gmp_pi1_t

[Niels Möller]

nisse at lysator.liu.se (Niels Möller) writes:

> I'm tempted to remove the inv21 member of the gmp_pi1_t type. It's used
> in exactly one place, and there it's most likely better to use 3/2
> division and inv32 anyway.

Here's a patch to dcpi1_div_qr.c, replacing the use of inv21 by inv32.
Some comments and questions:

* I require dn >= 2, I guess that requirement is part of the interface
  for the new division functions? Then I'd like to add an ASSERT to that
  effect at the top of the functions.

* The use of inv21 was in the case that the first block should generate
  a single quotient limb. I replaced the old code (based on
  udiv_qrnnd_preinv) by basically one iteration of the code in
  sbpi1_div_wr, which took me some time to get right due to the
  different pointer offsets used.

* I had do a separate update and adjustment step as well, using
  mpn_submul_1, since the general update code was for (2 qn) / qn
  division, while for qn = 1 I do a 3/2 division. The patch uses a goto
  for that, but I think one could get rid of that (didn't do that now,
  since I don't want to bloat the patch with indentation changes).

* Also in the general case, I think it would make sense to do (2qn+1) /
  (qn+1) divisions, to reduce the probability of expensive adjustments.

Feedback appreciated. I'd like to check this in in a day or two.
And after that, one can delete the inv21 member.

Regards,
/Niels
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