lib/libm/src/e_pow.c

   1 /* @(#)e_pow.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: e_pow.c,v 1.14 2008/04/25 22:21:53 christos Exp $");
  16 #endif
  17
  18 /* __ieee754_pow(x,y) return x**y
  19  *
  20  *                    n
  21  * Method:  Let x =  2   * (1+f)
  22  *      1. Compute and return log2(x) in two pieces:
  23  *              log2(x) = w1 + w2,
  24  *         where w1 has 53-24 = 29 bit trailing zeros.
  25  *      2. Perform y*log2(x) = n+y' by simulating multi-precision
  26  *         arithmetic, where |y'|<=0.5.
  27  *      3. Return x**y = 2**n*exp(y'*log2)
  28  *
  29  * Special cases:
  30  *      1.  (anything) ** 0  is 1
  31  *      2.  (anything) ** 1  is itself
  32  *      3.  (anything) ** NAN is NAN
  33  *      4.  NAN ** (anything except 0) is NAN
  34  *      5.  +-(|x| > 1) **  +INF is +INF
  35  *      6.  +-(|x| > 1) **  -INF is +0
  36  *      7.  +-(|x| < 1) **  +INF is +0
  37  *      8.  +-(|x| < 1) **  -INF is +INF
  38  *      9.  +-1         ** +-INF is NAN
  39  *      10. +0 ** (+anything except 0, NAN)               is +0
  40  *      11. -0 ** (+anything except 0, NAN, odd integer)  is +0
  41  *      12. +0 ** (-anything except 0, NAN)               is +INF
  42  *      13. -0 ** (-anything except 0, NAN, odd integer)  is +INF
  43  *      14. -0 ** (odd integer) = -( +0 ** (odd integer) )
  44  *      15. +INF ** (+anything except 0,NAN) is +INF
  45  *      16. +INF ** (-anything except 0,NAN) is +0
  46  *      17. -INF ** (anything)  = -0 ** (-anything)
  47  *      18. (-anything) ** (integer) is (-1)**(integer)*(+anything**integer)
  48  *      19. (-anything except 0 and inf) ** (non-integer) is NAN
  49  *
  50  * Accuracy:
  51  *      pow(x,y) returns x**y nearly rounded. In particular
  52  *                      pow(integer,integer)
  53  *      always returns the correct integer provided it is
  54  *      representable.
  55  *
  56  * Constants :
  57  * The hexadecimal values are the intended ones for the following
  58  * constants. The decimal values may be used, provided that the
  59  * compiler will convert from decimal to binary accurately enough
  60  * to produce the hexadecimal values shown.
  61  */
  62
  63 #include "math.h"
  64 #include "math_private.h"
  65
  66 static const double
  67 bp[] = {1.0, 1.5,},
  68 dp_h[] = { 0.0, 5.84962487220764160156e-01,}, /* 0x3FE2B803, 0x40000000 */
  69 dp_l[] = { 0.0, 1.35003920212974897128e-08,}, /* 0x3E4CFDEB, 0x43CFD006 */
  70 zero    =  0.0,
  71 one     =  1.0,
  72 two     =  2.0,
  73 two53   =  9007199254740992.0,  /* 0x43400000, 0x00000000 */
  74 huge    =  1.0e300,
  75 tiny    =  1.0e-300,
  76         /* poly coefs for (3/2)*(log(x)-2s-2/3*s**3 */
  77 L1  =  5.99999999999994648725e-01, /* 0x3FE33333, 0x33333303 */
  78 L2  =  4.28571428578550184252e-01, /* 0x3FDB6DB6, 0xDB6FABFF */
  79 L3  =  3.33333329818377432918e-01, /* 0x3FD55555, 0x518F264D */
  80 L4  =  2.72728123808534006489e-01, /* 0x3FD17460, 0xA91D4101 */
  81 L5  =  2.30660745775561754067e-01, /* 0x3FCD864A, 0x93C9DB65 */
  82 L6  =  2.06975017800338417784e-01, /* 0x3FCA7E28, 0x4A454EEF */
  83 P1   =  1.66666666666666019037e-01, /* 0x3FC55555, 0x5555553E */
  84 P2   = -2.77777777770155933842e-03, /* 0xBF66C16C, 0x16BEBD93 */
  85 P3   =  6.61375632143793436117e-05, /* 0x3F11566A, 0xAF25DE2C */
  86 P4   = -1.65339022054652515390e-06, /* 0xBEBBBD41, 0xC5D26BF1 */
  87 P5   =  4.13813679705723846039e-08, /* 0x3E663769, 0x72BEA4D0 */
  88 lg2  =  6.93147180559945286227e-01, /* 0x3FE62E42, 0xFEFA39EF */
  89 lg2_h  =  6.93147182464599609375e-01, /* 0x3FE62E43, 0x00000000 */
  90 lg2_l  = -1.90465429995776804525e-09, /* 0xBE205C61, 0x0CA86C39 */
  91 ovt =  8.0085662595372944372e-0017, /* -(1024-log2(ovfl+.5ulp)) */
  92 cp    =  9.61796693925975554329e-01, /* 0x3FEEC709, 0xDC3A03FD =2/(3ln2) */
  93 cp_h  =  9.61796700954437255859e-01, /* 0x3FEEC709, 0xE0000000 =(float)cp */
  94 cp_l  = -7.02846165095275826516e-09, /* 0xBE3E2FE0, 0x145B01F5 =tail of cp_h*/
  95 ivln2    =  1.44269504088896338700e+00, /* 0x3FF71547, 0x652B82FE =1/ln2 */
  96 ivln2_h  =  1.44269502162933349609e+00, /* 0x3FF71547, 0x60000000 =24b 1/ln2*/
  97 ivln2_l  =  1.92596299112661746887e-08; /* 0x3E54AE0B, 0xF85DDF44 =1/ln2 tail*/
  98
  99 double
 100 __ieee754_pow(double x, double y)
 101 {
 102         double z,ax,z_h,z_l,p_h,p_l;
 103         double yy1,t1,t2,r,s,t,u,v,w;
 104         int32_t i,j,k,yisint,n;
 105         int32_t hx,hy,ix,iy;
 106         u_int32_t lx,ly;
 107
 108         EXTRACT_WORDS(hx,lx,x);
 109         EXTRACT_WORDS(hy,ly,y);
 110         ix = hx&0x7fffffff;  iy = hy&0x7fffffff;
 111
 112     /* y==zero: x**0 = 1 */
 113         if((iy|ly)==0) return one;
 114
 115     /* +-NaN return x+y */
 116         if(ix > 0x7ff00000 || ((ix==0x7ff00000)&&(lx!=0)) ||
 117            iy > 0x7ff00000 || ((iy==0x7ff00000)&&(ly!=0)))
 118                 return x+y;
 119
 120     /* determine if y is an odd int when x < 0
 121      * yisint = 0       ... y is not an integer
 122      * yisint = 1       ... y is an odd int
 123      * yisint = 2       ... y is an even int
 124      */
 125         yisint  = 0;
 126         if(hx<0) {
 127             if(iy>=0x43400000) yisint = 2; /* even integer y */
 128             else if(iy>=0x3ff00000) {
 129                 k = (iy>>20)-0x3ff;        /* exponent */
 130                 if(k>20) {
 131                     j = ly>>(52-k);
 132                     if((uint32_t)(j<<(52-k))==ly) yisint = 2-(j&1);
 133                 } else if(ly==0) {
 134                     j = iy>>(20-k);
 135                     if((j<<(20-k))==iy) yisint = 2-(j&1);
 136                 }
 137             }
 138         }
 139
 140     /* special value of y */
 141         if(ly==0) {
 142             if (iy==0x7ff00000) {       /* y is +-inf */
 143                 if(((ix-0x3ff00000)|lx)==0)
 144                     return  y - y;      /* inf**+-1 is NaN */
 145                 else if (ix >= 0x3ff00000)/* (|x|>1)**+-inf = inf,0 */
 146                     return (hy>=0)? y: zero;
 147                 else                    /* (|x|<1)**-,+inf = inf,0 */
 148                     return (hy<0)?-y: zero;
 149             }
 150             if(iy==0x3ff00000) {        /* y is  +-1 */
 151                 if(hy<0) return one/x; else return x;
 152             }
 153             if(hy==0x40000000) return x*x; /* y is  2 */
 154             if(hy==0x3fe00000) {        /* y is  0.5 */
 155                 if(hx>=0)       /* x >= +0 */
 156                 return __ieee754_sqrt(x);
 157             }
 158         }
 159
 160         ax   = fabs(x);
 161     /* special value of x */
 162         if(lx==0) {
 163             if(ix==0x7ff00000||ix==0||ix==0x3ff00000){
 164                 z = ax;                 /*x is +-0,+-inf,+-1*/
 165                 if(hy<0) z = one/z;     /* z = (1/|x|) */
 166                 if(hx<0) {
 167                     if(((ix-0x3ff00000)|yisint)==0) {
 168                         z = (z-z)/(z-z); /* (-1)**non-int is NaN */
 169                     } else if(yisint==1)
 170                         z = -z;         /* (x<0)**odd = -(|x|**odd) */
 171                 }
 172                 return z;
 173             }
 174         }
 175
 176         n = (hx>>31)+1;
 177
 178     /* (x<0)**(non-int) is NaN */
 179         if((n|yisint)==0) return (x-x)/(x-x);
 180
 181         s = one; /* s (sign of result -ve**odd) = -1 else = 1 */
 182         if((n|(yisint-1))==0) s = -one;/* (-ve)**(odd int) */
 183
 184     /* |y| is huge */
 185         if(iy>0x41e00000) { /* if |y| > 2**31 */
 186             if(iy>0x43f00000){  /* if |y| > 2**64, must o/uflow */
 187                 if(ix<=0x3fefffff) return (hy<0)? huge*huge:tiny*tiny;
 188                 if(ix>=0x3ff00000) return (hy>0)? huge*huge:tiny*tiny;
 189             }
 190         /* over/underflow if x is not close to one */
 191             if(ix<0x3fefffff) return (hy<0)? s*huge*huge:s*tiny*tiny;
 192             if(ix>0x3ff00000) return (hy>0)? s*huge*huge:s*tiny*tiny;
 193         /* now |1-x| is tiny <= 2**-20, suffice to compute
 194            log(x) by x-x^2/2+x^3/3-x^4/4 */
 195             t = ax-one;         /* t has 20 trailing zeros */
 196             w = (t*t)*(0.5-t*(0.3333333333333333333333-t*0.25));
 197             u = ivln2_h*t;      /* ivln2_h has 21 sig. bits */
 198             v = t*ivln2_l-w*ivln2;
 199             t1 = u+v;
 200             SET_LOW_WORD(t1,0);
 201             t2 = v-(t1-u);
 202         } else {
 203             double ss,s2,s_h,s_l,t_h,t_l;
 204             n = 0;
 205         /* take care subnormal number */
 206             if(ix<0x00100000)
 207                 {ax *= two53; n -= 53; GET_HIGH_WORD(ix,ax); }
 208             n  += ((ix)>>20)-0x3ff;
 209             j  = ix&0x000fffff;
 210         /* determine interval */
 211             ix = j|0x3ff00000;          /* normalize ix */
 212             if(j<=0x3988E) k=0;         /* |x|<sqrt(3/2) */
 213             else if(j<0xBB67A) k=1;     /* |x|<sqrt(3)   */
 214             else {k=0;n+=1;ix -= 0x00100000;}
 215             SET_HIGH_WORD(ax,ix);
 216
 217         /* compute ss = s_h+s_l = (x-1)/(x+1) or (x-1.5)/(x+1.5) */
 218             u = ax-bp[k];               /* bp[0]=1.0, bp[1]=1.5 */
 219             v = one/(ax+bp[k]);
 220             ss = u*v;
 221             s_h = ss;
 222             SET_LOW_WORD(s_h,0);
 223         /* t_h=ax+bp[k] High */
 224             t_h = zero;
 225             SET_HIGH_WORD(t_h,((ix>>1)|0x20000000)+0x00080000+(k<<18));
 226             t_l = ax - (t_h-bp[k]);
 227             s_l = v*((u-s_h*t_h)-s_h*t_l);
 228         /* compute log(ax) */
 229             s2 = ss*ss;
 230             r = s2*s2*(L1+s2*(L2+s2*(L3+s2*(L4+s2*(L5+s2*L6)))));
 231             r += s_l*(s_h+ss);
 232             s2  = s_h*s_h;
 233             t_h = 3.0+s2+r;
 234             SET_LOW_WORD(t_h,0);
 235             t_l = r-((t_h-3.0)-s2);
 236         /* u+v = ss*(1+...) */
 237             u = s_h*t_h;
 238             v = s_l*t_h+t_l*ss;
 239         /* 2/(3log2)*(ss+...) */
 240             p_h = u+v;
 241             SET_LOW_WORD(p_h,0);
 242             p_l = v-(p_h-u);
 243             z_h = cp_h*p_h;             /* cp_h+cp_l = 2/(3*log2) */
 244             z_l = cp_l*p_h+p_l*cp+dp_l[k];
 245         /* log2(ax) = (ss+..)*2/(3*log2) = n + dp_h + z_h + z_l */
 246             t = (double)n;
 247             t1 = (((z_h+z_l)+dp_h[k])+t);
 248             SET_LOW_WORD(t1,0);
 249             t2 = z_l-(((t1-t)-dp_h[k])-z_h);
 250         }
 251
 252     /* split up y into yy1+y2 and compute (yy1+y2)*(t1+t2) */
 253         yy1  = y;
 254         SET_LOW_WORD(yy1,0);
 255         p_l = (y-yy1)*t1+y*t2;
 256         p_h = yy1*t1;
 257         z = p_l+p_h;
 258         EXTRACT_WORDS(j,i,z);
 259         if (j>=0x40900000) {                            /* z >= 1024 */
 260             if(((j-0x40900000)|i)!=0)                   /* if z > 1024 */
 261                 return s*huge*huge;                     /* overflow */
 262             else {
 263                 if(p_l+ovt>z-p_h) return s*huge*huge;   /* overflow */
 264             }
 265         } else if((j&0x7fffffff)>=0x4090cc00 ) {        /* z <= -1075 */
 266             if(((j-0xc090cc00)|i)!=0)           /* z < -1075 */
 267                 return s*tiny*tiny;             /* underflow */
 268             else {
 269                 if(p_l<=z-p_h) return s*tiny*tiny;      /* underflow */
 270             }
 271         }
 272     /*
 273      * compute 2**(p_h+p_l)
 274      */
 275         i = j&0x7fffffff;
 276         k = (i>>20)-0x3ff;
 277         n = 0;
 278         if(i>0x3fe00000) {              /* if |z| > 0.5, set n = [z+0.5] */
 279             n = j+(0x00100000>>(k+1));
 280             k = ((n&0x7fffffff)>>20)-0x3ff;     /* new k for n */
 281             t = zero;
 282             SET_HIGH_WORD(t,n&~(0x000fffff>>k));
 283             n = ((n&0x000fffff)|0x00100000)>>(20-k);
 284             if(j<0) n = -n;
 285             p_h -= t;
 286         }
 287         t = p_l+p_h;
 288         SET_LOW_WORD(t,0);
 289         u = t*lg2_h;
 290         v = (p_l-(t-p_h))*lg2+t*lg2_l;
 291         z = u+v;
 292         w = v-(z-u);
 293         t  = z*z;
 294         t1  = z - t*(P1+t*(P2+t*(P3+t*(P4+t*P5))));
 295         r  = (z*t1)/(t1-two)-(w+z*w);
 296         z  = one-(r-z);
 297         GET_HIGH_WORD(j,z);
 298         j += (n<<20);
 299         if((j>>20)<=0) z = scalbn(z,n); /* subnormal output */
 300         else SET_HIGH_WORD(z,j);
 301         return s*z;
 302 }