// ancestor is not a control flow op or a FuncOp, or of a single block region,
// an error will be returned.
LogicalResult GetControlFlowAncestors(
    Operation* op, llvm::SmallPtrSetImpl<Operation*>& control_flow_ops,
    llvm::SmallPtrSetImpl<Block*>& control_flow_blocks) {
  Block* block = op->getBlock();
  Operation* parent = block->getParentOp();
  while (block && parent && !isa<func::FuncOp>(parent)) {

}

// Visits an op's operand if it is an output of an Operation in the same
// tf_executor.graph.
void VisitOpOperand(GraphOp graph, Value operand,
                    llvm::SmallPtrSetImpl<Operation*>* reachable_ops,
                    llvm::SmallVectorImpl<Operation*>* ops_to_visit) {
  Operation* def = operand.getDefiningOp();
  if (def && def->getParentOp() == graph && reachable_ops->insert(def).second) {

 public:
  void runOnOperation() override;
};

void RecursiveRemove(Operation* op,
                     llvm::SmallVectorImpl<Operation*>& erase_list,
                     llvm::SmallPtrSetImpl<Operation*>& dead_ops) {
  for (mlir::Value res : op->getResults()) {
    for (Operation* user : res.getUsers()) {
      if (!dead_ops.insert(user).second) continue;
      RecursiveRemove(user, erase_list, dead_ops);

constexpr int kMaxIterations = 20;

// Populates `reachable_functions` with all functions that can be reached from
// device cluster ops.
void PopulateClusterReachableFunctions(
    ModuleOp module, SmallPtrSetImpl<Operation*>& reachable_functions) {
  SymbolTableCollection table;
  SymbolUserMap symbol_map(table, module);

  // Create map from caller to set of all callee(s).

  }
}

bool AddSpecialTpuOps(
    IslandOp candidate_island, llvm::StringRef cluster_name,
    llvm::SmallDenseMap<llvm::StringRef, llvm::SmallDenseSet<Operation*>>&
        cluster_to_tpu_op_map,
    SmallPtrSetImpl<Operation*>& visited_wrapped_ops, bool incoming) {
  std::queue<IslandOp> op_worklist;
  std::vector<IslandOp> ops;

  op_worklist.push(candidate_island);

  while (!op_worklist.empty()) {