Single superclass inheritance enables simple and efficient table-driven virtual method dispatch. However, virtual method table dispatch does not handle multiple inheritance and interfaces. This complication has led to a widespread misimpression that interface method dispatch is inherently inefficient. This paper argues that with proper implementation techniques, Java interfaces need not be a source of significant performance degradation.
We present an efficient interface method dispatch mechanism, associating a fixed-sized interface method table (IMT) with each class that implements an interface. Interface method signatures hash to an IMT slot, with any hashing collisions handled by custom-generated conflict resolution stubs. The dispatch mechanism is efficient in both time and space. Furthermore, with static analysis and online profile data, an optimizing compiler can inline the dominant target(s) of any frequently executed interface call.
Micro-benchmark results demonstrate that the expected cost of an interface method call dispatched via an IMT is comparable to the cost of a virtual method call. Experimental evaluation of a number of interface dispatch mechanisms on a suite of larger applications demonstrates that, even for applications that make only moderate use of interface methods, the choice of interface dispatching mechanism can significantly impact overall performance. Fortunately, several mechanisms provide good performance at a modest space cost.
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