xref: /titanic_51/usr/src/lib/libm/common/Q/log10l.c (revision b59c4a48daf5a1863ecac763711b497b2f8321e4)
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 log10l = __log10l
32 #endif
33 
34 /*
35  * log10l(X)
36  *
37  * Method :
38  *	Let log10_2hi = leading 98(SPARC)/49(x86) bits of log10(2) and
39  *	    log10_2lo = log10(2) - log10_2hi,
40  *	    ivln10   = 1/log(10) rounded.
41  *	Then
42  *		n = ilogb(x),
43  *		if (n<0)  n = n+1;
44  *		x = scalbn(x,-n);
45  *		LOG10(x) := n*log10_2hi + (n*log10_2lo + ivln10*log(x))
46  *
47  * Note1:
48  *	For fear of destroying log10(10**n)=n, the rounding mode is
49  *	set to Round-to-Nearest.
50  *
51  * Special cases:
52  *	log10(x) is NaN with signal if x < 0;
53  *	log10(+INF) is +INF with no signal; log10(0) is -INF with signal;
54  *	log10(NaN) is that NaN with no signal;
55  *	log10(10**N) = N  for N=0,1,...,22.
56  *
57  * Constants:
58  * The hexadecimal values are the intended ones for the following constants.
59  * The decimal values may be used, provided that the compiler will convert
60  * from decimal to binary accurately enough to produce the hexadecimal values
61  * shown.
62  */
63 
64 #include "libm.h"
65 #include "longdouble.h"
66 
67 #if defined(__x86)
68 #define	__swapRD	__swap87RD
69 #endif
70 extern enum fp_direction_type __swapRD(enum fp_direction_type);
71 
72 static const long double
73 	zero	  = 0.0L,
74 	ivln10    = 4.342944819032518276511289189166050822944e-0001L,
75 	one	  = 1.0L,
76 #if defined(__x86)
77 	log10_2hi = 3.010299956639803653501985536422580480576e-01L,
78 	log10_2lo = 8.298635403410822349787106337291183585413e-16L;
79 #elif defined(__sparc)
80 	log10_2hi = 3.010299956639811952137388947242098603469e-01L,
81 	log10_2lo = 2.831664213089468167896664371953210945664e-31L;
82 #else
83 #error Unknown Architecture!
84 #endif
85 
86 long double
87 log10l(long double x) {
88 	long double y, z;
89 	enum fp_direction_type rd;
90 	int n;
91 
92 	if (!finitel(x))
93 		return (x + fabsl(x));	/* x is +-INF or NaN */
94 	else if (x > zero) {
95 		n = ilogbl(x);
96 		if (n < 0)
97 			n += 1;
98 		rd = __swapRD(fp_nearest);
99 		y = n;
100 		x = scalbnl(x, -n);
101 		z = y * log10_2lo + ivln10 * logl(x);
102 		z += y * log10_2hi;
103 		if (rd != fp_nearest)
104 			(void) __swapRD(rd);
105 		return (z);
106 	} else if (x == zero)	/* -INF */
107 		return (-one / zero);
108 	else			/* x <0, return NaN */
109 		return (zero / zero);
110 }
111