glibc/sysdeps/i386/ldbl2mpn.c

/* Copyright (C) 1995, 1996 Free Software Foundation, Inc.
This file is part of the GNU C Library.

The GNU C Library is free software; you can redistribute it and/or
modify it under the terms of the GNU Library General Public License as
published by the Free Software Foundation; either version 2 of the
License, or (at your option) any later version.

The GNU C 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
Library General Public License for more details.

You should have received a copy of the GNU Library General Public
License along with the GNU C Library; see the file COPYING.LIB.  If
not, write to the Free Software Foundation, Inc., 675 Mass Ave,
Cambridge, MA 02139, USA.  */

#include "gmp.h"
#include "gmp-impl.h"
#include "longlong.h"
#include "ieee754.h"
#include <float.h>
#include <stdlib.h>

/* Convert a `long double' in IEEE854 standard double-precision format to a
   multi-precision integer representing the significand scaled up by its
   number of bits (64 for long double) and an integral power of two
   (MPN frexpl). */

mp_size_t
__mpn_extract_long_double (mp_ptr res_ptr, mp_size_t size,
			   int *expt, int *is_neg,
			   long double value)
{
  union ieee854_long_double u;
  u.d = value;

  *is_neg = u.ieee.negative;
  *expt = (int) u.ieee.exponent - IEEE854_LONG_DOUBLE_BIAS;

#if BITS_PER_MP_LIMB == 32
  res_ptr[0] = u.ieee.mantissa1; /* Low-order 32 bits of fraction.  */
  res_ptr[1] = u.ieee.mantissa0; /* High-order 32 bits.  */
  #define N 2
#elif BITS_PER_MP_LIMB == 64
  /* Hopefully the compiler will combine the two bitfield extracts
     and this composition into just the original quadword extract.  */
  res_ptr[0] = ((unsigned long int) u.ieee.mantissa0 << 32) | u.ieee.mantissa1;
  #define N 1
#else
  #error "mp_limb size " BITS_PER_MP_LIMB "not accounted for"
#endif

  if (u.ieee.exponent == 0)
    {
      /* A biased exponent of zero is a special case.
	 Either it is a zero or it is a denormal number.  */
      if (res_ptr[0] == 0 && res_ptr[N - 1] == 0) /* Assumes N<=2.  */
	/* It's zero.  */
	*expt = 0;
      else
	{
          /* It is a denormal number, meaning it has no implicit leading
  	     one bit, and its exponent is in fact the format minimum.  */
	  int cnt;

	  /* One problem with Intel's 80-bit format is that the explicit
	     leading one in the normalized representation has to be zero
	     for denormalized number.  If it is one, the number is according
	     to Intel's specification an invalid number.  We make the
	     representation unique by explicitly clearing this bit.  */
	  res_ptr[N - 1] &= ~(1L << ((LDBL_MANT_DIG - 1) % BITS_PER_MP_LIMB));

	  if (res_ptr[N - 1] != 0)
	    {
	      count_leading_zeros (cnt, res_ptr[N - 1]);
	      if (cnt != 0)
		{
#if N == 2
	          res_ptr[N - 1] = res_ptr[N - 1] << cnt
			           | (res_ptr[0] >> (BITS_PER_MP_LIMB - cnt));
	          res_ptr[0] <<= cnt;
#else
	          res_ptr[N - 1] <<= cnt;
#endif
		}
	      *expt = LDBL_MIN_EXP - 1 - cnt;
	    }
	  else
	    {
	      count_leading_zeros (cnt, res_ptr[0]);
	      res_ptr[N - 1] = res_ptr[0] << cnt;
	      res_ptr[0] = 0;
	      *expt = LDBL_MIN_EXP - 1 - BITS_PER_MP_LIMB - cnt;
	    }
	}
    }

  return N;
}