s_logb.c

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/* @(#)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 "math.h"
#include "math_private.h"

double
logb(double x)
{
    int32_t lx,ix;
    EXTRACT_WORDS(ix,lx,x);
    ix &= 0x7fffffff;           /* high |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); 
}