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Re: [c++std-parallel-1616] Re: Compilers and RCU readers: Once more unto the breach!
- From: "Paul E. McKenney" <paulmck at linux dot vnet dot ibm dot com>
- To: c++std-parallel at accu dot org
- Cc: Linus Torvalds <torvalds at linux-foundation dot org>, Linux Kernel Mailing List <linux-kernel at vger dot kernel dot org>, "linux-arch at vger dot kernel dot org" <linux-arch at vger dot kernel dot org>, "gcc at gcc dot gnu dot org" <gcc at gcc dot gnu dot org>, p796231 <Peter dot Sewell at cl dot cam dot ac dot uk>, "mark dot batty at cl dot cam dot ac dot uk" <Mark dot Batty at cl dot cam dot ac dot uk>, Peter Zijlstra <peterz at infradead dot org>, Will Deacon <will dot deacon at arm dot com>, Ramana Radhakrishnan <Ramana dot Radhakrishnan at arm dot com>, David Howells <dhowells at redhat dot com>, Andrew Morton <akpm at linux-foundation dot org>, Ingo Molnar <mingo at kernel dot org>, michaelw at ca dot ibm dot com
- Date: Wed, 20 May 2015 05:01:10 -0700
- Subject: Re: [c++std-parallel-1616] Re: Compilers and RCU readers: Once more unto the breach!
- Authentication-results: sourceware.org; auth=none
- References: <20150520005510 dot GA23559 at linux dot vnet dot ibm dot com> <CA+55aFy_8V-rbE9FQMHx6tXjj8HHKZuKSJvnRPVYvpk46EQA1g at mail dot gmail dot com> <20150520023402 dot GC6776 at linux dot vnet dot ibm dot com> <555C38F2 dot 7060402 at gmx dot net>
- Reply-to: paulmck at linux dot vnet dot ibm dot com
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