Re: Support constants for DW_AT_data_member_location

[Vladimir Prus <vladimir@codesourcery.com>]

On Saturday 01 December 2007 04:48:09 Jim Blandy wrote:

> > So, DW_FORM_data4 used for DW_AT_data_member_location is never interepreted
> > as constant. Maybe we should have "is_surely_constant" function?
> 
> Oh --- great catch, thanks.  (We don't support location lists for
> DW_AT_data_member_location yet, but we certainly should warn, not
> randomly misinterpret the value.)
> 
> How's this, then?
> 
> gdb/ChangeLog:
> 2007-11-30  Jim Blandy  <jimb@codesourcery.com>
> 
>         * dwarf2read.c (attr_form_is_constant): New function.
>         (dwarf2_add_field): Use it and attr_form_is_section_offset to
>         recognize DW_AT_data_member_location attributes.  Use
>         dwarf2_get_attr_constant_value when the attribute is a constant.
> 
>         * dwarf2read.c (attr_form_is_section_offset): New function.
>         (dwarf_add_member_fn, read_common_block, read_partial_die)
>         (dwarf2_symbol_mark_computed): Use it, instead of writing it out.
> 
> diff -r c4f654de59cf gdb/dwarf2read.c
> --- a/gdb/dwarf2read.c  Thu Nov 29 11:28:59 2007 -0800
> +++ b/gdb/dwarf2read.c  Fri Nov 30 17:32:46 2007 -0800
> @@ -1039,6 +1039,10 @@ static void dwarf_decode_macros (struct 
>                                   char *, bfd *, struct dwarf2_cu *);
>  
>  static int attr_form_is_block (struct attribute *);
> +
> +static int attr_form_is_section_offset (struct attribute *);
> +
> +static int attr_form_is_constant (struct attribute *);
>  
>  static void dwarf2_symbol_mark_computed (struct attribute *attr,
>                                          struct symbol *sym,
> @@ -3380,8 +3384,16 @@ dwarf2_add_field (struct field_info *fip
>        attr = dwarf2_attr (die, DW_AT_data_member_location, cu);
>        if (attr)
>         {
> -         FIELD_BITPOS (*fp) =
> -           decode_locdesc (DW_BLOCK (attr), cu) * bits_per_byte;
> +          if (attr_form_is_section_offset (attr))
> +            {
> +              dwarf2_complex_location_expr_complaint ();
> +              FIELD_BITPOS (*fp) = 0;
> +            }
> +          else if (attr_form_is_constant (attr))
> +            FIELD_BITPOS (*fp) = dwarf2_get_attr_constant_value (attr, 0);

You need "* 8" above, since the value of data_member_location is offset
in bytes.

> +          else
> +            FIELD_BITPOS (*fp) =
> +              decode_locdesc (DW_BLOCK (attr), cu) * bits_per_byte;

Or, if we really want GDB to support processors with 9-bit bytes, you
need "* bits_per_byte".


> +/* Return non-zero if ATTR's value is a section offset (classes
> +   lineptr, loclistptr, macptr or rangelistptr). 

I'd also note here that in all cases, an attribute value
may fall to only one class listed above.

> In this case, 
> +   you may use DW_UNSND (attr) to retrieve the offset.  */
> +static int
> +attr_form_is_section_offset (struct attribute *attr)
> +{
> +  return (attr->form == DW_FORM_data4
> +          || attr->form == DW_FORM_data8);

I'd probably add a comment here saying that in DWARF standand,
those are only two forms that can encode those classes. Not being
DWARF expert, it took me a bit of looking to verify that.  If you
explicitly mention this, then future readers can just trust the
comment ;-)

With the "* bits_per_byte" change, this patch fixes the problem I saw.
Assuming there are no regressions, can you check it in?

Thanks,
Volodya