sysdeps/ieee754/dbl2mpn.c

   1 /* Copyright (C) 1993, 1994, 1995, 1996, 1997 Free Software Foundation, Inc.
   2    This file is part of the GNU C Library.
   3
   4    The GNU C Library is free software; you can redistribute it and/or
   5    modify it under the terms of the GNU Library General Public License as
   6    published by the Free Software Foundation; either version 2 of the
   7    License, or (at your option) any later version.
   8
   9    The GNU C Library is distributed in the hope that it will be useful,
  10    but WITHOUT ANY WARRANTY; without even the implied warranty of
  11    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  12    Library General Public License for more details.
  13
  14    You should have received a copy of the GNU Library General Public
  15    License along with the GNU C Library; see the file COPYING.LIB.  If not,
  16    write to the Free Software Foundation, Inc., 59 Temple Place - Suite 330,
  17    Boston, MA 02111-1307, USA.  */
  18
  19 #include "gmp.h"
  20 #include "gmp-impl.h"
  21 #include "longlong.h"
  22 #include "ieee754.h"
  23 #include <float.h>
  24 #include <stdlib.h>
  25
  26 /* Convert a `double' in IEEE754 standard double-precision format to a
  27    multi-precision integer representing the significand scaled up by its
  28    number of bits (52 for double) and an integral power of two (MPN frexp). */
  29
  30 mp_size_t
  31 __mpn_extract_double (mp_ptr res_ptr, mp_size_t size,
  32                       int *expt, int *is_neg,
  33                       double value)
  34 {
  35   union ieee754_double u;
  36   u.d = value;
  37
  38   *is_neg = u.ieee.negative;
  39   *expt = (int) u.ieee.exponent - IEEE754_DOUBLE_BIAS;
  40
  41 #if BITS_PER_MP_LIMB == 32
  42   res_ptr[0] = u.ieee.mantissa1; /* Low-order 32 bits of fraction.  */
  43   res_ptr[1] = u.ieee.mantissa0; /* High-order 20 bits.  */
  44   #define N 2
  45 #elif BITS_PER_MP_LIMB == 64
  46   /* Hopefully the compiler will combine the two bitfield extracts
  47      and this composition into just the original quadword extract.  */
  48   res_ptr[0] = ((unsigned long int) u.ieee.mantissa0 << 32) | u.ieee.mantissa1;
  49   #define N 1
  50 #else
  51   #error "mp_limb size " BITS_PER_MP_LIMB "not accounted for"
  52 #endif
  53 /* The format does not fill the last limb.  There are some zeros.  */
  54 #define NUM_LEADING_ZEROS (BITS_PER_MP_LIMB \
  55                            - (DBL_MANT_DIG - ((N - 1) * BITS_PER_MP_LIMB)))
  56
  57   if (u.ieee.exponent == 0)
  58     {
  59       /* A biased exponent of zero is a special case.
  60          Either it is a zero or it is a denormal number.  */
  61       if (res_ptr[0] == 0 && res_ptr[N - 1] == 0) /* Assumes N<=2.  */
  62         /* It's zero.  */
  63         *expt = 0;
  64       else
  65         {
  66           /* It is a denormal number, meaning it has no implicit leading
  67              one bit, and its exponent is in fact the format minimum.  */
  68           int cnt;
  69
  70           if (res_ptr[N - 1] != 0)
  71             {
  72               count_leading_zeros (cnt, res_ptr[N - 1]);
  73               cnt -= NUM_LEADING_ZEROS;
  74 #if N == 2
  75               res_ptr[N - 1] = res_ptr[1] << cnt
  76                                | (N - 1)
  77                                  * (res_ptr[0] >> (BITS_PER_MP_LIMB - cnt));
  78               res_ptr[0] <<= cnt;
  79 #else
  80               res_ptr[N - 1] <<= cnt;
  81 #endif
  82               *expt = DBL_MIN_EXP - 1 - cnt;
  83             }
  84           else
  85             {
  86               count_leading_zeros (cnt, res_ptr[0]);
  87               if (cnt >= NUM_LEADING_ZEROS)
  88                 {
  89                   res_ptr[N - 1] = res_ptr[0] << (cnt - NUM_LEADING_ZEROS);
  90                   res_ptr[0] = 0;
  91                 }
  92               else
  93                 {
  94                   res_ptr[N - 1] = res_ptr[0] >> (NUM_LEADING_ZEROS - cnt);
  95                   res_ptr[0] <<= BITS_PER_MP_LIMB - (NUM_LEADING_ZEROS - cnt);
  96                 }
  97               *expt = DBL_MIN_EXP - 1
  98                       - (BITS_PER_MP_LIMB - NUM_LEADING_ZEROS) - cnt;
  99             }
 100         }
 101     }
 102   else
 103     /* Add the implicit leading one bit for a normalized number.  */
 104     res_ptr[N - 1] |= 1L << (DBL_MANT_DIG - 1 - ((N - 1) * BITS_PER_MP_LIMB));
 105
 106   return N;
 107 }