JSInternalFieldObjectImpl.h

/*
 * Copyright (C) 2019-2021 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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 * 1. Redistributions of source code must retain the above copyright
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 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 *
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 * THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
 * PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL APPLE INC. OR ITS CONTRIBUTORS
 * BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
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 * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
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 */

#pragma once

#include "JSObject.h"
#include <wtf/Atomics.h>

WTF_ALLOW_UNSAFE_BUFFER_USAGE_BEGIN

namespace JSC {


// This is used for sharing interface and implementation. It should not have its own classInfo.
template<unsigned passedNumberOfInternalFields = 1>
class JSInternalFieldObjectImpl : public JSNonFinalObject {
public:
    friend class LLIntOffsetsExtractor;

    using Base = JSNonFinalObject;
    static constexpr unsigned numberOfInternalFields = passedNumberOfInternalFields;

    template<typename CellType, SubspaceAccess>
    static void subspaceFor(VM&)
    {
        RELEASE_ASSERT_NOT_REACHED();
    }

    static size_t allocationSize(Checked<size_t> inlineCapacity)
    {
        ASSERT_UNUSED(inlineCapacity, !inlineCapacity);
        return sizeof(JSInternalFieldObjectImpl);
    }

    const WriteBarrier<Unknown>& internalField(unsigned index) const
    {
        ASSERT(index < numberOfInternalFields);
        return m_internalFields[index];
    }

    WriteBarrier<Unknown>& internalField(unsigned index)
    {
        ASSERT(index < numberOfInternalFields);
        return m_internalFields[index];
    }

    // SPEC-ungil §N.5 (annex N7 row R7): the generic atomic word view onto an
    // internal field, for the resume-claim CAS / release-publish primitives
    // (@atomicInternalFieldClaim/Publish in the spec; landed as host hooks —
    // see SPEC-ungil-history.md "§N.5 LANDED SHAPE"). Layout-only: every
    // WriteBarrier<Unknown> slot is exactly one EncodedJSValue word and
    // naturally aligned, so the bit_cast is the same word op_get/put_
    // internal_field touches in every tier. No write-barrier is implied — the
    // §N.5 claim word holds int32 jsNumbers only (states + tokens), never a
    // cell; callers storing a cell through this view would need their own
    // barrier. Flag-off byte-identical: the accessor is dead code (every
    // caller is behind the @gilOffProcess constant branch / vm.gilOff()).
    Atomic<EncodedJSValue>& atomicInternalField(unsigned index)
    {
        static_assert(sizeof(WriteBarrier<Unknown>) == sizeof(EncodedJSValue));
        static_assert(alignof(WriteBarrier<Unknown>) >= alignof(EncodedJSValue));
        ASSERT(index < numberOfInternalFields);
        return *std::bit_cast<Atomic<EncodedJSValue>*>(&m_internalFields[index]);
    }

    static constexpr ptrdiff_t offsetOfInternalFields() { return OBJECT_OFFSETOF(JSInternalFieldObjectImpl, m_internalFields); }
    static constexpr ptrdiff_t offsetOfInternalField(unsigned index) { return OBJECT_OFFSETOF(JSInternalFieldObjectImpl, m_internalFields) + index * sizeof(WriteBarrier<Unknown>); }

    DECLARE_VISIT_CHILDREN;

protected:
    JSInternalFieldObjectImpl(VM& vm, Structure* structure)
        : Base(vm, structure)
    {
    }

    WriteBarrier<Unknown> m_internalFields[numberOfInternalFields] { };
};

} // namespace JSC

WTF_ALLOW_UNSAFE_BUFFER_USAGE_END