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.
Richard.
