src/libm/e_pow.c
author Ozkan Sezer <sezeroz@gmail.com>
Sun, 18 Nov 2018 11:50:20 +0300
changeset 12420 4a6c91d9cc33
parent 11711 8a982ed61896
permissions -rw-r--r--
libm: Watcom defines huge=__huge: undefine it to fix build using Watcom.
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/*
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 * ====================================================
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 * Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved.
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 *
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 * Developed at SunPro, a Sun Microsystems, Inc. business.
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 * Permission to use, copy, modify, and distribute this
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 * software is freely granted, provided that this notice
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 * is preserved.
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 * ====================================================
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 */
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/* __ieee754_pow(x,y) return x**y
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 *
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 *		      n
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 * Method:  Let x =  2   * (1+f)
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 *	1. Compute and return log2(x) in two pieces:
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 *		log2(x) = w1 + w2,
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 *	   where w1 has 53-24 = 29 bit trailing zeros.
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 *	2. Perform y*log2(x) = n+y' by simulating muti-precision
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 *	   arithmetic, where |y'|<=0.5.
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 *	3. Return x**y = 2**n*exp(y'*log2)
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 *
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 * Special cases:
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 *	1.  +-1 ** anything  is 1.0
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 *	2.  +-1 ** +-INF     is 1.0
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 *	3.  (anything) ** 0  is 1
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 *	4.  (anything) ** 1  is itself
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 *	5.  (anything) ** NAN is NAN
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 *	6.  NAN ** (anything except 0) is NAN
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 *	7.  +-(|x| > 1) **  +INF is +INF
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 *	8.  +-(|x| > 1) **  -INF is +0
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 *	9.  +-(|x| < 1) **  +INF is +0
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 *	10  +-(|x| < 1) **  -INF is +INF
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 *	11. +0 ** (+anything except 0, NAN)               is +0
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 *	12. -0 ** (+anything except 0, NAN, odd integer)  is +0
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 *	13. +0 ** (-anything except 0, NAN)               is +INF
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 *	14. -0 ** (-anything except 0, NAN, odd integer)  is +INF
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 *	15. -0 ** (odd integer) = -( +0 ** (odd integer) )
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 *	16. +INF ** (+anything except 0,NAN) is +INF
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 *	17. +INF ** (-anything except 0,NAN) is +0
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 *	18. -INF ** (anything)  = -0 ** (-anything)
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 *	19. (-anything) ** (integer) is (-1)**(integer)*(+anything**integer)
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 *	20. (-anything except 0 and inf) ** (non-integer) is NAN
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 *
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 * Accuracy:
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 *	pow(x,y) returns x**y nearly rounded. In particular
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 *			pow(integer,integer)
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 *	always returns the correct integer provided it is
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 *	representable.
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 *
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 * Constants :
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 * The hexadecimal values are the intended ones for the following
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 * constants. The decimal values may be used, provided that the
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 * compiler will convert from decimal to binary accurately enough
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 * to produce the hexadecimal values shown.
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 */
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#include "math_libm.h"
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#include "math_private.h"
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#if defined(_MSC_VER)           /* Handle Microsoft VC++ compiler specifics. */
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/* C4756: overflow in constant arithmetic */
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#pragma warning ( disable : 4756 )
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#endif
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#ifdef __WATCOMC__ /* Watcom defines huge=__huge */
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#undef huge
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#endif
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static const double
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bp[] = {1.0, 1.5,},
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dp_h[] = { 0.0, 5.84962487220764160156e-01,}, /* 0x3FE2B803, 0x40000000 */
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dp_l[] = { 0.0, 1.35003920212974897128e-08,}, /* 0x3E4CFDEB, 0x43CFD006 */
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zero    =  0.0,
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one	=  1.0,
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two	=  2.0,
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two53	=  9007199254740992.0,	/* 0x43400000, 0x00000000 */
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huge	=  1.0e300,
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tiny    =  1.0e-300,
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	/* poly coefs for (3/2)*(log(x)-2s-2/3*s**3 */
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L1  =  5.99999999999994648725e-01, /* 0x3FE33333, 0x33333303 */
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L2  =  4.28571428578550184252e-01, /* 0x3FDB6DB6, 0xDB6FABFF */
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L3  =  3.33333329818377432918e-01, /* 0x3FD55555, 0x518F264D */
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L4  =  2.72728123808534006489e-01, /* 0x3FD17460, 0xA91D4101 */
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L5  =  2.30660745775561754067e-01, /* 0x3FCD864A, 0x93C9DB65 */
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L6  =  2.06975017800338417784e-01, /* 0x3FCA7E28, 0x4A454EEF */
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P1   =  1.66666666666666019037e-01, /* 0x3FC55555, 0x5555553E */
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P2   = -2.77777777770155933842e-03, /* 0xBF66C16C, 0x16BEBD93 */
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P3   =  6.61375632143793436117e-05, /* 0x3F11566A, 0xAF25DE2C */
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P4   = -1.65339022054652515390e-06, /* 0xBEBBBD41, 0xC5D26BF1 */
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P5   =  4.13813679705723846039e-08, /* 0x3E663769, 0x72BEA4D0 */
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lg2  =  6.93147180559945286227e-01, /* 0x3FE62E42, 0xFEFA39EF */
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lg2_h  =  6.93147182464599609375e-01, /* 0x3FE62E43, 0x00000000 */
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lg2_l  = -1.90465429995776804525e-09, /* 0xBE205C61, 0x0CA86C39 */
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ovt =  8.0085662595372944372e-0017, /* -(1024-log2(ovfl+.5ulp)) */
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cp    =  9.61796693925975554329e-01, /* 0x3FEEC709, 0xDC3A03FD =2/(3ln2) */
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cp_h  =  9.61796700954437255859e-01, /* 0x3FEEC709, 0xE0000000 =(float)cp */
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cp_l  = -7.02846165095275826516e-09, /* 0xBE3E2FE0, 0x145B01F5 =tail of cp_h*/
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ivln2    =  1.44269504088896338700e+00, /* 0x3FF71547, 0x652B82FE =1/ln2 */
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ivln2_h  =  1.44269502162933349609e+00, /* 0x3FF71547, 0x60000000 =24b 1/ln2*/
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ivln2_l  =  1.92596299112661746887e-08; /* 0x3E54AE0B, 0xF85DDF44 =1/ln2 tail*/
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double attribute_hidden __ieee754_pow(double x, double y)
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{
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	double z,ax,z_h,z_l,p_h,p_l;
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	double y1,t1,t2,r,s,t,u,v,w;
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	int32_t i,j,k,yisint,n;
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	int32_t hx,hy,ix,iy;
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	u_int32_t lx,ly;
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	EXTRACT_WORDS(hx,lx,x);
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    /* x==1: 1**y = 1 (even if y is NaN) */
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	if (hx==0x3ff00000 && lx==0) {
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		return x;
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	}
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	ix = hx&0x7fffffff;
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	EXTRACT_WORDS(hy,ly,y);
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	iy = hy&0x7fffffff;
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    /* y==zero: x**0 = 1 */
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	if((iy|ly)==0) return one;
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    /* +-NaN return x+y */
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	if(ix > 0x7ff00000 || ((ix==0x7ff00000)&&(lx!=0)) ||
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	   iy > 0x7ff00000 || ((iy==0x7ff00000)&&(ly!=0)))
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		return x+y;
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    /* determine if y is an odd int when x < 0
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     * yisint = 0	... y is not an integer
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     * yisint = 1	... y is an odd int
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     * yisint = 2	... y is an even int
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     */
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	yisint  = 0;
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	if(hx<0) {
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	    if(iy>=0x43400000) yisint = 2; /* even integer y */
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	    else if(iy>=0x3ff00000) {
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		k = (iy>>20)-0x3ff;	   /* exponent */
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		if(k>20) {
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		    j = ly>>(52-k);
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		    if((j<<(52-k))==ly) yisint = 2-(j&1);
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		} else if(ly==0) {
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		    j = iy>>(20-k);
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		    if((j<<(20-k))==iy) yisint = 2-(j&1);
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		}
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	    }
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	}
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    /* special value of y */
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	if(ly==0) {
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	    if (iy==0x7ff00000) {       /* y is +-inf */
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	        if (((ix-0x3ff00000)|lx)==0)
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		    return one;	        /* +-1**+-inf is 1 (yes, weird rule) */
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	        if (ix >= 0x3ff00000)   /* (|x|>1)**+-inf = inf,0 */
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		    return (hy>=0) ? y : zero;
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	        /* (|x|<1)**-,+inf = inf,0 */
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		return (hy<0) ? -y : zero;
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	    }
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	    if(iy==0x3ff00000) {	/* y is  +-1 */
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		if(hy<0) return one/x; else return x;
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	    }
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	    if(hy==0x40000000) return x*x; /* y is  2 */
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	    if(hy==0x3fe00000) {	/* y is  0.5 */
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		if(hx>=0)	/* x >= +0 */
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		    return __ieee754_sqrt(x);
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	    }
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	}
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	ax   = fabs(x);
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    /* special value of x */
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	if(lx==0) {
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	    if(ix==0x7ff00000||ix==0||ix==0x3ff00000){
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		z = ax;			/*x is +-0,+-inf,+-1*/
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		if(hy<0) z = one/z;	/* z = (1/|x|) */
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		if(hx<0) {
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		    if(((ix-0x3ff00000)|yisint)==0) {
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			z = (z-z)/(z-z); /* (-1)**non-int is NaN */
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		    } else if(yisint==1)
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			z = -z;		/* (x<0)**odd = -(|x|**odd) */
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		}
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		return z;
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	    }
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	}
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    /* (x<0)**(non-int) is NaN */
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	if(((((u_int32_t)hx>>31)-1)|yisint)==0) return (x-x)/(x-x);
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    /* |y| is huge */
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	if(iy>0x41e00000) { /* if |y| > 2**31 */
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	    if(iy>0x43f00000){	/* if |y| > 2**64, must o/uflow */
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		if(ix<=0x3fefffff) return (hy<0)? huge*huge:tiny*tiny;
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		if(ix>=0x3ff00000) return (hy>0)? huge*huge:tiny*tiny;
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	    }
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	/* over/underflow if x is not close to one */
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	    if(ix<0x3fefffff) return (hy<0)? huge*huge:tiny*tiny;
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	    if(ix>0x3ff00000) return (hy>0)? huge*huge:tiny*tiny;
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	/* now |1-x| is tiny <= 2**-20, suffice to compute
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	   log(x) by x-x^2/2+x^3/3-x^4/4 */
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	    t = x-1;		/* t has 20 trailing zeros */
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	    w = (t*t)*(0.5-t*(0.3333333333333333333333-t*0.25));
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	    u = ivln2_h*t;	/* ivln2_h has 21 sig. bits */
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	    v = t*ivln2_l-w*ivln2;
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	    t1 = u+v;
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	    SET_LOW_WORD(t1,0);
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	    t2 = v-(t1-u);
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	} else {
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	    double s2,s_h,s_l,t_h,t_l;
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	    n = 0;
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	/* take care subnormal number */
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	    if(ix<0x00100000)
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		{ax *= two53; n -= 53; GET_HIGH_WORD(ix,ax); }
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	    n  += ((ix)>>20)-0x3ff;
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	    j  = ix&0x000fffff;
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	/* determine interval */
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	    ix = j|0x3ff00000;		/* normalize ix */
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	    if(j<=0x3988E) k=0;		/* |x|<sqrt(3/2) */
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	    else if(j<0xBB67A) k=1;	/* |x|<sqrt(3)   */
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	    else {k=0;n+=1;ix -= 0x00100000;}
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	    SET_HIGH_WORD(ax,ix);
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	/* compute s = s_h+s_l = (x-1)/(x+1) or (x-1.5)/(x+1.5) */
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	    u = ax-bp[k];		/* bp[0]=1.0, bp[1]=1.5 */
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	    v = one/(ax+bp[k]);
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	    s = u*v;
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	    s_h = s;
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	    SET_LOW_WORD(s_h,0);
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	/* t_h=ax+bp[k] High */
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	    t_h = zero;
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	    SET_HIGH_WORD(t_h,((ix>>1)|0x20000000)+0x00080000+(k<<18));
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	    t_l = ax - (t_h-bp[k]);
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	    s_l = v*((u-s_h*t_h)-s_h*t_l);
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	/* compute log(ax) */
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	    s2 = s*s;
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	    r = s2*s2*(L1+s2*(L2+s2*(L3+s2*(L4+s2*(L5+s2*L6)))));
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	    r += s_l*(s_h+s);
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	    s2  = s_h*s_h;
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	    t_h = 3.0+s2+r;
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	    SET_LOW_WORD(t_h,0);
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	    t_l = r-((t_h-3.0)-s2);
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	/* u+v = s*(1+...) */
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	    u = s_h*t_h;
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	    v = s_l*t_h+t_l*s;
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	/* 2/(3log2)*(s+...) */
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	    p_h = u+v;
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	    SET_LOW_WORD(p_h,0);
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	    p_l = v-(p_h-u);
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	    z_h = cp_h*p_h;		/* cp_h+cp_l = 2/(3*log2) */
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	    z_l = cp_l*p_h+p_l*cp+dp_l[k];
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	/* log2(ax) = (s+..)*2/(3*log2) = n + dp_h + z_h + z_l */
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	    t = (double)n;
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	    t1 = (((z_h+z_l)+dp_h[k])+t);
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	    SET_LOW_WORD(t1,0);
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	    t2 = z_l-(((t1-t)-dp_h[k])-z_h);
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	}
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	s = one; /* s (sign of result -ve**odd) = -1 else = 1 */
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	if(((((u_int32_t)hx>>31)-1)|(yisint-1))==0)
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	    s = -one;/* (-ve)**(odd int) */
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    /* split up y into y1+y2 and compute (y1+y2)*(t1+t2) */
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	y1  = y;
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	SET_LOW_WORD(y1,0);
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	p_l = (y-y1)*t1+y*t2;
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	p_h = y1*t1;
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	z = p_l+p_h;
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	EXTRACT_WORDS(j,i,z);
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	if (j>=0x40900000) {				/* z >= 1024 */
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	    if(((j-0x40900000)|i)!=0)			/* if z > 1024 */
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		return s*huge*huge;			/* overflow */
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	    else {
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		if(p_l+ovt>z-p_h) return s*huge*huge;	/* overflow */
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	    }
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	} else if((j&0x7fffffff)>=0x4090cc00 ) {	/* z <= -1075 */
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	    if(((j-0xc090cc00)|i)!=0) 		/* z < -1075 */
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		return s*tiny*tiny;		/* underflow */
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	    else {
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		if(p_l<=z-p_h) return s*tiny*tiny;	/* underflow */
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	    }
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	}
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    /*
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     * compute 2**(p_h+p_l)
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     */
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	i = j&0x7fffffff;
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	k = (i>>20)-0x3ff;
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	n = 0;
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	if(i>0x3fe00000) {		/* if |z| > 0.5, set n = [z+0.5] */
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	    n = j+(0x00100000>>(k+1));
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	    k = ((n&0x7fffffff)>>20)-0x3ff;	/* new k for n */
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	    t = zero;
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	    SET_HIGH_WORD(t,n&~(0x000fffff>>k));
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	    n = ((n&0x000fffff)|0x00100000)>>(20-k);
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	    if(j<0) n = -n;
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	    p_h -= t;
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	}
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	t = p_l+p_h;
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	SET_LOW_WORD(t,0);
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	u = t*lg2_h;
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	v = (p_l-(t-p_h))*lg2+t*lg2_l;
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	z = u+v;
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	w = v-(z-u);
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	t  = z*z;
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	t1  = z - t*(P1+t*(P2+t*(P3+t*(P4+t*P5))));
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	r  = (z*t1)/(t1-two)-(w+z*w);
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	z  = one-(r-z);
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	GET_HIGH_WORD(j,z);
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	j += (n<<20);
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	if((j>>20)<=0) z = scalbn(z,n);	/* subnormal output */
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	else SET_HIGH_WORD(z,j);
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	return s*z;
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}
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/*
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 * wrapper pow(x,y) return x**y
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 */
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#ifndef _IEEE_LIBM
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double pow(double x, double y)
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{
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	double z = __ieee754_pow(x, y);
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	if (_LIB_VERSION == _IEEE_|| isnan(y))
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		return z;
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	if (isnan(x)) {
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		if (y == 0.0)
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			return __kernel_standard(x, y, 42); /* pow(NaN,0.0) */
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		return z;
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	}
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	if (x == 0.0) {
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		if (y == 0.0)
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	    		return __kernel_standard(x, y, 20); /* pow(0.0,0.0) */
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		if (isfinite(y) && y < 0.0)
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			return __kernel_standard(x,y,23); /* pow(0.0,negative) */
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		return z;
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	}
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   333
	if (!isfinite(z)) {
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		if (isfinite(x) && isfinite(y)) {
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			if (isnan(z))
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				return __kernel_standard(x, y, 24); /* pow neg**non-int */
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			return __kernel_standard(x, y, 21); /* pow overflow */
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		}
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   339
	}
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   340
	if (z == 0.0 && isfinite(x) && isfinite(y))
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		return __kernel_standard(x, y, 22); /* pow underflow */
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	return z;
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   343
}
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#else
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strong_alias(__ieee754_pow, pow)
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#endif
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   347
libm_hidden_def(pow)