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diff --git a/lib/signbitl.c b/lib/signbitl.c
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+++ b/lib/signbitl.c
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+/* signbit() macro: Determine the sign bit of a floating-point number.
+   Copyright (C) 2007, 2009 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 "isnanl-nolibm.h"
+#include "float+.h"
+
+#ifdef gl_signbitl_OPTIMIZED_MACRO
+# undef gl_signbitl
+#endif
+
+int
+gl_signbitl (long double arg)
+{
+#if defined LDBL_SIGNBIT_WORD && defined LDBL_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 (long double) + sizeof (unsigned int) - 1) / sizeof (unsigned int))
+  union { long double value; unsigned int word[NWORDS]; } m;
+  m.value = arg;
+  return (m.word[LDBL_SIGNBIT_WORD] >> LDBL_SIGNBIT_BIT) & 1;
+#elif HAVE_COPYSIGNL_IN_LIBC
+  return copysignl (1.0L, arg) < 0;
+#else
+  /* This does not do the right thing for NaN, but this is irrelevant for
+     most use cases.  */
+  if (isnanl (arg))
+    return 0;
+  if (arg < 0.0L)
+    return 1;
+  else if (arg == 0.0L)
+    {
+      /* Distinguish 0.0L and -0.0L.  */
+      static long double plus_zero = 0.0L;
+      long double arg_mem = arg;
+      return (memcmp (&plus_zero, &arg_mem, SIZEOF_LDBL) != 0);
+    }
+  else
+    return 0;
+#endif
+}