Re: [google/gcc-4_7, trunk] Fix problem with -fdebug-types-section and template instantiations

[Cary Coutant <ccoutant at google dot com>]

[dropping the bogus address I accidentally typed on the command line]

On Sat, Aug 25, 2012 at 7:08 PM, Cary Coutant <ccoutant@google.com> wrote:
> This patch is for trunk and the google/gcc-4_7 branch.
>
> When a class template instantiation is moved into a separate type unit,
> it can bring along a lot of other referenced types into the type unit,
> especially if the template is derived from another (large) type that
> does not have an actually have a type definition in a type unit of its
> own. When there are many instantiations of the same template, we get
> a lot of duplication, and in the worst case (a template with several
> parameters, instantiated multiple times along each dimension), GCC
> can end up taking a long time and exhausting available memory. Because
> instantiations typically happen in .cc files, and the resulting type
> definition is unlikely to be useful as a comdat type unit, this patch
> avoids the problem by choosing not to move template instantiations
> into type units.
>
> Bootstraps and passes regression tests. Also tested with a large
> internal test case that previously resulted in out-of-memory during
> compilation.
>
> Google ref b/7041390.
>
>
> 2012-08-25   Cary Coutant  <ccoutant@google.com>
>
> gcc/
>         * dwarf2out.c (is_template_instantiation): New function.
>         (should_move_die_to_comdat): Reject types that are template
>         instantiations.
>
>
> Index: gcc/dwarf2out.c
> ===================================================================
> --- gcc/dwarf2out.c     (revision 190681)
> +++ gcc/dwarf2out.c     (working copy)
> @@ -7277,6 +7277,21 @@ contains_subprogram_definition (dw_die_r
>    return 0;
>  }
>
> +/* Return non-zero if this DIE represents a template instantiation.  */
> +
> +static int
> +is_template_instantiation (dw_die_ref die)
> +{
> +  dw_die_ref c;
> +
> +  FOR_EACH_CHILD (die, c,
> +                 if (c->die_tag == DW_TAG_template_type_param
> +                     || c->die_tag == DW_TAG_template_value_param
> +                     || c->die_tag == DW_TAG_GNU_template_template_param)
> +                   return 1);
> +  return 0;
> +}
> +
>  /* Return non-zero if this is a type DIE that should be moved to a
>     COMDAT .debug_types section.  */
>
> @@ -7294,6 +7309,7 @@ should_move_die_to_comdat (dw_die_ref di
>        if (is_declaration_die (die)
>            || get_AT (die, DW_AT_abstract_origin)
>            || is_nested_in_subprogram (die)
> +         || is_template_instantiation (die)
>           || contains_subprogram_definition (die))
>          return 0;
>        return 1;