common/C/scalbn.c

/*
 * 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 __scalbn = scalbn

#include "libm.h"

static const double
    one = 1.0,
    huge    = 1.0e300,
    tiny    = 1.0e-300,
    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
scalbn(double x, int n) {
    int *px, ix, hx, k;

    px = (int *)&x;
    ix = px[HIWORD];
    hx = ix & ~0x80000000;
    k = hx >> 20;

    if (k == 0x7ff) /* x is inf or NaN */
        return (x * one);

    if (k == 0) {
        if ((hx | px[LOWORD]) == 0 || n == 0)
            return (x);
#if defined(__x86)
        x *= two52;
        ix = px[HIWORD];
        k = ((ix & ~0x80000000) >> 20) - 52;
#else
        k = ilogb_biased((unsigned *)px);
        ix = px[HIWORD];
#endif
        /* now k is in the range -51..0 */
        k += n;
        if (k > n)  /* integer overflow occurred */
            k = -100;
    } else {
        /* k is in the range 1..1023 */
        k += n;
        if (k < n)  /* integer overflow occurred */
            k = 0x7ff;
    }

    if (k > 0x7fe)
        return (huge * ((ix < 0)? -huge : huge));
    if (k < 1) {
        if (k <= -54)
            return (tiny * ((ix < 0)? -tiny : tiny));
        k += 54;
        px[HIWORD] = (ix & ~0x7ff00000) | (k << 20);
        return (x * twom54);
    }
    px[HIWORD] = (ix & ~0x7ff00000) | (k << 20);
    return (x);
}