js/src/jslog2.cpp

   1 /* -*- Mode: C++; tab-width: 4; indent-tabs-mode: nil; c-basic-offset: 4 -*- */
   2 /* ***** BEGIN LICENSE BLOCK *****
   3  * Version: MPL 1.1/GPL 2.0/LGPL 2.1
   4  *
   5  * The contents of this file are subject to the Mozilla Public License Version
   6  * 1.1 (the "License"); you may not use this file except in compliance with
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   8  * http://www.mozilla.org/MPL/
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  11  * WITHOUT WARRANTY OF ANY KIND, either express or implied. See the License
  12  * for the specific language governing rights and limitations under the
  13  * License.
  14  *
  15  * The Original Code is Mozilla Communicator client code, released
  16  * March 31, 1998.
  17  *
  18  * The Initial Developer of the Original Code is
  19  * Netscape Communications Corporation.
  20  * Portions created by the Initial Developer are Copyright (C) 1998
  21  * the Initial Developer. All Rights Reserved.
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  25  * Alternatively, the contents of this file may be used under the terms of
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  27  * or the GNU Lesser General Public License Version 2.1 or later (the "LGPL"),
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  37  * ***** END LICENSE BLOCK ***** */
  38
  39 #include "jsstddef.h"
  40 #include "jsbit.h"
  41 #include "jsutil.h"
  42
  43 /*
  44 ** Compute the log of the least power of 2 greater than or equal to n
  45 */
  46 JS_PUBLIC_API(JSIntn) JS_CeilingLog2(JSUint32 n)
  47 {
  48     JSIntn log2;
  49
  50     JS_CEILING_LOG2(log2, n);
  51     return log2;
  52 }
  53
  54 /*
  55 ** Compute the log of the greatest power of 2 less than or equal to n.
  56 ** This really just finds the highest set bit in the word.
  57 */
  58 JS_PUBLIC_API(JSIntn) JS_FloorLog2(JSUint32 n)
  59 {
  60     JSIntn log2;
  61
  62     JS_FLOOR_LOG2(log2, n);
  63     return log2;
  64 }
  65
  66 /*
  67  * js_FloorLog2wImpl has to be defined only for 64-bit non-GCC case.
  68  */
  69 #if !defined(JS_HAS_BUILTIN_BITSCAN64) && JS_BYTES_PER_WORD == 8
  70
  71 JSUword
  72 js_FloorLog2wImpl(JSUword n)
  73 {
  74     JSUword log2, m;
  75
  76     JS_ASSERT(n != 0);
  77
  78     log2 = 0;
  79     m = n >> 32;
  80     if (m != 0) { n = m; log2 = 32; }
  81     m = n >> 16;
  82     if (m != 0) { n = m; log2 |= 16; }
  83     m = n >> 8;
  84     if (m != 0) { n = m; log2 |= 8; }
  85     m = n >> 4;
  86     if (m != 0) { n = m; log2 |= 4; }
  87     m = n >> 2;
  88     if (m != 0) { n = m; log2 |= 2; }
  89     log2 |= (n >> 1);
  90
  91     return log2;
  92 }
  93
  94 #endif