This is the mail archive of the gcc@gcc.gnu.org mailing list for the GCC project.


Index Nav: [Date Index] [Subject Index] [Author Index] [Thread Index]
Message Nav: [Date Prev] [Date Next] [Thread Prev] [Thread Next]
Other format: [Raw text]

Re: increase alignment of global structs in increase_alignment pass


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.

> >
> > +  /* 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.

Thanks,
Richard.


Index Nav: [Date Index] [Subject Index] [Author Index] [Thread Index]
Message Nav: [Date Prev] [Date Next] [Thread Prev] [Thread Next]