s_ilogb.c

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/* @(#)s_ilogb.c 1.3 95/01/18 */
/*
 * ====================================================
 * Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved.
 *
 * Developed at SunSoft, a Sun Microsystems, Inc. business.
 * Permission to use, copy, modify, and distribute this
 * software is freely granted, provided that this notice 
 * is preserved.
 * ====================================================
 */

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

#include "fdlibm.h"

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

    hx  = (__HI(x))&0x7fffffff; /* high word of x */
    if(hx<0x00100000) {
        lx = __LO(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;
}