From: Richard Biener <richard.guenther@gmail.com>
To: Jens Maurer <Jens.Maurer@gmx.net>
Cc: c++std-parallel@accu.org,
"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 11:03:00 +0200 [thread overview]
Message-ID: <CAFiYyc2csMmcEJLRaoSExLkWGUw=zyL9L6i4VaNDyJ6H+rROYQ@mail.gmail.com> (raw)
In-Reply-To: <555C38F2.7060402@gmx.net>
On Wed, May 20, 2015 at 9:34 AM, Jens Maurer <Jens.Maurer@gmx.net> wrote:
> 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
Of course you can try to circumvent that by doing
(char*)((intptr_t)ptr - (intptr_t)ptr + (intptr_t)ptr)
(see https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65752 for extra fun).
Which (IMHO) gets you into the standard language that only makes conversion of
the exact same integer back to a pointer well-defined(?)
Richard.
> Jens
next prev parent reply other threads:[~2015-05-20 9:03 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 ` [c++std-parallel-1614] " Jens Maurer
2015-05-20 9:03 ` Richard Biener [this message]
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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