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-arch" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
