usr/src/lib/libm/common/C/ilogb.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 __ilogb = ilogb
  31
  32 #include "libm.h"
  33 #include "xpg6.h"       /* __xpg6 */
  34
  35 #if defined(__x86)
  36 static const double two52 = 4503599627370496.0;
  37 #else
  38 /*
  39  * v: high part of a non-zero subnormal |x|; w: low part of |x|
  40  */
  41 static int
  42 ilogb_subnormal(unsigned v, unsigned w) {
  43         int r = -1022 - 52;
  44
  45         if (v)
  46                 r += 32;
  47         else
  48                 v = w;
  49         if (v & 0xffff0000)
  50                 r += 16, v >>= 16;
  51         if (v & 0xff00)
  52                 r += 8, v >>= 8;
  53         if (v & 0xf0)
  54                 r += 4, v >>= 4;
  55         v <<= 1;
  56         return (r + ((0xffffaa50 >> v) & 0x3));
  57 }
  58 #endif  /* defined(__x86) */
  59
  60 static int
  61 raise_invalid(int v) {  /* SUSv3 requires ilogb(0,+/-Inf,NaN) raise invalid */
  62 #ifndef lint
  63         if ((__xpg6 & _C99SUSv3_ilogb_0InfNaN_raises_invalid) != 0) {
  64                 static const double zero = 0.0;
  65                 volatile double dummy __unused;
  66
  67                 dummy = zero / zero;
  68         }
  69 #endif
  70         return (v);
  71 }
  72
  73 int
  74 ilogb(double x) {
  75         int *px = (int *) &x, k = px[HIWORD] & ~0x80000000;
  76
  77         if (k < 0x00100000) {
  78                 if ((px[LOWORD] | k) == 0)
  79                         return (raise_invalid(0x80000001));
  80                 else {
  81 #if defined(__x86)
  82                         x *= two52;
  83                         return (((px[HIWORD] & 0x7ff00000) >> 20) - 1075);
  84 #else
  85                         return (ilogb_subnormal(k, px[LOWORD]));
  86 #endif
  87                 }
  88         } else if (k < 0x7ff00000)
  89                 return ((k >> 20) - 1023);
  90         else
  91                 return (raise_invalid(0x7fffffff));
  92 }