Merge Atomics branch into Master

[Jeffrey Walton]

Hi Everyone,

Crypto++ provides a Singleton class in misc.h. The original class suffered C++03 (and earlier) shortcomings. A design compromise was made to leak memory rather than using locks to increase portability.

We now have a portable implementation that does not depend on non-portable locks. It follows http://preshing.com/20130930/double-checked-locking-is-fixed-in-cpp11.

template <class T, class F, int instance>
  const T & Singleton<T, F, instance>::Ref(CRYPTOPP_NOINLINE_DOTDOTDOT) const
{
    static std::mutex s_mutex;
    static std::atomic<T*> s_pObject;

    T *p = s_pObject.load(std::memory_order_relaxed);
    std::atomic_thread_fence(std::memory_order_acquire);

    if (p)
        return *p;

    std::lock_guard<std::mutex> lock(s_mutex);
    p = s_pObject.load(std::memory_order_relaxed);
    std::atomic_thread_fence(std::memory_order_acquire);

    if (p)
        return *p;

    T *newObject = m_objectFactory();
    s_pObject.store(newObject, std::memory_order_relaxed);
    std::atomic_thread_fence(std::memory_order_release);

    return *newObject;
}

The implementation is guarded by both CRYPTOPP_CXX11_ATOMICS and CRYPTOPP_CXX11_SYNCHRONIZATION because atomics is not a proper subset of synchronization when it comes to compiler support and versions.

I'd like to merge it once it goes through a round of testing.

Are there any objections?

Jeff

[Jeffrey Walton]

Committed at http://github.com/weidai11/cryptopp/commit/e961c2da5b002c6e8601d88546e54c544794fdfd