LoopUnswitch.cpp

/// it must not be used.
///
/// FIXME: When the loop optimizer is more mature, separate this out to a new
/// pass.
///
void LoopUnswitch::SimplifyCode(std::vector<Instruction*> &Worklist) {
  while (!Worklist.empty()) {
    Instruction *I = Worklist.back();
    Worklist.pop_back();
    
    // Simple constant folding.
    if (Constant *C = ConstantFoldInstruction(I)) {
      ReplaceUsesOfWith(I, C, Worklist);
      continue;
    }
    
    // Simple DCE.
    if (isInstructionTriviallyDead(I)) {
      DEBUG(std::cerr << "Remove dead instruction '" << *I);
      
      // Add uses to the worklist, which may be dead now.
      for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i)
        if (Instruction *Use = dyn_cast<Instruction>(I->getOperand(i)))
          Worklist.push_back(Use);
      I->eraseFromParent();
      RemoveFromWorklist(I, Worklist);
      ++NumSimplify;
      continue;
    }
    
    // Special case hacks that appear commonly in unswitched code.
    switch (I->getOpcode()) {
    case Instruction::Select:
      if (ConstantBool *CB = dyn_cast<ConstantBool>(I->getOperand(0))) {
        ReplaceUsesOfWith(I, I->getOperand(!CB->getValue()+1), Worklist);
        continue;
      }
      break;
    case Instruction::And:
      if (isa<ConstantBool>(I->getOperand(0)))   // constant -> RHS
        cast<BinaryOperator>(I)->swapOperands();
      if (ConstantBool *CB = dyn_cast<ConstantBool>(I->getOperand(1))) {
        if (CB->getValue())   // X & 1 -> X
          ReplaceUsesOfWith(I, I->getOperand(0), Worklist);
        else                  // X & 0 -> 0
          ReplaceUsesOfWith(I, I->getOperand(1), Worklist);
        continue;
      }
      break;
    case Instruction::Or:
      if (isa<ConstantBool>(I->getOperand(0)))   // constant -> RHS
        cast<BinaryOperator>(I)->swapOperands();
      if (ConstantBool *CB = dyn_cast<ConstantBool>(I->getOperand(1))) {
        if (CB->getValue())   // X | 1 -> 1
          ReplaceUsesOfWith(I, I->getOperand(1), Worklist);
        else                  // X | 0 -> X
          ReplaceUsesOfWith(I, I->getOperand(0), Worklist);
        continue;
      }
      break;
    case Instruction::Br: {
      BranchInst *BI = cast<BranchInst>(I);
      if (BI->isUnconditional()) {
        // If BI's parent is the only pred of the successor, fold the two blocks
        // together.
        BasicBlock *Pred = BI->getParent();
        BasicBlock *Succ = BI->getSuccessor(0);
        BasicBlock *SinglePred = Succ->getSinglePredecessor();
        if (!SinglePred) continue;  // Nothing to do.
        assert(SinglePred == Pred && "CFG broken");

        DEBUG(std::cerr << "Merging blocks: " << Pred->getName() << " <- " 
                        << Succ->getName() << "\n");
        
        // Resolve any single entry PHI nodes in Succ.
        while (PHINode *PN = dyn_cast<PHINode>(Succ->begin()))
          ReplaceUsesOfWith(PN, PN->getIncomingValue(0), Worklist);
        
        // Move all of the successor contents from Succ to Pred.
        Pred->getInstList().splice(BI, Succ->getInstList(), Succ->begin(),
                                   Succ->end());
        BI->eraseFromParent();
        RemoveFromWorklist(BI, Worklist);
        
        // If Succ has any successors with PHI nodes, update them to have
        // entries coming from Pred instead of Succ.
        Succ->replaceAllUsesWith(Pred);
        
        // Remove Succ from the loop tree.
        LI->removeBlock(Succ);
        Succ->eraseFromParent();
        ++NumSimplify;
      } else if (ConstantBool *CB = dyn_cast<ConstantBool>(BI->getCondition())){
        // Conditional branch.  Turn it into an unconditional branch, then
        // remove dead blocks.
        break;  // FIXME: Enable.

        DEBUG(std::cerr << "Folded branch: " << *BI);
        BasicBlock *DeadSucc = BI->getSuccessor(CB->getValue());
        BasicBlock *LiveSucc = BI->getSuccessor(!CB->getValue());
        DeadSucc->removePredecessor(BI->getParent(), true);
        Worklist.push_back(new BranchInst(LiveSucc, BI));
        BI->eraseFromParent();
        RemoveFromWorklist(BI, Worklist);
        ++NumSimplify;

        RemoveBlockIfDead(DeadSucc, Worklist);
      }
      break;
    }
    }
  }
}