------- Comment #15 from rguenth at gcc dot gnu dot org 2010-02-09 23:11
-------
(In reply to comment #14)
> Subject: Re: vectorizer created unaligned vector
> insns
>
> On 01/18/10 05:17, irar at il dot ibm dot com wrote:
> > ------- Comment #13 from irar at il dot ibm dot com 2010-01-18 12:17
> > -------
> > Does something like this make sense? (With this patch we will never use
> > peeling
> > for function parameters, unless the builtin returns OK to peel for packed
> > types).
> >
> > Index: tree-vect-data-refs.c
> > ===================================================================
> > --- tree-vect-data-refs.c (revision 155880)
> > +++ tree-vect-data-refs.c (working copy)
> > @@ -1010,10 +1010,29 @@ vector_alignment_reachable_p (struct dat
> > tree type = (TREE_TYPE (DR_REF (dr)));
> > tree ba = DR_BASE_OBJECT (dr);
> > bool is_packed = false;
> > + tree tmp = TREE_TYPE (DR_BASE_ADDRESS (dr));
> >
> > if (ba)
> > is_packed = contains_packed_reference (ba);
> >
> > + is_packed = is_packed || contains_packed_reference (DR_BASE_ADDRESS
> > (dr));
> > +
> > + if (!is_packed)
> > + {
> > + while (tmp)
> > + {
> > + is_packed = TYPE_PACKED (tmp);
> > + if (is_packed)
> > + break;
> > +
> > + tmp = TREE_TYPE (tmp);
> > + }
> > + }
> > +
> > + if (TREE_CODE (DR_BASE_ADDRESS (dr)) == SSA_NAME
> > +&& TREE_CODE (SSA_NAME_VAR (DR_BASE_ADDRESS (dr))) == PARM_DECL)
> > + is_packed = true;
> > +
> > if (vect_print_dump_info (REPORT_DETAILS))
> > fprintf (vect_dump, "Unknown misalignment, is_packed =
> > %d",is_packed);
> > if (targetm.vectorize.vector_alignment_reachable (type, is_packed))
> >
> >
> I still don't see how this can be conservatively correct. The
> fundamental problem is the code assumes that if it doesn't find a packed
> attribute or the DR_BASE_ADDRESS isn't directly derived from a PARM_DECL
> then it's safe to vectorize. Instead the code really needs to operate
> by proving suitable alignment and if suitable alignment can't be proven,
> then vectorization is not possible without runtime alignment guards.
>
> As an example, consider the case where an unaligned packed address is
> passed as a parameter, but the target function does somethign like
>
> fubar (int *p, bool condition ) // where P is potentially unaligned
A pointer of type int * which is not aligned properly invokes undefined
behavior.
> {
> int *addr;
> if (condition)
> temp = malloc (whatever)
> addr = (condition ? temp : p);
>
> // addr used as base address in vectorizable loop here
> }
>
> Jeff
>
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=42652
