 simple and
     complex functions to find out.  Inlining <code>rep movs</code> would be
     desirable, it'd be both smaller and faster.
<li> Pentium P54: <code>mpn_lshift</code> and <code>mpn_rshift</code> can come
     down from 6.0 c/l to 5.5 or 5.375 by paying attention to pairing after
     <code>shrdl</code> and <code>shldl</code>, see mpn/x86/pentium/README.
<li> Pentium P55 MMX: <code>mpn_lshift</code> and <code>mpn_rshift</code>
     might benefit from some destination prefetching.
<li> PentiumPro