xref: /illumos-gate/usr/src/lib/libm/common/C/logb.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 __logb = logb
31 #pragma weak _logb = logb
32 
33 #include "libm.h"
34 #include "xpg6.h"	/* __xpg6 */
35 #define	_C99SUSv3_logb	_C99SUSv3_logb_subnormal_is_like_ilogb
36 
37 #if defined(__x86)
38 static const double two52 = 4503599627370496.0;
39 #else
40 /*
41  * v: high part of a non-zero subnormal |x|; w: low part of |x|
42  */
43 static int
ilogb_subnormal(unsigned v,unsigned w)44 ilogb_subnormal(unsigned v, unsigned w) {
45 	int r = -1022 - 52;
46 
47 	if (v)
48 		r += 32;
49 	else
50 		v = w;
51 	if (v & 0xffff0000)
52 		r += 16, v >>= 16;
53 	if (v & 0xff00)
54 		r += 8, v >>= 8;
55 	if (v & 0xf0)
56 		r += 4, v >>= 4;
57 	v <<= 1;
58 	return (r + ((0xffffaa50 >> v) & 0x3));
59 }
60 #endif	/* defined(__x86) */
61 
62 double
logb(double x)63 logb(double x) {
64 	int *px = (int *) &x, k = px[HIWORD] & ~0x80000000;
65 
66 	if (k < 0x00100000) {
67 		if ((px[LOWORD] | k) == 0)
68 			return (_SVID_libm_err(x, x, 45));
69 		else if ((__xpg6 & _C99SUSv3_logb) != 0) {
70 #if defined(__x86)
71 			x *= two52;
72 			return ((double) (((px[HIWORD] & 0x7ff00000) >> 20)
73 				- 1075));
74 #else
75 			return ((double) ilogb_subnormal(k, px[LOWORD]));
76 #endif
77 		} else
78 			return (-1022.0);
79 	} else if (k < 0x7ff00000)
80 		return ((double) ((k >> 20) - 1023));
81 	else
82 		return (x * x);
83 }
84