speed of unbalanced multiplication

[Torbjorn Granlund]

bodrato at mail.dm.unipi.it writes:

  If other developers does not dislike the changed meaning of the .<r>
  parameter to mpn_mul, this patch can be applied to the main repo...
  
I don't mind, since I never remember which way it is anyway.
Avoiding the local allocation is nice too.

It would be nice to understand the slowdown of before, though...

-- 
Torbjörn