'signbit' patch to use 'copysign' if available

[Paul Eggert]

Here's a proposed patch to 'signbit' to have it use 'copysign' if
available.

Normally the 'signbit' implementation relies on undefined behavior, as
it accesses the "wrong" member of a union; but when copysign is
available the implementation can use copysign's well-defined behavior
instead.  This is safer for traditional platforms like Solaris 9 that
have copysign but not signbit.

2007-04-06  Paul Eggert  <address@hidden>

        * lib/signbitd.c (gl_signbitd): Use copysign if available.
        This avoids relying on undefined behavior.
        * lib/signbitf.c (gl_signbitf): Use copysignf if available.
        * lib/signbitl.c (gl_signbitl): Use copysignl if available.
        * m4/signbit.m4 (gl_SIGNBIT): Test for copysignf, copysign,
        copysignl.

Index: lib/signbitd.c
===================================================================
RCS file: /cvsroot/gnulib/gnulib/lib/signbitd.c,v
retrieving revision 1.1
diff -u -p -r1.1 signbitd.c
--- lib/signbitd.c      6 Apr 2007 20:55:44 -0000       1.1
+++ lib/signbitd.c      7 Apr 2007 05:22:38 -0000
@@ -29,7 +29,9 @@
 int
 gl_signbitd (double arg)
 {
-#if defined DBL_SIGNBIT_WORD && defined DBL_SIGNBIT_BIT
+#if HAVE_DECL_COPYSIGN
+  return copysign (1, arg) < 0;
+#elif defined DBL_SIGNBIT_WORD && defined DBL_SIGNBIT_BIT
 # define NWORDS \
     ((sizeof (double) + sizeof (unsigned int) - 1) / sizeof (unsigned int))
   union { double value; unsigned int word[NWORDS]; } m;
Index: lib/signbitf.c
===================================================================
RCS file: /cvsroot/gnulib/gnulib/lib/signbitf.c,v
retrieving revision 1.1
diff -u -p -r1.1 signbitf.c
--- lib/signbitf.c      6 Apr 2007 20:55:44 -0000       1.1
+++ lib/signbitf.c      7 Apr 2007 05:22:38 -0000
@@ -29,7 +29,9 @@
 int
 gl_signbitf (float arg)
 {
-#if defined FLT_SIGNBIT_WORD && defined FLT_SIGNBIT_BIT
+#if HAVE_DECL_COPYSIGNF
+  return copysignf (1, arg) < 0;
+#elif defined FLT_SIGNBIT_WORD && defined FLT_SIGNBIT_BIT
 # define NWORDS \
     ((sizeof (float) + sizeof (unsigned int) - 1) / sizeof (unsigned int))
   union { float value; unsigned int word[NWORDS]; } m;
Index: lib/signbitl.c
===================================================================
RCS file: /cvsroot/gnulib/gnulib/lib/signbitl.c,v
retrieving revision 1.1
diff -u -p -r1.1 signbitl.c
--- lib/signbitl.c      6 Apr 2007 20:55:44 -0000       1.1
+++ lib/signbitl.c      7 Apr 2007 05:22:38 -0000
@@ -29,7 +29,9 @@
 int
 gl_signbitl (long double arg)
 {
-#if defined LDBL_SIGNBIT_WORD && defined LDBL_SIGNBIT_BIT
+#if HAVE_DECL_COPYSIGNL
+  return copysignl (1, arg) < 0;
+#elif defined LDBL_SIGNBIT_WORD && defined LDBL_SIGNBIT_BIT
 # define NWORDS \
     ((sizeof (long double) + sizeof (unsigned int) - 1) / sizeof (unsigned 
int))
   union { long double value; unsigned int word[NWORDS]; } m;
Index: m4/signbit.m4
===================================================================
RCS file: /cvsroot/gnulib/gnulib/m4/signbit.m4,v
retrieving revision 1.1
diff -u -p -r1.1 signbit.m4
--- m4/signbit.m4       6 Apr 2007 20:55:44 -0000       1.1
+++ m4/signbit.m4       7 Apr 2007 05:22:38 -0000
@@ -62,9 +62,12 @@ int main ()
     AC_LIBOBJ([signbitf])
     AC_LIBOBJ([signbitd])
     AC_LIBOBJ([signbitl])
-    gl_FLOAT_SIGN_LOCATION
-    gl_DOUBLE_SIGN_LOCATION
-    gl_LONG_DOUBLE_SIGN_LOCATION
+    AC_CHECK_DECLS([copysignf], [], [gl_FLOAT_SIGN_LOCATION],
+      [#include <math.h>])
+    AC_CHECK_DECLS([copysign], [], [gl_DOUBLE_SIGN_LOCATION],
+      [#include <math.h>])
+    AC_CHECK_DECLS([copysignl], [], [gl_LONG_DOUBLE_SIGN_LOCATION],
+      [#include <math.h>])
   fi
 ])