text: // Copyright 2017 The Abseil Authors.
  //
  // Licensed under the Apache License, Version 2.0 (the "License");
  // you may not use this file except in compliance with the License.
  // You may obtain a copy of the License at
  //
  //      https://www.apache.org/licenses/LICENSE-2.0
  //
  // Unless required by applicable law or agreed to in writing, software
  // distributed under the License is distributed on an "AS IS" BASIS,
  // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  // See the License for the specific language governing permissions and
  // limitations under the License.
  //
  
  #ifndef ABSL_BASE_INTERNAL_ATOMIC_HOOK_H_
  #define ABSL_BASE_INTERNAL_ATOMIC_HOOK_H_
  
  #include <atomic>
  #include <cassert>
  #include <cstdint>
  #include <utility>
  
  #ifdef _MSC_FULL_VER
  #define ABSL_HAVE_WORKING_ATOMIC_POINTER 0
  #else
  #define ABSL_HAVE_WORKING_ATOMIC_POINTER 1
  #endif
  
  namespace absl {
  namespace base_internal {
  
  template <typename T>
  class AtomicHook;
  
  // AtomicHook is a helper class, templatized on a raw function pointer type, for
  // implementing Abseil customization hooks.  It is a callable object that
  // dispatches to the registered hook.
  //
  // A default constructed object performs a no-op (and returns a default
  // constructed object) if no hook has been registered.
  //
  // Hooks can be pre-registered via constant initialization, for example,
  // ABSL_CONST_INIT static AtomicHook<void(*)()> my_hook(DefaultAction);
  // and then changed at runtime via a call to Store().
  //
  // Reads and writes guarantee memory_order_acquire/memory_order_release
  // semantics.
  template <typename ReturnType, typename... Args>
  class AtomicHook<ReturnType (*)(Args...)> {
   public:
    using FnPtr = ReturnType (*)(Args...);
  
    // Constructs an object that by default performs a no-op (and
    // returns a default constructed object) when no hook as been registered.
    constexpr AtomicHook() : AtomicHook(DummyFunction) {}
  
    // Constructs an object that by default dispatches to/returns the
    // pre-registered default_fn when no hook has been registered at runtime.
  #if ABSL_HAVE_WORKING_ATOMIC_POINTER
    explicit constexpr AtomicHook(FnPtr default_fn)
        : hook_(default_fn), default_fn_(default_fn) {}
  #else
    explicit constexpr AtomicHook(FnPtr default_fn)
        : hook_(kUninitialized), default_fn_(default_fn) {}
  #endif
  
    // Stores the provided function pointer as the value for this hook.
    //
    // This is intended to be called once.  Multiple calls are legal only if the
    // same function pointer is provided for each call.  The store is implemented
    // as a memory_order_release operation, and read accesses are implemented as
    // memory_order_acquire.
    void Store(FnPtr fn) {
      bool success = DoStore(fn);
      static_cast<void>(success);
      assert(success);
    }
  
    // Invokes the registered callback.  If no callback has yet been registered, a
    // default-constructed object of the appropriate type is returned instead.
    template <typename... CallArgs>
    ReturnType operator()(CallArgs&&... args) const {
      return DoLoad()(std::forward<CallArgs>(args)...);
    }
  
    // Returns the registered callback, or nullptr if none has been registered.
    // Useful if client code needs to conditionalize behavior based on whether a
    // callback was registered.
    //
    // Note that atomic_hook.Load()() and atomic_hook() have different semantics:
    // operator()() will perform a no-op if no callback was registered, while
    // Load()() will dereference a null function pointer.  Prefer operator()() to
    // Load()() unless you must conditionalize behavior on whether a hook was
    // registered.
    FnPtr Load() const {
      FnPtr ptr = DoLoad();
      return (ptr == DummyFunction) ? nullptr : ptr;
    }
  
   private:
    static ReturnType DummyFunction(Args...) {
      return ReturnType();
    }
  
    // Current versions of MSVC (as of September 2017) have a broken
    // implementation of std::atomic<T*>:  Its constructor attempts to do the
    // equivalent of a reinterpret_cast in a constexpr context, which is not
    // allowed.
    //
    // This causes an issue when building with LLVM under Windows.  To avoid this,
    // we use a less-efficient, intptr_t-based implementation on Windows.
  #if ABSL_HAVE_WORKING_ATOMIC_POINTER
    // Return the stored value, or DummyFunction if no value has been stored.
    FnPtr DoLoad() const { return hook_.load(std::memory_order_acquire); }
  
    // Store the given value.  Returns false if a different value was already
    // stored to this object.
    bool DoStore(FnPtr fn) {
      assert(fn);
      FnPtr expected = default_fn_;
      const bool store_succeeded = hook_.compare_exchange_strong(
          expected, fn, std::memory_order_acq_rel, std::memory_order_acquire);
      const bool same_value_already_stored = (expected == fn);
      return store_succeeded || same_value_already_stored;
    }
  
    std::atomic<FnPtr> hook_;
  #else  // !ABSL_HAVE_WORKING_ATOMIC_POINTER
    // Use a sentinel value unlikely to be the address of an actual function.
    static constexpr intptr_t kUninitialized = 0;
  
    static_assert(sizeof(intptr_t) >= sizeof(FnPtr),
                  "intptr_t can't contain a function pointer");
  
    FnPtr DoLoad() const {
      const intptr_t value = hook_.load(std::memory_order_acquire);
      if (value == kUninitialized) {
        return default_fn_;
      }
      return reinterpret_cast<FnPtr>(value);
    }
  
    bool DoStore(FnPtr fn) {
      assert(fn);
      const auto value = reinterpret_cast<intptr_t>(fn);
      intptr_t expected = kUninitialized;
      const bool store_succeeded = hook_.compare_exchange_strong(
          expected, value, std::memory_order_acq_rel, std::memory_order_acquire);
      const bool same_value_already_stored = (expected == value);
      return store_succeeded || same_value_already_stored;
    }
  
    std::atomic<intptr_t> hook_;
  #endif
  
    const FnPtr default_fn_;
  };
  
  #undef ABSL_HAVE_WORKING_ATOMIC_POINTER
  
  }  // namespace base_internal
  }  // namespace absl
  
  #endif  // ABSL_BASE_INTERNAL_ATOMIC_HOOK_H_
  