xref: /illumos-gate/usr/src/lib/libm/common/m9x/scalbln.c (revision 1ec68d33)

/*
 * CDDL HEADER START
 *
 * The contents of this file are subject to the terms of the
 * Common Development and Distribution License (the "License").
 * You may not use this file except in compliance with the License.
 *
 * You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE
 * or http://www.opensolaris.org/os/licensing.
 * See the License for the specific language governing permissions
 * and limitations under the License.
 *
 * When distributing Covered Code, include this CDDL HEADER in each
 * file and include the License file at usr/src/OPENSOLARIS.LICENSE.
 * If applicable, add the following below this CDDL HEADER, with the
 * fields enclosed by brackets "[]" replaced with your own identifying
 * information: Portions Copyright [yyyy] [name of copyright owner]
 *
 * CDDL HEADER END
 */

/*
 * Copyright 2011 Nexenta Systems, Inc.  All rights reserved.
 */
/*
 * Copyright 2006 Sun Microsystems, Inc.  All rights reserved.
 * Use is subject to license terms.
 */

#pragma weak scalbln = __scalbln

#include "libm.h"
#include <float.h>		/* DBL_MAX, DBL_MIN */

static const double twom54 = 5.5511151231257827021181583404541015625e-17;
#if defined(__x86)
static const double two52 = 4503599627370496.0;
#else
/*
 * Normalize non-zero subnormal x and return biased exponent of x in [-51,0]
 */
static int
ilogb_biased(unsigned *px) {
	int s = 52;
	unsigned v = px[HIWORD] & ~0x80000000, w = px[LOWORD], t = v;

	if (t)
		s -= 32;
	else
		t = w;
	if (t & 0xffff0000)
		s -= 16, t >>= 16;
	if (t & 0xff00)
		s -= 8, t >>= 8;
	if (t & 0xf0)
		s -= 4, t >>= 4;
	t <<= 1;
	s -= (0xffffaa50 >> t) & 0x3;
	if (s < 32) {
		v = (v << s) | w >> (32 - s);
		w <<= s;
	} else {
		v = w << (s - 32);
		w = 0;
	}
	px[HIWORD] = (px[HIWORD] & 0x80000000) | v;
	px[LOWORD] = w;
	return (1 - s);
}
#endif	/* defined(__x86) */

double
scalbln(double x, long n) {
	int *px = (int *) &x, ix, k;

	ix = px[HIWORD] & ~0x80000000;
	k = ix >> 20;
	if (k == 0x7ff)
#if defined(FPADD_TRAPS_INCOMPLETE_ON_NAN)
		return ((px[HIWORD] & 0x80000) != 0 ? x : x + x);
		/* assumes sparc-like QNaN */
#else
		return (x + x);
#endif
	if ((px[LOWORD] | ix) == 0 || n == 0)
		return (x);
	if (k == 0) {
#if defined(__x86)
		x *= two52;
		k = ((px[HIWORD] & ~0x80000000) >> 20) - 52;
#else
		k = ilogb_biased((unsigned *) px);
#endif
	}
	k += (int) n;
	if (n > 5000 || k > 0x7fe)
		return (DBL_MAX * copysign(DBL_MAX, x));
	if (n < -5000 || k <= -54)
		return (DBL_MIN * copysign(DBL_MIN, x));
	if (k > 0) {
		px[HIWORD] = (px[HIWORD] & ~0x7ff00000) | (k << 20);
		return (x);
	}
	k += 54;
	px[HIWORD] = (px[HIWORD] & ~0x7ff00000) | (k << 20);
	return (x * twom54);
}