5. Conclusion and Future Directions

A new declarative semantics for logic languages. In R. Optimizations of bottom-up evaluation with non-ground terms. In Miller 15], pages 557{574. 19 orthogonal to the type of program to which memoing is applied. With some care, they can be used in the context of full Prolog, speciied as we have suggested by a programmer controlled directive. The practicality of the special case of ground, multi-argument-indexed Delphi-lemmas has convinced us to add them as a standard extension to the BinProlog compiler, starting from version 5.00 (available by ftp from clement.info.umoncton.ca). Delphi-lemmas have also served as inspiration for BinProlog 5.00's dynamic re-compilation scheme, which moves`hot spots' of non-volatile interpreted code to compiled code on the y, based on update vs. call statistics 23]. Indexing in the case of non-ground answers in general can improve potential performance problems with abstract answers. The techniques described in 18] as well as the idea of using tries instead of hashing, as done in the latest version of XSB, can be adapted for this purpose. The program transformations we used can be modiied in the case of non-deterministic programs to avoid interference between lemma related pruning and the program's control structure. More work is needed on how this can be done in general and what are the limitations and the tradeoos. The latest version of the XSB system successfully deals with SLG-resolution which is a fairly realistic subset of Prolog including non-determinism and a restricted form of negation as failure. We plan to apply some of the techniques of XSB (like memoing of multiple answer substitutions) to extend delphi lemmas beyond deenite programs. Nevertheless, we consider our techniques useful for`real' programmers as it can accelerate the most critical parts of a program in a signiicant way and also because they can be fully automated. Delphi lemmas can also complement existing systems with memoing facilities having objectives that are orthogonal to ours, as 28]. Future work is planned to port Delphi lemmas to the latest version of XSB Prolog. We do not know about something similar to Delphi-lemmas in functional or procedural languages but the concept can be easily adapted. Although it is a proba-bilistic concept in the same sense as hashing or Ethernet collision avoidance, it gives very good performances for mostly the same reasons, especially in combination with the use of abstract answers. As our benchmark data on boyer shows, the technique looks particularly beneecial for …