[Bug tree-optimization/101610] CST - (x ^ (CST-1)) can be optimized to x + 1 if x < CST and CST is a power of 2

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101610

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           See Also|                            |https://gcc.gnu.org/bugzill
                   |                            |a/show_bug.cgi?id=96921
             Status|UNCONFIRMED                 |ASSIGNED
   Last reconfirmed|                            |2021-07-28
     Ever confirmed|0                           |1
           Assignee|unassigned at gcc dot gnu.org      |pinskia at gcc dot gnu.org

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
So looking at this one (and one which I just assigned myself):
unsigned long f(unsigned long x)
{
    if (x >= 64)__builtin_unreachable();
    x = x ^ 63;
unsigned long y = x; ;; Range is still [0,63]
unsigned long z = 64 - y; ;; is similar to (63 - y) +1 -> (y ^ 63) + 1 -> x + 1
return z;
}

So mine:

Something like:
(simplify
 (minus INTEGER_CST@0 SSA_NAME@1)
 (if (exact_power2(@0) && get_nonzero_bits(@1) == (@0 - 1)
  (add (bit_xor! @1 @0) {build_one_cst (type); }))