Managing stack frames in Smalltalk

Abstract

The Smalltalk programming language allows contexts (stack frames) to be accessed and manipulated in very general ways. This sometimes requires that contexts be retained even after they have terminated executing, and that they be reclaimed other than by LIFO stack discipline. The authoritative definition of Smalltalk [Goldberg and Robson 83] uses reference counting garbage collection to manage contexts, an approach found to be inadequate in practice [Krasner, et al. 83]. Deutsch and Schiffman have described a technique that uses an actual stack as much as possible [Deutsch and Schiffman 84]. Here we offer a less complex technique that we expect will have lower total overhead and reclaim many frames sooner and more easily. We are implementing our technique as part of a state of the art Smalltalk interpreter. The approach may apply to other languages that allow indefinite lifetimes for execution contexts, be they interpreted or compiled.