Efficient Method Dispatch in PCL by Gregor J. Kiczales and Luis H. Rodriguez Jr. is an all-out awesome yet compact paper about generic functions in their portable CLOS implementation.
Some salient points:
They add a level of indirection between objects and their classes, called wrappers. An object points to a wrapper and that points to the class of the object. Inside each wrapper they store a random value for use by the memoization tables of generic functions.
Each generic function has a small memoization table that maps the hashed random value of a wrapper to the program counter (PC) of the corresponding method defined for the wrapper. The memoization tables use a simple random % memotablesize computation, and linear scanning after that.
How they handle class redefinition is swell: they simply set the wrapper's random value to zero, thereby invalidating the wrapper. They always leave the first entry of generic functions' memoization tables empty, and because an invalidated wrapper's random value will now hash to that empty entry, they only need to check for an invalid wrapper after a memoization miss occurs. Genius.
Generic functions run through a kind of state machine, depending on how they're used. (Similar to how polymorphic inline caches switch between mono-, poly-, and mega-morphic states, but with additional states tuned to CLOS semantics.) For example, if a generic function is only ever used with two different classes it can use a simple IF to test for these two classes. Here's another kicker: if a generic function's state machine detects that it's used as a slot accessor, it can directly store the slot offset instead of the method's PC in the memoization table. AWESOME.