xref: /freebsd/lib/msun/src/s_logb.c (revision acd3428b7d3e94cef0e1881c868cb4b131d4ff41)
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
14 static char rcsid[] = "$FreeBSD$";
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 
26 static const double
27 two54 = 1.80143985094819840000e+16;	/* 43500000 00000000 */
28 
29 double
30 logb(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