e_log2f.c revision 216248 
118334Speter/*
218334Speter * ====================================================
318334Speter * Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved.
418334Speter *
518334Speter * Developed at SunPro, a Sun Microsystems, Inc. business.
6 * Permission to use, copy, modify, and distribute this
7 * software is freely granted, provided that this notice
8 * is preserved.
9 * ====================================================
10 */
11
12#include <sys/cdefs.h>
13__FBSDID("$FreeBSD: head/lib/msun/src/e_log2f.c 216248 2010-12-07 03:29:36Z das $");
14
15/*
16 * Return the base 2 logarithm of x. See k_log.c for details on the algorithm.
17 */
18
19#include "math.h"
20#include "math_private.h"
21#include "k_logf.h"
22
23static const float
24two25      =  3.3554432000e+07, /* 0x4c000000 */
25ivln2hi    =  1.4428710938e+00, /* 0x3fb8b000 */
26ivln2lo    = -1.7605285393e-04; /* 0xb9389ad4 */
27
28static const float zero   =  0.0;
29
30float
31__ieee754_log2f(float x)
32{
33	float f,hi,lo;
34	int32_t i,k,hx;
35
36	GET_FLOAT_WORD(hx,x);
37
38        k=0;
39        if (hx < 0x00800000) {                  /* x < 2**-126  */
40            if ((hx&0x7fffffff)==0)
41                return -two25/zero;             /* log(+-0)=-inf */
42            if (hx<0) return (x-x)/zero;        /* log(-#) = NaN */
43            k -= 25; x *= two25; /* subnormal number, scale up x */
44	    GET_FLOAT_WORD(hx,x);
45        }
46	if (hx >= 0x7f800000) return x+x;
47	k += (hx>>23)-127;
48	hx &= 0x007fffff;
49	i = (hx+(0x4afb0d))&0x800000;
50	SET_FLOAT_WORD(x,hx|(i^0x3f800000));	/* normalize x or x/2 */
51	k += (i>>23);
52	f = __kernel_logf(x);
53	x = x - (float)1.0;
54	GET_FLOAT_WORD(hx,x);
55	SET_FLOAT_WORD(hi,hx&0xfffff000);
56	lo = x - hi;
57	return (x+f)*ivln2lo + (lo+f)*ivln2hi + hi*ivln2hi + k;
58}
59