Source/JavaScriptCore/runtime/JSNonDestructibleProxy.h

/*
 * Copyright (C) 2019 Apple Inc. All rights reserved.
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 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
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 * THIS SOFTWARE IS PROVIDED BY APPLE INC. ``AS IS'' AND ANY
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#pragma once

#include "JSProxy.h"

namespace JSC {

class JSNonDestructibleProxy : public JSProxy {
public:
    using Base = JSProxy;
    static constexpr unsigned StructureFlags = Base::StructureFlags;
    static constexpr bool needsDestruction = false;

    template<typename CellType, SubspaceAccess mode>
    static CompleteSubspace* subspaceFor(VM& vm)
    {
        // JSProxy is JSDestrucitbleObject, but we make this JSNonDestructibleProxy non-destructible by using non-destructible subspace.
        // The motivation behind this is (1) except for JSWindowProxy JSProxy does not need to be destructible, and (2) subspace of destructible
        // and non-destructible objects are separated and JSProxy is using one MarkedBlock only for JSProxy class in the JSC framework and wasting memory.
        // Basically, to make objects destructible, objects need to inherit JSDestructibleObject. It holds a classInfo at a specific offset
        // so that Heap can get methodTable::destroy even if structures held by objects are destroyed before objects' destructions. But this
        // requirement forces JSProxy to inherit JSDestructibleObject for JSWindowProxy even while the other JSProxy does not need to be
        // destructible. We create JSNonDestructibleProxy, which is a subclass of JSProxy, and make it non-destructible so that we still keep
        // JSWindowProxy destructible while making JSNonDestructibleProxy non-destructible.
        return JSNonFinalObject::subspaceFor<CellType, mode>(vm);
    }

    static JSNonDestructibleProxy* create(VM& vm, Structure* structure, JSObject* target)
    {
        JSNonDestructibleProxy* proxy = new (NotNull, allocateCell<JSNonDestructibleProxy>(vm.heap)) JSNonDestructibleProxy(vm, structure);
        proxy->finishCreation(vm, target);
        return proxy;
    }

    static JSNonDestructibleProxy* create(VM& vm, Structure* structure)
    {
        JSNonDestructibleProxy* proxy = new (NotNull, allocateCell<JSNonDestructibleProxy>(vm.heap)) JSNonDestructibleProxy(vm, structure);
        proxy->finishCreation(vm);
        return proxy;
    }

    static Structure* createStructure(VM& vm, JSGlobalObject* globalObject, JSValue prototype, JSType proxyType)
    {
        ASSERT(proxyType == ImpureProxyType || proxyType == PureForwardingProxyType);
        return Structure::create(vm, globalObject, prototype, TypeInfo(proxyType, StructureFlags), info());
    }

    DECLARE_EXPORT_INFO;

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

} // namespace JSC