Efficient Subtyping Tests with PQ-Encoding
Abstract:
Subtyping tests, i.e., determining whether one type is a subtype of another, are a frequent operation during the execution of objectoriented programs. The challenge is in encoding the hierarchy in a small space, while simultaneously making sure that subtyping tests have efficient implementation. We present a new scheme for encoding multiple and single inheritance hierarchies, which, in the standardized hierarchies, reduces the footprint of all previously published schemes. The scheme is called PQ-encoding after PQ-trees, a data structure previously used in graph theory for finding the orderings that satisfy a collection of constraints. In particular, we show that in the traditional object layout model, the extra memory requirements for single inheritance hierarchies is zero. In the PQ-encoding subtyping tests are constant time, and use only two comparisons. Other than PQ-trees, PQ-encoding uses several novel optimization techniques. These techniques are applicable also in improving the performance of other, previously published, encoding schemes. 1.
