Caffeine: A parallel runtime library for supporting modern Fortran compilers

The CoArray Fortran Framework of Efficient Interfaces to Network Environments (Caffeine) provides a runtime library that supports Fortran's SPMD features. Caffeine implements inter-process communication by building atop the GASNet-EX exascale networking middleware library. Caffeine is the first implementation of the compiler- and runtime-agnostic Parallel Runtime Interface for Fortran (PRIF) specification.  Any compiler that targets PRIF can use any runtime that supports PRIF.  Caffeine supports researching the novel approach of writing most of a compiler's parallel runtime library in the language being compiled: Caffeine is primarily implemented using Fortran's non-parallel features, with a thin C-language layer that invokes the external GASNet-EX communication library. Exploring this approach in open source lowers a barrier to contributions from the compiler's users: Fortran programmers.  Caffeine also facilitates research such as investigating various optimization opportunities that exploit specific hardware such as shared memory or specific interconnects.