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

[aldyh at gcc dot gnu.org]

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

--- Comment #4 from Aldy Hernandez <aldyh at gcc dot gnu.org> 2012-03-12 15:29:06 UTC ---

> No, we don't want to fix this for 4.7 as this is not a regression.
> 
> Yes, LIM only avoids introducing traps, not data-races.  This was discussed
> in the past already, btw, and we do not want to generally disallow this
> optimization.  [The C++ memory model is stupid here, it should not treat
> every variable raceable but only specially marked ones, oh well ...]
> 
> There will be very many other passes that are affected by this, and even more
> very many passes that will be affected by load data-races.
> 
> You will for example slow down SPEC CPU 2006 quite a bit (though technically
> it does not include C++11 benchmarks).

I thought we ignored *load* data races, 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.