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=-2.3 required=3.0 tests=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 8DEBFC10F14 for ; Tue, 8 Oct 2019 14:34:08 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 6ED3B20815 for ; Tue, 8 Oct 2019 14:34:08 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726948AbfJHOeH (ORCPT ); Tue, 8 Oct 2019 10:34:07 -0400 Received: from foss.arm.com ([217.140.110.172]:38216 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725839AbfJHOeH (ORCPT ); Tue, 8 Oct 2019 10:34:07 -0400 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 8B5AE1570; Tue, 8 Oct 2019 07:34:06 -0700 (PDT) Received: from [10.1.194.37] (e113632-lin.cambridge.arm.com [10.1.194.37]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 50A223F703; Tue, 8 Oct 2019 07:34:05 -0700 (PDT) Subject: Re: [PATCH v3 04/10] sched/fair: rework load_balance To: Peter Zijlstra Cc: Vincent Guittot , linux-kernel , Ingo Molnar , Phil Auld , Srikar Dronamraju , Quentin Perret , Dietmar Eggemann , Morten Rasmussen , Hillf Danton References: <1568878421-12301-1-git-send-email-vincent.guittot@linaro.org> <1568878421-12301-5-git-send-email-vincent.guittot@linaro.org> <31cac0c1-98e4-c70e-e156-51a70813beff@arm.com> <20191008141642.GQ2294@hirez.programming.kicks-ass.net> From: Valentin Schneider Message-ID: Date: Tue, 8 Oct 2019 15:34:04 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.8.0 MIME-Version: 1.0 In-Reply-To: <20191008141642.GQ2294@hirez.programming.kicks-ass.net> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 08/10/2019 15:16, Peter Zijlstra wrote: > On Wed, Oct 02, 2019 at 11:47:59AM +0100, Valentin Schneider wrote: > >> Yeah, right shift on signed negative values are implementation defined. > > Seriously? Even under -fno-strict-overflow? There is a perfectly > sensible operation for signed shift right, this stuff should not be > undefined. > Mmm good point. I didn't see anything relevant in the description of that flag. All my copy of the C99 standard (draft) says at 6.5.7.5 is: """ The result of E1 >> E2 [...] If E1 has a signed type and a negative value, the resulting value is implementation-defined. """ Arithmetic shift would make sense, but I think this stems from twos' complement not being imposed: 6.2.6.2.2 says sign can be done with sign + magnitude, twos complement or ones' complement... I suppose when you really just want a division you should ask for division semantics - i.e. use '/'. I'd expect compilers to be smart enough to turn that into a shift if a power of 2 is involved, and to do something else if negative values can be involved.