xref: /illumos-gate/usr/src/lib/libm/common/LD/log1pl.c (revision 1ed6b69a5ca1ca3ee5e9a4931f74e2237c7e1c9f)
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 #if defined(ELFOBJ)
31 #pragma weak log1pl = __log1pl
32 #endif
33 
34 /*
35  * log1pl(x)
36  * Kahan's trick based on log(1+x)/x being a slow varying function.
37  */
38 
39 #include "libm.h"
40 
41 #if defined(__x86)
42 #define	__swapRD	__swap87RD
43 #endif
44 extern enum fp_direction_type __swapRD(enum fp_direction_type);
45 
46 long double
47 log1pl(long double x) {
48 	long double y;
49 	enum fp_direction_type rd;
50 
51 	if (x != x)
52 		return (x + x);
53 	if (x < -1.L)
54 		return (logl(x));
55 	rd = __swapRD(fp_nearest);
56 	y = 1.L + x;
57 	if (y != 1.L) {
58 		if (y == x)
59 			x = logl(x);
60 		else
61 			x *= logl(y) / (y - 1.L);
62 	}
63 	if (rd != fp_nearest)
64 		(void) __swapRD(rd);
65 	return (x);
66 }
67