On Mon, Jul 5, 2010 at 1:54 PM, Tom de Vries <tjvr...@xs4all.nl> wrote:
>> Interesting. My first reaction is that this is an invalid use of the
>> garbage collector. I think there is really only one valid function
>> that can be used as an if_marked function: one which checks
>> ggc_marked_p on the structure.
>
>
> Then how about tree_map_base_marked_p, the if_marked function for
> value_expr_for_decl?
>
> tree.h:
> ...
> struct GTY(()) tree_map_base {
> tree from;
> };
>
> struct GTY(()) tree_decl_map {
> struct tree_map_base base;
> tree to;
> };
>
> #define tree_decl_map_marked_p tree_map_base_marked_p
> ...
>
> tree.c:
> ...
> static GTY ((if_marked ("tree_decl_map_marked_p"), param_is (struct
> tree_decl_map)))
> htab_t value_expr_for_decl;
>
> int
> tree_map_base_marked_p (const void *p)
> {
> return ggc_marked_p (((const struct tree_map_base *) p)->from);
> }
> ...
>
> The tree_map_base_marked_p checks ggc_marked_p on the from field. During
> ggc_scan_cache_tab, if the from field is live, also the to field is marked
> live.
> I wrote some code to do sanity testing and found a similar scenario as
> before:
> - a register attribute is not marked live during root marking
> - reg_attrs_htab is traversed, and the hash table entry corresponding to the
> register attribute is removed
> - value_expr_for_decl is traversed, a from field is found live, so the to
> field is also marked live, marking the register attribute live.
>
> Is this valid use of the garbage collector?
Originally the if_marked hook was supposed to be used only for
caching purposes. So it doesn't matter whether an element is
collected or not for correctness. If we now have accumulated other
uses we indeed have to worry about this scenario (and I think it
won't work very well there).
Richard.
