lib/msun/src/s_ilogb.c

/* @(#)s_ilogb.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.
 * ====================================================
 */

#ifndef lint
static char rcsid[] = "$Id: s_ilogb.c,v 1.4 1994/03/03 17:04:38 jtc Exp $";
#endif

/* ilogb(double x)
 * return the binary exponent of non-zero x
 * ilogb(0) = 0x80000001
 * ilogb(inf/NaN) = 0x7fffffff (no signal is raised)
 */

#include "math.h"
#include <machine/endian.h>

#if BYTE_ORDER == LITTLE_ENDIAN
#define n0      1
#else
#define n0      0
#endif

#ifdef __STDC__
        int ilogb(double x)
#else
        int ilogb(x)
        double x;
#endif
{
        int hx,lx,ix;

        hx  = (*(n0+(unsigned*)&x))&0x7fffffff; /* high word of x */
        if(hx<0x00100000) {
            lx = *(1-n0+(int*)&x);
            if((hx|lx)==0)
                return 0x80000001;      /* ilogb(0) = 0x80000001 */
            else                        /* subnormal x */
                if(hx==0) {
                    for (ix = -1043; lx>0; lx<<=1) ix -=1;
                } else {
                    for (ix = -1022,hx<<=11; hx>0; hx<<=1) ix -=1;
                }
            return ix;
        }
        else if (hx<0x7ff00000) return (hx>>20)-1023;
        else return 0x7fffffff;
}