e_log2f.c revision 216211 
1/*
2 * ====================================================
3 * Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved.
4 *
5 * 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 216211 2010-12-05 22:11:22Z das $");
14
15#include "math.h"
16#include "math_private.h"
17#include "k_logf.h"
18
19static const float
20two25      =  3.3554432000e+07, /* 0x4c000000 */
21ivln2hi    =  0x1.716p+0f,
22ivln2lo    = -0x1.7135a8fa03d11p-13;
23
24static const float zero   =  0.0;
25
26float
27__ieee754_log2f(float x)
28{
29	float f,hi,lo;
30	int32_t i,k,hx;
31
32	GET_FLOAT_WORD(hx,x);
33
34        k=0;
35        if (hx < 0x00800000) {                  /* x < 2**-126  */
36            if ((hx&0x7fffffff)==0)
37                return -two25/zero;             /* log(+-0)=-inf */
38            if (hx<0) return (x-x)/zero;        /* log(-#) = NaN */
39            k -= 25; x *= two25; /* subnormal number, scale up x */
40	    GET_FLOAT_WORD(hx,x);
41        }
42	if (hx >= 0x7f800000) return x+x;
43	k += (hx>>23)-127;
44	hx &= 0x007fffff;
45	i = (hx+(0x4afb0d))&0x800000;
46	SET_FLOAT_WORD(x,hx|(i^0x3f800000));	/* normalize x or x/2 */
47	k += (i>>23);
48	f = __kernel_logf(x);
49	x = x - 1;
50	GET_FLOAT_WORD(hx,x);
51	SET_FLOAT_WORD(hi,hx&0xfffff000);
52	lo = x - hi;
53	return (x+f)*ivln2lo + (lo+f)*ivln2hi + hi*ivln2hi + k;
54}
55