> 2010/5/21 Jan Hubicka <hubi...@ucw.cz>:
> >> On Thu, May 20, 2010 at 11:21 PM, Xinliang David Li <davi...@google.com>
> >> wrote:
> >> > On Thu, May 20, 2010 at 2:18 PM, Steven Bosscher <stevenb....@gmail.com>
> >> > wrote:
> >> >> On Thu, May 20, 2010 at 11:14 PM, Xinliang David Li
> >> >> <davi...@google.com> wrote:
> >> >>> stack variable overlay and stack slot assignments is here too.
> >> >>
> >> >> Yes, and for these I would like to add a separate timevar. Agree?
> >> >
> >> > Yes. (By the way, we are rewriting this pass to eliminate the code
> >> > motion/aliasing problem -- but that is a different topic).
> >>
> >> Btw, we want to address the same problem by representing the
> >> points where (big) variables go out-of scope in the IL, also to
> >> help DSE. The original idea was to simply drop in an aggregate
> >> assignment from an undefined value at the end of the scope
> >> during lowering, like
> >>
> >> var = {undefined};
> >>
> >> which we'd expand to nothing. Of course shifting the problem to
> >> the RTL optimizers, so better expand to a similar RTL construct.
> >> But then are you addressing the similar problem on the RTL side?
> >
> > Well, RTL already has the clobber insns, that are pretty much what you need
> > (and RTL land plays with this at least for double word arithmetic). But
> > I am not quite sure how much of DSE we want to deffer to RTL.
>
> But a clobber for (stack) memory objects doesn't work in the granularity
> we want it to be applied, does it?
Clobber takes memory expression that has alias info in it, so you can make it
specific to single var. But I guess problem will be that memory clobber
(unlike reg clobber) has by default the "may" interpretation (i.e. it may or
may not change the memory in random way).
Honza
