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/* @(#)s_cbrt.c 5.1 93/09/24 */
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* ====================================================
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* Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved.
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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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* ====================================================
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* Optimized by Bruce D. Evans.
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static char rcsid[] = "$FreeBSD: src/lib/msun/src/s_cbrt.c,v 1.10 2005/12/13 20:17:23 bde Exp $";
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#include "math_private.h"
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* Return cube root of x
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static const u_int32_t
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B1 = 715094163, /* B1 = (1023-1023/3-0.03306235651)*2**20 */
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B2 = 696219795; /* B2 = (1023-1023/3-54/3-0.03306235651)*2**20 */
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C = 5.42857142857142815906e-01, /* 19/35 = 0x3FE15F15, 0xF15F15F1 */
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D = -7.05306122448979611050e-01, /* -864/1225 = 0xBFE691DE, 0x2532C834 */
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E = 1.41428571428571436819e+00, /* 99/70 = 0x3FF6A0EA, 0x0EA0EA0F */
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F = 1.60714285714285720630e+00, /* 45/28 = 0x3FF9B6DB, 0x6DB6DB6E */
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G = 3.57142857142857150787e-01; /* 5/14 = 0x3FD6DB6D, 0xB6DB6DB7 */
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sign=hx&0x80000000; /* sign= sign(x) */
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if(hx>=0x7ff00000) return(x+x); /* cbrt(NaN,INF) is itself */
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return(x); /* cbrt(0) is itself */
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* Rough cbrt to 5 bits:
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* cbrt(2**e*(1+m) ~= 2**(e/3)*(1+(e%3+m)/3)
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* where e is integral and >= 0, m is real and in [0, 1), and "/" and
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* "%" are integer division and modulus with rounding towards minus
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* infinity. The RHS is always >= the LHS and has a maximum relative
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* error of about 1 in 16. Adding a bias of -0.03306235651 to the
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* (e%3+m)/3 term reduces the error to about 1 in 32. With the IEEE
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* floating point representation, for finite positive normal values,
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* ordinary integer divison of the value in bits magically gives
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* almost exactly the RHS of the above provided we first subtract the
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* exponent bias (1023 for doubles) and later add it back. We do the
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* subtraction virtually to keep e >= 0 so that ordinary integer
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* division rounds towards minus infinity; this is also efficient.
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if(hx<0x00100000) { /* subnormal number */
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SET_HIGH_WORD(t,0x43500000); /* set t= 2**54 */
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GET_HIGH_WORD(high,t);
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SET_HIGH_WORD(t,sign|((high&0x7fffffff)/3+B2));
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SET_HIGH_WORD(t,sign|(hx/3+B1));
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/* new cbrt to 23 bits; may be implemented in single precision */
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/* chop t to 20 bits and make it larger in magnitude than cbrt(x) */
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GET_HIGH_WORD(high,t);
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INSERT_WORDS(t,high+0x00000001,0);
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/* one step Newton iteration to 53 bits with error less than 0.667 ulps */
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s=t*t; /* t*t is exact */
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r=(r-t)/(w+r); /* r-t is exact */