usr/src/lib/libm/common/C/logb.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 __logb = logb
  31 #pragma weak _logb = logb
  32
  33 #include "libm.h"
  34 #include "xpg6.h"       /* __xpg6 */
  35 #define _C99SUSv3_logb  _C99SUSv3_logb_subnormal_is_like_ilogb
  36
  37 #if defined(__x86)
  38 static const double two52 = 4503599627370496.0;
  39 #else
  40 /*
  41  * v: high part of a non-zero subnormal |x|; w: low part of |x|
  42  */
  43 static int
  44 ilogb_subnormal(unsigned v, unsigned w) {
  45         int r = -1022 - 52;
  46
  47         if (v)
  48                 r += 32;
  49         else
  50                 v = w;
  51         if (v & 0xffff0000)
  52                 r += 16, v >>= 16;
  53         if (v & 0xff00)
  54                 r += 8, v >>= 8;
  55         if (v & 0xf0)
  56                 r += 4, v >>= 4;
  57         v <<= 1;
  58         return (r + ((0xffffaa50 >> v) & 0x3));
  59 }
  60 #endif  /* defined(__x86) */
  61
  62 double
  63 logb(double x) {
  64         int *px = (int *) &x, k = px[HIWORD] & ~0x80000000;
  65
  66         if (k < 0x00100000) {
  67                 if ((px[LOWORD] | k) == 0)
  68                         return (_SVID_libm_err(x, x, 45));
  69                 else if ((__xpg6 & _C99SUSv3_logb) != 0) {
  70 #if defined(__x86)
  71                         x *= two52;
  72                         return ((double) (((px[HIWORD] & 0x7ff00000) >> 20)
  73                                 - 1075));
  74 #else
  75                         return ((double) ilogb_subnormal(k, px[LOWORD]));
  76 #endif
  77                 } else
  78                         return (-1022.0);
  79         } else if (k < 0x7ff00000)
  80                 return ((double) ((k >> 20) - 1023));
  81         else
  82                 return (x * x);
  83 }