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=-3.8 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS 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 6C4AEC4743F for ; Tue, 8 Jun 2021 12:48:49 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 4EB1F60FF0 for ; Tue, 8 Jun 2021 12:48:49 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232608AbhFHMuk convert rfc822-to-8bit (ORCPT ); Tue, 8 Jun 2021 08:50:40 -0400 Received: from eu-smtp-delivery-151.mimecast.com ([185.58.85.151]:25623 "EHLO eu-smtp-delivery-151.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232299AbhFHMuj (ORCPT ); Tue, 8 Jun 2021 08:50:39 -0400 Received: from AcuMS.aculab.com (156.67.243.121 [156.67.243.121]) (Using TLS) by relay.mimecast.com with ESMTP id uk-mta-286-rjzIJM_MPTyFs9Jyebup6g-1; Tue, 08 Jun 2021 13:48:41 +0100 X-MC-Unique: rjzIJM_MPTyFs9Jyebup6g-1 Received: from AcuMS.Aculab.com (fd9f:af1c:a25b:0:994c:f5c2:35d6:9b65) by AcuMS.aculab.com (fd9f:af1c:a25b:0:994c:f5c2:35d6:9b65) with Microsoft SMTP Server (TLS) id 15.0.1497.18; Tue, 8 Jun 2021 13:48:40 +0100 Received: from AcuMS.Aculab.com ([fe80::994c:f5c2:35d6:9b65]) by AcuMS.aculab.com ([fe80::994c:f5c2:35d6:9b65%12]) with mapi id 15.00.1497.018; Tue, 8 Jun 2021 13:48:40 +0100 From: David Laight To: 'Peter Zijlstra' , Linus Torvalds , "will@kernel.org" , "paulmck@kernel.org" , "stern@rowland.harvard.edu" , "parri.andrea@gmail.com" , "boqun.feng@gmail.com" , "npiggin@gmail.com" , "dhowells@redhat.com" , "j.alglave@ucl.ac.uk" , "luc.maranget@inria.fr" , "akiyks@gmail.com" CC: "linux-kernel@vger.kernel.org" , "linux-toolchains@vger.kernel.org" , "linux-arch@vger.kernel.org" Subject: RE: [RFC] LKMM: Add volatile_if() Thread-Topic: [RFC] LKMM: Add volatile_if() Thread-Index: AQHXWSohzZ+wiGIEgk6EAayn44xGXqsKFedw Date: Tue, 8 Jun 2021 12:48:40 +0000 Message-ID: <208f76bd2815443981e73d790b1b7174@AcuMS.aculab.com> References: In-Reply-To: Accept-Language: en-GB, en-US X-MS-Has-Attach: X-MS-TNEF-Correlator: x-ms-exchange-transport-fromentityheader: Hosted x-originating-ip: [10.202.205.107] MIME-Version: 1.0 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=C51A453 smtp.mailfrom=david.laight@aculab.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: aculab.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8BIT Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: Peter Zijlstra > Sent: 04 June 2021 11:12 > > Hi! > > With optimizing compilers becoming more and more agressive and C so far > refusing to acknowledge the concept of control-dependencies even while > we keep growing the amount of reliance on them, things will eventually > come apart. > > There have been talks with toolchain people on how to resolve this; one > suggestion was allowing the volatile qualifier on branch statements like > 'if', but so far no actual compiler has made any progress on this. > > Rather than waiting any longer, provide our own construct based on that > suggestion. The idea is by Alan Stern and refined by Paul and myself. > > Code generation is sub-optimal (for the weak architectures) since we're > forced to convert the condition into another and use a fixed conditional > branch instruction, but shouldn't be too bad. What happens on mips-like architectures (I think includes riscv) that have 'compare two registers and branch' instructions rather than a more traditional 'flags register'? The generated code it likely to be somewhat different. David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)