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

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Date:   Fri, 30 Oct 2020 19:16:50 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     David Laight <David.Laight@...LAB.COM>,
        'Arvind Sankar' <nivedita@...m.mit.edu>,
        Thomas Gleixner <tglx@...utronix.de>
Cc:     '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 29/10/20 23:12, David Laight wrote:
>> 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.

Yeah, setting

	p = (struct s *) ((char *)&p) - 8;

invokes undefined behavior _for a different reason than strict aliasing_
(it's a pointer that is based on "p" but points before its start or
after one byte past its end).  So the compiler assumes that only the
first few bytes of a global can overlap it.

If you change the size of the fields from long to char in the compiler
explorer link above, every field forces a reload of the global.

Paolo