usr/src/lib/libm/common/Q/log10l.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 __log10l = log10l
  31
  32 /*
  33  * log10l(X)
  34  *
  35  * Method :
  36  *      Let log10_2hi = leading 98(SPARC)/49(x86) bits of log10(2) and
  37  *          log10_2lo = log10(2) - log10_2hi,
  38  *          ivln10   = 1/log(10) rounded.
  39  *      Then
  40  *              n = ilogb(x),
  41  *              if (n<0)  n = n+1;
  42  *              x = scalbn(x,-n);
  43  *              LOG10(x) := n*log10_2hi + (n*log10_2lo + ivln10*log(x))
  44  *
  45  * Note1:
  46  *      For fear of destroying log10(10**n)=n, the rounding mode is
  47  *      set to Round-to-Nearest.
  48  *
  49  * Special cases:
  50  *      log10(x) is NaN with signal if x < 0;
  51  *      log10(+INF) is +INF with no signal; log10(0) is -INF with signal;
  52  *      log10(NaN) is that NaN with no signal;
  53  *      log10(10**N) = N  for N=0,1,...,22.
  54  *
  55  * Constants:
  56  * The hexadecimal values are the intended ones for the following constants.
  57  * The decimal values may be used, provided that the compiler will convert
  58  * from decimal to binary accurately enough to produce the hexadecimal values
  59  * shown.
  60  */
  61
  62 #include "libm.h"
  63 #include "longdouble.h"
  64
  65 #if defined(__x86)
  66 #define __swapRD        __swap87RD
  67 #endif
  68 extern enum fp_direction_type __swapRD(enum fp_direction_type);
  69
  70 static const long double
  71         zero      = 0.0L,
  72         ivln10    = 4.342944819032518276511289189166050822944e-0001L,
  73         one       = 1.0L,
  74 #if defined(__x86)
  75         log10_2hi = 3.010299956639803653501985536422580480576e-01L,
  76         log10_2lo = 8.298635403410822349787106337291183585413e-16L;
  77 #elif defined(__sparc)
  78         log10_2hi = 3.010299956639811952137388947242098603469e-01L,
  79         log10_2lo = 2.831664213089468167896664371953210945664e-31L;
  80 #else
  81 #error Unknown Architecture!
  82 #endif
  83
  84 long double
  85 log10l(long double x) {
  86         long double y, z;
  87         enum fp_direction_type rd;
  88         int n;
  89
  90         if (!finitel(x))
  91                 return (x + fabsl(x));  /* x is +-INF or NaN */
  92         else if (x > zero) {
  93                 n = ilogbl(x);
  94                 if (n < 0)
  95                         n += 1;
  96                 rd = __swapRD(fp_nearest);
  97                 y = n;
  98                 x = scalbnl(x, -n);
  99                 z = y * log10_2lo + ivln10 * logl(x);
 100                 z += y * log10_2hi;
 101                 if (rd != fp_nearest)
 102                         (void) __swapRD(rd);
 103                 return (z);
 104         } else if (x == zero)   /* -INF */
 105                 return (-one / zero);
 106         else                    /* x <0, return NaN */
 107                 return (zero / zero);
 108 }