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: Fri, 19 Apr 2019 10:33:43 -0700 Organization: UCLA Computer Science Department Message-ID: 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> <2dfb837d-989d-c736-b6e6-b20c0e940596@cs.ucla.edu> <87o956c4n4.fsf@gmail.com> <1fbd2fca-18f0-0a90-7a45-58419a9e11ee@cs.ucla.edu> <1555450070.23658.4@yandex.ru> <66b74701-012a-902e-4a5b-6bc30efa87c0@cs.ucla.edu> <87tveu85xt.fsf@gmail.com> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Injection-Info: blaine.gmane.org; posting-host="blaine.gmane.org:195.159.176.226"; logging-data="147600"; 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, Stefan Monnier , Konstantin Kharlamov To: Alex Gramiak Original-X-From: emacs-devel-bounces+ged-emacs-devel=m.gmane.org@gnu.org Fri Apr 19 19:34:02 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 1hHXOk-000cGb-4W for ged-emacs-devel@m.gmane.org; Fri, 19 Apr 2019 19:34:02 +0200 Original-Received: from localhost ([127.0.0.1]:59382 helo=lists.gnu.org) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1hHXOj-0002uK-3T for ged-emacs-devel@m.gmane.org; Fri, 19 Apr 2019 13:34:01 -0400 Original-Received: from eggs.gnu.org ([209.51.188.92]:38010) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1hHXOW-0002t0-VI for emacs-devel@gnu.org; Fri, 19 Apr 2019 13:33:49 -0400 Original-Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1hHXOV-0005Yk-LP for emacs-devel@gnu.org; Fri, 19 Apr 2019 13:33:48 -0400 Original-Received: from zimbra.cs.ucla.edu ([131.179.128.68]:53590) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1hHXOV-0005YH-6t for emacs-devel@gnu.org; Fri, 19 Apr 2019 13:33:47 -0400 Original-Received: from localhost (localhost [127.0.0.1]) by zimbra.cs.ucla.edu (Postfix) with ESMTP id 2361216179B; Fri, 19 Apr 2019 10:33:45 -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 GWljCZXEZyJm; Fri, 19 Apr 2019 10:33:44 -0700 (PDT) Original-Received: from localhost (localhost [127.0.0.1]) by zimbra.cs.ucla.edu (Postfix) with ESMTP id 418E7161777; Fri, 19 Apr 2019 10:33:44 -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 qSlXpzQR5Umh; Fri, 19 Apr 2019 10:33:44 -0700 (PDT) Original-Received: from Penguin.CS.UCLA.EDU (Penguin.CS.UCLA.EDU [131.179.64.200]) by zimbra.cs.ucla.edu (Postfix) with ESMTPSA id 16630160F1A; Fri, 19 Apr 2019 10:33:44 -0700 (PDT) Openpgp: preference=signencrypt Autocrypt: addr=eggert@cs.ucla.edu; prefer-encrypt=mutual; keydata= xsFNBEyAcmQBEADAAyH2xoTu7ppG5D3a8FMZEon74dCvc4+q1XA2J2tBy2pwaTqfhpxxdGA9 Jj50UJ3PD4bSUEgN8tLZ0san47l5XTAFLi2456ciSl5m8sKaHlGdt9XmAAtmXqeZVIYX/UFS 96fDzf4xhEmm/y7LbYEPQdUdxu47xA5KhTYp5bltF3WYDz1Ygd7gx07Auwp7iw7eNvnoDTAl KAl8KYDZzbDNCQGEbpY3efZIvPdeI+FWQN4W+kghy+P6au6PrIIhYraeua7XDdb2LS1en3Ss mE3QjqfRqI/A2ue8JMwsvXe/WK38Ezs6x74iTaqI3AFH6ilAhDqpMnd/msSESNFt76DiO1ZK QMr9amVPknjfPmJISqdhgB1DlEdw34sROf6V8mZw0xfqT6PKE46LcFefzs0kbg4GORf8vjG2 Sf1tk5eU8MBiyN/bZ03bKNjNYMpODDQQwuP84kYLkX2wBxxMAhBxwbDVZudzxDZJ1C2VXujC OJVxq2kljBM9ETYuUGqd75AW2LXrLw6+MuIsHFAYAgRr7+KcwDgBAfwhPBYX34nSSiHlmLC+ KaHLeCLF5ZI2vKm3HEeCTtlOg7xZEONgwzL+fdKo+D6SoC8RRxJKs8a3sVfI4t6CnrQzvJbB n6gxdgCu5i29J1QCYrCYvql2UyFPAK+do99/1jOXT4m2836j1wARAQABzSBQYXVsIEVnZ2Vy dCA8ZWdnZXJ0QGNzLnVjbGEuZWR1PsLBfgQTAQIAKAUCTIByZAIbAwUJEswDAAYLCQgHAwIG FQgCCQoLBBYCAwECH In-Reply-To: <87tveu85xt.fsf@gmail.com> 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:235666 Archived-At: On 4/19/19 6:45 AM, Alex Gramiak wrote: > What do you think about marking a few bytecode cases as > cold/unused? I've attached a diff below that does that. Does it make a > difference for you on your setup? No, it generates the same machine code for me on Fedora 29 x86-64, gcc 8.3.1 20190223 (Red Hat 8.3.1-2), with gcc -O2. > It seems to slow Emacs down a slight > bit for me, but I was hoping you might know why it would do so. Since > the newly cold attributes should be unused, is this perhaps a GCC bug? Possibly. Or it could be just bad luck. I've often made small changes and noted slight slowdowns or speedups that are due to luck rather than any real effects. Sometimes a simple 'make bootstrap' reverses the slowdown or speedup; sometimes it's more complicated than that. I suppose it could be related to lucky alignment in executables, or to code that just happens to exceed the associativity of my L1 or TLB caches, or whatever. So one must view these performance measurements with some skepticism. >> This patch also adds a convenience macro AVOID for the now-common pattern >> '_Noreturn ATTRIBUTE_COLD void'. > I'm not sure about the name. If I wasn't part of this discussion I might > have thought AVOID meant that one should avoid usage of the procedure in > new code. Not a big deal, of course. Yes, I considered other names but they all had problems too, and AVOID worked the best for me of the names that I thought of. A mnemonic is that 'extern AVOID time_overflow (void);' means that execution typically avoids calling time_overflow. If someone can think of a better name that would be good. Or we can just get accustomed to AVOID; as you say, it's not that big a deal.