1  /* Copyright (C) 1993-2023 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     <https://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  }