Implementation of sensitivity analysis for automatic parallelization

Sensitivity Analysis (SA) is a novel compiler technique that complements, and integrates with, static automatic parallelization analysis for the cases when program behavior is input sensitive. SA can extract all the input dependent, statically unavailable, conditions for which loops can be dynamically parallelized. SA generates a sequence of sufficient conditions which, when evaluated dynamically in order of their complexity, can each validate the dynamic parallel execution of the corresponding loop. While SA's principles are fairly simple, implementing it in a real compiler and obtaining good experimental results on benchmark codes is a difficult task. In this paper we present some of the most important implementation issues that we had to overcome in order to achieve a fairly successful automatic parallelizer. We present techniques related to validating dependence removing transformations, e.g., privatization or pushback parallelization, and static and dynamic evaluation of complex conditions for loop parallelization. We concern ourselves with multi-version and parallel code generation as well as the use of speculative parallelization when other, less costly options fail. We present a summary table of the contributions of our techniques to the successful parallelization of 22 industry benchmark codes. We also report speedups and parallel coverage of these codes on two multicore based systems and compare them to results obtained by the Ifort compiler.