sysdeps/ieee754/dbl-64/dbl2mpn.c

   1 /* Copyright (C) 1993-2016 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 Lesser General Public
   6    License as published by the Free Software Foundation; either
   7    version 2.1 of the 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    Lesser General Public License for more details.
  13
  14    You should have received a copy of the GNU Lesser General Public
  15    License along with the GNU C Library; if not, see
  16    <http://www.gnu.org/licenses/>.  */
  17
  18 #include "gmp.h"
  19 #include "gmp-impl.h"
  20 #include "longlong.h"
  21 #include <ieee754.h>
  22 #include <float.h>
  23 #include <stdlib.h>
  24
  25 /* Convert a `double' in IEEE754 standard double-precision format to a
  26    multi-precision integer representing the significand scaled up by its
  27    number of bits (52 for double) and an integral power of two (MPN frexp). */
  28
  29 mp_size_t
  30 __mpn_extract_double (mp_ptr res_ptr, mp_size_t size,
  31                       int *expt, int *is_neg,
  32                       double value)
  33 {
  34   union ieee754_double u;
  35   u.d = value;
  36
  37   *is_neg = u.ieee.negative;
  38   *expt = (int) u.ieee.exponent - IEEE754_DOUBLE_BIAS;
  39
  40 #if BITS_PER_MP_LIMB == 32
  41   res_ptr[0] = u.ieee.mantissa1; /* Low-order 32 bits of fraction.  */
  42   res_ptr[1] = u.ieee.mantissa0; /* High-order 20 bits.  */
  43   # define N 2
  44 #elif BITS_PER_MP_LIMB == 64
  45   /* Hopefully the compiler will combine the two bitfield extracts
  46      and this composition into just the original quadword extract.  */
  47   res_ptr[0] = ((mp_limb_t) u.ieee.mantissa0 << 32) | u.ieee.mantissa1;
  48   # define N 1
  49 #else
  50   # error "mp_limb size " BITS_PER_MP_LIMB "not accounted for"
  51 #endif
  52 /* The format does not fill the last limb.  There are some zeros.  */
  53 #define NUM_LEADING_ZEROS (BITS_PER_MP_LIMB \
  54                            - (DBL_MANT_DIG - ((N - 1) * BITS_PER_MP_LIMB)))
  55
  56   if (u.ieee.exponent == 0)
  57     {
  58       /* A biased exponent of zero is a special case.
  59          Either it is a zero or it is a denormal number.  */
  60       if (res_ptr[0] == 0 && res_ptr[N - 1] == 0) /* Assumes N<=2.  */
  61         /* It's zero.  */
  62         *expt = 0;
  63       else
  64         {
  65           /* It is a denormal number, meaning it has no implicit leading
  66              one bit, and its exponent is in fact the format minimum.  */
  67           int cnt;
  68
  69           if (res_ptr[N - 1] != 0)
  70             {
  71               count_leading_zeros (cnt, res_ptr[N - 1]);
  72               cnt -= NUM_LEADING_ZEROS;
  73 #if N == 2
  74               res_ptr[N - 1] = res_ptr[1] << cnt
  75                                | (N - 1)
  76                                * (res_ptr[0] >> (BITS_PER_MP_LIMB - cnt));
  77               res_ptr[0] <<= cnt;
  78 #else
  79               res_ptr[N - 1] <<= cnt;
  80 #endif
  81               *expt = DBL_MIN_EXP - 1 - cnt;
  82             }
  83           else
  84             {
  85               count_leading_zeros (cnt, res_ptr[0]);
  86               if (cnt >= NUM_LEADING_ZEROS)
  87                 {
  88                   res_ptr[N - 1] = res_ptr[0] << (cnt - NUM_LEADING_ZEROS);
  89                   res_ptr[0] = 0;
  90                 }
  91               else
  92                 {
  93                   res_ptr[N - 1] = res_ptr[0] >> (NUM_LEADING_ZEROS - cnt);
  94                   res_ptr[0] <<= BITS_PER_MP_LIMB - (NUM_LEADING_ZEROS - cnt);
  95                 }
  96               *expt = DBL_MIN_EXP - 1
  97                       - (BITS_PER_MP_LIMB - NUM_LEADING_ZEROS) - cnt;
  98             }
  99         }
 100     }
 101   else
 102     /* Add the implicit leading one bit for a normalized number.  */
 103     res_ptr[N - 1] |= (mp_limb_t) 1 << (DBL_MANT_DIG - 1
 104                                         - ((N - 1) * BITS_PER_MP_LIMB));
 105
 106   return N;
 107 }