Software-Defined Vector Processing on Manycore Fabrics

We describe a tiled architecture that can fluidly transition between manycore (MIMD) and vector (SIMD) execution. The hardware provides a software-defined vector programming model that lets applications aggregate groups of manycore tiles into logical vector engines. In manycore mode, the machine behaves as a standard parallel processor. In vector mode, groups of tiles repurpose their functional units as vector execution lanes and scratchpads as vector memory banks. The key mechanism is an instruction forwarding network: a single tile fetches instructions and sends them to other trailing cores. Most cores disable their frontends and instruction caches, so vector groups amortize the intrinsic hardware costs of von Neumann control. Vector groups also use a decoupled access/execute scheme to centralize their memory requests and issue coalesced, wide loads. We augment an existing RISC-V manycore design with a minimal hardware extension to implement software-defined vectors. Cycle-level simulation results show that software-defined vectors improve performance by an average of 1.7 × over standard MIMD execution while saving 22% of the energy. Compared to a similarly configured GPU, the architecture improves performance by 1.9 ×.