[Bug c/63303] Pointer subtraction is broken when using -fsanitize=undefined

["rguenth at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63303

--- Comment #12 from Richard Biener <rguenth at gcc dot gnu.org> ---
(In reply to Jakub Jelinek from comment #3)
> The problem is that we don't have a POINTER_DIFF_EXPR similar to
> POINTER_PLUS_EXPR, which would take two pointers and return an integer, and
> the FEs emit pointer difference as cast of both the pointers to signed
> integral type
> and subtracts the integers.
> If
> ssize_t foo (char *p, char *q) { return p - q; }
> is changed into
> ssize_t foo (char *p, char *q) { return (ssize_t) p - (ssize_t) q; }
> by the FE, then indeed if you have array starting at 0x7fff0000 and ending
> at 0x80010000 and subtract those two pointers, you get undefined behavior.
> That is undefined behavior not just for ubsan, but for anything else in the
> middle-end.
> So, if pointer difference is supposed to behave differently, then
> we'd either need to represent pointer difference as
> ssize_t foo (char *p, char *q) { return (ssize_t) ((size_t) p - (size_t) q);
> }
> (but we risk missed optimizations that way I'd say), or we'd need a better
> representation of it in the middle-end.

Note that apart from missing POINTER_DIFF this isn't a middle-end but a
frontend
issue.