Ax-BxP: Approximate Blocked Computation for Precision-reconfigurable Deep Neural Network Acceleration

Precision scaling has emerged as a popular technique to optimize the compute and storage requirements of Deep Neural Networks (DNNs). Efforts toward creating ultra-low-precision (sub-8-bit) DNNs for efficient inference suggest that the minimum precision required to achieve a given network-level accuracy varies considerably across networks, and even across layers within a network. This translates to a need to support variable precision computation in DNN hardware. Previous proposals for precision-reconfigurable hardware, such as bit-serial architectures, incur high overheads, significantly diminishing the benefits of lower precision. We propose Ax-BxP, a method for approximate blocked computation wherein each multiply-accumulate operation is performed block-wise (a block is a group of bits), facilitating re-configurability at the granularity of blocks. Further, approximations are introduced by only performing a subset of the required block-wise computations to realize precision re-configurability with high efficiency. We design a DNN accelerator that embodies approximate blocked computation and propose a method to determine a suitable approximation configuration for any given DNN. For the AlexNet, ResNet50, and MobileNetV2 DNNs, Ax-BxP achieves improvement in system energy and performance, respectively, over an 8-bit fixed-point (FxP8) baseline, with minimal loss (<1% on average) in classification accuracy. Further, by varying the approximation configurations at a finer granularity across layers and data-structures within a DNN, we achieve improvement in system energy and performance, respectively.