Comment by hinkley
9 hours ago
The problem they were trying to solve was multiple inheritance, and by nominal type not by code reuse. So interfaces, basically.
So these guys essentially assigned a hashcode to every function of every interface and then you would do dispatch instead of obj.vtable[12] you would do modular math x = singature.hash % len(obj.vtable) and call that.
I believe this was sometime around 2005-2008 and they found that it was fast enough on hardware of that era to be usable.
Thanks, I think I get it now. The hash value would be a pure function of the method's signature (argument types and return type) and its name, so that two interfaces with a same-name, same-signature method would hash to the same value and thus invoke the same underlying method; the constraints would be that, after modulo, different methods must map to different indices; and the objective function to minimise would be the vtable size (which I think would be common across all classes).
But maybe I don't get it, since this would require knowledge of all interfaces, and as soon as you require that, it's straightforward to build a minimal-size mapping from method name+signature to integer index: e.g., just form the union of all method declarations appearing in any interface, sort them lexicographically, and use a method's position in this sorted list as its index. Lookups in this map are only ever done at compile time so there's no runtime inefficiency to worry about.