Rethinking integer divider design for FPGA-based soft-processors

Most existing soft-processors on FPGAs today support a fixed-latency instruction pipeline. Therefore, for integer division, a simple fixed-latency radix-2 integer divider is typically used, or algorithm-level changes are made to avoid integer divisions. However, for certain important application domains the simple radix-2 integer divider becomes the performance bottleneck, as every 32-bit division operation takes 32 cycles. In this paper, we explore integer divider designs for FPGA-based soft-processors, by leveraging the recent support of variable-latency execution units in their instruction pipeline. We implement a high-performance, data-dependent, variable-latency integer divider called Quick-Div, optimize its performance on FPGAs, and integrate it into a RISC-V soft-processor called Taiga that supports a variable-latency instruction pipeline. We perform a comprehensive analysis and comparison-in terms of cycles, clock frequency, and resource usage-for both the fixed-latency radix-2/4/8/16 dividers and our variable-latency Quick-Div divider with various optimizations. Experimental results on a Xilinx Virtex UltraScale+ VCU118 FPGA board show that our Quick-Div divider can provide over 5x better performance and over 4x better performance/LUT compared to a radix-2 divider for certain applications like random number generation. Finally, through a case study of integer square root, we demonstrate that our Quick-Div divider provides opportunities for reconsidering simpler and faster algorithmic choices.