Re: stack slot reuse

[Richard Guenther <richard dot guenther at gmail dot com>]

On Fri, May 21, 2010 at 7:30 PM, Xinliang David Li <davidxl@google.com> wrote:
> On Fri, May 21, 2010 at 2:24 AM, Richard Guenther
> <richard.guenther@gmail.com> wrote:
>> On Thu, May 20, 2010 at 11:21 PM, Xinliang David Li <davidxl@google.com> wrote:
>>> On Thu, May 20, 2010 at 2:18 PM, Steven Bosscher <stevenb.gcc@gmail.com> wrote:
>>>> On Thu, May 20, 2010 at 11:14 PM, Xinliang David Li <davidxl@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};
>>
>
> This looks like a very interesting approach. ?Do you see any downside
> of this approach? ?What is the problem of handling (nullifying) the
> dummy statement in expansion pass?

That is what I'd have done initially.  I could in theory see RTL
code motion optimizations move stuff in an invalid way after that
(but we try to avoid this by properly sharing TBAA compatible
slots only and fixing up points-to information as well).

So in the end it'll probably just work dropping the assignments
on the floor during expansion to RTL.

> The approach we took is different --- we move this overlay/packing
> earlier (after ipa-inlining). One of the other motivation for doing
> this is due to the limitation in current implementation that leaves
> out many overlaying opportunities (e.g. structs with union members can
> not share slots etc), but this is a probably independent issue.

Yes, one earlier idea would have unified stack slots at gimple lowering
time.  I'm not sure that after ipa-inlining is early enough (probably
it is due to the lack of code motion optimizations).

With the extra assignments I was also hoping to help analysis phases
to note that for example in

  {
    int a[10];
    foo (a);
   }
   bar ();

a is not live over the call to bar as it can't validly escape out of
its scope.

Thanks,
Richard.

> Thanks,
>
> David
>
>> 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?
>>
>> Richard.
>>
>