> On Tue, 14 Apr 2020, Richard Sandiford wrote:
>
> > Richard Biener <rguent...@suse.de> writes:
> > > This makes same_type_for_tbaa_p conservative in the same way
> > > get_alias_set is about void * which we allow to alias all other
> > > pointers.
> > >
> > > Bootstrap & regtest running on x86_64-unknown-linux-gnu.
> > >
> > > Honza, is this what you had in mind?
> > >
> > > Thanks,
> > > Richard.
> > >
> > > 2020-04-14 Richard Biener <rguent...@suse.de>
> > >
> > > PR middle-end/94539
> > > * tree-ssa-alias.c (same_type_for_tbaa): Handle void *
> > > pointers the same as get_alias_set, returning -1.
> > >
> > > * gcc.dg/alias-14.c: Make dg-do run.
> > > ---
> > > gcc/testsuite/gcc.dg/alias-14.c | 2 +-
> > > gcc/tree-ssa-alias.c | 9 +++++++++
> > > 2 files changed, 10 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/gcc/testsuite/gcc.dg/alias-14.c
> > > b/gcc/testsuite/gcc.dg/alias-14.c
> > > index 1ca1c09d5e3..24f0d1c1168 100644
> > > --- a/gcc/testsuite/gcc.dg/alias-14.c
> > > +++ b/gcc/testsuite/gcc.dg/alias-14.c
> > > @@ -1,4 +1,4 @@
> > > -/* { dg-do compile } */
> > > +/* { dg-do run } */
> > > /* { dg-options "-O2" } */
> > > #include <stddef.h>
> > > void *a;
> > > diff --git a/gcc/tree-ssa-alias.c b/gcc/tree-ssa-alias.c
> > > index df9ba0de0d6..2850141303e 100644
> > > --- a/gcc/tree-ssa-alias.c
> > > +++ b/gcc/tree-ssa-alias.c
> > > @@ -831,6 +831,15 @@ same_type_for_tbaa (tree type1, tree type2)
> > > && TREE_CODE (type2) == ARRAY_TYPE)
> > > return -1;
> > >
> > > + /* void * is compatible with all other pointers. */
> > > + if (POINTER_TYPE_P (type1)
> > > + && POINTER_TYPE_P (type2)
> > > + && (TREE_CODE (TREE_TYPE (type1)) == VOID_TYPE
> > > + || TYPE_STRUCTURAL_EQUALITY_P (TREE_TYPE (type1))
> > > + || TREE_CODE (TREE_TYPE (type2)) == VOID_TYPE
> > > + || TYPE_STRUCTURAL_EQUALITY_P (TREE_TYPE (type2))))
> > > + return -1;
> > > +
> >
> > Could you add a comment about the TYPE_STRUCTURAL_EQUALITY_P checks?
> > Was surprised by them at first, since in aarch64 we use the flag to keep
> > a distinct ABI identity between ACLE vector types and other vector types.
> > I'm not sure we expected that to affect the aliasing rules between the
> > vector types and (say) arbitrary structures.
>
> I copied that from the get_alias_set handling:
>
> /* Make void * compatible with char * and also void **.
> Programs are commonly violating TBAA by this.
>
> We also make void * to conflict with every pointer
> (see record_component_aliases) and thus it is safe it to use it
> for
> pointers to types with TYPE_STRUCTURAL_EQUALITY_P. */
> if (TREE_CODE (p) == VOID_TYPE || TYPE_STRUCTURAL_EQUALITY_P (p))
> set = get_alias_set (ptr_type_node);
>
> and the reason is that while we keep struct S * and struct R * have
> different alias sets the case where 'S' requires structural equality
> means we have to use an alias-set that conflicts with any other
> pointer. I'll add a pointer to get_alias_set in the comment.
>
> Note that structures with TYPE_STRUCTURAL_EQUALITY_P use alias-set zero,
> thus have TBAA disabled. Note for vector types this only matters as
> of the pointer case (we're maybe missing to look one level deeper
> here), vector types themselves use the alias-set of the component type.
Thanks. I had same patch but failed to send it earlier. I believe it is
safe to return 1 since the types really are considered same by TBAA.
Honza
>
> Richard.
