From: Jens Maurer <Jens.Maurer@gmx.net>
To: c++std-parallel@accu.org
Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
Linus Torvalds <torvalds@linux-foundation.org>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
"linux-arch@vger.kernel.org" <linux-arch@vger.kernel.org>,
"gcc@gcc.gnu.org" <gcc@gcc.gnu.org>,
p796231 <Peter.Sewell@cl.cam.ac.uk>,
"mark.batty@cl.cam.ac.uk" <Mark.Batty@cl.cam.ac.uk>,
Peter Zijlstra <peterz@infradead.org>,
Will Deacon <will.deacon@arm.com>,
Ramana Radhakrishnan <Ramana.Radhakrishnan@arm.com>,
David Howells <dhowells@redhat.com>,
Andrew Morton <akpm@linux-foundation.org>,
Ingo Molnar <mingo@kernel.org>,
michaelw@ca.ibm.com
Subject: Re: [c++std-parallel-1614] Re: Compilers and RCU readers: Once more unto the breach!
Date: Wed, 20 May 2015 09:34:10 +0200 [thread overview]
Message-ID: <555C38F2.7060402@gmx.net> (raw)
In-Reply-To: <20150520023402.GC6776@linux.vnet.ibm.com>
On 05/20/2015 04:34 AM, Paul E. McKenney wrote:
> On Tue, May 19, 2015 at 06:57:02PM -0700, Linus Torvalds wrote:
>> - the "you can add/subtract integral values" still opens you up to
>> language lawyers claiming "(char *)ptr - (intptr_t)ptr" preserving the
>> dependency, which it clearly doesn't. But language-lawyering it does,
>> since all those operations (cast to pointer, cast to integer,
>> subtracting an integer) claim to be dependency-preserving operations.
[...]
> There are some stranger examples, such as "(char *)ptr - ((intptr_t)ptr)/7",
> but in that case, if the resulting pointer happens by chance to reference
> valid memory, I believe a dependency would still be carried.
[...]
>From a language lawyer standpoint, pointer arithmetic is only valid
within an array. These examples seem to go beyond the bounds of the
array and therefore have undefined behavior.
C++ standard section 5.7 paragraph 4
"If both the pointer operand and the result point to elements of the
same array object, or one past the last element of the array object,
the evaluation shall not produce an overflow; otherwise, the behavior
is undefined."
C99 and C11
identical phrasing in 6.5.6 paragraph 8
Jens
next prev parent reply other threads:[~2015-05-20 7:35 UTC|newest]
Thread overview: 44+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-05-20 0:55 Compilers and RCU readers: Once more unto the breach! Paul E. McKenney
2015-05-20 1:57 ` Linus Torvalds
2015-05-20 2:10 ` Linus Torvalds
2015-05-20 2:41 ` Paul E. McKenney
2015-05-20 11:47 ` Will Deacon
2015-05-20 12:15 ` Paul E. McKenney
2015-05-20 15:46 ` Will Deacon
2015-05-20 15:54 ` Andrew Haley
2015-05-20 18:16 ` [c++std-parallel-1632] " Paul E. McKenney
2015-05-21 14:22 ` Michael Matz
2015-05-21 15:10 ` Paul E. McKenney
2015-05-21 16:17 ` Michael Matz
2015-05-21 18:37 ` Paul E. McKenney
2015-05-20 18:16 ` Paul E. McKenney
2015-05-21 19:24 ` Will Deacon
2015-05-21 20:02 ` Paul E. McKenney
2015-05-21 20:42 ` Linus Torvalds
2015-05-21 22:02 ` Paul E. McKenney
2015-05-22 6:43 ` Ingo Molnar
2015-05-22 10:43 ` Richard Kenner
2015-05-22 13:11 ` Paul E. McKenney
2015-05-22 13:12 ` Paul E. McKenney
2015-05-26 17:37 ` [c++std-parallel-1641] " Torvald Riegel
2015-05-22 17:30 ` Will Deacon
2015-05-22 18:55 ` Paul E. McKenney
2015-05-20 13:18 ` David Howells
2015-05-20 13:30 ` Paul E. McKenney
2015-05-20 13:37 ` David Howells
2015-05-20 13:44 ` Ramana Radhakrishnan
2015-05-20 14:03 ` Paul E. McKenney
2015-05-20 14:15 ` Ramana Radhakrishnan
2015-05-20 15:12 ` Paul E. McKenney
2015-05-20 15:46 ` David Howells
2015-05-20 14:02 ` [c++std-parallel-1624] " Paul E. McKenney
2015-05-20 2:34 ` Paul E. McKenney
2015-05-20 7:34 ` Jens Maurer [this message]
2015-05-20 9:03 ` [c++std-parallel-1614] " Richard Biener
2015-05-20 12:02 ` Paul E. McKenney
2015-05-20 12:01 ` [c++std-parallel-1616] " Paul E. McKenney
2015-05-26 17:08 ` [c++std-parallel-1611] " Torvald Riegel
2015-05-27 1:41 ` [c++std-parallel-1651] " Paul E. McKenney
2015-07-14 0:44 ` Paul E. McKenney
2015-09-22 17:00 ` Paul E. McKenney
[not found] ` <CAPUmR1aqV_cQWjE8qC9x2sfmW-1ocKKMtCgNbjZH0cJ-AO2WTg@mail.gmail.com>
2015-09-23 23:26 ` [c++std-parallel-2008] " Paul E. McKenney
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