libm/upstream-freebsd/lib/msun/src/s_logb.c

/*
 * ====================================================
 * 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 <float.h>

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

static const double
two54 = 1.80143985094819840000e+16;	/* 43500000 00000000 */

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<0x00100000) {
		x *= two54;		 /* convert subnormal x to normal */
		GET_HIGH_WORD(ix,x);
		ix &= 0x7fffffff;
		return (double) ((ix>>20)-1023-54);
	} else
		return (double) ((ix>>20)-1023);
}

#if (LDBL_MANT_DIG == 53)
__weak_reference(logb, logbl);
#endif