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+/* signbit() macro: Determine the sign bit of a floating-point number.
+ Copyright (C) 2007-2016 Free Software Foundation, Inc.
+
+ This program is free software: you can redistribute it and/or modify
+ it under the terms of the GNU General Public License as published by
+ the Free Software Foundation; either version 3 of the License, or
+ (at your option) any later version.
+
+ This program 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 General Public License for more details.
+
+ You should have received a copy of the GNU General Public License
+ along with this program. If not, see <http://www.gnu.org/licenses/>. */
+
+#include <config.h>
+
+/* Specification. */
+#include <math.h>
+
+#include <string.h>
+#include "isnand-nolibm.h"
+#include "float+.h"
+
+#ifdef gl_signbitd_OPTIMIZED_MACRO
+# undef gl_signbitd
+#endif
+
+int
+gl_signbitd (double arg)
+{
+#if defined DBL_SIGNBIT_WORD && defined DBL_SIGNBIT_BIT
+ /* The use of a union to extract the bits of the representation of a
+ 'long double' is safe in practice, despite of the "aliasing rules" of
+ C99, because the GCC docs say
+ "Even with '-fstrict-aliasing', type-punning is allowed, provided the
+ memory is accessed through the union type."
+ and similarly for other compilers. */
+# define NWORDS \
+ ((sizeof (double) + sizeof (unsigned int) - 1) / sizeof (unsigned int))
+ union { double value; unsigned int word[NWORDS]; } m;
+ m.value = arg;
+ return (m.word[DBL_SIGNBIT_WORD] >> DBL_SIGNBIT_BIT) & 1;
+#elif HAVE_COPYSIGN_IN_LIBC
+ return copysign (1.0, arg) < 0;
+#else
+ /* This does not do the right thing for NaN, but this is irrelevant for
+ most use cases. */
+ if (isnand (arg))
+ return 0;
+ if (arg < 0.0)
+ return 1;
+ else if (arg == 0.0)
+ {
+ /* Distinguish 0.0 and -0.0. */
+ static double plus_zero = 0.0;
+ double arg_mem = arg;
+ return (memcmp (&plus_zero, &arg_mem, SIZEOF_DBL) != 0);
+ }
+ else
+ return 0;
+#endif
+}