X-Git-Url: http://plrg.eecs.uci.edu/git/?a=blobdiff_plain;f=lib%2FTransforms%2FScalar%2FDeadStoreElimination.cpp;h=03a557e0600a184ed9bcf3bd02501ea2fa62ba77;hb=5ccb0825ed1bdf6271ef451b8239e86d4ff635b1;hp=2d38e76dbe84342b8fdec30a7539f5a7193f3bd7;hpb=ceb4d1aecb9deffe59b3dcdc9a783ffde8477be9;p=oota-llvm.git diff --git a/lib/Transforms/Scalar/DeadStoreElimination.cpp b/lib/Transforms/Scalar/DeadStoreElimination.cpp index 2d38e76dbe8..03a557e0600 100644 --- a/lib/Transforms/Scalar/DeadStoreElimination.cpp +++ b/lib/Transforms/Scalar/DeadStoreElimination.cpp @@ -19,159 +19,677 @@ #include "llvm/Transforms/Scalar.h" #include "llvm/Constants.h" #include "llvm/Function.h" +#include "llvm/GlobalVariable.h" #include "llvm/Instructions.h" #include "llvm/IntrinsicInst.h" #include "llvm/Pass.h" -#include "llvm/ADT/SmallPtrSet.h" -#include "llvm/ADT/Statistic.h" #include "llvm/Analysis/AliasAnalysis.h" +#include "llvm/Analysis/CaptureTracking.h" #include "llvm/Analysis/Dominators.h" +#include "llvm/Analysis/MemoryBuiltins.h" #include "llvm/Analysis/MemoryDependenceAnalysis.h" +#include "llvm/Analysis/ValueTracking.h" #include "llvm/Target/TargetData.h" #include "llvm/Transforms/Utils/Local.h" -#include "llvm/Support/Compiler.h" +#include "llvm/Support/Debug.h" +#include "llvm/ADT/SmallPtrSet.h" +#include "llvm/ADT/Statistic.h" +#include "llvm/ADT/STLExtras.h" using namespace llvm; STATISTIC(NumFastStores, "Number of stores deleted"); STATISTIC(NumFastOther , "Number of other instrs removed"); namespace { - struct VISIBILITY_HIDDEN DSE : public FunctionPass { + struct DSE : public FunctionPass { + AliasAnalysis *AA; + MemoryDependenceAnalysis *MD; + DominatorTree *DT; + static char ID; // Pass identification, replacement for typeid - DSE() : FunctionPass(&ID) {} + DSE() : FunctionPass(ID), AA(0), MD(0), DT(0) { + initializeDSEPass(*PassRegistry::getPassRegistry()); + } virtual bool runOnFunction(Function &F) { + AA = &getAnalysis(); + MD = &getAnalysis(); + DT = &getAnalysis(); + bool Changed = false; for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I) - Changed |= runOnBasicBlock(*I); + // Only check non-dead blocks. Dead blocks may have strange pointer + // cycles that will confuse alias analysis. + if (DT->isReachableFromEntry(I)) + Changed |= runOnBasicBlock(*I); + + AA = 0; MD = 0; DT = 0; return Changed; } - + bool runOnBasicBlock(BasicBlock &BB); - bool handleFreeWithNonTrivialDependency(FreeInst *F, MemDepResult Dep); + bool HandleFree(CallInst *F); bool handleEndBlock(BasicBlock &BB); - bool RemoveUndeadPointers(Value* Ptr, uint64_t killPointerSize, - BasicBlock::iterator& BBI, - SmallPtrSet& deadPointers); - void DeleteDeadInstruction(Instruction *I, - SmallPtrSet *deadPointers = 0); - - - // getAnalysisUsage - We require post dominance frontiers (aka Control - // Dependence Graph) + void RemoveAccessedObjects(const AliasAnalysis::Location &LoadedLoc, + SmallPtrSet &DeadStackObjects); + virtual void getAnalysisUsage(AnalysisUsage &AU) const { AU.setPreservesCFG(); AU.addRequired(); - AU.addRequired(); AU.addRequired(); AU.addRequired(); - AU.addPreserved(); AU.addPreserved(); + AU.addPreserved(); AU.addPreserved(); } }; } char DSE::ID = 0; -static RegisterPass X("dse", "Dead Store Elimination"); +INITIALIZE_PASS_BEGIN(DSE, "dse", "Dead Store Elimination", false, false) +INITIALIZE_PASS_DEPENDENCY(DominatorTree) +INITIALIZE_PASS_DEPENDENCY(MemoryDependenceAnalysis) +INITIALIZE_AG_DEPENDENCY(AliasAnalysis) +INITIALIZE_PASS_END(DSE, "dse", "Dead Store Elimination", false, false) FunctionPass *llvm::createDeadStoreEliminationPass() { return new DSE(); } -bool DSE::runOnBasicBlock(BasicBlock &BB) { - MemoryDependenceAnalysis& MD = getAnalysis(); - TargetData &TD = getAnalysis(); +//===----------------------------------------------------------------------===// +// Helper functions +//===----------------------------------------------------------------------===// - bool MadeChange = false; +/// DeleteDeadInstruction - Delete this instruction. Before we do, go through +/// and zero out all the operands of this instruction. If any of them become +/// dead, delete them and the computation tree that feeds them. +/// +/// If ValueSet is non-null, remove any deleted instructions from it as well. +/// +static void DeleteDeadInstruction(Instruction *I, + MemoryDependenceAnalysis &MD, + SmallPtrSet *ValueSet = 0) { + SmallVector NowDeadInsts; + + NowDeadInsts.push_back(I); + --NumFastOther; + + // Before we touch this instruction, remove it from memdep! + do { + Instruction *DeadInst = NowDeadInsts.pop_back_val(); + ++NumFastOther; + + // This instruction is dead, zap it, in stages. Start by removing it from + // MemDep, which needs to know the operands and needs it to be in the + // function. + MD.removeInstruction(DeadInst); + + for (unsigned op = 0, e = DeadInst->getNumOperands(); op != e; ++op) { + Value *Op = DeadInst->getOperand(op); + DeadInst->setOperand(op, 0); + + // If this operand just became dead, add it to the NowDeadInsts list. + if (!Op->use_empty()) continue; + + if (Instruction *OpI = dyn_cast(Op)) + if (isInstructionTriviallyDead(OpI)) + NowDeadInsts.push_back(OpI); + } + + DeadInst->eraseFromParent(); + + if (ValueSet) ValueSet->erase(DeadInst); + } while (!NowDeadInsts.empty()); +} + + +/// hasMemoryWrite - Does this instruction write some memory? This only returns +/// true for things that we can analyze with other helpers below. +static bool hasMemoryWrite(Instruction *I) { + if (isa(I)) + return true; + if (IntrinsicInst *II = dyn_cast(I)) { + switch (II->getIntrinsicID()) { + default: + return false; + case Intrinsic::memset: + case Intrinsic::memmove: + case Intrinsic::memcpy: + case Intrinsic::init_trampoline: + case Intrinsic::lifetime_end: + return true; + } + } + return false; +} + +/// getLocForWrite - Return a Location stored to by the specified instruction. +/// If isRemovable returns true, this function and getLocForRead completely +/// describe the memory operations for this instruction. +static AliasAnalysis::Location +getLocForWrite(Instruction *Inst, AliasAnalysis &AA) { + if (StoreInst *SI = dyn_cast(Inst)) + return AA.getLocation(SI); + + if (MemIntrinsic *MI = dyn_cast(Inst)) { + // memcpy/memmove/memset. + AliasAnalysis::Location Loc = AA.getLocationForDest(MI); + // If we don't have target data around, an unknown size in Location means + // that we should use the size of the pointee type. This isn't valid for + // memset/memcpy, which writes more than an i8. + if (Loc.Size == AliasAnalysis::UnknownSize && AA.getTargetData() == 0) + return AliasAnalysis::Location(); + return Loc; + } + + IntrinsicInst *II = dyn_cast(Inst); + if (II == 0) return AliasAnalysis::Location(); + + switch (II->getIntrinsicID()) { + default: return AliasAnalysis::Location(); // Unhandled intrinsic. + case Intrinsic::init_trampoline: + // If we don't have target data around, an unknown size in Location means + // that we should use the size of the pointee type. This isn't valid for + // init.trampoline, which writes more than an i8. + if (AA.getTargetData() == 0) return AliasAnalysis::Location(); + + // FIXME: We don't know the size of the trampoline, so we can't really + // handle it here. + return AliasAnalysis::Location(II->getArgOperand(0)); + case Intrinsic::lifetime_end: { + uint64_t Len = cast(II->getArgOperand(0))->getZExtValue(); + return AliasAnalysis::Location(II->getArgOperand(1), Len); + } + } +} + +/// getLocForRead - Return the location read by the specified "hasMemoryWrite" +/// instruction if any. +static AliasAnalysis::Location +getLocForRead(Instruction *Inst, AliasAnalysis &AA) { + assert(hasMemoryWrite(Inst) && "Unknown instruction case"); + + // The only instructions that both read and write are the mem transfer + // instructions (memcpy/memmove). + if (MemTransferInst *MTI = dyn_cast(Inst)) + return AA.getLocationForSource(MTI); + return AliasAnalysis::Location(); +} + + +/// isRemovable - If the value of this instruction and the memory it writes to +/// is unused, may we delete this instruction? +static bool isRemovable(Instruction *I) { + // Don't remove volatile/atomic stores. + if (StoreInst *SI = dyn_cast(I)) + return SI->isUnordered(); + + IntrinsicInst *II = cast(I); + switch (II->getIntrinsicID()) { + default: assert(0 && "doesn't pass 'hasMemoryWrite' predicate"); + case Intrinsic::lifetime_end: + // Never remove dead lifetime_end's, e.g. because it is followed by a + // free. + return false; + case Intrinsic::init_trampoline: + // Always safe to remove init_trampoline. + return true; + + case Intrinsic::memset: + case Intrinsic::memmove: + case Intrinsic::memcpy: + // Don't remove volatile memory intrinsics. + return !cast(II)->isVolatile(); + } +} + + +/// isShortenable - Returns true if this instruction can be safely shortened in +/// length. +static bool isShortenable(Instruction *I) { + // Don't shorten stores for now + if (isa(I)) + return false; + + IntrinsicInst *II = cast(I); + switch (II->getIntrinsicID()) { + default: return false; + case Intrinsic::memset: + case Intrinsic::memcpy: + // Do shorten memory intrinsics. + return true; + } +} + +/// getStoredPointerOperand - Return the pointer that is being written to. +static Value *getStoredPointerOperand(Instruction *I) { + if (StoreInst *SI = dyn_cast(I)) + return SI->getPointerOperand(); + if (MemIntrinsic *MI = dyn_cast(I)) + return MI->getDest(); + + IntrinsicInst *II = cast(I); + switch (II->getIntrinsicID()) { + default: assert(false && "Unexpected intrinsic!"); + case Intrinsic::init_trampoline: + return II->getArgOperand(0); + } +} + +static uint64_t getPointerSize(Value *V, AliasAnalysis &AA) { + const TargetData *TD = AA.getTargetData(); + + if (CallInst *CI = dyn_cast(V)) { + assert(isMalloc(CI) && "Expected Malloc call!"); + if (ConstantInt *C = dyn_cast(CI->getArgOperand(0))) + return C->getZExtValue(); + return AliasAnalysis::UnknownSize; + } + + if (TD == 0) + return AliasAnalysis::UnknownSize; + + if (AllocaInst *A = dyn_cast(V)) { + // Get size information for the alloca + if (ConstantInt *C = dyn_cast(A->getArraySize())) + return C->getZExtValue() * TD->getTypeAllocSize(A->getAllocatedType()); + return AliasAnalysis::UnknownSize; + } + + assert(isa(V) && "Expected AllocaInst, malloc call or Argument!"); + PointerType *PT = cast(V->getType()); + return TD->getTypeAllocSize(PT->getElementType()); +} + +/// isObjectPointerWithTrustworthySize - Return true if the specified Value* is +/// pointing to an object with a pointer size we can trust. +static bool isObjectPointerWithTrustworthySize(const Value *V) { + if (const AllocaInst *AI = dyn_cast(V)) + return !AI->isArrayAllocation(); + if (const GlobalVariable *GV = dyn_cast(V)) + return !GV->mayBeOverridden(); + if (const Argument *A = dyn_cast(V)) + return A->hasByValAttr(); + if (isMalloc(V)) + return true; + return false; +} + +namespace { + enum OverwriteResult + { + OverwriteComplete, + OverwriteEnd, + OverwriteUnknown + }; +} + +/// isOverwrite - Return 'OverwriteComplete' if a store to the 'Later' location +/// completely overwrites a store to the 'Earlier' location. +/// 'OverwriteEnd' if the end of the 'Earlier' location is completely +/// overwritten by 'Later', or 'OverwriteUnknown' if nothing can be determined +static OverwriteResult isOverwrite(const AliasAnalysis::Location &Later, + const AliasAnalysis::Location &Earlier, + AliasAnalysis &AA, + int64_t& EarlierOff, + int64_t& LaterOff) { + const Value *P1 = Earlier.Ptr->stripPointerCasts(); + const Value *P2 = Later.Ptr->stripPointerCasts(); + + // If the start pointers are the same, we just have to compare sizes to see if + // the later store was larger than the earlier store. + if (P1 == P2) { + // If we don't know the sizes of either access, then we can't do a + // comparison. + if (Later.Size == AliasAnalysis::UnknownSize || + Earlier.Size == AliasAnalysis::UnknownSize) { + // If we have no TargetData information around, then the size of the store + // is inferrable from the pointee type. If they are the same type, then + // we know that the store is safe. + if (AA.getTargetData() == 0 && + Later.Ptr->getType() == Earlier.Ptr->getType()) + return OverwriteComplete; + + return OverwriteUnknown; + } + + // Make sure that the Later size is >= the Earlier size. + if (Later.Size >= Earlier.Size) + return OverwriteComplete; + } + + // Otherwise, we have to have size information, and the later store has to be + // larger than the earlier one. + if (Later.Size == AliasAnalysis::UnknownSize || + Earlier.Size == AliasAnalysis::UnknownSize || + AA.getTargetData() == 0) + return OverwriteUnknown; + + // Check to see if the later store is to the entire object (either a global, + // an alloca, or a byval argument). If so, then it clearly overwrites any + // other store to the same object. + const TargetData &TD = *AA.getTargetData(); + + const Value *UO1 = GetUnderlyingObject(P1, &TD), + *UO2 = GetUnderlyingObject(P2, &TD); + + // If we can't resolve the same pointers to the same object, then we can't + // analyze them at all. + if (UO1 != UO2) + return OverwriteUnknown; + + // If the "Later" store is to a recognizable object, get its size. + if (isObjectPointerWithTrustworthySize(UO2)) { + uint64_t ObjectSize = + TD.getTypeAllocSize(cast(UO2->getType())->getElementType()); + if (ObjectSize == Later.Size) + return OverwriteComplete; + } + + // Okay, we have stores to two completely different pointers. Try to + // decompose the pointer into a "base + constant_offset" form. If the base + // pointers are equal, then we can reason about the two stores. + EarlierOff = 0; + LaterOff = 0; + const Value *BP1 = GetPointerBaseWithConstantOffset(P1, EarlierOff, TD); + const Value *BP2 = GetPointerBaseWithConstantOffset(P2, LaterOff, TD); + + // If the base pointers still differ, we have two completely different stores. + if (BP1 != BP2) + return OverwriteUnknown; + + // The later store completely overlaps the earlier store if: + // + // 1. Both start at the same offset and the later one's size is greater than + // or equal to the earlier one's, or + // + // |--earlier--| + // |-- later --| + // + // 2. The earlier store has an offset greater than the later offset, but which + // still lies completely within the later store. + // + // |--earlier--| + // |----- later ------| + // + // We have to be careful here as *Off is signed while *.Size is unsigned. + if (EarlierOff >= LaterOff && + Later.Size > Earlier.Size && + uint64_t(EarlierOff - LaterOff) + Earlier.Size <= Later.Size) + return OverwriteComplete; - // Do a top-down walk on the BB + // The other interesting case is if the later store overwrites the end of + // the earlier store + // + // |--earlier--| + // |-- later --| + // + // In this case we may want to trim the size of earlier to avoid generating + // writes to addresses which will definitely be overwritten later + if (LaterOff > EarlierOff && + LaterOff < int64_t(EarlierOff + Earlier.Size) && + LaterOff + Later.Size >= EarlierOff + Earlier.Size) + return OverwriteEnd; + + // Otherwise, they don't completely overlap. + return OverwriteUnknown; +} + +/// isPossibleSelfRead - If 'Inst' might be a self read (i.e. a noop copy of a +/// memory region into an identical pointer) then it doesn't actually make its +/// input dead in the traditional sense. Consider this case: +/// +/// memcpy(A <- B) +/// memcpy(A <- A) +/// +/// In this case, the second store to A does not make the first store to A dead. +/// The usual situation isn't an explicit A<-A store like this (which can be +/// trivially removed) but a case where two pointers may alias. +/// +/// This function detects when it is unsafe to remove a dependent instruction +/// because the DSE inducing instruction may be a self-read. +static bool isPossibleSelfRead(Instruction *Inst, + const AliasAnalysis::Location &InstStoreLoc, + Instruction *DepWrite, AliasAnalysis &AA) { + // Self reads can only happen for instructions that read memory. Get the + // location read. + AliasAnalysis::Location InstReadLoc = getLocForRead(Inst, AA); + if (InstReadLoc.Ptr == 0) return false; // Not a reading instruction. + + // If the read and written loc obviously don't alias, it isn't a read. + if (AA.isNoAlias(InstReadLoc, InstStoreLoc)) return false; + + // Okay, 'Inst' may copy over itself. However, we can still remove a the + // DepWrite instruction if we can prove that it reads from the same location + // as Inst. This handles useful cases like: + // memcpy(A <- B) + // memcpy(A <- B) + // Here we don't know if A/B may alias, but we do know that B/B are must + // aliases, so removing the first memcpy is safe (assuming it writes <= # + // bytes as the second one. + AliasAnalysis::Location DepReadLoc = getLocForRead(DepWrite, AA); + + if (DepReadLoc.Ptr && AA.isMustAlias(InstReadLoc.Ptr, DepReadLoc.Ptr)) + return false; + + // If DepWrite doesn't read memory or if we can't prove it is a must alias, + // then it can't be considered dead. + return true; +} + + +//===----------------------------------------------------------------------===// +// DSE Pass +//===----------------------------------------------------------------------===// + +bool DSE::runOnBasicBlock(BasicBlock &BB) { + bool MadeChange = false; + + // Do a top-down walk on the BB. for (BasicBlock::iterator BBI = BB.begin(), BBE = BB.end(); BBI != BBE; ) { Instruction *Inst = BBI++; - - // If we find a store or a free, get it's memory dependence. - if (!isa(Inst) && !isa(Inst)) - continue; - - // Don't molest volatile stores or do queries that will return "clobber". - if (StoreInst *SI = dyn_cast(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(Inst)) { - MadeChange |= handleFreeWithNonTrivialDependency(FI, InstDep); + // Handle 'free' calls specially. + if (CallInst *F = isFreeCall(Inst)) { + MadeChange |= HandleFree(F); continue; } - - StoreInst *SI = cast(Inst); - - // If not a definite must-alias dependency, ignore it. - if (!InstDep.isDef()) + + // If we find something that writes memory, get its memory dependence. + if (!hasMemoryWrite(Inst)) 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(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; - } - + + MemDepResult InstDep = MD->getDependency(Inst); + + // Ignore any store where we can't find a local dependence. + // FIXME: cross-block DSE would be fun. :) + if (!InstDep.isDef() && !InstDep.isClobber()) + 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(InstDep.getInst())) { - if (SI->getPointerOperand() == DepLoad->getPointerOperand() && - SI->getOperand(0) == DepLoad) { - DeleteDeadInstruction(SI); - if (BBI != BB.begin()) - --BBI; - NumFastStores++; - MadeChange = true; - continue; + if (StoreInst *SI = dyn_cast(Inst)) { + if (LoadInst *DepLoad = dyn_cast(InstDep.getInst())) { + if (SI->getPointerOperand() == DepLoad->getPointerOperand() && + SI->getOperand(0) == DepLoad && isRemovable(SI)) { + DEBUG(dbgs() << "DSE: Remove Store Of Load from same pointer:\n " + << "LOAD: " << *DepLoad << "\n STORE: " << *SI << '\n'); + + // DeleteDeadInstruction can delete the current instruction. Save BBI + // in case we need it. + WeakVH NextInst(BBI); + + DeleteDeadInstruction(SI, *MD); + + if (NextInst == 0) // Next instruction deleted. + BBI = BB.begin(); + else if (BBI != BB.begin()) // Revisit this instruction if possible. + --BBI; + ++NumFastStores; + MadeChange = true; + continue; + } } } + + // Figure out what location is being stored to. + AliasAnalysis::Location Loc = getLocForWrite(Inst, *AA); + + // If we didn't get a useful location, fail. + if (Loc.Ptr == 0) + continue; + + while (InstDep.isDef() || InstDep.isClobber()) { + // Get the memory clobbered by the instruction we depend on. MemDep will + // skip any instructions that 'Loc' clearly doesn't interact with. If we + // end up depending on a may- or must-aliased load, then we can't optimize + // away the store and we bail out. However, if we depend on on something + // that overwrites the memory location we *can* potentially optimize it. + // + // Find out what memory location the dependent instruction stores. + Instruction *DepWrite = InstDep.getInst(); + AliasAnalysis::Location DepLoc = getLocForWrite(DepWrite, *AA); + // If we didn't get a useful location, or if it isn't a size, bail out. + if (DepLoc.Ptr == 0) + break; + + // If we find a write that is a) removable (i.e., non-volatile), b) is + // completely obliterated by the store to 'Loc', and c) which we know that + // 'Inst' doesn't load from, then we can remove it. + if (isRemovable(DepWrite) && + !isPossibleSelfRead(Inst, Loc, DepWrite, *AA)) { + int64_t InstWriteOffset, DepWriteOffset; + OverwriteResult OR = isOverwrite(Loc, DepLoc, *AA, + DepWriteOffset, InstWriteOffset); + if (OR == OverwriteComplete) { + DEBUG(dbgs() << "DSE: Remove Dead Store:\n DEAD: " + << *DepWrite << "\n KILLER: " << *Inst << '\n'); + + // Delete the store and now-dead instructions that feed it. + DeleteDeadInstruction(DepWrite, *MD); + ++NumFastStores; + MadeChange = true; + + // DeleteDeadInstruction can delete the current instruction in loop + // cases, reset BBI. + BBI = Inst; + if (BBI != BB.begin()) + --BBI; + break; + } else if (OR == OverwriteEnd && isShortenable(DepWrite)) { + // TODO: base this on the target vector size so that if the earlier + // store was too small to get vector writes anyway then its likely + // a good idea to shorten it + // Power of 2 vector writes are probably always a bad idea to optimize + // as any store/memset/memcpy is likely using vector instructions so + // shortening it to not vector size is likely to be slower + MemIntrinsic* DepIntrinsic = cast(DepWrite); + unsigned DepWriteAlign = DepIntrinsic->getAlignment(); + if (llvm::isPowerOf2_64(InstWriteOffset) || + ((DepWriteAlign != 0) && InstWriteOffset % DepWriteAlign == 0)) { + + DEBUG(dbgs() << "DSE: Remove Dead Store:\n OW END: " + << *DepWrite << "\n KILLER (offset " + << InstWriteOffset << ", " + << DepLoc.Size << ")" + << *Inst << '\n'); + + Value* DepWriteLength = DepIntrinsic->getLength(); + Value* TrimmedLength = ConstantInt::get(DepWriteLength->getType(), + InstWriteOffset - + DepWriteOffset); + DepIntrinsic->setLength(TrimmedLength); + MadeChange = true; + } + } + } + + // If this is a may-aliased store that is clobbering the store value, we + // can keep searching past it for another must-aliased pointer that stores + // to the same location. For example, in: + // store -> P + // store -> Q + // store -> P + // we can remove the first store to P even though we don't know if P and Q + // alias. + if (DepWrite == &BB.front()) break; + + // Can't look past this instruction if it might read 'Loc'. + if (AA->getModRefInfo(DepWrite, Loc) & AliasAnalysis::Ref) + break; + + InstDep = MD->getPointerDependencyFrom(Loc, false, DepWrite, &BB); + } } - + // If this block ends in a return, unwind, or unreachable, all allocas are // dead at its end, which means stores to them are also dead. if (BB.getTerminator()->getNumSuccessors() == 0) MadeChange |= handleEndBlock(BB); - + return MadeChange; } -/// 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(); - - StoreInst *Dependency = dyn_cast_or_null(Dep.getInst()); - if (!Dependency || Dependency->isVolatile()) - return false; - - Value *DepPointer = Dependency->getPointerOperand()->getUnderlyingObject(); +/// Find all blocks that will unconditionally lead to the block BB and append +/// them to F. +static void FindUnconditionalPreds(SmallVectorImpl &Blocks, + BasicBlock *BB, DominatorTree *DT) { + for (pred_iterator I = pred_begin(BB), E = pred_end(BB); I != E; ++I) { + BasicBlock *Pred = *I; + TerminatorInst *PredTI = Pred->getTerminator(); + if (PredTI->getNumSuccessors() != 1) + continue; - // 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; + if (DT->isReachableFromEntry(Pred)) + Blocks.push_back(Pred); + } +} + +/// HandleFree - Handle frees of entire structures whose dependency is a store +/// to a field of that structure. +bool DSE::HandleFree(CallInst *F) { + bool MadeChange = false; + + AliasAnalysis::Location Loc = AliasAnalysis::Location(F->getOperand(0)); + SmallVector Blocks; + Blocks.push_back(F->getParent()); + + while (!Blocks.empty()) { + BasicBlock *BB = Blocks.pop_back_val(); + Instruction *InstPt = BB->getTerminator(); + if (BB == F->getParent()) InstPt = F; + + MemDepResult Dep = MD->getPointerDependencyFrom(Loc, false, InstPt, BB); + while (Dep.isDef() || Dep.isClobber()) { + Instruction *Dependency = Dep.getInst(); + if (!hasMemoryWrite(Dependency) || !isRemovable(Dependency)) + break; + + Value *DepPointer = + GetUnderlyingObject(getStoredPointerOperand(Dependency)); + + // Check for aliasing. + if (!AA->isMustAlias(F->getArgOperand(0), DepPointer)) + break; + + Instruction *Next = llvm::next(BasicBlock::iterator(Dependency)); + + // DCE instructions only used to calculate that store + DeleteDeadInstruction(Dependency, *MD); + ++NumFastStores; + MadeChange = true; + + // Inst's old Dependency is now deleted. Compute the next dependency, + // which may also be dead, as in + // s[0] = 0; + // s[1] = 0; // This has just been deleted. + // free(s); + Dep = MD->getPointerDependencyFrom(Loc, false, Next, BB); + } + + if (Dep.isNonLocal()) + FindUnconditionalPreds(Blocks, BB, DT); + } + + return MadeChange; } /// handleEndBlock - Remove dead stores to stack-allocated locations in the @@ -181,281 +699,168 @@ bool DSE::handleFreeWithNonTrivialDependency(FreeInst *F, MemDepResult Dep) { /// store i32 1, i32* %A /// ret void bool DSE::handleEndBlock(BasicBlock &BB) { - TargetData &TD = getAnalysis(); - AliasAnalysis &AA = getAnalysis(); - bool MadeChange = false; - - // Pointers alloca'd in this function are dead in the end block - SmallPtrSet deadPointers; - + + // Keep track of all of the stack objects that are dead at the end of the + // function. + SmallPtrSet DeadStackObjects; + // Find all of the alloca'd pointers in the entry block. BasicBlock *Entry = BB.getParent()->begin(); - for (BasicBlock::iterator I = Entry->begin(), E = Entry->end(); I != E; ++I) + for (BasicBlock::iterator I = Entry->begin(), E = Entry->end(); I != E; ++I) { if (AllocaInst *AI = dyn_cast(I)) - deadPointers.insert(AI); - + DeadStackObjects.insert(AI); + + // Okay, so these are dead heap objects, but if the pointer never escapes + // then it's leaked by this function anyways. + if (CallInst *CI = extractMallocCall(I)) + if (!PointerMayBeCaptured(CI, true, true)) + DeadStackObjects.insert(CI); + } + // 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); - + DeadStackObjects.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(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(BBI)) { - Value *dest = M->getDest()->getUnderlyingObject(); - - if (deadPointers.count(dest)) { - BBI++; - DeleteDeadInstruction(M, &deadPointers); - NumFastOther++; + + // If we find a store, check to see if it points into a dead stack value. + if (hasMemoryWrite(BBI) && isRemovable(BBI)) { + // See through pointer-to-pointer bitcasts + Value *Pointer = GetUnderlyingObject(getStoredPointerOperand(BBI)); + + // Stores to stack values are valid candidates for removal. + if (DeadStackObjects.count(Pointer)) { + Instruction *Dead = BBI++; + + DEBUG(dbgs() << "DSE: Dead Store at End of Block:\n DEAD: " + << *Dead << "\n Object: " << *Pointer << '\n'); + + // DCE instructions only used to calculate that store. + DeleteDeadInstruction(Dead, *MD, &DeadStackObjects); + ++NumFastStores; MadeChange = true; continue; } - - // 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(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(BBI)) { - killPointer = V->getOperand(0); - } else if (isa(BBI) && - isa(cast(BBI)->getLength())) { - killPointer = cast(BBI)->getSource(); - killPointerSize = cast( - cast(BBI)->getLength())->getZExtValue(); - } else if (AllocaInst* A = dyn_cast(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; - } - + + // Remove any dead non-memory-mutating instructions. + if (isInstructionTriviallyDead(BBI)) { + Instruction *Inst = BBI++; + DeleteDeadInstruction(Inst, *MD, &DeadStackObjects); + ++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)) + } + + if (AllocaInst *A = dyn_cast(BBI)) { + DeadStackObjects.erase(A); + continue; + } + + if (CallInst *CI = extractMallocCall(BBI)) { + DeadStackObjects.erase(CI); + continue; + } + + if (CallSite CS = cast(BBI)) { + // If this call does not access memory, it can't be loading 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 dead; - for (SmallPtrSet::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(*I)) { - if (ConstantInt* C = dyn_cast(A->getArraySize())) - pointerSize = C->getZExtValue() * - TD.getTypePaddedSize(A->getAllocatedType()); - } else { - const PointerType* PT = cast( - cast(*I)->getType()); - pointerSize = TD.getTypePaddedSize(PT->getElementType()); - } + // If the call might load from any of our allocas, then any store above + // the call is live. + SmallVector LiveAllocas; + for (SmallPtrSet::iterator I = DeadStackObjects.begin(), + E = DeadStackObjects.end(); I != E; ++I) { + // See if the call site touches it. + AliasAnalysis::ModRefResult A = + AA->getModRefInfo(CS, *I, getPointerSize(*I, *AA)); - // 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); + LiveAllocas.push_back(*I); } - for (std::vector::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; + for (SmallVector::iterator I = LiveAllocas.begin(), + E = LiveAllocas.end(); I != E; ++I) + DeadStackObjects.erase(*I); + + // If all of the allocas were clobbered by the call then we're not going + // to find anything else to process. + if (DeadStackObjects.empty()) + return MadeChange; + continue; } - - if (!killPointer) + + AliasAnalysis::Location LoadedLoc; + + // If we encounter a use of the pointer, it is no longer considered dead + if (LoadInst *L = dyn_cast(BBI)) { + if (!L->isUnordered()) // Be conservative with atomic/volatile load + break; + LoadedLoc = AA->getLocation(L); + } else if (VAArgInst *V = dyn_cast(BBI)) { + LoadedLoc = AA->getLocation(V); + } else if (MemTransferInst *MTI = dyn_cast(BBI)) { + LoadedLoc = AA->getLocationForSource(MTI); + } else if (!BBI->mayReadFromMemory()) { + // Instruction doesn't read memory. Note that stores that weren't removed + // above will hit this case. continue; + } else { + // Unknown inst; assume it clobbers everything. + break; + } - killPointer = killPointer->getUnderlyingObject(); + // Remove any allocas from the DeadPointer set that are loaded, as this + // makes any stores above the access live. + RemoveAccessedObjects(LoadedLoc, DeadStackObjects); - // Deal with undead pointers - MadeChange |= RemoveUndeadPointers(killPointer, killPointerSize, BBI, - deadPointers); + // If all of the allocas were clobbered by the access then we're not going + // to find anything else to process. + if (DeadStackObjects.empty()) + break; } - + return MadeChange; } -/// 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& deadPointers) { - TargetData &TD = getAnalysis(); - AliasAnalysis &AA = getAnalysis(); - - // 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(killPointer)) - return false; - - bool MadeChange = false; - - SmallVector undead; - - for (SmallPtrSet::iterator I = deadPointers.begin(), - E = deadPointers.end(); I != E; ++I) { - // Get size information for the alloca. - unsigned pointerSize = ~0U; - if (AllocaInst* A = dyn_cast(*I)) { - if (ConstantInt* C = dyn_cast(A->getArraySize())) - pointerSize = C->getZExtValue() * - TD.getTypePaddedSize(A->getAllocatedType()); - } else { - const PointerType* PT = cast(cast(*I)->getType()); - pointerSize = TD.getTypePaddedSize(PT->getElementType()); - } - - // See if this pointer could alias it - AliasAnalysis::AliasResult A = AA.alias(*I, pointerSize, - killPointer, killPointerSize); +/// RemoveAccessedObjects - Check to see if the specified location may alias any +/// of the stack objects in the DeadStackObjects set. If so, they become live +/// because the location is being loaded. +void DSE::RemoveAccessedObjects(const AliasAnalysis::Location &LoadedLoc, + SmallPtrSet &DeadStackObjects) { + const Value *UnderlyingPointer = GetUnderlyingObject(LoadedLoc.Ptr); - // If it must-alias and a store, we can delete it - if (isa(BBI) && A == AliasAnalysis::MustAlias) { - StoreInst* S = cast(BBI); + // A constant can't be in the dead pointer set. + if (isa(UnderlyingPointer)) + return; - // Remove it! - BBI++; - DeleteDeadInstruction(S, &deadPointers); - NumFastStores++; - MadeChange = true; - - continue; + // If the kill pointer can be easily reduced to an alloca, don't bother doing + // extraneous AA queries. + if (isa(UnderlyingPointer) || isa(UnderlyingPointer)) { + DeadStackObjects.erase(const_cast(UnderlyingPointer)); + return; + } - // Otherwise, it is undead - } else if (A != AliasAnalysis::NoAlias) - undead.push_back(*I); + SmallVector NowLive; + for (SmallPtrSet::iterator I = DeadStackObjects.begin(), + E = DeadStackObjects.end(); I != E; ++I) { + // See if the loaded location could alias the stack location. + AliasAnalysis::Location StackLoc(*I, getPointerSize(*I, *AA)); + if (!AA->isNoAlias(StackLoc, LoadedLoc)) + NowLive.push_back(*I); } - for (SmallVector::iterator I = undead.begin(), E = undead.end(); + for (SmallVector::iterator I = NowLive.begin(), E = NowLive.end(); I != E; ++I) - deadPointers.erase(*I); - - return MadeChange; + DeadStackObjects.erase(*I); } -/// 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 *ValueSet) { - SmallVector NowDeadInsts; - - NowDeadInsts.push_back(I); - --NumFastOther; - - // Before we touch this instruction, remove it from memdep! - MemoryDependenceAnalysis &MDA = getAnalysis(); - 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(Op)) - if (isInstructionTriviallyDead(OpI)) - NowDeadInsts.push_back(OpI); - } - - DeadInst->eraseFromParent(); - - if (ValueSet) ValueSet->erase(DeadInst); - } -}