From mboxrd@z Thu Jan 1 00:00:00 1970 Path: news.gmane.io!.POSTED.blaine.gmane.org!not-for-mail From: Andrea Corallo via "Emacs development discussions." Newsgroups: gmane.emacs.devel Subject: Re: What happened to TCO? Date: Thu, 18 Mar 2021 10:01:56 +0000 Message-ID: References: <87k0q58kvy.fsf@gmail.com> Reply-To: Andrea Corallo Mime-Version: 1.0 Content-Type: text/plain Injection-Info: ciao.gmane.io; posting-host="blaine.gmane.org:116.202.254.214"; logging-data="960"; mail-complaints-to="usenet@ciao.gmane.io" User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/28.0.50 (gnu/linux) Cc: Troy Hinckley , emacs-devel@gnu.org To: Stefan Monnier Original-X-From: emacs-devel-bounces+ged-emacs-devel=m.gmane-mx.org@gnu.org Thu Mar 18 11:03:29 2021 Return-path: Envelope-to: ged-emacs-devel@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 1lMpV2-00008Y-Pr for ged-emacs-devel@m.gmane-mx.org; Thu, 18 Mar 2021 11:03:28 +0100 Original-Received: from localhost ([::1]:36204 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1lMpV1-0006a9-QI for ged-emacs-devel@m.gmane-mx.org; Thu, 18 Mar 2021 06:03:27 -0400 Original-Received: from eggs.gnu.org ([2001:470:142:3::10]:47898) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1lMpTq-0005Rz-AZ for emacs-devel@gnu.org; Thu, 18 Mar 2021 06:02:14 -0400 Original-Received: from mx.sdf.org ([205.166.94.24]:61989) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1lMpTi-000511-6o for emacs-devel@gnu.org; Thu, 18 Mar 2021 06:02:14 -0400 Original-Received: from mab (ma.sdf.org [205.166.94.33]) by mx.sdf.org (8.15.2/8.14.5) with ESMTPS id 12IA1uuK016368 (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256 bits) verified NO); Thu, 18 Mar 2021 10:01:57 GMT In-Reply-To: (Stefan Monnier's message of "Wed, 17 Mar 2021 22:30:13 -0400") Received-SPF: pass client-ip=205.166.94.24; envelope-from=akrl@sdf.org; helo=mx.sdf.org 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 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: emacs-devel@gnu.org X-Mailman-Version: 2.1.23 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-mx.org@gnu.org Original-Sender: "Emacs-devel" Xref: news.gmane.io gmane.emacs.devel:266552 Archived-At: Stefan Monnier writes: >> I see two different patches from 2012 >> https://lists.gnu.org/archive/html/emacs-devel/2012-12/msg00283.html >> https://lists.gnu.org/archive/html/emacs-devel/2012-09/msg00477.html >> Neither was merged but I don't see any reason presented in the mailing >> list. Why were these changes not accepted? > > I think the lukewarm reception discouraged them. > > Also, the benefits were not made very clear: as Daniel pointed out back > then, if the TCO is only applied when byte-compiled, then you can't > always rely on it (while you can argue that interpreted code doesn't > matter to some extent, we still rely crucially on interpreted code > during the bootstrap and during Edebug sessions. We could fairly easily > circumvent the bootstrap problem, but for Edebug it requires a lot more > work). Also reliance on TCO means use of recursive calls, which may be > undesirable even with TCO if recursive calls are more expensive than > equivalent `while` loops. > > So, I think TCO with the current ELisp implementation should be seen > first and foremost as an "opportunistic optimization" rather than a new > semantic feature on which code can rely. Which begs for > benchmarks showing how it affects existing code. > But we haven't seen any, AFAICT. > > There are of course also potential other side-effects (e.g. impacts on > backtraces). I think these are hard to judge without actually using > such a patch for a while, so we'd probably want it to be conditional > at first. > > BTW, in the meant time (i.e. quite recently), I implemented another form > of TCO (much more limited than Chris Gray's patch, since it only applies > to self recursion and only for functions defined with `cl-labels`), but > one that works both for bytecode and for interpreted code (because the > optimization is done during macroexpansion). For completeness I just wanted to add that Tail Recursion Elimination is already present in the native-comp branch when compiling at speed 3. Andrea