Since the early days of logic programming, researchers in the field realized the potential for exploitation of parallelism present in the execution of logic programs. Their high-level nature, the presence of non-determinism, and their referential transparency, among other characteristics, make logic programs interesting candidates for obtaining speedups through parallel execution. At the same time, the fact that the typical applications of logic programming frequently involve irregular computations, make heavy use of dynamic data structures with logical variables, and involve search and speculation, makes the techniques used in the corresponding parallelizing compilers and run-time systems potentially interesting even outside the field. The objective of this paper is to provide a comprehensive survey of the issues arising in parallel execution of logic programming languages along with the most relevant approaches explored to date in the field. Focus is mostly given to the challenges emerging from the parallel execution of Prolog programs. The paper describes the major techniques used for shared memory implementation of Or-parallelism, And-parallelism, and combinations of the two. We also explore some related issues, such as memory

. The WAM allows within its framework many variations e.g. regarding the term representation, the instruction set and the memory organization. Consequently several Prolog systems have implemented successful variants of the WAM. While these variants are eective within their own context, it is dicult to assess the merit of their particular variation. In this work, four term representations that were used by at least one successful system are compared empirically within dProlog, one basic implementation which keeps all other things equal. We also report on dierent implementation choices in the dProlog emulator itself. dProlog is reasonably ecient, so it makes sense to use it for these experiments. 1

This paper addresses the design of visual paradigms for studying the parallel execution of logic programs. First, an intuitive method is proposed for arriving at the design of a paradigm and its implementation as a tool for a given model of parallelism. This method is based on stepwise refinement starting from the definition of basic notions such as events and observables and of some precedence relationships among events which hold for the given model of parallelism. The method is then applied to several types of parallel execution models for logic programs (Or-- parallelism, Restricted And--parallelism, Determinate Dependent And parallelism) for which visualization paradigms are designed. Finally, VisAndOr, a tool which implements all of these paradigms is presented, together with a discussion of its usefulness through examples. 1 Introduction Writing programs for parallel hardware has traditionally been considered a difficult task both because of the intrinsic difficulty of having ...

In this paper we describe the design and the implementation of the GNU Prolog system. This system draws on our previous experience of compiling Prolog to C in the wamcc system and of compiling finite domain constraints in the clp(FD) system. The compilation scheme has however been redesigned in order to overcome the drawbacks of compiling to C. In particular, GNU-Prolog is based on a low-level mini-assembly platform-independent language that makes it possible to avoid compiling C code, and thus drastically reduces compilation time. It also makes it possible to produce small stand-alone executable files as the result of the compilation process. Interestingly, GNU Prolog is now compliant to the ISO standard, includes several extensions (OS interface, sockets, global variables, etc) and integrates a powerful constraint solver over finite domains. The system is efficient and in terms of performance is comparable with commercial systems for both the Prolog and constraint aspects.

We present an extension of Logic Programming (LP) which, in addition to ordinary LP clauses, also includes integrity constraints, explicit representation of disjunction in the bodies of clauses and in goals, and suspension of atoms as in concurrent logic languages. The resulting framework aims to unify Constraint Logic Programming (CLP), Abductive Logic Programming (ALP) and Semantic Query Optimisation (SQO) in deductive databases. We present a proof procedure for the new framework, simplifying and generalising previously proposed proof procedures for ALP. We discuss applications of the framework, formulating traditional problems from LP, ALP, CLP and SQO.

We describe in this paper the GNU-Prolog system, a free system consisting of a Prolog compiler and a constraint solver on finite domains. GNU-Prolog is based on a low-level mini-assembly platform-independent language that makes it possible for efficient compilation time, and allows to produce small stand alone executable files as the result of the compilation process. Interestingly, the Prolog part is compliant to the ISO standard, and the constraint part includes several extensions, such as an efficient handling of reified constraints. The overall system is efficient and comparable in performances with commercial systems, both for the Prolog and constraint parts. 1. INTRODUCTION GNU Prolog 1 is a free Prolog compiler supported by the GNU organization. It was released in April 1999 and more than 2500 copies have been downloaded up to now from the INRIA ftp mirror site 2 . It is built on previous systems developed at INRIA, namely wamcc [4] for Prolog and clp(FD) [5] for constrain...

Machine (WAM), the most widely used abstract machine for Prolog implementation, arguments are passed through argument registers, and the information associated with procedure calls is stored in possibly two frames. Although accessing registers is faster than accessing memory, this scheme requires the argument registers to be saved and restored for backtracking and makes it difficult to implement full tail recursion elimination. These disadvantages may far outweigh the advantage in emulator-based implementations because registers are actually simulated by using memory. In this article, we reconsider the two crucial issues and describe a new abstract machine called ATOAM (yet Another Tree-Oriented Abstract Machine) . The ATOAM differs from the WAM mainly in that (1) arguments are passed directly into stack frames, (2) only one frame is used for each procedure call, and (3) procedures are translated into matching trees if possible, and clauses in each procedure are indexed on all input ar...

Much work has been done in the areas of and--parallelism and data parallelism in Logic Programs. Such work has proceeded to a certain extent in an independent fashion. Both types of parallelism offer advantages and disadvantages. Traditional (and--) parallel models offer generality, being able to exploit parallelism in a large class of programs (including that exploited by data parallelism techniques). Data parallelism techniques on the other hand offer increased performance for a restricted class of programs. The thesis of this paper is that these two forms of parallelism are not fundamentally different and that relating them opens the possibility of obtaining the advantages of both within the same system. Some relevant issues are discussed and solutions proposed. The discussion is illustrated through visualizations of actual parallel executions implementing the ideas proposed. Keywords: Parallel Logic Programming, And-Parallelism, Data-Parallelism, Fast Task Startup, Scheduling. 1 I...

We describe in this paper the compilation scheme of the GNU Prolog system. This system is built on our previous experience of compiling Prolog to C in wamcc. The compilation scheme has been however redesigned to overcome drawbacks of the compilation to C. In particular, GNU-Prolog is based on a low-level miniassembly platform-independent language that makes it possible to avoid the phase of compiling C code, and thus speeds up drastically compilation time. It also makes it possible to produce small stand alone executable les as the result of the compilation process. Interestingly, GNU Prolog is now compliant to the ISO standard and includes several extensions (constraint solving, OS interface, sockets, global variables, etc). The overall system is ecient and comparable in performances with commercial systems.

The parallel execution of logic programs, in particular written in Prolog, has great potential for efficient high-level parallel programming. Previous efforts have targeted the parallel execution of conjunctions and disjunctions of programs, so-called And-Or parallelism. A recent, promising approach is that of recursion-parallelism, where structurally recursive programs are exploited to yield large amounts of parallelism. In this work, we describe techniques for compiling recursion-parallel Prolog programs. We define a restriction of general recursion-parallelism that is suitable for efficient parallel implementation. The compilation of this language consists of compile-time analysis by abstract interpretation followed by compilation. The analysis is done to ensure that programs satisfy the required properties of efficient implementation and to locate opportunities of optimization. In particular, the analysis distinguishes a hierarchy of data sharing that the compiler attempts to expl...