LoopStrengthReduce.cpp

/// Rewrite - Emit instructions for the leading candidate expression for this
/// LSRUse (this is called "expanding"), and update the UserInst to reference
/// the newly expanded value.
void LSRInstance::Rewrite(const LSRFixup &LF,
                          const Formula &F,
                          Loop *L, Instruction *IVIncInsertPos,
                          SCEVExpander &Rewriter,
                          SmallVectorImpl<WeakVH> &DeadInsts,
                          ScalarEvolution &SE, DominatorTree &DT,
                          Pass *P) const {
  // First, find an insertion point that dominates UserInst. For PHI nodes,
  // find the nearest block which dominates all the relevant uses.
  if (PHINode *PN = dyn_cast<PHINode>(LF.UserInst)) {
    RewriteForPHI(PN, LF, F, L, IVIncInsertPos, Rewriter, DeadInsts, SE, DT, P);
  } else {
    Value *FullV = Expand(LF, F, LF.UserInst, L, IVIncInsertPos,
                          Rewriter, DeadInsts, SE, DT);

    // If this is reuse-by-noop-cast, insert the noop cast.
    const Type *OpTy = LF.OperandValToReplace->getType();
    if (FullV->getType() != OpTy) {
      Instruction *Cast =
        CastInst::Create(CastInst::getCastOpcode(FullV, false, OpTy, false),
                         FullV, OpTy, "tmp", LF.UserInst);
      FullV = Cast;
    }

    // Update the user. ICmpZero is handled specially here (for now) because
    // Expand may have updated one of the operands of the icmp already, and
    // its new value may happen to be equal to LF.OperandValToReplace, in
    // which case doing replaceUsesOfWith leads to replacing both operands
    // with the same value. TODO: Reorganize this.
    if (Uses[LF.LUIdx].Kind == LSRUse::ICmpZero)
      LF.UserInst->setOperand(0, FullV);
    else
      LF.UserInst->replaceUsesOfWith(LF.OperandValToReplace, FullV);
  }

  DeadInsts.push_back(LF.OperandValToReplace);
}

void
LSRInstance::ImplementSolution(const SmallVectorImpl<const Formula *> &Solution,
                               Pass *P) {
  // Keep track of instructions we may have made dead, so that
  // we can remove them after we are done working.
  SmallVector<WeakVH, 16> DeadInsts;

  SCEVExpander Rewriter(SE);
  Rewriter.disableCanonicalMode();
  Rewriter.setIVIncInsertPos(L, IVIncInsertPos);

  // Expand the new value definitions and update the users.
  for (size_t i = 0, e = Fixups.size(); i != e; ++i) {
    size_t LUIdx = Fixups[i].LUIdx;

    Rewrite(Fixups[i], *Solution[LUIdx], L, IVIncInsertPos, Rewriter,
            DeadInsts, SE, DT, P);

    Changed = true;
  }

  // Clean up after ourselves. This must be done before deleting any
  // instructions.
  Rewriter.clear();

  Changed |= DeleteTriviallyDeadInstructions(DeadInsts);
}

LSRInstance::LSRInstance(const TargetLowering *tli, Loop *l, Pass *P)
  : IU(P->getAnalysis<IVUsers>()),
    SE(P->getAnalysis<ScalarEvolution>()),
    DT(P->getAnalysis<DominatorTree>()),
    TLI(tli), L(l), Changed(false), IVIncInsertPos(0) {

  // If LoopSimplify form is not available, stay out of trouble.
  if (!L->isLoopSimplifyForm()) return;

  // If there's no interesting work to be done, bail early.
  if (IU.empty()) return;

  DEBUG(dbgs() << "\nLSR on loop ";
        WriteAsOperand(dbgs(), L->getHeader(), /*PrintType=*/false);
        dbgs() << ":\n");

  /// OptimizeShadowIV - If IV is used in a int-to-float cast
  /// inside the loop then try to eliminate the cast opeation.
  OptimizeShadowIV();

  // Change loop terminating condition to use the postinc iv when possible.
  Changed |= OptimizeLoopTermCond();

  CollectInterestingTypesAndFactors();
  CollectFixupsAndInitialFormulae();
  CollectLoopInvariantFixupsAndFormulae();

  DEBUG(dbgs() << "LSR found " << Uses.size() << " uses:\n";
        print_uses(dbgs()));

  // Now use the reuse data to generate a bunch of interesting ways
  // to formulate the values needed for the uses.
  GenerateAllReuseFormulae();

  DEBUG(dbgs() << "\n"
                  "After generating reuse formulae:\n";
        print_uses(dbgs()));

  FilterOutUndesirableDedicatedRegisters();
  NarrowSearchSpaceUsingHeuristics();

  SmallVector<const Formula *, 8> Solution;
  Solve(Solution);
  assert(Solution.size() == Uses.size() && "Malformed solution!");

  // Release memory that is no longer needed.
  Factors.clear();
  Types.clear();
  RegUses.clear();

#ifndef NDEBUG
  // Formulae should be legal.
  for (SmallVectorImpl<LSRUse>::const_iterator I = Uses.begin(),
       E = Uses.end(); I != E; ++I) {
     const LSRUse &LU = *I;
     for (SmallVectorImpl<Formula>::const_iterator J = LU.Formulae.begin(),
          JE = LU.Formulae.end(); J != JE; ++J)
        assert(isLegalUse(J->AM, LU.MinOffset, LU.MaxOffset,
                          LU.Kind, LU.AccessTy, TLI) &&
               "Illegal formula generated!");
  };
#endif

  // Now that we've decided what we want, make it so.
  ImplementSolution(Solution, P);
}

void LSRInstance::print_factors_and_types(raw_ostream &OS) const {
  if (Factors.empty() && Types.empty()) return;

  OS << "LSR has identified the following interesting factors and types: ";
  bool First = true;

  for (SmallSetVector<int64_t, 8>::const_iterator
       I = Factors.begin(), E = Factors.end(); I != E; ++I) {
    if (!First) OS << ", ";
    First = false;
    OS << '*' << *I;
  }

  for (SmallSetVector<const Type *, 4>::const_iterator
       I = Types.begin(), E = Types.end(); I != E; ++I) {
    if (!First) OS << ", ";
    First = false;
    OS << '(' << **I << ')';
  }
  OS << '\n';
}

void LSRInstance::print_fixups(raw_ostream &OS) const {
  OS << "LSR is examining the following fixup sites:\n";
  for (SmallVectorImpl<LSRFixup>::const_iterator I = Fixups.begin(),
       E = Fixups.end(); I != E; ++I) {
    const LSRFixup &LF = *I;
    dbgs() << "  ";
    LF.print(OS);
    OS << '\n';
  }
}

void LSRInstance::print_uses(raw_ostream &OS) const {
  OS << "LSR is examining the following uses:\n";
  for (SmallVectorImpl<LSRUse>::const_iterator I = Uses.begin(),
       E = Uses.end(); I != E; ++I) {
    const LSRUse &LU = *I;
    dbgs() << "  ";
    LU.print(OS);
    OS << '\n';
    for (SmallVectorImpl<Formula>::const_iterator J = LU.Formulae.begin(),
         JE = LU.Formulae.end(); J != JE; ++J) {
      OS << "    ";
      J->print(OS);
      OS << '\n';
    }
  }
}

void LSRInstance::print(raw_ostream &OS) const {
  print_factors_and_types(OS);
  print_fixups(OS);
  print_uses(OS);
}

void LSRInstance::dump() const {
  print(errs()); errs() << '\n';
}

namespace {

class LoopStrengthReduce : public LoopPass {
  /// TLI - Keep a pointer of a TargetLowering to consult for determining
  /// transformation profitability.
  const TargetLowering *const TLI;

public:
  static char ID; // Pass ID, replacement for typeid
  explicit LoopStrengthReduce(const TargetLowering *tli = 0);

private:
  bool runOnLoop(Loop *L, LPPassManager &LPM);
  void getAnalysisUsage(AnalysisUsage &AU) const;
};

}

char LoopStrengthReduce::ID = 0;
static RegisterPass<LoopStrengthReduce>
X("loop-reduce", "Loop Strength Reduction");

Pass *llvm::createLoopStrengthReducePass(const TargetLowering *TLI) {
  return new LoopStrengthReduce(TLI);
}

LoopStrengthReduce::LoopStrengthReduce(const TargetLowering *tli)
  : LoopPass(&ID), TLI(tli) {}

void LoopStrengthReduce::getAnalysisUsage(AnalysisUsage &AU) const {
  // We split critical edges, so we change the CFG.  However, we do update
  // many analyses if they are around.
  AU.addPreservedID(LoopSimplifyID);
  AU.addPreserved<LoopInfo>();
  AU.addPreserved("domfrontier");

  AU.addRequiredID(LoopSimplifyID);
  AU.addRequired<DominatorTree>();
  AU.addPreserved<DominatorTree>();
  AU.addRequired<ScalarEvolution>();
  AU.addPreserved<ScalarEvolution>();
  AU.addRequired<IVUsers>();
  AU.addPreserved<IVUsers>();
}

bool LoopStrengthReduce::runOnLoop(Loop *L, LPPassManager & /*LPM*/) {
  bool Changed = false;

  // Run the main LSR transformation.
  Changed |= LSRInstance(TLI, L, this).getChanged();

  // At this point, it is worth checking to see if any recurrence PHIs are also
  // dead, so that we can remove them as well.
  Changed |= DeleteDeadPHIs(L->getHeader());

  return Changed;
}