bool made_changes = false;

    if (failed(CheckNoRecursion(module, getAnalysis<CallGraph>())))
      return failure();

    do {
      SymbolUserMap symbol_users(symbol_table_collection, module);

      made_changes = false;
      for (auto func :
           llvm::make_early_inc_range(module.getOps<func::FuncOp>())) {

// 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).
  llvm::DenseMap<func::FuncOp, llvm::DenseSet<func::FuncOp>> caller_callee_map;

  module.walk([&](Operation* op) {
    if (IsCommunicationOp(op)) {
      ops_with_tokens.insert(op);
      worklist.push_back(op);
    }
  });

  SymbolTableCollection table;
  SymbolUserMap symbol_map(table, module);

  // Regions that contains ops requiring token input attributes.
  absl::flat_hash_set<Region*> regions_with_token;
  while (!worklist.empty()) {
    Operation* op = worklist.back();

// the uses is as a while condition, an empty vector is returned.
SmallVector<TF::WhileOp> GetWhileCallers(func::FuncOp func,
                                         SymbolUserMap& symbol_map) {
  SmallVector<TF::WhileOp> while_callers;
  for (auto user : symbol_map.getUsers(func)) {
    if (auto while_caller = dyn_cast<TF::WhileOp>(user)) {

}

// Returns all functions that can be reached from TPUPartitionedCall ops.
SmallPtrSet<Operation*, 16> FindTPUPartitionedCallReachableFunctions(
    ModuleOp module) {
  SymbolTableCollection table;
  SymbolUserMap symbol_map(table, module);
  llvm::DenseMap<func::FuncOp, llvm::DenseSet<func::FuncOp>> caller_callee_map;
  // Creates work queue for determining reachability below.
  std::queue<func::FuncOp> function_worklist;

  // corresponds to a constant value.
  ValuePortResultMap results_;

  // Map from a function to the callers of that function.
  SymbolTableCollection symbol_table_;
  SymbolUserMap symbol_users_;

  // Queue of functions being processed.
  llvm::DenseSet<func::FuncOp> queue_set_;
  std::queue<func::FuncOp> queue_;

  int64_t graph_version_;