A Reconfigurable Approximate Multiplier for Quantized CNN Applications

Quantized CNNs, featured with different bit-widths at different layers, have been widely deployed in mobile and embedded applications. The implementation of a quantized CNN may have multiple multipliers at different precisions with limited resource reuse or one multiplier at higher precision than needed causing area overhead. It is then highly desired to design a multiplier by accounting for the characteristics of quantized CNNs to ensure both flexibility and energy efficiency. In this work, we present a reconfigurable approximate multiplier to support multiplications at various precisions, i.e., bit-widths. Moreover, unlike prior works assuming uniform distribution with bit-wise independence, a quantized CNN may have centralized weight distribution and hence follow a Gaussian-like distribution with correlated adjacent bits. Thus, a new block-based approximate adder is also proposed as part of the multiplier to ensure energy efficient operation with awareness of bit-wise correlation. Our experimental results show that the proposed adder significantly reduces the error rate by 76-98% over a state-of-the-art approximate adder for such scenarios. Moreover, with the deployment of the proposed multiplier, which is 17% faster and 22% more power saving than a Xilinx multiplier IP at the same precision, a quantized CNN implemented in FPGA achieves 17% latency reduction and 15% power saving compared with a full precision case.