Translation and Run-Time Validation of Optimized Code

Abstract The paper presents approaches to the validation of optimizing compilers. The emphasis is on aggressive and architecture-targeted optimizations which try to obtain the highest performance from modern architectures, in particular EPIC-like micro-processors. Rather than verify the compiler, the approach of translation validation performs a validation check after every run of the compiler, producing a formal proof that the produced target code is a correct implementation of the source code. First we survey the standard approach to validation of optimizations which preserve the loop structure of the code (though they may move code in and out of loops and radically modify individual statements), present a simulation-based general technique for validating such optimizations, and describe a tool, VOC-64, which implements these technique. For more aggressive optimizations which, typically, alter the loop structure of the code, such as loop distribution and fusion, loop tiling, and loop interchanges, we present a set of permutation rules which establish that the transformed code satisfies all the implied data dependencies necessary for the validity of the considered transformation. We describe the necessary extensions to the VOC-64 in order to validate these structure-modifying optimizations. Finally, the paper discusses preliminary work on run-time validation of speculative loop optimizations, that involves using run-time tests to ensure the correctness of loop optimizations which neither the compiler nor compiler-validation techniques can guarantee the correctness of. Unlike compiler validation, run-time validation has not only the task of determining when an optimization has generated incorrect code, but also has the task of recovering from the optimization without aborting the program or producing an incorrect result. This technique has been applied to several loop optimizations, including loop interchange, loop tiling, and software pipelining and appears to be quite promising.