On Wed, May 20, 2015 at 11:03:00AM +0200, Richard Biener wrote: > On Wed, May 20, 2015 at 9:34 AM, Jens Maurer <Jens.Maurer@xxxxxxx> 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 > > Of course you can try to circumvent that by doing > (char*)((intptr_t)ptr - (intptr_t)ptr + (intptr_t)ptr) > (see https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65752 for extra fun). > > Which (IMHO) gets you into the standard language that only makes conversion of > the exact same integer back to a pointer well-defined(?) I am feeling good about leaving the restriction and calling out the two paragraphs in a footnote, then. ;-) Thanx, 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
