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 /*
  27  * Copyright 2006 Sun Microsystems, Inc.  All rights reserved.
  28  * Use is subject to license terms.
  29  */
  30 
  31 #pragma weak __log1pl = log1pl
  32 
  33 /*
  34  * log1pl(x)
  35  * Kahan's trick based on log(1+x)/x being a slow varying function.
  36  */
  37 
  38 #include "libm.h"
  39 
  40 #if defined(__x86)
  41 #define __swapRD        __swap87RD
  42 #endif
  43 
  44 extern enum fp_direction_type __swapRD(enum fp_direction_type);
  45 
  46 long double
  47 log1pl(long double x)
  48 {
  49         long double y;
  50         enum fp_direction_type rd;
  51 
  52         if (x != x)
  53                 return (x + x);
  54 
  55         if (x < -1.L)
  56                 return (logl(x));
  57 
  58         rd = __swapRD(fp_nearest);
  59         y = 1.L + x;
  60 
  61         if (y != 1.L) {
  62                 if (y == x)
  63                         x = logl(x);
  64                 else
  65                         x *= logl(y) / (y - 1.L);
  66         }
  67 
  68         if (rd != fp_nearest)
  69                 (void) __swapRD(rd);
  70 
  71         return (x);
  72 }