Source/JavaScriptCore/bytecode/ArrayAllocationProfile.cpp

/*
 * Copyright (C) 2012, 2013 Apple Inc. All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
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 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 *
 * THIS SOFTWARE IS PROVIDED BY APPLE INC. ``AS IS'' AND ANY
 * EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
 * PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL APPLE INC. OR
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#include "config.h"
#include "ArrayAllocationProfile.h"

#include "JSCInlines.h"

namespace JSC {

void ArrayAllocationProfile::updateIndexingType()
{
    // This is awkwardly racy but totally sound even when executed concurrently. The
    // worst cases go something like this:
    //
    // - Two threads race to execute this code; one of them succeeds in updating the
    //   m_currentIndexingType and the other either updates it again, or sees a null
    //   m_lastArray; if it updates it again then at worst it will cause the profile
    //   to "forget" some array. That's still sound, since we don't promise that
    //   this profile is a reflection of any kind of truth.
    //
    // - A concurrent thread reads m_lastArray, but that array is now dead. While
    //   it's possible for that array to no longer be reachable, it cannot actually
    //   be freed, since we require the GC to wait until all concurrent JITing
    //   finishes.
    
    JSArray* lastArray = m_lastArray;
    if (!lastArray)
        return;
    m_currentIndexingType = leastUpperBoundOfIndexingTypes(m_currentIndexingType, lastArray->indexingType());
    m_lastArray = 0;
}

} // namespace JSC