Towards Efficient Hash Maps in Functional Array Languages

We present a systematic derivation of a data-parallel implementation of two-level, static and collision-free hash maps, by giving a functional formulation of the Fredman et al. construction, and then flattening it. We discuss the challenges of providing a flexible, polymorphic, and abstract interface to hash maps in a functional array language, with particular attention paid to the problem of dynamically sized keys, which we address by associating each hash map with an arbitrary context. The algorithm is implemented in Futhark, and the achieved GPU execution performance is compared on simple benchmark problems. We find that our hash maps outperform conventional tree/search-based approaches. Furthermore, our implementation is compared against the state-of-the-art cuCollections library, which is significantly faster for hash map construction, and to a lesser degree for lookups. We explain to which extent the performance difference is due to low-level code generation limitation in the Futhark compiler, and to which extent it can be attributed to the data-parallel programming vocabulary not providing the constructs necessary to express the equivalent of the algorithms used by cuCollections. We end by reflecting to which extent the functional array language programming model could, or should, be extended to address these weaknesses.