linux-kernel - Re: [c++std-parallel-1616] Re: Compilers and RCU readers: Once more unto the breach!

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Date:	Wed, 20 May 2015 05:01:10 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	c++std-parallel@...u.org
Cc:	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-1616] Re: Compilers and RCU readers: Once more
 unto the breach!

On Wed, May 20, 2015 at 09:34:10AM +0200, Jens Maurer 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

Even better!  I added a footnote calling out these two paragraphs.

							Thax, Paul

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