llvm/lib/Transforms/Scalar/DeadStoreElimination.cpp

//===- 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.setPreservesCFG();
      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<FreeInst>(I) &&
        (!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;
    Value *Ptr;
    if (isa<StoreInst>(I)) {
      Ptr = I->getOperand(1);
      ValSize = TD.getTypeSize(I->getOperand(0)->getType());
    } else {
      Ptr = I->getOperand(0);
      ValSize = ~0;
    }

    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(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;
    }
}