Program slicing is a method for automatically decomposing programs by analyzing their data flow and control flow. Starting from a subset of a program's behavior, slicing reduces that program to a minimal form which still produces that behavior. The reduced program, called a ``slice,'' is an independent program guaranteed to represent faithfully the original program within the domain of the specified subset of behavior. Some properties of slices are presented. In particular, finding statement-minimal slices is in general unsolvable, but using data flow analysis is sufficient to find approximate slices. Potential applications include automatic slicing tools for debuggng and parallel processing of slices.

Program Slicing

In this paper we present an intermediate program representation, called the program dependence graph (PDG), that makes explicit both the data and control dependences for each operation in a program. Data dependences have been used to represent only the relevant data flow relationships of a program. Control dependences are introduced to analogously represent only the essential control flow relationships of a program. Control dependences are derived from the usual control flow graph. Many traditional optimizations operate more efficiently on the PDG. Since dependences in the PDG connect computationally related parts of the program, a single walk of these dependences is sufficient to perform many optimizations. The PDG allows transformations such as vectorization, that previously required special treatment of control dependence, to be performed in a manner that is uniform for both control and data dependences. Program transformations that require interaction of the two dependence types can also be easily handled with our representation. As an example, an incremental approach to modifying data dependences resulting from branch deletion or loop unrolling is introduced. The PDG supports incremental optimization, permitting transformations to be triggered by one another and applied only to affected dependences.

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The program dependence graph and its use in optimization

Program slicing is a decomposition technique that slides program components not relevant to a chosen computation, referred to as a slicing criterion. The remaining components form an executable program called a slice that computes a projection of the original programpsilas semantics. Using examples coupled with fundamental principles, a tutorial introduction to program slicing is presented. Then applications of program slicing are surveyed, ranging from its first use as a debugging technique to current applications in property verification using finite state models. Finally, a summary of research challenges for the slicing community is discussed.

Program slicing

The notion of a program slice, originally introduced by Mark Weiser, is useful in program debugging, automatic parallelization, and program integration. A slice of a program is taken with respect to a program point p and a variable x; the slice consists of all statements of the program that might affect the value of x at point p. This paper concerns the problem of interprocedural slicing—generating a slice of an entire program, where the slice crosses the boundaries of procedure calls. To solve this problem, we introduce a new kind of graph to represent programs, called a system dependence graph, which extends previous dependence representations to incorporate collections of procedures (with procedure calls) rather than just monolithic programs. Our main result is an algorithm for interprocedural slicing that uses the new representation. (It should be noted that our work concerns a somewhat restricted kind of slice: rather than permitting a program to b e sliced with respect to program point p and an arbitrary variable, a slice must be taken with respect to a variable that is defined or used at p.)The chief difficulty in interprocedural slicing is correctly accounting for the calling context of a called procedure. To handle this problem, system dependence graphs include some data dependence edges that represent transitive dependences due to the effects of procedure calls, in addition to the conventional direct-dependence edges. These edges are constructed with the aid of an auxiliary structure that represents calling and parameter-linkage relationships. This structure takes the form of an attribute grammar. The step of computing the required transitive-dependence edges is reduced to the construction of the subordinate characteristic graphs for the grammar's nonterminals.

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Interprocedural slicing using dependence graphs

A program slice consists of the parts of a program that (potentially) affect the values computed at some point of interest Such a point of interest is referred to as a slicing criterion, and is typically specified by a location in the program in combination with a subset of the program’s variables The task of computing program slices is called program slicing The original definition of a program slice was presented by Weiser in 1979 Since then, various slightly different notions of program slices have been proposed, as well as a number of methods to compute them An important distinction is that between a static and a dynamic slice Static slices are computed without making assumptions regarding a program’s input, whereas the computation of dynamic slices relies on a specific test case This survey presents an overview of program slicing, including the various general approaches used to compute slices, as well as the specific techniques used to address a variety of language features such as procedures, unstructured control flow, composite data types and pointers, and concurrency Static and dynamic slicing methods for each of these features are compared and classified in terms of their accuracy and efficiency Moreover, the possibilities for combining solutions for different features are investigated Recent work on the use of compiler-optimization and symbolic execution techniques for obtaining more accurate slices is discussed The paper concludes with an overview of the applications of program slicing, which include debugging, program integration, dataflow testing, and software maintenance

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A Survey of Program Slicing Techniques.

Computer programmers break apart large programs into smaller coherent pieces. Each of these pieces: functions, subroutines, modules, or abstract datatypes, is usually a contiguous piece of program text. The experiment reported here shows that programmers also routinely break programs into one kind of coherent piece which is not coniguous. When debugging unfamiliar programs programmers use program pieces called slices which are sets of statements related by their flow of data. The statements in a slice are not necessarily textually contiguous, but may be scattered through a program.

Programmers use slices when debugging

Program slicing is a method used by experienced computer programmers for abstracting from programs. Starting from a subset of a program's behavior, slicing reduces that program to a minimal form which still produces that behavior. The reduced program, called a “slice”, is an independent program guaranteed to faithfully represent the original program within the domain of the specified subset of behavior.Finding a slice is in general unsolvable. A dataflow algorithm is presented for approximating slices when the behavior subset is specified as the values of a set of variables at a statement. Experimental evidence is presented that these slices are used by programmers during debugging. Experience with two automatic slicing tools is summarized. New measures of program complexity are suggested based on the organization of a program's slices.

Textnet is a new system for structuring text. The Textnet approach uses one uniform data structure to capture graphlike pools of text, as well as embedded hierarchical structures. By using a semantic network formalism of nodes connected by typed links, the relationships between neighboring pieces of text are made explicit. Also described is our partial implementation of the Textnet approach, which makes use of an object-oriented window/menu-driven user interface. Users peruse the network by moving among object menus or by reading text along a path through the network. In addition, critiquing, reader linking, searching, and jumping are easily accessible operations. Finally, the results of a short trial with users are presented.

TEXTNET: a network-based approach to text handling

The representation of computer programming problems in relation to the organization of programming knowledge is examined. An experiment previously done for physics knowledge is replicated to examine differences in the categories used for problem representation by novice and expert programmers. Results from sorting tasks show experts and novices begin their problem representations with specific different problem categories. Experts initially abstract an algorithm to solve a problem, whereas novices base their approach on a problem's literal features. A preliminary study of programming managers indicates an abstraction different from that used by programmers.

Mark Weiser

Papers

Program Slicing

Programmers use slices when debugging

Program slicing

TEXTNET: a network-based approach to text handling

Programming problem representation in novice and expert programmers