  Not sure how
     much this can improve the speed, though, since the symmetry that we rely
     on is lost.  Perhaps we can just gain cycles when s2 &lt; 2^16, or more
     accurately, when two 16-bit s2 chunks which are 16 bits apart are zero.
<li> UltraSPARC/64: Write native <code>mpn_submul_1</code>, analogous to
     <code>mpn_addmul_1</code>.
<li> UltraSPARC/64: Write <code>umul_ppmm</code>.  Using four
     "<code>mulx</code>"s either with an asm block or via the generic C code is
     about 90 cycl