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

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

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

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jason at gcc dot gnu.org,
                   |                            |jsm28 at gcc dot gnu.org,
                   |                            |rguenth at gcc dot gnu.org

--- Comment #3 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
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.