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We describe a facility for improving optimization of Haskell programs using rewrite rules. Library authors can use rules to express domain-specific optimizations that the compiler cannot discover for itself. The compiler can also generate rules internally to propagate information obtained from automated analyses. The rewrite mechanism is fully implemented in the released Glasgow Haskell Compiler. Our system is very simple, but can be effective in optimizing real programs. We describe two practical applications involving short-cut deforestation, for lists and for rose trees, and document substantial performance improvements on a range of programs. 1 Introduction Optimising compilers perform program transformations that improve the efficiency of the program. However, a compiler can only use relatively shallow reasoning to guarantee the correctness of its optimisations. In contrast, the programmer has much deeper information about the program and its intended behaviour. For example, a programmer may know that

Program transformation is used in a wide range of applications including compiler construction, optimization, program synthesis, refactoring, software renovation, and reverse engineering. Complex program transformations are achieved through a number of consecutive modifications of a program. Transformation rules define basic modifications. A transformation strategy is an algorithm for choosing a path in the rewrite relation induced by a set of rules. This paper surveys the support for the definition of strategies in program transformation systems. After a discussion of kinds of program transformation and choices in program representation, the basic elements of a strategy system are discussed and the choices in the design of a strategy language are considered. Several styles of strategy systems as provided in existing languages are then analyzed.

Automating software evolution requires both identifying precisely the affected program points and selecting the appropriate modification at each point. This task is particularly complicated when considering a large program, even when the modifications appear to be systematic. We illustrate this situation in the context of evolving the Linux kernel to support Bossa, an event-based framework for process-scheduler development. To support Bossa, events must be added at points scattered throughout the kernel. In each case, the choice of event depends on properties of one or a sequence of instructions. To describe precisely the choice of event, we propose to guide the event insertion by using a set of rules, amounting to an aspect, that describes the control-flow contexts in which each event should be generated. In this paper, we present our approach and describe the set of rules that allows proper event insertion. These rules use temporal logic to describe sequences of instructions that require events to be inserted. We also give an overview of an implementation that we have developed to automatically perform this evolution. 1.

We aim to specify program transformations in a declarative style, and then to generate executable program transformers from such specifications. Many transformations require non-trivial program analysis to check their applicability, and it is prohibitively expensive to re-run such analyses after each transformation. It is desirable, therefore, that the analysis information is incrementally updated. We achieve this by drawing on two pieces of previous work: first, Bernhard Steffen's proposal to use model checking for certain analysis problems, and second, John Conway's theory of language factors. The first allows the neat specification of transformations, while the second opens the way for an incremental implementation. The two ideas are linked by using regular patterns instead of Steffen's modal logic: these patterns can be viewed as queries on the set of program paths.

Path logic programming is a modest extension of Prolog for the specification of program transformations. We give an informal introduction to this extension, and we show how it can be used in coding standard compiler optimisations, and also a number of obfuscating transformations. The object language is the Microsoft .NET intermediate language (IL).

Abstract. Data-flow transformations used in optimizing compilers are also useful in other programming tools such as code generators, aspect weavers, domain- and application-specific optimizers, and refactoring tools. These applications require source-to-source transformations rather than transformations on a low-level intermediate representation. In this paper we describe the composition of source-to-source dataflow transformations in the program transformation language Stratego. The language supports the high-level specification of transformations by means of rewriting strategy combinators that allow a natural modeling of data- and control-flow without committing to a specific source language. Data-flow facts are propagated using dynamic rewriting rules. In particular, we introduce the concept of dependent dynamic rewrite rules, for modeling the dependencies of data-flow facts on program entitities such as variables. The approach supports the combination of analysis and transformation, the combination of multiple transformations, the combination with other types of transformations, and the correct treatment of variable binding constructs and lexical scope to avoid variable capture. 1

While software methodology encourages the use of libraries and advocates architectures of layered libraries, in practice the composition of libraries is not always seamless and the combination of two well-designed libraries not necessarily well designed, since it could result in suboptimal call sequences, lost functionality, or avoidable overhead. In this paper we introduce Simplicissimus, a framework for rewrite-based source code transformations that allows for code replacement in a systematic and safe manner. We discuss the design and implementation of the framework and illustrate its functionality with applications in several areas. 1

Given are a directed edge-labelled graph G with a distinguished node n0, and a regular expression P which may contain variables. We wish to compute all substitutions φ (of symbols for variables), together with all nodes n such that all paths n0 → n are in φ(P). We derive an algorithm for this problem using relational algebra, and show how it may be implemented in Prolog. The motivation for the problem derives from a declarative framework for specifying compiler optimisations. 1 Bob Paige and IFIP WG 2.1 Bob Paige was a long-standing member of IFIP Working Group 2.1 on Algorithmic Languages and Calculi. In recent years, the main aim of this group has been to investigate the derivation of algorithms from specifications by program transformation. Already in the mid-eighties, Bob was way ahead of the pack: instead of applying transformational techniques to well-worn examples, he was applying his theories of program transformation to new problems, and discovering new algorithms [16, 48, 52]. The secret of his success lay partly in his insistence on the study of general algorithm design strategies (in particular

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