--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,10 @@
+2008-01-26  Bruno Haible  <bruno@clisp.org>
+
+	* m4/isnanl.m4 (gl_HAVE_ISNANL_NO_LIBM, gl_HAVE_ISNANL_IN_LIBM,
+	gl_FUNC_ISNANL_WORKS): Test the GCC >= 4.0 built-in.
+	* lib/isnanl.h (isnanl): Use the GCC >= 4.0 built-in.
+	* lib/isnanl-nolibm.h (isnanl): Likewise.
+
 2008-01-26  Bruno Haible  <bruno@clisp.org>
 
 	* m4/isnand.m4 (gl_FUNC_ISNAND_NO_LIBM): Test the GCC >= 4.0 built-in.

--- a/lib/isnanl-nolibm.h
+++ b/lib/isnanl-nolibm.h
@@ -1,5 +1,5 @@
 /* Test for NaN that does not need libm.
-   Copyright (C) 2007 Free Software Foundation, Inc.
+   Copyright (C) 2007-2008 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
@@ -17,7 +17,11 @@
 #if HAVE_ISNANL_IN_LIBC
 /* Get declaration of isnan macro or (older) isnanl function.  */
 # include <math.h>
-# ifdef isnan
+# if __GNUC__ >= 4
+   /* GCC 4.0 and newer provides three built-ins for isnan.  */
+#  undef isnanl
+#  define isnanl(x) __builtin_isnanl ((long double)(x))
+# elif defined isnan
 #  undef isnanl
 #  define isnanl(x) isnan ((long double)(x))
 # endif

--- a/lib/isnanl.h
+++ b/lib/isnanl.h
@@ -1,5 +1,5 @@
 /* Test for NaN.
-   Copyright (C) 2007 Free Software Foundation, Inc.
+   Copyright (C) 2007-2008 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
@@ -17,7 +17,11 @@
 #if HAVE_ISNANL
 /* Get declaration of isnan macro or (older) isnanl function.  */
 # include <math.h>
-# ifdef isnan
+# if __GNUC__ >= 4
+   /* GCC 4.0 and newer provides three built-ins for isnan.  */
+#  undef isnanl
+#  define isnanl(x) __builtin_isnanl ((long double)(x))
+# elif defined isnan
 #  undef isnanl
 #  define isnanl(x) isnan ((long double)(x))
 # endif

--- a/m4/isnanl.m4
+++ b/m4/isnanl.m4
@@ -1,5 +1,5 @@
-# isnanl.m4 serial 5
-dnl Copyright (C) 2007 Free Software Foundation, Inc.
+# isnanl.m4 serial 6
+dnl Copyright (C) 2007-2008 Free Software Foundation, Inc.
 dnl This file is free software; the Free Software Foundation
 dnl gives unlimited permission to copy and/or distribute it,
 dnl with or without modifications, as long as this notice is preserved.
@@ -64,7 +64,10 @@
     [gl_cv_func_isnanl_no_libm],
     [
       AC_TRY_LINK([#include <math.h>
-                   #ifdef isnan
+                   #if __GNUC__ >= 4
+                   # undef isnanl
+                   # define isnanl(x) __builtin_isnanl ((long double)(x))
+                   #elif defined isnan
                    # undef isnanl
                    # define isnanl(x) isnan ((long double)(x))
                    #endif
@@ -84,7 +87,10 @@
       save_LIBS="$LIBS"
       LIBS="$LIBS -lm"
       AC_TRY_LINK([#include <math.h>
-                   #ifdef isnan
+                   #if __GNUC__ >= 4
+                   # undef isnanl
+                   # define isnanl(x) __builtin_isnanl ((long double)(x))
+                   #elif defined isnan
                    # undef isnanl
                    # define isnanl(x) isnan ((long double)(x))
                    #endif
@@ -98,6 +104,9 @@
 
 dnl Test whether isnanl() recognizes all numbers which are neither finite nor
 dnl infinite. This test fails e.g. on NetBSD/i386 and on glibc/ia64.
+dnl Also, the GCC >= 4.0 built-in __builtin_isnanl does not pass the tests
+dnl - for pseudo-denormals on i686 and x86_64,
+dnl - for pseudo-zeroes, unnormalized numbers, and pseudo-denormals on ia64.
 AC_DEFUN([gl_FUNC_ISNANL_WORKS],
 [
   AC_REQUIRE([AC_PROG_CC])
@@ -109,7 +118,10 @@
 #include <float.h>
 #include <limits.h>
 #include <math.h>
-#ifdef isnan
+#if __GNUC__ >= 4
+# undef isnanl
+# define isnanl(x) __builtin_isnanl ((long double)(x))
+#elif defined isnan
 # undef isnanl
 # define isnanl(x) isnan ((long double)(x))
 #endif