Code motion of control structures in high-level languages

• Authors: Ron Cytron;Andy Lowry;F. Kenneth Zadeck
• Venue: POPL '86 Proceedings of the 13th ACM SIGACT-SIGPLAN symposium on Principles of programming languages
• Year: 1986

One trend among programmers is the increased use of abstractions. Through encapsulation techniques, abstractions extend the repertory of data structures and their concomitant operations that are processed directly by a compiler. For example, a compiler might not offer sets or set operations in its base language, but abstractions allow a programmer to define sets in terms of constructs already recognized by the compiler. In particular, abstractions can allow new constructs to be defined in terms of other abstractions. Although significant power is gained through the use of layered abstractions, object code quality suffers as increasingly less of a program's data structures and operations are exposed to the optimization phase of a compiler. Multiple references to abstractions are also inefficient, since the interaction between abstractions is often complex yet hidden from a compiler. Abstractions are most flexible when they are cast in general terms; a specific invocation is then tailored by the abstraction to obtain the appropriate code. A sequence of references to such abstractions can be inefficient due to functional redundancy that cannot be detected at compile-time. By integrating the references, the offending segments of code can be moved to a more advantageous position. Although procedure integration materializes abstracted constructs, the abstractions can still be ineligible for optimization using current techniques; in particular, abstractions often involve loops and conditional branches that can obscure code that would otherwise be eligible for code motion.