[Bug tree-optimization/79119] absolute value of a pointer difference can be assumed to be less than or equal to PTRDIFF_MAX

[glisse at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79119

--- Comment #2 from Marc Glisse <glisse at gcc dot gnu.org> ---
With POINTER_DIFF_EXPR, things may be easier. One of the things I suggested
when introducing it was that VRP could always use a range of [ -PTRDIFF_MAX,
PTRDIFF_MAX ] for them. If you had written

  __PTRDIFF_TYPE__ d = q - p;
  __SIZE_TYPE__ n = (d < 0) ? -d : d;

that would suffice to simplify f. (With the code above, g is already optimized)

We could improve phiopt abs_replacement so it understands versions with
MINUS_EXPR (that's necessary for other PRs anyway), and versions with pointers.

Teaching VRP that p<q implies q-p>0 looks like more work...