-//===- 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
+// This file was developed by Owen Anderson and 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/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/AliasSetTracker.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;
-STATISTIC(NumStores, "Number of stores deleted");
-STATISTIC(NumOther , "Number of other instrs removed");
+STATISTIC(NumFastStores, "Number of stores deleted");
+STATISTIC(NumFastOther , "Number of other instrs removed");
namespace {
struct VISIBILITY_HIDDEN DSE : public FunctionPass {
- static const char ID; // Pass identifcation, replacement for typeid
+ static char ID; // Pass identification, replacement for typeid
DSE() : FunctionPass((intptr_t)&ID) {}
virtual bool runOnFunction(Function &F) {
}
bool runOnBasicBlock(BasicBlock &BB);
-
+ bool handleFreeWithNonTrivialDependency(FreeInst* F,
+ Instruction* dependency,
+ SetVector<Instruction*>& possiblyDead);
+ bool handleEndBlock(BasicBlock& BB, SetVector<Instruction*>& possiblyDead);
+ bool RemoveUndeadPointers(Value* pointer, unsigned pointerSize,
+ BasicBlock::iterator& BBI,
+ SmallPtrSet<AllocaInst*, 4>& deadPointers,
+ SetVector<Instruction*>& possiblyDead);
void DeleteDeadInstructionChains(Instruction *I,
SetVector<Instruction*> &DeadInsts);
+ void TranslatePointerBitCasts(Value*& v) {
+ assert(isa<PointerType>(v->getType()) &&
+ "Translating a non-pointer type?");
+
+ // See through pointer-to-pointer bitcasts
+ while (isa<BitCastInst>(v) || isa<GetElementPtrInst>(v))
+ if (BitCastInst* C = dyn_cast<BitCastInst>(v))
+ v = C->getOperand(0);
+ else if (GetElementPtrInst* G = dyn_cast<GetElementPtrInst>(v))
+ v = G->getOperand(0);
+ }
// getAnalysisUsage - We require post dominance frontiers (aka Control
// Dependence Graph)
AU.setPreservesCFG();
AU.addRequired<TargetData>();
AU.addRequired<AliasAnalysis>();
+ AU.addRequired<MemoryDependenceAnalysis>();
AU.addPreserved<AliasAnalysis>();
+ AU.addPreserved<MemoryDependenceAnalysis>();
}
};
- const char DSE::ID = 0;
+ char DSE::ID = 0;
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);
-
+ MemoryDependenceAnalysis& MD = getAnalysis<MemoryDependenceAnalysis>();
+
+ // Record the last-seen store to this pointer
+ DenseMap<Value*, StoreInst*> lastStore;
+ // Record instructions possibly made dead by deleting a store
+ SetVector<Instruction*> possiblyDead;
+
+ bool MadeChange = false;
+
+ // Do a top-down walk on the BB
+ for (BasicBlock::iterator BBI = BB.begin(), BBE = BB.end();
+ BBI != BBE; ++BBI) {
+ // If we find a store or a free...
+ if (isa<StoreInst>(BBI) || isa<FreeInst>(BBI)) {
+ Value* pointer = 0;
+ if (StoreInst* S = dyn_cast<StoreInst>(BBI))
+ pointer = S->getPointerOperand();
+ else if (FreeInst* F = dyn_cast<FreeInst>(BBI))
+ pointer = F->getPointerOperand();
+
+ assert(pointer && "Not a free or a store?");
+
+ 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) {
+
+ // 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;
+ } else {
+ dep = MD.getDependency(BBI, dep);
+ }
+ }
+ }
+
+ // Handle frees whose dependencies are non-trivial
+ if (FreeInst* F = dyn_cast<FreeInst>(BBI))
+ if (!deletedStore)
+ MadeChange |= handleFreeWithNonTrivialDependency(F,
+ MD.getDependency(F),
+ possiblyDead);
+
+ // Update our most-recent-store map
+ if (StoreInst* S = dyn_cast<StoreInst>(BBI))
+ last = S;
+ else
+ last = 0;
+ }
+ }
+
// 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);
- }
+ 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;
+}
- // 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, 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;
+
+ Value* depPointer = dependency->getPointerOperand();
+ const Type* depType = dependency->getOperand(0)->getType();
+ unsigned depPointerSize = TD.getTypeSize(depType);
+
+ // Check for aliasing
+ AliasAnalysis::AliasResult A = AA.alias(F->getPointerOperand(), ~0UL,
+ depPointer, depPointerSize);
+
+ if (A == AliasAnalysis::MustAlias) {
+ // Remove it!
+ MD.removeInstruction(dependency);
- bool MadeChange = false;
- for (BasicBlock::iterator BBI = BB.end(); BBI != BB.begin(); ) {
- Instruction *I = --BBI; // Keep moving iterator backwards
+ // 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);
- // 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);
+ dependency->eraseFromParent();
+ NumFastStores++;
+ return true;
+ }
+
+ return false;
+}
+
+/// handleEndBlock - Remove dead stores to stack-allocated locations in the
+/// function end block
+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*, 4> 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;
+
+ Value* killPointer = 0;
+ unsigned killPointerSize = 0;
+
+ // If we find a store whose pointer is dead...
+ if (StoreInst* S = dyn_cast<StoreInst>(BBI)) {
+ Value* pointerOperand = S->getPointerOperand();
+ // See through pointer-to-pointer bitcasts
+ TranslatePointerBitCasts(pointerOperand);
+
+ if (deadPointers.count(pointerOperand)){
+ // Remove it!
+ MD.removeInstruction(S);
+
+ // DCE instructions only used to calculate that store
+ if (Instruction* D = dyn_cast<Instruction>(S->getOperand(0)))
+ possiblyDead.insert(D);
+ if (Instruction* D = dyn_cast<Instruction>(S->getOperand(1)))
+ possiblyDead.insert(D);
+
+ BBI++;
+ S->eraseFromParent();
+ NumFastStores++;
+ MadeChange = true;
+ }
+
+ // If we encounter a use of the pointer, it is no longer considered dead
+ } else if (LoadInst* L = dyn_cast<LoadInst>(BBI)) {
+ killPointer = L->getPointerOperand();
+ killPointerSize = TD.getTypeSize(L->getType());
+ } else if (VAArgInst* V = dyn_cast<VAArgInst>(BBI)) {
+ killPointer = V->getOperand(0);
+ killPointerSize = TD.getTypeSize(V->getType());
+ } else if (FreeInst* F = dyn_cast<FreeInst>(BBI)) {
+ killPointer = F->getPointerOperand();
+ killPointerSize = ~0UL;
+ } else if (AllocaInst* A = dyn_cast<AllocaInst>(BBI)) {
+ deadPointers.erase(A);
continue;
- }
+ } else if (CallSite::get(BBI).getInstruction() != 0) {
+ // Remove any pointers made undead by the call from the dead set
+ std::vector<Instruction*> dead;
+ for (SmallPtrSet<AllocaInst*, 4>::iterator I = deadPointers.begin(),
+ E = deadPointers.end(); I != E; ++I) {
+ // Get size information for the alloca
+ unsigned pointerSize = ~0UL;
+ if (ConstantInt* C = dyn_cast<ConstantInt>((*I)->getArraySize()))
+ pointerSize = C->getZExtValue() * \
+ TD.getTypeSize((*I)->getAllocatedType());
+
+ // See if the call site touches it
+ AliasAnalysis::ModRefResult A = AA.getModRefInfo(CallSite::get(BBI),
+ *I, pointerSize);
+ if (A == AliasAnalysis::ModRef || A == AliasAnalysis::Ref)
+ dead.push_back(*I);
+ }
- 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();
- continue;
- }
+ for (std::vector<Instruction*>::iterator I = dead.begin(), E = dead.end();
+ I != E; ++I)
+ deadPointers.erase(*I);
- // 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;
}
+
+ if (!killPointer)
+ continue;
+
+ // Deal with undead pointers
+ MadeChange |= RemoveUndeadPointers(killPointer, killPointerSize, BBI,
+ deadPointers, possiblyDead);
+ }
+
+ 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;
- }
+bool DSE::RemoveUndeadPointers(Value* killPointer, unsigned killPointerSize,
+ BasicBlock::iterator& BBI,
+ SmallPtrSet<AllocaInst*, 4>& deadPointers,
+ SetVector<Instruction*>& possiblyDead) {
+ TargetData &TD = getAnalysis<TargetData>();
+ AliasAnalysis &AA = getAnalysis<AliasAnalysis>();
+ MemoryDependenceAnalysis& MD = getAnalysis<MemoryDependenceAnalysis>();
+
+ bool MadeChange = false;
+
+ std::vector<Instruction*> undead;
+
+ for (SmallPtrSet<AllocaInst*, 4>::iterator I = deadPointers.begin(),
+ E = deadPointers.end(); I != E; ++I) {
+ // Get size information for the alloca
+ unsigned pointerSize = ~0UL;
+ if (ConstantInt* C = dyn_cast<ConstantInt>((*I)->getArraySize()))
+ pointerSize = C->getZExtValue() * \
+ TD.getTypeSize((*I)->getAllocatedType());
+
+ // See if this pointer could alias it
+ AliasAnalysis::AliasResult A = AA.alias(*I, pointerSize,
+ killPointer, killPointerSize);
- // Otherwise, this is a non-dead store just add it to the set of dead
- // locations.
- KillLocs.add(cast<StoreInst>(I));
- BigContinue:;
- }
+ // If it must-alias and a store, we can delete it
+ if (isa<StoreInst>(BBI) && A == AliasAnalysis::MustAlias) {
+ StoreInst* S = cast<StoreInst>(BBI);
- while (!PotentiallyDeadInsts.empty()) {
- Instruction *I = PotentiallyDeadInsts.back();
- PotentiallyDeadInsts.pop_back();
- DeleteDeadInstructionChains(I, PotentiallyDeadInsts);
+ // Remove it!
+ MD.removeInstruction(S);
+
+ // DCE instructions only used to calculate that store
+ if (Instruction* D = dyn_cast<Instruction>(S->getOperand(0)))
+ possiblyDead.insert(D);
+ if (Instruction* D = dyn_cast<Instruction>(S->getOperand(1)))
+ possiblyDead.insert(D);
+
+ BBI++;
+ S->eraseFromParent();
+ NumFastStores++;
+ MadeChange = true;
+
+ continue;
+
+ // Otherwise, it is undead
+ } else if (A != AliasAnalysis::NoAlias)
+ undead.push_back(*I);
}
+
+ for (std::vector<Instruction*>::iterator I = undead.begin(), E = undead.end();
+ I != E; ++I)
+ deadPointers.erase(*I);
+
return MadeChange;
}
// 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);
+ // 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 (Instruction *Op = dyn_cast<Instruction>(I->getOperand(i)))
- DeadInsts.insert(Op); // Attempt to nuke it later.
+ 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();
- ++NumOther;
+ ++NumFastOther;
}
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