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

[Paul E. McKenney]

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

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/