-//===- DeadStoreElimination.cpp - Dead Store Elimination ------------------===//
+//===- DeadStoreElimination.cpp - Fast Dead Store Elimination -------------===//
//
// The LLVM Compiler Infrastructure
//
-// This file was developed by the LLVM research group and is distributed under
-// the University of Illinois Open Source License. See LICENSE.TXT for details.
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
//
//===----------------------------------------------------------------------===//
+#define DEBUG_TYPE "dse"
#include "llvm/Transforms/Scalar.h"
-#include "llvm/DerivedTypes.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/AliasSetTracker.h"
+#include "llvm/Analysis/Dominators.h"
+#include "llvm/Analysis/MemoryDependenceAnalysis.h"
#include "llvm/Target/TargetData.h"
#include "llvm/Transforms/Utils/Local.h"
-#include "llvm/ADT/SetVector.h"
-#include "llvm/ADT/Statistic.h"
+#include "llvm/Support/Compiler.h"
using namespace llvm;
-namespace {
- Statistic<> NumStores("dse", "Number of stores deleted");
- Statistic<> NumOther ("dse", "Number of other instrs removed");
+STATISTIC(NumFastStores, "Number of stores deleted");
+STATISTIC(NumFastOther , "Number of other instrs removed");
- struct DSE : public FunctionPass {
+namespace {
+ struct VISIBILITY_HIDDEN DSE : public FunctionPass {
+ static char ID; // Pass identification, replacement for typeid
+ DSE() : FunctionPass(&ID) {}
virtual bool runOnFunction(Function &F) {
bool Changed = false;
Changed |= runOnBasicBlock(*I);
return Changed;
}
-
+
bool runOnBasicBlock(BasicBlock &BB);
-
- void DeleteDeadInstructionChains(Instruction *I,
- SetVector<Instruction*> &DeadInsts);
+ bool handleFreeWithNonTrivialDependency(FreeInst *F, MemDepResult Dep);
+ bool handleEndBlock(BasicBlock &BB);
+ bool RemoveUndeadPointers(Value* Ptr, uint64_t killPointerSize,
+ BasicBlock::iterator& BBI,
+ SmallPtrSet<Value*, 64>& deadPointers);
+ void DeleteDeadInstruction(Instruction *I,
+ SmallPtrSet<Value*, 64> *deadPointers = 0);
+
// getAnalysisUsage - We require post dominance frontiers (aka Control
// Dependence Graph)
virtual void getAnalysisUsage(AnalysisUsage &AU) const {
AU.setPreservesCFG();
+ AU.addRequired<DominatorTree>();
AU.addRequired<TargetData>();
AU.addRequired<AliasAnalysis>();
+ AU.addRequired<MemoryDependenceAnalysis>();
+ AU.addPreserved<DominatorTree>();
AU.addPreserved<AliasAnalysis>();
+ AU.addPreserved<MemoryDependenceAnalysis>();
}
};
- RegisterPass<DSE> X("dse", "Dead Store Elimination");
}
+char DSE::ID = 0;
+static RegisterPass<DSE> X("dse", "Dead Store Elimination");
+
FunctionPass *llvm::createDeadStoreEliminationPass() { return new DSE(); }
bool DSE::runOnBasicBlock(BasicBlock &BB) {
- TargetData &TD = getAnalysis<TargetData>();
- AliasAnalysis &AA = getAnalysis<AliasAnalysis>();
- AliasSetTracker KillLocs(AA);
-
- // 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) {
- BasicBlock *Entry = BB.getParent()->begin();
- for (BasicBlock::iterator I = Entry->begin(), E = Entry->end(); I != E; ++I)
- if (AllocaInst *AI = dyn_cast<AllocaInst>(I)) {
- unsigned Size = ~0U;
- if (!AI->isArrayAllocation() &&
- AI->getType()->getElementType()->isSized())
- Size = (unsigned)TD.getTypeSize(AI->getType()->getElementType());
- KillLocs.add(AI, Size);
+ MemoryDependenceAnalysis& MD = getAnalysis<MemoryDependenceAnalysis>();
+ TargetData &TD = getAnalysis<TargetData>();
+
+ bool MadeChange = false;
+
+ // Do a top-down walk on the BB
+ for (BasicBlock::iterator BBI = BB.begin(), BBE = BB.end(); BBI != BBE; ) {
+ Instruction *Inst = BBI++;
+
+ // If we find a store or a free, get it's memory dependence.
+ if (!isa<StoreInst>(Inst) && !isa<FreeInst>(Inst))
+ continue;
+
+ // Don't molest volatile stores or do queries that will return "clobber".
+ if (StoreInst *SI = dyn_cast<StoreInst>(Inst))
+ if (SI->isVolatile())
+ continue;
+
+ MemDepResult InstDep = MD.getDependency(Inst);
+
+ // Ignore non-local stores.
+ // FIXME: cross-block DSE would be fun. :)
+ if (InstDep.isNonLocal()) continue;
+
+ // Handle frees whose dependencies are non-trivial.
+ if (FreeInst *FI = dyn_cast<FreeInst>(Inst)) {
+ MadeChange |= handleFreeWithNonTrivialDependency(FI, InstDep);
+ continue;
+ }
+
+ StoreInst *SI = cast<StoreInst>(Inst);
+
+ // If not a definite must-alias dependency, ignore it.
+ if (!InstDep.isDef())
+ continue;
+
+ // If this is a store-store dependence, then the previous store is dead so
+ // long as this store is at least as big as it.
+ if (StoreInst *DepStore = dyn_cast<StoreInst>(InstDep.getInst()))
+ if (TD.getTypeStoreSize(DepStore->getOperand(0)->getType()) <=
+ TD.getTypeStoreSize(SI->getOperand(0)->getType())) {
+ // Delete the store and now-dead instructions that feed it.
+ DeleteDeadInstruction(DepStore);
+ NumFastStores++;
+ MadeChange = true;
+
+ if (BBI != BB.begin())
+ --BBI;
+ continue;
}
+
+ // If we're storing the same value back to a pointer that we just
+ // loaded from, then the store can be removed.
+ if (LoadInst *DepLoad = dyn_cast<LoadInst>(InstDep.getInst())) {
+ if (SI->getPointerOperand() == DepLoad->getPointerOperand() &&
+ SI->getOperand(0) == DepLoad) {
+ DeleteDeadInstruction(SI);
+ if (BBI != BB.begin())
+ --BBI;
+ NumFastStores++;
+ MadeChange = true;
+ continue;
+ }
+ }
}
+
+ // If this block ends in a return, unwind, or unreachable, all allocas are
+ // dead at its end, which means stores to them are also dead.
+ if (BB.getTerminator()->getNumSuccessors() == 0)
+ MadeChange |= handleEndBlock(BB);
+
+ return MadeChange;
+}
- // PotentiallyDeadInsts - Deleting dead stores from the program can make other
- // instructions die if they were only used as operands to stores. Keep track
- // of the operands to stores so that we can try deleting them at the end of
- // the traversal.
- SetVector<Instruction*> PotentiallyDeadInsts;
+/// handleFreeWithNonTrivialDependency - Handle frees of entire structures whose
+/// dependency is a store to a field of that structure.
+bool DSE::handleFreeWithNonTrivialDependency(FreeInst *F, MemDepResult Dep) {
+ AliasAnalysis &AA = getAnalysis<AliasAnalysis>();
+
+ StoreInst *Dependency = dyn_cast_or_null<StoreInst>(Dep.getInst());
+ if (!Dependency || Dependency->isVolatile())
+ return false;
+
+ Value *DepPointer = Dependency->getPointerOperand()->getUnderlyingObject();
+ // Check for aliasing.
+ if (AA.alias(F->getPointerOperand(), 1, DepPointer, 1) !=
+ AliasAnalysis::MustAlias)
+ return false;
+
+ // DCE instructions only used to calculate that store
+ DeleteDeadInstruction(Dependency);
+ NumFastStores++;
+ return true;
+}
+
+/// handleEndBlock - Remove dead stores to stack-allocated locations in the
+/// function end block. Ex:
+/// %A = alloca i32
+/// ...
+/// store i32 1, i32* %A
+/// ret void
+bool DSE::handleEndBlock(BasicBlock &BB) {
+ TargetData &TD = getAnalysis<TargetData>();
+ AliasAnalysis &AA = getAnalysis<AliasAnalysis>();
+
bool MadeChange = false;
- for (BasicBlock::iterator BBI = BB.end(); BBI != BB.begin(); ) {
- Instruction *I = --BBI; // Keep moving iterator backwards
+
+ // Pointers alloca'd in this function are dead in the end block
+ SmallPtrSet<Value*, 64> deadPointers;
+
+ // Find all of the alloca'd pointers in the entry block.
+ BasicBlock *Entry = BB.getParent()->begin();
+ for (BasicBlock::iterator I = Entry->begin(), E = Entry->end(); I != E; ++I)
+ if (AllocaInst *AI = dyn_cast<AllocaInst>(I))
+ deadPointers.insert(AI);
+
+ // Treat byval arguments the same, stores to them are dead at the end of the
+ // function.
+ for (Function::arg_iterator AI = BB.getParent()->arg_begin(),
+ AE = BB.getParent()->arg_end(); AI != AE; ++AI)
+ if (AI->hasByValAttr())
+ deadPointers.insert(AI);
+
+ // Scan the basic block backwards
+ for (BasicBlock::iterator BBI = BB.end(); BBI != BB.begin(); ){
+ --BBI;
+
+ // If we find a store whose pointer is dead.
+ if (StoreInst* S = dyn_cast<StoreInst>(BBI)) {
+ if (!S->isVolatile()) {
+ // See through pointer-to-pointer bitcasts
+ Value* pointerOperand = S->getPointerOperand()->getUnderlyingObject();
- // If this is a free instruction, it makes the free'd location dead!
- if (FreeInst *FI = dyn_cast<FreeInst>(I)) {
- // Free instructions make any stores to the free'd location dead.
- KillLocs.add(FI);
+ // Alloca'd pointers or byval arguments (which are functionally like
+ // alloca's) are valid candidates for removal.
+ if (deadPointers.count(pointerOperand)) {
+ // DCE instructions only used to calculate that store.
+ BBI++;
+ DeleteDeadInstruction(S, &deadPointers);
+ NumFastStores++;
+ MadeChange = true;
+ }
+ }
+
continue;
}
+
+ // We can also remove memcpy's to local variables at the end of a function.
+ if (MemCpyInst *M = dyn_cast<MemCpyInst>(BBI)) {
+ Value *dest = M->getDest()->getUnderlyingObject();
- if (!isa<StoreInst>(I) || cast<StoreInst>(I)->isVolatile()) {
- // If this is a vaarg instruction, it reads its operand. We don't model
- // it correctly, so just conservatively remove all entries.
- if (isa<VAArgInst>(I)) {
- KillLocs.clear();
+ if (deadPointers.count(dest)) {
+ BBI++;
+ DeleteDeadInstruction(M, &deadPointers);
+ NumFastOther++;
+ MadeChange = true;
continue;
- }
+ }
- // If this is a non-store instruction, it makes everything referenced no
- // longer killed. Remove anything aliased from the alias set tracker.
- KillLocs.remove(I);
- continue;
+ // Because a memcpy is also a load, we can't skip it if we didn't remove
+ // it.
}
+
+ Value* killPointer = 0;
+ uint64_t killPointerSize = ~0UL;
+
+ // If we encounter a use of the pointer, it is no longer considered dead
+ if (LoadInst *L = dyn_cast<LoadInst>(BBI)) {
+ // However, if this load is unused and not volatile, we can go ahead and
+ // remove it, and not have to worry about it making our pointer undead!
+ if (L->use_empty() && !L->isVolatile()) {
+ BBI++;
+ DeleteDeadInstruction(L, &deadPointers);
+ NumFastOther++;
+ MadeChange = true;
+ continue;
+ }
+
+ killPointer = L->getPointerOperand();
+ } else if (VAArgInst* V = dyn_cast<VAArgInst>(BBI)) {
+ killPointer = V->getOperand(0);
+ } else if (isa<MemCpyInst>(BBI) &&
+ isa<ConstantInt>(cast<MemCpyInst>(BBI)->getLength())) {
+ killPointer = cast<MemCpyInst>(BBI)->getSource();
+ killPointerSize = cast<ConstantInt>(
+ cast<MemCpyInst>(BBI)->getLength())->getZExtValue();
+ } else if (AllocaInst* A = dyn_cast<AllocaInst>(BBI)) {
+ deadPointers.erase(A);
+
+ // Dead alloca's can be DCE'd when we reach them
+ if (A->use_empty()) {
+ BBI++;
+ DeleteDeadInstruction(A, &deadPointers);
+ NumFastOther++;
+ MadeChange = true;
+ }
+
+ continue;
+ } else if (CallSite::get(BBI).getInstruction() != 0) {
+ // If this call does not access memory, it can't
+ // be undeadifying any of our pointers.
+ CallSite CS = CallSite::get(BBI);
+ if (AA.doesNotAccessMemory(CS))
+ continue;
+
+ unsigned modRef = 0;
+ unsigned other = 0;
+
+ // Remove any pointers made undead by the call from the dead set
+ std::vector<Value*> dead;
+ for (SmallPtrSet<Value*, 64>::iterator I = deadPointers.begin(),
+ E = deadPointers.end(); I != E; ++I) {
+ // HACK: if we detect that our AA is imprecise, it's not
+ // worth it to scan the rest of the deadPointers set. Just
+ // assume that the AA will return ModRef for everything, and
+ // go ahead and bail.
+ if (modRef >= 16 && other == 0) {
+ deadPointers.clear();
+ return MadeChange;
+ }
- // If this is a non-volatile store instruction, and if it is already in
- // the stored location is already in the tracker, then this is a dead
- // store. We can just delete it here, but while we're at it, we also
- // delete any trivially dead expression chains.
- unsigned ValSize = (unsigned)TD.getTypeSize(I->getOperand(0)->getType());
- Value *Ptr = I->getOperand(1);
-
- if (AliasSet *AS = KillLocs.getAliasSetForPointerIfExists(Ptr, ValSize))
- for (AliasSet::iterator ASI = AS->begin(), E = AS->end(); ASI != E; ++ASI)
- if (ASI.getSize() >= ValSize && // Overwriting all of this store.
- AA.alias(ASI.getPointer(), ASI.getSize(), Ptr, ValSize)
- == AliasAnalysis::MustAlias) {
- // If we found a must alias in the killed set, then this store really
- // is dead. Remember that the various operands of the store now have
- // fewer users. At the end we will see if we can delete any values
- // that are dead as part of the store becoming dead.
- if (Instruction *Op = dyn_cast<Instruction>(I->getOperand(0)))
- PotentiallyDeadInsts.insert(Op);
- if (Instruction *Op = dyn_cast<Instruction>(Ptr))
- PotentiallyDeadInsts.insert(Op);
-
- // Delete it now.
- ++BBI; // Don't invalidate iterator.
- BB.getInstList().erase(I); // Nuke the store!
- ++NumStores;
- MadeChange = true;
- goto BigContinue;
+ // Get size information for the alloca
+ unsigned pointerSize = ~0U;
+ if (AllocaInst* A = dyn_cast<AllocaInst>(*I)) {
+ if (ConstantInt* C = dyn_cast<ConstantInt>(A->getArraySize()))
+ pointerSize = C->getZExtValue() *
+ TD.getTypePaddedSize(A->getAllocatedType());
+ } else {
+ const PointerType* PT = cast<PointerType>(
+ cast<Argument>(*I)->getType());
+ pointerSize = TD.getTypePaddedSize(PT->getElementType());
}
- // Otherwise, this is a non-dead store just add it to the set of dead
- // locations.
- KillLocs.add(cast<StoreInst>(I));
- BigContinue:;
- }
+ // 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);
+ }
- while (!PotentiallyDeadInsts.empty()) {
- Instruction *I = PotentiallyDeadInsts.back();
- PotentiallyDeadInsts.pop_back();
- DeleteDeadInstructionChains(I, PotentiallyDeadInsts);
+ 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;
}
-void DSE::DeleteDeadInstructionChains(Instruction *I,
- SetVector<Instruction*> &DeadInsts) {
- // Instruction must be dead.
- if (!I->use_empty() || !isInstructionTriviallyDead(I)) return;
-
- // Let the alias analysis know that we have nuked a value.
- getAnalysis<AliasAnalysis>().deleteValue(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 (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.
+/// RemoveUndeadPointers - check for uses of a pointer that make it
+/// undead when scanning for dead stores to alloca's.
+bool DSE::RemoveUndeadPointers(Value* killPointer, uint64_t killPointerSize,
+ BasicBlock::iterator &BBI,
+ SmallPtrSet<Value*, 64>& deadPointers) {
+ TargetData &TD = getAnalysis<TargetData>();
+ AliasAnalysis &AA = getAnalysis<AliasAnalysis>();
+
+ // If the kill pointer can be easily reduced to an alloca,
+ // don't bother doing extraneous AA queries.
+ if (deadPointers.count(killPointer)) {
+ deadPointers.erase(killPointer);
+ return false;
+ }
+
+ // A global can't be in the dead pointer set.
+ if (isa<GlobalValue>(killPointer))
+ return false;
+
+ bool MadeChange = false;
+
+ SmallVector<Value*, 16> undead;
+
+ for (SmallPtrSet<Value*, 64>::iterator I = deadPointers.begin(),
+ E = deadPointers.end(); I != E; ++I) {
+ // Get size information for the alloca.
+ unsigned pointerSize = ~0U;
+ if (AllocaInst* A = dyn_cast<AllocaInst>(*I)) {
+ if (ConstantInt* C = dyn_cast<ConstantInt>(A->getArraySize()))
+ pointerSize = C->getZExtValue() *
+ TD.getTypePaddedSize(A->getAllocatedType());
+ } else {
+ const PointerType* PT = cast<PointerType>(cast<Argument>(*I)->getType());
+ pointerSize = TD.getTypePaddedSize(PT->getElementType());
+ }
+
+ // See if this pointer could alias it
+ AliasAnalysis::AliasResult A = AA.alias(*I, pointerSize,
+ killPointer, killPointerSize);
+
+ // If it must-alias and a store, we can delete it
+ if (isa<StoreInst>(BBI) && A == AliasAnalysis::MustAlias) {
+ StoreInst* S = cast<StoreInst>(BBI);
+
+ // Remove it!
+ BBI++;
+ DeleteDeadInstruction(S, &deadPointers);
+ NumFastStores++;
+ MadeChange = true;
+
+ continue;
+
+ // Otherwise, it is undead
+ } else if (A != AliasAnalysis::NoAlias)
+ undead.push_back(*I);
}
- I->eraseFromParent();
- ++NumOther;
+ for (SmallVector<Value*, 16>::iterator I = undead.begin(), E = undead.end();
+ I != E; ++I)
+ deadPointers.erase(*I);
+
+ return MadeChange;
+}
+
+/// DeleteDeadInstruction - Delete this instruction. Before we do, go through
+/// and zero out all the operands of this instruction. If any of them become
+/// dead, delete them and the computation tree that feeds them.
+///
+/// If ValueSet is non-null, remove any deleted instructions from it as well.
+///
+void DSE::DeleteDeadInstruction(Instruction *I,
+ SmallPtrSet<Value*, 64> *ValueSet) {
+ SmallVector<Instruction*, 32> NowDeadInsts;
+
+ NowDeadInsts.push_back(I);
+ --NumFastOther;
+
+ // Before we touch this instruction, remove it from memdep!
+ MemoryDependenceAnalysis &MDA = getAnalysis<MemoryDependenceAnalysis>();
+ while (!NowDeadInsts.empty()) {
+ Instruction *DeadInst = NowDeadInsts.back();
+ NowDeadInsts.pop_back();
+
+ ++NumFastOther;
+
+ // This instruction is dead, zap it, in stages. Start by removing it from
+ // MemDep, which needs to know the operands and needs it to be in the
+ // function.
+ MDA.removeInstruction(DeadInst);
+
+ for (unsigned op = 0, e = DeadInst->getNumOperands(); op != e; ++op) {
+ Value *Op = DeadInst->getOperand(op);
+ DeadInst->setOperand(op, 0);
+
+ // If this operand just became dead, add it to the NowDeadInsts list.
+ if (!Op->use_empty()) continue;
+
+ if (Instruction *OpI = dyn_cast<Instruction>(Op))
+ if (isInstructionTriviallyDead(OpI))
+ NowDeadInsts.push_back(OpI);
+ }
+
+ DeadInst->eraseFromParent();
+
+ if (ValueSet) ValueSet->erase(DeadInst);
+ }
}
projects / oota-llvm.git / blobdiff