InferredValue.h   [plain text]

/*
 * Copyright (C) 2015 Apple Inc. All rights reserved.
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 * modification, are permitted provided that the following conditions
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 *    notice, this list of conditions and the following disclaimer in the
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 *
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 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
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#ifndef InferredValue_h
#define InferredValue_h

#include "JSCell.h"
#include "Watchpoint.h"
#include "WriteBarrier.h"

namespace JSC {

// Allocate one of these if you'd like to infer a constant value. Writes to the value should use
// notifyWrite(). So long as exactly one value had ever been written and invalidate() has never been
// called, and you register a watchpoint, you can rely on the inferredValue() being the one true
// value.
//
// Commonly used for inferring singletons - in that case each allocation does notifyWrite(). But you
// can use it for other things as well.

class InferredValue : public JSCell {
public:
    typedef JSCell Base;
    
    static InferredValue* create(VM&);
    
    static const bool needsDestruction = true;
    static void destroy(JSCell*);
    
    static Structure* createStructure(VM&, JSGlobalObject*, JSValue prototype);
    
    static void visitChildren(JSCell*, SlotVisitor&);
    
    DECLARE_INFO;
    
    // For the purpose of deciding whether or not to watch this variable, you only need
    // to inspect inferredValue(). If this returns something other than the empty
    // value, then it means that at all future safepoints, this watchpoint set will be
    // in one of these states:
    //
    //    IsWatched: in this case, the variable's value must still be the
    //        inferredValue.
    //
    //    IsInvalidated: in this case the variable's value may be anything but you'll
    //        either notice that it's invalidated and not install the watchpoint, or
    //        you will have been notified that the watchpoint was fired.
    JSValue inferredValue() { return m_value.get(); }

    // Forwards some WatchpointSet methods.
    WatchpointState state() const { return m_set.state(); }
    bool isStillValid() const { return m_set.isStillValid(); }
    bool hasBeenInvalidated() const { return m_set.hasBeenInvalidated(); }
    void add(Watchpoint* watchpoint) { m_set.add(watchpoint); }
    
    void notifyWrite(VM& vm, JSValue value, const FireDetail& detail)
    {
        if (LIKELY(m_set.stateOnJSThread() == IsInvalidated))
            return;
        notifyWriteSlow(vm, value, detail);
    }
    
    void notifyWrite(VM& vm, JSValue value, const char* reason)
    {
        if (LIKELY(m_set.stateOnJSThread() == IsInvalidated))
            return;
        notifyWriteSlow(vm, value, reason);
    }
    
    void invalidate(const FireDetail& detail)
    {
        m_value.clear();
        m_set.invalidate(detail);
    }
    
    static const unsigned StructureFlags = StructureIsImmortal | Base::StructureFlags;
    
    // We could have used Weak<>. But we want arbitrary JSValues, not just cells. It's also somewhat
    // convenient to have eager notification of death.
    //
    // Also note that this should be a private class, but it isn't because Windows.
    class ValueCleanup : public UnconditionalFinalizer {
        WTF_MAKE_FAST_ALLOCATED;
        
    public:
        ValueCleanup(InferredValue*);
        virtual ~ValueCleanup();
        
    protected:
        void finalizeUnconditionally() override;
        
    private:
        InferredValue* m_owner;
    };
    
private:
    InferredValue(VM&);
    ~InferredValue();
    
    JS_EXPORT_PRIVATE void notifyWriteSlow(VM&, JSValue, const FireDetail&);
    JS_EXPORT_PRIVATE void notifyWriteSlow(VM&, JSValue, const char* reason);
    
    friend class ValueCleanup;
    
    InlineWatchpointSet m_set;
    WriteBarrier<Unknown> m_value;
    std::unique_ptr<ValueCleanup> m_cleanup;
};

// FIXME: We could have an InlineInferredValue, which only allocates the InferredValue object when
// a notifyWrite() transitions us towards watching, and then clears the reference (allowing the object
// to die) when we get invalidated.

} // namespace JSC

#endif // InferredValue_h