Compiler Automatic Discovery of OmpSs Task Dependencies

Dependence analysis is an essential step for many compiler optimizations, from simple loop transformations to automatic parallelization. Parallel programming models require specific dependence analyses that take into account multi-threaded execution. Furthermore, asynchronous parallelism introduced by OpenMP tasks has promoted the development of new dependency analysis techniques. In these terms, OmpSs parallel programming model extends OpenMP tasks with the definition of intertask dependencies. This extension allows run-time dependency detection, which potentially improves the performance when load balancing or locality rule the execution time. On the other side, the extension requires the user to figure out data-sharing attributes and the type of access to each data in all tasks in order to correctly specify the dependencies. We aim to enhance the programmability of OmpSs with a new methodology that enables the compiler to automatically determine the dependencies of OmpSs tasks, thus releasing users from the task of manually defining these dependencies. In this context, we have developed an algorithm based on the discovery of code concurrent to a task and liveness analysis. The algorithm first finds out all code concurrent with a given task. Then, it computes the data-sharing attributes of the variables appearing in the task. Finally, it analyzes the liveness properties of the task’s shared variables. With this information, the algorithm figures out the proper dependencies of the task. We have implemented this algorithm in the Mercurium source-to-source compiler. We have tested the results with several benchmarks proving that the algorithm is able to correctly find a large number of dependency expressions.