Slides/objects (#76)

* started slides for objects

* slides for objects

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@@ -28,3 +28,6 @@ Table of contents for the course.
 
 0. Values in VMs - [slides]({{< ref "/slides/06_vm_values" >}}),
    [doc](https://github.com/SofiaCPP/IPL/blob/master/website/content/slides/06_vm_values/_index.md)
+
+0. Objects in VMs - [slides]({{< ref "/slides/08_vm_objects" >}}),
+   [doc](https://github.com/SofiaCPP/IPL/blob/master/website/content/slides/08_vm_objects/_index.md)
diff --git a/website/content/slides/08_vm_objects/_index.md b/website/content/slides/08_vm_objects/_index.md
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+---
+title: "Objects in the VM"
+date: 2018-12-05T17:45:53+03:00
+draft: false
+outputs: ["Reveal"]
+---
+# Objects in the VM
+
+---
+## Contents
+
+1. JavaScript Language Features
+2. Getting properties
+3. Objects
+4. Optimizations
+4. 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](www.filpizlo.com/papers.html)
+
+---
+## 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:
+            // ...
+        }
+    }
+
+---
+### ?
+
+- 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;
+    };
+
+---
+# ?