Some plugins may want to be informed when GGC (the GCC Garbage
Collector) is running. They can register callbacks for the
PLUGIN_GGC_START and PLUGIN_GGC_END events (for which
the callback is called with a null gcc_data) to be notified of
the start or end of the GCC garbage collection.
Some plugins may need to have GGC mark additional data. This can be
done by registering a callback (called with a null gcc_data)
for the PLUGIN_GGC_MARKING event. Such callbacks can call the
ggc_set_mark routine, preferably through the ggc_mark macro
(and conversely, these routines should usually not be used in plugins
outside of the PLUGIN_GGC_MARKING event). Plugins that wish to hold
weak references to gc data may also use this event to drop weak references when
the object is about to be collected. The ggc_marked_p function can be
used to tell if an object is marked, or is about to be collected. The
gt_clear_cache overloads which some types define may also be of use in
managing weak references.
Some plugins may need to add extra GGC root tables, e.g. to handle their own
GTY-ed data. This can be done with the PLUGIN_REGISTER_GGC_ROOTS
pseudo-event with a null callback and the extra root table (of type struct
ggc_root_tab*) as user_data. Running the
gengtype -p source-dir file-list plugin*.c ...
utility generates these extra root tables.
You should understand the details of memory management inside GCC
before using PLUGIN_GGC_MARKING or PLUGIN_REGISTER_GGC_ROOTS.