Add access through parameter derference tracking to modref

[Richard Biener]

On Thu, Sep 24, 2020 at 12:54 PM Jan Hubicka <hubicka@ucw.cz> wrote:
>
> > > +  else if (TREE_CODE (op) == SSA_NAME
> > > +          && POINTER_TYPE_P (TREE_TYPE (op)))
> > > +    {
> > > +      if (DECL_P (base) && !ptr_deref_may_alias_decl_p (op, base))
> > > +       return false;
> > > +      if (TREE_CODE (base) == SSA_NAME
> > > +         && !ptr_derefs_may_alias_p (op, base))
> > > +       return false;
> > > +    }
> >
> > this all looks redundant - why is it important to look at the
> > base of ref, why not simply ask below (*)
> >
> > > +         modref_access_node *access_node;
> > > +         FOR_EACH_VEC_SAFE_ELT (ref_node->accesses, k, access_node)
> > > +           {
> > > +             if (num_tests >= max_tests)
> > > +               return true;
> > > +
> > > +             if (access_node->parm_index == -1
> > > +                 || (unsigned)access_node->parm_index
> > > +                    >= gimple_call_num_args (stmt))
> > > +               return true;
> > > +
> > > +             tree op = gimple_call_arg (stmt, access_node->parm_index);
> > > +
> > > +             alias_stats.modref_baseptr_tests++;
> > > +
> > > +             /* Lookup base, if this is the first time we compare bases.  */
> > > +             if (!base)
> >
> > Meh, so this function is a bit confusing with base_node, ref_node,
> > access_node and now 'base' and 'op'.  The loop now got a
> > new nest as well.
> >
> > I'm looking for a high-level description of the modref_tree <>
> > but cannot find any which makes reviewing this quite difficult...
>
> There is a description in ipa-modref.c though it may need a bit of expanding.
> Basically the modref summary represents a decision tree for
> tree-ssa-alias that has three levels
>   1) base which records base alias set,
>   2) ref which records ref alias set and
>   3) access wich presently records info whether the access is a
>   dreference of pointer passed by parameter. In future I will re-add
>   info about offset/size and base type. It would be posisble to record
>   the access path though I am not sure if that it is worth the effort
> >
> > > +               {
> > > +                 base = ref->ref;
> > > +                 while (handled_component_p (base))
> > > +                   base = TREE_OPERAND (base, 0);
> >
> > ao_ref_base (ref)?  OK, that might strip an inner
> > MEM_REF, yielding in a decl, but ...
> >
> > > +                 if (TREE_CODE (base) == MEM_REF
> > > +                     || TREE_CODE (base) == TARGET_MEM_REF)
> > > +                   base = TREE_OPERAND (base, 0);
> >
> > that might happen here, too.  But in the MEM_REF case base
> > is a pointer.
> >
> > > +               }
> > > +
> > > +             if (base_may_alias_with_dereference_p (base, op))
> >
> > So this is a query purely at the caller side - whether 'ref' may
> > alias 'op'.
> >
> > ---
> >
> > (*) ptr_deref_may_alias_ref_p_1 (op, ref)
> >
> > without any of the magic?
>
> Hmm, it may actually just work, I did not know that looks through
> memrefs, let me re-test the patch.
> >
> > Can you please amend ipa-modref-tree.h/c with a toplevel comment
> > layint out the data structure and what is recorded?
>
> I will do (but need to think bit of a redundancy between comment in
> ipa-modref and ipa-modref-tree)

One place is enough - just add a pointer to the other place.

Richard.

> Honza
> >
> > Thanks,
> > Richard.
> >
> > > +               return true;
> > > +             num_tests++;
> > > +           }
> > >         }
> > >      }
> > >    return false;
> > > @@ -2510,7 +2584,7 @@ ref_maybe_used_by_call_p_1 (gcall *call, ao_ref *ref, bool tbaa_p)
> > >           modref_summary *summary = get_modref_function_summary (node);
> > >           if (summary)
> > >             {
> > > -             if (!modref_may_conflict (summary->loads, ref, tbaa_p))
> > > +             if (!modref_may_conflict (call, summary->loads, ref, tbaa_p))
> > >                 {
> > >                   alias_stats.modref_use_no_alias++;
> > >                   if (dump_file && (dump_flags & TDF_DETAILS))
> > > @@ -2934,7 +3008,7 @@ call_may_clobber_ref_p_1 (gcall *call, ao_ref *ref, bool tbaa_p)
> > >           modref_summary *summary = get_modref_function_summary (node);
> > >           if (summary)
> > >             {
> > > -             if (!modref_may_conflict (summary->stores, ref, tbaa_p))
> > > +             if (!modref_may_conflict (call, summary->stores, ref, tbaa_p))
> > >                 {
> > >                   alias_stats.modref_clobber_no_alias++;
> > >                   if (dump_file && (dump_flags & TDF_DETAILS))