usr/src/lib/libm/common/LD/cbrtl.c

   1 /*
   2  * CDDL HEADER START
   3  *
   4  * The contents of this file are subject to the terms of the
   5  * Common Development and Distribution License (the "License").
   6  * You may not use this file except in compliance with the License.
   7  *
   8  * You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE
   9  * or http://www.opensolaris.org/os/licensing.
  10  * See the License for the specific language governing permissions
  11  * and limitations under the License.
  12  *
  13  * When distributing Covered Code, include this CDDL HEADER in each
  14  * file and include the License file at usr/src/OPENSOLARIS.LICENSE.
  15  * If applicable, add the following below this CDDL HEADER, with the
  16  * fields enclosed by brackets "[]" replaced with your own identifying
  17  * information: Portions Copyright [yyyy] [name of copyright owner]
  18  *
  19  * CDDL HEADER END
  20  */
  21
  22 /*
  23  * Copyright 2011 Nexenta Systems, Inc.  All rights reserved.
  24  */
  25 /*
  26  * Copyright 2006 Sun Microsystems, Inc.  All rights reserved.
  27  * Use is subject to license terms.
  28  */
  29
  30 #pragma weak __cbrtl = cbrtl
  31
  32 #include "libm.h"
  33 #include "longdouble.h"
  34
  35 static const double d_one = 1.0;
  36
  37 long double
  38 cbrtl(long double x) {
  39         long double s, t, r, w, y;
  40         double dx, dy;
  41         int *py = (int *) &dy;
  42         int n, m, m3, n0, sx;
  43
  44         if (!finitel(x))
  45                 return (x + x);
  46         if (iszerol(x))
  47                 return (x);
  48         n0 = 0;
  49         if (*((int *) &d_one) == 0)
  50                 n0 = 1;
  51         sx = signbitl(x);
  52         x = fabsl(x);
  53         n = ilogbl(x);
  54         m = n / 3;
  55         m3 = m + m + m;
  56         y = scalbnl(x, -m3);
  57         dx = (double) y;
  58         dy = cbrt(dx);
  59         py[1 - n0] += 2;
  60         if (py[1 - n0] == 0)
  61                 py[n0] += 1;
  62
  63         /* one step newton iteration to 113 bits with error < 0.667ulps */
  64         t = (long double) dy;
  65         t = scalbnl(t, m);
  66         s = t * t;
  67         r = x / s;
  68         w = t + t;
  69         r = (r - t) / (w + r);
  70         t += t * r;
  71
  72         return (sx == 0 ? t : -t);
  73 }