Approaching overhead-free execution on FPGA soft-processors

Abstract

Implementing systems on FPGA soft-processors, rather than as custom hardware, eases and accelerates the development process, but at the cost of a great reduction in performance. Orthogonal to limitations in parallelism or clock frequency, this reduction in performance primarily originates in the intrinsic addressing and flow-control overheads of scalar microprocessors, which expend a considerable number of cycles interleaving address calculations and branch decisions within the actual useful work. We present an improved FPGA soft-processor architecture which statically overlaps "overhead" computations and executes them in parallel with the "useful" computations, significantly reducing the number of processor cycles needed to execute sequential programs, while reducing maximum clock frequency to 0.939x of its original value. In addition to eliminating almost all overhead computations, the proposed soft-processor can operate at 500 MHz on the Altera Stratix IV FPGA - 0.909x of the absolute maximum rating. Combined, the high speed and execution efficiency increase the range of FPGA designs amenable to soft-processors rather than custom hardware. We evaluate our cycle count improvements with multiple benchmarks, achieving speedups ranging from 1.07x for control-heavy code, to 1.92x for looping code, never performing worse than the original sequential code, and always performing better than a totally unrolled loop.