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Re: PATCH: tree-ssa-sink breaks stack layout


On Wed, Apr 1, 2009 at 3:58 PM, Daniel Berlin <dberlin@dberlin.org> wrote:
> On Wed, Apr 1, 2009 at 5:12 AM, Richard Guenther
> <richard.guenther@gmail.com> wrote:
>> On Wed, Apr 1, 2009 at 2:27 AM, Daniel Berlin <dberlin@dberlin.org> wrote:
>>> On Tue, Mar 31, 2009 at 6:04 PM, Eric Botcazou <ebotcazou@adacore.com> wrote:
>>>>> Well, no, it is a *real* bug that it claims the two objects
>>>>> must-conflict and decides they can share a space when we have no
>>>>> informatin available to substantiate this
>>>>
>>>> We're at -O1 so it's true that the objects must-conflict in the alias.c sense.
>>>
>>> alias.c uses conflict as a synonym for the word "alias" in almost all
>>> places, and there is no documentation elsewhere, so uh, claiming i'm
>>> "changing the definition" seems a bit much, that said ...
>>>
>>>> The code thinks that if the blocks are different and the objects must-conflict
>>>> then it's enough to conclude that the slots can be shared. ?That's wrong.
>>>>
>>>> You seem to be proposing to change the definition of must-conflict;
>>>> ?it seems
>>>> to me (and that was the conclusion of the discussion in PR middle-end/32327)
>>>> that the problem is rather in the "if the blocks are different".
>>> This is independent of the fact that it's clearly missing an aliasing
>>> conflict above.
>>> Then again, I have no dog in this fight, I can happily wait till you
>>> fix this with live range info and then discover it's still borked
>>> because it is using alias information wrong :)
>>
>> This alias sets are conflicting check was added for the very same reason - to
>> paper over bugs that may appear otherwise due to RTL scheduling of loads/stores
>> which may make life-ranges of stack variables which share their stack slot
>> overlapping. ?The theory is that the scheduler won't do such thing if the
>> alias sets conflict (which is of course not exactly true if you can disambiguate
>> using offsets, so you can still get overlapping lifetime of a whole struct just
>> not individual parts ... if that will break anything is another question).
>>
>
> All true, but look at this way:
>
> At O1, objects_must_conflict is returning 1 (causing it to not add an
> aliasing conflict and share the slots)
> At O2, objects_must_conflict is returning 0 (causing it to add an
> aliasing conflict and not share the slots)
>
> Either the objects conflict or they don't. Conservatively, it needs to
> return the answer that *doesn't* allow sharing, which it clearly is
> *not* doing.

The naming may be completely bogus, but this code only adds
"conflicts" in the sense that two variables that conflict may not share
stack slots.  This happens in the case if the two variables objects
are not "must conflict" (thus, their alias sets are not must conflict).
This is only to make sure that accesses to objects that share a stack
slot will return true for alias_sets_conflict_p () on any (sub-)accesses
so their lifetime will never be caused to overlap by RTL code motion.

At -O1 we have -fno-strict-aliasing and RTL will not perform any
code motion for memory operations (unless the alias-export patch
will come along - which is when all this will break down.  Yay.)

Richard.

> --Dan
>


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