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

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Date:   Thu, 29 Oct 2020 13:20:48 -0400
From:   Arvind Sankar <nivedita@...m.mit.edu>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Arvind Sankar <nivedita@...m.mit.edu>,
        David Laight <David.Laight@...LAB.COM>,
        'Arnd Bergmann' <arnd@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        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 05:59:54PM +0100, 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.
> 
> Paolo
> 
> (*) if it is, it might also be due to Linux using -fno-strict-aliasing
> 

Nope, it doesn't read it multiple times. I guess it still assumes that
apic isn't in the middle of what it points to: it would reload the
address if the first element of *apic was modified, but doesn't for
other elements. Interesting.