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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  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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  37  * ***** END LICENSE BLOCK ***** */
  38
  39 #include "jsstdint.h"
  40 #include "jsbit.h"
  41 #include "jsutil.h"
  42
  43 /*
  44  * Check that we can use js_bitscan_clz32 to implement JS_FLOOR_LOG2 and
  45  * JS_FLOOR_LOG2W and js_bitscan_clz64 to implement JS_FLOOR_LOG2W on 64-bit
  46  * systems.
  47  */
  48 #ifdef JS_HAS_BUILTIN_BITSCAN32
  49 JS_STATIC_ASSERT(sizeof(unsigned int) == sizeof(JSUint32));
  50 JS_STATIC_ASSERT_IF(JS_BYTES_PER_WORD == 4,
  51                     sizeof(unsigned int) == sizeof(JSUword));
  52 #endif
  53 #ifdef JS_HAS_BUILTIN_BITSCAN64
  54 JS_STATIC_ASSERT_IF(JS_BYTES_PER_WORD == 8,
  55                     sizeof(unsigned long long) == sizeof(JSUword));
  56 #endif
  57
  58 /*
  59  * Compute the log of the least power of 2 greater than or equal to n
  60  */
  61 JS_PUBLIC_API(JSIntn)
  62 JS_CeilingLog2(JSUint32 n)
  63 {
  64     JSIntn log2;
  65
  66     JS_CEILING_LOG2(log2, n);
  67     return log2;
  68 }
  69
  70 /*
  71  * Compute the log of the greatest power of 2 less than or equal to n.
  72  * This really just finds the highest set bit in the word.
  73  */
  74 JS_PUBLIC_API(JSIntn)
  75 JS_FloorLog2(JSUint32 n)
  76 {
  77     JSIntn log2;
  78
  79     JS_FLOOR_LOG2(log2, n);
  80     return log2;
  81 }
  82
  83 /*
  84  * js_FloorLog2wImpl has to be defined only for 64-bit non-GCC case.
  85  */
  86 #if !defined(JS_HAS_BUILTIN_BITSCAN64) && JS_BYTES_PER_WORD == 8
  87
  88 size_t
  89 js_FloorLog2wImpl(size_t n)
  90 {
  91     size_t log2, m;
  92
  93     JS_ASSERT(n != 0);
  94
  95     log2 = 0;
  96     m = n >> 32;
  97     if (m != 0) { n = m; log2 = 32; }
  98     m = n >> 16;
  99     if (m != 0) { n = m; log2 |= 16; }
 100     m = n >> 8;
 101     if (m != 0) { n = m; log2 |= 8; }
 102     m = n >> 4;
 103     if (m != 0) { n = m; log2 |= 4; }
 104     m = n >> 2;
 105     if (m != 0) { n = m; log2 |= 2; }
 106     log2 |= (n >> 1);
 107
 108     return log2;
 109 }
 110
 111 #endif