sysdeps/i386/ldbl2mpn.c

   1 /* Copyright (C) 1995-2014 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 `long double' in IEEE854 standard double-precision format to a
  26    multi-precision integer representing the significand scaled up by its
  27    number of bits (64 for long double) and an integral power of two
  28    (MPN frexpl). */
  29
  30 mp_size_t
  31 __mpn_extract_long_double (mp_ptr res_ptr, mp_size_t size,
  32                            int *expt, int *is_neg,
  33                            long double value)
  34 {
  35   union ieee854_long_double u;
  36   u.d = value;
  37
  38   *is_neg = u.ieee.negative;
  39   *expt = (int) u.ieee.exponent - IEEE854_LONG_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 32 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] = ((mp_limb_t) 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
  54   if (u.ieee.exponent == 0)
  55     {
  56       /* A biased exponent of zero is a special case.
  57          Either it is a zero or it is a denormal number.  */
  58       if (res_ptr[0] == 0 && res_ptr[N - 1] == 0) /* Assumes N<=2.  */
  59         /* It's zero.  */
  60         *expt = 0;
  61       else
  62         {
  63           /* It is a denormal number, meaning it has no implicit leading
  64              one bit, and its exponent is in fact the format minimum.  */
  65           int cnt;
  66
  67           /* One problem with Intel's 80-bit format is that the explicit
  68              leading one in the normalized representation has to be zero
  69              for denormalized number.  If it is one, the number is according
  70              to Intel's specification an invalid number.  We make the
  71              representation unique by explicitly clearing this bit.  */
  72           res_ptr[N - 1] &= ~(1L << ((LDBL_MANT_DIG - 1) % BITS_PER_MP_LIMB));
  73
  74           if (res_ptr[N - 1] != 0)
  75             {
  76               count_leading_zeros (cnt, res_ptr[N - 1]);
  77               if (cnt != 0)
  78                 {
  79 #if N == 2
  80                   res_ptr[N - 1] = res_ptr[N - 1] << cnt
  81                                    | (res_ptr[0] >> (BITS_PER_MP_LIMB - cnt));
  82                   res_ptr[0] <<= cnt;
  83 #else
  84                   res_ptr[N - 1] <<= cnt;
  85 #endif
  86                 }
  87               *expt = LDBL_MIN_EXP - 1 - cnt;
  88             }
  89           else if (res_ptr[0] != 0)
  90             {
  91               count_leading_zeros (cnt, res_ptr[0]);
  92               res_ptr[N - 1] = res_ptr[0] << cnt;
  93               res_ptr[0] = 0;
  94               *expt = LDBL_MIN_EXP - 1 - BITS_PER_MP_LIMB - cnt;
  95             }
  96           else
  97             {
  98               /* This is the special case of the pseudo denormal number
  99                  with only the implicit leading bit set.  The value is
 100                  in fact a normal number and so we have to treat this
 101                  case differently.  */
 102 #if N == 2
 103               res_ptr[N - 1] = 0x80000000ul;
 104 #else
 105               res_ptr[0] = 0x8000000000000000ul;
 106 #endif
 107               *expt = LDBL_MIN_EXP - 1;
 108             }
 109         }
 110     }
 111   else if (u.ieee.exponent < 0x7fff
 112 #if N == 2
 113            && res_ptr[0] == 0
 114 #endif
 115            && res_ptr[N - 1] == 0)
 116     /* Pseudo zero.  */
 117     *expt = 0;
 118
 119   return N;
 120 }