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//===- DeadStoreElimination.cpp - 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 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/DerivedTypes.h"
#include "llvm/Function.h"
#include "llvm/Instructions.h"
#include "llvm/Analysis/AliasAnalysis.h"
#include "llvm/Analysis/AliasSetTracker.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");
namespace {
struct VISIBILITY_HIDDEN DSE : public FunctionPass {
static const char ID; // Pass identifcation, 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);
void DeleteDeadInstructionChains(Instruction *I,
SetVector<Instruction*> &DeadInsts);
// 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.addPreserved<AliasAnalysis>();
}
};
const 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);
// 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);
}
}
// 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;
bool MadeChange = false;
for (BasicBlock::iterator BBI = BB.end(); BBI != BB.begin(); ) {
Instruction *I = --BBI; // Keep moving iterator backwards
// 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);
continue;
}
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;
}
// 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 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;
}
// Otherwise, this is a non-dead store just add it to the set of dead
// locations.
KillLocs.add(cast<StoreInst>(I));
BigContinue:;
}
while (!PotentiallyDeadInsts.empty()) {
Instruction *I = PotentiallyDeadInsts.back();
PotentiallyDeadInsts.pop_back();
DeleteDeadInstructionChains(I, PotentiallyDeadInsts);
}
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.
}
I->eraseFromParent();
++NumOther;
}
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