Memory allocation and higher-order functions

Abstract

This paper presents a constant-time marking-collecting algorithm to efficiently implement recursion with a general heap memory rather than with a vectorial stack, in a context of frequent captures of continuations. It has been seen to reduce the 80% garbage collection overhead to less than 5% on average. The algorithm has been built into a virtual machine to efficiently implement at the assembly level the Actor language PLASMA, an Actor-oriented version of PROLOG and a variant of SCHEME, currently in use on 8086, 68000 and Vax. The rationale to use the heap memory is that continuations are available via a single pointer in a unified memory and can be shared optimally when recurrently captured, which is simply impossible using a strategy based on stack recopy. Further, non-captured continuations can be incrementally garbage collected on the fly. Part I describes the elementary recursive instructions of the virtual machine. Part II presents and proves the marking-collecting strategy. Part III safely generalizes the transformation "call + return = branch" in a way compatible with the possible capture of the current continuation. An appendix relates its integration in the Virtual Scheme Machine supporting Scheme 84.