[Bug libstdc++/56538] no way to disable shared_ptr's use of atomics

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=56538

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Should be fixed with glibc 2.32 by checking __libc_single_threaded before using
atomics in shared_ptr.

I don't understand why the original example would use atomics anyway though,
__gthread_once should have been false. I think what this example is showing is
not that the atomics are expensive, but that the compiler can optimise away the
dispatching code if both branches use the same instructions.