s_cbrtf.c revision 97409 
1/* s_cbrtf.c -- float version of s_cbrt.c.
2 * Conversion to float by Ian Lance Taylor, Cygnus Support, ian@cygnus.com.
3 */
4
5/*
6 * ====================================================
7 * Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved.
8 *
9 * Developed at SunPro, a Sun Microsystems, Inc. business.
10 * Permission to use, copy, modify, and distribute this
11 * software is freely granted, provided that this notice
12 * is preserved.
13 * ====================================================
14 */
15
16#ifndef lint
17static char rcsid[] = "$FreeBSD: head/lib/msun/src/s_cbrtf.c 97409 2002-05-28 17:51:46Z alfred $";
18#endif
19
20#include "math.h"
21#include "math_private.h"
22
23/* cbrtf(x)
24 * Return cube root of x
25 */
26static const unsigned
27	B1 = 709958130, /* B1 = (84+2/3-0.03306235651)*2**23 */
28	B2 = 642849266; /* B2 = (76+2/3-0.03306235651)*2**23 */
29
30static const float
31C =  5.4285717010e-01, /* 19/35     = 0x3f0af8b0 */
32D = -7.0530611277e-01, /* -864/1225 = 0xbf348ef1 */
33E =  1.4142856598e+00, /* 99/70     = 0x3fb50750 */
34F =  1.6071428061e+00, /* 45/28     = 0x3fcdb6db */
35G =  3.5714286566e-01; /* 5/14      = 0x3eb6db6e */
36
37	float cbrtf(float x)
38{
39	float r,s,t;
40	int32_t hx;
41	u_int32_t sign;
42	u_int32_t high;
43
44	GET_FLOAT_WORD(hx,x);
45	sign=hx&0x80000000; 		/* sign= sign(x) */
46	hx  ^=sign;
47	if(hx>=0x7f800000) return(x+x); /* cbrt(NaN,INF) is itself */
48	if(hx==0)
49	    return(x);		/* cbrt(0) is itself */
50
51	SET_FLOAT_WORD(x,hx);	/* x <- |x| */
52    /* rough cbrt to 5 bits */
53	if(hx<0x00800000) 		/* subnormal number */
54	  {SET_FLOAT_WORD(t,0x4b800000); /* set t= 2**24 */
55	   t*=x; GET_FLOAT_WORD(high,t); SET_FLOAT_WORD(t,high/3+B2);
56	  }
57	else
58	  SET_FLOAT_WORD(t,hx/3+B1);
59
60
61    /* new cbrt to 23 bits */
62	r=t*t/x;
63	s=C+r*t;
64	t*=G+F/(s+E+D/s);
65
66    /* retore the sign bit */
67	GET_FLOAT_WORD(high,t);
68	SET_FLOAT_WORD(t,high|sign);
69	return(t);
70}
71