usr/src/lib/libm/common/C/scalbn.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  * Copyright 2011 Nexenta Systems, Inc.  All rights reserved.
  23  */
  24 /*
  25  * Copyright 2006 Sun Microsystems, Inc.  All rights reserved.
  26  * Use is subject to license terms.
  27  */
  28
  29 #pragma weak __scalbn = scalbn
  30
  31 #include "libm.h"
  32
  33 static const double
  34         one     = 1.0,
  35         huge    = 1.0e300,
  36         tiny    = 1.0e-300,
  37         twom54  = 5.5511151231257827021181583404541015625e-17;
  38
  39 #if defined(__x86)
  40 static const double two52 = 4503599627370496.0;
  41 #else
  42 /*
  43  * Normalize non-zero subnormal x and return biased exponent of x in [-51,0]
  44  */
  45 static int
  46 ilogb_biased(unsigned *px) {
  47         int s = 52;
  48         unsigned v = px[HIWORD] & ~0x80000000, w = px[LOWORD], t = v;
  49
  50         if (t)
  51                 s -= 32;
  52         else
  53                 t = w;
  54         if (t & 0xffff0000)
  55                 s -= 16, t >>= 16;
  56         if (t & 0xff00)
  57                 s -= 8, t >>= 8;
  58         if (t & 0xf0)
  59                 s -= 4, t >>= 4;
  60         t <<= 1;
  61         s -= (0xffffaa50 >> t) & 0x3;
  62         if (s < 32) {
  63                 v = (v << s) | w >> (32 - s);
  64                 w <<= s;
  65         } else {
  66                 v = w << (s - 32);
  67                 w = 0;
  68         }
  69         px[HIWORD] = (px[HIWORD] & 0x80000000) | v;
  70         px[LOWORD] = w;
  71         return (1 - s);
  72 }
  73 #endif  /* defined(__x86) */
  74
  75 double
  76 scalbn(double x, int n) {
  77         int     *px, ix, hx, k;
  78
  79         px = (int *)&x;
  80         ix = px[HIWORD];
  81         hx = ix & ~0x80000000;
  82         k = hx >> 20;
  83
  84         if (k == 0x7ff) /* x is inf or NaN */
  85                 return (x * one);
  86
  87         if (k == 0) {
  88                 if ((hx | px[LOWORD]) == 0 || n == 0)
  89                         return (x);
  90 #if defined(__x86)
  91                 x *= two52;
  92                 ix = px[HIWORD];
  93                 k = ((ix & ~0x80000000) >> 20) - 52;
  94 #else
  95                 k = ilogb_biased((unsigned *)px);
  96                 ix = px[HIWORD];
  97 #endif
  98                 /* now k is in the range -51..0 */
  99                 k += n;
 100                 if (k > n)      /* integer overflow occurred */
 101                         k = -100;
 102         } else {
 103                 /* k is in the range 1..1023 */
 104                 k += n;
 105                 if (k < n)      /* integer overflow occurred */
 106                         k = 0x7ff;
 107         }
 108
 109         if (k > 0x7fe)
 110                 return (huge * ((ix < 0)? -huge : huge));
 111         if (k < 1) {
 112                 if (k <= -54)
 113                         return (tiny * ((ix < 0)? -tiny : tiny));
 114                 k += 54;
 115                 px[HIWORD] = (ix & ~0x7ff00000) | (k << 20);
 116                 return (x * twom54);
 117         }
 118         px[HIWORD] = (ix & ~0x7ff00000) | (k << 20);
 119         return (x);
 120 }