getType())) {
         // Delete the store and now-dead instructions that feed it.
         DeleteDeadInstruction(DepStore);
         NumFastStores++;
         MadeChange = true;

         if (BBI != BB.begin())
           --BBI;
         continue;
       }

     // 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>(InstDep.getInst())) {
       if (SI->getPointerOperand() == DepLoad->getPointerOperand() &&
           SI->getOperand(0) == DepLoad) {
         DeleteDeadInstruction(SI);
         if (BBI != BB.begin())
           --BBI;
         NumFastStores++;
         MadeChange = true;
         continue;
       }
     }
   }

   // 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;
 }

 /// handleFreeWithNonTrivialDependency - Handle frees of entire structures whose
 /// dependency is a store to a field of that structure.
 bool DSE::handleFreeWithNonTrivialDependency(FreeInst *F, MemDepResult Dep) {
   AliasAnalysis &AA = getAnalysis<AliasAnalysis>();

   StoreInst *Dependency = dyn_cast_or_null<StoreInst>(Dep.getInst());
   if (!Dependency || Dependency->isVolatile())
     return false;

   Value *DepPointer = Dependency->getPointerOperand()->getUnderlyingObject();

   // Check for aliasing.
   if (AA.alias(F->getPointerOperand(), 1, DepPointer, 1) !=
          AliasAnalysis::MustAlias)
     return false;

   // DCE instructions only used to calculate that store
   DeleteDeadInstruction(Dependency);
   NumFastStores++;
   return true;
 }

 /// 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) {
   TargetData &TD = getAnalysis<TargetData>();
   AliasAnalysis &AA = getAnalysis<AliasAnalysis>();

   bool MadeChange = false;

   // Pointers alloca'd in this function are dead in the end block
   SmallPtrSet<Value*, 64> deadPointers;

   // 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))
       deadPointers.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())
       deadPointers.insert(AI);

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

     // If we find a store whose pointer is dead.
     if (StoreInst* S = dyn_cast<StoreInst>(BBI)) {
       if (!S->isVolatile()) {
         // See through pointer-to-pointer bitcasts
         Value* pointerOperand = S->getPointerOperand()->getUnderlyingObject();

         // Alloca'd pointers or byval arguments (which are functionally like
         // alloca's) are valid candidates for removal.
         if (deadPointers.count(pointerOperand)) {
           // DCE instructions only used to calculate that store.
           BBI++;
           DeleteDeadInstruction(S, &deadPointers);
           NumFastStores++;
           MadeChange = true;
         }
       }

       continue;
     }

     // We can also remove memcpy's to local variables at the end of a function.
     if (MemCpyInst *M = dyn_cast<MemCpyInst>(BBI)) {
       Value *dest = M->getDest()->getUnderlyingObject();

       if (deadPointers.count(dest)) {
         BBI++;
         DeleteDeadInstruction(M, &deadPointers);
         NumFastOther++;
         MadeChange = true;
         continue;
       }

       // Because a memcpy is also a load, we can't skip it if we didn't remove
       // it.
     }

     Value* killPointer = 0;
     uint64_t killPointerSize = ~0UL;

     // If we encounter a use of the pointer, it is no longer considered dead
     if (LoadInst *L = dyn_cast<LoadInst>(BBI)) {
       // However, if this load is unused and not volatile, we can go ahead and
       // remove it, and not have to worry about it making our pointer undead!
       if (L->use_empty() && !L->isVolatile()) {
         BBI++;
         DeleteDeadInstruction(L, &deadPointers);
         NumFastOther++;
         MadeChange = true;
         continue;
       }

       killPointer = L->getPointerOperand();
     } else if (VAArgInst* V = dyn_cast<VAArgInst>(BBI)) {
       killPointer = V->getOperand(0);
     } else if (isa<MemCpyInst>(BBI) &&
                isa<ConstantInt>(cast<MemCpyInst>(BBI)->getLength())) {
       killPointer = cast<MemCpyInst>(BBI)->getSource();
       killPointerSize = cast<ConstantInt>(
                             cast<MemCpyInst>(BBI)->getLength())->getZExtValue();
     } else if (AllocaInst* A = dyn_cast<AllocaInst>(BBI)) {
       deadPointers.erase(A);

       // Dead alloca's can be DCE'd when we reach them
       if (A->use_empty()) {
         BBI++;
         DeleteDeadInstruction(A, &deadPointers);
         NumFastOther++;
         MadeChange = true;
       }

       continue;
     } else if (CallSite::get(BBI).getInstruction() != 0) {
       // If this call does not access memory, it can't
       // be undeadifying any of our pointers.
       CallSite CS = CallSite::get(BBI);
       if (AA.doesNotAccessMemory(CS))
         continue;

       unsigned modRef = 0;
       unsigned other = 0;

       // Remove any pointers made undead by the call from the dead set
       std::vector<Value*> dead;
       for (SmallPtrSet<Value*, 64>::iterator I = deadPointers.begin(),
            E = deadPointers.end(); I != E; ++I) {
         // HACK: if we detect that our AA is imprecise, it's not
         // worth it to scan the rest of the deadPointers set.  Just
         // assume that the AA will return ModRef for everything, and
         // go ahead and bail.
         if (modRef >= 16 && other == 0) {
           deadPointers.clear();
           return MadeChange;
         }

         // Get size information for the alloca
         unsigned pointerSize = ~0U;
         if (AllocaInst* A = dyn_cast<AllocaInst>(*I)) {
           if (ConstantInt* C = dyn_cast<ConstantInt>(A->getArraySize()))
             pointerSize = C->getZExtValue() *
                           TD.getTypePaddedSize(A->getAllocatedType());
         } else {
           const PointerType* PT = cast<PointerType>(
                                                  cast<Argument>(*I)->getType());
           pointerSize = TD.getTypePaddedSize(PT->getElementType());
         }

         // See if the call site touches it
         AliasAnalysis::ModRefResult A = AA.getModRefInfo(CS, *I, pointerSize);

         if (A == AliasAnalysis::ModRef)
           modRef++;
         else
           other++;

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

       for (std::vector<Value*>::iterator I = dead.begin(), E = dead.end();
            I != E; ++I)
         deadPointers.erase(*I);

       continue;
     } else if (isInstructionTriviallyDead(BBI)) {
       // For any non-memory-affecting non-terminators, DCE them as we reach them
       Instruction *Inst = BBI;
       BBI++;
       DeleteDeadInstruction(Inst, &deadPointers);
       NumFastOther++;
       MadeChange = true;
       continue;
     }

     if (!killPointer)
       continue;

     killPointer = killPointer->getUnderlyingObject();

     // Deal with undead pointers
     MadeChange |= RemoveUndeadPointers(killPointer, killPointerSize, BBI,
                                        deadPointers);
   }

   return MadeChange;
 }

 /// RemoveUndeadPointers - check for uses of a pointer that make it
 /// undead when scanning for dead stores to alloca's.
 bool DSE::RemoveUndeadPointers(Value* killPointer, uint64_t killPointerSize,
                                BasicBlock::iterator &BBI,
                                SmallPtrSet<Value*, 64>& deadPointers) {
   TargetData &TD = getAnalysis<TargetData>();
   AliasAnalysis &AA = getAnalysis<AliasAnalysis>();

   // If the kill pointer can be easily reduced to an alloca,
   // don't bother doing extraneous AA queries.
   if (deadPointers.count(killPointer)) {
     deadPointers.erase(killPointer);
     return false;
   }

   // A global can't be in the dead pointer set.
   if (isa<GlobalValue>(killPointer))
     return false;

   bool MadeChange = false;

   SmallVector<Value*, 16> undead;

   for (SmallPtrSet<Value*, 64>::iterator I = deadPointers.begin(),
       E = deadPointers.end(); I != E; ++I) {
     // Get size information for the alloca.
     unsigned pointerSize = ~0U;
     if (AllocaInst* A = dyn_cast<AllocaInst>(*I)) {
       if (ConstantInt* C = dyn_cast<ConstantInt>(A->getArraySize()))
         pointerSize = C->getZExtValue() *
                       TD.getTypePaddedSize(A->getAllocatedType());
     } else {
       const PointerType* PT = cast<PointerType>(cast<Argument>(*I)->getType());
       pointerSize = TD.getTypePaddedSize(PT->getElementType());
     }

     // See if this pointer could alias it
     AliasAnalysis::AliasResult A = AA.alias(*I, pointerSize,
                                             killPointer, killPointerSize);

     // If it must-alias and a store, we can delete it
     if (isa<StoreInst>(BBI) && A == AliasAnalysis::MustAlias) {
       StoreInst* S = cast<StoreInst>(BBI);

       // Remove it!
       BBI++;
       DeleteDeadInstruction(S, &deadPointers);
       NumFastStores++;
       MadeChange = true;

       continue;

       // Otherwise, it is undead
     } else if (A != AliasAnalysis::NoAlias)
       undead.push_back(*I);
   }

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

   return MadeChange;
 }

 /// 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.
 ///
 void DSE::DeleteDeadInstruction(Instruction *I,
                                 SmallPtrSet<Value*, 64> *ValueSet) {
   SmallVector<Instruction*, 32> NowDeadInsts;

   NowDeadInsts.push_back(I);
   --NumFastOther;

   // Before we touch this instruction, remove it from memdep!
   MemoryDependenceAnalysis &MDA = getAnalysis<MemoryDependenceAnalysis>();
   while (!NowDeadInsts.empty()) {
     Instruction *DeadInst = NowDeadInsts.back();
     NowDeadInsts.pop_back();

     ++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.
     MDA.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);
   }
 }
	//===- 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/Instructions.h"
	#include "llvm/IntrinsicInst.h"
	#include "llvm/Pass.h"
	#include "llvm/ADT/SmallPtrSet.h"
	#include "llvm/ADT/Statistic.h"
	#include "llvm/Analysis/AliasAnalysis.h"
	#include "llvm/Analysis/Dominators.h"
	#include "llvm/Analysis/MemoryDependenceAnalysis.h"
	#include "llvm/Target/TargetData.h"
	#include "llvm/Transforms/Utils/Local.h"
	#include "llvm/Support/Compiler.h"
	using namespace llvm;

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

	namespace {
	struct VISIBILITY_HIDDEN DSE : public FunctionPass {
	static char ID; // Pass identification, replacement for typeid
	DSE() : FunctionPass(&ID) {}

	virtual bool runOnFunction(Function &F) {
	bool Changed = false;
	for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I)
	Changed \|= runOnBasicBlock(*I);
	return Changed;
	}

	bool runOnBasicBlock(BasicBlock &BB);
	bool handleFreeWithNonTrivialDependency(FreeInst *F, MemDepResult Dep);
	bool handleEndBlock(BasicBlock &BB);
	bool RemoveUndeadPointers(Value* Ptr, uint64_t killPointerSize,
	BasicBlock::iterator& BBI,
	SmallPtrSet<Value*, 64>& deadPointers);
	void DeleteDeadInstruction(Instruction *I,
	SmallPtrSet<Value, 64> deadPointers = 0);


	// getAnalysisUsage - We require post dominance frontiers (aka Control
	// Dependence Graph)
	virtual void getAnalysisUsage(AnalysisUsage &AU) const {
	AU.setPreservesCFG();
	AU.addRequired<DominatorTree>();
	AU.addRequired<TargetData>();
	AU.addRequired<AliasAnalysis>();
	AU.addRequired<MemoryDependenceAnalysis>();
	AU.addPreserved<DominatorTree>();
	AU.addPreserved<AliasAnalysis>();
	AU.addPreserved<MemoryDependenceAnalysis>();
	}
	};
	}

	char DSE::ID = 0;
	static RegisterPass<DSE> X("dse", "Dead Store Elimination");

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

	bool DSE::runOnBasicBlock(BasicBlock &BB) {
	MemoryDependenceAnalysis& MD = getAnalysis<MemoryDependenceAnalysis>();
	TargetData &TD = getAnalysis<TargetData>();

	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++;

	// If we find a store or a free, get it's memory dependence.
	if (!isa<StoreInst>(Inst) && !isa<FreeInst>(Inst))
	continue;

	// Don't molest volatile stores or do queries that will return "clobber".
	if (StoreInst *SI = dyn_cast<StoreInst>(Inst))
	if (SI->isVolatile())
	continue;

	MemDepResult InstDep = MD.getDependency(Inst);

	// Ignore non-local stores.
	// FIXME: cross-block DSE would be fun. :)
	if (InstDep.isNonLocal()) continue;

	// Handle frees whose dependencies are non-trivial.
	if (FreeInst *FI = dyn_cast<FreeInst>(Inst)) {
	MadeChange \|= handleFreeWithNonTrivialDependency(FI, InstDep);
	continue;
	}

	StoreInst *SI = cast<StoreInst>(Inst);

	// If not a definite must-alias dependency, ignore it.
	if (!InstDep.isDef())
	continue;

	// If this is a store-store dependence, then the previous store is dead so
	// long as this store is at least as big as it.
	if (StoreInst *DepStore = dyn_cast<StoreInst>(InstDep.getInst()))
	if (TD.getTypeStoreSize(DepStore->getOperand(0)->getType()) <=
	TD.getTypeStoreSize(SI->getOperand(0)->getType())) {
	// Delete the store and now-dead instructions that feed it.
	DeleteDeadInstruction(DepStore);
	NumFastStores++;
	MadeChange = true;

	if (BBI != BB.begin())
	--BBI;
	continue;
	}

	// 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>(InstDep.getInst())) {
	if (SI->getPointerOperand() == DepLoad->getPointerOperand() &&
	SI->getOperand(0) == DepLoad) {
	DeleteDeadInstruction(SI);
	if (BBI != BB.begin())
	--BBI;
	NumFastStores++;
	MadeChange = true;
	continue;
	}
	}
	}

	// 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;
	}

	/// handleFreeWithNonTrivialDependency - Handle frees of entire structures whose
	/// dependency is a store to a field of that structure.
	bool DSE::handleFreeWithNonTrivialDependency(FreeInst *F, MemDepResult Dep) {
	AliasAnalysis &AA = getAnalysis<AliasAnalysis>();

	StoreInst *Dependency = dyn_cast_or_null<StoreInst>(Dep.getInst());
	if (!Dependency \|\| Dependency->isVolatile())
	return false;

	Value *DepPointer = Dependency->getPointerOperand()->getUnderlyingObject();

	// Check for aliasing.
	if (AA.alias(F->getPointerOperand(), 1, DepPointer, 1) !=
	AliasAnalysis::MustAlias)
	return false;

	// DCE instructions only used to calculate that store
	DeleteDeadInstruction(Dependency);
	NumFastStores++;
	return true;
	}

	/// 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) {
	TargetData &TD = getAnalysis<TargetData>();
	AliasAnalysis &AA = getAnalysis<AliasAnalysis>();

	bool MadeChange = false;

	// Pointers alloca'd in this function are dead in the end block
	SmallPtrSet<Value*, 64> deadPointers;

	// 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))
	deadPointers.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())
	deadPointers.insert(AI);

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

	// If we find a store whose pointer is dead.
	if (StoreInst* S = dyn_cast<StoreInst>(BBI)) {
	if (!S->isVolatile()) {
	// See through pointer-to-pointer bitcasts
	Value* pointerOperand = S->getPointerOperand()->getUnderlyingObject();

	// Alloca'd pointers or byval arguments (which are functionally like
	// alloca's) are valid candidates for removal.
	if (deadPointers.count(pointerOperand)) {
	// DCE instructions only used to calculate that store.
	BBI++;
	DeleteDeadInstruction(S, &deadPointers);
	NumFastStores++;
	MadeChange = true;
	}
	}

	continue;
	}

	// We can also remove memcpy's to local variables at the end of a function.
	if (MemCpyInst *M = dyn_cast<MemCpyInst>(BBI)) {
	Value *dest = M->getDest()->getUnderlyingObject();

	if (deadPointers.count(dest)) {
	BBI++;
	DeleteDeadInstruction(M, &deadPointers);
	NumFastOther++;
	MadeChange = true;
	continue;
	}

	// Because a memcpy is also a load, we can't skip it if we didn't remove
	// it.
	}

	Value* killPointer = 0;
	uint64_t killPointerSize = ~0UL;

	// If we encounter a use of the pointer, it is no longer considered dead
	if (LoadInst *L = dyn_cast<LoadInst>(BBI)) {
	// However, if this load is unused and not volatile, we can go ahead and
	// remove it, and not have to worry about it making our pointer undead!
	if (L->use_empty() && !L->isVolatile()) {
	BBI++;
	DeleteDeadInstruction(L, &deadPointers);
	NumFastOther++;
	MadeChange = true;
	continue;
	}

	killPointer = L->getPointerOperand();
	} else if (VAArgInst* V = dyn_cast<VAArgInst>(BBI)) {
	killPointer = V->getOperand(0);
	} else if (isa<MemCpyInst>(BBI) &&
	isa<ConstantInt>(cast<MemCpyInst>(BBI)->getLength())) {
	killPointer = cast<MemCpyInst>(BBI)->getSource();
	killPointerSize = cast<ConstantInt>(
	cast<MemCpyInst>(BBI)->getLength())->getZExtValue();
	} else if (AllocaInst* A = dyn_cast<AllocaInst>(BBI)) {
	deadPointers.erase(A);

	// Dead alloca's can be DCE'd when we reach them
	if (A->use_empty()) {
	BBI++;
	DeleteDeadInstruction(A, &deadPointers);
	NumFastOther++;
	MadeChange = true;
	}

	continue;
	} else if (CallSite::get(BBI).getInstruction() != 0) {
	// If this call does not access memory, it can't
	// be undeadifying any of our pointers.
	CallSite CS = CallSite::get(BBI);
	if (AA.doesNotAccessMemory(CS))
	continue;

	unsigned modRef = 0;
	unsigned other = 0;

	// Remove any pointers made undead by the call from the dead set
	std::vector<Value*> dead;
	for (SmallPtrSet<Value*, 64>::iterator I = deadPointers.begin(),
	E = deadPointers.end(); I != E; ++I) {
	// HACK: if we detect that our AA is imprecise, it's not
	// worth it to scan the rest of the deadPointers set. Just
	// assume that the AA will return ModRef for everything, and
	// go ahead and bail.
	if (modRef >= 16 && other == 0) {
	deadPointers.clear();
	return MadeChange;
	}

	// Get size information for the alloca
	unsigned pointerSize = ~0U;
	if (AllocaInst* A = dyn_cast<AllocaInst>(*I)) {
	if (ConstantInt* C = dyn_cast<ConstantInt>(A->getArraySize()))
	pointerSize = C->getZExtValue() *
	TD.getTypePaddedSize(A->getAllocatedType());
	} else {
	const PointerType* PT = cast<PointerType>(
	cast<Argument>(*I)->getType());
	pointerSize = TD.getTypePaddedSize(PT->getElementType());
	}

	// See if the call site touches it
	AliasAnalysis::ModRefResult A = AA.getModRefInfo(CS, *I, pointerSize);

	if (A == AliasAnalysis::ModRef)
	modRef++;
	else
	other++;

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

	for (std::vector<Value*>::iterator I = dead.begin(), E = dead.end();
	I != E; ++I)
	deadPointers.erase(*I);

	continue;
	} else if (isInstructionTriviallyDead(BBI)) {
	// For any non-memory-affecting non-terminators, DCE them as we reach them
	Instruction *Inst = BBI;
	BBI++;
	DeleteDeadInstruction(Inst, &deadPointers);
	NumFastOther++;
	MadeChange = true;
	continue;
	}

	if (!killPointer)
	continue;

	killPointer = killPointer->getUnderlyingObject();

	// Deal with undead pointers
	MadeChange \|= RemoveUndeadPointers(killPointer, killPointerSize, BBI,
	deadPointers);
	}

	return MadeChange;
	}

	/// RemoveUndeadPointers - check for uses of a pointer that make it
	/// undead when scanning for dead stores to alloca's.
	bool DSE::RemoveUndeadPointers(Value* killPointer, uint64_t killPointerSize,
	BasicBlock::iterator &BBI,
	SmallPtrSet<Value*, 64>& deadPointers) {
	TargetData &TD = getAnalysis<TargetData>();
	AliasAnalysis &AA = getAnalysis<AliasAnalysis>();

	// If the kill pointer can be easily reduced to an alloca,
	// don't bother doing extraneous AA queries.
	if (deadPointers.count(killPointer)) {
	deadPointers.erase(killPointer);
	return false;
	}

	// A global can't be in the dead pointer set.
	if (isa<GlobalValue>(killPointer))
	return false;

	bool MadeChange = false;

	SmallVector<Value*, 16> undead;

	for (SmallPtrSet<Value*, 64>::iterator I = deadPointers.begin(),
	E = deadPointers.end(); I != E; ++I) {
	// Get size information for the alloca.
	unsigned pointerSize = ~0U;
	if (AllocaInst* A = dyn_cast<AllocaInst>(*I)) {
	if (ConstantInt* C = dyn_cast<ConstantInt>(A->getArraySize()))
	pointerSize = C->getZExtValue() *
	TD.getTypePaddedSize(A->getAllocatedType());
	} else {
	const PointerType* PT = cast<PointerType>(cast<Argument>(*I)->getType());
	pointerSize = TD.getTypePaddedSize(PT->getElementType());
	}

	// See if this pointer could alias it
	AliasAnalysis::AliasResult A = AA.alias(*I, pointerSize,
	killPointer, killPointerSize);

	// If it must-alias and a store, we can delete it
	if (isa<StoreInst>(BBI) && A == AliasAnalysis::MustAlias) {
	StoreInst* S = cast<StoreInst>(BBI);

	// Remove it!
	BBI++;
	DeleteDeadInstruction(S, &deadPointers);
	NumFastStores++;
	MadeChange = true;

	continue;

	// Otherwise, it is undead
	} else if (A != AliasAnalysis::NoAlias)
	undead.push_back(*I);
	}

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

	return MadeChange;
	}

	/// 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.
	///
	void DSE::DeleteDeadInstruction(Instruction *I,
	SmallPtrSet<Value, 64> ValueSet) {
	SmallVector<Instruction*, 32> NowDeadInsts;

	NowDeadInsts.push_back(I);
	--NumFastOther;

	// Before we touch this instruction, remove it from memdep!
	MemoryDependenceAnalysis &MDA = getAnalysis<MemoryDependenceAnalysis>();
	while (!NowDeadInsts.empty()) {
	Instruction *DeadInst = NowDeadInsts.back();
	NowDeadInsts.pop_back();

	++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.
	MDA.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);
	}
	}