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Re: increase alignment of global structs in increase_alignment pass


On Thu, 19 May 2016, Prathamesh Kulkarni wrote:

> On 18 May 2016 at 19:38, Richard Biener <rguenther@suse.de> wrote:
> > On Wed, 18 May 2016, Prathamesh Kulkarni wrote:
> >
> >> On 17 May 2016 at 18:36, Richard Biener <rguenther@suse.de> wrote:
> >> > On Wed, 11 May 2016, Prathamesh Kulkarni wrote:
> >> >
> >> >> On 6 May 2016 at 17:20, Richard Biener <rguenther@suse.de> wrote:
> >> >> >
> >> >> > You can't simply use
> >> >> >
> >> >> > +      offset = int_byte_position (field);
> >> >> >
> >> >> > as it can be placed at variable offset which will make int_byte_position
> >> >> > ICE.  Note it also returns a truncated byte position (bit position
> >> >> > stripped) which may be undesirable here.  I think you want to use
> >> >> > bit_position and if and only if DECL_FIELD_OFFSET and
> >> >> > DECL_FIELD_BIT_OFFSET are INTEGER_CST.
> >> >> oops, I didn't realize offsets could be variable.
> >> >> Will that be the case only for VLA member inside struct ?
> >> >
> >> > And non-VLA members after such member.
> >> >
> >> >> > Your observation about the expensiveness of the walk still stands I guess
> >> >> > and eventually you should at least cache the
> >> >> > get_vec_alignment_for_record_decl cases.  Please make those workers
> >> >> > _type rather than _decl helpers.
> >> >> Done
> >> >> >
> >> >> > You seem to simply get at the maximum vectorized field/array element
> >> >> > alignment possible for all arrays - you could restrict that to
> >> >> > arrays with at least vector size (as followup).
> >> >> Um sorry, I didn't understand this part.
> >> >
> >> > It doesn't make sense to align
> >> >
> >> > struct { int a; int b; int c; int d; float b[3]; int e; };
> >> >
> >> > because we have a float[3] member.  There is no vector size that
> >> > would cover the float[3] array.
> >> Thanks for the explanation.
> >> So we do not want to align struct if sizeof (array_field) < sizeof
> >> (vector_type).
> >> This issue is also present without patch for global arrays, so I modified
> >> get_vec_alignment_for_array_type, to return 0 if sizeof (array_type) <
> >> sizeof (vectype).
> >> >
> >> >> >
> >> >> > +  /* Skip artificial decls like typeinfo decls or if
> >> >> > +     record is packed.  */
> >> >> > +  if (DECL_ARTIFICIAL (record_decl) || TYPE_PACKED (type))
> >> >> > +    return 0;
> >> >> >
> >> >> > I think we should honor DECL_USER_ALIGN as well and not mess with those
> >> >> > decls.
> >> >> Done
> >> >> >
> >> >> > Given the patch now does quite some extra work it might make sense
> >> >> > to split the symtab part out of the vect_can_force_dr_alignment_p
> >> >> > predicate and call that early.
> >> >> In the patch I call symtab_node::can_increase_alignment_p early. I tried
> >> >> moving that to it's callers - vect_compute_data_ref_alignment and
> >> >> increase_alignment::execute, however that failed some tests in vect, and
> >> >> hence I didn't add the following hunk in the patch. Did I miss some
> >> >> check ?
> >> >
> >> > Not sure.
> >> >
> >> >> diff --git a/gcc/tree-vect-data-refs.c b/gcc/tree-vect-data-refs.c
> >> >> index 7652e21..2c1acee 100644
> >> >> --- a/gcc/tree-vect-data-refs.c
> >> >> +++ b/gcc/tree-vect-data-refs.c
> >> >> @@ -795,7 +795,10 @@ vect_compute_data_ref_alignment (struct data_reference *dr)
> >> >>    && TREE_CODE (TREE_OPERAND (base, 0)) == ADDR_EXPR)
> >> >>   base = TREE_OPERAND (TREE_OPERAND (base, 0), 0);
> >> >>
> >> >> -      if (!vect_can_force_dr_alignment_p (base, TYPE_ALIGN (vectype)))
> >> >> +      if (!(TREE_CODE (base) == VAR_DECL
> >> >> +            && decl_in_symtab_p (base)
> >> >> +            && symtab_node::get (base)->can_increase_alignment_p ()
> >> >> +            && vect_can_force_dr_alignment_p (base, TYPE_ALIGN (vectype))))
> >> >>   {
> >> >>    if (dump_enabled_p ())
> >> >>      {
> >> >
> >> > +  for (tree field = first_field (type);
> >> > +       field != NULL_TREE;
> >> > +       field = DECL_CHAIN (field))
> >> > +    {
> >> > +      /* Skip if not FIELD_DECL or has variable offset.  */
> >> > +      if (TREE_CODE (field) != FIELD_DECL
> >> > +         || TREE_CODE (DECL_FIELD_OFFSET (field)) != INTEGER_CST
> >> > +         || TREE_CODE (DECL_FIELD_BIT_OFFSET (field)) != INTEGER_CST
> >> > +         || DECL_USER_ALIGN (field)
> >> > +         || DECL_ARTIFICIAL (field))
> >> > +       continue;
> >> >
> >> > you can stop processing the type and return 0 here if the offset
> >> > is not an INTEGER_CST.  All following fields will have the same issue.
> >> >
> >> > +      /* FIXME: is this check necessary since we check for variable
> >> > offset above ?  */
> >> > +      if (TREE_CODE (offset_tree) != INTEGER_CST)
> >> > +       continue;
> >> >
> >> > the check should not be necessary.
> >> >
> >> >       offset = tree_to_uhwi (offset_tree);
> >> >
> >> > but in theory offset_tree might not fit a unsigned HOST_WIDE_INT, so
> >> > instead of for INTEGER_CST please check ! tree_fits_uhwi_p (offset_tree).
> >> > As above all following fields will also fail the check if this fails so
> >> > you can return 0 early.
> >> >
> >> > +static unsigned
> >> > +get_vec_alignment_for_type (tree type)
> >> > +{
> >> > +  if (type == NULL_TREE)
> >> > +    return 0;
> >> > +
> >> > +  gcc_assert (TYPE_P (type));
> >> > +
> >> > +  unsigned *slot = type_align_map->get (type);
> >> > +  if (slot)
> >> > +    return *slot;
> >> >
> >> > I suggest to apply the caching only for the RECORD_TYPE case to keep
> >> > the size of the map low.
> >> >
> >> > Otherwise the patch looks ok now.
> >> I have done the suggested changes in attached patch.
> >> Is this version OK to commit after bootstrap+test ?
> >> The patch survives cross-testing on arm*-*-* and aarch64*-*-*.
> >
> > +  tree type_size_tree = TYPE_SIZE (type);
> > +  if (!type_size_tree)
> > +    return TYPE_ALIGN (vectype);
> > +
> > +  if (TREE_CODE (type_size_tree) != INTEGER_CST)
> > +    return TYPE_ALIGN (vectype);
> > +
> > +  /* If array has constant size, ensure that it's at least equal to
> > +     size of vector type.  */
> > +  if (!tree_fits_uhwi_p (type_size_tree))
> > +    return TYPE_ALIGN (vectype);
> > +  unsigned HOST_WIDE_INT type_size = tree_to_uhwi (type_size_tree);
> > +
> > +  tree vectype_size_tree = TYPE_SIZE (vectype);
> > +  if (!tree_fits_uhwi_p (vectype_size_tree))
> > +    return TYPE_ALIGN (vectype);
> > +
> > +  unsigned HOST_WIDE_INT vectype_size = tree_to_uhwi (vectype_size_tree);
> > +  return (type_size > vectype_size) ? TYPE_ALIGN (vectype) : 0;
> >
> > please change this to
> >
> >   if (! TYPE_SIZE (type)
> >       || TREE_CODE (TYPE_SIZE (type)) != INTEGER_CST
> >       || tree_int_cst_lt (TYPE_SIZE (type), TYPE_SIZE (vectype)))
> >     return 0;
> >
> >   return TYPE_ALIGN (vectype);
> >
> > consistent with the handling for "VLA" records.
> >
> > +  unsigned *slot = type_align_map->get (type);
> > +  if (slot)
> > +    return *slot;
> > +
> > +  unsigned max_align = 0, alignment;
> > +  HOST_WIDE_INT offset;
> > +  tree offset_tree;
> > +
> > +  if (TYPE_PACKED (type))
> > +    return 0;
> > +
> >
> > move the tye_align_map query after the TYPE_PACKED check.
> >
> > +      /* We don't need to process the type further if offset is variable,
> > +        since the offsets of remaining members will also be variable.  */
> > +      if (TREE_CODE (DECL_FIELD_OFFSET (field)) != INTEGER_CST
> > +         || TREE_CODE (DECL_FIELD_BIT_OFFSET (field)) != INTEGER_CST)
> > +       return 0;
> >
> > just break; here (and in the other case returning zero).  If a
> > previous record field said we'd want to align we still should align.
> > Also we want to store 0 into the type_align_map as well.
> >
> > Ok with those changes.
> Hi,
> Is this version OK ?
> Cross tested on arm*-*-* and aarch64*-*-*

Yes.  Btw, please always do a regular bootstrap and regtest, cross-testing
alone is _not_ sufficient!

Thanks,
Richard.

> Thanks,
> Prathamesh
> >
> > Thanks,
> > Richard.
> 

-- 
Richard Biener <rguenther@suse.de>
SUSE LINUX GmbH, GF: Felix Imendoerffer, Jane Smithard, Graham Norton, HRB 21284 (AG Nuernberg)


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