65 lines
2.1 KiB
C
65 lines
2.1 KiB
C
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/* signbit() macro: Determine the sign bit of a floating-point number.
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Copyright (C) 2007-2020 Free Software Foundation, Inc.
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This program is free software: you can redistribute it and/or modify
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it under the terms of the GNU General Public License as published by
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the Free Software Foundation; either version 3 of the License, or
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(at your option) any later version.
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This program 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
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GNU General Public License for more details.
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You should have received a copy of the GNU General Public License
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along with this program. If not, see <https://www.gnu.org/licenses/>. */
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#include <config.h>
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/* Specification. */
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#include <math.h>
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#include <string.h>
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#include "isnand-nolibm.h"
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#include "float+.h"
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#ifdef gl_signbitd_OPTIMIZED_MACRO
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# undef gl_signbitd
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#endif
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int
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gl_signbitd (double arg)
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{
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#if defined DBL_SIGNBIT_WORD && defined DBL_SIGNBIT_BIT
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/* The use of a union to extract the bits of the representation of a
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'long double' is safe in practice, despite of the "aliasing rules" of
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C99, because the GCC docs say
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"Even with '-fstrict-aliasing', type-punning is allowed, provided the
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memory is accessed through the union type."
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and similarly for other compilers. */
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# define NWORDS \
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((sizeof (double) + sizeof (unsigned int) - 1) / sizeof (unsigned int))
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union { double value; unsigned int word[NWORDS]; } m;
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m.value = arg;
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return (m.word[DBL_SIGNBIT_WORD] >> DBL_SIGNBIT_BIT) & 1;
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#elif HAVE_COPYSIGN_IN_LIBC
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return copysign (1.0, arg) < 0;
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#else
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/* This does not do the right thing for NaN, but this is irrelevant for
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most use cases. */
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if (isnand (arg))
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return 0;
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if (arg < 0.0)
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return 1;
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else if (arg == 0.0)
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{
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/* Distinguish 0.0 and -0.0. */
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static double plus_zero = 0.0;
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double arg_mem = arg;
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return (memcmp (&plus_zero, &arg_mem, SIZEOF_DBL) != 0);
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}
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else
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return 0;
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#endif
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}
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