From: Paolo Bonzini <pbonzini@redhat.com>
To: David Laight <David.Laight@ACULAB.COM>,
'Arvind Sankar' <nivedita@alum.mit.edu>,
Thomas Gleixner <tglx@linutronix.de>
Cc: 'Arnd Bergmann' <arnd@kernel.org>, Ingo Molnar <mingo@redhat.com>,
Borislav Petkov <bp@alien8.de>, "x86@kernel.org" <x86@kernel.org>,
Arnd Bergmann <arnd@arndb.de>,
"K. Y. Srinivasan" <kys@microsoft.com>,
Haiyang Zhang <haiyangz@microsoft.com>,
Stephen Hemminger <sthemmin@microsoft.com>,
"H. Peter Anvin" <hpa@zytor.com>,
"Rafael J. Wysocki" <rjw@rjwysocki.net>,
Vitaly Kuznetsov <vkuznets@redhat.com>,
Wanpeng Li <wanpengli@tencent.com>,
Jim Mattson <jmattson@google.com>, Joerg Roedel <joro@8bytes.org>,
"linux-hyperv@vger.kernel.org" <linux-hyperv@vger.kernel.org>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
"kvm@vger.kernel.org" <kvm@vger.kernel.org>,
"platform-driver-x86@vger.kernel.org"
<platform-driver-x86@vger.kernel.org>,
"xen-devel@lists.xenproject.org" <xen-devel@lists.xenproject.org>,
"iommu@lists.linux-foundation.org"
<iommu@lists.linux-foundation.org>
Subject: Re: [PATCH] [v2] x86: apic: avoid -Wshadow warning in header
Date: Fri, 30 Oct 2020 19:16:50 +0100 [thread overview]
Message-ID: <070f590f-b702-35f0-0b6c-c6455f08e9d5@redhat.com> (raw)
In-Reply-To: <ad73f56e79d249b1b3614bccc85e2ca5@AcuMS.aculab.com>
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
prev parent reply other threads:[~2020-10-30 18:17 UTC|newest]
Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-10-28 21:20 Arnd Bergmann
2020-10-29 7:04 ` Paolo Bonzini
2020-10-29 9:51 ` Arnd Bergmann
2020-10-29 16:30 ` David Laight
2020-10-29 16:52 ` Thomas Gleixner
2020-10-29 15:05 ` David Laight
2020-10-29 16:56 ` Arvind Sankar
2020-10-29 16:59 ` Paolo Bonzini
2020-10-29 17:20 ` Arvind Sankar
2020-10-29 20:41 ` Thomas Gleixner
2020-10-29 21:35 ` Arvind Sankar
2020-10-29 22:12 ` David Laight
2020-10-30 18:16 ` Paolo Bonzini [this message]
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