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([2001:b07:6468:f312:c8dd:75d4:99ab:290a]) by smtp.gmail.com with ESMTPSA id r24sm3338060eds.67.2020.10.30.11.16.50 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 30 Oct 2020 11:16:51 -0700 (PDT) Subject: Re: [PATCH] [v2] x86: apic: avoid -Wshadow warning in header To: David Laight , 'Arvind Sankar' , Thomas Gleixner Cc: 'Arnd Bergmann' , Ingo Molnar , Borislav Petkov , "x86@kernel.org" , Arnd Bergmann , "K. Y. Srinivasan" , Haiyang Zhang , Stephen Hemminger , "H. Peter Anvin" , "Rafael J. Wysocki" , Vitaly Kuznetsov , Wanpeng Li , Jim Mattson , Joerg Roedel , "linux-hyperv@vger.kernel.org" , "linux-kernel@vger.kernel.org" , "kvm@vger.kernel.org" , "platform-driver-x86@vger.kernel.org" , "xen-devel@lists.xenproject.org" , "iommu@lists.linux-foundation.org" References: <20201028212417.3715575-1-arnd@kernel.org> <38b11ed3fec64ebd82d6a92834a4bebe@AcuMS.aculab.com> <20201029165611.GA2557691@rani.riverdale.lan> <93180c2d-268c-3c33-7c54-4221dfe0d7ad@redhat.com> <87v9esojdi.fsf@nanos.tec.linutronix.de> <20201029213512.GA34524@rani.riverdale.lan> From: Paolo Bonzini Message-ID: <070f590f-b702-35f0-0b6c-c6455f08e9d5@redhat.com> Date: Fri, 30 Oct 2020 19:16:50 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.3.1 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: platform-driver-x86@vger.kernel.org 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