Edit: The code here still has some bugs in it, and it could do better in the performance department, but instead of trying to fix this, for the record I took the problem over to the Intel discussion groups and got lots of great feedback, and if all goes well a polished version of Atomic float will be included in a near future release of Intel's Threading Building Blocks

好的，这是一个棘手的问题，我想要一个原子 float ，不是为了超快的图形性能，而是作为类的数据成员常规使用。而且我不想为在这些类上使用锁付出代价，因为它没有为我的需求提供额外的好处。

现在使用英特尔的 tbb 和我见过的其他原子库，支持整数类型，但不支持浮点。所以我继续实现了一个，它有效......但我不确定它是否真的有效，或者我很幸运它有效。

这里有人知道这不是某种形式的线程异端吗？

typedef unsigned int uint_32;

  struct AtomicFloat
  {
    private:
    tbb::atomic<uint_32> atomic_value_;

    public:
    template<memory_semantics M>
    float fetch_and_store( float value ) 
    {
        const uint_32 value_ = atomic_value_.tbb::atomic<uint_32>::fetch_and_store<M>((uint_32&)value);
        return reinterpret_cast<const float&>(value_);
    }

    float fetch_and_store( float value ) 
    {
        const uint_32 value_ = atomic_value_.tbb::atomic<uint_32>::fetch_and_store((uint_32&)value);
        return reinterpret_cast<const float&>(value_);
    }

    template<memory_semantics M>
    float compare_and_swap( float value, float comparand ) 
    {
        const uint_32 value_ = atomic_value_.tbb::atomic<uint_32>::compare_and_swap<M>((uint_32&)value,(uint_32&)compare);
        return reinterpret_cast<const float&>(value_);
    }

    float compare_and_swap(float value, float compare)
    {
        const uint_32 value_ = atomic_value_.tbb::atomic<uint_32>::compare_and_swap((uint_32&)value,(uint_32&)compare);
        return reinterpret_cast<const float&>(value_);
    }

    operator float() const volatile // volatile qualifier here for backwards compatibility 
    {
        const uint_32 value_ = atomic_value_;
        return reinterpret_cast<const float&>(value_);
    }

    float operator=(float value)
    {
        const uint_32 value_ = atomic_value_.tbb::atomic<uint_32>::operator =((uint_32&)value);
        return reinterpret_cast<const float&>(value_);
    }

    float operator+=(float value)
    {
        volatile float old_value_, new_value_;
        do
        {
            old_value_ = reinterpret_cast<float&>(atomic_value_);
            new_value_ = old_value_ + value;
        } while(compare_and_swap(new_value_,old_value_) != old_value_);
        return (new_value_);
    }

    float operator*=(float value)
    {
        volatile float old_value_, new_value_;
        do
        {
            old_value_ = reinterpret_cast<float&>(atomic_value_);
            new_value_ = old_value_ * value;
        } while(compare_and_swap(new_value_,old_value_) != old_value_);
        return (new_value_);
    }

    float operator/=(float value)
    {
        volatile float old_value_, new_value_;
        do
        {
            old_value_ = reinterpret_cast<float&>(atomic_value_);
            new_value_ = old_value_ / value;
        } while(compare_and_swap(new_value_,old_value_) != old_value_);
        return (new_value_);
    }

    float operator-=(float value)
    {
        return this->operator+=(-value);
    }

    float operator++() 
    {
        return this->operator+=(1);
    }

    float operator--() 
    {
        return this->operator+=(-1);
    }

    float fetch_and_add( float addend ) 
    {
        return this->operator+=(-addend);
    }

    float fetch_and_increment() 
    {
        return this->operator+=(1);
    }

    float fetch_and_decrement() 
    {
        return this->operator+=(-1);
    }
   };

谢谢!

编辑:按照 Greg Rogers 的建议将 size_t 更改为 uint32_t，这样更便携

编辑:为整个事物添加了列表，并进行了一些修复。

更多编辑: 性能方面，在我的机器上使用锁定 float 进行 5.000.000 += 操作和 100 个线程需要 3.6 秒，而我的原子 float 即使是愚蠢的 do-while 也需要 0.2 秒做同样的工作。因此，> 30 倍的性能提升意味着它是值得的，(这就是问题所在)如果它是正确的。

更多编辑: 正如 Awgn 指出的那样，我的 fetch_and_xxxx 部分都是错误的。修复了这个问题并删除了我不确定的部分 API(模板化内存模型)。并在运算符+=方面实现了其他操作，避免代码重复

添加:添加了运算符 *= 和运算符/=，因为没有它们， float 就不是 float 。感谢 Peterchen 的评论，注意到这一点

编辑:最新版本的代码如下(不过我会留下旧版本以供引用)

  #include <tbb/atomic.h>
  typedef unsigned int      uint_32;
  typedef __TBB_LONG_LONG       uint_64;

  template<typename FLOATING_POINT,typename MEMORY_BLOCK>
  struct atomic_float_
  {
    /*  CRC Card -----------------------------------------------------
    |   Class:          atmomic float template class
    |
    |   Responsability: handle integral atomic memory as it were a float,
    |                   but partially bypassing FPU, SSE/MMX, so it is
    |                   slower than a true float, but faster and smaller
    |                   than a locked float.
    |                       *Warning* If your float usage is thwarted by
    |                   the A-B-A problem this class isn't for you
    |                       *Warning* Atomic specification says we return,
    |                   values not l-values. So  (i = j) = k doesn't work.
    |
    |   Collaborators:  intel's tbb::atomic handles memory atomicity
    ----------------------------------------------------------------*/
    typedef typename atomic_float_<FLOATING_POINT,MEMORY_BLOCK> self_t;

    tbb::atomic<MEMORY_BLOCK> atomic_value_;

    template<memory_semantics M>
    FLOATING_POINT fetch_and_store( FLOATING_POINT value ) 
    {
        const MEMORY_BLOCK value_ = 
            atomic_value_.tbb::atomic<MEMORY_BLOCK>::fetch_and_store<M>((MEMORY_BLOCK&)value);
        //atomic specification requires returning old value, not new one
        return reinterpret_cast<const FLOATING_POINT&>(value_);
    }

    FLOATING_POINT fetch_and_store( FLOATING_POINT value ) 
    {
        const MEMORY_BLOCK value_ = 
            atomic_value_.tbb::atomic<MEMORY_BLOCK>::fetch_and_store((MEMORY_BLOCK&)value);
        //atomic specification requires returning old value, not new one
        return reinterpret_cast<const FLOATING_POINT&>(value_);
    }

    template<memory_semantics M>
    FLOATING_POINT compare_and_swap( FLOATING_POINT value, FLOATING_POINT comparand ) 
    {
        const MEMORY_BLOCK value_ = 
            atomic_value_.tbb::atomic<MEMORY_BLOCK>::compare_and_swap<M>((MEMORY_BLOCK&)value,(MEMORY_BLOCK&)compare);
        //atomic specification requires returning old value, not new one
        return reinterpret_cast<const FLOATING_POINT&>(value_);
    }

    FLOATING_POINT compare_and_swap(FLOATING_POINT value, FLOATING_POINT compare)
    {
        const MEMORY_BLOCK value_ = 
            atomic_value_.tbb::atomic<MEMORY_BLOCK>::compare_and_swap((MEMORY_BLOCK&)value,(MEMORY_BLOCK&)compare);
        //atomic specification requires returning old value, not new one
        return reinterpret_cast<const FLOATING_POINT&>(value_);
    }

    operator FLOATING_POINT() const volatile // volatile qualifier here for backwards compatibility 
    {
        const MEMORY_BLOCK value_ = atomic_value_;
        return reinterpret_cast<const FLOATING_POINT&>(value_);
    }

    //Note: atomic specification says we return the a copy of the base value not an l-value
    FLOATING_POINT operator=(FLOATING_POINT rhs) 
    {
        const MEMORY_BLOCK value_ = atomic_value_.tbb::atomic<MEMORY_BLOCK>::operator =((MEMORY_BLOCK&)rhs);
        return reinterpret_cast<const FLOATING_POINT&>(value_);
    }

    //Note: atomic specification says we return an l-value when operating among atomics
    self_t& operator=(self_t& rhs) 
    {
        const MEMORY_BLOCK value_ = atomic_value_.tbb::atomic<MEMORY_BLOCK>::operator =((MEMORY_BLOCK&)rhs);
        return *this;
    }

    FLOATING_POINT& _internal_reference() const
    {
        return reinterpret_cast<FLOATING_POINT&>(atomic_value_.tbb::atomic<MEMORY_BLOCK>::_internal_reference());
    }

    FLOATING_POINT operator+=(FLOATING_POINT value)
    {
        FLOATING_POINT old_value_, new_value_;
        do
        {
            old_value_ = reinterpret_cast<FLOATING_POINT&>(atomic_value_);
            new_value_ = old_value_ + value;
        //floating point binary representation is not an issue because
        //we are using our self's compare and swap, thus comparing floats and floats
        } while(self_t::compare_and_swap(new_value_,old_value_) != old_value_);
        return (new_value_); //return resulting value
    }

    FLOATING_POINT operator*=(FLOATING_POINT value)
    {
        FLOATING_POINT old_value_, new_value_;
        do
        {
            old_value_ = reinterpret_cast<FLOATING_POINT&>(atomic_value_);
            new_value_ = old_value_ * value;
        //floating point binary representation is not an issue becaus
        //we are using our self's compare and swap, thus comparing floats and floats
        } while(self_t::compare_and_swap(new_value_,old_value_) != old_value_);
        return (new_value_); //return resulting value
    }

    FLOATING_POINT operator/=(FLOATING_POINT value)
    {
        FLOATING_POINT old_value_, new_value_;
        do
        {
            old_value_ = reinterpret_cast<FLOATING_POINT&>(atomic_value_);
            new_value_ = old_value_ / value;
        //floating point binary representation is not an issue because
        //we are using our self's compare and swap, thus comparing floats and floats
        } while(self_t::compare_and_swap(new_value_,old_value_) != old_value_);
        return (new_value_); //return resulting value
    }

    FLOATING_POINT operator-=(FLOATING_POINT value)
    {
        return this->operator+=(-value); //return resulting value
    }

    //Prefix operator
    FLOATING_POINT operator++()
    {
        return this->operator+=(1); //return resulting value
    }

    //Prefix operator
    FLOATING_POINT operator--() 
    {
        return this->operator+=(-1); //return resulting value
    }

    //Postfix operator
    FLOATING_POINT operator++(int)
    {
        const FLOATING_POINT temp = this;
        this->operator+=(1);
        return temp//return resulting value
    }

    //Postfix operator
    FLOATING_POINT operator--(int) 
    {
        const FLOATING_POINT temp = this;
        this->operator+=(1);
        return temp//return resulting value
    }

    FLOATING_POINT fetch_and_add( FLOATING_POINT addend ) 
    {
        const FLOATING_POINT old_value_ = atomic_value_;
        this->operator+=(addend);
        //atomic specification requires returning old value, not new one as in operator x=
        return old_value_; 
    }

    FLOATING_POINT fetch_and_increment() 
    {
        const FLOATING_POINT old_value_ = atomic_value_;
        this->operator+=(+1);
        //atomic specification requires returning old value, not new one as in operator x=
        return old_value_; 
    }

    FLOATING_POINT fetch_and_decrement() 
    {
        const FLOATING_POINT old_value_ = atomic_value_;
        this->operator+=(-1);
        //atomic specification requires returning old value, not new one as in operator x=
        return old_value_; 
    }
  };

  typedef atomic_float_<float,uint_32> AtomicFloat;
  typedef atomic_float_<double,uint_64> AtomicDouble;

最佳答案

我强烈建议不要进行公共(public)继承。我不知道原子实现是什么样的，但我假设它具有将其用作整数类型的重载运算符，这意味着在许多(也许是大多数？)情况下，将使用这些提升而不是你的 float 。

我看不出有什么不可行的理由，但像你一样，我必须证明这一点......

注意:您的 operator float() 例程没有加载获取语义，不应该将其标记为 const volatile(或绝对至少是 const)吗？

编辑:如果你要提供 operator--() 你应该提供前缀/后缀形式。

关于c++ - 这个用于原子浮点的 C++ 实现安全吗？，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/242172/