Inline Caching - Runtime Method Binding

Runtime Method Binding

The following ECMAScript function receives an object, invokes its toString-method and displays the results on the page the script is embedded in.

function dump(obj) { document.write(obj.toString); }

Since the type of the object is not specified and because of potential method overloading, it is impossible to decide ahead of time, which concrete implementation of the toString-method is going to be invoked. Instead, a dynamic lookup has to be performed at runtime. In language runtimes that do not employ some form of caching, this lookup is performed every time a method is invoked. Because methods may be defined several steps down the inheritance chain, a dynamic lookup can be an expensive operation.

To achieve better performance, many language runtimes employ some form of non-inline caching where the results of a limited number of method lookups are stored in an associative data structure. This can greatly increase performance, provided that the programs executed are "cache friendly" (i.e. there is a limited set of methods that is invoked frequently). This data structure is typically called the first-level method lookup cache.