SRC Research Report 4
SRC Research Report 4
Suppose that we want to eliminate the local go to statements in a PASCAL program by replacing them with multilevel loop exit statements. There is a standard technique for doing so that succeeds if and only if the flow graph of the PASCAL program is reducible. This technique assumes that we don't allow ourselves either to introduce new variables or to replicate code, but that we do allow ourselves to reorder the atomic tests and actions within the text of the program and to rewrite the connecting control structures from scratch. In this paper, we shall investigate the extent to which go tos can be replaced with exits while preserving as much as possible of the program's original structure. On the negative side, we shall find that there are programs whose flow graphs are reducible but whose go tos cannot be eliminated without reordering their tests and actions. That is, programs with go tos can have their atomic elements in some weird static order, an order that doesn't correspond in any structured way to the dynamic flow of control. We shall analyze this situation by augmenting our flow graphs with edges that encode the static order of the atomic elements and then showing that the augmented flow graphs of programs with exits are always reducible. On the positive side, given a program with go tos whose augmented flow graph is reducible, we shall show that we can replace its go tos with exits while preserving essentially all of its structure. In fact, we can simply delete the go to statements and the labels they jump to and insert various exit statements and labeled Repeat-Endloop pairs for them to jump out of, without changing the rest of the program text in any way.
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