[PATCH 2/4] Add atomic operations similar to those provided by C11.

[Torvald Riegel]

On Wed, 2014-10-29 at 22:00 +0000, Joseph S. Myers wrote:
> On Wed, 29 Oct 2014, Torvald Riegel wrote:
> 
> > This patch adds atomic operations similar to C11.
> > 
> > The function naming is essentially the C11 names, but with the memory
> > order argument removed and added as a suffix.  For example, C11's
> >   atomic_store_explicit(&foo, 23, memory_order_release)
> > becomes
> >   atomic_store_relaxed (&foo, 23);
> 
> As previously discussed, I'm concerned about the explicit relaxed loads 
> and stores being defined in terms of __atomic_* (see 
> <https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63273>).  Unless and until 
> __atomic_* implements relaxed atomics as plain loads/stores not inhibiting 
> optimization (as far as compatible with standard semantics), as evidenced 
> by the change not significantly changing code generated for uses in glibc, 
> I think the glibc implementation should be using plain loads and stores 
> rather than __atomic_*.

Yes, see Patch 4/4 for how this looks with pthread_once.  (You review
faster than I can write emails -- nice! :)
I've chosen to keep the builtins for relaxed loads/stores for now, but
we can revisit later when we have transformed more code.