| //===- 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, bool zeroGepsOnly = false) { |
| 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)) |
| if (!zeroGepsOnly || G->hasAllZeroIndices()) { |
| v = G->getOperand(0); |
| } else { |
| break; |
| } |
| 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>(); |
| TargetData &TD = getAnalysis<TargetData>(); |
| |
| // 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(); |
| |
| TranslatePointerBitCasts(pointer, true); |
| 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 || |
| TD.getTypeStoreSize(last->getOperand(0)->getType()) > |
| TD.getTypeStoreSize(BBI->getOperand(0)->getType())) { |
| 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.getTypeStoreSize(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 (isa<AllocaInst>(pointerOperand) && |
| deadPointers.count(cast<AllocaInst>(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 (AA.doesNotAccessMemory(CS)) |
| 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.getABITypeSize((*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) |
| if (AllocaInst *AI = dyn_cast<AllocaInst>(*I)) |
| deadPointers.erase(AI); |
| |
| 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.getABITypeSize((*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) |
| if (AllocaInst *AI = dyn_cast<AllocaInst>(*I)) |
| deadPointers.erase(AI); |
| |
| 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; |
| } |