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diff --git a/mpfr/ufloor_log2.c b/mpfr/ufloor_log2.c
new file mode 100644
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--- /dev/null
+++ b/mpfr/ufloor_log2.c
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+/* __gmpfr_floor_log2 - returns floor(log(d)/log(2))
+
+Copyright 1999, 2000, 2001, 2002, 2003, 2004, 2006, 2007, 2008, 2009 Free Software Foundation, Inc.
+Contributed by the Arenaire and Cacao projects, INRIA.
+
+This file is part of the GNU MPFR Library.
+
+The GNU MPFR 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 MPFR 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 MPFR Library; see the file COPYING.LIB.  If not, write to
+the Free Software Foundation, Inc., 51 Franklin St, Fifth Floor, Boston,
+MA 02110-1301, USA. */
+
+#include "mpfr-impl.h"
+
+/* returns floor(log2(d)) */
+long
+__gmpfr_floor_log2 (double d)
+{
+#if _GMP_IEEE_FLOATS
+  union ieee_double_extract x;
+
+  x.d = d;
+  return (long) x.s.exp - 1023;
+#else
+  long exp;
+  double m;
+
+  MPFR_ASSERTD (d >= 0);
+  if (d >= 1.0)
+    {
+      exp = -1;
+      for( m= 1.0 ; m <= d ; m *=2.0 )
+        exp++;
+    }
+  else
+    {
+      exp = 0;
+      for( m= 1.0 ; m > d ; m *= (1.0/2.0) )
+        exp--;
+    }
+  return exp;
+#endif
+}