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dbl2mpn.c
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1994-01-09
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/* Copyright (C) 1993, 1994 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 "ieee754.h"
#include <stdlib.h>
/* Convert a `double' in IEEE754 standard double-precision format to a
multi-precision integer representing the significand scaled up by its
number of bits (52 for double) and an integral power of two (MPN frexp). */
mp_size_t
__mpn_extract_double (mp_ptr res_ptr, mp_size_t size,
int *expt, int *is_neg,
double value)
{
union ieee754_double u;
u.d = value;
*is_neg = u.ieee.negative;
*expt = (int) u.ieee.exponent - 1022;
#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 20 bits. */
#define N 2
#define IMPL1 1 << 20
#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] = (u.ieee.mantissa0 << 20) | 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. Just one limb records that. */
return 1;
/* It is a denormal number, meaning it has no implicit leading
one bit, and its exponent is in fact the format minimum. */
*expt = -1021;
#if N == 2
if (res_ptr[1] == 0)
/* No point in using an extra high-order limb that is zero. */
return 1;
#endif
}
else
/* Add the implicit leading one bit for a normalized number. */
res_ptr[N - 1] |= 1 << (52 - ((N - 1) * BITS_PER_MP_LIMB));
return N;
}