Inline Caching - Monomorphic Inline Caching

Monomorphic Inline Caching

If a particular call site frequently sees different types of objects, the performance benefits of inline caching can easily be nullified by the overhead induced by the frequent changes in state of the call site. The following example constitutes a worst case scenario for monomorphic inline caching:

var values = ; for (var i in values) { document.write(values.toString); }

Again, the method toString is invoked on an object whose type is not known in advance. More importantly though, the type of the object changes with every iteration of the surrounding loop. A naive implementation of monomorphic inline caching would therefore constantly cycle through the "uninitialized" and "monomorphic" states. In order to prevent this from happening, most implementations of monomorphic inline caching support a third state often referred to as the "megamorphic" state. This state is entered when a particular call site has seen a predetermined number of different types. Once a call site has entered the "megamorphic" state, it will behave just as it did in the "uninitialized" state with the exception that it will not enter the "monomorphic" state ever again (some implementations of monomorphic inline caching will change "megamorphic" call sites back to being "uninitialized" after a certain amount of time has passed or once a full garbage collection cycle is performed).