You can version manually or automatically in the optimizing compiler inner loops of those builtins depending on the denseness / representation of the array back

You can version manually or automatically in the optimizing compiler inner loops of those builtins depending on the denseness / representation of the array backing store. Something along this lines: http://gist.io/7050013 http://gist.io/7050013. Trace compiler could actually give such versioning for free automatically. So I always putting this into not-done-yet category for JS VM related work and it is a very interesting problem to tackle.