CHAT: The Copy-Hybrid Approach to Tabling

The copying approach to tabling (CAT) is an alternative to SLG-WAM and based on incrementally copying the areas that the SLG-WAM freezes to preserve execution states of suspended computations. The main advantage of CAT over the SLG-WAM is that support for tabling does not affect the speed of the underlying abstract machine for strictly non-tabled execution. The disadvantage of CAT as pointed out in a previous paper is that in the worst case, CAT must copy so much that its tabled execution becomes arbitrarily worse than that of the SLG-WAM. Remedies to this problem have been studied, but a completely satisfactory solution has not emerged. Here, a hybrid approach is presented: abbreviated as CHAT. Its design was guided by the requirement that for non-tabled (i.e. Prolog) execution no changes to the underlying WAM engine need to be made. CHAT not only combines certain features of the SLG-WAM with features of CAT, but also introduces a technique for freezing WAM stacks without the use of the SLG-WAM’s freeze registers that is of independent interest. This article describes only the basic CHAT mechanism which allows for programs that perform arbitrarily worse than under the SLG-WAM. However, empirical results indicate that even basic CHAT is a better choice for implementing the control of tabling than SLG-WAM or CAT. © 2000 Elsevier Science B.V. All rights reserved.