Program Decision Logic Optimization using Predication and Control Speculation [abstract] (IEEE Xplore, PDF, PostScript, IEEE Xplore)
Wen-mei W. Hwu, David I. August, and John W. Sias
Proceedings of the IEEE, Volume 89, Number 11, November 2001.

The mainstream arrival of predication, a means other than branching of selecting instructions for execution, has required compiler architects to reformulate fundamental analyses and transformations. Traditionally, the compiler has generated branches straightforwardly to implement control flow designed by the programmer and has then performed sophisticated global optimizations to move and optimize code around them. In this model, the inherent tie between the control state of the program and the location of the single instruction pointer serialized runtime evaluation of control and limited the extent to which the compiler could optimize the control structure of the program (without extensive code replication). Predication provides a means of control independent of branches and instruction fetch location, freeing both compiler and architecture from these restrictions; effective compilation of predicated code, however, requires sophisticated understanding of the program s control structure. This article explores a representational technique which, through direct code analysis, maps the program's control component into a canonical database, a reduced ordered binary decision diagram (ROBDD), which fully enables the compiler to utilize and manipulate predication. This abstraction is then applied to optimize the program s control component, transforming it into a form more amenable to instruction-level parallel (ILP) execution.