[Bug c++/52558] write introduction incorrect wrt the C++11 memory model

["rguenther at suse dot de" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52558

--- Comment #7 from rguenther at suse dot de <rguenther at suse dot de> 2012-03-12 15:45:39 UTC ---
On Mon, 12 Mar 2012, aldyh at redhat dot com wrote:

> http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52558
> 
> --- Comment #6 from Aldy Hernandez <aldyh at redhat dot com> 2012-03-12 15:42:45 UTC ---
> On 03/12/12 10:32, rguenther at suse dot de wrote:
> es, but still cared about introducing write
> >> data races.  This test case has both.  I don't understand why we would allow
> >> introducing writes on paths that did not have it, but I will defer to you.
> >
> > Why should we avoid them if we know they cannot cause problems?  This
> > will happen for every loop where we do not know if it iterates at least
> > once.  Store-motion is a very important optimization.
> >
> > Richard.
> >
> 
> They won't cause problems in a single threaded environment, but will 
> cause problems in a multi threaded environment.  Even if you're writing 
> the value g_2 originally had, another thread may have written to g_2 
> right before.

Yes, that's clear to me.

> Just to get this straight, am I to assume that the default code 
> generation for GCC is a single threaded environment?  I just want to 
> make sure I get these variants right in my head, and can properly 
> separate what is and is not allowed in default GCC, and what only 
> pertains to the C11 memory model (or transactional stuff).

It certainly is, though we are getting more careful about this stuff
in recent years and generally only read data-races are ok.

Richard.