Index launches: scalable, flexible representation of parallel task groups

It's common to see specialized language constructs in modern task-based programming systems for reasoning about groups of independent tasks intended for parallel execution. However, most systems use an ad-hoc representation that limits expressiveness and often overfits for a given application domain. We introduce index launches, a scalable and flexible representation of a group of tasks. Index launches use a flexible mechanism to indicate the data required for a given task, allowing them to be used for a much broader set of use cases while maintaining an efficient representation. We present a hybrid design for index launches, involving static and dynamic program analyses, along with a characterization of how they're used in Legion and Regent, and show how they generalize constructs found in other task-based systems. Finally, we present results of scaling experiments which demonstrate that index launches are crucial for the efficient distributed execution of several scientific codes in Regent.