Source/JavaScriptCore/bytecode/ArrayAllocationProfile.cpp

/*
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#include "config.h"
#include "ArrayAllocationProfile.h"

#include "JSCInlines.h"

#include <algorithm>

namespace JSC {

void ArrayAllocationProfile::updateProfile()
{
    // 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.
    //
    // But one exception is vector length. We access vector length to get the vector
    // length hint. However vector length can be accessible only from the main
    // thread because large butterfly can be realloced in the main thread.
    // So for now, we update the allocation profile only from the main thread.
    
    ASSERT(!isCompilationThread());
    JSArray* lastArray = m_lastArray;
    if (!lastArray)
        return;
    if (LIKELY(Options::useArrayAllocationProfiling())) {
        // The basic model here is that we will upgrade ourselves to whatever the CoW version of lastArray is except ArrayStorage since we don't have CoW ArrayStorage.
        IndexingType indexingType = leastUpperBoundOfIndexingTypes(m_currentIndexingType & IndexingTypeMask, lastArray->indexingType());
        if (isCopyOnWrite(m_currentIndexingType)) {
            if (indexingType > ArrayWithContiguous)
                indexingType = ArrayWithContiguous;
            indexingType |= CopyOnWrite;
        }
        m_currentIndexingType = indexingType;
        m_largestSeenVectorLength = std::min(std::max(m_largestSeenVectorLength, lastArray->getVectorLength()), BASE_CONTIGUOUS_VECTOR_LEN_MAX);
    }
    m_lastArray = nullptr;
}

} // namespace JSC