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 /*
14 * double logb(x)
15 * IEEE 754 logb. Included to pass IEEE test suite. Not recommend.
16 * Use ilogb instead.
17 */
18
19 #include <float.h>
20
21 #include "math.h"
22 #include "math_private.h"
23
24 static const double
25 two54 = 1.80143985094819840000e+16; /* 43500000 00000000 */
26
27 double
logb(double x)28 logb(double x)
29 {
30 int32_t lx,ix;
31 EXTRACT_WORDS(ix,lx,x);
32 ix &= 0x7fffffff; /* high |x| */
33 if((ix|lx)==0) return -1.0/fabs(x);
34 if(ix>=0x7ff00000) return x*x;
35 if(ix<0x00100000) {
36 x *= two54; /* convert subnormal x to normal */
37 GET_HIGH_WORD(ix,x);
38 ix &= 0x7fffffff;
39 return (double) ((ix>>20)-1023-54);
40 } else
41 return (double) ((ix>>20)-1023);
42 }
43
44 #if (LDBL_MANT_DIG == 53)
45 __weak_reference(logb, logbl);
46 #endif
47