Objects in the virtual machine

Representing objects inside the virtual machine.

Slideshow

Objects in the VM

Contents

1. JavaScript Language Features
2. Getting properties
3. Objects
4. Optimizations
5. Talking to the world

JavaScript Language Features

We will be discussing only EcmaScript 3 inheritance.

1. Prototype-based inheritance
2. Everything is an object

Inheritance

JavaScript is dynamically typed, so inheritance is about properties

console.log(answer.value)

Property access

When looking-up a property in a object:

1. Look for the property in the object, if not found
2. Look for the property in the object’s prototype, if not found
3. Look for the property in the object’s prototype’s prototype …

Value GetProperty(object, name) {
    Value result = object.GetProps().GetProperty(name);
    while (result == undefined && object.GetPrototype()) {
        object = object.GetPrototype();
        result = object.GetProps().GetProperty(name);
    }
    return result;
}

Properties

We need to map from string to a value.

struct Properties
{
    typedef std::unordered_map<StringValue* name, Value> PropertyMap;
    PropertyMap m_Props;

    Value GetProperty(StringValue* name) {
        const auto i = m_Props.find(name);
        return i != m_Props.end()? i->second : Value::kUndefined;
    }
};

Settting a property?

Set on the object directly

void SetProperty(Object* object, StringValue* name,
                 Value value)
{
    object->GetProps().Set(name, value);
}

Prototypes

struct Object {
    Value GetProperty(StringValue* name);
    Object* GetPrototype() { return m_Prototype; }
    Properties& GetProps() { return m_OwnProps;}

    Object* m_Prototype;
    Properties m_OwnProps;
};

Optimizations

Inline Cache

Skip the name look up for a property at the place where the look is done

var s = 0;
for (var i = 0; i < strings.length; ++i) {
    s += strings[i].length; // << Here is the interesting part
}

getprop $t, $strings, $i
getprop $r, $t, "length" // Goes in to the runtime and does the hash lookup
add $s, $s, $r

getprop $t, $strings, $i
if (typeof $t == "string") {
    $r = getfast($t, 0); // (size_t*)(*t+8);
} else {
    getprop $r, $t, "length" 
}

JSC impl

Hidden structures

Implementation

Enhancing Value

• toString
• getProperty
• setProperty

toString

Value Value::toString(Spasm* vm) const
{
    switch(get_type())
    {
        case Number:
        // ...
    }
}

getProperty

Value Value::getProperty(Spasm* vm, Value key) const
{
    switch(get_type())
    {
        case Number:
        // ...
    }
}

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• lots of duplication
• lots of casting

Visitor again

template <typename V>
typename V::ResultType Visit(V& v) {
    switch(get_type())
    {
        case Number: return v.Visit(get_double());
        case Null: return v.VisitNull();
        case Undefined: return v.VisitUndefined();
        case String: return v.Visit(get_string());
        // ...
    }
}

toString

struct ToStringVisitor
{
    typedef Value ResultType;

    Value Visit(double d) const;
    Value VisitUndefined() const;
    Value VisitNull() const;
    Value Visit(const StringValue& value) const;
    Value Visit(const ArrayValue& value) const;
    Value Visit(const ObjectValue& value) const;
    Value Visit(const FunctionValue& value) const;
};

Talking to the world

typedef Value (*Function)(Value thisObject, Value function, Value* args,
        size_t count, Value* exception)

typedef Value (*Getter)(Value thisObject, StringValue* name,
        Value* exception);

typedef void (*Setter)(Value thisObject, StringValue* name, Value value,
        Value* exception);

Property descriptors

Special kind of object that has:

struct PropertyDescriptor {
    Getter m_Getter;
    Setter m_Setter;
};

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