sing one int + two fp).
<li> PPC630: <code>mpn_rshift</code> could do the same sort of unrolled loop
     as <code>mpn_lshift</code>.  Some judicious use of m4 might let the two
     share source code, or with a register to control the loop direction
     perhaps even share object code.
<li> Implement <code>mpn_mul_basecase</code> and <code>mpn_sqr_basecase</code>
     for important machines.  Helping the generic sqr_basecase.c with an
     <code>mpn_sqr_diagonal</code> might be enough for some of the RISCs