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Re: Hash Tables and GC
Date: Wed, 7 Sep 88 15:49 EDT
From: Michael Greenwald <Greenwald@STONY-BROOK.SCRC.Symbolics.COM>
Date: Wed, 7 Sep 88 11:52 PDT
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
How does this tell you whether there *are* any non-weak pointers to the
object in question?
The main point of the desire for weak pointers is to free storage being
held by the pointers. SI:GC-EVERY-FLIP-LIST can't tell you that
removing a pointer will definitely free storage, but it does let you
remove pointers that you don't need and that you know will probably free
For example, I've used it to empty the free pool of a resource, and I've
used it to "clean" a cache of all obsolete items.