linux-kernel - Re: [PATCH] [v2] x86: apic: avoid -Wshadow warning in header

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Message-ID: <87v9esojdi.fsf@nanos.tec.linutronix.de>
Date:   Thu, 29 Oct 2020 21:41:13 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Paolo Bonzini <pbonzini@...hat.com>,
        Arvind Sankar <nivedita@...m.mit.edu>,
        David Laight <David.Laight@...LAB.COM>
Cc:     'Arnd Bergmann' <arnd@...nel.org>, Ingo Molnar <mingo@...hat.com>,
        Borislav Petkov <bp@...en8.de>,
        "x86\@kernel.org" <x86@...nel.org>, Arnd Bergmann <arnd@...db.de>,
        "K. Y. Srinivasan" <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        "linux-hyperv\@vger.kernel.org" <linux-hyperv@...r.kernel.org>,
        "linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>,
        "kvm\@vger.kernel.org" <kvm@...r.kernel.org>,
        "platform-driver-x86\@vger.kernel.org" 
        <platform-driver-x86@...r.kernel.org>,
        "xen-devel\@lists.xenproject.org" <xen-devel@...ts.xenproject.org>,
        "iommu\@lists.linux-foundation.org" 
        <iommu@...ts.linux-foundation.org>
Subject: Re: [PATCH] [v2] x86: apic: avoid -Wshadow warning in header

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