Unifying std::atomic_int and std::atomic<int>

[Jonathan Wakely <jwakely at redhat dot com>]

Our <atomic> was implemented (by Benjamin IIRC) based on an early
C++0x draft when the spec was still trying to be valid for both C and
C++. Part of the C compatibility aspect was that std::atomic_int is
allowed to be either a typedef for std::atomic<int> or a base class of
it, so that a C library could define std::atomic_int and then the C++
library could make std::atomic<int> derive from that.

In the final C11 spec atomics work completely differently, and
atomic_int is a typedef for _Atomic int, which is not a valid base
class. So the old C++0x draft's compatibility aim is impossible,
atomic_int can never be the same type in C and C++.

In our implementation, std::atomic_int is a base class of
std::atomic<int>, which has no benefit I can see, but causes
https://gcc.gnu.org/PR60940

Rather than overloading every atomic_op() non-member function to
handle the derived class and the base class, it would be simpler to
just get rid of the base classes and make atomic_xxx a typedef for
atomic<xxx>, as the attached patch does for atomic_{bool,char,schar}.

Does anyone object to that change?

If you object, are you prepared to do the work to fix PR60940? :-)



[Note:- it could probably be simplified even further so atomic<char>
is just:

 template<>
   struct atomic<char> : public __atomic_base<char>
   {
     using __atomic_base<char>::__atomic_base;
   };

But that could be done later as it wouldn't change anything
observable, making atomic_char a typedef for atomic<char> is the
observable and IMHO important change. -end note]

Attachment: atomics.txt
Description: Text document