xref: /src/lib/libc/compat/gen/compat_ldexp_ieee754.c

/* $NetBSD: compat_ldexp_ieee754.c,v 1.9 2024/06/17 18:16:58 riastradh Exp $ */

/*-
 * Copyright (c) 1999 The NetBSD Foundation, Inc.
 * All rights reserved.
 *
 * This code is derived from software contributed to The NetBSD Foundation
 * by Charles M. Hannum.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 *
 * THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. AND CONTRIBUTORS
 * ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
 * TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
 * PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE FOUNDATION OR CONTRIBUTORS
 * BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
 * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
 * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
 * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
 * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
 * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
 * POSSIBILITY OF SUCH DAMAGE.
 */

#include <sys/cdefs.h>
#if defined(LIBC_SCCS) && !defined(lint)
__RCSID("$NetBSD: compat_ldexp_ieee754.c,v 1.9 2024/06/17 18:16:58 riastradh Exp $");
#endif /* LIBC_SCCS and not lint */

#include <sys/types.h>

#include <machine/ieee.h>

#include <errno.h>
#include <float.h>
#include <math.h>

static volatile const double tiny = DBL_MIN, huge = DBL_MAX;

static double
underflow(double val)
{

	errno = ERANGE;
	return (val < 0 ? -tiny*tiny : tiny*tiny);
}

static double
overflow(double val)
{

	errno = ERANGE;
	return (val < 0 ? -huge*huge : huge*huge);
}

/*
 * Multiply the given value by 2^expon.
 */
double
ldexp(double val, int expon)
{
	int oldexp, newexp;
	union ieee_double_u u, mul;

	u.dblu_d = val;
	oldexp = u.dblu_dbl.dbl_exp;

	/*
	 * If input is zero, Inf or NaN, just return it, but raise
	 * invalid exception if it is a signalling NaN: adding any of
	 * these inputs to itself gives itself as output; arithmetic on
	 * a signalling NaN additionally raises invalid-operation.
	 */
	if (u.dblu_d == 0.0 || oldexp == DBL_EXP_INFNAN)
		return (val + val);

	if (oldexp == 0) {
		/*
		 * u.v is denormal.  We must adjust it so that the exponent
		 * arithmetic below will work.
		 */
		if (expon <= DBL_EXP_BIAS) {
			/*
			 * Optimization: if the scaling can be done in a single
			 * multiply, or underflows, just do it now.
			 */
			if (expon <= -DBL_FRACBITS)
				return underflow(val);
			mul.dblu_d = 0.0;
			mul.dblu_dbl.dbl_exp = expon + DBL_EXP_BIAS;
			u.dblu_d *= mul.dblu_d;
			if (u.dblu_d == 0.0)
				return underflow(val);
			return (u.dblu_d);
		} else {
			/*
			 * We know that expon is very large, and therefore the
			 * result cannot be denormal (though it may be Inf).
			 * Shift u.v by just enough to make it normal.
			 */
			mul.dblu_d = 0.0;
			mul.dblu_dbl.dbl_exp = DBL_FRACBITS + DBL_EXP_BIAS;
			u.dblu_d *= mul.dblu_d;
			expon -= DBL_FRACBITS;
			oldexp = u.dblu_dbl.dbl_exp;
		}
	}

	/*
	 * u.v is now normalized and oldexp has been adjusted if necessary.
	 * We have
	 *
	 *	0 <= oldexp <= DBL_EXP_INFNAN,
	 *
	 * but
	 *
	 *	INT_MIN <= expon <= INT_MAX.
	 *
	 * Check for underflow and overflow, and if none, calculate the
	 * new exponent.
	 */
	if (expon >= DBL_EXP_INFNAN - oldexp) {
		/*
		 * The result overflowed; return +/-Inf.
		 */
		return overflow(val);
	}

	/*
	 * We now have INT_MIN <= oldexp + expon <= DBL_EXP_INFNAN <= INT_MAX,
	 * so the arithmetic is safe.
	 */
	newexp = oldexp + expon;

	if (newexp <= 0) {
		/*
		 * The output number is either denormal or underflows (see
		 * comments in machine/ieee.h).
		 */
		if (newexp <= -DBL_FRACBITS)
			return underflow(val);
		/*
		 * Denormalize the result.  We do this with a multiply.  If
		 * expon is very large, it won't fit in a double, so we have
		 * to adjust the exponent first.  This is safe because we know
		 * that u.v is normal at this point.
		 */
		if (expon <= -DBL_EXP_BIAS) {
			u.dblu_dbl.dbl_exp = 1;
			expon += oldexp - 1;
		}
		mul.dblu_d = 0.0;
		mul.dblu_dbl.dbl_exp = expon + DBL_EXP_BIAS;
		u.dblu_d *= mul.dblu_d;
		return (u.dblu_d);
	} else {
		/*
		 * The result is normal; just replace the old exponent with the
		 * new one.
		 */
		u.dblu_dbl.dbl_exp = newexp;
		return (u.dblu_d);
	}
}