An extension of C++ with memory-centric specifications for HPC to reduce memory footprints and streamline MPI development

Abstract: The C++ programming language and its cousins lean towards a memory-inefficient storage of structs: The compiler inserts helper bits such that individual instance variables fit to byte or cache boundaries, while it is not able to exploit knowledge about the range of integers, enums or bitsets. Furthermore, the language provides neither support for data exchange via MPI nor for arbitrary floating-point precisions. We propose C++ attributes through which developers can guide the compiler what memory arrangements would be beneficial: Can multiple booleans or integers with limited range be squeezed into one bit field, do floating point numbers hold fewer significant bits than in the IEEE standard, or does the code benefit from a MPI datatype for subsets of attributes? The extension offers the opportunity to fall back to normal alignment via plain C++ assignments, no dependencies upon external libraries are introduced, and the resulting code remains standard C++ subject to some weakened guarantees on addresses and pointer arithmetics. Our work implements the language annotations within LLVM and demonstrates their potential impact, both upon the runtime and the memory footprint, through smoothed particle hydrodynamics (SPH) benchmarks. They uncover the potential gains in terms of performance and development productivity.