From mboxrd@z Thu Jan 1 00:00:00 1970 Path: news.gmane.io!.POSTED.blaine.gmane.org!not-for-mail From: Stefan Monnier via "Bug reports for GNU Emacs, the Swiss army knife of text editors" Newsgroups: gmane.emacs.bugs Subject: bug#58513: describe-function should say that defconst variables are constant Date: Fri, 14 Oct 2022 12:44:01 -0400 Message-ID: References: <87zgdy8ug8.fsf@gnus.org> <8fae3292e6c1b175853b557060d05487@webmail.orcon.net.nz> <87v8om8tok.fsf@gnus.org> <87edva8seu.fsf@gnus.org> <87sfjq7a91.fsf@gnus.org> Reply-To: Stefan Monnier Mime-Version: 1.0 Content-Type: text/plain Injection-Info: ciao.gmane.io; posting-host="blaine.gmane.org:116.202.254.214"; logging-data="14776"; mail-complaints-to="usenet@ciao.gmane.io" User-Agent: Gnus/5.13 (Gnus v5.13) Cc: Phil Sainty , Stefan Kangas , 58513@debbugs.gnu.org To: Lars Ingebrigtsen Original-X-From: bug-gnu-emacs-bounces+geb-bug-gnu-emacs=m.gmane-mx.org@gnu.org Fri Oct 14 18:45:16 2022 Return-path: Envelope-to: geb-bug-gnu-emacs@m.gmane-mx.org Original-Received: from lists.gnu.org ([209.51.188.17]) by ciao.gmane.io with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.92) (envelope-from ) id 1ojNoB-0003Xa-MA for geb-bug-gnu-emacs@m.gmane-mx.org; Fri, 14 Oct 2022 18:45:15 +0200 Original-Received: from localhost ([::1]:55890 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1ojNo9-0001dx-NV for geb-bug-gnu-emacs@m.gmane-mx.org; Fri, 14 Oct 2022 12:45:13 -0400 Original-Received: from eggs.gnu.org ([2001:470:142:3::10]:54854) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1ojNny-0001bf-Bs for bug-gnu-emacs@gnu.org; Fri, 14 Oct 2022 12:45:02 -0400 Original-Received: from debbugs.gnu.org ([209.51.188.43]:39392) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_128_GCM_SHA256:128) (Exim 4.90_1) (envelope-from ) id 1ojNny-0000SQ-39 for bug-gnu-emacs@gnu.org; Fri, 14 Oct 2022 12:45:02 -0400 Original-Received: from Debian-debbugs by debbugs.gnu.org with local (Exim 4.84_2) (envelope-from ) id 1ojNnx-0000jj-MX for bug-gnu-emacs@gnu.org; Fri, 14 Oct 2022 12:45:01 -0400 X-Loop: help-debbugs@gnu.org Resent-From: Stefan Monnier Original-Sender: "Debbugs-submit" Resent-CC: bug-gnu-emacs@gnu.org Resent-Date: Fri, 14 Oct 2022 16:45:01 +0000 Resent-Message-ID: Resent-Sender: help-debbugs@gnu.org X-GNU-PR-Message: followup 58513 X-GNU-PR-Package: emacs Original-Received: via spool by 58513-submit@debbugs.gnu.org id=B58513.16657658532758 (code B ref 58513); Fri, 14 Oct 2022 16:45:01 +0000 Original-Received: (at 58513) by debbugs.gnu.org; 14 Oct 2022 16:44:13 +0000 Original-Received: from localhost ([127.0.0.1]:38470 helo=debbugs.gnu.org) by debbugs.gnu.org with esmtp (Exim 4.84_2) (envelope-from ) id 1ojNnA-0000iQ-KT for submit@debbugs.gnu.org; Fri, 14 Oct 2022 12:44:12 -0400 Original-Received: from mailscanner.iro.umontreal.ca ([132.204.25.50]:16154) by debbugs.gnu.org with esmtp (Exim 4.84_2) (envelope-from ) id 1ojNn8-0000i6-0i for 58513@debbugs.gnu.org; Fri, 14 Oct 2022 12:44:10 -0400 Original-Received: from pmg3.iro.umontreal.ca (localhost [127.0.0.1]) by pmg3.iro.umontreal.ca (Proxmox) with ESMTP id 2DE29440F7D; Fri, 14 Oct 2022 12:44:04 -0400 (EDT) Original-Received: from mail01.iro.umontreal.ca (unknown [172.31.2.1]) by pmg3.iro.umontreal.ca (Proxmox) with ESMTP id 6C05D4411C8; Fri, 14 Oct 2022 12:44:02 -0400 (EDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=iro.umontreal.ca; s=mail; t=1665765842; bh=saDz1ipLnSBgbmiLVG8mf0oWRJGqYwQJEVQwmiHLxdo=; h=From:To:Cc:Subject:In-Reply-To:References:Date:From; b=Uy3AHVhzi4ZF/+qvRV2KGpUWTilZRwpO7afR9U4Vtl+GB9N5jJ4EU3GMNiWSKgbzi sKby+UFiHmsKWW61lun/NKi6uZwOTki/Q1h8UHca/ak66Qtqc0DghC0I8TLGGe5uMy uKkRLxq4SmlaDNZWnImUkPZVmYxjkCJLY/RxWxtJB4AORBb1+XZcHM3VXjhT7sP8cg BMRceX3A0WR+WPcCrd3iV0q+rgqBkic2VqIkpovHcUgyrxJFO2TNYzqAnzXvDvXM+d e/X+P4VPxjh/ROgXCIFvbpcBuDHsQ7T9+2I41nfKVY+de9/+CxQ+zuNtWPO85VuWzW Rq+LcFVRuEkmA== Original-Received: from alfajor (unknown [45.44.229.252]) by mail01.iro.umontreal.ca (Postfix) with ESMTPSA id 43556120F05; Fri, 14 Oct 2022 12:44:02 -0400 (EDT) In-Reply-To: <87sfjq7a91.fsf@gnus.org> (Lars Ingebrigtsen's message of "Fri, 14 Oct 2022 16:25:30 +0200") X-BeenThere: debbugs-submit@debbugs.gnu.org X-Mailman-Version: 2.1.18 Precedence: list X-BeenThere: bug-gnu-emacs@gnu.org List-Id: "Bug reports for GNU Emacs, the Swiss army knife of text editors" List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: bug-gnu-emacs-bounces+geb-bug-gnu-emacs=m.gmane-mx.org@gnu.org Original-Sender: "bug-gnu-emacs" Xref: news.gmane.io gmane.emacs.bugs:245463 Archived-At: Lars Ingebrigtsen [2022-10-14 16:25:30] wrote: > Stefan Monnier writes: >> FWIW, my own local Emacs has `defconst` create actually >> constant variables by adding a `SYMBOL_DEFCONST` to `enum >> symbol_trapped_write`. Admittedly I didn't make them really constant: >> I allow a subsequent `defconst` to change the value set by a previous >> `defconst`, but `setq` signals an error just like it does if you try to >> `setq` on `enable-multibyte-characters` or on `nil`. > > Hm... do you know if there's a performance impact from doing this? In terms of memory use, we already use 2 bits for that `enum symbol_trapped_write`, so adding a new value doesn't cost anything at all. In terms of run-time, it adds a 4th branch to a few 3-branch `switch` statements, so it can have a performance impact, most importantly in `set_internal`, I guess, but I'd be very surprised if it's ever not lost in the noise: - it only affects `setq` on global/dynamic vars which should be a small fraction of the overall `setq` we perform nowadays. - since that new branch always signals an error (well, in my own code, that is: I don't know what that branch would end up doing on `master` where we'd want to be more careful about backward compatibility), it's expected to be more-or-less never taken and even the most naive branch predictors should handle it "perfectly". Stefan