[9fans] vlong double-spill botch

[Fernan Bolando]

what does vlong double-spill botch mean in
r = (a * b) + (((a * u) + (b * t)) << 18);          /* low is only 35b */

a larger section of the code is shown below


t_uint64 a = ABS (s1);
t_uint64 b = ABS (s2);
t_uint64 t, u, r;

if ((a == 0) || (b == 0)) {                             /* operand = 0? */
    rs[0] = rs[1] = 0;                                  /* result 0 */
    return;
    }
if ((a & FIT32) || (b & FIT32)) {                       /* fit in 64b? */
    t = a >> 18;                                        /* no, split in half */
    a = a & RMASK;                                      /* "dp" multiply */
    u = b >> 18;
    b = b & RMASK;
    r = (a * b) + (((a * u) + (b * t)) << 18);          /* low is only 35b */
    rs[0] = ((t * u) << 1) + (r >> 35);                 /* so lsh hi 1 */
    rs[1] = r & MMASK;
    }