We describe a method for reducing the complexity of temporal logic model checking in systems composed of many parallel processes. The goal is to check properties of the components of a system and then deduce global properties from these local properties. The main difficulty with this type of approach is that local properties are often not preserved at the global level. We present a general framework for using additional interface processes to model the environment for a component. These interface processes are typically much simpler than the full environment of the component. By composing a component with its interface processes and then checking properties of this composition, we can guarantee that these properties will be preserved at the global level. We give two example compositional systems based on the logic CTL*.

In semistructured databases there is no schema fixed in advance. To provide the benefits of a schema in such environments, we introduce DataGuides: concise and accurate structural summaries of semistructured databases. DataGuides serve as dynamic schemas, generated from the database; they are useful for browsing database structure, formulating queries, storing information such as statistics and sample values, and enabling query optimization. This paper presents the theoretical foundations of DataGuides along with an algorithm for their creation and an overview of incremental maintenance. We provide performance results based on our implementation of DataGuides in the Lore DBMS for semistructured data. We also describe the use of DataGuides in Lore, both in the user interface to enable structure browsing and query formulation, and as a means of guiding the query processor and optimizing query execution.

With advanced computer technology, systems are getting larger to fulfill more complicated tasks, however, they are also becoming less reliable. Consequently, testing is an indispensable part of system design and implementation; yet it has proved to be a formidable task for complex systems. This motivates the study of testing finite state machines to ensure the correct functioning of systems and to discover aspects of their behavior. A finite state machine contains a finite number of states and produces outputs on state transitions after receiving inputs. Finite state machines are widely used to model systems in diverse areas, including sequential circuits, certain types of programs, and, more recently, communication protocols. In a testing problem we have a machine about which we lack some information; we would like to deduce this information by providing a sequence of inputs to the machine and observing the outputs produced. Because of its practical importance and theoretical intere...

Set-based analysis (SBA) produces good predictions about the behavior of functional and objectoriented programs. The analysis proceeds by inferring constraints that characterize the data flow relationships of the analyzed program. Experiences with MrSpidey, a static debugger based on SBA, indicate that SBA can adequately deal with programs of up to a couple of thousand lines of code. SBA fails, however, to cope with larger programs because it generates systems of constraints that are at least linear, and possibly quadratic, in the size of the analyzed program. This article presents theoretical and practical results concerning methods for reducing the size of constraint systems. The theoretical results include a proof-theoretic characterization of the observable behavior of constraint systems for program components, and a complete algorithm for deciding the observable equivalence of constraint systems. In the course of this development we establish a close connection between the observable equivalence of constraint systems and the equivalence of regular-tree grammars. We then exploit this connection to adapt a variety of algorithms for simplifying grammars to the problem of simplifying constraint systems. Based on the resulting algorithms, we have developed componential set-based analysis, a modular and polymorphic variant of SBA. Experimental results verify the effectiveness of the simplification

The purpose of this article is to introduce Monadic Second-order Logic as a practical means of specifying regularity. The logic is a highly succinct alternative to the use of regular expressions. We have built a tool MONA, which acts as a decision procedure and as a translator to finite-state automata. The tool is based on new algorithms for minimizing finitestate automata that use binary decision diagrams (BDDs) to represent transition functions in compressed form. A byproduct of this work is a new bottom-up algorithm to reduce BDDs in linear time without hashing. The potential

Abstract. We study congruences on words in order to characterize the class of visibly pushdown languages (Vpl), a subclass of context-free languages. For any language L, we define a natural congruence on words that resembles the syntactic congruence for regular languages, such that this congruence is of finite index if, and only if, L is a Vpl. We then study the problem of finding canonical minimal deterministic automata for Vpls. Though Vpls in general do not have unique minimal automata, we consider a subclass of VPAs called k-module single-entry VPAs that correspond to programs with recursive procedures without input parameters, and show that the class of well-matched Vpls do indeed have unique minimal k-module single-entry automata. We also give a polynomial time algorithm that minimizes such k-module single-entry VPAs. 1 Introduction The class of visibly pushdown languages (Vpl), introduced in [1], is a subclassof context-free languages accepted by pushdown automata in which the input letter determines the type of operation permitted on the stack. Visibly push-down languages are closed under all boolean operations, and problems such as inclusion, that are undecidable for context-free languages, are decidable for Vpl. Vpls are relevant to several applications that use context-free languages suchas the model-checking of software programs using their pushdown models [1-3]. Recent work has shown applications in other contexts: in modeling semanticsof effects in processing XML streams [4], in game semantics for programming languages [5], and in identifying larger classes of pushdown specifications thatadmit decidable problems for infinite games on pushdown graphs [6].

This paper describes a language-independent program representation-the program dependence graph—and &mCusses how program dependence graphs, together with operations such as program slicing, can provide the basis for powerfid programming tools that address important software-engineering problems, such as understanding what an existing program does and how it works, understanding the differences between several versions of a program, and creating new programs by combining pieces of old programs. The paper primarily surveys work in this mea that has been czried out at the University of Wisconsin during the past five years.

Text-based file comparators (e.g., the Unix utility diff), are very general tools that can be applied to arbitrary files. However, using such tools to compare programs can be unsatisfactory because their only notion of change is based on program text rather than program behavior. This paper describes a technique for comparing two versions of a program, determining which program components represent changes, and classifying each changed component as representing either a semantic or a textual change. ######################## This work was supported in part by the Defense Advanced Research Projects Agency, monitored by the Office of Naval Research under contract N00014-88-K, by the National Science Foundation under grant CCR8958530, and by grants from Xerox, Kodak, and Cray. Author's address: Computer Sciences Department, Univ. of Wisconsin, 1210 W. Dayton St., Madison, WI 53706. Permission to copy without fee all or part of this material is granted provided that the copies are not made...

In this paper we introduce the representative object, which uncovers the inherent schema(s) in semistructured, hierarchical data sources and provides a concise description of the structure of the data. Semistructured data, unlike data stored in typical relational or object-oriented databases, does not have fixed schema that is known in advance and stored separately from the data. Withthe rapid growth of the World Wide Web, semistructured hierarchical data sources are becoming widely available to the casual user. The lack of external schema information currently makes browsing and querying these data sources inefficient at best, and impossible at worst. We show how representative objects make schema discovery efficient and facilitate the generation of meaningful queries over the data. 1.

This thesis presents a framework for describing optimizations. It shows how to combine two such frameworks and how to reason about the properties of the resulting framework. The structure of the framework provides insight into when a combination yields better results. Also presented is a simple iterative algorithm for solving these frameworks. A framework is shown that combines Constant Propagation, Unreachable Code Elimination, Global Congruence Finding and Global Value Numbering. For these optimizations, the iterative algorithm runs in O(n^2) time. This thesis then presents an O(n log n) algorithm for combining the same optimizations. This technique also finds many of the common subexpressions found by Partial Redundancy Elimination. However, it requires a global code motion pass to make the optimized code correct, also presented. The global code motion algorithm removes some Partially Dead Code as a side-effect. An implementation demonstrates that the algorithm has shorter compile times than repeated passes of the separate optimizations while producing run-time speedups of 4%–7%. While global analyses are stronger, peephole analyses can be unexpectedly powerful. This thesis demonstrates parse-time peephole optimizations that find more than 95% of the constants and common subexpressions found by the best combined analysis. Finding constants and common subexpressions while parsing reduces peak intermediate representation size. This speeds up the later global analyses, reducing total compilation time by 10%. In conjunction with global code motion, these peephole optimizations generate excellent code very quickly, a useful feature for compilers that stress compilation speed over code quality.