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Hash Tables and GC
re: Just to throw a little light into this discussion: On Symbolics systems
there is a list of forms to be evaluated before any gc flip (ephemeral or
dynamic). It's possible to implement whatever kind of weak pointers
strategy you want by explicitly clearing whatever pointers are "weak"
before the flip occurs. The list is called SI:GC-EVERY-FLIP-LIST
Hmmmmm. As barmar points out, this could't work _in general_ for weak
pointers unless you also restore the non-GC'd entries that you deleted.
PDP10 MacLisp provided a different variant of this -- the infamous GC
hook [sigh, there were so many "hooks" in MacLisp!]. Just _after_ a GC
finishs, but before any more consing is done, the user's "hook" function
is run. At this unique interval in time, it is possible to ask of a random
address/pointer "Were you just GC'd"; [in fact, this capability is independent
of whether it is a copying, or collecting or refcnt'ing GC]. Then using NIL
type arrays, pointers could be kept around for inspection after the GC; the
"NIL type" array simply means that the GC doesn't mark or descend the entries
(just like XPOINTER type arrays in Interlisp-D].
GJS used this hook to good advantage in MicroPlanner and early Scheme
implementations. In effect, I think he implemnted :temporary type
-- JonL --