PIR classes can override VTABLE entries with PIR functions, but detecting this is costly: it requires every Object VTABLE entry to perform a check for a VTABLE override on every VTABLE invocation.

Most Objects never override any VTABLE entries.

We can detect whether a class has overridden a VTABLE entry; the add_vtable_override() method in the Class PMC is a good place to perform a bit of magic.

For this trick, we need:

  • a comprehensive set of VTABLE entries which *only* dispatch to the given PMC Object's Class's PIR VTABLE override
  • a way of mapping the name of a VTABLE entry (passed to add_vtable_override()) to the appropriate slot in the fake VTABLE
  • a way of swapping the standard Object VTABLE for a custom VTABLE for all Object PMCs created by the instantiate() of a Class with overridden VTABLE entries
  • (likely) a way of identifying whether any *parent* Class in a Class's MRO has overridden VTABLE entries

Thus every time something adds a new VTABLE entry to a Class, add_vtable_override() copies the current object VTABLE, swaps in the appropriate "dispatch to the PIR entry instead" VTABLE entry, and uses that new VTABLE for all Objects instantiated from that Class.

We can then remove all "check for override" code from every other part of the system.

Volunteers welcome for any or all parts of this.