From mboxrd@z Thu Jan 1 00:00:00 1970 Path: news.gmane.org!.POSTED!not-for-mail From: Paul Eggert Newsgroups: gmane.emacs.devel Subject: Re: Conservative GC isn't safe Date: Sat, 26 Nov 2016 00:30:21 -0800 Message-ID: <66485157-00cd-4704-a421-cbfe84299cae@cs.ucla.edu> References: NNTP-Posting-Host: blaine.gmane.org Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 7bit X-Trace: blaine.gmane.org 1480149083 32524 195.159.176.226 (26 Nov 2016 08:31:23 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Sat, 26 Nov 2016 08:31:23 +0000 (UTC) User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.4.0 To: Daniel Colascione , Emacs developers Original-X-From: emacs-devel-bounces+ged-emacs-devel=m.gmane.org@gnu.org Sat Nov 26 09:31:18 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 1cAYOD-0007Li-Bf for ged-emacs-devel@m.gmane.org; Sat, 26 Nov 2016 09:31:17 +0100 Original-Received: from localhost ([::1]:49647 helo=lists.gnu.org) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1cAYOG-0007fU-RR for ged-emacs-devel@m.gmane.org; Sat, 26 Nov 2016 03:31:20 -0500 Original-Received: from eggs.gnu.org ([2001:4830:134:3::10]:35044) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1cAYNU-0007fC-Av for emacs-devel@gnu.org; Sat, 26 Nov 2016 03:30:33 -0500 Original-Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1cAYNR-0001zd-1N for emacs-devel@gnu.org; Sat, 26 Nov 2016 03:30:32 -0500 Original-Received: from zimbra.cs.ucla.edu ([131.179.128.68]:59090) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1cAYNQ-0001z1-Qz for emacs-devel@gnu.org; Sat, 26 Nov 2016 03:30:28 -0500 Original-Received: from localhost (localhost [127.0.0.1]) by zimbra.cs.ucla.edu (Postfix) with ESMTP id 086EA1600EC; Sat, 26 Nov 2016 00:30:26 -0800 (PST) Original-Received: from zimbra.cs.ucla.edu ([127.0.0.1]) by localhost (zimbra.cs.ucla.edu [127.0.0.1]) (amavisd-new, port 10032) with ESMTP id 5DwbWgF5Dzfu; Sat, 26 Nov 2016 00:30:25 -0800 (PST) Original-Received: from localhost (localhost [127.0.0.1]) by zimbra.cs.ucla.edu (Postfix) with ESMTP id 0EC7E1600ED; Sat, 26 Nov 2016 00:30:25 -0800 (PST) X-Virus-Scanned: amavisd-new at zimbra.cs.ucla.edu Original-Received: from zimbra.cs.ucla.edu ([127.0.0.1]) by localhost (zimbra.cs.ucla.edu [127.0.0.1]) (amavisd-new, port 10026) with ESMTP id isW1FmQiKnz7; Sat, 26 Nov 2016 00:30:24 -0800 (PST) Original-Received: from [192.168.0.104] (unknown [47.153.178.162]) by zimbra.cs.ucla.edu (Postfix) with ESMTPSA id A644D1600EC; Sat, 26 Nov 2016 00:30:24 -0800 (PST) In-Reply-To: X-detected-operating-system: by eggs.gnu.org: GNU/Linux 3.x [fuzzy] X-Received-From: 131.179.128.68 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:209606 Archived-At: On 11/26/2016 12:11 AM, Daniel Colascione wrote: > > 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.) > > 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.)