It should be noted that hidden classes have at least two advantages: 1) they go beyond simple index to property mappings and can capture many different aspects (types of backing stores, constant function properties, type specific optimized construction stubs, estimated number of properties that will be added to an object).
2) allow to optimize memory usage --- dictionary based backing stores a much less compact. Your suggestion of course is viable (and well known, e.g. similar approach is used for example by LuaJIT2) optimization that can be used to make monomorphic dictionary lookups faster. However it cannot replace hidden classes entirely as noted above. -- Vyacheslav Egorov On Sat, Dec 17, 2011 at 11:31 AM, Vladimir Nesterovsky < [email protected]> wrote: > Yesterday I've seen an article about some design principles of V8 > JavaScript Engine (http://code.google.com/apis/v8/design.html). In > particular V8 engine optimizes property access using "dynamically > created hidden classes", they are derived when a new property is > created (deleted) on the object. > > 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: > > a) object is implemented as a hash map of property id to property > value: Map<ID, Value>; > b) it stores data as an array of pairs and can be accessed directly: > Pair<ID, Value> values[]; > c) 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. > > See also > http://www.nesterovsky-bros.com/weblog/2011/12/17/FastPropertyAccessInV8JavaScriptEngine.aspx > -- > Vladimir Nesterovsky > > -- > v8-users mailing list > [email protected] > http://groups.google.com/group/v8-users > -- v8-users mailing list [email protected] http://groups.google.com/group/v8-users
