Previously every time a method was defined on a module, we would recursively walk all subclasses to see if the module was included in a class which the VM optimizes for (such as Integer#+). For most method definitions we can tell immediately that this won't be the case based on the method's name. To do this we just keep a hash with method IDs of optimized methods and if our new method isn't in that list we don't need to check subclasses at all.