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=-4.0 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 6E01DC433E3 for ; Thu, 23 Jul 2020 15:36:58 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 53509206D8 for ; Thu, 23 Jul 2020 15:36:58 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729497AbgGWPg5 convert rfc822-to-8bit (ORCPT ); Thu, 23 Jul 2020 11:36:57 -0400 Received: from eu-smtp-delivery-151.mimecast.com ([207.82.80.151]:33514 "EHLO eu-smtp-delivery-151.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727108AbgGWPg5 (ORCPT ); Thu, 23 Jul 2020 11:36:57 -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-70-7crC5ShNNamsn-Pz86RPdg-1; Thu, 23 Jul 2020 16:36:53 +0100 X-MC-Unique: 7crC5ShNNamsn-Pz86RPdg-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, 23 Jul 2020 16:36:52 +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, 23 Jul 2020 16:36:52 +0100 From: David Laight To: 'Al Viro' CC: Linus Torvalds , "linux-kernel@vger.kernel.org" , "linux-arch@vger.kernel.org" Subject: RE: [PATCH 04/18] csum_and_copy_..._user(): pass 0xffffffff instead of 0 as initial sum Thread-Topic: [PATCH 04/18] csum_and_copy_..._user(): pass 0xffffffff instead of 0 as initial sum Thread-Index: AQHWX51MlcPCEWebQUuN/OB/armWnKkTU0FggABJU4CAABlpkP//+uQAgAAU8xCAAAgogIABYH9ggAAcDMyAAAKlwA== Date: Thu, 23 Jul 2020 15:36:52 +0000 Message-ID: <1ba695b2264349f187f825ff2c308624@AcuMS.aculab.com> References: <20200721202549.4150745-1-viro@ZenIV.linux.org.uk> <20200721202549.4150745-4-viro@ZenIV.linux.org.uk> <2d85ebb8ea2248c8a14f038a0c60297e@AcuMS.aculab.com> <20200722144213.GE2786714@ZenIV.linux.org.uk> <4e03cce8ed184d40bb0ea40fd3d51000@AcuMS.aculab.com> <20200722155452.GF2786714@ZenIV.linux.org.uk> <20200722173903.GG2786714@ZenIV.linux.org.uk> <02938acd78fd40beb02ffc5a1b803d85@AcuMS.aculab.com> <20200723145342.GH2786714@ZenIV.linux.org.uk> <20200723152101.GI2786714@ZenIV.linux.org.uk> In-Reply-To: <20200723152101.GI2786714@ZenIV.linux.org.uk> 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 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: aculab.com Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8BIT Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: Al Viro > Sent: 23 July 2020 16:21 ... > The point is, your "~4.5 cycles per vector" is pretty much noise and the > difference between the 3-argument and 4-argument variants could easily be > in the same range. It might be a valid microoptimization, it might be not. > 3-argument variant is simpler and IMO in absence of strong data we ought > to go with that. There is definitely more to be gained by rewriting the x86-86 asm. David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)