diff --git a/lib/Transforms/Scalar/DeadStoreElimination.cpp b/lib/Transforms/Scalar/DeadStoreElimination.cpp new file mode 100644 index 00000000000..b972dda3a1c --- /dev/null +++ b/lib/Transforms/Scalar/DeadStoreElimination.cpp @@ -0,0 +1,139 @@ +//===- 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(); + AU.addRequired(); + AU.addPreserved(); + } + }; + RegisterOpt X("dse", "Dead Store Elimination"); +} + +Pass *llvm::createDeadStoreEliminationPass() { return new DSE(); } + +bool DSE::runOnBasicBlock(BasicBlock &BB) { + TargetData &TD = getAnalysis(); + AliasAnalysis &AA = getAnalysis(); + 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(I)) { + // Free instructions make any stores to the free'd location dead. + KillLocs.insert(I); + } +#endif + + if (!isa(I) || cast(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(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(V)) + if (isInstructionTriviallyDead(I)) { + AST.deleteValue(I); + getAnalysis().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; + } +}