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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.
//
//===----------------------------------------------------------------------===//
#include "llvm/Transforms/Scalar.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 "Support/Statistic.h"
using namespace llvm;
namespace {
Statistic<> NumStores("dse", "Number of stores deleted");
Statistic<> NumOther ("dse", "Number of other instrs removed");
struct DSE : public FunctionPass {
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 DeleteDeadValueChains(Value *V, AliasSetTracker &AST);
// getAnalysisUsage - We require post dominance frontiers (aka Control
// Dependence Graph)
virtual void getAnalysisUsage(AnalysisUsage &AU) const {
AU.addRequired<TargetData>();
AU.addRequired<AliasAnalysis>();
AU.addPreserved<AliasAnalysis>();
}
};
RegisterOpt<DSE> X("dse", "Dead Store Elimination");
}
Pass *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, and eventually tailcall/barrier,
// then all allocas are dead at its end.
if (BB.getTerminator()->getNumSuccessors() == 0) {
}
bool MadeChange = false;
for (BasicBlock::iterator BBI = BB.end(); BBI != BB.begin(); ) {
Instruction *I = --BBI; // Keep moving iterator backwards
#if 0
// AST doesn't support malloc/free/alloca???
if (isa<FreeInst>(I)) {
// Free instructions make any stores to the free'd location dead.
KillLocs.insert(I);
}
#endif
if (!isa<StoreInst>(I) || cast<StoreInst>(I)->isVolatile()) {
// 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 = 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 (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. Delete it now.
++BBI; // Don't invalidate iterator.
Value *Val = I->getOperand(0);
BB.getInstList().erase(I); // Nuke the store!
++NumStores;
DeleteDeadValueChains(Val, KillLocs); // Delete any now-dead instrs
DeleteDeadValueChains(Ptr, KillLocs); // Delete any now-dead instrs
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:;
}
return MadeChange;
}
void DSE::DeleteDeadValueChains(Value *V, AliasSetTracker &AST) {
// Value must be dead.
if (!V->use_empty()) return;
if (Instruction *I = dyn_cast<Instruction>(V))
if (isInstructionTriviallyDead(I)) {
AST.deleteValue(I);
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.
while (unsigned NumOps = I->getNumOperands()) {
Value *Op = I->getOperand(NumOps-1);
I->op_erase(I->op_end()-1); // Drop from the operand list.
DeleteDeadValueChains(Op, AST); // Attempt to nuke it.
}
I->getParent()->getInstList().erase(I);
++NumOther;
}
}
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