Small toom43 cleanup

[Torbjorn Granlund]

bodrato at mail.dm.unipi.it writes:

  Ciao!
  
  > Marco, Torbjörn, does this look ok?
  
  Perfect.
  
  But I have a question:
  
  > +mpn_toom43_mul_itch (mp_size_t an, mp_size_t bn)
  > +{
  > +  mp_size_t n = 1 + (3 * an >= 4 * bn ? (an - 1) >> 2 : (bn - 1) /
  > (unsigned long) 3);
  
  > +static inline mp_size_t
  >  mpn_toom53_mul_itch (mp_size_t an, mp_size_t bn)
  >  {
  >    mp_size_t n = 1 + (3 * an >= 5 * bn ? (an - 1) / (size_t) 5 : (bn - 1)
  > / (size_t) 3);
  
  You cast the divisor in toom43_itch to (unsigned long); toom53_itch cast
  divisors do (size_t). It my last code I used a cast to (mp_size_t)...
  The question is, do we really need a cast for a single digit?
  
It is an efficiency thing.

mp_size_t is a signed type, for various reasons.  size_t is an unsigned
type.  Unsigned division by constants is a few cycles faster.

Casting to "unsigned long" is not correct, it is not always large
enough.  But size_t is safe.

-- 
Torbjörn