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.
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.
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]
22 * Copyright 2011 Nexenta Systems, Inc. All rights reserved.
25 * Copyright 2005 Sun Microsystems, Inc. All rights reserved.
26 * Use is subject to license terms.
29 #pragma weak __log1p = log1p
34 * 1. Argument Reduction: find k and f such that
36 * where sqrt(2)/2 < 1+f < sqrt(2) .
38 * Note. If k=0, then f=x is exact. However, if k != 0, then f
39 * may not be representable exactly. In that case, a correction
40 * term is need. Let u=1+x rounded. Let c = (1+x)-u, then
41 * log(1+x) - log(u) ~ c/u. Thus, we proceed to compute log(u),
42 * and add back the correction term c/u.
43 * (Note: when x > 2**53, one can simply return log(x))
45 * 2. Approximation of log1p(f).
46 * Let s = f/(2+f) ; based on log(1+f) = log(1+s) - log(1-s)
47 * = 2s + 2/3 s**3 + 2/5 s**5 + .....,
49 * We use a special Reme algorithm on [0,0.1716] to generate
50 * a polynomial of degree 14 to approximate R The maximum error
51 * of this polynomial approximation is bounded by 2**-58.45. In
54 * R(z) ~ Lp1*s +Lp2*s +Lp3*s +Lp4*s +Lp5*s +Lp6*s +Lp7*s
55 * (the values of Lp1 to Lp7 are listed in the program)
58 * | Lp1*s +...+Lp7*s - R(z) | <= 2
60 * Note that 2s = f - s*f = f - hfsq + s*hfsq, where hfsq = f*f/2.
61 * In order to guarantee error in log below 1ulp, we compute log
63 * log1p(f) = f - (hfsq - s*(hfsq+R)).
65 * 3. Finally, log1p(x) = k*ln2 + log1p(f).
66 * = k*ln2_hi+(f-(hfsq-(s*(hfsq+R)+k*ln2_lo)))
67 * Here ln2 is splitted into two floating point number:
69 * where n*ln2_hi is always exact for |n| < 2000.
72 * log1p(x) is NaN with signal if x < -1 (including -INF) ;
73 * log1p(+INF) is +INF; log1p(-1) is -INF with signal;
74 * log1p(NaN) is that NaN with no signal.
77 * according to an error analysis, the error is always less than
78 * 1 ulp (unit in the last place).
81 * The hexadecimal values are the intended ones for the following
82 * constants. The decimal values may be used, provided that the
83 * compiler will convert from decimal to binary accurately enough
84 * to produce the hexadecimal values shown.
86 * Note: Assuming log() return accurate answer, the following
87 * algorithm can be used to compute log1p(x) to within a few ULP:
90 * if (u == 1.0) return x ; else
91 * return log(u)*(x/(u-1.0));
93 * See HP-15C Advanced Functions Handbook, p.193.
99 static const double xxx
[] = {
100 /* ln2_hi */ 6.93147180369123816490e-01, /* 3fe62e42 fee00000 */
101 /* ln2_lo */ 1.90821492927058770002e-10, /* 3dea39ef 35793c76 */
102 /* two54 */ 1.80143985094819840000e+16, /* 43500000 00000000 */
103 /* Lp1 */ 6.666666666666735130e-01, /* 3FE55555 55555593 */
104 /* Lp2 */ 3.999999999940941908e-01, /* 3FD99999 9997FA04 */
105 /* Lp3 */ 2.857142874366239149e-01, /* 3FD24924 94229359 */
106 /* Lp4 */ 2.222219843214978396e-01, /* 3FCC71C5 1D8E78AF */
107 /* Lp5 */ 1.818357216161805012e-01, /* 3FC74664 96CB03DE */
108 /* Lp6 */ 1.531383769920937332e-01, /* 3FC39A09 D078C69F */
109 /* Lp7 */ 1.479819860511658591e-01, /* 3FC2F112 DF3E5244 */
112 #define ln2_hi xxx[0]
113 #define ln2_lo xxx[1]
126 double hfsq
, f
, c
= 0.0, s
, z
, R
, u
;
129 hx
= ((int *)&x
)[HIWORD
]; /* high word of x */
130 ax
= hx
& 0x7fffffff;
132 if (ax
>= 0x7ff00000) { /* x is inf or nan */
133 if (((hx
- 0xfff00000) | ((int *)&x
)[LOWORD
]) == 0) /* -inf */
134 return (_SVID_libm_err(x
, x
, 44));
139 if (hx
< 0x3FDA827A) { /* x < 0.41422 */
140 if (ax
>= 0x3ff00000) /* x <= -1.0 */
141 return (_SVID_libm_err(x
, x
, x
== -1.0 ? 43 : 44));
142 if (ax
< 0x3e200000) { /* |x| < 2**-29 */
143 if (two54
+ x
> zero
&& /* raise inexact */
144 ax
< 0x3c900000) /* |x| < 2**-54 */
147 return (x
- x
* x
* 0.5);
149 if (hx
> 0 || hx
<= (int)0xbfd2bec3) { /* -0.2929<x<0.41422 */
155 /* We will initialize 'c' here. */
157 if (hx
< 0x43400000) {
159 hu
= ((int *)&u
)[HIWORD
]; /* high word of u */
160 k
= (hu
>> 20) - 1023;
164 c
= k
> 0 ? 1.0 - (u
- x
) : x
- (u
- 1.0);
168 hu
= ((int *)&u
)[HIWORD
]; /* high word of u */
169 k
= (hu
>> 20) - 1023;
173 if (hu
< 0x6a09e) { /* normalize u */
174 ((int *)&u
)[HIWORD
] = hu
| 0x3ff00000;
175 } else { /* normalize u/2 */
177 ((int *)&u
)[HIWORD
] = hu
| 0x3fe00000;
178 hu
= (0x00100000 - hu
) >> 2;
183 if (hu
== 0) { /* |f| < 2**-20 */
187 /* We already initialized 'c' before, when (k != 0) */
189 return (k
* ln2_hi
+ c
);
191 R
= hfsq
* (1.0 - 0.66666666666666666 * f
);
194 return (k
* ln2_hi
- ((R
- (k
* ln2_lo
+ c
)) - f
));
198 R
= z
* (Lp1
+ z
* (Lp2
+ z
* (Lp3
+ z
* (Lp4
+ z
* (Lp5
+
199 z
* (Lp6
+ z
* Lp7
))))));
201 return (f
- (hfsq
- s
* (hfsq
+ R
)));
202 return (k
* ln2_hi
- ((hfsq
- (s
* (hfsq
+ R
) +
203 (k
* ln2_lo
+ c
))) - f
));