source: vendor/FreeBSD-msun/5.x-2005-06-xx/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[] = "$FreeBSD: src/lib/msun/src/s_ilogb.c,v 1.9 2004/10/09 17:14:28 stefanf Exp $";
#endif

/* ilogb(double x)
 * return the binary exponent of non-zero x
 * ilogb(0) = FP_ILOGB0
 * ilogb(NaN) = FP_ILOGBNAN (no signal is raised)
 * ilogb(inf) = INT_MAX (no signal is raised)
 */

#include <limits.h>

#include "math.h"
#include "math_private.h"

        int ilogb(double x)
{
        int32_t hx,lx,ix;

        EXTRACT_WORDS(hx,lx,x);
        hx &= 0x7fffffff;
        if(hx<0x00100000) {
            if((hx|lx)==0)
                return FP_ILOGB0;
            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 if (hx>0x7ff00000 || lx!=0) return FP_ILOGBNAN;
        else return INT_MAX;
}