s_logb.c revision a27d2baa0c1a2ec70f47ea9199b1dd6762c8a349 
1/* @(#)s_logb.c 5.1 93/09/24 */
2/*
3 * ====================================================
4 * Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved.
5 *
6 * Developed at SunPro, a Sun Microsystems, Inc. business.
7 * Permission to use, copy, modify, and distribute this
8 * software is freely granted, provided that this notice
9 * is preserved.
10 * ====================================================
11 */
12
13#ifndef lint
14static char rcsid[] = "$FreeBSD: src/lib/msun/src/s_logb.c,v 1.10 2005/12/03 11:57:19 bde Exp $";
15#endif
16
17/*
18 * double logb(x)
19 * IEEE 754 logb. Included to pass IEEE test suite. Not recommend.
20 * Use ilogb instead.
21 */
22
23#include "math.h"
24#include "math_private.h"
25
26static const double
27two54 = 1.80143985094819840000e+16;	/* 43500000 00000000 */
28
29double
30logb(double x)
31{
32	int32_t lx,ix;
33	EXTRACT_WORDS(ix,lx,x);
34	ix &= 0x7fffffff;			/* high |x| */
35	if((ix|lx)==0) return -1.0/fabs(x);
36	if(ix>=0x7ff00000) return x*x;
37	if(ix<0x00100000) {
38		x *= two54;		 /* convert subnormal x to normal */
39		GET_FLOAT_WORD(ix,x);
40		ix &= 0x7fffffff;
41		return (float) ((ix>>20)-1023-54);
42	} else
43		return (double) ((ix>>20)-1023);
44}
45