xref: /illumos-gate/usr/src/lib/libm/common/m9x/scalbln.c (revision ddc0e0b5)
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 __scalbln = scalbln
31 
32 #include "libm.h"
33 #include <float.h>		/* DBL_MAX, DBL_MIN */
34 
35 static const double twom54 = 5.5511151231257827021181583404541015625e-17;
36 #if defined(__x86)
37 static const double two52 = 4503599627370496.0;
38 #else
39 /*
40  * Normalize non-zero subnormal x and return biased exponent of x in [-51,0]
41  */
42 static int
ilogb_biased(unsigned * px)43 ilogb_biased(unsigned *px) {
44 	int s = 52;
45 	unsigned v = px[HIWORD] & ~0x80000000, w = px[LOWORD], t = v;
46 
47 	if (t)
48 		s -= 32;
49 	else
50 		t = w;
51 	if (t & 0xffff0000)
52 		s -= 16, t >>= 16;
53 	if (t & 0xff00)
54 		s -= 8, t >>= 8;
55 	if (t & 0xf0)
56 		s -= 4, t >>= 4;
57 	t <<= 1;
58 	s -= (0xffffaa50 >> t) & 0x3;
59 	if (s < 32) {
60 		v = (v << s) | w >> (32 - s);
61 		w <<= s;
62 	} else {
63 		v = w << (s - 32);
64 		w = 0;
65 	}
66 	px[HIWORD] = (px[HIWORD] & 0x80000000) | v;
67 	px[LOWORD] = w;
68 	return (1 - s);
69 }
70 #endif	/* defined(__x86) */
71 
72 double
scalbln(double x,long n)73 scalbln(double x, long n) {
74 	int *px = (int *) &x, ix, k;
75 
76 	ix = px[HIWORD] & ~0x80000000;
77 	k = ix >> 20;
78 	if (k == 0x7ff)
79 #if defined(FPADD_TRAPS_INCOMPLETE_ON_NAN)
80 		return ((px[HIWORD] & 0x80000) != 0 ? x : x + x);
81 		/* assumes sparc-like QNaN */
82 #else
83 		return (x + x);
84 #endif
85 	if ((px[LOWORD] | ix) == 0 || n == 0)
86 		return (x);
87 	if (k == 0) {
88 #if defined(__x86)
89 		x *= two52;
90 		k = ((px[HIWORD] & ~0x80000000) >> 20) - 52;
91 #else
92 		k = ilogb_biased((unsigned *) px);
93 #endif
94 	}
95 	k += (int) n;
96 	if (n > 5000 || k > 0x7fe)
97 		return (DBL_MAX * copysign(DBL_MAX, x));
98 	if (n < -5000 || k <= -54)
99 		return (DBL_MIN * copysign(DBL_MIN, x));
100 	if (k > 0) {
101 		px[HIWORD] = (px[HIWORD] & ~0x7ff00000) | (k << 20);
102 		return (x);
103 	}
104 	k += 54;
105 	px[HIWORD] = (px[HIWORD] & ~0x7ff00000) | (k << 20);
106 	return (x * twom54);
107 }
108