Coloring, a versatile technique for implementing object-oriented languages

Late binding and subtyping create run-time overhead for object-oriented languages. Dynamic typing and multiple inheritance create even more overhead. Static typing and single inheritance lead to two major invariants—of reference and position—that make the implementation as efficient as possible. Coloring is a technique that preserves these invariants for dynamic typing or multiple inheritance at minimal spatial cost. Coloring has been introduced and applied, more or less independently, to method invocation—under the name of selector coloring—to subtype tests under the name of pack encoding and to attribute access. This paper reviews a number of uses of coloring for optimizing object-oriented programming and generalizes them. It specifies several variations of coloring, such as bidirectional coloring and n-dimensional coloring. Coloring is NP-hard, so compilers that use it depend on heuristics. The paper describes some experimental results which indicate that bidirectional coloring gives the best results.