[Bug target/65697] __atomic memory barriers not strong enough for __sync builtins

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65697

--- Comment #42 from Richard Henderson <rth at gcc dot gnu.org> ---
(In reply to Andrew Macleod from comment #39)
> no, __sync was simply an implementation of psABI back when it was new... I'm
> not aware of any additions, enhancements or guarantees that were added when
> it was ported to other arch's.  
> 
> Terminology was much looser 14 years ago :-)  That's one of the reasons we
> want to migrate to __atomic... it is supposedly more precisely defined,
> whereas __sync had some hand-waving.  We're now experiencing some different
> interpretations of that.    Regardless of the documentation, we didn't think
> we'd be supporting something stronger than SEQ_CST since they were suppose
> to be equivalent... 

Exactly right.  I don't believe there's anywhere we can look for 
more definitive semantics than the psABI.  And as already explored
here, that's not entirely helpful.