1 //===- DeadStoreElimination.cpp - Dead Store Elimination ------------------===//
3 // The LLVM Compiler Infrastructure
5 // This file was developed by the LLVM research group and is distributed under
6 // the University of Illinois Open Source License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
10 // This file implements a trivial dead store elimination that only considers
11 // basic-block local redundant stores.
13 // FIXME: This should eventually be extended to be a post-dominator tree
14 // traversal. Doing so would be pretty trivial.
16 //===----------------------------------------------------------------------===//
18 #define DEBUG_TYPE "dse"
19 #include "llvm/Transforms/Scalar.h"
20 #include "llvm/DerivedTypes.h"
21 #include "llvm/Function.h"
22 #include "llvm/Instructions.h"
23 #include "llvm/Analysis/AliasAnalysis.h"
24 #include "llvm/Analysis/AliasSetTracker.h"
25 #include "llvm/Target/TargetData.h"
26 #include "llvm/Transforms/Utils/Local.h"
27 #include "llvm/ADT/SetVector.h"
28 #include "llvm/ADT/Statistic.h"
29 #include "llvm/Support/Compiler.h"
32 STATISTIC(NumStores, "Number of stores deleted");
33 STATISTIC(NumOther , "Number of other instrs removed");
36 struct VISIBILITY_HIDDEN DSE : public FunctionPass {
38 virtual bool runOnFunction(Function &F) {
40 for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I)
41 Changed |= runOnBasicBlock(*I);
45 bool runOnBasicBlock(BasicBlock &BB);
47 void DeleteDeadInstructionChains(Instruction *I,
48 SetVector<Instruction*> &DeadInsts);
50 // getAnalysisUsage - We require post dominance frontiers (aka Control
52 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
54 AU.addRequired<TargetData>();
55 AU.addRequired<AliasAnalysis>();
56 AU.addPreserved<AliasAnalysis>();
59 RegisterPass<DSE> X("dse", "Dead Store Elimination");
62 FunctionPass *llvm::createDeadStoreEliminationPass() { return new DSE(); }
64 bool DSE::runOnBasicBlock(BasicBlock &BB) {
65 TargetData &TD = getAnalysis<TargetData>();
66 AliasAnalysis &AA = getAnalysis<AliasAnalysis>();
67 AliasSetTracker KillLocs(AA);
69 // If this block ends in a return, unwind, unreachable, and eventually
70 // tailcall, then all allocas are dead at its end.
71 if (BB.getTerminator()->getNumSuccessors() == 0) {
72 BasicBlock *Entry = BB.getParent()->begin();
73 for (BasicBlock::iterator I = Entry->begin(), E = Entry->end(); I != E; ++I)
74 if (AllocaInst *AI = dyn_cast<AllocaInst>(I)) {
76 if (!AI->isArrayAllocation() &&
77 AI->getType()->getElementType()->isSized())
78 Size = (unsigned)TD.getTypeSize(AI->getType()->getElementType());
79 KillLocs.add(AI, Size);
83 // PotentiallyDeadInsts - Deleting dead stores from the program can make other
84 // instructions die if they were only used as operands to stores. Keep track
85 // of the operands to stores so that we can try deleting them at the end of
87 SetVector<Instruction*> PotentiallyDeadInsts;
89 bool MadeChange = false;
90 for (BasicBlock::iterator BBI = BB.end(); BBI != BB.begin(); ) {
91 Instruction *I = --BBI; // Keep moving iterator backwards
93 // If this is a free instruction, it makes the free'd location dead!
94 if (FreeInst *FI = dyn_cast<FreeInst>(I)) {
95 // Free instructions make any stores to the free'd location dead.
100 if (!isa<StoreInst>(I) || cast<StoreInst>(I)->isVolatile()) {
101 // If this is a vaarg instruction, it reads its operand. We don't model
102 // it correctly, so just conservatively remove all entries.
103 if (isa<VAArgInst>(I)) {
108 // If this is a non-store instruction, it makes everything referenced no
109 // longer killed. Remove anything aliased from the alias set tracker.
114 // If this is a non-volatile store instruction, and if it is already in
115 // the stored location is already in the tracker, then this is a dead
116 // store. We can just delete it here, but while we're at it, we also
117 // delete any trivially dead expression chains.
118 unsigned ValSize = (unsigned)TD.getTypeSize(I->getOperand(0)->getType());
119 Value *Ptr = I->getOperand(1);
121 if (AliasSet *AS = KillLocs.getAliasSetForPointerIfExists(Ptr, ValSize))
122 for (AliasSet::iterator ASI = AS->begin(), E = AS->end(); ASI != E; ++ASI)
123 if (ASI.getSize() >= ValSize && // Overwriting all of this store.
124 AA.alias(ASI.getPointer(), ASI.getSize(), Ptr, ValSize)
125 == AliasAnalysis::MustAlias) {
126 // If we found a must alias in the killed set, then this store really
127 // is dead. Remember that the various operands of the store now have
128 // fewer users. At the end we will see if we can delete any values
129 // that are dead as part of the store becoming dead.
130 if (Instruction *Op = dyn_cast<Instruction>(I->getOperand(0)))
131 PotentiallyDeadInsts.insert(Op);
132 if (Instruction *Op = dyn_cast<Instruction>(Ptr))
133 PotentiallyDeadInsts.insert(Op);
136 ++BBI; // Don't invalidate iterator.
137 BB.getInstList().erase(I); // Nuke the store!
143 // Otherwise, this is a non-dead store just add it to the set of dead
145 KillLocs.add(cast<StoreInst>(I));
149 while (!PotentiallyDeadInsts.empty()) {
150 Instruction *I = PotentiallyDeadInsts.back();
151 PotentiallyDeadInsts.pop_back();
152 DeleteDeadInstructionChains(I, PotentiallyDeadInsts);
157 void DSE::DeleteDeadInstructionChains(Instruction *I,
158 SetVector<Instruction*> &DeadInsts) {
159 // Instruction must be dead.
160 if (!I->use_empty() || !isInstructionTriviallyDead(I)) return;
162 // Let the alias analysis know that we have nuked a value.
163 getAnalysis<AliasAnalysis>().deleteValue(I);
165 // See if this made any operands dead. We do it this way in case the
166 // instruction uses the same operand twice. We don't want to delete a
167 // value then reference it.
168 for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i) {
169 if (Instruction *Op = dyn_cast<Instruction>(I->getOperand(i)))
170 DeadInsts.insert(Op); // Attempt to nuke it later.
171 I->setOperand(i, 0); // Drop from the operand list.
174 I->eraseFromParent();