Saumya K. Debray
Abstract
Although the ability to simulate nondeterminism and to compute multiple solutions for a single query is a powerful and attractive feature of logic programming languages, it is expensive in both time and space. Since programs in such languages are very often functional, that is, they do not produce more than one distinct solution for a single input, this overhead is especially undesirable. This paper describes how programs may be analyzed statically to determine which literals and predicates are functional, and how the program may then be optimized using this information. Our notion of “functionality” subsumes the notion of “determinacy” that has been considered by various researchers. Our algorithm is less reliant on language features such as the cut, and thus extends more easily to parallel execution strategies, than others that have been proposed.
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Index Terms
- Functional computations in logic programs
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Reviews
Francois Aribaud
Because it is well known that full backtracking is the cause of inefficiency in the execution of Prolog programs, Prolog has a special feature designed to reduce the running time, namely, the highly controversial cut, which is not a logical operation but which conveys certain information to the system. In this paper, the authors start with the observation that programs are often functional: if all alternatives produce the same result, then repeated computation (the gist of backtracking) is unnecessary. Therefore, a prior analysis of the functional components of the program is a major improvement. Because functionality is a matter of semantics, the problem of the exact determination of what is functional is generally undecidable. Instead, the authors look at syntactic functionalities. They describe some rules which, from functional predicates, lead to other functional predicates. An algorithm running in linear time determines these functionalities. One may take advantage of this knowledge and optimize the programs by inserting suitable cuts, the control status of which is then clearly established. The authors note that “some predicates which are actually functional, are inferred to be relational (i.e., not functional).” One may hope that for usual programs the part of the forgotten functional components is small.
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