Cyclic reference counting for combinator machines

Abstract

This new algorithm deals correctly and automatically with the kind of cyclic (i.e. self-referencing) structures which arise in a combinator graph reduction machine. By extending the standard reference count algorithm, cycles can be handled safely at little extra cost. Cyclic reference counting uses one extra bit per pointer and per object and one extra reference count per object. Extra computation amounts to inspection of the objects in a cyclic structure from time to time before the structure becomes free. In the absence of cycles, no computation overhead is incurred. When executing some typical recursive programs, the number of objects inspected is approximately equal to the number of new objects used during execution.