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# Saturday, December 17, 2011

Yesterday, by accident, we've seen an article about some design principles of V8 JavaScript Engine. It made clearer what techniques are used in today's script implementations.

In particular V8 engine optimizes property access using "dynamically created hidden classes". These are structures to store object's layout, they are derived when ש new property is created (deleted) on the object. When code accesses a property, and if a cached object's dynamic hidden class is available at the code point then access time is comparable to one of native fields.

In our opinion this tactics might lead to a proliferation of such dynamic hidden classes, which requires a considerable housekeeping, which also slows property write access, especially when it's written for the first time.

We would like to suggest a slightly different strategy, which exploits the cache matches, and does not require a dynamic hidden classes.

Consider an implementation data type with following characteristics:

  • object is implemented as a hash map of property id to property value: Map<ID, Value>;
  • it stores data as an array of pairs and can be accessed directly: Pair<ID, Value> values[];
  • property index can be acquired with a method: int index(ID);

A pseudo code for the property access looks like this:

pair = object.values[cachedIndex];

if (pair.ID == propertyID)
{
   value = pair.Value;
}
else
{
  // Cache miss.
  cachedIndex = object.index(propertyID);
  value = objec.values[cachedIndex].Value;
}

This approach brings us back to dictionary like implementation but with important optimization of array speed access when property index is cached, and with no dynamic hidden classes.

Saturday, December 17, 2011 10:18:14 AM UTC  #    Comments [0] -
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