https://gcc.gnu.org/bugzilla/show_bug.cgi?id=55394
--- Comment #11 from Jonathan Wakely <redi at gcc dot gnu.org> --- It's not plausible because it doesn't work for non-pthreads targets where gthr-default.h is not gthr-posix.h We can't use pthread_once anyway, see PR 66146, so I'm rewriting it entirely in terms of either futexes or condition variables.
