linux-kernel - Re: [c++std-parallel-1614] Re: Compilers and RCU readers: Once more unto the breach!

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Message-ID: <CAFiYyc2csMmcEJLRaoSExLkWGUw=zyL9L6i4VaNDyJ6H+rROYQ@mail.gmail.com>
Date:	Wed, 20 May 2015 11:03:00 +0200
From:	Richard Biener <richard.guenther@...il.com>
To:	Jens Maurer <Jens.Maurer@....net>
Cc:	c++std-parallel@...u.org,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
	"gcc@....gnu.org" <gcc@....gnu.org>,
	p796231 <Peter.Sewell@...cam.ac.uk>,
	"mark.batty@...cam.ac.uk" <Mark.Batty@...cam.ac.uk>,
	Peter Zijlstra <peterz@...radead.org>,
	Will Deacon <will.deacon@....com>,
	Ramana Radhakrishnan <Ramana.Radhakrishnan@....com>,
	David Howells <dhowells@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>, michaelw@...ibm.com
Subject: Re: [c++std-parallel-1614] Re: Compilers and RCU readers: Once more
 unto the breach!

On Wed, May 20, 2015 at 9:34 AM, Jens Maurer <Jens.Maurer@....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
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