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: `aset` on strings, changing the size in bytes Date: Sun, 09 Sep 2018 20:18:04 -0400 Message-ID: References: <88must56x4.fsf@gnu.org> <83efe4trsa.fsf@gnu.org> <83lg8bsvfm.fsf@gnu.org> <83k1nvso1o.fsf@gnu.org> <83y3cbqm81.fsf@gnu.org> NNTP-Posting-Host: blaine.gmane.org Mime-Version: 1.0 Content-Type: text/plain X-Trace: blaine.gmane.org 1536538852 18132 195.159.176.226 (10 Sep 2018 00:20:52 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Mon, 10 Sep 2018 00:20:52 +0000 (UTC) User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/27.0.50 (gnu/linux) To: emacs-devel@gnu.org Original-X-From: emacs-devel-bounces+ged-emacs-devel=m.gmane.org@gnu.org Mon Sep 10 02:20:47 2018 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 1fz9wd-0004dd-Eu for ged-emacs-devel@m.gmane.org; Mon, 10 Sep 2018 02:20:47 +0200 Original-Received: from localhost ([::1]:49137 helo=lists.gnu.org) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1fz9yj-0005T5-Ug for ged-emacs-devel@m.gmane.org; Sun, 09 Sep 2018 20:22:58 -0400 Original-Received: from eggs.gnu.org ([208.118.235.92]:47250) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1fz9yU-0005PM-0r for emacs-devel@gnu.org; Sun, 09 Sep 2018 20:22:44 -0400 Original-Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1fz9yQ-0001hc-Vm for emacs-devel@gnu.org; Sun, 09 Sep 2018 20:22:41 -0400 Original-Received: from [195.159.176.226] (port=40634 helo=blaine.gmane.org) by eggs.gnu.org with esmtps (TLS1.0:RSA_AES_128_CBC_SHA1:16) (Exim 4.71) (envelope-from ) id 1fz9yQ-0001SF-JC for emacs-devel@gnu.org; Sun, 09 Sep 2018 20:22:38 -0400 Original-Received: from list by blaine.gmane.org with local (Exim 4.84_2) (envelope-from ) id 1fz9vv-0003uW-KR for emacs-devel@gnu.org; Mon, 10 Sep 2018 02:20:03 +0200 X-Injected-Via-Gmane: http://gmane.org/ Original-Lines: 29 Original-X-Complaints-To: usenet@blaine.gmane.org Cancel-Lock: sha1:y/Ft7u9wmODeBi40GOvur6Lvtxo= X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] 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:229581 Archived-At: >> >> The need for an indirection (a String_Object has to hold a pointer to an >> >> sdata object rather than being able to keep its payload directly in the >> >> Lisp_String object (using FLEXIBLE_ARRAY_MEMBER)) >> > Why is that a problem? >> It slows down every string access, and increases the heap size of every >> string (currently they're something like N bytes of payload plus >> 5 words where 2 of those 5 words are due to the extra indirection). >> For a feature that's almost never used, I think it's pretty costly. > It can be (and was) used by memory-allocation infrastructure, > especially with very large strings. Are we sure we want to lose it > for slowdown that should be hardly perceptible? As mentioned in my original message, this indirection is used currently for 2 purposes: 1- to implement `aset`. 2- to implement string compaction in the GC. For this reason, either one of the two may be considered to "come for free" if you presume the other one as a given. Point nb 2 is a purely internal implementation detail, which we could change whenever we feel like this decision gets in the way of something preferable. Point nb 1 OTOH cannot be changed so easily because it would be a backward-incompatible change. Stefan