Eliminating go to's while preserving program structure

Abstract

Suppose we want to eliminate the local go to statements of a Pascal program by replacing them with multilevel loop exit statements. The standard ground rules for eliminating go to's require that we preserve the flow graph of the program, but they allow us to completely rewrite the control structures that glue together the program's atomic tests and actions. The go to's can be eliminated from a program under those ground rules if and only if the flow graph of that program has the graph-theoretic property named reducibility. This paper considers a stricter set of ground rules, introduced by Peterson, Kasami, and Tokura, which demand that we preserve the program's original control structures, as well as its flow graph, while we eliminate its go to's. In particular, we are allowed to delete the go to statements and the labels that they jump to and to insert various exit statements and labeled repeat-endloop pairs for them to jump out of. But we are forbidden to change the rest of the program text in any way. The critical issue that determines whether go to's can be eliminated under these stricter rules turns out to be the static order of the atomic tests and actions in the program text. This static order can be encoded in the program's flow graph by augmenting it with extra edges. It can then be shown that the reducibility of a program's augmented flow graph, augmenting edges and all, is a necessary and sufficient condition for the eliminability of go to's from that program under the stricter rules.