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=-3.7 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED 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 A6636C433B4 for ; Sat, 17 Apr 2021 13:59:38 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 854E0611ED for ; Sat, 17 Apr 2021 13:59:38 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S236397AbhDQOAD convert rfc822-to-8bit (ORCPT ); Sat, 17 Apr 2021 10:00:03 -0400 Received: from eu-smtp-delivery-151.mimecast.com ([185.58.86.151]:20565 "EHLO eu-smtp-delivery-151.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230442AbhDQOAC (ORCPT ); Sat, 17 Apr 2021 10:00:02 -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-213-vl0FeB5ZN9KlyvtC7G0NGQ-1; Sat, 17 Apr 2021 14:59:33 +0100 X-MC-Unique: vl0FeB5ZN9KlyvtC7G0NGQ-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 15.0.1497.2; Sat, 17 Apr 2021 14:59:32 +0100 Received: from AcuMS.Aculab.com ([fe80::994c:f5c2:35d6:9b65]) by AcuMS.aculab.com ([fe80::994c:f5c2:35d6:9b65%12]) with mapi id 15.00.1497.015; Sat, 17 Apr 2021 14:59:32 +0100 From: David Laight To: 'Al Viro' , Eric Dumazet CC: Thomas Gleixner , Linus Torvalds , linux-kernel , Eric Dumazet Subject: RE: [PATCH] x86/uaccess: small optimization in unsafe_copy_to_user() Thread-Topic: [PATCH] x86/uaccess: small optimization in unsafe_copy_to_user() Thread-Index: AQHXMvjk6jXIkdHnG0KLlGcZVxHm0qq4vBHA Date: Sat, 17 Apr 2021 13:59:32 +0000 Message-ID: References: <20210416192413.1514419-1-eric.dumazet@gmail.com> 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: Al Viro On Behalf Of Al Viro > Sent: 16 April 2021 20:44 > On Fri, Apr 16, 2021 at 12:24:13PM -0700, Eric Dumazet wrote: > > From: Eric Dumazet > > > > We have to loop only to copy u64 values. > > After this first loop, we copy at most one u32, one u16 and one byte. > > Does it actually yield a better code? > > FWIW, this > void bar(unsigned); > void foo(unsigned n) > { > while (n >= 8) { > bar(n); > n -= 8; > } > while (n >= 4) { > bar(n); > n -= 4; > } > while (n >= 2) { > bar(n); > n -= 2; > } > while (n >= 1) { > bar(n); > n -= 1; > } > } This variant might be better: void foo(unsigned n) { while (n >= 8) { bar(8); n -= 8; } if (likely(!n)) return; if (n & 4) bar(4); if (n & 2) bar(2); if (n & 1) bar(1); } I think Al's version might have optimised down to this, but Eric's asm contains the n -= 4/2/1; OTOH gcc can make a real pig's breakfast of code like this! David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)