Scale-CIM: Precision-scalable computing-in-memory for energy-efficient quantized neural networks

Quantized neural networks (QNNs), which perform multiply-accumulate (MAC) operations with low-precision weights or activations, have been widely exploited to reduce energy consumption. QNNs usually have a tradeoff between energy consumption and accuracy depending on the quantized precision, so that it is necessary to select an appropriate precision for energy efficiency. Nevertheless, the conventional hardware accelerators such as Google TPU are typically designed and optimized for a specific precision (e.g., 8-bit), which may degrade energy efficiency for other precisions. Though an analog-based computing-in-memory (CIM) technology supporting variable precision has been proposed to improve energy efficiency, its implementation requires extremely large and power-consuming analog-to-digital converters (ADCs). In this paper, we propose Scale-CIM, a precision-scalable CIM architecture which supports MAC operations based on digital computations (not analog computations). Scale-CIM performs binary MAC operations with high parallelism, by executing digital-based multiplication operations in the CIM array and accumulation operations in the peripheral logic. In addition, Scale-CIM supports multi-bit MAC operations without ADCs, based on the binary MAC operations and shift operations depending on the precision. Since Scale-CIM fully utilizes the CIM array for various quantized precisions (not for a specific precision), it achieves high compute-throughput. Consequently, Scale-CIM enables precision-scalable CIM-based MAC operations with high parallelism. Our simulation results show that Scale-CIM achieves 1.5-15.8 x speedup and reduces system energy consumption by 53.7-95.7% across different quantized precisions, compared to the state-of-the-art precision-scalable accelerator.