2 /* @(#)e_log10.c 1.3 95/01/18 */
4 * ====================================================
5 * Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved.
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
11 * ====================================================
14 #include <sys/cdefs.h>
15 __FBSDID("$FreeBSD$");
18 * Return the base 2 logarithm of x. See k_log.c for details on the algorithm.
22 #include "math_private.h"
26 two54 = 1.80143985094819840000e+16, /* 0x43500000, 0x00000000 */
27 ivln2hi = 1.44269504072144627571e+00, /* 0x3ff71547, 0x65200000 */
28 ivln2lo = 1.67517131648865118353e-10; /* 0x3de705fc, 0x2eefa200 */
30 static const double zero = 0.0;
33 __ieee754_log2(double x)
39 EXTRACT_WORDS(hx,lx,x);
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 */
49 if (hx >= 0x7ff00000) return x+x;
52 i = (hx+0x95f64)&0x100000;
53 SET_HIGH_WORD(x,hx|(i^0x3ff00000)); /* normalize x or x/2 */
59 return (x+f)*ivln2lo + (lo+f)*ivln2hi + hi*ivln2hi + k;