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 * Copyright 2006 Sun Microsystems, Inc. All rights reserved.
27 * Use is subject to license terms.
28 */
29
30 /*
31 * log2l(x)
32 * RETURN THE BASE 2 LOGARITHM OF X
33 *
34 * Method:
35 * purge off 0,INF, and NaN.
36 * n = ilogb(x)
37 * if (n<0) n+=1
38 * z = scalbn(x,-n)
39 * LOG2(x) = n + (1/ln2)*log(x)
40 */
41
42 #pragma weak __log2l = log2l
43
44 #include "libm.h"
45 #include "longdouble.h"
46
47 static const long double
48 zero = 0.0L,
49 half = 0.5L,
50 one = 1.0L,
51 invln2 = 1.442695040888963407359924681001892137427e+0000L;
52
53 long double
log2l(long double x)54 log2l(long double x) {
55 int n;
56
57 if (x == zero || !finitel(x))
58 return (logl(x));
59 n = ilogbl(x);
60 if (n < 0)
61 n += 1;
62 x = scalbnl(x, -n);
63 if (x == half)
64 return (n - one);
65 return (n + invln2 * logl(x));
66 }
67