为什么没有完全实现原子双重 [英] Why isn't atomic double fully implemented
问题描述
我的问题很简单.为什么std::atomic<double> 完全实现?我知道这与原子RMW(读-修改-写)访问有关.但我真的不明白，为什么在double上不可能做到这一点.
已指定可以使用任何普通可复制类型.当然，double就在其中.因此，C ++ 11要求您可以对任何类类型使用的基本操作(加载，存储，CAS，交换等).
但是，在整数上可以进行一组额外的操作(fetch_add，++，+=等).
A double与这些类型的区别很小.它是本机的，可复制的，等等.为什么标准不包括这些类型的double?
更新:C ++ 20确实将fc_add和sub专用于std::atomic<T>用于浮点类型. C ++ 20 std :: atomic< float>-std :: atomic< double> .specializations 但不是原子绝对值(AND)或取反(XOR).
编者注:如果没有C ++ 20，您可以将自己的软件从CAS中移出.参见原子双浮点或SSE/AVX向量在x86_64上加载/存储(用于可移植示例)； atomic<double>和float在大多数C ++实现中都是无锁的.
std::atomic<double>，它可以在C ++ 11的规则下工作.您可以执行加载和存储操作，并进行比较交换等.
该标准规定仅对整数类型"的原子提供算术运算(+，*，+ =，&等)，因此std::atomic<double>不会定义任何这些运算./p>
我的理解是，由于当今使用的硬件几乎不支持对浮点类型进行fetch-add或任何其他原子算术运算的支持，因此C ++标准未为其提供运算符，因为它们必须是实施效率低下.
(编辑).顺便说一句，VS2015RC中的std::atomic<double>是无锁的.
My question is quite simple. Why isn't std::atomic<double> implemented completely? I know it has to do with atomic RMW (read-modify-write) access. But I really don't see, why this shouldn't be possible on a double.
It's specified that any trivially copyable type can be used. And of course double is among them. So C++11 requires the basic operations (load, store, CAS, exchange, etc.) that you can use with any class type.
However, on integers an extra set of operations is possible (fetch_add, ++, +=, etc).
A double differs very little from these types. It's native, trivially copyable, etc. Why didn't the standard include the double with these types?
Update: C++20 does specialize std::atomic<T> for floating-point types, with fetch_add and sub. C++20 std::atomic<float>- std::atomic<double>.specializations But not atomic absolute-value (AND) or negate (XOR).
Editor's note: Without C++20 you can roll your own out of CAS; see Atomic double floating point or SSE/AVX vector load/store on x86_64 for portable examples; atomic<double> and float are lock-free on most C++ implementations.
std::atomic<double> is supported in the sense that you can create one in your program and it will work under the rules of C++11. You can perform loads and stores with it and do compare-exchange and the like.
The standard specifies that arithmetic operations (+, *, +=, &, etc.) are only provided for atomics of "integral types", so an std::atomic<double> won't have any of those operations defined.
My understanding is that, because there is little support for fetch-add or any other atomic arithmetic operations for floating point types in hardware in use today, the C++ standard doesn't provide the operators for them because they would have to be implemented inefficiently.
(edit). As an aside, std::atomic<double> in VS2015RC is lock-free.
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