Look in the Object PMC's generated C code.  See all of the calls to Parrot_oo_find_vtable_override()?  Isn't that ridiculous?

Some ~6% of all of the time spent running Rakudo, if not more, goes to checking if an object's class has a VTABLE override written in PIR.  Most don't.  Very few do.  Yet these get checked on *every* invocation of each of those VTABLE entries.

We can fix this:

 * create a dummy VTABLE, where each entry automatically, irrevocably delegates to an overridden PIR VTABLE
 * add code to the NameSpace PMC (or wherever's most appropriate) to duplicate the current class VTABLE and swap in the appropriate override pointer when something adds a VTABLE override
 * only duplicate that VTABLE once
 * make sure to free that duplicate VTABLE when appropriate, lest we leak memory at the end of the program

We may run a small risk of not being able to add overrides when we've already instantiated objects of the given class, but I think that's a low risk (and we can test for that anyway).  We already forbid modifications to the class if we've instantiated objects.

This is a fairly simple task which will have positive performance implications for not too much work.  Takers welcome.

Tasks:

  * Write a small program (Perl 5 is fine, NQP-rx may be better) which parses src/vtable.tbl
    * generate a dummy VTABLE
    * each VTABLE entry should only fetch a PIR sub override and invoke it with the given arguments
    * see lib/Parrot/Pmc2c/PMC/Object.pm around line 68
    * store this dummy VTABLE somewhere where the interpreter can get to it
  * Add code to the NameSpace PMC to detect adding a vtable override
    * when this happens, clone the current VTABLE (unless it's already been cloned once)
    * swap in the appropriate pointer from the dummy VTABLE
    * store the new VTABLE where instances of the class can use it
    * (hands wave here; probably Class needs to install it for Object)
    * add code to Class (or whatever stores the cloned version) to delete the cloned VTABLE
  * Remove the special cases from lib/Parrot/Pmc2c/PMC/Object.pm

That's it!