Sub-Word Parallel Precision-Scalable MAC Engines for Efficient Embedded DNN Inference

To enable energy-efficient embedded execution of Deep Neural Networks (DNNs), the critical sections of these workloads, their multiply-accumulate (MAC) operations, need to be carefully optimized. The SotA pursues this through run-time precision-scalable MAC operators, which can support the varying precision needs of DNNs in an energy-efficient way. Yet, to implement the adaptable precision MAC operation, most SotA solutions rely on separately optimized low precision multipliers and a precision-variable accumulation scheme, with the possible disadvantages of a high control complexity and degraded throughput. This paper, first optimizes one of the most effective SotA techniques to support fully-connected DNN layers. This mode, exploiting the transformation of a high precision multiplier into independent parallel low-precision multipliers, will be called the Sum Separate (SS) mode. In addition, this work suggests an alternative low-precision scheme, i.e. the implicit accumulation of multiple low precision products within the multiplier itself, called the Sum Together (ST) mode. Based on the two types of MAC arrangements explored, corresponding architectures have been proposed to implement DNN processing. The two architectures, yielding the same throughput, are compared in different working precisions (2/4/8/16-bit), based on Post-Synthesis simulation. The result shows that the proposed ST-Mode based architecture outperforms the earlier SS-Mode by up to ×1.6 on Energy Efficiency (TOPS/W) and ×1.5 on Area Efficiency (GOPS/mm
²
).