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,DKIM_SIGNED, DKIM_VALID,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 89A55C4741F for ; Thu, 29 Oct 2020 21:35:21 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 258CB20838 for ; Thu, 29 Oct 2020 21:35:21 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=gmail.com header.i=@gmail.com header.b="gGuTqV/6" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1725813AbgJ2VfT (ORCPT ); Thu, 29 Oct 2020 17:35:19 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:47134 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725379AbgJ2VfS (ORCPT ); Thu, 29 Oct 2020 17:35:18 -0400 Received: from mail-io1-xd2f.google.com (mail-io1-xd2f.google.com [IPv6:2607:f8b0:4864:20::d2f]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id E78F8C0613CF; Thu, 29 Oct 2020 14:35:16 -0700 (PDT) Received: by mail-io1-xd2f.google.com with SMTP id u62so5270256iod.8; Thu, 29 Oct 2020 14:35:16 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=sender:from:date:to:cc:subject:message-id:references:mime-version :content-disposition:in-reply-to; bh=o/fZRhurDRoh3x2KXwZGFT2Yi8JsbQLcBDCy2LAivAY=; b=gGuTqV/6UjMeGtDxjXYhwQPHTl/X7tzC8nyDkopIigNmkrO3mCPbuH/1MhlZEXs368 ilsmPmjF8/8PcamJS4tzXcIssUUJKuowmvAaZpvnATyfAuLpyzByye2ob5V27GVE8qIT KZP17N/xFAWoIKkVe0RuMkFayeP8BWuJYRQfihoxN9IcITuyVnsNMyFFHot0k3ChkDxn nFZSLwwceJfQxsoqw1O2SZk7WDZNX5WeTFIk5dkyBtO3OtvmEbw8mG/eVrbi8eoLhfOn 0GZOx0BOnNgBQieydv87h3zVPKI0C/CSIt6VEOi3C7rjEkzw5bkznPJoY5+aGh/Uu3xA /g8g== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:sender:from:date:to:cc:subject:message-id :references:mime-version:content-disposition:in-reply-to; bh=o/fZRhurDRoh3x2KXwZGFT2Yi8JsbQLcBDCy2LAivAY=; b=KinZ6hrbkXJ3h3DclDuBAp8Nhpaw739QgjhpYXN2kOq16mF6EKn8Ncz8nxq1cB+RdC loTEqq5lPm4snNchDLwNgIUKdnDY5cHv1ycThuX+/LrmKeNpMBzlZD88dUtS8DRtT0fS O1WK6EcWuJg5q8WjYvucVAqpNoOV23EL3FR22XeNqy3YpSc45KlMYLS6anuZH/AbWyao HIC/o+eJgLkW6NEezx/TYHyDJpi9pUkGguel+AXgTrEL14OcltJbt+1MeSDaBytRBaLM 2Og8NWATW8Ty1bf4EMJL9uBGBpDN6puw4xlIk1fX0FCDL3Y3UVJeo0WcBT3Ak+eGyphi 48Uw== X-Gm-Message-State: AOAM530rBehHEPLKdCyAg69CswAyeQ1Lz1y2STmerKwBdnO3UkUCVFrP 9pbUYcrhb5F1jAkIq7T++vY= X-Google-Smtp-Source: ABdhPJyTkgnuu9ypsiIO3xCkmwdOh4o3IIPvbsxF62gfZSdk4g2zCe1B1vFwKdXsPs4FzLAVBSR7HA== X-Received: by 2002:a05:6602:22cf:: with SMTP id e15mr5087695ioe.1.1604007316212; Thu, 29 Oct 2020 14:35:16 -0700 (PDT) Received: from rani.riverdale.lan ([2001:470:1f07:5f3::b55f]) by smtp.gmail.com with ESMTPSA id r3sm3065346iog.55.2020.10.29.14.35.13 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 29 Oct 2020 14:35:15 -0700 (PDT) Sender: Arvind Sankar From: Arvind Sankar X-Google-Original-From: Arvind Sankar Date: Thu, 29 Oct 2020 17:35:12 -0400 To: Thomas Gleixner Cc: Paolo Bonzini , Arvind Sankar , David Laight , 'Arnd Bergmann' , Ingo Molnar , Borislav Petkov , "x86@kernel.org" , Arnd Bergmann , "K. Y. Srinivasan" , Haiyang Zhang , Stephen Hemminger , "H. Peter Anvin" , "Rafael J. Wysocki" , Vitaly Kuznetsov , Wanpeng Li , Jim Mattson , Joerg Roedel , "linux-hyperv@vger.kernel.org" , "linux-kernel@vger.kernel.org" , "kvm@vger.kernel.org" , "platform-driver-x86@vger.kernel.org" , "xen-devel@lists.xenproject.org" , "iommu@lists.linux-foundation.org" Subject: Re: [PATCH] [v2] x86: apic: avoid -Wshadow warning in header Message-ID: <20201029213512.GA34524@rani.riverdale.lan> References: <20201028212417.3715575-1-arnd@kernel.org> <38b11ed3fec64ebd82d6a92834a4bebe@AcuMS.aculab.com> <20201029165611.GA2557691@rani.riverdale.lan> <93180c2d-268c-3c33-7c54-4221dfe0d7ad@redhat.com> <87v9esojdi.fsf@nanos.tec.linutronix.de> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <87v9esojdi.fsf@nanos.tec.linutronix.de> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Oct 29, 2020 at 09:41:13PM +0100, Thomas Gleixner wrote: > On Thu, Oct 29 2020 at 17:59, Paolo Bonzini wrote: > > On 29/10/20 17:56, Arvind Sankar wrote: > >>> For those two just add: > >>> struct apic *apic = x86_system_apic; > >>> before all the assignments. > >>> Less churn and much better code. > >>> > >> Why would it be better code? > >> > > > > I think he means the compiler produces better code, because it won't > > read the global variable repeatedly. Not sure if that's true,(*) but I > > think I do prefer that version if Arnd wants to do that tweak. > > It's not true. > > foo *p = bar; > > p->a = 1; > p->b = 2; > > The compiler is free to reload bar after accessing p->a and with > > bar->a = 1; > bar->b = 1; > > it can either cache bar in a register or reread it after bar->a > > The generated code is the same as long as there is no reason to reload, > e.g. register pressure. > > Thanks, > > tglx It's not quite the same. https://godbolt.org/z/4dzPbM With -fno-strict-aliasing, the compiler reloads the pointer if you write to the start of what it points to, but not if you write to later elements.