if (a == b) {
       return mp_sqr(a,c);
   } else if (MP_HAS(S_MP_BALANCE_MUL) &&
       /* Check sizes. The smaller one needs to be larger than the Karatsuba cut-off.
        * The bigger one needs to be at least about one MP_KARATSUBA_MUL_CUTOFF bigger
        * to make some sense, but it depends on architecture, OS, position of the
        * stars... so YMMV.
        * Using it to cut the input into slices small enough for fast_s_mp_mul_digs
        * Using it to cut the input into slices small enough for s_mp_mul_digs_fast
        * was actually slower on the author's machine, but YMMV.
        */
       (min_len >= MP_KARATSUBA_MUL_CUTOFF) &&
       ((max_len / 2) >= MP_KARATSUBA_MUL_CUTOFF) &&
       /* Not much effect was observed below a ratio of 1:2, but again: YMMV. */
       (max_len >= (2 * min_len))) {
      err = s_mp_balance_mul(a,b,c);