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Register a new userStepping Lazy Programs
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Added by
Stephen Chang
Publication date
2011
fields
Informatics Engineering
and research's language is
English
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Debugging lazy functional programs poses serious challenges. In support of the stop, examine, and resume debugging style of imperative languages, some debugging tools abandon lazy evaluation. Other debuggers preserve laziness but present it in a way that may confuse programmers because the focus of evaluation jumps around in a seemingly random manner. In this paper, we introduce a supplemental tool, the algebraic program stepper. An algebraic stepper shows computation as a mathematical calculation. Algebraic stepping could be particularly useful for novice programmers or programmers new to lazy programming. Mathematically speaking, an algebraic stepper renders computation as the standard rewriting sequence of a lazy lambda-calculus. Our novel lazy semantics introduces lazy evaluation as a form of parallel program rewriting. It represents a compromise between Launchburys store-based semantics and a simple, axiomatic description of lazy computation as sharing-via-parameters. Finally, we prove that the steppers run-time machinery correctly reconstructs the standard rewriting sequence.
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Partial Redundancy Elimination (PRE) is a compiler optimization that eliminates expressions that are redundant on some but not necessarily all paths through a program. In this project, we implemented a PRE optimization pass in LLVM and measured results on a variety of applications. We chose PRE because it is a powerful technique that subsumes Common Subexpression Elimination (CSE) and Loop Invariant Code Motion (LICM), and hence has the potential to greatly improve performance.
The possibility of translating logic programs into functional ones has long been a subject of investigation. Common to the many approaches is that the original logic program, in order to be translated, needs to be well-moded and this has led to the common understanding that these programs can be considered to be the ``functional part of logic programs. As a consequence of this it has become widely accepted that ``complex logical variables, the possibility of a dynamic selection rule, and general properties of non-well-moded programs are exclusive features of logic programs. This is not quite true, as some of these features are naturally found in lazy functional languages. We readdress the old question of what features are exclusive to the logic programming paradigm by defining a simple translation applicable to a wider range of logic programs, and demonstrate that the current circumscription is unreasonably restrictive.
We extend a technique called Compiling Control. The technique transforms coroutining logic programs into logic programs that, when executed under the standard left-to-right selection rule (and not using any delay features) have the same computational behavior as the coroutining program. In recent work, we revised Compiling Control and reformulated it as an instance of Abstract Conjunctive Partial Deduction. This work was mostly focused on the program analysis performed in Compiling Control. In the current paper, we focus on the synthesis of the transformed program. Instead of synthesizing a new logic program, we synthesize a CHR(Prolog) program which mimics the coroutining program. The synthesis to CHR yields programs containing only simplification rules, which are particularly amenable to certain static analysis techniques. The programs are also more concise and readable and can be ported to CHR implementations embedded in other languages than Prolog.
Frama-C is a software analysis framework that provides a common infrastructure and a common behavioral specification language to plugins that implement various static and dynamic analyses of C programs. Most plugins do not support concurrency. We have proposed Conc2Seq, a Frama-C plugin based on program transformation, capable to leverage the existing huge code base of plugins and to handle concurrent C programs. In this paper we formalize and sketch the proof of correctness of the program transformation principle behind Conc2Seq, and present an effort towards the full mechanization of both the formalization and proofs with the proof assistant Coq.
Program transformation is an appealing technique which allows to improve run-time efficiency, space-consumption, and more generally to optimize a given program. Essentially, it consists of a sequence of syntactic program manipulations which preserves some kind of semantic equivalence. Unfolding is one of the basic operations which is used by most program transformation systems and which consists in the replacement of a procedure call by its definition. While there is a large body of literature on transformation and unfolding of sequential programs, very few papers have addressed this issue for concurrent languages. This paper defines an unfolding system for CHR programs. We define an unfolding rule, show its correctness and discuss some conditions which can be used to delete an unfolded rule while preserving the program meaning. We also prove that, under some suitable conditions, confluence and termination are preserved by the above transformation. To appear in Theory and Practice of Logic Programming (TPLP)
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