From mboxrd@z Thu Jan 1 00:00:00 1970 Path: news.gmane.org!.POSTED.blaine.gmane.org!not-for-mail From: Paul Eggert Newsgroups: gmane.emacs.devel Subject: Re: Using __builtin_expect (likely/unlikely macros) Date: Tue, 16 Apr 2019 08:22:36 -0700 Organization: UCLA Computer Science Department Message-ID: <2dfb837d-989d-c736-b6e6-b20c0e940596@cs.ucla.edu> References: <87a7gst973.fsf@gmail.com> <875zrgt12q.fsf@gmail.com> <6919a4c8-df76-ea1e-34db-1fa62a360e5a@cs.ucla.edu> <87h8aykdod.fsf@gmail.com> <4fa7885e-8c66-c7c4-ff71-a013505863af@cs.ucla.edu> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 7bit Injection-Info: blaine.gmane.org; posting-host="blaine.gmane.org:195.159.176.226"; logging-data="130665"; mail-complaints-to="usenet@blaine.gmane.org" User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.6.1 Cc: emacs-devel@gnu.org To: Stefan Monnier Original-X-From: emacs-devel-bounces+ged-emacs-devel=m.gmane.org@gnu.org Tue Apr 16 17:24:34 2019 Return-path: Envelope-to: ged-emacs-devel@m.gmane.org Original-Received: from lists.gnu.org ([209.51.188.17]) by blaine.gmane.org with esmtps (TLS1.0:RSA_AES_256_CBC_SHA1:256) (Exim 4.89) (envelope-from ) id 1hGPwo-000XrD-5s for ged-emacs-devel@m.gmane.org; Tue, 16 Apr 2019 17:24:34 +0200 Original-Received: from localhost ([127.0.0.1]:38357 helo=lists.gnu.org) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1hGPwn-0007la-2I for ged-emacs-devel@m.gmane.org; Tue, 16 Apr 2019 11:24:33 -0400 Original-Received: from eggs.gnu.org ([209.51.188.92]:38276) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1hGPv5-0006bi-Ii for emacs-devel@gnu.org; Tue, 16 Apr 2019 11:22:51 -0400 Original-Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1hGPv3-0004VA-PU for emacs-devel@gnu.org; Tue, 16 Apr 2019 11:22:46 -0400 Original-Received: from zimbra.cs.ucla.edu ([131.179.128.68]:50254) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1hGPv1-0004Rr-6x for emacs-devel@gnu.org; Tue, 16 Apr 2019 11:22:43 -0400 Original-Received: from localhost (localhost [127.0.0.1]) by zimbra.cs.ucla.edu (Postfix) with ESMTP id 2FD5716171C; Tue, 16 Apr 2019 08:22:41 -0700 (PDT) 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 kZqYXGzEKbBp; Tue, 16 Apr 2019 08:22:40 -0700 (PDT) Original-Received: from localhost (localhost [127.0.0.1]) by zimbra.cs.ucla.edu (Postfix) with ESMTP id 4ABF416172B; Tue, 16 Apr 2019 08:22:40 -0700 (PDT) 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 BlXqA-7ks4Hv; Tue, 16 Apr 2019 08:22:40 -0700 (PDT) Original-Received: from [192.168.1.9] (cpe-23-242-74-103.socal.res.rr.com [23.242.74.103]) by zimbra.cs.ucla.edu (Postfix) with ESMTPSA id 26D281616C4; Tue, 16 Apr 2019 08:22:40 -0700 (PDT) In-Reply-To: Content-Language: en-US X-detected-operating-system: by eggs.gnu.org: GNU/Linux 3.x 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:235524 Archived-At: Stefan Monnier wrote: > FWIW, the fact that a function is "_Noreturn" doesn't necessarily mean > that a call to it is unlikely (in many cases it is, I guess, but > definitely not all), so maybe GCC maintainers consciously decided not to > link the two. Possibly they did. In hindsight I'd argue that was a mistake. If one has no other evidence about the likelihood of a branch, a branch to a _Noreturn call should default to being unlikely. > BTW I think instead of marking branches as likely or unlikely, I'd > prefer to tell GCC that some functions "should be slow" > (e.g. emacs_abort) so it optimizes the code paths that don't go through > those functions to the detriment of those that do. GCC has the function attribute 'cold' for that. This is less intrusive than __builtin_expect and so would be preferable. Still, the GCC manual says that __attribute__ ((cold)) is ignored when profile feedback is available, which is another indication that people who care about performance should be using -fprofile-use etc. And as far as I know __attribute__ ((cold)) is rarely used: even glibc uses it only once, in obscure code never used on GNU/Linux. Presumably this is partly because the attribute didn't exist until about five years ago.