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git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@15162 91177308-0d34-0410-b5e6-96231b3b80d8
141 lines
5.0 KiB
C++
141 lines
5.0 KiB
C++
//===- DeadStoreElimination.cpp - Dead Store Elimination ------------------===//
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//
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// The LLVM Compiler Infrastructure
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//
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// This file was developed by the LLVM research group and is distributed under
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// the University of Illinois Open Source License. See LICENSE.TXT for details.
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//
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//===----------------------------------------------------------------------===//
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//
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// This file implements a trivial dead store elimination that only considers
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// basic-block local redundant stores.
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//
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// FIXME: This should eventually be extended to be a post-dominator tree
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// traversal. Doing so would be pretty trivial.
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//
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//===----------------------------------------------------------------------===//
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#include "llvm/Transforms/Scalar.h"
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#include "llvm/Function.h"
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#include "llvm/Instructions.h"
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#include "llvm/Analysis/AliasAnalysis.h"
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#include "llvm/Analysis/AliasSetTracker.h"
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#include "llvm/Target/TargetData.h"
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#include "llvm/Transforms/Utils/Local.h"
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#include "Support/Statistic.h"
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using namespace llvm;
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namespace {
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Statistic<> NumStores("dse", "Number of stores deleted");
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Statistic<> NumOther ("dse", "Number of other instrs removed");
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struct DSE : public FunctionPass {
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virtual bool runOnFunction(Function &F) {
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bool Changed = false;
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for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I)
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Changed |= runOnBasicBlock(*I);
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return Changed;
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}
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bool runOnBasicBlock(BasicBlock &BB);
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void DeleteDeadValueChains(Value *V, AliasSetTracker &AST);
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// getAnalysisUsage - We require post dominance frontiers (aka Control
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// Dependence Graph)
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virtual void getAnalysisUsage(AnalysisUsage &AU) const {
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AU.setPreservesCFG();
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AU.addRequired<TargetData>();
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AU.addRequired<AliasAnalysis>();
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AU.addPreserved<AliasAnalysis>();
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}
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};
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RegisterOpt<DSE> X("dse", "Dead Store Elimination");
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}
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Pass *llvm::createDeadStoreEliminationPass() { return new DSE(); }
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bool DSE::runOnBasicBlock(BasicBlock &BB) {
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TargetData &TD = getAnalysis<TargetData>();
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AliasAnalysis &AA = getAnalysis<AliasAnalysis>();
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AliasSetTracker KillLocs(AA);
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// If this block ends in a return, unwind, and eventually tailcall/barrier,
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// then all allocas are dead at its end.
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if (BB.getTerminator()->getNumSuccessors() == 0) {
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}
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bool MadeChange = false;
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for (BasicBlock::iterator BBI = BB.end(); BBI != BB.begin(); ) {
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Instruction *I = --BBI; // Keep moving iterator backwards
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#if 0
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// AST doesn't support malloc/free/alloca???
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if (isa<FreeInst>(I)) {
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// Free instructions make any stores to the free'd location dead.
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KillLocs.insert(I);
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}
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#endif
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if (!isa<StoreInst>(I) || cast<StoreInst>(I)->isVolatile()) {
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// If this is a non-store instruction, it makes everything referenced no
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// longer killed. Remove anything aliased from the alias set tracker.
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KillLocs.remove(I);
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continue;
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}
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// If this is a non-volatile store instruction, and if it is already in
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// the stored location is already in the tracker, then this is a dead
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// store. We can just delete it here, but while we're at it, we also
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// delete any trivially dead expression chains.
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unsigned ValSize = TD.getTypeSize(I->getOperand(0)->getType());
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Value *Ptr = I->getOperand(1);
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if (AliasSet *AS = KillLocs.getAliasSetForPointerIfExists(Ptr, ValSize))
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for (AliasSet::iterator ASI = AS->begin(), E = AS->end(); ASI != E; ++ASI)
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if (AA.alias(ASI.getPointer(), ASI.getSize(), Ptr, ValSize)
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== AliasAnalysis::MustAlias) {
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// If we found a must alias in the killed set, then this store really
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// is dead. Delete it now.
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++BBI; // Don't invalidate iterator.
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Value *Val = I->getOperand(0);
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BB.getInstList().erase(I); // Nuke the store!
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++NumStores;
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DeleteDeadValueChains(Val, KillLocs); // Delete any now-dead instrs
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DeleteDeadValueChains(Ptr, KillLocs); // Delete any now-dead instrs
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MadeChange = true;
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goto BigContinue;
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}
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// Otherwise, this is a non-dead store just add it to the set of dead
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// locations.
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KillLocs.add(cast<StoreInst>(I));
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BigContinue:;
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}
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return MadeChange;
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}
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void DSE::DeleteDeadValueChains(Value *V, AliasSetTracker &AST) {
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// Value must be dead.
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if (!V->use_empty()) return;
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if (Instruction *I = dyn_cast<Instruction>(V))
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if (isInstructionTriviallyDead(I)) {
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AST.deleteValue(I);
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getAnalysis<AliasAnalysis>().deleteValue(I);
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// See if this made any operands dead. We do it this way in case the
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// instruction uses the same operand twice. We don't want to delete a
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// value then reference it.
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while (unsigned NumOps = I->getNumOperands()) {
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Value *Op = I->getOperand(NumOps-1);
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I->op_erase(I->op_end()-1); // Drop from the operand list.
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DeleteDeadValueChains(Op, AST); // Attempt to nuke it.
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}
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I->getParent()->getInstList().erase(I);
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++NumOther;
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}
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}
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