2 /* @(#)e_exp.c 1.6 04/04/22 */
4 * ====================================================
5 * Copyright (C) 2004 by Sun Microsystems, Inc. All rights reserved.
7 * Permission to use, copy, modify, and distribute this
8 * software is freely granted, provided that this notice
10 * ====================================================
14 * Returns the exponential of x.
17 * 1. Argument reduction:
18 * Reduce x to an r so that |r| <= 0.5*ln2 ~ 0.34658.
19 * Given x, find r and integer k such that
21 * x = k*ln2 + r, |r| <= 0.5*ln2.
23 * Here r will be represented as r = hi-lo for better
26 * 2. Approximation of exp(r) by a special rational function on
27 * the interval [0,0.34658]:
29 * R(r**2) = r*(exp(r)+1)/(exp(r)-1) = 2 + r*r/6 - r**4/360 + ...
30 * We use a special Remes algorithm on [0,0.34658] to generate
31 * a polynomial of degree 5 to approximate R. The maximum error
32 * of this polynomial approximation is bounded by 2**-59. In
34 * R(z) ~ 2.0 + P1*z + P2*z**2 + P3*z**3 + P4*z**4 + P5*z**5
35 * (where z=r*r, and the values of P1 to P5 are listed below)
38 * | 2.0+P1*z+...+P5*z - R(z) | <= 2
40 * The computation of exp(r) thus becomes
42 * exp(r) = 1 + -------
45 * = 1 + r + ----------- (for better accuracy)
49 * R1(r) = r - (P1*r + P2*r + ... + P5*r ).
51 * 3. Scale back to obtain exp(x):
52 * From step 1, we have
53 * exp(x) = 2^k * exp(r)
56 * exp(INF) is INF, exp(NaN) is NaN;
58 * for finite argument, only exp(0)=1 is exact.
61 * according to an error analysis, the error is always less than
62 * 1 ulp (unit in the last place).
66 * if x > 7.09782712893383973096e+02 then exp(x) overflow
67 * if x < -7.45133219101941108420e+02 then exp(x) underflow
70 * The hexadecimal values are the intended ones for the following
71 * constants. The decimal values may be used, provided that the
72 * compiler will convert from decimal to binary accurately enough
73 * to produce the hexadecimal values shown.
77 #include "math_private.h"
81 halF[2] = {0.5,-0.5,},
83 twom1000= 9.33263618503218878990e-302, /* 2**-1000=0x01700000,0*/
84 o_threshold= 7.09782712893383973096e+02, /* 0x40862E42, 0xFEFA39EF */
85 u_threshold= -7.45133219101941108420e+02, /* 0xc0874910, 0xD52D3051 */
86 ln2HI[2] ={ 6.93147180369123816490e-01, /* 0x3fe62e42, 0xfee00000 */
87 -6.93147180369123816490e-01,},/* 0xbfe62e42, 0xfee00000 */
88 ln2LO[2] ={ 1.90821492927058770002e-10, /* 0x3dea39ef, 0x35793c76 */
89 -1.90821492927058770002e-10,},/* 0xbdea39ef, 0x35793c76 */
90 invln2 = 1.44269504088896338700e+00, /* 0x3ff71547, 0x652b82fe */
91 P1 = 1.66666666666666019037e-01, /* 0x3FC55555, 0x5555553E */
92 P2 = -2.77777777770155933842e-03, /* 0xBF66C16C, 0x16BEBD93 */
93 P3 = 6.61375632143793436117e-05, /* 0x3F11566A, 0xAF25DE2C */
94 P4 = -1.65339022054652515390e-06, /* 0xBEBBBD41, 0xC5D26BF1 */
95 P5 = 4.13813679705723846039e-08; /* 0x3E663769, 0x72BEA4D0 */
99 exp(double x) /* default IEEE double exp */
101 double y,hi=0.0,lo=0.0,c,t;
106 xsb = (hx>>31)&1; /* sign bit of x */
107 hx &= 0x7fffffff; /* high word of |x| */
109 /* filter out non-finite argument */
110 if(hx >= 0x40862E42) { /* if |x|>=709.78... */
114 if(((hx&0xfffff)|lx)!=0)
115 return x+x; /* NaN */
116 else return (xsb==0)? x:0.0; /* exp(+-inf)={inf,0} */
118 if(x > o_threshold) return huge*huge; /* overflow */
119 if(x < u_threshold) return twom1000*twom1000; /* underflow */
122 /* argument reduction */
123 if(hx > 0x3fd62e42) { /* if |x| > 0.5 ln2 */
124 if(hx < 0x3FF0A2B2) { /* and |x| < 1.5 ln2 */
125 hi = x-ln2HI[xsb]; lo=ln2LO[xsb]; k = 1-xsb-xsb;
127 k = (int)(invln2*x+halF[xsb]);
129 hi = x - t*ln2HI[0]; /* t*ln2HI is exact here */
134 else if(hx < 0x3e300000) { /* when |x|<2**-28 */
135 if(huge+x>one) return one+x;/* trigger inexact */
139 /* x is now in primary range */
141 c = x - t*(P1+t*(P2+t*(P3+t*(P4+t*P5))));
142 if(k==0) return one-((x*c)/(c-2.0)-x);
143 else y = one-((lo-(x*c)/(2.0-c))-hi);
147 SET_HIGH_WORD(y,hy+(k<<20)); /* add k to y's exponent */
152 SET_HIGH_WORD(y,hy+((k+1000)<<20)); /* add k to y's exponent */