Dynamic dispatch in object-oriented languages

Abstract

Dynamic binding in object-oriented languages is perhaps the most important semantic aspect of these languages. At the same time it can contribute to inefficiency and lack of robustness because it incurs lookup overheads on function calls and hinders the compiler determining the exact type of objects held in variables or returned by functions. This may, for instance, preclude inlining of small functions or attribute offset computation at compile time. Yet attribute accesses are the most frequently executed operations. As a result, to regain lost performance, OO programmers are tempted to break the encapsulation of classes or want explicit control over dynamic dispatch, trading off extensibility. In the implementation of parallel object-oriented languages the additional complication arises that object accesses may require more expensive remote memory accesses. Lookup at the call may be inappropriate if the code has to be executed on a different processor and there perhaps has a different address. This paper summarizes dispatching as addressed in several modern object-oriented languages. We then describe and benchmark fast and exible dispatch schemes that we are currently implementing on SPARC based workstations and multi-processors. These involve elements of C++ virtual function tables and Eiffel's and Sather's ability to redefine abstract functions as attributes. Initial benchmarks seem to promise improved efficiency on a range of modern RISC based architectures.