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108 lines
3.4 KiB
C
108 lines
3.4 KiB
C
/* Copyright (C) 1993-2016 Free Software Foundation, Inc.
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This file is part of the GNU C Library.
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The GNU C Library is free software; you can redistribute it and/or
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modify it under the terms of the GNU Lesser General Public
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License as published by the Free Software Foundation; either
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version 2.1 of the License, or (at your option) any later version.
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The GNU C Library is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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Lesser General Public License for more details.
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You should have received a copy of the GNU Lesser General Public
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License along with the GNU C Library; if not, see
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<http://www.gnu.org/licenses/>. */
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#include "gmp.h"
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#include "gmp-impl.h"
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#include "longlong.h"
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#include <ieee754.h>
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#include <float.h>
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#include <stdlib.h>
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/* Convert a `double' in IEEE754 standard double-precision format to a
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multi-precision integer representing the significand scaled up by its
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number of bits (52 for double) and an integral power of two (MPN frexp). */
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mp_size_t
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__mpn_extract_double (mp_ptr res_ptr, mp_size_t size,
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int *expt, int *is_neg,
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double value)
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{
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union ieee754_double u;
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u.d = value;
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*is_neg = u.ieee.negative;
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*expt = (int) u.ieee.exponent - IEEE754_DOUBLE_BIAS;
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#if BITS_PER_MP_LIMB == 32
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res_ptr[0] = u.ieee.mantissa1; /* Low-order 32 bits of fraction. */
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res_ptr[1] = u.ieee.mantissa0; /* High-order 20 bits. */
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# define N 2
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#elif BITS_PER_MP_LIMB == 64
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/* Hopefully the compiler will combine the two bitfield extracts
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and this composition into just the original quadword extract. */
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res_ptr[0] = ((mp_limb_t) u.ieee.mantissa0 << 32) | u.ieee.mantissa1;
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# define N 1
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#else
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# error "mp_limb size " BITS_PER_MP_LIMB "not accounted for"
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#endif
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/* The format does not fill the last limb. There are some zeros. */
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#define NUM_LEADING_ZEROS (BITS_PER_MP_LIMB \
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- (DBL_MANT_DIG - ((N - 1) * BITS_PER_MP_LIMB)))
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if (u.ieee.exponent == 0)
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{
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/* A biased exponent of zero is a special case.
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Either it is a zero or it is a denormal number. */
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if (res_ptr[0] == 0 && res_ptr[N - 1] == 0) /* Assumes N<=2. */
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/* It's zero. */
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*expt = 0;
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else
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{
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/* It is a denormal number, meaning it has no implicit leading
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one bit, and its exponent is in fact the format minimum. */
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int cnt;
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if (res_ptr[N - 1] != 0)
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{
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count_leading_zeros (cnt, res_ptr[N - 1]);
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cnt -= NUM_LEADING_ZEROS;
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#if N == 2
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res_ptr[N - 1] = res_ptr[1] << cnt
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| (N - 1)
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* (res_ptr[0] >> (BITS_PER_MP_LIMB - cnt));
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res_ptr[0] <<= cnt;
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#else
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res_ptr[N - 1] <<= cnt;
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#endif
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*expt = DBL_MIN_EXP - 1 - cnt;
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}
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else
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{
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count_leading_zeros (cnt, res_ptr[0]);
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if (cnt >= NUM_LEADING_ZEROS)
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{
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res_ptr[N - 1] = res_ptr[0] << (cnt - NUM_LEADING_ZEROS);
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res_ptr[0] = 0;
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}
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else
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{
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res_ptr[N - 1] = res_ptr[0] >> (NUM_LEADING_ZEROS - cnt);
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res_ptr[0] <<= BITS_PER_MP_LIMB - (NUM_LEADING_ZEROS - cnt);
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}
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*expt = DBL_MIN_EXP - 1
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- (BITS_PER_MP_LIMB - NUM_LEADING_ZEROS) - cnt;
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}
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}
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}
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else
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/* Add the implicit leading one bit for a normalized number. */
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res_ptr[N - 1] |= (mp_limb_t) 1 << (DBL_MANT_DIG - 1
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- ((N - 1) * BITS_PER_MP_LIMB));
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return N;
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}
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