SCCP.cpp

    // Now that all instructions in the function are constant folded, erase dead
    // blocks, because we can now use ConstantFoldTerminator to get rid of
    // in-edges.
    for (unsigned i = 0, e = BlocksToErase.size(); i != e; ++i) {
      // If there are any PHI nodes in this successor, drop entries for BB now.
      BasicBlock *DeadBB = BlocksToErase[i];
      for (Value::use_iterator UI = DeadBB->use_begin(), UE = DeadBB->use_end();
           UI != UE; ) {
        // Grab the user and then increment the iterator early, as the user
        // will be deleted. Step past all adjacent uses from the same user.
        Instruction *I = dyn_cast<Instruction>(*UI);
        do { ++UI; } while (UI != UE && *UI == I);

        // Ignore blockaddress users; BasicBlock's dtor will handle them.
        if (!I) continue;

        bool Folded = ConstantFoldTerminator(I->getParent());
        if (!Folded) {
          // The constant folder may not have been able to fold the terminator
          // if this is a branch or switch on undef.  Fold it manually as a
          // branch to the first successor.
#ifndef NDEBUG
          if (BranchInst *BI = dyn_cast<BranchInst>(I)) {
            assert(BI->isConditional() && isa<UndefValue>(BI->getCondition()) &&
                   "Branch should be foldable!");
          } else if (SwitchInst *SI = dyn_cast<SwitchInst>(I)) {
            assert(isa<UndefValue>(SI->getCondition()) && "Switch should fold");
          } else {
            llvm_unreachable("Didn't fold away reference to block!");
          }
#endif
          
          // Make this an uncond branch to the first successor.
          TerminatorInst *TI = I->getParent()->getTerminator();
          BranchInst::Create(TI->getSuccessor(0), TI);
          
          // Remove entries in successor phi nodes to remove edges.
          for (unsigned i = 1, e = TI->getNumSuccessors(); i != e; ++i)
            TI->getSuccessor(i)->removePredecessor(TI->getParent());
          
          // Remove the old terminator.
          TI->eraseFromParent();
        }
      }

      // Finally, delete the basic block.
      F->getBasicBlockList().erase(DeadBB);
    }
    BlocksToErase.clear();
  }

  // If we inferred constant or undef return values for a function, we replaced
  // all call uses with the inferred value.  This means we don't need to bother
  // actually returning anything from the function.  Replace all return
  // instructions with return undef.
  //
  // Do this in two stages: first identify the functions we should process, then
  // actually zap their returns.  This is important because we can only do this
  // if the address of the function isn't taken.  In cases where a return is the
  // last use of a function, the order of processing functions would affect
  // whether other functions are optimizable.
  SmallVector<ReturnInst*, 8> ReturnsToZap;
  
  // TODO: Process multiple value ret instructions also.
  const DenseMap<Function*, LatticeVal> &RV = Solver.getTrackedRetVals();
  for (DenseMap<Function*, LatticeVal>::const_iterator I = RV.begin(),
       E = RV.end(); I != E; ++I) {
    Function *F = I->first;
    if (I->second.isOverdefined() || F->getReturnType()->isVoidTy())
      continue;
  
    // We can only do this if we know that nothing else can call the function.
    if (!F->hasLocalLinkage() || AddressTakenFunctions.count(F))
      continue;
    
    for (Function::iterator BB = F->begin(), E = F->end(); BB != E; ++BB)
      if (ReturnInst *RI = dyn_cast<ReturnInst>(BB->getTerminator()))
        if (!isa<UndefValue>(RI->getOperand(0)))
          ReturnsToZap.push_back(RI);
  }

  // Zap all returns which we've identified as zap to change.
  for (unsigned i = 0, e = ReturnsToZap.size(); i != e; ++i) {
    Function *F = ReturnsToZap[i]->getParent()->getParent();
    ReturnsToZap[i]->setOperand(0, UndefValue::get(F->getReturnType()));
  }
    
  // If we inferred constant or undef values for globals variables, we can delete
  // the global and any stores that remain to it.
  const DenseMap<GlobalVariable*, LatticeVal> &TG = Solver.getTrackedGlobals();
  for (DenseMap<GlobalVariable*, LatticeVal>::const_iterator I = TG.begin(),
         E = TG.end(); I != E; ++I) {
    GlobalVariable *GV = I->first;
    assert(!I->second.isOverdefined() &&
           "Overdefined values should have been taken out of the map!");
    DEBUG(dbgs() << "Found that GV '" << GV->getName() << "' is constant!\n");
    while (!GV->use_empty()) {
      StoreInst *SI = cast<StoreInst>(GV->use_back());
      SI->eraseFromParent();
    }
    M.getGlobalList().erase(GV);
    ++IPNumGlobalConst;
  }

  return MadeChanges;
}