//
    // func.func @f(...) -> ... {
    //     yield %x : foo
    // }
    // ...
    // func.call f(...) -> foo
    auto symbol_uses = f.getSymbolUses(f->getParentOp());
    if (!symbol_uses.has_value()) {
      return failure();
    }
    for (auto use : *symbol_uses) {
      Operation* caller = use.getUser();
      bool changed = false;
      rewriter.startOpModification(caller);

  // 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_;