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=-1.0 required=3.0 tests=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 74583C433E1 for ; Thu, 2 Jul 2020 09:37:33 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 4964C20874 for ; Thu, 2 Jul 2020 09:37:33 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728279AbgGBJhc convert rfc822-to-8bit (ORCPT ); Thu, 2 Jul 2020 05:37:32 -0400 Received: from eu-smtp-delivery-151.mimecast.com ([207.82.80.151]:28594 "EHLO eu-smtp-delivery-151.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728241AbgGBJhb (ORCPT ); Thu, 2 Jul 2020 05:37:31 -0400 Received: from AcuMS.aculab.com (156.67.243.126 [156.67.243.126]) (Using TLS) by relay.mimecast.com with ESMTP id uk-mta-132-_943ttHWM7K-SXNAkBuyNw-1; Thu, 02 Jul 2020 10:37:27 +0100 X-MC-Unique: _943ttHWM7K-SXNAkBuyNw-1 Received: from AcuMS.Aculab.com (fd9f:af1c:a25b:0:43c:695e:880f:8750) by AcuMS.aculab.com (fd9f:af1c:a25b:0:43c:695e:880f:8750) with Microsoft SMTP Server (TLS) id 15.0.1347.2; Thu, 2 Jul 2020 10:37:26 +0100 Received: from AcuMS.Aculab.com ([fe80::43c:695e:880f:8750]) by AcuMS.aculab.com ([fe80::43c:695e:880f:8750%12]) with mapi id 15.00.1347.000; Thu, 2 Jul 2020 10:37:26 +0100 From: David Laight To: "'paulmck@kernel.org'" CC: 'Peter Zijlstra' , Marco Elver , Nick Desaulniers , Sami Tolvanen , Masahiro Yamada , "Will Deacon" , Greg Kroah-Hartman , Kees Cook , clang-built-linux , Kernel Hardening , linux-arch , Linux ARM , Linux Kbuild mailing list , LKML , "linux-pci@vger.kernel.org" , "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" Subject: RE: [PATCH 00/22] add support for Clang LTO Thread-Topic: [PATCH 00/22] add support for Clang LTO Thread-Index: AQHWT4eVR3DE4y9c50++UkzL75GurajywsMggAAQjQCAATOsAA== Date: Thu, 2 Jul 2020 09:37:26 +0000 Message-ID: References: <20200624211540.GS4817@hirez.programming.kicks-ass.net> <20200625080313.GY4817@hirez.programming.kicks-ass.net> <20200625082433.GC117543@hirez.programming.kicks-ass.net> <20200625085745.GD117543@hirez.programming.kicks-ass.net> <20200630191931.GA884155@elver.google.com> <20200630201243.GD4817@hirez.programming.kicks-ass.net> <20200630203016.GI9247@paulmck-ThinkPad-P72> <20200701091054.GW4781@hirez.programming.kicks-ass.net> <4427b0f825324da4b1640e32265b04bd@AcuMS.aculab.com> <20200701160624.GO9247@paulmck-ThinkPad-P72> In-Reply-To: <20200701160624.GO9247@paulmck-ThinkPad-P72> Accept-Language: en-GB, en-US Content-Language: 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-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8BIT Sender: linux-pci-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-pci@vger.kernel.org From: Paul E. McKenney > Sent: 01 July 2020 17:06 ... > > Would an asm statement that uses the same 'register' for input and > > output but doesn't actually do anything help? > > It won't generate any code, but the compiler ought to assume that > > it might change the value - so can't do optimisations that track > > the value across the call. > > It might replace the volatile load, but there are optimizations that > apply to the downstream code as well. > > Or are you suggesting periodically pushing the dependent variable > through this asm? That might work, but it would be easier and > more maintainable to just mark the variable. Marking the variable requires compiler support. Although what 'volatile register int foo;' means might be interesting. So I was thinking that in the case mentioned earlier you do: ptr += LAUNDER(offset & 1); to ensure the compiler didn't convert to: if (offset & 1) ptr++; (Which is probably a pessimisation - the reverse is likely better.) David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)