xref: /illumos-gate/usr/src/lib/libm/common/Q/log10l.c (revision ddc0e0b53c661f6e439e3b7072b3ef353eadb4af)
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 #pragma weak __log10l = log10l
31 
32 /*
33  * log10l(X)
34  *
35  * Method :
36  *	Let log10_2hi = leading 98(SPARC)/49(x86) bits of log10(2) and
37  *	    log10_2lo = log10(2) - log10_2hi,
38  *	    ivln10   = 1/log(10) rounded.
39  *	Then
40  *		n = ilogb(x),
41  *		if (n<0)  n = n+1;
42  *		x = scalbn(x,-n);
43  *		LOG10(x) := n*log10_2hi + (n*log10_2lo + ivln10*log(x))
44  *
45  * Note1:
46  *	For fear of destroying log10(10**n)=n, the rounding mode is
47  *	set to Round-to-Nearest.
48  *
49  * Special cases:
50  *	log10(x) is NaN with signal if x < 0;
51  *	log10(+INF) is +INF with no signal; log10(0) is -INF with signal;
52  *	log10(NaN) is that NaN with no signal;
53  *	log10(10**N) = N  for N=0,1,...,22.
54  *
55  * Constants:
56  * The hexadecimal values are the intended ones for the following constants.
57  * The decimal values may be used, provided that the compiler will convert
58  * from decimal to binary accurately enough to produce the hexadecimal values
59  * shown.
60  */
61 
62 #include "libm.h"
63 #include "longdouble.h"
64 
65 #if defined(__x86)
66 #define	__swapRD	__swap87RD
67 #endif
68 extern enum fp_direction_type __swapRD(enum fp_direction_type);
69 
70 static const long double
71 	zero	  = 0.0L,
72 	ivln10    = 4.342944819032518276511289189166050822944e-0001L,
73 	one	  = 1.0L,
74 #if defined(__x86)
75 	log10_2hi = 3.010299956639803653501985536422580480576e-01L,
76 	log10_2lo = 8.298635403410822349787106337291183585413e-16L;
77 #elif defined(__sparc)
78 	log10_2hi = 3.010299956639811952137388947242098603469e-01L,
79 	log10_2lo = 2.831664213089468167896664371953210945664e-31L;
80 #else
81 #error Unknown Architecture!
82 #endif
83 
84 long double
log10l(long double x)85 log10l(long double x) {
86 	long double y, z;
87 	enum fp_direction_type rd;
88 	int n;
89 
90 	if (!finitel(x))
91 		return (x + fabsl(x));	/* x is +-INF or NaN */
92 	else if (x > zero) {
93 		n = ilogbl(x);
94 		if (n < 0)
95 			n += 1;
96 		rd = __swapRD(fp_nearest);
97 		y = n;
98 		x = scalbnl(x, -n);
99 		z = y * log10_2lo + ivln10 * logl(x);
100 		z += y * log10_2hi;
101 		if (rd != fp_nearest)
102 			(void) __swapRD(rd);
103 		return (z);
104 	} else if (x == zero)	/* -INF */
105 		return (-one / zero);
106 	else			/* x <0, return NaN */
107 		return (zero / zero);
108 }
109