1 /* @(#)s_expm1.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
13 #include <sys/cdefs.h>
14 #if defined(LIBM_SCCS) && !defined(lint)
15 __RCSID("$NetBSD: s_expm1.c,v 1.13 2017/02/09 22:11:09 maya Exp $");
16 #endif
17
18 /* expm1(x)
19 * Returns exp(x)-1, the exponential of x minus 1.
20 *
21 * Method
22 * 1. Argument reduction:
23 * Given x, find r and integer k such that
24 *
25 * x = k*ln2 + r, |r| <= 0.5*ln2 ~ 0.34658
26 *
27 * Here a correction term c will be computed to compensate
28 * the error in r when rounded to a floating-point number.
29 *
30 * 2. Approximating expm1(r) by a special rational function on
31 * the interval [0,0.34658]:
32 * Since
33 * r*(exp(r)+1)/(exp(r)-1) = 2+ r^2/6 - r^4/360 + ...
34 * we define R1(r*r) by
35 * r*(exp(r)+1)/(exp(r)-1) = 2+ r^2/6 * R1(r*r)
36 * That is,
37 * R1(r**2) = 6/r *((exp(r)+1)/(exp(r)-1) - 2/r)
38 * = 6/r * ( 1 + 2.0*(1/(exp(r)-1) - 1/r))
39 * = 1 - r^2/60 + r^4/2520 - r^6/100800 + ...
40 * We use a special Reme algorithm on [0,0.347] to generate
41 * a polynomial of degree 5 in r*r to approximate R1. The
42 * maximum error of this polynomial approximation is bounded
43 * by 2**-61. In other words,
44 * R1(z) ~ 1.0 + Q1*z + Q2*z**2 + Q3*z**3 + Q4*z**4 + Q5*z**5
45 * where Q1 = -1.6666666666666567384E-2,
46 * Q2 = 3.9682539681370365873E-4,
47 * Q3 = -9.9206344733435987357E-6,
48 * Q4 = 2.5051361420808517002E-7,
49 * Q5 = -6.2843505682382617102E-9;
50 * (where z=r*r, and the values of Q1 to Q5 are listed below)
51 * with error bounded by
52 * | 5 | -61
53 * | 1.0+Q1*z+...+Q5*z - R1(z) | <= 2
54 * | |
55 *
56 * expm1(r) = exp(r)-1 is then computed by the following
57 * specific way which minimize the accumulation rounding error:
58 * 2 3
59 * r r [ 3 - (R1 + R1*r/2) ]
60 * expm1(r) = r + --- + --- * [--------------------]
61 * 2 2 [ 6 - r*(3 - R1*r/2) ]
62 *
63 * To compensate the error in the argument reduction, we use
64 * expm1(r+c) = expm1(r) + c + expm1(r)*c
65 * ~ expm1(r) + c + r*c
66 * Thus c+r*c will be added in as the correction terms for
67 * expm1(r+c). Now rearrange the term to avoid optimization
68 * screw up:
69 * ( 2 2 )
70 * ({ ( r [ R1 - (3 - R1*r/2) ] ) } r )
71 * expm1(r+c)~r - ({r*(--- * [--------------------]-c)-c} - --- )
72 * ({ ( 2 [ 6 - r*(3 - R1*r/2) ] ) } 2 )
73 * ( )
74 *
75 * = r - E
76 * 3. Scale back to obtain expm1(x):
77 * From step 1, we have
78 * expm1(x) = either 2^k*[expm1(r)+1] - 1
79 * = or 2^k*[expm1(r) + (1-2^-k)]
80 * 4. Implementation notes:
81 * (A). To save one multiplication, we scale the coefficient Qi
82 * to Qi*2^i, and replace z by (x^2)/2.
83 * (B). To achieve maximum accuracy, we compute expm1(x) by
84 * (i) if x < -56*ln2, return -1.0, (raise inexact if x!=inf)
85 * (ii) if k=0, return r-E
86 * (iii) if k=-1, return 0.5*(r-E)-0.5
87 * (iv) if k=1 if r < -0.25, return 2*((r+0.5)- E)
88 * else return 1.0+2.0*(r-E);
89 * (v) if (k<-2||k>56) return 2^k(1-(E-r)) - 1 (or exp(x)-1)
90 * (vi) if k <= 20, return 2^k((1-2^-k)-(E-r)), else
91 * (vii) return 2^k(1-((E+2^-k)-r))
92 *
93 * Special cases:
94 * expm1(INF) is INF, expm1(NaN) is NaN;
95 * expm1(-INF) is -1, and
96 * for finite argument, only expm1(0)=0 is exact.
97 *
98 * Accuracy:
99 * according to an error analysis, the error is always less than
100 * 1 ulp (unit in the last place).
101 *
102 * Misc. info.
103 * For IEEE double
104 * if x > 7.09782712893383973096e+02 then expm1(x) overflow
105 *
106 * Constants:
107 * The hexadecimal values are the intended ones for the following
108 * constants. The decimal values may be used, provided that the
109 * compiler will convert from decimal to binary accurately enough
110 * to produce the hexadecimal values shown.
111 */
112
113 #include "math.h"
114 #include "math_private.h"
115
116 static const double
117 one = 1.0,
118 huge = 1.0e+300,
119 tiny = 1.0e-300,
120 o_threshold = 7.09782712893383973096e+02,/* 0x40862E42, 0xFEFA39EF */
121 ln2_hi = 6.93147180369123816490e-01,/* 0x3fe62e42, 0xfee00000 */
122 ln2_lo = 1.90821492927058770002e-10,/* 0x3dea39ef, 0x35793c76 */
123 invln2 = 1.44269504088896338700e+00,/* 0x3ff71547, 0x652b82fe */
124 /* scaled coefficients related to expm1 */
125 Q1 = -3.33333333333331316428e-02, /* BFA11111 111110F4 */
126 Q2 = 1.58730158725481460165e-03, /* 3F5A01A0 19FE5585 */
127 Q3 = -7.93650757867487942473e-05, /* BF14CE19 9EAADBB7 */
128 Q4 = 4.00821782732936239552e-06, /* 3ED0CFCA 86E65239 */
129 Q5 = -2.01099218183624371326e-07; /* BE8AFDB7 6E09C32D */
130
131 double
expm1(double x)132 expm1(double x)
133 {
134 double y,hi,lo,c,t,e,hxs,hfx,r1;
135 int32_t k,xsb;
136 u_int32_t hx;
137
138 c = 0;
139 GET_HIGH_WORD(hx,x);
140 xsb = hx&0x80000000; /* sign bit of x */
141 #ifdef DEAD_CODE
142 if(xsb==0) y=x; else y= -x; /* y = |x| */
143 #endif
144 hx &= 0x7fffffff; /* high word of |x| */
145
146 /* filter out huge and non-finite argument */
147 if(hx >= 0x4043687A) { /* if |x|>=56*ln2 */
148 if(hx >= 0x40862E42) { /* if |x|>=709.78... */
149 if(hx>=0x7ff00000) {
150 u_int32_t low;
151 GET_LOW_WORD(low,x);
152 if(((hx&0xfffff)|low)!=0)
153 return x+x; /* NaN */
154 else return (xsb==0)? x:-1.0;/* exp(+-inf)={inf,-1} */
155 }
156 if(x > o_threshold) return huge*huge; /* overflow */
157 }
158 if(xsb!=0) { /* x < -56*ln2, return -1.0 with inexact */
159 if(x+tiny<0.0) /* raise inexact */
160 return tiny-one; /* return -1 */
161 }
162 }
163
164 /* argument reduction */
165 if(hx > 0x3fd62e42) { /* if |x| > 0.5 ln2 */
166 if(hx < 0x3FF0A2B2) { /* and |x| < 1.5 ln2 */
167 if(xsb==0)
168 {hi = x - ln2_hi; lo = ln2_lo; k = 1;}
169 else
170 {hi = x + ln2_hi; lo = -ln2_lo; k = -1;}
171 } else {
172 k = invln2*x+((xsb==0)?0.5:-0.5);
173 t = k;
174 hi = x - t*ln2_hi; /* t*ln2_hi is exact here */
175 lo = t*ln2_lo;
176 }
177 x = hi - lo;
178 c = (hi-x)-lo;
179 }
180 else if(hx < 0x3c900000) { /* when |x|<2**-54, return x */
181 t = huge+x; /* return x with inexact flags when x!=0 */
182 return x - (t-(huge+x));
183 }
184 else k = 0;
185
186 /* x is now in primary range */
187 hfx = 0.5*x;
188 hxs = x*hfx;
189 r1 = one+hxs*(Q1+hxs*(Q2+hxs*(Q3+hxs*(Q4+hxs*Q5))));
190 t = 3.0-r1*hfx;
191 e = hxs*((r1-t)/(6.0 - x*t));
192 if(k==0) return x - (x*e-hxs); /* c is 0 */
193 else {
194 e = (x*(e-c)-c);
195 e -= hxs;
196 if(k== -1) return 0.5*(x-e)-0.5;
197 if(k==1) {
198 if(x < -0.25) return -2.0*(e-(x+0.5));
199 else return one+2.0*(x-e);
200 }
201 if (k <= -2 || k>56) { /* suffice to return exp(x)-1 */
202 u_int32_t high;
203 y = one-(e-x);
204 GET_HIGH_WORD(high,y);
205 SET_HIGH_WORD(y,high+(k<<20)); /* add k to y's exponent */
206 return y-one;
207 }
208 t = one;
209 if(k<20) {
210 u_int32_t high;
211 SET_HIGH_WORD(t,0x3ff00000 - (0x200000>>k)); /* t=1-2^-k */
212 y = t-(e-x);
213 GET_HIGH_WORD(high,y);
214 SET_HIGH_WORD(y,high+(k<<20)); /* add k to y's exponent */
215 } else {
216 u_int32_t high;
217 SET_HIGH_WORD(t,((0x3ff-k)<<20)); /* 2^-k */
218 y = x-(e+t);
219 y += one;
220 GET_HIGH_WORD(high,y);
221 SET_HIGH_WORD(y,high+(k<<20)); /* add k to y's exponent */
222 }
223 }
224 return y;
225 }
226