uox3/spidermonkey/jslog2.cpp

/* -*- Mode: C++; tab-width: 4; indent-tabs-mode: nil; c-basic-offset: 4 -*- */
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 * Version: MPL 1.1/GPL 2.0/LGPL 2.1
 *
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 * 1.1 (the "License"); you may not use this file except in compliance with
 * the License. You may obtain a copy of the License at
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 *
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 * WITHOUT WARRANTY OF ANY KIND, either express or implied. See the License
 * for the specific language governing rights and limitations under the
 * License.
 *
 * The Original Code is Mozilla Communicator client code, released
 * March 31, 1998.
 *
 * The Initial Developer of the Original Code is
 * Netscape Communications Corporation.
 * Portions created by the Initial Developer are Copyright (C) 1998
 * the Initial Developer. All Rights Reserved.
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 * Contributor(s):
 *
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 * either of the GNU General Public License Version 2 or later (the "GPL"),
 * or the GNU Lesser General Public License Version 2.1 or later (the "LGPL"),
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#include "jsstdint.h"
#include "jsbit.h"
#include "jsutil.h"

/*
 * Check that we can use js_bitscan_clz32 to implement JS_FLOOR_LOG2 and
 * JS_FLOOR_LOG2W and js_bitscan_clz64 to implement JS_FLOOR_LOG2W on 64-bit
 * systems.
 */
#ifdef JS_HAS_BUILTIN_BITSCAN32
JS_STATIC_ASSERT(sizeof(unsigned int) == sizeof(JSUint32));
JS_STATIC_ASSERT_IF(JS_BYTES_PER_WORD == 4,
                    sizeof(unsigned int) == sizeof(JSUword));
#endif
#ifdef JS_HAS_BUILTIN_BITSCAN64
JS_STATIC_ASSERT_IF(JS_BYTES_PER_WORD == 8,
                    sizeof(unsigned long long) == sizeof(JSUword));
#endif

/*
 * Compute the log of the least power of 2 greater than or equal to n
 */
JS_PUBLIC_API(JSIntn)
JS_CeilingLog2(JSUint32 n)
{
    JSIntn log2;

    JS_CEILING_LOG2(log2, n);
    return log2;
}

/*
 * Compute the log of the greatest power of 2 less than or equal to n.
 * This really just finds the highest set bit in the word.
 */
JS_PUBLIC_API(JSIntn)
JS_FloorLog2(JSUint32 n)
{
    JSIntn log2;

    JS_FLOOR_LOG2(log2, n);
    return log2;
}

/*
 * js_FloorLog2wImpl has to be defined only for 64-bit non-GCC case.
 */
#if !defined(JS_HAS_BUILTIN_BITSCAN64) && JS_BYTES_PER_WORD == 8

size_t
js_FloorLog2wImpl(size_t n)
{
    size_t log2, m;

    JS_ASSERT(n != 0);

    log2 = 0;
    m = n >> 32;
    if (m != 0) { n = m; log2 = 32; }
    m = n >> 16;
    if (m != 0) { n = m; log2 |= 16; }
    m = n >> 8;
    if (m != 0) { n = m; log2 |= 8; }
    m = n >> 4;
    if (m != 0) { n = m; log2 |= 4; }
    m = n >> 2;
    if (m != 0) { n = m; log2 |= 2; }
    log2 |= (n >> 1);

    return log2;
}

#endif