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 #include "llvm/Transforms/Scalar.h"
19 #include "llvm/DerivedTypes.h"
20 #include "llvm/Function.h"
21 #include "llvm/Instructions.h"
22 #include "llvm/Analysis/AliasAnalysis.h"
23 #include "llvm/Analysis/AliasSetTracker.h"
24 #include "llvm/Target/TargetData.h"
25 #include "llvm/Transforms/Utils/Local.h"
26 #include "llvm/ADT/SetVector.h"
27 #include "llvm/ADT/Statistic.h"
31 Statistic<> NumStores("dse", "Number of stores deleted");
32 Statistic<> NumOther ("dse", "Number of other instrs removed");
34 struct DSE : public FunctionPass {
36 virtual bool runOnFunction(Function &F) {
38 for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I)
39 Changed |= runOnBasicBlock(*I);
43 bool runOnBasicBlock(BasicBlock &BB);
45 void DeleteDeadInstructionChains(Instruction *I,
46 SetVector<Instruction*> &DeadInsts);
48 // getAnalysisUsage - We require post dominance frontiers (aka Control
50 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
52 AU.addRequired<TargetData>();
53 AU.addRequired<AliasAnalysis>();
54 AU.addPreserved<AliasAnalysis>();
57 RegisterOpt<DSE> X("dse", "Dead Store Elimination");
60 FunctionPass *llvm::createDeadStoreEliminationPass() { return new DSE(); }
62 bool DSE::runOnBasicBlock(BasicBlock &BB) {
63 TargetData &TD = getAnalysis<TargetData>();
64 AliasAnalysis &AA = getAnalysis<AliasAnalysis>();
65 AliasSetTracker KillLocs(AA);
67 // If this block ends in a return, unwind, unreachable, and eventually
68 // tailcall, then all allocas are dead at its end.
69 if (BB.getTerminator()->getNumSuccessors() == 0) {
70 BasicBlock *Entry = BB.getParent()->begin();
71 for (BasicBlock::iterator I = Entry->begin(), E = Entry->end(); I != E; ++I)
72 if (AllocaInst *AI = dyn_cast<AllocaInst>(I)) {
74 if (!AI->isArrayAllocation() &&
75 AI->getType()->getElementType()->isSized())
76 Size = (unsigned)TD.getTypeSize(AI->getType()->getElementType());
77 KillLocs.add(AI, Size);
81 // PotentiallyDeadInsts - Deleting dead stores from the program can make other
82 // instructions die if they were only used as operands to stores. Keep track
83 // of the operands to stores so that we can try deleting them at the end of
85 SetVector<Instruction*> PotentiallyDeadInsts;
87 bool MadeChange = false;
88 for (BasicBlock::iterator BBI = BB.end(); BBI != BB.begin(); ) {
89 Instruction *I = --BBI; // Keep moving iterator backwards
91 // If this is a free instruction, it makes the free'd location dead!
92 if (FreeInst *FI = dyn_cast<FreeInst>(I)) {
93 // Free instructions make any stores to the free'd location dead.
98 if (!isa<StoreInst>(I) || cast<StoreInst>(I)->isVolatile()) {
99 // If this is a non-store instruction, it makes everything referenced no
100 // longer killed. Remove anything aliased from the alias set tracker.
105 // If this is a non-volatile store instruction, and if it is already in
106 // the stored location is already in the tracker, then this is a dead
107 // store. We can just delete it here, but while we're at it, we also
108 // delete any trivially dead expression chains.
109 unsigned ValSize = (unsigned)TD.getTypeSize(I->getOperand(0)->getType());
110 Value *Ptr = I->getOperand(1);
112 if (AliasSet *AS = KillLocs.getAliasSetForPointerIfExists(Ptr, ValSize))
113 for (AliasSet::iterator ASI = AS->begin(), E = AS->end(); ASI != E; ++ASI)
114 if (ASI.getSize() >= ValSize && // Overwriting all of this store.
115 AA.alias(ASI.getPointer(), ASI.getSize(), Ptr, ValSize)
116 == AliasAnalysis::MustAlias) {
117 // If we found a must alias in the killed set, then this store really
118 // is dead. Remember that the various operands of the store now have
119 // fewer users. At the end we will see if we can delete any values
120 // that are dead as part of the store becoming dead.
121 if (Instruction *Op = dyn_cast<Instruction>(I->getOperand(0)))
122 PotentiallyDeadInsts.insert(Op);
123 if (Instruction *Op = dyn_cast<Instruction>(Ptr))
124 PotentiallyDeadInsts.insert(Op);
127 ++BBI; // Don't invalidate iterator.
128 BB.getInstList().erase(I); // Nuke the store!
134 // Otherwise, this is a non-dead store just add it to the set of dead
136 KillLocs.add(cast<StoreInst>(I));
140 while (!PotentiallyDeadInsts.empty()) {
141 Instruction *I = PotentiallyDeadInsts.back();
142 PotentiallyDeadInsts.pop_back();
143 DeleteDeadInstructionChains(I, PotentiallyDeadInsts);
148 void DSE::DeleteDeadInstructionChains(Instruction *I,
149 SetVector<Instruction*> &DeadInsts) {
150 // Instruction must be dead.
151 if (!I->use_empty() || !isInstructionTriviallyDead(I)) return;
153 // Let the alias analysis know that we have nuked a value.
154 getAnalysis<AliasAnalysis>().deleteValue(I);
156 // See if this made any operands dead. We do it this way in case the
157 // instruction uses the same operand twice. We don't want to delete a
158 // value then reference it.
159 for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i) {
160 if (Instruction *Op = dyn_cast<Instruction>(I->getOperand(i)))
161 DeadInsts.insert(Op); // Attempt to nuke it later.
162 I->setOperand(i, 0); // Drop from the operand list.
165 I->eraseFromParent();