[Bug libstdc++/33578] __gnu_parallel::yield means what?

["redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=33578

--- Comment #10 from Jonathan Wakely <redi at gcc dot gnu.org> ---
For std::this_thread::yield() we do:

    inline void
    yield() noexcept
    {
#if defined _GLIBCXX_HAS_GTHREADS && defined _GLIBCXX_USE_SCHED_YIELD
      __gthread_yield();
#endif
    }

And gthr-win32.h has:

__GTHREAD_WIN32_INLINE int
__gthread_yield (void)
{
  Sleep (0);
  return 0;
}

For atomics we also support using the x86 pause instruction

    inline void
    __thread_yield() noexcept
    {
#if defined _GLIBCXX_HAS_GTHREADS && defined _GLIBCXX_USE_SCHED_YIELD
     __gthread_yield();
#endif
    }

    inline void
    __thread_relax() noexcept
    {
#if defined __i386__ || defined __x86_64__
      __builtin_ia32_pause();
#else
      __thread_yield();
#endif
    }



For Parallel Mode maybe we should just use __gthread_yield() everywhere?
gthr-win32.h already takes care of hiding the Win32 API. Since Parallel Mode is
likely to get removed at some point (now that we have the parallel STL algos
from C++17) I don't see any point in revisiting its implementation w.r.t using
of yield at all (comment 1).

tl;dr let's not reinvent the wheel when we already have portable code for
yielding, and don't want to redesign the parallel mode.