On Sat, 4 May 2024, Martin Uecker wrote:
> Am Freitag, dem 03.05.2024 um 21:16 +0200 schrieb Jakub Jelinek:
> > > On Fri, May 03, 2024 at 09:11:20PM +0200, Martin Uecker wrote:
> > > > > > > TYPE_CANONICAL as used by the middle-end cannot express this but
> > > > >
> > > > > Hm. so how does it work now for arrays?
> > >
> > > Do you have a testcase which doesn't work correctly with the arrays?
>
> I am mostly trying to understand better how this works. But
> if I am not mistaken, the following example would indeed
> indicate that we do incorrect aliasing decisions for types
> derived from arrays:
>
> https://godbolt.org/z/rTsE3PhKc
This example is about pointer-to-array types, int (*)[2] and
int (*)[1] are supposed to be compatible as in receive the same alias
set. This is ensured by get_alias_set POINTER_TYPE_P handling,
the alias set is supposed to be the same as that of int *. It seems
we do restrict the handling a bit, the code does
/* Unnest all pointers and references.
We also want to make pointer to array/vector equivalent to
pointer to
its element (see the reasoning above). Skip all those types, too.
*/
for (p = t; POINTER_TYPE_P (p)
|| (TREE_CODE (p) == ARRAY_TYPE
&& (!TYPE_NONALIASED_COMPONENT (p)
|| !COMPLETE_TYPE_P (p)
|| TYPE_STRUCTURAL_EQUALITY_P (p)))
|| TREE_CODE (p) == VECTOR_TYPE;
p = TREE_TYPE (p))
where the comment doesn't exactly match the code - but C should
never have TYPE_NONALIASED_COMPONENT (p).
But maybe I misread the example or it goes wrong elsewhere.
Richard.
> Martin
>
> > >
> > > E.g. same_type_for_tbaa has
> > > type1 = TYPE_MAIN_VARIANT (type1);
> > > type2 = TYPE_MAIN_VARIANT (type2);
> > >
> > > /* Handle the most common case first. */
> > > if (type1 == type2)
> > > return 1;
> > >
> > > /* If we would have to do structural comparison bail out. */
> > > if (TYPE_STRUCTURAL_EQUALITY_P (type1)
> > > || TYPE_STRUCTURAL_EQUALITY_P (type2))
> > > return -1;
> > >
> > > /* Compare the canonical types. */
> > > if (TYPE_CANONICAL (type1) == TYPE_CANONICAL (type2))
> > > return 1;
> > >
> > > /* ??? Array types are not properly unified in all cases as we have
> > > spurious changes in the index types for example. Removing this
> > > causes all sorts of problems with the Fortran frontend. */
> > > if (TREE_CODE (type1) == ARRAY_TYPE
> > > && TREE_CODE (type2) == ARRAY_TYPE)
> > > return -1;
> > > ...
> > > and later compares alias sets and the like.
> > > So, even if int[] and int[0] have different TYPE_CANONICAL, they
> > > will be considered maybe the same. Also, guess get_alias_set
> > > has some ARRAY_TYPE handling...
> > >
> > > Anyway, I think we should just go with Richi's patch.
> > >
> > > Jakub
> > >
>
>
>
--
Richard Biener <[email protected]>
SUSE Software Solutions Germany GmbH,
Frankenstrasse 146, 90461 Nuernberg, Germany;
GF: Ivo Totev, Andrew McDonald, Werner Knoblich; (HRB 36809, AG Nuernberg)
