From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-2.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id AEEA6C7618B for ; Thu, 25 Jul 2019 13:23:04 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 8483A2190F for ; Thu, 25 Jul 2019 13:23:04 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1730036AbfGYNXD (ORCPT ); Thu, 25 Jul 2019 09:23:03 -0400 Received: from gate.crashing.org ([63.228.1.57]:43593 "EHLO gate.crashing.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725852AbfGYNXC (ORCPT ); Thu, 25 Jul 2019 09:23:02 -0400 Received: from gate.crashing.org (localhost.localdomain [127.0.0.1]) by gate.crashing.org (8.14.1/8.14.1) with ESMTP id x6PDMX5o003118; Thu, 25 Jul 2019 08:22:34 -0500 Received: (from segher@localhost) by gate.crashing.org (8.14.1/8.14.1/Submit) id x6PDMWpp003114; Thu, 25 Jul 2019 08:22:32 -0500 X-Authentication-Warning: gate.crashing.org: segher set sender to segher@kernel.crashing.org using -f Date: Thu, 25 Jul 2019 08:22:32 -0500 From: Segher Boessenkool To: Michael Ellerman Cc: Nathan Chancellor , Christophe Leroy , Benjamin Herrenschmidt , Paul Mackerras , linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org, clang-built-linux@googlegroups.com Subject: Re: [PATCH v2] powerpc: slightly improve cache helpers Message-ID: <20190725132232.GQ20882@gate.crashing.org> References: <20190709064952.GA40851@archlinux-threadripper> <20190719032456.GA14108@archlinux-threadripper> <20190719152303.GA20882@gate.crashing.org> <20190719160455.GA12420@archlinux-threadripper> <20190721075846.GA97701@archlinux-threadripper> <20190721180150.GN20882@gate.crashing.org> <87imru74ul.fsf@concordia.ellerman.id.au> <20190722151801.GC20882@gate.crashing.org> <875znt7izy.fsf@concordia.ellerman.id.au> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <875znt7izy.fsf@concordia.ellerman.id.au> User-Agent: Mutt/1.4.2.3i Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Jul 23, 2019 at 09:21:53AM +1000, Michael Ellerman wrote: > Segher Boessenkool writes: > >> can use both RA and RB to compute the address, rather than us forcing RA > >> to 0. > >> > >> But at least with my compiler here (GCC 8 vintage) I don't actually see > >> GCC ever using both GPRs even with the patch. Or at least, there's no > >> difference before/after the patch as far as I can see. > > > > The benefit is small, certainly. > > Zero is small, but I guess some things are smaller? :P Heh. 0 out of 12 is small. It actually is quite easy to do trigger the macros to generate two-reg dcb* instructions; but all the places where that is especially useful, in loops for example, already use hand-written assembler code (and yes, using two-reg forms). You probably will not want to write those routines as plain C ever given how important those are for performance (memset, clear-a-page), so the dcb* macros won't ever be very hot, oh well. > >> So my inclination is to revert the original patch. We can try again in a > >> few years :D > >> > >> Thoughts? > > > > I think you should give the clang people time to figure out what is > > going on. > > Yeah fair enough, will wait and see what their diagnosis is. Thanks! Segher