The Design and Implementation of the Self Compiler, an Optimizing Compiler for Object-Oriented Programming Languages¶
Object-oriented programming languages promise to improve programmer productivity by supporting abstract data types, inheritance, and message passing directly within the language. Unfortunately, traditional implementations of object-oriented language features, particularly message passing, have been much slower than traditional implementations of their non-object-oriented counterparts: the fastest existing implementation of Smalltalk-80 runs at only a tenth the speed of an optimizing C implementation. The dearth of suitable implementation technology has forced most object-oriented languages to be designed as hybrids with traditional non-object-oriented languages, complicating the languages and making programs harder to extend and reuse.
This dissertation describes a collection of implementation techniques that can improve the run-time performance of object-oriented languages, in hopes of reducing the need for hybrid languages and encouraging wider spread of purely object-oriented languages. The purpose of the new techniques is to identify those messages whose receiver can only be of a single representation and eliminate the overhead of message passing by replacing the message with a normal direct procedure call; these direct procedure calls are then amenable to traditional inline-expansion. The techniques include a type analysis component that analyzes the procedures being compiled and extracts representation-level type information about the receivers of messages. To enable more messages to be optimized away, the techniques include a number of transformations which can increase the number of messages with a single receiver type. Customization transforms a single source method into several compiled versions, each version specific to a particular inheriting receiver type; customization allows all messages to self to be inlined away (or at least replaced with direct procedure calls). To avoid generating too much compiled code, the compiler is invoked at run-time, generating customized versions only for those method/receiver type pairs used by a particular program. Splitting transforms a single path through a source method into multiple separate fragments of compiled code, each fragment specific to a particular combination of run-time types. Messages to expressions of these discriminated types can then be optimized away in the split versions. The techniques are designed to coexist with other requirements of the language and programming environment, such as generic arithmetic, user-defined control structures, robust error-checking language primitives, source-level debugging, and automatic recompilation of out-of-date methods after a programming change. These techniques have been implemented as part of the compiler for the Self language, a purely object-oriented language designed as a refinement of Smalltalk-80. If only pre-existing implementation technology were used for Self, programs in Self would run one to two orders of magnitude slower than their counterparts written in a traditional non-object-oriented language. However, by applying the techniques described in this dissertation, the performance of the Self system is five times better than the fastest Smalltalk-80 system, better than that of an optimizing Scheme implementation, and close to half that of an optimizing C implementation.
These techniques could be applied to other object-oriented languages to boost their performance or enable a more object-oriented programming style. They also are applicable to non-object-oriented languages incorporating generic arithmetic or other generic operations, including Lisp, Icon, and APL. Finally, they might be applicable to languages that include multiple representations or states of a single program structure, such as logic variables in Prolog and futures in Multilisp. <hr>
Ph. D. dissertation, Computer Science Department, Stanford University, March 1992.
(Not available online)