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Re: [PATCH 2/9] operand_equal_p: add support for FIELD_DECL


On Thu, Aug 15, 2019 at 4:17 PM Jan Hubicka <hubicka@ucw.cz> wrote:
>
> > On Tue, Aug 6, 2019 at 5:44 PM Martin Liska <mliska@suse.cz> wrote:
> > >
> > >
> > > gcc/ChangeLog:
> >
> > So I suppose this isn't to call operand_equal_p on two FIELD_DECLs
> > but to make two COMPONENT_REFs "more equal"?  If so I then
>
> yes. The patch originates from my original patchset I believe and it is
> what ICF does.
> > I suggest to make this change "local" to the COMPONENT_REF handling.
> > This also interacts with path-based disambiguation so you want to make
> > sure to only make things equal here iff it wouldn't change the outcome
> > of path-based analysis.  Honza?
>
> Indeed this can be handled as part of COMPONENT_REF match.
> Access path oracle here basically checks:
>  1) that MEM_REF type matches (we want predicate for this)
>  2) if it finds type match via same_type_for_tbaa and then it applies
>     the assumption about disjointness or overlap
>
> So I guess ideally we should
>
>  1) do matching part of COMPONENT_REF
>  2) compare OFFSET, BIT_OFFSET
>     This establishes that the access has same semantics.
>  3) for -fno-strict-aliasing be happy
>  4) for -fstrict-aliaisng check if access path applies (we should export
>     predicate from tree-ssa-alias as discussed earlier)
>  5) compare types by same_type_for_tbaa_p

Ick.  This smells like a layering violation to me.  IMHO this extended
equality handling should be handled with the overloading/callback
and not in native operand_equal_p.  Either on the level of the
COMPONENT_REF itself (sounds like that would be needed)
or the FIELD_DECL.  Not sure if the above suggestions make
it neccessary to look at more than a single COMPONENT_REF/FIELD_DECL
in the access path.  If so then watch out for quadraticness as operand_equal_p
traverses a reference chain...

Richard.

> Honza


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