329 lines
10 KiB
C
329 lines
10 KiB
C
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/* origin: FreeBSD /usr/src/lib/msun/src/e_pow.c */
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/*
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* ====================================================
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* Copyright (C) 2004 by Sun Microsystems, Inc. All rights reserved.
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*
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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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/* 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. (anything) ** 0 is 1
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* 2. 1 ** (anything) is 1
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* 3. (anything except 1) ** NAN is NAN
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* 4. NAN ** (anything except 0) is NAN
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* 5. +-(|x| > 1) ** +INF is +INF
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* 6. +-(|x| > 1) ** -INF is +0
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* 7. +-(|x| < 1) ** +INF is +0
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* 8. +-(|x| < 1) ** -INF is +INF
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* 9. -1 ** +-INF is 1
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* 10. +0 ** (+anything except 0, NAN) is +0
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* 11. -0 ** (+anything except 0, NAN, odd integer) is +0
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* 12. +0 ** (-anything except 0, NAN) is +INF, raise divbyzero
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* 13. -0 ** (-anything except 0, NAN, odd integer) is +INF, raise divbyzero
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* 14. -0 ** (+odd integer) is -0
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* 15. -0 ** (-odd integer) is -INF, raise divbyzero
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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 ** (+odd integer) is -INF
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* 19. -INF ** (anything) = -0 ** (-anything), (anything except odd integer)
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* 20. (anything) ** 1 is (anything)
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* 21. (anything) ** -1 is 1/(anything)
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* 22. (-anything) ** (integer) is (-1)**(integer)*(+anything**integer)
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* 23. (-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 "libm.h"
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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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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-017, /* -(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 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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uint32_t lx,ly;
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EXTRACT_WORDS(hx, lx, x);
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EXTRACT_WORDS(hy, ly, y);
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ix = hx & 0x7fffffff;
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iy = hy & 0x7fffffff;
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/* x**0 = 1, even if x is NaN */
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if ((iy|ly) == 0)
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return 1.0;
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/* 1**y = 1, even if y is NaN */
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if (hx == 0x3ff00000 && lx == 0)
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return 1.0;
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/* NaN if either arg is NaN */
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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)
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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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uint32_t j = ly>>(52-k);
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if ((j<<(52-k)) == ly)
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yisint = 2 - (j&1);
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} else if (ly == 0) {
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uint32_t j = iy>>(20-k);
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if ((j<<(20-k)) == iy)
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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) /* (-1)**+-inf is 1 */
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return 1.0;
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else if (ix >= 0x3ff00000) /* (|x|>1)**+-inf = inf,0 */
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return hy >= 0 ? y : 0.0;
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else /* (|x|<1)**+-inf = 0,inf */
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return hy >= 0 ? 0.0 : -y;
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}
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if (iy == 0x3ff00000) { /* y is +-1 */
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if (hy >= 0)
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return x;
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y = 1/x;
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#if FLT_EVAL_METHOD!=0
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{
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union {double f; uint64_t i;} u = {y};
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uint64_t i = u.i & -1ULL/2;
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if (i>>52 == 0 && (i&(i-1)))
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FORCE_EVAL((float)y);
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}
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#endif
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return y;
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}
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if (hy == 0x40000000) /* y is 2 */
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return x*x;
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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 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) { /* x is +-0,+-inf,+-1 */
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z = ax;
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if (hy < 0) /* z = (1/|x|) */
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z = 1.0/z;
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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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s = 1.0; /* sign of result */
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if (hx < 0) {
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if (yisint == 0) /* (x<0)**(non-int) is NaN */
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return (x-x)/(x-x);
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if (yisint == 1) /* (x<0)**(odd int) */
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s = -1.0;
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}
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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)
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return hy < 0 ? huge*huge : tiny*tiny;
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if (ix >= 0x3ff00000)
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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)
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return hy < 0 ? s*huge*huge : s*tiny*tiny;
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if (ix > 0x3ff00000)
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return hy > 0 ? s*huge*huge : s*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 = ax - 1.0; /* 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 ss,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;
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n -= 53;
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GET_HIGH_WORD(ix,ax);
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}
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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) /* |x|<sqrt(3/2) */
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k = 0;
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else if (j < 0xBB67A) /* |x|<sqrt(3) */
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k = 1;
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else {
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k = 0;
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n += 1;
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ix -= 0x00100000;
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}
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SET_HIGH_WORD(ax, ix);
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/* compute ss = 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 = 1.0/(ax+bp[k]);
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ss = u*v;
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s_h = ss;
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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 = 0.0;
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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 = ss*ss;
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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+ss);
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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 = ss*(1+...) */
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u = s_h*t_h;
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v = s_l*t_h + t_l*ss;
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/* 2/(3log2)*(ss+...) */
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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) = (ss+..)*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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/* 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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if (p_l + ovt > z - p_h)
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return s*huge*huge; /* overflow */
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} else if ((j&0x7fffffff) >= 0x4090cc00) { /* z <= -1075 */ // FIXME: instead of abs(j) use unsigned j
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if (((j-0xc090cc00)|i) != 0) /* z < -1075 */
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return s*tiny*tiny; /* underflow */
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if (p_l <= z - p_h)
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return s*tiny*tiny; /* underflow */
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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] */
|
||
|
|
n = j + (0x00100000>>(k+1));
|
||
|
|
k = ((n&0x7fffffff)>>20) - 0x3ff; /* new k for n */
|
||
|
|
t = 0.0;
|
||
|
|
SET_HIGH_WORD(t, n & ~(0x000fffff>>k));
|
||
|
|
n = ((n&0x000fffff)|0x00100000)>>(20-k);
|
||
|
|
if (j < 0)
|
||
|
|
n = -n;
|
||
|
|
p_h -= t;
|
||
|
|
}
|
||
|
|
t = p_l + p_h;
|
||
|
|
SET_LOW_WORD(t, 0);
|
||
|
|
u = t*lg2_h;
|
||
|
|
v = (p_l-(t-p_h))*lg2 + t*lg2_l;
|
||
|
|
z = u + v;
|
||
|
|
w = v - (z-u);
|
||
|
|
t = z*z;
|
||
|
|
t1 = z - t*(P1+t*(P2+t*(P3+t*(P4+t*P5))));
|
||
|
|
r = (z*t1)/(t1-2.0) - (w + z*w);
|
||
|
|
z = 1.0 - (r-z);
|
||
|
|
GET_HIGH_WORD(j, z);
|
||
|
|
j += n<<20;
|
||
|
|
if ((j>>20) <= 0) /* subnormal output */
|
||
|
|
z = scalbn(z,n);
|
||
|
|
else
|
||
|
|
SET_HIGH_WORD(z, j);
|
||
|
|
return s*z;
|
||
|
|
}
|