From: Stefan Monnier <monnier@iro.umontreal.ca>
To: Lynn Winebarger <owinebar@gmail.com>
Cc: emacs-devel <emacs-devel@gnu.org>
Subject: Re: GC mark stack
Date: Mon, 15 Aug 2022 14:17:11 -0400 [thread overview]
Message-ID: <jwvy1vp2xkv.fsf-monnier+emacs@gnu.org> (raw)
In-Reply-To: <CAM=F=bAFKWt+qtUfaQFn+yw-TLi=_ta29UoyVJh2jo-KTYFdEg@mail.gmail.com> (Lynn Winebarger's message of "Mon, 15 Aug 2022 13:52:23 -0400")
> If the loop can be expressed by a relational expression consisting of
> a concrete relation (like "references"), composition, union, and
> transitive closure, then there is a systematic way to construct an
> algorithm of the type I described.
I think the core of the problem is the following:
In a "key weak" hash table, if a reachable object points (transitively) to
an object that is a key in that table, then the value associated with
that key becomes reachable. So conceptually we have a reference (an
edge) from the key *object* to the value, but this reference is not
actually represented in the heap.
More specifically, the code that does the marking (i.e. `mark_object`)
only knows about the key object itself: it doesn't know that this object
is a key in a weak hash table nor what is the associated value.
We could try and reify those conceptual references, e.g. in some kind of
auxiliary table, but the added cost of making `mark_object` consult this
table for every object it marks (at least while marking weak-refs)
seems prohibitive.
Stefan
next prev parent reply other threads:[~2022-08-15 18:17 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-06-26 15:31 GC mark stack Lynn Winebarger
2022-06-26 15:50 ` Lynn Winebarger
2022-08-15 14:01 ` Stefan Monnier
2022-08-15 14:51 ` Lynn Winebarger
2022-08-15 16:28 ` Stefan Monnier
2022-08-15 17:52 ` Lynn Winebarger
2022-08-15 18:17 ` Stefan Monnier [this message]
2022-08-16 1:53 ` Lynn Winebarger
2022-08-16 9:50 ` Lynn Winebarger
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