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 1.12 2002/05/26 22:01:55 wiz 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 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 if(xsb==0) y=x; else y= -x; /* y = |x| */ 142 hx &= 0x7fffffff; /* high word of |x| */ 143 144 /* filter out huge and non-finite argument */ 145 if(hx >= 0x4043687A) { /* if |x|>=56*ln2 */ 146 if(hx >= 0x40862E42) { /* if |x|>=709.78... */ 147 if(hx>=0x7ff00000) { 148 u_int32_t low; 149 GET_LOW_WORD(low,x); 150 if(((hx&0xfffff)|low)!=0) 151 return x+x; /* NaN */ 152 else return (xsb==0)? x:-1.0;/* exp(+-inf)={inf,-1} */ 153 } 154 if(x > o_threshold) return huge*huge; /* overflow */ 155 } 156 if(xsb!=0) { /* x < -56*ln2, return -1.0 with inexact */ 157 if(x+tiny<0.0) /* raise inexact */ 158 return tiny-one; /* return -1 */ 159 } 160 } 161 162 /* argument reduction */ 163 if(hx > 0x3fd62e42) { /* if |x| > 0.5 ln2 */ 164 if(hx < 0x3FF0A2B2) { /* and |x| < 1.5 ln2 */ 165 if(xsb==0) 166 {hi = x - ln2_hi; lo = ln2_lo; k = 1;} 167 else 168 {hi = x + ln2_hi; lo = -ln2_lo; k = -1;} 169 } else { 170 k = invln2*x+((xsb==0)?0.5:-0.5); 171 t = k; 172 hi = x - t*ln2_hi; /* t*ln2_hi is exact here */ 173 lo = t*ln2_lo; 174 } 175 x = hi - lo; 176 c = (hi-x)-lo; 177 } 178 else if(hx < 0x3c900000) { /* when |x|<2**-54, return x */ 179 t = huge+x; /* return x with inexact flags when x!=0 */ 180 return x - (t-(huge+x)); 181 } 182 else k = 0; 183 184 /* x is now in primary range */ 185 hfx = 0.5*x; 186 hxs = x*hfx; 187 r1 = one+hxs*(Q1+hxs*(Q2+hxs*(Q3+hxs*(Q4+hxs*Q5)))); 188 t = 3.0-r1*hfx; 189 e = hxs*((r1-t)/(6.0 - x*t)); 190 if(k==0) return x - (x*e-hxs); /* c is 0 */ 191 else { 192 e = (x*(e-c)-c); 193 e -= hxs; 194 if(k== -1) return 0.5*(x-e)-0.5; 195 if(k==1) { 196 if(x < -0.25) return -2.0*(e-(x+0.5)); 197 else return one+2.0*(x-e); 198 } 199 if (k <= -2 || k>56) { /* suffice to return exp(x)-1 */ 200 u_int32_t high; 201 y = one-(e-x); 202 GET_HIGH_WORD(high,y); 203 SET_HIGH_WORD(y,high+(k<<20)); /* add k to y's exponent */ 204 return y-one; 205 } 206 t = one; 207 if(k<20) { 208 u_int32_t high; 209 SET_HIGH_WORD(t,0x3ff00000 - (0x200000>>k)); /* t=1-2^-k */ 210 y = t-(e-x); 211 GET_HIGH_WORD(high,y); 212 SET_HIGH_WORD(y,high+(k<<20)); /* add k to y's exponent */ 213 } else { 214 u_int32_t high; 215 SET_HIGH_WORD(t,((0x3ff-k)<<20)); /* 2^-k */ 216 y = x-(e+t); 217 y += one; 218 GET_HIGH_WORD(high,y); 219 SET_HIGH_WORD(y,high+(k<<20)); /* add k to y's exponent */ 220 } 221 } 222 return y; 223 } 224