Taming Control Flow: A Structured Approach to Eliminating Goto Statements

1994 IEEE. Personal use of this material is permitted. However, permission to reprint/republish this material for advertising or promotional purposes or for creating new collective works for resale or redistribution to servers or lists, or to reuse any copyrighted component of this work in other works must be obtained from the IEEE. Abstract In designing optimizing and parallelizing compilers, it is often simpler and more eecient to deal with programs that have structured control ow. Although most programmers naturally program in a structured fashion, there remain many important programs and benchmarks that include some number of goto statements , thus rendering the entire program unstruc-tured. Such unstructured programs cannot be handled with compilers built with analyses and transformations for structured programs. In this paper we present a straightforward algorithm to structure C programs by eliminating all goto statements. The method works directly on a high-level abstract syntax tree (AST) representation of the program and could easily be integrated into any compiler that uses an AST-based intermediate representation. The actual algorithm proceeds by eliminating each goto by rst applying a sequence of goto-movement transformations followed by the appropriate goto-elimination transformation. We have implemented the method in the Mc-CAT (McGill Compiler Architecture Testbed) opti-mizing/parallelizing C compiler and we present experimental results that demonstrate that the method is both eecient and eeective.