logb: Provide replacement and workarounds.

* lib/math.in.h (logb): Ensure declaration. Replace if REPLACE_LOGB
is 1.
* lib/logb.c: New file.
* m4/logb.m4 (gl_FUNC_LOGB_WORKS): New macro.
(gl_FUNC_LOGB): Invoke it. Set HAVE_LOGB, REPLACE_LOGB.
* m4/math_h.m4 (gl_MATH_H_DEFAULTS): Initialize REPLACE_LOGB.
* modules/math (Makefile.am): Substitute REPLACE_LOGB.
* modules/logb (Files): Add lib/logb.c.
(Depends-on): Add isfinite, frexp, isnand.
(configure.ac): Compile the replacement code logb.c if needed.
* tests/test-math-c++.cc: Check the declaration of logb.
* doc/posix-functions/logb.texi: Mention the replacement and the bug
with subnormal numbers.

1 files changed, 71 insertions, 0 deletions

diff --git a/lib/logb.c b/lib/logb.c
new file mode 100644
index 0000000000..d56be557a7
--- /dev/null
+++ b/lib/logb.c
@@ -0,0 +1,71 @@
+/* Floating-point exponent.
+   Copyright (C) 2012 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/>.  */
+
+#if ! (defined USE_LONG_DOUBLE || defined USE_FLOAT)
+# include <config.h>
+#endif
+
+/* Specification.  */
+#include <math.h>
+
+#ifdef USE_LONG_DOUBLE
+# define LOGB logbl
+# define DOUBLE long double
+# define L_(literal) literal##L
+# define HUGEVAL HUGE_VALL
+# define FREXP frexpl
+# define ISNAN isnanl
+#elif ! defined USE_FLOAT
+# define LOGB logb
+# define DOUBLE double
+# define L_(literal) literal
+# define HUGEVAL HUGE_VAL
+# define FREXP frexp
+# define ISNAN isnand
+#else /* defined USE_FLOAT */
+# define LOGB logbf
+# define DOUBLE float
+# define L_(literal) literal##f
+# define HUGEVAL HUGE_VALF
+# define FREXP frexpf
+# define ISNAN isnanf
+#endif
+
+DOUBLE
+LOGB (DOUBLE x)
+{
+  if (isfinite (x))
+    {
+      if (x == L_(0.0))
+        /* Return -Infinity.  */
+        return - HUGEVAL;
+      else
+        {
+          int e;
+
+          (void) FREXP (x, &e);
+          return (DOUBLE) (e - 1);
+        }
+    }
+  else
+    {
+      if (ISNAN (x))
+        return x; /* NaN */
+      else
+        /* Return +Infinity.  */
+        return HUGEVAL;
+    }
+}