gcc/mpfr/ufloor_log2.c

/* __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
}