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=-5.2 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, 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 C7512C433E7 for ; Tue, 20 Oct 2020 11:56:50 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 63266221FC for ; Tue, 20 Oct 2020 11:56:50 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2393913AbgJTL4t (ORCPT ); Tue, 20 Oct 2020 07:56:49 -0400 Received: from gate.crashing.org ([63.228.1.57]:35087 "EHLO gate.crashing.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2393831AbgJTL4t (ORCPT ); Tue, 20 Oct 2020 07:56:49 -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 09KBpIuN022840; Tue, 20 Oct 2020 06:51:18 -0500 Received: (from segher@localhost) by gate.crashing.org (8.14.1/8.14.1/Submit) id 09KBpGRn022835; Tue, 20 Oct 2020 06:51:16 -0500 X-Authentication-Warning: gate.crashing.org: segher set sender to segher@kernel.crashing.org using -f Date: Tue, 20 Oct 2020 06:51:16 -0500 From: Segher Boessenkool To: Christophe Leroy Cc: Benjamin Herrenschmidt , Paul Mackerras , Michael Ellerman , linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH 3/3] powerpc: Fix pre-update addressing in inline assembly Message-ID: <20201020115116.GY2672@gate.crashing.org> References: <5ffcb064f695d5285bf1faab91bffa3f9245fc26.1603109522.git.christophe.leroy@csgroup.eu> <20201019202441.GU2672@gate.crashing.org> Mime-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: User-Agent: Mutt/1.4.2.3i Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Oct 20, 2020 at 09:44:33AM +0200, Christophe Leroy wrote: > Le 19/10/2020 à 22:24, Segher Boessenkool a écrit : > >>but the associated "<>" constraint is missing. > > > >But that is just fine. Pointless, sure, but not a bug. > > Most of those are from prehistoric code. So at some point in time it was > effective. Then one day GCC changed it's way and they became pointless. So, > not a software bug, but still a regression at some point. > > >>Use UPD_CONSTR macro everywhere %Un modifier is used. > > > >Eww. My poor stomach. > > There are not that many :) Heh, your pain threshold is much higher than mine I guess :-) > >Have you verified that update form is *correct* in all these, and that > >we even *want* this there? > > I can't see anything that would militate against it, do you ? > > I guess if the elders have put %Us there, it was wanted. On old CPUs, update form load/stores actually executed faster than a "normal" memory access and an addi (or plain add). But on more recent stuff it mostly saves code size. Which is nice of course, and can speed up your code a bit, in theory at least. It is quite hard to trigger the compiler to generate update form insns in asm, sigh. So testing will probably not show anything either way. Oh well :-) Segher