From mboxrd@z Thu Jan 1 00:00:00 1970 Path: news.gmane.org!.POSTED!not-for-mail From: Stefan Monnier Newsgroups: gmane.emacs.devel Subject: Re: Conservative GC isn't safe Date: Sat, 26 Nov 2016 10:03:39 -0500 Message-ID: References: <66485157-00cd-4704-a421-cbfe84299cae@cs.ucla.edu> NNTP-Posting-Host: blaine.gmane.org Mime-Version: 1.0 Content-Type: text/plain X-Trace: blaine.gmane.org 1480172708 16571 195.159.176.226 (26 Nov 2016 15:05:08 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Sat, 26 Nov 2016 15:05:08 +0000 (UTC) User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/25.1.50 (gnu/linux) To: emacs-devel@gnu.org Original-X-From: emacs-devel-bounces+ged-emacs-devel=m.gmane.org@gnu.org Sat Nov 26 16:05:01 2016 Return-path: Envelope-to: ged-emacs-devel@m.gmane.org Original-Received: from lists.gnu.org ([208.118.235.17]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1cAeXF-0003It-Fw for ged-emacs-devel@m.gmane.org; Sat, 26 Nov 2016 16:05:01 +0100 Original-Received: from localhost ([::1]:50824 helo=lists.gnu.org) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1cAeXJ-00014l-8J for ged-emacs-devel@m.gmane.org; Sat, 26 Nov 2016 10:05:05 -0500 Original-Received: from eggs.gnu.org ([2001:4830:134:3::10]:58398) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1cAeW6-00013L-RR for emacs-devel@gnu.org; Sat, 26 Nov 2016 10:03:52 -0500 Original-Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1cAeW2-0001gm-Sc for emacs-devel@gnu.org; Sat, 26 Nov 2016 10:03:50 -0500 Original-Received: from [195.159.176.226] (port=43862 helo=blaine.gmane.org) by eggs.gnu.org with esmtps (TLS1.0:RSA_AES_128_CBC_SHA1:16) (Exim 4.71) (envelope-from ) id 1cAeW2-0001d0-Lc for emacs-devel@gnu.org; Sat, 26 Nov 2016 10:03:46 -0500 Original-Received: from list by blaine.gmane.org with local (Exim 4.84_2) (envelope-from ) id 1cAeVw-0000Fl-S9 for emacs-devel@gnu.org; Sat, 26 Nov 2016 16:03:40 +0100 X-Injected-Via-Gmane: http://gmane.org/ Original-Lines: 27 Original-X-Complaints-To: usenet@blaine.gmane.org Cancel-Lock: sha1:U54sK/3VoX5cyDNduURZ9OrDgWI= X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] [fuzzy] X-Received-From: 195.159.176.226 X-BeenThere: emacs-devel@gnu.org X-Mailman-Version: 2.1.21 Precedence: list List-Id: "Emacs development discussions." List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: emacs-devel-bounces+ged-emacs-devel=m.gmane.org@gnu.org Original-Sender: "Emacs-devel" Xref: news.gmane.org gmane.emacs.devel:209616 Archived-At: >> 1) mark_maybe_pointer looks only for exact matches on object start. It's >> perfectly legal for the compiler to keep an interior object pointer and >> discard the pointer to the object start. > Yes, just as it's perfectly legal for the compiler to subtract 42 from every > pointer before putting it in a register or storing it into memory. In > practice, though, compilers don't do this around calls to the garbage > collector. (True, this assumption should be documented better.) Indeed. Hans Boehm's done a fair bit of research in this issue, including discussing the underlying assumptions and arguing that compilers should (and usually do) guarantee those assumptions. >> 2) INTERVAL is GCed, but it's not represented in the memory tree: struct >> interval isn't a real lisp object and it's allocated as >> MEM_TYPE_NON_LISP. Even a direct pointer to the start of an interval won't >> protect it from GC. Shouldn't we treat intervals like conses? > Does the code ever create an interval that is accessible only via locals > when a GC occurs? If not, Emacs should be OK. (This should also be > documented better.) Indeed, this is a fairly delicate assumption that we don't check. It's fairly rare to manipulate "struct interval" directly, so I think the assumption is probably acceptable, but we should maybe document it more prominently. Stefan