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

 //===- 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/MemoryBuiltins.h"
 #include "llvm/Analysis/MemoryDependenceAnalysis.h"
 #include "llvm/Target/TargetData.h"
 #include "llvm/Transforms/Utils/Local.h"
 using namespace llvm;

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

 namespace {
   struct DSE : public FunctionPass {
     TargetData *TD;

     static char ID; // Pass identification, replacement for typeid
     DSE() : FunctionPass(ID) {}

     virtual bool runOnFunction(Function &F) {
       bool Changed = false;

       DominatorTree &DT = getAnalysis<DominatorTree>();

       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);
       return Changed;
     }

     bool runOnBasicBlock(BasicBlock &BB);
     bool handleFreeWithNonTrivialDependency(const CallInst *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<AliasAnalysis>();
       AU.addRequired<MemoryDependenceAnalysis>();
       AU.addPreserved<DominatorTree>();
       AU.addPreserved<MemoryDependenceAnalysis>();
     }

     unsigned getPointerSize(Value *V) const;
   };
 }

 char DSE::ID = 0;
 INITIALIZE_PASS(DSE, "dse", "Dead Store Elimination", false, false);

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

 /// doesClobberMemory - Does this instruction clobber (write without reading)
 /// some memory?
 static bool doesClobberMemory(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;
 }

 /// isElidable - If the value of this instruction and the memory it writes to is
 /// unused, may we delete this instrtction?
 static bool isElidable(Instruction *I) {
   assert(doesClobberMemory(I));
   if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(I))
     return II->getIntrinsicID() != Intrinsic::lifetime_end;
   if (StoreInst *SI = dyn_cast<StoreInst>(I))
     return !SI->isVolatile();
   return true;
 }

 /// getPointerOperand - Return the pointer that is being clobbered.
 static Value *getPointerOperand(Instruction *I) {
   assert(doesClobberMemory(I));
   if (StoreInst *SI = dyn_cast<StoreInst>(I))
     return SI->getPointerOperand();
   if (MemIntrinsic *MI = dyn_cast<MemIntrinsic>(I))
     return MI->getArgOperand(0);

   IntrinsicInst *II = cast<IntrinsicInst>(I);
   switch (II->getIntrinsicID()) {
   default: assert(false && "Unexpected intrinsic!");
   case Intrinsic::init_trampoline:
     return II->getArgOperand(0);
   case Intrinsic::lifetime_end:
     return II->getArgOperand(1);
   }
 }

 /// getStoreSize - Return the length in bytes of the write by the clobbering
 /// instruction. If variable or unknown, returns -1.
 static unsigned getStoreSize(Instruction *I, const TargetData *TD) {
   assert(doesClobberMemory(I));
   if (StoreInst *SI = dyn_cast<StoreInst>(I)) {
     if (!TD) return -1u;
     return TD->getTypeStoreSize(SI->getOperand(0)->getType());
   }

   Value *Len;
   if (MemIntrinsic *MI = dyn_cast<MemIntrinsic>(I)) {
     Len = MI->getLength();
   } else {
     IntrinsicInst *II = cast<IntrinsicInst>(I);
     switch (II->getIntrinsicID()) {
     default: assert(false && "Unexpected intrinsic!");
     case Intrinsic::init_trampoline:
       return -1u;
     case Intrinsic::lifetime_end:
       Len = II->getArgOperand(0);
       break;
     }
   }
   if (ConstantInt *LenCI = dyn_cast<ConstantInt>(Len))
     if (!LenCI->isAllOnesValue())
       return LenCI->getZExtValue();
   return -1u;
 }

 /// isStoreAtLeastAsWideAs - Return true if the size of the store in I1 is
 /// greater than or equal to the store in I2.  This returns false if we don't
 /// know.
 ///
 static bool isStoreAtLeastAsWideAs(Instruction *I1, Instruction *I2,
                                    const TargetData *TD) {
   const Type *I1Ty = getPointerOperand(I1)->getType();
   const Type *I2Ty = getPointerOperand(I2)->getType();

   // Exactly the same type, must have exactly the same size.
   if (I1Ty == I2Ty) return true;

   int I1Size = getStoreSize(I1, TD);
   int I2Size = getStoreSize(I2, TD);

   return I1Size != -1 && I2Size != -1 && I1Size >= I2Size;
 }

 bool DSE::runOnBasicBlock(BasicBlock &BB) {
   MemoryDependenceAnalysis &MD = getAnalysis<MemoryDependenceAnalysis>();
   TD = getAnalysisIfAvailable<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 its memory dependence.
     if (!doesClobberMemory(Inst) && !isFreeCall(Inst))
       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 (const CallInst *F = isFreeCall(Inst)) {
       MadeChange |= handleFreeWithNonTrivialDependency(F, InstDep);
       continue;
     }

     // 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 (doesClobberMemory(InstDep.getInst())) {
       Instruction *DepStore = InstDep.getInst();
       if (isStoreAtLeastAsWideAs(Inst, DepStore, TD) &&
           isElidable(DepStore)) {
         // Delete the store and now-dead instructions that feed it.
         DeleteDeadInstruction(DepStore);
         ++NumFastStores;
         MadeChange = true;

         // DeleteDeadInstruction can delete the current instruction in loop
         // cases, reset BBI.
         BBI = Inst;
         if (BBI != BB.begin())
           --BBI;
         continue;
       }
     }

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

           DeleteDeadInstruction(SI);

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

     // If this is a lifetime end marker, we can throw away the store.
     if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(InstDep.getInst())) {
       if (II->getIntrinsicID() == Intrinsic::lifetime_end) {
         // Delete the store and now-dead instructions that feed it.
         // DeleteDeadInstruction can delete the current instruction.  Save BBI
         // in case we need it.
         WeakVH NextInst(BBI);

         DeleteDeadInstruction(Inst);

         if (NextInst == 0)  // Next instruction deleted.
           BBI = BB.begin();
         else if (BBI != BB.begin())  // Revisit this instruction if possible.
           --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(const CallInst *F,
                                              MemDepResult Dep) {
   AliasAnalysis &AA = getAnalysis<AliasAnalysis>();

   Instruction *Dependency = Dep.getInst();
   if (!Dependency || !doesClobberMemory(Dependency) || !isElidable(Dependency))
     return false;

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

   // Check for aliasing.
   if (AA.alias(F->getArgOperand(0), 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) {
   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 (doesClobberMemory(BBI)) {
       if (isElidable(BBI)) {
         // See through pointer-to-pointer bitcasts
         Value *pointerOperand = getPointerOperand(BBI)->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.
           Instruction *Dead = BBI;
           ++BBI;
           DeleteDeadInstruction(Dead, &deadPointers);
           ++NumFastStores;
           MadeChange = true;
           continue;
         }
       }

       // Because a memcpy or memmove is also a load, we can't skip it if we
       // didn't remove it.
       if (!isa<MemTransferInst>(BBI))
         continue;
     }

     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<MemTransferInst>(BBI) &&
                isa<ConstantInt>(cast<MemTransferInst>(BBI)->getLength())) {
       killPointer = cast<MemTransferInst>(BBI)->getSource();
       killPointerSize = cast<ConstantInt>(
                        cast<MemTransferInst>(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 CS = cast<Value>(BBI)) {
       // If this call does not access memory, it can't
       // be undeadifying any of our pointers.
       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;
         }

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

         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) {
   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) {
     // See if this pointer could alias it
     AliasAnalysis::AliasResult A = AA.alias(*I, getPointerSize(*I),
                                             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>();
   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.
     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);
   } while (!NowDeadInsts.empty());
 }

 unsigned DSE::getPointerSize(Value *V) const {
   if (TD) {
     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());
     } else {
       assert(isa<Argument>(V) && "Expected AllocaInst or Argument!");
       const PointerType *PT = cast<PointerType>(V->getType());
       return TD->getTypeAllocSize(PT->getElementType());
     }
   }
   return ~0U;
 }
	//===- 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/MemoryBuiltins.h"
	#include "llvm/Analysis/MemoryDependenceAnalysis.h"
	#include "llvm/Target/TargetData.h"
	#include "llvm/Transforms/Utils/Local.h"
	using namespace llvm;

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

	namespace {
	struct DSE : public FunctionPass {
	TargetData *TD;

	static char ID; // Pass identification, replacement for typeid
	DSE() : FunctionPass(ID) {}

	virtual bool runOnFunction(Function &F) {
	bool Changed = false;

	DominatorTree &DT = getAnalysis<DominatorTree>();

	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);
	return Changed;
	}

	bool runOnBasicBlock(BasicBlock &BB);
	bool handleFreeWithNonTrivialDependency(const CallInst *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<AliasAnalysis>();
	AU.addRequired<MemoryDependenceAnalysis>();
	AU.addPreserved<DominatorTree>();
	AU.addPreserved<MemoryDependenceAnalysis>();
	}

	unsigned getPointerSize(Value *V) const;
	};
	}

	char DSE::ID = 0;
	INITIALIZE_PASS(DSE, "dse", "Dead Store Elimination", false, false);

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

	/// doesClobberMemory - Does this instruction clobber (write without reading)
	/// some memory?
	static bool doesClobberMemory(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;
	}

	/// isElidable - If the value of this instruction and the memory it writes to is
	/// unused, may we delete this instrtction?
	static bool isElidable(Instruction *I) {
	assert(doesClobberMemory(I));
	if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(I))
	return II->getIntrinsicID() != Intrinsic::lifetime_end;
	if (StoreInst *SI = dyn_cast<StoreInst>(I))
	return !SI->isVolatile();
	return true;
	}

	/// getPointerOperand - Return the pointer that is being clobbered.
	static Value getPointerOperand(Instruction I) {
	assert(doesClobberMemory(I));
	if (StoreInst *SI = dyn_cast<StoreInst>(I))
	return SI->getPointerOperand();
	if (MemIntrinsic *MI = dyn_cast<MemIntrinsic>(I))
	return MI->getArgOperand(0);

	IntrinsicInst *II = cast<IntrinsicInst>(I);
	switch (II->getIntrinsicID()) {
	default: assert(false && "Unexpected intrinsic!");
	case Intrinsic::init_trampoline:
	return II->getArgOperand(0);
	case Intrinsic::lifetime_end:
	return II->getArgOperand(1);
	}
	}

	/// getStoreSize - Return the length in bytes of the write by the clobbering
	/// instruction. If variable or unknown, returns -1.
	static unsigned getStoreSize(Instruction I, const TargetData TD) {
	assert(doesClobberMemory(I));
	if (StoreInst *SI = dyn_cast<StoreInst>(I)) {
	if (!TD) return -1u;
	return TD->getTypeStoreSize(SI->getOperand(0)->getType());
	}

	Value *Len;
	if (MemIntrinsic *MI = dyn_cast<MemIntrinsic>(I)) {
	Len = MI->getLength();
	} else {
	IntrinsicInst *II = cast<IntrinsicInst>(I);
	switch (II->getIntrinsicID()) {
	default: assert(false && "Unexpected intrinsic!");
	case Intrinsic::init_trampoline:
	return -1u;
	case Intrinsic::lifetime_end:
	Len = II->getArgOperand(0);
	break;
	}
	}
	if (ConstantInt *LenCI = dyn_cast<ConstantInt>(Len))
	if (!LenCI->isAllOnesValue())
	return LenCI->getZExtValue();
	return -1u;
	}

	/// isStoreAtLeastAsWideAs - Return true if the size of the store in I1 is
	/// greater than or equal to the store in I2. This returns false if we don't
	/// know.
	///
	static bool isStoreAtLeastAsWideAs(Instruction I1, Instruction I2,
	const TargetData *TD) {
	const Type *I1Ty = getPointerOperand(I1)->getType();
	const Type *I2Ty = getPointerOperand(I2)->getType();

	// Exactly the same type, must have exactly the same size.
	if (I1Ty == I2Ty) return true;

	int I1Size = getStoreSize(I1, TD);
	int I2Size = getStoreSize(I2, TD);

	return I1Size != -1 && I2Size != -1 && I1Size >= I2Size;
	}

	bool DSE::runOnBasicBlock(BasicBlock &BB) {
	MemoryDependenceAnalysis &MD = getAnalysis<MemoryDependenceAnalysis>();
	TD = getAnalysisIfAvailable<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 its memory dependence.
	if (!doesClobberMemory(Inst) && !isFreeCall(Inst))
	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 (const CallInst *F = isFreeCall(Inst)) {
	MadeChange \|= handleFreeWithNonTrivialDependency(F, InstDep);
	continue;
	}

	// 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 (doesClobberMemory(InstDep.getInst())) {
	Instruction *DepStore = InstDep.getInst();
	if (isStoreAtLeastAsWideAs(Inst, DepStore, TD) &&
	isElidable(DepStore)) {
	// Delete the store and now-dead instructions that feed it.
	DeleteDeadInstruction(DepStore);
	++NumFastStores;
	MadeChange = true;

	// DeleteDeadInstruction can delete the current instruction in loop
	// cases, reset BBI.
	BBI = Inst;
	if (BBI != BB.begin())
	--BBI;
	continue;
	}
	}

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

	DeleteDeadInstruction(SI);

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

	// If this is a lifetime end marker, we can throw away the store.
	if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(InstDep.getInst())) {
	if (II->getIntrinsicID() == Intrinsic::lifetime_end) {
	// Delete the store and now-dead instructions that feed it.
	// DeleteDeadInstruction can delete the current instruction. Save BBI
	// in case we need it.
	WeakVH NextInst(BBI);

	DeleteDeadInstruction(Inst);

	if (NextInst == 0) // Next instruction deleted.
	BBI = BB.begin();
	else if (BBI != BB.begin()) // Revisit this instruction if possible.
	--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(const CallInst *F,
	MemDepResult Dep) {
	AliasAnalysis &AA = getAnalysis<AliasAnalysis>();

	Instruction *Dependency = Dep.getInst();
	if (!Dependency \|\| !doesClobberMemory(Dependency) \|\| !isElidable(Dependency))
	return false;

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

	// Check for aliasing.
	if (AA.alias(F->getArgOperand(0), 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) {
	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 (doesClobberMemory(BBI)) {
	if (isElidable(BBI)) {
	// See through pointer-to-pointer bitcasts
	Value *pointerOperand = getPointerOperand(BBI)->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.
	Instruction *Dead = BBI;
	++BBI;
	DeleteDeadInstruction(Dead, &deadPointers);
	++NumFastStores;
	MadeChange = true;
	continue;
	}
	}

	// Because a memcpy or memmove is also a load, we can't skip it if we
	// didn't remove it.
	if (!isa<MemTransferInst>(BBI))
	continue;
	}

	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<MemTransferInst>(BBI) &&
	isa<ConstantInt>(cast<MemTransferInst>(BBI)->getLength())) {
	killPointer = cast<MemTransferInst>(BBI)->getSource();
	killPointerSize = cast<ConstantInt>(
	cast<MemTransferInst>(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 CS = cast<Value>(BBI)) {
	// If this call does not access memory, it can't
	// be undeadifying any of our pointers.
	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;
	}

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

	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) {
	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) {
	// See if this pointer could alias it
	AliasAnalysis::AliasResult A = AA.alias(I, getPointerSize(I),
	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>();
	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.
	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);
	} while (!NowDeadInsts.empty());
	}

	unsigned DSE::getPointerSize(Value *V) const {
	if (TD) {
	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());
	} else {
	assert(isa<Argument>(V) && "Expected AllocaInst or Argument!");
	const PointerType *PT = cast<PointerType>(V->getType());
	return TD->getTypeAllocSize(PT->getElementType());
	}
	}
	return ~0U;
	}