gcc/gmp/mpn/generic/toom2_sqr.c

/* mpn_toom2_sqr -- Square {ap,an}.

   Contributed to the GNU project by Torbjorn Granlund.

   THE FUNCTION IN THIS FILE IS INTERNAL WITH A MUTABLE INTERFACE.  IT IS ONLY
   SAFE TO REACH IT THROUGH DOCUMENTED INTERFACES.  IN FACT, IT IS ALMOST
   GUARANTEED THAT IT WILL CHANGE OR DISAPPEAR IN A FUTURE GNU MP RELEASE.

Copyright 2006, 2007, 2008 Free Software Foundation, Inc.

This file is part of the GNU MP Library.

The GNU MP Library is free software; you can redistribute it and/or modify
it under the terms of the GNU Lesser General Public License as published by
the Free Software Foundation; either version 3 of the License, or (at your
option) any later version.

The GNU MP Library is distributed in the hope that it will be useful, but
WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU Lesser General Public
License for more details.

You should have received a copy of the GNU Lesser General Public License
along with the GNU MP Library.  If not, see http://www.gnu.org/licenses/.  */


#include "gmp.h"
#include "gmp-impl.h"

/* Evaluate in: -1, 0, +inf

  <-s--><--n-->
   ____ ______
  |_a1_|___a0_|
   |b1_|___b0_|
   <-t-><--n-->

  v0  =  a0     * b0       #   A(0)*B(0)
  vm1 = (a0- a1)*(b0- b1)  #  A(-1)*B(-1)
  vinf=      a1 *     b1   # A(inf)*B(inf)
*/

#if TUNE_PROGRAM_BUILD
#define MAYBE_sqr_toom2   1
#else
#define MAYBE_sqr_toom2							\
  (SQR_TOOM3_THRESHOLD >= 2 * SQR_TOOM2_THRESHOLD)
#endif

#define TOOM2_SQR_N_REC(p, a, n, ws)					\
  do {									\
    if (! MAYBE_sqr_toom2						\
	|| BELOW_THRESHOLD (n, SQR_KARATSUBA_THRESHOLD))		\
      mpn_sqr_basecase (p, a, n);					\
    else								\
      mpn_toom2_sqr (p, a, n, ws);					\
  } while (0)

void
mpn_toom2_sqr (mp_ptr pp,
	       mp_srcptr ap, mp_size_t an,
	       mp_ptr scratch)
{
  mp_size_t n, s;
  mp_limb_t cy, cy2;
  mp_ptr asm1;

#define a0  ap
#define a1  (ap + n)

  s = an >> 1;
  n = an - s;

  ASSERT (0 < s && s <= n);

  asm1 = pp;

  /* Compute asm1.  */
  if (s == n)
    {
      if (mpn_cmp (a0, a1, n) < 0)
	{
	  mpn_sub_n (asm1, a1, a0, n);
	}
      else
	{
	  mpn_sub_n (asm1, a0, a1, n);
	}
    }
  else
    {
      if (mpn_zero_p (a0 + s, n - s) && mpn_cmp (a0, a1, s) < 0)
	{
	  mpn_sub_n (asm1, a1, a0, s);
	  MPN_ZERO (asm1 + s, n - s);
	}
      else
	{
	  mpn_sub (asm1, a0, n, a1, s);
	}
    }

#define v0	pp				/* 2n */
#define vinf	(pp + 2 * n)			/* s+s */
#define vm1	scratch				/* 2n */

  /* vm1, 2n limbs */
  TOOM2_SQR_N_REC (vm1, asm1, n, scratch);

  /* vinf, s+s limbs */
  TOOM2_SQR_N_REC (vinf, a1, s, scratch);

  /* v0, 2n limbs */
  TOOM2_SQR_N_REC (v0, ap, n, scratch);

  /* H(v0) + L(vinf) */
  cy = mpn_add_n (pp + 2 * n, v0 + n, vinf, n);

  /* L(v0) + H(v0) */
  cy2 = cy + mpn_add_n (pp + n, pp + 2 * n, v0, n);

  /* L(vinf) + H(vinf) */
  cy += mpn_add (pp + 2 * n, pp + 2 * n, n, vinf + n, s + s - n);

  cy -= mpn_sub_n (pp + n, pp + n, vm1, 2 * n);

  ASSERT (cy + 1  <= 3);
  ASSERT (cy2 <= 2);

  mpn_incr_u (pp + 2 * n, cy2);
  if (LIKELY (cy <= 2))
    mpn_incr_u (pp + 3 * n, cy);
  else
    mpn_decr_u (pp + 3 * n, 1);
}