[Bug libstdc++/66146] call_once not C++11-compliant on ppc64le

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66146

--- Comment #20 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Anthony Williams from comment #16)
> Would it be worth ignoring pthread_once and using an implementation of
> call_once based on Mike Burrows' algorithm?
> 
> http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2007/n2444.html

That seems to work for pthread_once but not std::call_once, due to this
assumption:

  The number of pthread_once_t variables in a programme is bounded and can be
  counted in an instance of type T without overflow. If T is 32-bits, this
  implementation requires that the number of pthread_once_t variables in a
  programme not exceed 2**31-3. (Recall that pthread_once_t variables are
  intended to have static storage class, so it would be remarkable for such
  a huge number to exist.) 

Nothing prevents an unbounded number of std::once_flag objects in C++:

#include <mutex>

int main()
{
  while (true)
  {
    std::once_flag f;
    std::call_once(f, [](){});
  }
}