s_logb.c revision 174698 
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: head/lib/msun/src/s_logb.c 174698 2007-12-17 03:53:38Z das $";
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 <float.h>
24
25#include "math.h"
26#include "math_private.h"
27
28static const double
29two54 = 1.80143985094819840000e+16;	/* 43500000 00000000 */
30
31double
32logb(double x)
33{
34	int32_t lx,ix;
35	EXTRACT_WORDS(ix,lx,x);
36	ix &= 0x7fffffff;			/* high |x| */
37	if((ix|lx)==0) return -1.0/fabs(x);
38	if(ix>=0x7ff00000) return x*x;
39	if(ix<0x00100000) {
40		x *= two54;		 /* convert subnormal x to normal */
41		GET_FLOAT_WORD(ix,x);
42		ix &= 0x7fffffff;
43		return (float) ((ix>>20)-1023-54);
44	} else
45		return (double) ((ix>>20)-1023);
46}
47
48#if (LDBL_MANT_DIG == 53)
49__weak_reference(logb, logbl);
50#endif
51