he useful bits at each point in the calculation,
     since <code>mulx</code> speed depends on the highest bit set.  Either
     explicit masks or small types like <code>short</code> and
     <code>int</code> ought to work.
<li> Sparc64 HAL R1 <code>popc</code>: This chip reputedly implements
     <code>popc</code> properly (see gcc sparc.md).  Would need to recognise
     it as <code>sparchalr1</code> or something in configure / config.sub /
     config.guess.  <code>popc_limb</code> in gmp-impl.h could us