I can't remember how much of the data we put into the original paper, but we found that in practice idiomatic Ruby objects do generally have stable instance var

I can't remember how much of the data we put into the original paper, but we found that in practice idiomatic Ruby objects do generally have stable instance variable insertion order and so stable shapes. Unstable insertion order does introduce polymorphism that's true, but that's a fairly linear and gentle performance overhead. I wouldn't characterise this as a huge problem myself. Mis-predicting object layout if you try to do it statically like Rubinius does is a full-on performance cliff and pretty catastrophic.