FPGA Logic Block Architectures for Efficient Deep Learning Inference

Reducing the precision of deep neural network (DNN) inference accelerators can yield large efficiency gains with little or no accuracy degradation compared to half or single precision floating-point by enabling more multiplication operations per unit area. A wide range of precisions fall on the pareto-optimal curve of hardware efficiency vs. accuracy with no single precision dominating, making the variable precision capabilities of FPGAs very valuable. We propose three types of logic block architectural enhancements and fully evaluate a total of six architectures that improve the area efficiency of multiplications and additions implemented in the soft fabric. Increasing the LUT fracturability and adding two adders to the ALM (4-bit Adder Double Chain architecture) leads to a 1.5× area reduction for arithmetic heavy machine learning (ML) kernels, while increasing their speed. In addition, this architecture also reduces the logic area of general applications by 6%, while increasing the critical path delay by only 1%. However, our highest impact option, which adds a 9-bit shadow multiplier to the logic clusters, reduces the area and critical path delay of ML kernels by 2.4× and 1.2×, respectively. These large gains come at a cost of 15% logic area increase for general applications.