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

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Message-ID: <ad73f56e79d249b1b3614bccc85e2ca5@AcuMS.aculab.com>
Date:   Thu, 29 Oct 2020 22:12:59 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Arvind Sankar' <nivedita@...m.mit.edu>,
        Thomas Gleixner <tglx@...utronix.de>
CC:     Paolo Bonzini <pbonzini@...hat.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

From: Arvind Sankar
> Sent: 29 October 2020 21:35
> 
> 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.

I guess it assumes that global data doesn't overlap.

But in general they are sort of opposites:

With the local variable it can reload if it knows the write
cannot have affected the global - but is unlikely to do so.

Using the global it must reload if it is possible the write
might have affected the global.

	David

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