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=-5.3 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 C829EC433ED for ; Wed, 14 Apr 2021 15:24:52 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id A9AFB61164 for ; Wed, 14 Apr 2021 15:24:52 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S234117AbhDNPZL (ORCPT ); Wed, 14 Apr 2021 11:25:11 -0400 Received: from gate.crashing.org ([63.228.1.57]:38657 "EHLO gate.crashing.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232358AbhDNPZF (ORCPT ); Wed, 14 Apr 2021 11:25:05 -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 13EFJM4R012182; Wed, 14 Apr 2021 10:19:22 -0500 Received: (from segher@localhost) by gate.crashing.org (8.14.1/8.14.1/Submit) id 13EFJLFh012181; Wed, 14 Apr 2021 10:19:21 -0500 X-Authentication-Warning: gate.crashing.org: segher set sender to segher@kernel.crashing.org using -f Date: Wed, 14 Apr 2021 10:19:21 -0500 From: Segher Boessenkool To: Christophe Leroy Cc: Nicholas Piggin , linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, Paul Mackerras Subject: Re: [PATCH v1 1/2] powerpc/bitops: Use immediate operand when possible Message-ID: <20210414151921.GW26583@gate.crashing.org> References: <09da6fec57792d6559d1ea64e00be9870b02dab4.1617896018.git.christophe.leroy@csgroup.eu> <20210412215428.GM26583@gate.crashing.org> <20210413215803.GT26583@gate.crashing.org> <1618365589.67fxh7cot9.astroid@bobo.none> <20210414122409.GV26583@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 Wed, Apr 14, 2021 at 02:42:51PM +0200, Christophe Leroy wrote: > Le 14/04/2021 à 14:24, Segher Boessenkool a écrit : > >On Wed, Apr 14, 2021 at 12:01:21PM +1000, Nicholas Piggin wrote: > >>Would be nice if we could let the compiler deal with it all... > >> > >>static inline unsigned long lr(unsigned long *mem) > >>{ > >> unsigned long val; > >> > >> /* > >> * This doesn't clobber memory but want to avoid memory > >> operations > >> * moving ahead of it > >> */ > >> asm volatile("ldarx %0, %y1" : "=r"(val) : "Z"(*mem) : > >> "memory"); > >> > >> return val; > >>} > > > >(etc.) > > > >That can not work reliably: the compiler can put random instructions > >between the larx and stcx. this way, and you then do not have guaranteed > >forward progress anymore. It can put the two in different routines > >(after inlining and other interprocedural optimisations), duplicate > >them, make a different number of copies of them, etc. > > > >Nothing of that is okay if you want to guarantee forward progress on all > >implementations, and also not if you want to have good performance > >everywhere (or anywhere even). Unfortunately you have to write all > >larx/stcx. loops as one block of assembler, so that you know exactly > >what instructions will end up in your binary. > > > >If you don't, it will fail mysteriously after random recompilations, or > >have performance degradations, etc. You don't want to go there :-) > > > > Could the kernel use GCC builtin atomic functions instead ? > > https://gcc.gnu.org/onlinedocs/gcc/_005f_005fatomic-Builtins.html Certainly that should work fine for the simpler cases that the atomic operations are meant to provide. But esp. for not-so-simple cases the kernel may require some behaviour provided by the existing assembler implementation, and not by the atomic builtins. I'm not saying this cannot work, just that some serious testing will be needed. If it works it should be the best of all worlds, so then it is a really good idea yes :-) Segher