stack slot reuse

[Xinliang David Li <davidxl at google dot com>]

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?

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.

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.
>