From mboxrd@z Thu Jan 1 00:00:00 1970 Path: news.gmane.org!.POSTED.blaine.gmane.org!not-for-mail From: =?utf-8?Q?=C3=93scar_Fuentes?= Newsgroups: gmane.emacs.devel Subject: Re: Using __builtin_expect (likely/unlikely macros) Date: Sat, 20 Apr 2019 21:35:47 +0200 Message-ID: <87zhoko4fg.fsf@telefonica.net> 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> <86ef5wd7az.fsf@gmail.com> <9461246c-409b-15fd-943b-3d673c679870@cs.ucla.edu> <87imv8prov.fsf@telefonica.net> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8bit Injection-Info: blaine.gmane.org; posting-host="blaine.gmane.org:195.159.176.226"; logging-data="209981"; mail-complaints-to="usenet@blaine.gmane.org" User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/27.0.50 (gnu/linux) To: emacs-devel@gnu.org Original-X-From: emacs-devel-bounces+ged-emacs-devel=m.gmane.org@gnu.org Sat Apr 20 21:36:08 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 1hHvmQ-000sNN-O4 for ged-emacs-devel@m.gmane.org; Sat, 20 Apr 2019 21:36:06 +0200 Original-Received: from localhost ([127.0.0.1]:44711 helo=lists.gnu.org) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1hHvmP-0002ao-Pw for ged-emacs-devel@m.gmane.org; Sat, 20 Apr 2019 15:36:05 -0400 Original-Received: from eggs.gnu.org ([209.51.188.92]:33302) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1hHvmJ-0002aO-Gd for emacs-devel@gnu.org; Sat, 20 Apr 2019 15:36:00 -0400 Original-Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1hHvmI-00023e-Gw for emacs-devel@gnu.org; Sat, 20 Apr 2019 15:35:59 -0400 Original-Received: from [195.159.176.226] (port=41620 helo=blaine.gmane.org) by eggs.gnu.org with esmtps (TLS1.0:RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1hHvmH-00021s-PX for emacs-devel@gnu.org; Sat, 20 Apr 2019 15:35:58 -0400 Original-Received: from list by blaine.gmane.org with local (Exim 4.89) (envelope-from ) id 1hHvmG-000s7f-2P for emacs-devel@gnu.org; Sat, 20 Apr 2019 21:35:56 +0200 X-Injected-Via-Gmane: http://gmane.org/ Cancel-Lock: sha1:bZ6YUNChVb2+J+bBai+cQ2PVPR8= X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] X-Received-From: 195.159.176.226 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:235713 Archived-At: Paul Eggert writes: > Óscar Fuentes wrote: >>> For improvements where the >>> generated code is "obviously" faster (fewer and simpler instructions, >>> say), I typically don't bother with measurements as my own time is >>> limited too. >> You will be surprised. >> >> Modern hardware is complex. > > I'm often surprised :-), but I don't expect to be surprised when the > generated code seems obviously faster to me. Although I'm not as > expert as the computer architecture researcher whose office sits next > to mine, I know how modern hardware works reasonably well and > regularly give lectures on topics like μops and TLBs so I won't be > surprised as often as a naive programmer might be. I served many years as an Assembler programmer replacing C code where it was considered too slow so, excuse my arrogance, I'm far from being a naive programmer. I've seen many times cases where removing code (deleting unnecessary instructions and even full procedure calls) made the program run noticeably slower. "noticeably" as in "you don't need a stopwatch to notice it". Also, I've seen plenty of speedups on my development machine that vanished or even reversed when tried on a different microarchitecture, or the same microarchitecture with different cache sizes, or even the same CPU with different RAM modules. On our time of multi-level caches, branch prediction, speculative execution and what-not, acting as if your hard-earned 1% speedup will apply to most machines out there is a risky bet. >> If the 1.3% improvement in performance requires non-minimal source >> code complexity growth > > It doesn't. In this case the source code shrank slightly. Only very > slightly: just by 0.001% (this counts all files under Git control). > Still, a win's a win. Yea, but now everybody has to learn about and, worse, deal with those non-standard macros. That's added complexity. And I'm pretty sure that the speedup will vanish as code is modified and compilers and CPUs change.