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

[aldyh at redhat dot com]

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.

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).

Thanks.