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      1 /* @(#)e_pow.c 1.5 04/04/22 SMI */
      2 /*
      3  * ====================================================
      4  * Copyright (C) 2004 by Sun Microsystems, Inc. All rights reserved.
      5  *
      6  * Permission to use, copy, modify, and distribute this
      7  * software is freely granted, provided that this notice
      8  * is preserved.
      9  * ====================================================
     10  */
     11 
     12 #ifndef lint
     13 static char rcsid[] = "$FreeBSD: src/lib/msun/src/e_pow.c,v 1.11 2005/02/04 18:26:06 das Exp $";
     14 #endif
     15 
     16 /* __ieee754_pow(x,y) return x**y
     17  *
     18  *		      n
     19  * Method:  Let x =  2   * (1+f)
     20  *	1. Compute and return log2(x) in two pieces:
     21  *		log2(x) = w1 + w2,
     22  *	   where w1 has 53-24 = 29 bit trailing zeros.
     23  *	2. Perform y*log2(x) = n+y' by simulating muti-precision
     24  *	   arithmetic, where |y'|<=0.5.
     25  *	3. Return x**y = 2**n*exp(y'*log2)
     26  *
     27  * Special cases:
     28  *	1.  (anything) ** 0  is 1
     29  *	2.  (anything) ** 1  is itself
     30  *	3.  (anything) ** NAN is NAN
     31  *	4.  NAN ** (anything except 0) is NAN
     32  *	5.  +-(|x| > 1) **  +INF is +INF
     33  *	6.  +-(|x| > 1) **  -INF is +0
     34  *	7.  +-(|x| < 1) **  +INF is +0
     35  *	8.  +-(|x| < 1) **  -INF is +INF
     36  *	9.  +-1         ** +-INF is NAN
     37  *	10. +0 ** (+anything except 0, NAN)               is +0
     38  *	11. -0 ** (+anything except 0, NAN, odd integer)  is +0
     39  *	12. +0 ** (-anything except 0, NAN)               is +INF
     40  *	13. -0 ** (-anything except 0, NAN, odd integer)  is +INF
     41  *	14. -0 ** (odd integer) = -( +0 ** (odd integer) )
     42  *	15. +INF ** (+anything except 0,NAN) is +INF
     43  *	16. +INF ** (-anything except 0,NAN) is +0
     44  *	17. -INF ** (anything)  = -0 ** (-anything)
     45  *	18. (-anything) ** (integer) is (-1)**(integer)*(+anything**integer)
     46  *	19. (-anything except 0 and inf) ** (non-integer) is NAN
     47  *
     48  * Accuracy:
     49  *	pow(x,y) returns x**y nearly rounded. In particular
     50  *			pow(integer,integer)
     51  *	always returns the correct integer provided it is
     52  *	representable.
     53  *
     54  * Constants :
     55  * The hexadecimal values are the intended ones for the following
     56  * constants. The decimal values may be used, provided that the
     57  * compiler will convert from decimal to binary accurately enough
     58  * to produce the hexadecimal values shown.
     59  */
     60 
     61 #include "math.h"
     62 #include "math_private.h"
     63 
     64 static const double
     65 bp[] = {1.0, 1.5,},
     66 dp_h[] = { 0.0, 5.84962487220764160156e-01,}, /* 0x3FE2B803, 0x40000000 */
     67 dp_l[] = { 0.0, 1.35003920212974897128e-08,}, /* 0x3E4CFDEB, 0x43CFD006 */
     68 zero    =  0.0,
     69 one	=  1.0,
     70 two	=  2.0,
     71 two53	=  9007199254740992.0,	/* 0x43400000, 0x00000000 */
     72 huge	=  1.0e300,
     73 tiny    =  1.0e-300,
     74 	/* poly coefs for (3/2)*(log(x)-2s-2/3*s**3 */
     75 L1  =  5.99999999999994648725e-01, /* 0x3FE33333, 0x33333303 */
     76 L2  =  4.28571428578550184252e-01, /* 0x3FDB6DB6, 0xDB6FABFF */
     77 L3  =  3.33333329818377432918e-01, /* 0x3FD55555, 0x518F264D */
     78 L4  =  2.72728123808534006489e-01, /* 0x3FD17460, 0xA91D4101 */
     79 L5  =  2.30660745775561754067e-01, /* 0x3FCD864A, 0x93C9DB65 */
     80 L6  =  2.06975017800338417784e-01, /* 0x3FCA7E28, 0x4A454EEF */
     81 P1   =  1.66666666666666019037e-01, /* 0x3FC55555, 0x5555553E */
     82 P2   = -2.77777777770155933842e-03, /* 0xBF66C16C, 0x16BEBD93 */
     83 P3   =  6.61375632143793436117e-05, /* 0x3F11566A, 0xAF25DE2C */
     84 P4   = -1.65339022054652515390e-06, /* 0xBEBBBD41, 0xC5D26BF1 */
     85 P5   =  4.13813679705723846039e-08, /* 0x3E663769, 0x72BEA4D0 */
     86 lg2  =  6.93147180559945286227e-01, /* 0x3FE62E42, 0xFEFA39EF */
     87 lg2_h  =  6.93147182464599609375e-01, /* 0x3FE62E43, 0x00000000 */
     88 lg2_l  = -1.90465429995776804525e-09, /* 0xBE205C61, 0x0CA86C39 */
     89 ovt =  8.0085662595372944372e-0017, /* -(1024-log2(ovfl+.5ulp)) */
     90 cp    =  9.61796693925975554329e-01, /* 0x3FEEC709, 0xDC3A03FD =2/(3ln2) */
     91 cp_h  =  9.61796700954437255859e-01, /* 0x3FEEC709, 0xE0000000 =(float)cp */
     92 cp_l  = -7.02846165095275826516e-09, /* 0xBE3E2FE0, 0x145B01F5 =tail of cp_h*/
     93 ivln2    =  1.44269504088896338700e+00, /* 0x3FF71547, 0x652B82FE =1/ln2 */
     94 ivln2_h  =  1.44269502162933349609e+00, /* 0x3FF71547, 0x60000000 =24b 1/ln2*/
     95 ivln2_l  =  1.92596299112661746887e-08; /* 0x3E54AE0B, 0xF85DDF44 =1/ln2 tail*/
     96 
     97 double
     98 __ieee754_pow(double x, double y)
     99 {
    100 	double z,ax,z_h,z_l,p_h,p_l;
    101 	double y1,t1,t2,r,s,t,u,v,w;
    102 	int32_t i,j,k,yisint,n;
    103 	int32_t hx,hy,ix,iy;
    104 	u_int32_t lx,ly;
    105 
    106 	EXTRACT_WORDS(hx,lx,x);
    107 	EXTRACT_WORDS(hy,ly,y);
    108 	ix = hx&0x7fffffff;  iy = hy&0x7fffffff;
    109 
    110     /* y==zero: x**0 = 1 */
    111 	if((iy|ly)==0) return one;
    112 
    113     /* +-NaN return x+y */
    114 	if(ix > 0x7ff00000 || ((ix==0x7ff00000)&&(lx!=0)) ||
    115 	   iy > 0x7ff00000 || ((iy==0x7ff00000)&&(ly!=0)))
    116 		return x+y;
    117 
    118     /* determine if y is an odd int when x < 0
    119      * yisint = 0	... y is not an integer
    120      * yisint = 1	... y is an odd int
    121      * yisint = 2	... y is an even int
    122      */
    123 	yisint  = 0;
    124 	if(hx<0) {
    125 	    if(iy>=0x43400000) yisint = 2; /* even integer y */
    126 	    else if(iy>=0x3ff00000) {
    127 		k = (iy>>20)-0x3ff;	   /* exponent */
    128 		if(k>20) {
    129 		    j = ly>>(52-k);
    130 		    if((j<<(52-k))==ly) yisint = 2-(j&1);
    131 		} else if(ly==0) {
    132 		    j = iy>>(20-k);
    133 		    if((j<<(20-k))==iy) yisint = 2-(j&1);
    134 		}
    135 	    }
    136 	}
    137 
    138     /* special value of y */
    139 	if(ly==0) {
    140 	    if (iy==0x7ff00000) {	/* y is +-inf */
    141 	        if(((ix-0x3ff00000)|lx)==0)
    142 		    return  y - y;	/* inf**+-1 is NaN */
    143 	        else if (ix >= 0x3ff00000)/* (|x|>1)**+-inf = inf,0 */
    144 		    return (hy>=0)? y: zero;
    145 	        else			/* (|x|<1)**-,+inf = inf,0 */
    146 		    return (hy<0)?-y: zero;
    147 	    }
    148 	    if(iy==0x3ff00000) {	/* y is  +-1 */
    149 		if(hy<0) return one/x; else return x;
    150 	    }
    151 	    if(hy==0x40000000) return x*x; /* y is  2 */
    152 	    if(hy==0x3fe00000) {	/* y is  0.5 */
    153 		if(hx>=0)	/* x >= +0 */
    154 		return sqrt(x);
    155 	    }
    156 	}
    157 
    158 	ax   = fabs(x);
    159     /* special value of x */
    160 	if(lx==0) {
    161 	    if(ix==0x7ff00000||ix==0||ix==0x3ff00000){
    162 		z = ax;			/*x is +-0,+-inf,+-1*/
    163 		if(hy<0) z = one/z;	/* z = (1/|x|) */
    164 		if(hx<0) {
    165 		    if(((ix-0x3ff00000)|yisint)==0) {
    166 			z = (z-z)/(z-z); /* (-1)**non-int is NaN */
    167 		    } else if(yisint==1)
    168 			z = -z;		/* (x<0)**odd = -(|x|**odd) */
    169 		}
    170 		return z;
    171 	    }
    172 	}
    173 
    174     /* CYGNUS LOCAL + fdlibm-5.3 fix: This used to be
    175 	n = (hx>>31)+1;
    176        but ANSI C says a right shift of a signed negative quantity is
    177        implementation defined.  */
    178 	n = ((u_int32_t)hx>>31)-1;
    179 
    180     /* (x<0)**(non-int) is NaN */
    181 	if((n|yisint)==0) return (x-x)/(x-x);
    182 
    183 	s = one; /* s (sign of result -ve**odd) = -1 else = 1 */
    184 	if((n|(yisint-1))==0) s = -one;/* (-ve)**(odd int) */
    185 
    186     /* |y| is huge */
    187 	if(iy>0x41e00000) { /* if |y| > 2**31 */
    188 	    if(iy>0x43f00000){	/* if |y| > 2**64, must o/uflow */
    189 		if(ix<=0x3fefffff) return (hy<0)? huge*huge:tiny*tiny;
    190 		if(ix>=0x3ff00000) return (hy>0)? huge*huge:tiny*tiny;
    191 	    }
    192 	/* over/underflow if x is not close to one */
    193 	    if(ix<0x3fefffff) return (hy<0)? s*huge*huge:s*tiny*tiny;
    194 	    if(ix>0x3ff00000) return (hy>0)? s*huge*huge:s*tiny*tiny;
    195 	/* now |1-x| is tiny <= 2**-20, suffice to compute
    196 	   log(x) by x-x^2/2+x^3/3-x^4/4 */
    197 	    t = ax-one;		/* t has 20 trailing zeros */
    198 	    w = (t*t)*(0.5-t*(0.3333333333333333333333-t*0.25));
    199 	    u = ivln2_h*t;	/* ivln2_h has 21 sig. bits */
    200 	    v = t*ivln2_l-w*ivln2;
    201 	    t1 = u+v;
    202 	    SET_LOW_WORD(t1,0);
    203 	    t2 = v-(t1-u);
    204 	} else {
    205 	    double ss,s2,s_h,s_l,t_h,t_l;
    206 	    n = 0;
    207 	/* take care subnormal number */
    208 	    if(ix<0x00100000)
    209 		{ax *= two53; n -= 53; GET_HIGH_WORD(ix,ax); }
    210 	    n  += ((ix)>>20)-0x3ff;
    211 	    j  = ix&0x000fffff;
    212 	/* determine interval */
    213 	    ix = j|0x3ff00000;		/* normalize ix */
    214 	    if(j<=0x3988E) k=0;		/* |x|<sqrt(3/2) */
    215 	    else if(j<0xBB67A) k=1;	/* |x|<sqrt(3)   */
    216 	    else {k=0;n+=1;ix -= 0x00100000;}
    217 	    SET_HIGH_WORD(ax,ix);
    218 
    219 	/* compute ss = s_h+s_l = (x-1)/(x+1) or (x-1.5)/(x+1.5) */
    220 	    u = ax-bp[k];		/* bp[0]=1.0, bp[1]=1.5 */
    221 	    v = one/(ax+bp[k]);
    222 	    ss = u*v;
    223 	    s_h = ss;
    224 	    SET_LOW_WORD(s_h,0);
    225 	/* t_h=ax+bp[k] High */
    226 	    t_h = zero;
    227 	    SET_HIGH_WORD(t_h,((ix>>1)|0x20000000)+0x00080000+(k<<18));
    228 	    t_l = ax - (t_h-bp[k]);
    229 	    s_l = v*((u-s_h*t_h)-s_h*t_l);
    230 	/* compute log(ax) */
    231 	    s2 = ss*ss;
    232 	    r = s2*s2*(L1+s2*(L2+s2*(L3+s2*(L4+s2*(L5+s2*L6)))));
    233 	    r += s_l*(s_h+ss);
    234 	    s2  = s_h*s_h;
    235 	    t_h = 3.0+s2+r;
    236 	    SET_LOW_WORD(t_h,0);
    237 	    t_l = r-((t_h-3.0)-s2);
    238 	/* u+v = ss*(1+...) */
    239 	    u = s_h*t_h;
    240 	    v = s_l*t_h+t_l*ss;
    241 	/* 2/(3log2)*(ss+...) */
    242 	    p_h = u+v;
    243 	    SET_LOW_WORD(p_h,0);
    244 	    p_l = v-(p_h-u);
    245 	    z_h = cp_h*p_h;		/* cp_h+cp_l = 2/(3*log2) */
    246 	    z_l = cp_l*p_h+p_l*cp+dp_l[k];
    247 	/* log2(ax) = (ss+..)*2/(3*log2) = n + dp_h + z_h + z_l */
    248 	    t = (double)n;
    249 	    t1 = (((z_h+z_l)+dp_h[k])+t);
    250 	    SET_LOW_WORD(t1,0);
    251 	    t2 = z_l-(((t1-t)-dp_h[k])-z_h);
    252 	}
    253 
    254     /* split up y into y1+y2 and compute (y1+y2)*(t1+t2) */
    255 	y1  = y;
    256 	SET_LOW_WORD(y1,0);
    257 	p_l = (y-y1)*t1+y*t2;
    258 	p_h = y1*t1;
    259 	z = p_l+p_h;
    260 	EXTRACT_WORDS(j,i,z);
    261 	if (j>=0x40900000) {				/* z >= 1024 */
    262 	    if(((j-0x40900000)|i)!=0)			/* if z > 1024 */
    263 		return s*huge*huge;			/* overflow */
    264 	    else {
    265 		if(p_l+ovt>z-p_h) return s*huge*huge;	/* overflow */
    266 	    }
    267 	} else if((j&0x7fffffff)>=0x4090cc00 ) {	/* z <= -1075 */
    268 	    if(((j-0xc090cc00)|i)!=0) 		/* z < -1075 */
    269 		return s*tiny*tiny;		/* underflow */
    270 	    else {
    271 		if(p_l<=z-p_h) return s*tiny*tiny;	/* underflow */
    272 	    }
    273 	}
    274     /*
    275      * compute 2**(p_h+p_l)
    276      */
    277 	i = j&0x7fffffff;
    278 	k = (i>>20)-0x3ff;
    279 	n = 0;
    280 	if(i>0x3fe00000) {		/* if |z| > 0.5, set n = [z+0.5] */
    281 	    n = j+(0x00100000>>(k+1));
    282 	    k = ((n&0x7fffffff)>>20)-0x3ff;	/* new k for n */
    283 	    t = zero;
    284 	    SET_HIGH_WORD(t,n&~(0x000fffff>>k));
    285 	    n = ((n&0x000fffff)|0x00100000)>>(20-k);
    286 	    if(j<0) n = -n;
    287 	    p_h -= t;
    288 	}
    289 	t = p_l+p_h;
    290 	SET_LOW_WORD(t,0);
    291 	u = t*lg2_h;
    292 	v = (p_l-(t-p_h))*lg2+t*lg2_l;
    293 	z = u+v;
    294 	w = v-(z-u);
    295 	t  = z*z;
    296 	t1  = z - t*(P1+t*(P2+t*(P3+t*(P4+t*P5))));
    297 	r  = (z*t1)/(t1-two)-(w+z*w);
    298 	z  = one-(r-z);
    299 	GET_HIGH_WORD(j,z);
    300 	j += (n<<20);
    301 	if((j>>20)<=0) z = scalbn(z,n);	/* subnormal output */
    302 	else SET_HIGH_WORD(z,j);
    303 	return s*z;
    304 }
    305