xref: /dflybsd-src/contrib/mpfr/src/subnormal.c (revision 2786097444a0124b5d33763854de247e230c6629)

/* mpfr_subnormalize -- Subnormalize a floating point number
   emulating sub-normal numbers.

Copyright 2005, 2006, 2007, 2008, 2009, 2010, 2011, 2012, 2013 Free Software Foundation, Inc.
Contributed by the AriC and Caramel projects, INRIA.

This file is part of the GNU MPFR Library.

The GNU MPFR 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 MPFR 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 MPFR Library; see the file COPYING.LESSER.  If not, see
http://www.gnu.org/licenses/ or write to the Free Software Foundation, Inc.,
51 Franklin St, Fifth Floor, Boston, MA 02110-1301, USA. */

#include "mpfr-impl.h"

/* For MPFR_RNDN, we can have a problem of double rounding.
   In such a case, this table helps to conclude what to do (y positive):
     Rounding Bit |  Sticky Bit | inexact  | Action    | new inexact
     0            |   ?         |  ?       | Trunc     | sticky
     1            |   0         |  1       | Trunc     |
     1            |   0         |  0       | Trunc if even |
     1            |   0         | -1       | AddOneUlp |
     1            |   1         |  ?       | AddOneUlp |

   For other rounding mode, there isn't such a problem.
   Just round it again and merge the ternary values.

   Set the inexact flag if the returned ternary value is non-zero.
   Set the underflow flag if a second rounding occurred (whether this
   rounding is exact or not). See
     https://sympa.inria.fr/sympa/arc/mpfr/2009-06/msg00000.html
     https://sympa.inria.fr/sympa/arc/mpfr/2009-06/msg00008.html
     https://sympa.inria.fr/sympa/arc/mpfr/2009-06/msg00010.html
*/

int
mpfr_subnormalize (mpfr_ptr y, int old_inexact, mpfr_rnd_t rnd)
{
  int sign;

  /* The subnormal exponent range is [ emin, emin + MPFR_PREC(y) - 2 ] */
  if (MPFR_LIKELY (MPFR_IS_SINGULAR (y)
                   || (MPFR_GET_EXP (y) >=
                       __gmpfr_emin + (mpfr_exp_t) MPFR_PREC (y) - 1)))
    MPFR_RET (old_inexact);

  mpfr_set_underflow ();
  sign = MPFR_SIGN (y);

  /* We have to emulate one bit rounding if EXP(y) = emin */
  if (MPFR_GET_EXP (y) == __gmpfr_emin)
    {
      /* If this is a power of 2, we don't need rounding.
         It handles cases when |y| = 0.1 * 2^emin */
      if (mpfr_powerof2_raw (y))
        MPFR_RET (old_inexact);

      /* We keep the same sign for y.
         Assuming Y is the real value and y the approximation
         and since y is not a power of 2:  0.5*2^emin < Y < 1*2^emin
         We also know the direction of the error thanks to ternary value. */

      if (rnd == MPFR_RNDN)
        {
          mp_limb_t *mant, rb ,sb;
          mp_size_t s;
          /* We need the rounding bit and the sticky bit. Read them
             and use the previous table to conclude. */
          s = MPFR_LIMB_SIZE (y) - 1;
          mant = MPFR_MANT (y) + s;
          rb = *mant & (MPFR_LIMB_HIGHBIT >> 1);
          if (rb == 0)
            goto set_min;
          sb = *mant & ((MPFR_LIMB_HIGHBIT >> 1) - 1);
          while (sb == 0 && s-- != 0)
            sb = *--mant;
          if (sb != 0)
            goto set_min_p1;
          /* Rounding bit is 1 and sticky bit is 0.
             We need to examine old inexact flag to conclude. */
          if ((old_inexact > 0 && sign > 0) ||
              (old_inexact < 0 && sign < 0))
            goto set_min;
          /* If inexact != 0, return 0.1*2^(emin+1).
             Otherwise, rounding bit = 1, sticky bit = 0 and inexact = 0
             So we have 0.1100000000000000000000000*2^emin exactly.
             We return 0.1*2^(emin+1) according to the even-rounding
             rule on subnormals. */
          goto set_min_p1;
        }
      else if (MPFR_IS_LIKE_RNDZ (rnd, MPFR_IS_NEG (y)))
        {
        set_min:
          mpfr_setmin (y, __gmpfr_emin);
          MPFR_RET (-sign);
        }
      else
        {
        set_min_p1:
          /* Note: mpfr_setmin will abort if __gmpfr_emax == __gmpfr_emin. */
          mpfr_setmin (y, __gmpfr_emin + 1);
          MPFR_RET (sign);
        }
    }
  else /* Hard case: It is more or less the same problem than mpfr_cache */
    {
      mpfr_t dest;
      mpfr_prec_t q;
      int inexact, inex2;

      MPFR_ASSERTD (MPFR_GET_EXP (y) > __gmpfr_emin);

      /* Compute the intermediary precision */
      q = (mpfr_uexp_t) MPFR_GET_EXP (y) - __gmpfr_emin + 1;
      MPFR_ASSERTD (q >= MPFR_PREC_MIN && q < MPFR_PREC (y));

      /* TODO: perform the rounding in place. */
      mpfr_init2 (dest, q);
      /* Round y in dest */
      MPFR_SET_EXP (dest, MPFR_GET_EXP (y));
      MPFR_SET_SIGN (dest, sign);
      MPFR_RNDRAW_EVEN (inexact, dest,
                        MPFR_MANT (y), MPFR_PREC (y), rnd, sign,
                        MPFR_SET_EXP (dest, MPFR_GET_EXP (dest) + 1));
      if (MPFR_LIKELY (old_inexact != 0))
        {
          if (MPFR_UNLIKELY (rnd == MPFR_RNDN &&
                             (inexact == MPFR_EVEN_INEX ||
                              inexact == -MPFR_EVEN_INEX)))
            {
              /* if both roundings are in the same direction, we have to go
                 back in the other direction */
              if (SAME_SIGN (inexact, old_inexact))
                {
                  if (SAME_SIGN (inexact, MPFR_INT_SIGN (y)))
                    mpfr_nexttozero (dest);
                  else
                    mpfr_nexttoinf (dest);
                  inexact = -inexact;
                }
            }
          else if (MPFR_UNLIKELY (inexact == 0))
            inexact = old_inexact;
        }

      inex2 = mpfr_set (y, dest, rnd);
      MPFR_ASSERTN (inex2 == 0);
      MPFR_ASSERTN (MPFR_IS_PURE_FP (y));
      mpfr_clear (dest);

      MPFR_RET (inexact);
    }
}