Tile-based bottom-up compilation of custom mesh-of-functional-units FPGA overlays

Mesh-of-functional-units (mesh-of-FUs) overlays can deliver high-performance because they expose the massively parallel FPGA fabric and have the ability to be customized for different applications. However, a key challenge is how to quickly compile a number of custom mesh-of-FUs overlays to FPGA fabric such that they achieve high f_MAX and scale to large mesh sizes. We propose a tile-based bottom-up CAD flow that utilizes the hierarchical physical design techniques of partitioning and floorplanning. Our flow partitions the overlay circuit into tiles, groups of adjacent overlay cells, and then compiles the tiles to a rectangular coarse-grain floorplan. Independent compilation of tiles is made possible by inserting complementary elastic buffers on inter-tile paths to ensure that these paths are not a bottleneck for f_MAX. As a result, an overlay can be formed by only “stitching” a set of pre-compiled tiles.We show that compared to the flat flow, our bottom-up flow results in higher f_MAX that degrades little with increasing overlay size. Further, our flow can generate a library of pre-compiled tiles that can be reused - it can stitch a set of library tiles into a new overlay in only 35 minutes. It also allows a divide-and-conquer overlay compilation flow by compiling its tiles in parallel on multiple machines.