1 /*
   2  * CDDL HEADER START
   3  *
   4  * The contents of this file are subject to the terms of the
   5  * Common Development and Distribution License (the "License").
   6  * You may not use this file except in compliance with the License.
   7  *
   8  * You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE
   9  * or http://www.opensolaris.org/os/licensing.
  10  * See the License for the specific language governing permissions
  11  * and limitations under the License.
  12  *
  13  * When distributing Covered Code, include this CDDL HEADER in each
  14  * file and include the License file at usr/src/OPENSOLARIS.LICENSE.
  15  * If applicable, add the following below this CDDL HEADER, with the
  16  * fields enclosed by brackets "[]" replaced with your own identifying
  17  * information: Portions Copyright [yyyy] [name of copyright owner]
  18  *
  19  * CDDL HEADER END
  20  */
  21 
  22 /*
  23  * Copyright 2011 Nexenta Systems, Inc.  All rights reserved.
  24  */
  25 
  26 /*
  27  * Copyright 2006 Sun Microsystems, Inc.  All rights reserved.
  28  * Use is subject to license terms.
  29  */
  30 
  31 /*
  32  * log2l(x)
  33  * RETURN THE BASE 2 LOGARITHM OF X
  34  *
  35  * Method:
  36  *      purge off 0,INF, and NaN.
  37  *      n = ilogb(x)
  38  *      if (n<0) n+=1
  39  *      z = scalbn(x,-n)
  40  *      LOG2(x) = n + (1/ln2)*log(x)
  41  */
  42 
  43 #pragma weak __log2l = log2l
  44 
  45 #include "libm.h"
  46 #include "longdouble.h"
  47 
  48 static const long double zero = 0.0L,
  49         half = 0.5L,
  50         one = 1.0L,
  51         invln2 = 1.442695040888963407359924681001892137427e+0000L;
  52 
  53 long double
  54 log2l(long double x)
  55 {
  56         int n;
  57 
  58         if (x == zero || !finitel(x))
  59                 return (logl(x));
  60 
  61         n = ilogbl(x);
  62 
  63         if (n < 0)
  64                 n += 1;
  65 
  66         x = scalbnl(x, -n);
  67 
  68         if (x == half)
  69                 return (n - one);
  70 
  71         return (n + invln2 * logl(x));
  72 }