xref: /illumos-gate/usr/src/lib/libm/common/C/ilogb.c (revision 862246c1)
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 __ilogb = ilogb
31 
32 #include "libm.h"
33 #include "xpg6.h"	/* __xpg6 */
34 
35 #if defined(__x86)
36 static const double two52 = 4503599627370496.0;
37 #else
38 /*
39  * v: high part of a non-zero subnormal |x|; w: low part of |x|
40  */
41 static int
ilogb_subnormal(unsigned v,unsigned w)42 ilogb_subnormal(unsigned v, unsigned w) {
43 	int r = -1022 - 52;
44 
45 	if (v)
46 		r += 32;
47 	else
48 		v = w;
49 	if (v & 0xffff0000)
50 		r += 16, v >>= 16;
51 	if (v & 0xff00)
52 		r += 8, v >>= 8;
53 	if (v & 0xf0)
54 		r += 4, v >>= 4;
55 	v <<= 1;
56 	return (r + ((0xffffaa50 >> v) & 0x3));
57 }
58 #endif	/* defined(__x86) */
59 
60 static int
raise_invalid(int v)61 raise_invalid(int v) {	/* SUSv3 requires ilogb(0,+/-Inf,NaN) raise invalid */
62 #ifndef lint
63 	if ((__xpg6 & _C99SUSv3_ilogb_0InfNaN_raises_invalid) != 0) {
64 		static const double zero = 0.0;
65 		volatile double dummy __unused;
66 
67 		dummy = zero / zero;
68 	}
69 #endif
70 	return (v);
71 }
72 
73 int
ilogb(double x)74 ilogb(double x) {
75 	int *px = (int *) &x, k = px[HIWORD] & ~0x80000000;
76 
77 	if (k < 0x00100000) {
78 		if ((px[LOWORD] | k) == 0)
79 			return (raise_invalid(0x80000001));
80 		else {
81 #if defined(__x86)
82 			x *= two52;
83 			return (((px[HIWORD] & 0x7ff00000) >> 20) - 1075);
84 #else
85 			return (ilogb_subnormal(k, px[LOWORD]));
86 #endif
87 		}
88 	} else if (k < 0x7ff00000)
89 		return ((k >> 20) - 1023);
90 	else
91 		return (raise_invalid(0x7fffffff));
92 }
93