source: trunk/sys/libm/s_logb.c @ 80

/* @(#)s_logb.c 5.1 93/09/24 */
/*
 * ====================================================
 * Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved.
 *
 * Developed at SunPro, a Sun Microsystems, Inc. business.
 * Permission to use, copy, modify, and distribute this
 * software is freely granted, provided that this notice 
 * is preserved.
 * ====================================================
 */

/*
 * double logb(x)
 * IEEE 754 logb. Included to pass IEEE test suite. Not recommend.
 * Use ilogb instead.
 */

#include <libm/fdlibm.h>

#ifdef __STDC__
static const double one = 1.0;
#else
static double one = 1.0;
#endif

#ifdef __STDC__
        double logb(double x)
#else
        double logb(x)
        double x;
#endif
{
        int n0,lx,ix;
        n0 = ((*(int*)&one)>>29)^1;
        ix = (*(n0+(int*)&x))&0x7fffffff;       /* high |x| */
        lx = *(1-n0+(int*)&x);                  /* low x */
        if((ix|lx)==0) return -1.0/fabs(x);
        if(ix>=0x7ff00000) return x*x;
        if((ix>>=20)==0)                        /* IEEE 754 logb */
                return -1022.0; 
        else
                return (double) (ix-1023); 
}