1/* @(#)s_log1p.c 5.1 93/09/24 */
2/*
3 * ====================================================
4 * Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved.
5 *
6 * Developed at SunPro, a Sun Microsystems, Inc. business.
7 * Permission to use, copy, modify, and distribute this
8 * software is freely granted, provided that this notice
9 * is preserved.
10 * ====================================================
11 */
12/* Modified by Naohiko Shimizu/Tokai University, Japan 1997/08/25,
13 for performance improvement on pipelined processors.
14 */
15
16/* double log1p(double x)
17 *
18 * Method :
19 * 1. Argument Reduction: find k and f such that
20 * 1+x = 2^k * (1+f),
21 * where sqrt(2)/2 < 1+f < sqrt(2) .
22 *
23 * Note. If k=0, then f=x is exact. However, if k!=0, then f
24 * may not be representable exactly. In that case, a correction
25 * term is need. Let u=1+x rounded. Let c = (1+x)-u, then
26 * log(1+x) - log(u) ~ c/u. Thus, we proceed to compute log(u),
27 * and add back the correction term c/u.
28 * (Note: when x > 2**53, one can simply return log(x))
29 *
30 * 2. Approximation of log1p(f).
31 * Let s = f/(2+f) ; based on log(1+f) = log(1+s) - log(1-s)
32 * = 2s + 2/3 s**3 + 2/5 s**5 + .....,
33 * = 2s + s*R
34 * We use a special Reme algorithm on [0,0.1716] to generate
35 * a polynomial of degree 14 to approximate R The maximum error
36 * of this polynomial approximation is bounded by 2**-58.45. In
37 * other words,
38 * 2 4 6 8 10 12 14
39 * R(z) ~ Lp1*s +Lp2*s +Lp3*s +Lp4*s +Lp5*s +Lp6*s +Lp7*s
40 * (the values of Lp1 to Lp7 are listed in the program)
41 * and
42 * | 2 14 | -58.45
43 * | Lp1*s +...+Lp7*s - R(z) | <= 2
44 * | |
45 * Note that 2s = f - s*f = f - hfsq + s*hfsq, where hfsq = f*f/2.
46 * In order to guarantee error in log below 1ulp, we compute log
47 * by
48 * log1p(f) = f - (hfsq - s*(hfsq+R)).
49 *
50 * 3. Finally, log1p(x) = k*ln2 + log1p(f).
51 * = k*ln2_hi+(f-(hfsq-(s*(hfsq+R)+k*ln2_lo)))
52 * Here ln2 is split into two floating point number:
53 * ln2_hi + ln2_lo,
54 * where n*ln2_hi is always exact for |n| < 2000.
55 *
56 * Special cases:
57 * log1p(x) is NaN with signal if x < -1 (including -INF) ;
58 * log1p(+INF) is +INF; log1p(-1) is -INF with signal;
59 * log1p(NaN) is that NaN with no signal.
60 *
61 * Accuracy:
62 * according to an error analysis, the error is always less than
63 * 1 ulp (unit in the last place).
64 *
65 * Constants:
66 * The hexadecimal values are the intended ones for the following
67 * constants. The decimal values may be used, provided that the
68 * compiler will convert from decimal to binary accurately enough
69 * to produce the hexadecimal values shown.
70 *
71 * Note: Assuming log() return accurate answer, the following
72 * algorithm can be used to compute log1p(x) to within a few ULP:
73 *
74 * u = 1+x;
75 * if(u==1.0) return x ; else
76 * return log(u)*(x/(u-1.0));
77 *
78 * See HP-15C Advanced Functions Handbook, p.193.
79 */
80
81#include <float.h>
82#include <math.h>
83#include <math_private.h>
84
85static const double
86 ln2_hi = 6.93147180369123816490e-01, /* 3fe62e42 fee00000 */
87 ln2_lo = 1.90821492927058770002e-10, /* 3dea39ef 35793c76 */
88 two54 = 1.80143985094819840000e+16, /* 43500000 00000000 */
89 Lp[] = { 0.0, 6.666666666666735130e-01, /* 3FE55555 55555593 */
90 3.999999999940941908e-01, /* 3FD99999 9997FA04 */
91 2.857142874366239149e-01, /* 3FD24924 94229359 */
92 2.222219843214978396e-01, /* 3FCC71C5 1D8E78AF */
93 1.818357216161805012e-01, /* 3FC74664 96CB03DE */
94 1.531383769920937332e-01, /* 3FC39A09 D078C69F */
95 1.479819860511658591e-01 }; /* 3FC2F112 DF3E5244 */
96
97static const double zero = 0.0;
98
99double
100__log1p (double x)
101{
102 double hfsq, f, c, s, z, R, u, z2, z4, z6, R1, R2, R3, R4;
103 int32_t k, hx, hu, ax;
104
105 GET_HIGH_WORD (hx, x);
106 ax = hx & 0x7fffffff;
107
108 k = 1;
109 if (hx < 0x3FDA827A) /* x < 0.41422 */
110 {
111 if (__glibc_unlikely (ax >= 0x3ff00000)) /* x <= -1.0 */
112 {
113 if (x == -1.0)
114 return -two54 / zero; /* log1p(-1)=-inf */
115 else
116 return (x - x) / (x - x); /* log1p(x<-1)=NaN */
117 }
118 if (__glibc_unlikely (ax < 0x3e200000)) /* |x| < 2**-29 */
119 {
120 math_force_eval (two54 + x); /* raise inexact */
121 if (ax < 0x3c900000) /* |x| < 2**-54 */
122 {
123 math_check_force_underflow (x);
124 return x;
125 }
126 else
127 return x - x * x * 0.5;
128 }
129 if (hx > 0 || hx <= ((int32_t) 0xbfd2bec3))
130 {
131 k = 0; f = x; hu = 1;
132 } /* -0.2929<x<0.41422 */
133 }
134 else if (__glibc_unlikely (hx >= 0x7ff00000))
135 return x + x;
136 if (k != 0)
137 {
138 if (hx < 0x43400000)
139 {
140 u = 1.0 + x;
141 GET_HIGH_WORD (hu, u);
142 k = (hu >> 20) - 1023;
143 c = (k > 0) ? 1.0 - (u - x) : x - (u - 1.0); /* correction term */
144 c /= u;
145 }
146 else
147 {
148 u = x;
149 GET_HIGH_WORD (hu, u);
150 k = (hu >> 20) - 1023;
151 c = 0;
152 }
153 hu &= 0x000fffff;
154 if (hu < 0x6a09e)
155 {
156 SET_HIGH_WORD (u, hu | 0x3ff00000); /* normalize u */
157 }
158 else
159 {
160 k += 1;
161 SET_HIGH_WORD (u, hu | 0x3fe00000); /* normalize u/2 */
162 hu = (0x00100000 - hu) >> 2;
163 }
164 f = u - 1.0;
165 }
166 hfsq = 0.5 * f * f;
167 if (hu == 0) /* |f| < 2**-20 */
168 {
169 if (f == zero)
170 {
171 if (k == 0)
172 return zero;
173 else
174 {
175 c += k * ln2_lo; return k * ln2_hi + c;
176 }
177 }
178 R = hfsq * (1.0 - 0.66666666666666666 * f);
179 if (k == 0)
180 return f - R;
181 else
182 return k * ln2_hi - ((R - (k * ln2_lo + c)) - f);
183 }
184 s = f / (2.0 + f);
185 z = s * s;
186 R1 = z * Lp[1]; z2 = z * z;
187 R2 = Lp[2] + z * Lp[3]; z4 = z2 * z2;
188 R3 = Lp[4] + z * Lp[5]; z6 = z4 * z2;
189 R4 = Lp[6] + z * Lp[7];
190 R = R1 + z2 * R2 + z4 * R3 + z6 * R4;
191 if (k == 0)
192 return f - (hfsq - s * (hfsq + R));
193 else
194 return k * ln2_hi - ((hfsq - (s * (hfsq + R) + (k * ln2_lo + c))) - f);
195}
196