From mboxrd@z Thu Jan 1 00:00:00 1970 Path: news.gmane.org!not-for-mail From: Neil Jerram Newsgroups: gmane.lisp.guile.devel Subject: Re: SCM_BOOL_F == 0 and BDW-GC Date: Thu, 17 Sep 2009 23:00:33 +0100 Message-ID: <87iqfhgrjy.fsf@ossau.uklinux.net> References: <87k52uvhnt.fsf@arudy.ossau.uklinux.net> <20090702142823.GA1401@fibril.netris.org> <877hyqk8bx.fsf@arudy.ossau.uklinux.net> <20090703153218.GA1382@fibril.netris.org> <20090705024135.GA2363@fibril.netris.org> <20090707111406.GA1388@fibril.netris.org> <87iqg5y2o8.fsf@arudy.ossau.uklinux.net> <87zl9fps1m.fsf_-_@gnu.org> NNTP-Posting-Host: lo.gmane.org Mime-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Transfer-Encoding: quoted-printable X-Trace: ger.gmane.org 1253224892 30582 80.91.229.12 (17 Sep 2009 22:01:32 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Thu, 17 Sep 2009 22:01:32 +0000 (UTC) Cc: guile-devel@gnu.org To: ludo@gnu.org (Ludovic =?iso-8859-1?Q?Court=E8s?=) Original-X-From: guile-devel-bounces+guile-devel=m.gmane.org@gnu.org Fri Sep 18 00:01:25 2009 Return-path: Envelope-to: guile-devel@m.gmane.org Original-Received: from lists.gnu.org ([199.232.76.165]) by lo.gmane.org with esmtp (Exim 4.50) id 1MoP2b-0008FL-3g for guile-devel@m.gmane.org; Fri, 18 Sep 2009 00:01:25 +0200 Original-Received: from localhost ([127.0.0.1]:33071 helo=lists.gnu.org) by lists.gnu.org with esmtp (Exim 4.43) id 1MoP2a-00067F-F7 for guile-devel@m.gmane.org; Thu, 17 Sep 2009 18:01:24 -0400 Original-Received: from mailman by lists.gnu.org with tmda-scanned (Exim 4.43) id 1MoP2W-00066M-DA for guile-devel@gnu.org; Thu, 17 Sep 2009 18:01:20 -0400 Original-Received: from exim by lists.gnu.org with spam-scanned (Exim 4.43) id 1MoP2S-00065V-UP for guile-devel@gnu.org; Thu, 17 Sep 2009 18:01:20 -0400 Original-Received: from [199.232.76.173] (port=57514 helo=monty-python.gnu.org) by lists.gnu.org with esmtp (Exim 4.43) id 1MoP2S-00065S-Or for guile-devel@gnu.org; Thu, 17 Sep 2009 18:01:16 -0400 Original-Received: from mail3.uklinux.net ([80.84.72.33]:50308) by monty-python.gnu.org with esmtp (Exim 4.60) (envelope-from ) id 1MoP2N-00061q-MV; Thu, 17 Sep 2009 18:01:12 -0400 Original-Received: from arudy (host86-147-112-99.range86-147.btcentralplus.com [86.147.112.99]) by mail3.uklinux.net (Postfix) with ESMTP id C2D691F686A; Thu, 17 Sep 2009 23:00:34 +0100 (BST) Original-Received: from arudy (arudy [127.0.0.1]) by arudy (Postfix) with ESMTP id 2907B38021; Thu, 17 Sep 2009 23:00:34 +0100 (BST) In-Reply-To: <87zl9fps1m.fsf_-_@gnu.org> ("Ludovic =?iso-8859-1?Q?Court=E8?= =?iso-8859-1?Q?s=22's?= message of "Mon, 31 Aug 2009 23:55:17 +0200") User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/23.1 (gnu/linux) X-detected-operating-system: by monty-python.gnu.org: GNU/Linux 2.4-2.6 X-BeenThere: guile-devel@gnu.org X-Mailman-Version: 2.1.5 Precedence: list List-Id: "Developers list for Guile, the GNU extensibility library" List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Original-Sender: guile-devel-bounces+guile-devel=m.gmane.org@gnu.org Errors-To: guile-devel-bounces+guile-devel=m.gmane.org@gnu.org Xref: news.gmane.org gmane.lisp.guile.devel:9342 Archived-At: ludo@gnu.org (Ludovic Court=E8s) writes: > Hello! > > Neil Jerram writes: > >> Just checking this because Ludovic said recently that (SCM_BOOL_F =3D=3D >> 0) would have nice properties for BDW-GC. > > Actually he wasn't quite right when he said that. :-) > > The issue with BDW-GC is that "disappearing links" (weak pointers in > libgc parlance) replace pointers to objects that have been reclaimed by > NULL, and there's no way to tell it to use some other value. > > That leads to insanities in the weak hash table implementation [0,=A01], > which I thought could somehow vanish if SCM_BOOL_F =3D=3D 0. They're not so bad. But I agree that it would be nicer not to have to use SCM_WEAK_PAIR_CAR, and just use SCM_CAR instead. > Unfortunately that's not true; it would even make things worse because > NULL would now be a valid Scheme value. Yes, I see. > Instead what's really needed is a special pointer-to-reclaimed-object > value that can be distinguished from valid Scheme values since that > value ends up in the car or cdr of weak pairs in hash table buckets. As > such, SCM_PACK=A0(NULL) was a good choice until now. Here I'm confused again. I thought we now had no choice about the pointer-to-reclaimed-object value, because BDW-GC always uses NULL. > SCM_UNDEFINED =3D=3D 0 would work fine because SCM_UNDEFINED is not a val= id > Scheme value, but it wouldn't change the implementation. I'm afraid I don't understand "but it wouldn't change the implementation". 0 is also used for procedures that haven't been extended to primitive-generic - see SCM_SUBR_GENERIC - but I think making SCM_UNDEFINED =3D=3D 0 would be fine there too. > Thoughts? SCM_UNDEFINED =3D=3D 0 is sounding promising... Neil