e_log2.c revision 216211 
1
2/* @(#)e_log10.c 1.3 95/01/18 */
3/*
4 * ====================================================
5 * Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved.
6 *
7 * Developed at SunSoft, a Sun Microsystems, Inc. business.
8 * Permission to use, copy, modify, and distribute this
9 * software is freely granted, provided that this notice
10 * is preserved.
11 * ====================================================
12 */
13
14#include <sys/cdefs.h>
15__FBSDID("$FreeBSD: head/lib/msun/src/e_log2.c 216211 2010-12-05 22:11:22Z das $");
16
17/* log2(x)
18 * Return the base 2 logarithm of x.
19 */
20
21#include "math.h"
22#include "math_private.h"
23#include "k_log.h"
24
25static const double
26two54      =  1.80143985094819840000e+16, /* 0x43500000, 0x00000000 */
27ivln2hi    =  0x1.71547652000p+0,
28ivln2lo    =  0x1.705fc2eefa2p-33;
29
30static const double zero   =  0.0;
31
32double
33__ieee754_log2(double x)
34{
35	double f,hi,lo;
36	int32_t i,k,hx;
37	u_int32_t lx;
38
39	EXTRACT_WORDS(hx,lx,x);
40
41        k=0;
42        if (hx < 0x00100000) {                  /* x < 2**-1022  */
43            if (((hx&0x7fffffff)|lx)==0)
44                return -two54/zero;             /* log(+-0)=-inf */
45            if (hx<0) return (x-x)/zero;        /* log(-#) = NaN */
46            k -= 54; x *= two54; /* subnormal number, scale up x */
47	    GET_HIGH_WORD(hx,x);
48        }
49	if (hx >= 0x7ff00000) return x+x;
50	k += (hx>>20)-1023;
51	hx &= 0x000fffff;
52	i = (hx+0x95f64)&0x100000;
53	SET_HIGH_WORD(x,hx|(i^0x3ff00000));	/* normalize x or x/2 */
54	k += (i>>20);
55	f = __kernel_log(x);
56	hi = x = x - 1;
57	SET_LOW_WORD(hi,0);
58	lo = x - hi;
59	return (x+f)*ivln2lo + (lo+f)*ivln2hi + hi*ivln2hi + k;
60}
61