llvm/lib/Transforms/Scalar/DeadStoreElimination.cpp - toolchain/llvm-project - Gitiles

 //===- DeadStoreElimination.cpp - Fast Dead Store Elimination -------------===//
 //
 //                     The LLVM Compiler Infrastructure
 //
 // This file was developed by Owen Anderson and 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/Pass.h"
 #include "llvm/ADT/SetVector.h"
 #include "llvm/ADT/SmallPtrSet.h"
 #include "llvm/ADT/Statistic.h"
 #include "llvm/Analysis/AliasAnalysis.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((intptr_t)&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,
                                             Instruction* dependency,
                                         SetVector<Instruction*>& possiblyDead);
     bool handleEndBlock(BasicBlock& BB, SetVector<Instruction*>& possiblyDead);
     bool RemoveUndeadPointers(Value* pointer,
                               BasicBlock::iterator& BBI,
                               SmallPtrSet<AllocaInst*, 64>& deadPointers,
                               SetVector<Instruction*>& possiblyDead);
     void DeleteDeadInstructionChains(Instruction *I,
                                      SetVector<Instruction*> &DeadInsts);

     /// Find the base pointer that a pointer came from
     /// Because this is used to find pointers that originate
     /// from allocas, it is safe to ignore GEP indices, since
     /// either the store will be in the alloca, and thus dead,
     /// or beyond the end of the alloca, and thus undefined.
     void TranslatePointerBitCasts(Value*& v) {
       assert(isa<PointerType>(v->getType()) &&
              "Translating a non-pointer type?");
       while (true) {
         if (BitCastInst* C = dyn_cast<BitCastInst>(v))
           v = C->getOperand(0);
         else if (GetElementPtrInst* G = dyn_cast<GetElementPtrInst>(v))
           v = G->getOperand(0);
         else
           break;
       }
     }

     // getAnalysisUsage - We require post dominance frontiers (aka Control
     // Dependence Graph)
     virtual void getAnalysisUsage(AnalysisUsage &AU) const {
       AU.setPreservesCFG();
       AU.addRequired<TargetData>();
       AU.addRequired<AliasAnalysis>();
       AU.addRequired<MemoryDependenceAnalysis>();
       AU.addPreserved<AliasAnalysis>();
       AU.addPreserved<MemoryDependenceAnalysis>();
     }
   };
   char DSE::ID = 0;
   RegisterPass<DSE> X("dse", "Dead Store Elimination");
 }

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

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

   // Record the last-seen store to this pointer
   DenseMap<Value*, StoreInst*> lastStore;
   // Record instructions possibly made dead by deleting a store
   SetVector<Instruction*> possiblyDead;

   bool MadeChange = false;

   // Do a top-down walk on the BB
   for (BasicBlock::iterator BBI = BB.begin(), BBE = BB.end();
        BBI != BBE; ++BBI) {
     // If we find a store or a free...
     if (!isa<StoreInst>(BBI) && !isa<FreeInst>(BBI))
       continue;

     Value* pointer = 0;
     if (StoreInst* S = dyn_cast<StoreInst>(BBI)) {
       if (!S->isVolatile())
         pointer = S->getPointerOperand();
       else
         continue;
     } else
       pointer = cast<FreeInst>(BBI)->getPointerOperand();

     StoreInst*& last = lastStore[pointer];
     bool deletedStore = false;

     // ... to a pointer that has been stored to before...
     if (last) {
       Instruction* dep = MD.getDependency(BBI);

       // ... and no other memory dependencies are between them....
       while (dep != MemoryDependenceAnalysis::None &&
              dep != MemoryDependenceAnalysis::NonLocal &&
              isa<StoreInst>(dep)) {
         if (dep != last) {
           dep = MD.getDependency(BBI, dep);
           continue;
         }

         // Remove it!
         MD.removeInstruction(last);

         // DCE instructions only used to calculate that store
         if (Instruction* D = dyn_cast<Instruction>(last->getOperand(0)))
           possiblyDead.insert(D);
         if (Instruction* D = dyn_cast<Instruction>(last->getOperand(1)))
           possiblyDead.insert(D);

         last->eraseFromParent();
         NumFastStores++;
         deletedStore = true;
         MadeChange = true;

         break;
       }
     }

     // Handle frees whose dependencies are non-trivial.
     if (FreeInst* F = dyn_cast<FreeInst>(BBI)) {
       if (!deletedStore)
         MadeChange |= handleFreeWithNonTrivialDependency(F,
                                                          MD.getDependency(F),
                                                          possiblyDead);
       // No known stores after the free
       last = 0;
     } else {
       // Update our most-recent-store map.
       last = cast<StoreInst>(BBI);
     }
   }

   // If this block ends in a return, unwind, unreachable, and eventually
   // tailcall, then all allocas are dead at its end.
   if (BB.getTerminator()->getNumSuccessors() == 0)
     MadeChange |= handleEndBlock(BB, possiblyDead);

   // Do a trivial DCE
   while (!possiblyDead.empty()) {
     Instruction *I = possiblyDead.back();
     possiblyDead.pop_back();
     DeleteDeadInstructionChains(I, possiblyDead);
   }

   return MadeChange;
 }

 /// handleFreeWithNonTrivialDependency - Handle frees of entire structures whose
 /// dependency is a store to a field of that structure
 bool DSE::handleFreeWithNonTrivialDependency(FreeInst* F, Instruction* dep,
                                        SetVector<Instruction*>& possiblyDead) {
   TargetData &TD = getAnalysis<TargetData>();
   AliasAnalysis &AA = getAnalysis<AliasAnalysis>();
   MemoryDependenceAnalysis& MD = getAnalysis<MemoryDependenceAnalysis>();

   if (dep == MemoryDependenceAnalysis::None ||
       dep == MemoryDependenceAnalysis::NonLocal)
     return false;

   StoreInst* dependency = dyn_cast<StoreInst>(dep);
   if (!dependency)
     return false;
   else if (dependency->isVolatile())
     return false;

   Value* depPointer = dependency->getPointerOperand();
   const Type* depType = dependency->getOperand(0)->getType();
   unsigned depPointerSize = TD.getTypeSize(depType);

   // Check for aliasing
   AliasAnalysis::AliasResult A = AA.alias(F->getPointerOperand(), ~0UL,
                                           depPointer, depPointerSize);

   if (A == AliasAnalysis::MustAlias) {
     // Remove it!
     MD.removeInstruction(dependency);

     // DCE instructions only used to calculate that store
     if (Instruction* D = dyn_cast<Instruction>(dependency->getOperand(0)))
       possiblyDead.insert(D);
     if (Instruction* D = dyn_cast<Instruction>(dependency->getOperand(1)))
       possiblyDead.insert(D);

     dependency->eraseFromParent();
     NumFastStores++;
     return true;
   }

   return false;
 }

 /// 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,
                          SetVector<Instruction*>& possiblyDead) {
   TargetData &TD = getAnalysis<TargetData>();
   AliasAnalysis &AA = getAnalysis<AliasAnalysis>();
   MemoryDependenceAnalysis& MD = getAnalysis<MemoryDependenceAnalysis>();

   bool MadeChange = false;

   // Pointers alloca'd in this function are dead in the end block
   SmallPtrSet<AllocaInst*, 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);

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

     if (deadPointers.empty())
       break;

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

         if (deadPointers.count(pointerOperand)){
           // Remove it!
           MD.removeInstruction(S);

           // DCE instructions only used to calculate that store
           if (Instruction* D = dyn_cast<Instruction>(S->getOperand(0)))
             possiblyDead.insert(D);
           if (Instruction* D = dyn_cast<Instruction>(S->getOperand(1)))
             possiblyDead.insert(D);

           BBI++;
           S->eraseFromParent();
           NumFastStores++;
           MadeChange = true;
         }
       }

       continue;
     }

     Value* killPointer = 0;

     // If we encounter a use of the pointer, it is no longer considered dead
     if (LoadInst* L = dyn_cast<LoadInst>(BBI)) {
       killPointer = L->getPointerOperand();
     } else if (VAArgInst* V = dyn_cast<VAArgInst>(BBI)) {
       killPointer = V->getOperand(0);
     } else if (AllocaInst* A = dyn_cast<AllocaInst>(BBI)) {
       deadPointers.erase(A);
       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 (CS.getCalledFunction() &&
           AA.doesNotAccessMemory(CS.getCalledFunction()))
         continue;

       unsigned modRef = 0;
       unsigned other = 0;

       // Remove any pointers made undead by the call from the dead set
       std::vector<Instruction*> dead;
       for (SmallPtrSet<AllocaInst*, 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 = ~0UL;
         if (ConstantInt* C = dyn_cast<ConstantInt>((*I)->getArraySize()))
           pointerSize = C->getZExtValue() * \
                         TD.getTypeSize((*I)->getAllocatedType());

         // 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<Instruction*>::iterator I = dead.begin(), E = dead.end();
            I != E; ++I)
         deadPointers.erase(*I);

       continue;
     }

     if (!killPointer)
       continue;

     TranslatePointerBitCasts(killPointer);

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

   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,
                                 BasicBlock::iterator& BBI,
                                 SmallPtrSet<AllocaInst*, 64>& deadPointers,
                                 SetVector<Instruction*>& possiblyDead) {
   TargetData &TD = getAnalysis<TargetData>();
   AliasAnalysis &AA = getAnalysis<AliasAnalysis>();
   MemoryDependenceAnalysis& MD = getAnalysis<MemoryDependenceAnalysis>();

   // If the kill pointer can be easily reduced to an alloca,
   // don't bother doing extraneous AA queries
   if (AllocaInst* A = dyn_cast<AllocaInst>(killPointer)) {
     if (deadPointers.count(A))
       deadPointers.erase(A);
     return false;
   } else if (isa<GlobalValue>(killPointer)) {
     // A global can't be in the dead pointer set
     return false;
   }

   bool MadeChange = false;

   std::vector<Instruction*> undead;

   for (SmallPtrSet<AllocaInst*, 64>::iterator I = deadPointers.begin(),
       E = deadPointers.end(); I != E; ++I) {
     // Get size information for the alloca
     unsigned pointerSize = ~0UL;
     if (ConstantInt* C = dyn_cast<ConstantInt>((*I)->getArraySize()))
       pointerSize = C->getZExtValue() * \
                     TD.getTypeSize((*I)->getAllocatedType());

     // See if this pointer could alias it
     AliasAnalysis::AliasResult A = AA.alias(*I, pointerSize,
                                             killPointer, ~0UL);

     // 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!
       MD.removeInstruction(S);

       // DCE instructions only used to calculate that store
       if (Instruction* D = dyn_cast<Instruction>(S->getOperand(0)))
         possiblyDead.insert(D);
       if (Instruction* D = dyn_cast<Instruction>(S->getOperand(1)))
         possiblyDead.insert(D);

       BBI++;
       S->eraseFromParent();
       NumFastStores++;
       MadeChange = true;

       continue;

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

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

   return MadeChange;
 }

 /// DeleteDeadInstructionChains - takes an instruction and a setvector of
 /// dead instructions.  If I is dead, it is erased, and its operands are
 /// checked for deadness.  If they are dead, they are added to the dead
 /// setvector.
 void DSE::DeleteDeadInstructionChains(Instruction *I,
                                       SetVector<Instruction*> &DeadInsts) {
   // Instruction must be dead.
   if (!I->use_empty() || !isInstructionTriviallyDead(I)) return;

   // Let the memory dependence know
   getAnalysis<MemoryDependenceAnalysis>().removeInstruction(I);

   // See if this made any operands dead.  We do it this way in case the
   // instruction uses the same operand twice.  We don't want to delete a
   // value then reference it.
   for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i) {
     if (I->getOperand(i)->hasOneUse())
       if (Instruction* Op = dyn_cast<Instruction>(I->getOperand(i)))
         DeadInsts.insert(Op);      // Attempt to nuke it later.

     I->setOperand(i, 0);         // Drop from the operand list.
   }

   I->eraseFromParent();
   ++NumFastOther;
 }
	//===- DeadStoreElimination.cpp - Fast Dead Store Elimination -------------===//
	//
	// The LLVM Compiler Infrastructure
	//
	// This file was developed by Owen Anderson and 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/Pass.h"
	#include "llvm/ADT/SetVector.h"
	#include "llvm/ADT/SmallPtrSet.h"
	#include "llvm/ADT/Statistic.h"
	#include "llvm/Analysis/AliasAnalysis.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((intptr_t)&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,
	Instruction* dependency,
	SetVector<Instruction*>& possiblyDead);
	bool handleEndBlock(BasicBlock& BB, SetVector<Instruction*>& possiblyDead);
	bool RemoveUndeadPointers(Value* pointer,
	BasicBlock::iterator& BBI,
	SmallPtrSet<AllocaInst*, 64>& deadPointers,
	SetVector<Instruction*>& possiblyDead);
	void DeleteDeadInstructionChains(Instruction *I,
	SetVector<Instruction*> &DeadInsts);

	/// Find the base pointer that a pointer came from
	/// Because this is used to find pointers that originate
	/// from allocas, it is safe to ignore GEP indices, since
	/// either the store will be in the alloca, and thus dead,
	/// or beyond the end of the alloca, and thus undefined.
	void TranslatePointerBitCasts(Value*& v) {
	assert(isa<PointerType>(v->getType()) &&
	"Translating a non-pointer type?");
	while (true) {
	if (BitCastInst* C = dyn_cast<BitCastInst>(v))
	v = C->getOperand(0);
	else if (GetElementPtrInst* G = dyn_cast<GetElementPtrInst>(v))
	v = G->getOperand(0);
	else
	break;
	}
	}

	// getAnalysisUsage - We require post dominance frontiers (aka Control
	// Dependence Graph)
	virtual void getAnalysisUsage(AnalysisUsage &AU) const {
	AU.setPreservesCFG();
	AU.addRequired<TargetData>();
	AU.addRequired<AliasAnalysis>();
	AU.addRequired<MemoryDependenceAnalysis>();
	AU.addPreserved<AliasAnalysis>();
	AU.addPreserved<MemoryDependenceAnalysis>();
	}
	};
	char DSE::ID = 0;
	RegisterPass<DSE> X("dse", "Dead Store Elimination");
	}

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

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

	// Record the last-seen store to this pointer
	DenseMap<Value, StoreInst> lastStore;
	// Record instructions possibly made dead by deleting a store
	SetVector<Instruction*> possiblyDead;

	bool MadeChange = false;

	// Do a top-down walk on the BB
	for (BasicBlock::iterator BBI = BB.begin(), BBE = BB.end();
	BBI != BBE; ++BBI) {
	// If we find a store or a free...
	if (!isa<StoreInst>(BBI) && !isa<FreeInst>(BBI))
	continue;

	Value* pointer = 0;
	if (StoreInst* S = dyn_cast<StoreInst>(BBI)) {
	if (!S->isVolatile())
	pointer = S->getPointerOperand();
	else
	continue;
	} else
	pointer = cast<FreeInst>(BBI)->getPointerOperand();

	StoreInst*& last = lastStore[pointer];
	bool deletedStore = false;

	// ... to a pointer that has been stored to before...
	if (last) {
	Instruction* dep = MD.getDependency(BBI);

	// ... and no other memory dependencies are between them....
	while (dep != MemoryDependenceAnalysis::None &&
	dep != MemoryDependenceAnalysis::NonLocal &&
	isa<StoreInst>(dep)) {
	if (dep != last) {
	dep = MD.getDependency(BBI, dep);
	continue;
	}

	// Remove it!
	MD.removeInstruction(last);

	// DCE instructions only used to calculate that store
	if (Instruction* D = dyn_cast<Instruction>(last->getOperand(0)))
	possiblyDead.insert(D);
	if (Instruction* D = dyn_cast<Instruction>(last->getOperand(1)))
	possiblyDead.insert(D);

	last->eraseFromParent();
	NumFastStores++;
	deletedStore = true;
	MadeChange = true;

	break;
	}
	}

	// Handle frees whose dependencies are non-trivial.
	if (FreeInst* F = dyn_cast<FreeInst>(BBI)) {
	if (!deletedStore)
	MadeChange \|= handleFreeWithNonTrivialDependency(F,
	MD.getDependency(F),
	possiblyDead);
	// No known stores after the free
	last = 0;
	} else {
	// Update our most-recent-store map.
	last = cast<StoreInst>(BBI);
	}
	}

	// If this block ends in a return, unwind, unreachable, and eventually
	// tailcall, then all allocas are dead at its end.
	if (BB.getTerminator()->getNumSuccessors() == 0)
	MadeChange \|= handleEndBlock(BB, possiblyDead);

	// Do a trivial DCE
	while (!possiblyDead.empty()) {
	Instruction *I = possiblyDead.back();
	possiblyDead.pop_back();
	DeleteDeadInstructionChains(I, possiblyDead);
	}

	return MadeChange;
	}

	/// handleFreeWithNonTrivialDependency - Handle frees of entire structures whose
	/// dependency is a store to a field of that structure
	bool DSE::handleFreeWithNonTrivialDependency(FreeInst* F, Instruction* dep,
	SetVector<Instruction*>& possiblyDead) {
	TargetData &TD = getAnalysis<TargetData>();
	AliasAnalysis &AA = getAnalysis<AliasAnalysis>();
	MemoryDependenceAnalysis& MD = getAnalysis<MemoryDependenceAnalysis>();

	if (dep == MemoryDependenceAnalysis::None \|\|
	dep == MemoryDependenceAnalysis::NonLocal)
	return false;

	StoreInst* dependency = dyn_cast<StoreInst>(dep);
	if (!dependency)
	return false;
	else if (dependency->isVolatile())
	return false;

	Value* depPointer = dependency->getPointerOperand();
	const Type* depType = dependency->getOperand(0)->getType();
	unsigned depPointerSize = TD.getTypeSize(depType);

	// Check for aliasing
	AliasAnalysis::AliasResult A = AA.alias(F->getPointerOperand(), ~0UL,
	depPointer, depPointerSize);

	if (A == AliasAnalysis::MustAlias) {
	// Remove it!
	MD.removeInstruction(dependency);

	// DCE instructions only used to calculate that store
	if (Instruction* D = dyn_cast<Instruction>(dependency->getOperand(0)))
	possiblyDead.insert(D);
	if (Instruction* D = dyn_cast<Instruction>(dependency->getOperand(1)))
	possiblyDead.insert(D);

	dependency->eraseFromParent();
	NumFastStores++;
	return true;
	}

	return false;
	}

	/// 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,
	SetVector<Instruction*>& possiblyDead) {
	TargetData &TD = getAnalysis<TargetData>();
	AliasAnalysis &AA = getAnalysis<AliasAnalysis>();
	MemoryDependenceAnalysis& MD = getAnalysis<MemoryDependenceAnalysis>();

	bool MadeChange = false;

	// Pointers alloca'd in this function are dead in the end block
	SmallPtrSet<AllocaInst*, 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);

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

	if (deadPointers.empty())
	break;

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

	if (deadPointers.count(pointerOperand)){
	// Remove it!
	MD.removeInstruction(S);

	// DCE instructions only used to calculate that store
	if (Instruction* D = dyn_cast<Instruction>(S->getOperand(0)))
	possiblyDead.insert(D);
	if (Instruction* D = dyn_cast<Instruction>(S->getOperand(1)))
	possiblyDead.insert(D);

	BBI++;
	S->eraseFromParent();
	NumFastStores++;
	MadeChange = true;
	}
	}

	continue;
	}

	Value* killPointer = 0;

	// If we encounter a use of the pointer, it is no longer considered dead
	if (LoadInst* L = dyn_cast<LoadInst>(BBI)) {
	killPointer = L->getPointerOperand();
	} else if (VAArgInst* V = dyn_cast<VAArgInst>(BBI)) {
	killPointer = V->getOperand(0);
	} else if (AllocaInst* A = dyn_cast<AllocaInst>(BBI)) {
	deadPointers.erase(A);
	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 (CS.getCalledFunction() &&
	AA.doesNotAccessMemory(CS.getCalledFunction()))
	continue;

	unsigned modRef = 0;
	unsigned other = 0;

	// Remove any pointers made undead by the call from the dead set
	std::vector<Instruction*> dead;
	for (SmallPtrSet<AllocaInst*, 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 = ~0UL;
	if (ConstantInt* C = dyn_cast<ConstantInt>((*I)->getArraySize()))
	pointerSize = C->getZExtValue() * \
	TD.getTypeSize((*I)->getAllocatedType());

	// 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<Instruction*>::iterator I = dead.begin(), E = dead.end();
	I != E; ++I)
	deadPointers.erase(*I);

	continue;
	}

	if (!killPointer)
	continue;

	TranslatePointerBitCasts(killPointer);

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

	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,
	BasicBlock::iterator& BBI,
	SmallPtrSet<AllocaInst*, 64>& deadPointers,
	SetVector<Instruction*>& possiblyDead) {
	TargetData &TD = getAnalysis<TargetData>();
	AliasAnalysis &AA = getAnalysis<AliasAnalysis>();
	MemoryDependenceAnalysis& MD = getAnalysis<MemoryDependenceAnalysis>();

	// If the kill pointer can be easily reduced to an alloca,
	// don't bother doing extraneous AA queries
	if (AllocaInst* A = dyn_cast<AllocaInst>(killPointer)) {
	if (deadPointers.count(A))
	deadPointers.erase(A);
	return false;
	} else if (isa<GlobalValue>(killPointer)) {
	// A global can't be in the dead pointer set
	return false;
	}

	bool MadeChange = false;

	std::vector<Instruction*> undead;

	for (SmallPtrSet<AllocaInst*, 64>::iterator I = deadPointers.begin(),
	E = deadPointers.end(); I != E; ++I) {
	// Get size information for the alloca
	unsigned pointerSize = ~0UL;
	if (ConstantInt* C = dyn_cast<ConstantInt>((*I)->getArraySize()))
	pointerSize = C->getZExtValue() * \
	TD.getTypeSize((*I)->getAllocatedType());

	// See if this pointer could alias it
	AliasAnalysis::AliasResult A = AA.alias(*I, pointerSize,
	killPointer, ~0UL);

	// 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!
	MD.removeInstruction(S);

	// DCE instructions only used to calculate that store
	if (Instruction* D = dyn_cast<Instruction>(S->getOperand(0)))
	possiblyDead.insert(D);
	if (Instruction* D = dyn_cast<Instruction>(S->getOperand(1)))
	possiblyDead.insert(D);

	BBI++;
	S->eraseFromParent();
	NumFastStores++;
	MadeChange = true;

	continue;

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

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

	return MadeChange;
	}

	/// DeleteDeadInstructionChains - takes an instruction and a setvector of
	/// dead instructions. If I is dead, it is erased, and its operands are
	/// checked for deadness. If they are dead, they are added to the dead
	/// setvector.
	void DSE::DeleteDeadInstructionChains(Instruction *I,
	SetVector<Instruction*> &DeadInsts) {
	// Instruction must be dead.
	if (!I->use_empty() \|\| !isInstructionTriviallyDead(I)) return;

	// Let the memory dependence know
	getAnalysis<MemoryDependenceAnalysis>().removeInstruction(I);

	// See if this made any operands dead. We do it this way in case the
	// instruction uses the same operand twice. We don't want to delete a
	// value then reference it.
	for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i) {
	if (I->getOperand(i)->hasOneUse())
	if (Instruction* Op = dyn_cast<Instruction>(I->getOperand(i)))
	DeadInsts.insert(Op); // Attempt to nuke it later.

	I->setOperand(i, 0); // Drop from the operand list.
	}

	I->eraseFromParent();
	++NumFastOther;
	}