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 A7FFBC433ED for ; Wed, 14 Apr 2021 17:25:43 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 7C35561179 for ; Wed, 14 Apr 2021 17:25:43 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1350215AbhDNR0E (ORCPT ); Wed, 14 Apr 2021 13:26:04 -0400 Received: from gate.crashing.org ([63.228.1.57]:53260 "EHLO gate.crashing.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231475AbhDNR0C (ORCPT ); Wed, 14 Apr 2021 13:26:02 -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 13EHK42L023677; Wed, 14 Apr 2021 12:20:04 -0500 Received: (from segher@localhost) by gate.crashing.org (8.14.1/8.14.1/Submit) id 13EHK3HZ023672; Wed, 14 Apr 2021 12:20:03 -0500 X-Authentication-Warning: gate.crashing.org: segher set sender to segher@kernel.crashing.org using -f Date: Wed, 14 Apr 2021 12:20:03 -0500 From: Segher Boessenkool To: David Laight Cc: Christophe Leroy , Paul Mackerras , "linuxppc-dev@lists.ozlabs.org" , "linux-kernel@vger.kernel.org" , Nicholas Piggin Subject: Re: [PATCH v1 1/2] powerpc/bitops: Use immediate operand when possible Message-ID: <20210414172003.GX26583@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> <20210414151921.GW26583@gate.crashing.org> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline 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 03:32:04PM +0000, David Laight wrote: > From: Segher Boessenkool > > Sent: 14 April 2021 16:19 > ... > > > 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 :-) > > I suspect they just add an extra layer of abstraction that makes it > even more difficult to verify and could easily get broken by a compiler > update (etc). I would say it uses an existing facility, instead of creating a kernel- specific one. > The other issue is that the code needs to be correct with compiled > with (for example) -O0. > That could very easily break anything except the asm implementation > if additional memory accesses and/or increased code size cause grief. The compiler generates correct code. New versions of the compiler or old, -O0 or not, under any phase of the moon. Of course sometimes the compiler is broken, but there are pre-existing ways of dealing with that, and there is no reason at all to think this would break more often than random other code. Segher