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Subject: Re: [PATCH] [v2] x86: apic: avoid -Wshadow warning in header
To:     David Laight <David.Laight@ACULAB.COM>,
        'Arvind Sankar' <nivedita@alum.mit.edu>,
        Thomas Gleixner <tglx@linutronix.de>
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From:   Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <070f590f-b702-35f0-0b6c-c6455f08e9d5@redhat.com>
Date:   Fri, 30 Oct 2020 19:16:50 +0100
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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