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
