Optimize logb code for 64-bit machines

ChangeLog

@@ -1,3 +1,7 @@
+2011-09-12  Ulrich Drepper  <drepper@gmail.com>
+
+	* sysdeps/ieee754/dbl-64/wordsize-64/s_logb.c: New file.
+
 2011-09-12  Andreas Schwab  <schwab@redhat.com>
 
 	* sysdeps/unix/sysv/linux/bits/socket.h (__cmsg_nxthdr): Cast to

sysdeps/ieee754/dbl-64/wordsize-64/s_logb.c

@@ -0,0 +1,44 @@
+/* Compute radix independent exponent.
+   Copyright (C) 2011 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+   Contributed by Ulrich Drepper <drepper@gmail.com>, 2011.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library 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
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, write to the Free
+   Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA
+   02111-1307 USA.  */
+
+#include <math.h>
+
+#include "math_private.h"
+
+
+double
+__logb (double x)
+{
+  int64_t ix;
+
+  EXTRACT_WORDS64 (ix, x);
+  ix &= UINT64_C(0x7fffffffffffffff);
+  if (ix == 0)
+    return -1.0 / fabs (x);
+  unsigned int ex = ix >> 52;
+  if (ex == 0x7ff)
+    return x * x;
+  return ex == 0 ? -1022.0 : (double) (ex - 1023);
+}
+weak_alias (__logb, logb)
+#ifdef NO_LONG_DOUBLE
+strong_alias (__logb, __logbl)
+weak_alias (__logb, logbl)
+#endif