Selective Recomputation for Handling Side-Effects in Parallel Logic Programs

In contrast to merely AND-and merely OR-parallel execution models/systems, the side-eeect problem in AND/OR parallel execution of Prolog programs is intricate and need to be carefully investigated. To decrease the non-trivial recomputation occurred in previous approach, this paper presents a Selective Recomputation(SR) approach for handling side-eeects in the OR-forest model which can exploit both AND-and OR-parallelism. Firstly, the background and motivation is introduced. Secondly , the complex side-eeects orderings in AND/OR parallel execution models/systems are clearly described using an innovative concept { side-eeect execution permit token. Thirdly, recomputation line is proposed to divide an AND-parallel child tree into non-recomputation part and re-computation part. Therefore, the recomputation is only connned to the recomputation part of an AND-parallel child tree, instead of the entire tree. And the non-recomputation part can be explored in parallel as pure child tree. Finally, by adopting the delayed execution of soft side-eeect built-ins, the non-recomputation part is enlarged and thus the recompu-tation is minimized and parallelism is maximized. Through comparison and analysis, we conclude the SR approach can solve the side-eeect problem with minimum recomputation and maximum parallelism. The idea of SR is applicable to other AND/OR parallel execution models/systems as well.