Reassociate.cpp

  std::stable_sort(Ops.begin(), Ops.end());
  
  // OptimizeExpression - Now that we have the expression tree in a convenient
  // sorted form, optimize it globally if possible.
  if (Value *V = OptimizeExpression(I, Ops)) {
    // This expression tree simplified to something that isn't a tree,
    // eliminate it.
    DEBUG(dbgs() << "Reassoc to scalar: " << *V << '\n');
    I->replaceAllUsesWith(V);
    RemoveDeadBinaryOp(I);
    ++NumAnnihil;
    return V;
  }
  
  // We want to sink immediates as deeply as possible except in the case where
  // this is a multiply tree used only by an add, and the immediate is a -1.
  // In this case we reassociate to put the negation on the outside so that we
  // can fold the negation into the add: (-X)*Y + Z -> Z-X*Y
  if (I->getOpcode() == Instruction::Mul && I->hasOneUse() &&
      cast<Instruction>(I->use_back())->getOpcode() == Instruction::Add &&
      isa<ConstantInt>(Ops.back().Op) &&
      cast<ConstantInt>(Ops.back().Op)->isAllOnesValue()) {
    ValueEntry Tmp = Ops.pop_back_val();
    Ops.insert(Ops.begin(), Tmp);
  }
  
  DEBUG(dbgs() << "RAOut:\t"; PrintOps(I, Ops); dbgs() << '\n');
  
  if (Ops.size() == 1) {
    // This expression tree simplified to something that isn't a tree,
    // eliminate it.
    I->replaceAllUsesWith(Ops[0].Op);
    RemoveDeadBinaryOp(I);
    return Ops[0].Op;
  }
  
  // Now that we ordered and optimized the expressions, splat them back into
  // the expression tree, removing any unneeded nodes.
  RewriteExprTree(I, Ops);
  return I;
}


bool Reassociate::runOnFunction(Function &F) {
  // Recalculate the rank map for F
  BuildRankMap(F);

  MadeChange = false;
  for (Function::iterator FI = F.begin(), FE = F.end(); FI != FE; ++FI)
    ReassociateBB(FI);

  // We are done with the rank map.
  RankMap.clear();
  ValueRankMap.clear();
  return MadeChange;
}