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

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Date:   Thu, 29 Oct 2020 17:35:12 -0400
From:   Arvind Sankar <nivedita@...m.mit.edu>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Arvind Sankar <nivedita@...m.mit.edu>,
        David Laight <David.Laight@...LAB.COM>,
        'Arnd Bergmann' <arnd@...nel.org>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "x86@...nel.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@...r.kernel.org" <linux-hyperv@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "platform-driver-x86@...r.kernel.org" 
        <platform-driver-x86@...r.kernel.org>,
        "xen-devel@...ts.xenproject.org" <xen-devel@...ts.xenproject.org>,
        "iommu@...ts.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 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.