Re: stack slot reuse

[Easwaran Raman <eraman at google dot com>]

On Wed, May 26, 2010 at 8:42 AM, Xinliang David Li <davidxl@google.com> wrote:
> On Wed, May 26, 2010 at 2:58 AM, Richard Guenther
> <richard.guenther@gmail.com> wrote:
>> On Tue, May 25, 2010 at 10:02 PM, Easwaran Raman <eraman@google.com> wrote:
>>> On Fri, May 21, 2010 at 10:30 AM, 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};
>>>> >
>>>>
>>>
>>> Is there something that prevents store sinking (or similar passes)
>>> from moving this 'var = {undefined};' statement outside the scope? Or
>>> should store sinking be taught to treat this as a barrier?
>>
>> Not at the moment (if indeed that assignment looks as a regular one).
>> Passes should be taught that it's not worthwhile to sink a
>> no-op. ?IIRC no pass currently would sink aggregate copies anyway.
>
> Other issues to consider: 1) how does it affect SRA decisions? 2)
> inline summary also needs to be taught to not include size of those
> fake instructions; 3) why only aggregates? For scalars that live in
> stack, they also need barriers if slot sharing pick them as
> candidates, etc.
>
>
>>
>>>> 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).
>>>
>>> To elaborate further, we use the current stack-slot sharing heuristics
>>> in cfgexpand.c to decide what variables can share stack slots,
>>> synthesize union variables with those variables as fields and replace
>>> references to those variables with field references. We have an
>>> initial implementation and are evaluating the performance impact of
>>> making the sharing decisions early.
>>
>> Note that using union variables will pessimize alias analysis
>> as we allow type-punning with unions. ?How do you address
>> the issue of debug information?
>>
>
> Debug information is handled. Easwaran can fill in the details.

We retain the original VAR_DECL even after synthesizing the union
variables and keep a map between the synthesized union variable to the
set of original variables corresponding to the synthesized union. In
cfgexpand.c, when RTL is generated for the synthesized variable, we
set the RTL to all of the original variables using SET_DECL_RTL
instead of setting the RTL to the synthesized union variable.


thanks,
Easwaran

>
> Thanks,
>
> David
>
>> Some time ago I had the very simple idea to merge identically
>> typed variables that do not have overlapping life-ranges into
>> a single variable (avoiding the union issue). ?That would not
>> catch all cases cfgexpand catches but may even re-use
>> common initializations. ?Of course the debug information
>> issues would be the same.
>>
>> I think we want the clobbering stores anyway, for optimization
>> purposes, even if we do not end up using them for the
>> stack slot sharing problem.
>>
>> Richard.
>>
>