Efficiency Considerations In Implementing Dijkstra's Guarded Command Constructs
McGuire, Marguerite Elaine
The guarded command alternative and iterative constructs proposed by E. W. Dijkstra subsume the conventional alternative and iterative constructs. The extra flexibility of these guarded command constructs enables the programmer to express his ideas more directly and clearly. Moreover, Dijkstra has developed a calculus for the derivation of correct programs that utilizes these guarded command constructs. This thesis addresses the problem of efficiently implementing these guarded command constructs. Several new optimizations that are particularly well suited to the guarded command constructs are described. The most useful is the elimination of redundant boolean expressions. This optimization provides a means of implementing the guarded command alternative statement with efficiency comparable to the IF-THEN-ELSE statement. The main contribution of this thesis is a detailed description of an algorithm for eliminating redundant boolean expressions. The algorithm itself is presented in a program written in a PASCAL supplemented with the guarded command constructs. The basis method involves considering individual execution paths through the guarded command construct and applying rules of inference to recognize and avoid evaluation of many boolean expressions. It is shown that the number of execution paths through a guarded command construct remains small enough to make this method practical.
computer science; technical report
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