From mboxrd@z Thu Jan 1 00:00:00 1970 Path: news.gmane.io!.POSTED.blaine.gmane.org!not-for-mail From: Alan Mackenzie Newsgroups: gmane.emacs.bugs Subject: bug#67116: byte-compile-let: reversing the order of evaluation of the clauses CAN make a difference. Date: Sun, 12 Nov 2023 14:54:40 +0000 Message-ID: References: Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Injection-Info: ciao.gmane.io; posting-host="blaine.gmane.org:116.202.254.214"; logging-data="40543"; mail-complaints-to="usenet@ciao.gmane.io" Cc: acm@muc.de, 67116@debbugs.gnu.org To: Stefan Monnier Original-X-From: bug-gnu-emacs-bounces+geb-bug-gnu-emacs=m.gmane-mx.org@gnu.org Sun Nov 12 15:55:49 2023 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 1r2BsL-000AOk-1y for geb-bug-gnu-emacs@m.gmane-mx.org; Sun, 12 Nov 2023 15:55:49 +0100 Original-Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1r2Brz-0007zJ-5V; Sun, 12 Nov 2023 09:55:27 -0500 Original-Received: from eggs.gnu.org ([2001:470:142:3::10]) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1r2Bru-0007yx-Lt for bug-gnu-emacs@gnu.org; Sun, 12 Nov 2023 09:55:22 -0500 Original-Received: from debbugs.gnu.org ([2001:470:142:5::43]) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_128_GCM_SHA256:128) (Exim 4.90_1) (envelope-from ) id 1r2Brt-0004JY-UK for bug-gnu-emacs@gnu.org; Sun, 12 Nov 2023 09:55:22 -0500 Original-Received: from Debian-debbugs by debbugs.gnu.org with local (Exim 4.84_2) (envelope-from ) id 1r2BsY-00089d-HO for bug-gnu-emacs@gnu.org; Sun, 12 Nov 2023 09:56:02 -0500 X-Loop: help-debbugs@gnu.org Resent-From: Alan Mackenzie Original-Sender: "Debbugs-submit" Resent-CC: bug-gnu-emacs@gnu.org Resent-Date: Sun, 12 Nov 2023 14:56:02 +0000 Resent-Message-ID: Resent-Sender: help-debbugs@gnu.org X-GNU-PR-Message: followup 67116 X-GNU-PR-Package: emacs X-Debbugs-Original-Cc: acm@muc.de, bug-gnu-emacs@gnu.org Original-Received: via spool by submit@debbugs.gnu.org id=B.169980094631053 (code B ref -1); Sun, 12 Nov 2023 14:56:02 +0000 Original-Received: (at submit) by debbugs.gnu.org; 12 Nov 2023 14:55:46 +0000 Original-Received: from localhost ([127.0.0.1]:56452 helo=debbugs.gnu.org) by debbugs.gnu.org with esmtp (Exim 4.84_2) (envelope-from ) id 1r2BsI-00084U-3b for submit@debbugs.gnu.org; Sun, 12 Nov 2023 09:55:46 -0500 Original-Received: from lists.gnu.org ([2001:470:142::17]:56794) by debbugs.gnu.org with esmtp (Exim 4.84_2) (envelope-from ) id 1r2BsC-0007uq-Ly for submit@debbugs.gnu.org; Sun, 12 Nov 2023 09:55:44 -0500 Original-Received: from eggs.gnu.org ([2001:470:142:3::10]) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1r2BrS-0007wP-2K for bug-gnu-emacs@gnu.org; Sun, 12 Nov 2023 09:54:54 -0500 Original-Received: from mail.muc.de ([193.149.48.3]) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1r2BrL-0003fu-Tw for bug-gnu-emacs@gnu.org; Sun, 12 Nov 2023 09:54:53 -0500 Original-Received: (qmail 84687 invoked by uid 3782); 12 Nov 2023 15:54:44 +0100 Original-Received: from acm.muc.de (pd953a91f.dip0.t-ipconnect.de [217.83.169.31]) (using STARTTLS) by colin.muc.de (tmda-ofmipd) with ESMTP; Sun, 12 Nov 2023 15:54:44 +0100 Original-Received: (qmail 22204 invoked by uid 1000); 12 Nov 2023 14:54:40 -0000 Content-Disposition: inline In-Reply-To: X-Submission-Agent: TMDA/1.3.x (Ph3nix) X-Primary-Address: acm@muc.de Received-SPF: pass client-ip=193.149.48.3; envelope-from=acm@muc.de; helo=mail.muc.de X-Spam_score_int: -18 X-Spam_score: -1.9 X-Spam_bar: - X-Spam_report: (-1.9 / 5.0 requ) BAYES_00=-1.9, SPF_HELO_PASS=-0.001, SPF_PASS=-0.001, T_SCC_BODY_TEXT_LINE=-0.01 autolearn=ham autolearn_force=no X-Spam_action: no action 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-bounces+geb-bug-gnu-emacs=m.gmane-mx.org@gnu.org Xref: news.gmane.io gmane.emacs.bugs:274206 Archived-At: Hello, Stefan. On Sat, Nov 11, 2023 at 23:52:38 -0500, Stefan Monnier wrote: > > In lisp/emacs-lisp/bytecomp.el (byte-compile-let), when the following > > form (from jit-lock--debug-fontify): > > (let > > ((beg pos) > > (end (setq pos > > (next-single-property-change > > pos 'fontified > > nil (point-max))))) > > (put-text-property beg end 'fontified nil) (message "jit-lock-fontify-now %s %s" beg end) > > (jit-lock-fontify-now beg end)) > > gets byte compiled, the order of evaluating BEG and END gets reversed so > > that END gets evaluated first. > Sounds like a bug. Do you have some recipe to reproduce it? Certainly! In the code fragment above in jit-lock--debug-fontify, insert the diagnostic line as above. Byte compile this function. Then M-x jit-lock-debug-mode. Now scrolling any (previously unfontified) sections of a buffer will fail to fontify those sections. In *Messages* it can be seen that the printed values of BEG and END are identical, hence the call to jit-lock-fontify-now does nothing. > I looked at the bytecode but it's a bit hard to tell what's going on > there, since the var names are lost along the way. > > The comment in byte-compile-let: > > ;; Bind the variables. > > ;; For `let', do it in reverse order, because it makes no > > ;; semantic difference, but it is a lot more efficient since the > > ;; values are now in reverse order on the stack. > > , is not true. It can make a semantic difference. So doing the binding > > in reverse order is a bug. > Note that this is talking about the actual binding operations, which is > separate from the computation of the values that are to be given. > What this is saying is that > (let ((X1 E1) > (X2 E2)) > ...) > can be compiled to > > > varbind X2 > varbind X1 > since computing E pushes it value on the stack, so after the two > "compute" we have the values of E1 and E2 on the stack and we can pop > them in reverse order. It seems apparent that the computations are being done in reverse order, too. That can be seen in the above *Messages* output as well as in byte-compile-let in bytecomp.el: (dolist (var (if is-let (reverse clauses) clauses)) (unless is-let (push (byte-compile-push-binding-init var) init-lexenv)) (let ((var (if (consp var) (car var) var))) (if (byte-compile-bind var init-lexenv) (pop init-lexenv)))) .. (reverse clauses) happens before the code for any of the value forms gets generated. > And indeed it should make no difference if we > do the `varbind X1` before or after `varbind X2` as long as they get > the right value and as long as we don't compute anything which depends > on those vars in-between. Yes, that is all true. But the byte compiler generates code which does the _evaluation_ of the values in the wrong order, according to the description of let on page "Local Variables" of the elisp manual. This is a bug in either the byte compiler or the documentation of let. I would tend towards the first of these alternatives. Whatever, that let in jit-lock--debug-fontify would probably be safer if it were a let*. > Stefan -- Alan Mackenzie (Nuremberg, Germany).