  CNNs outperform traditional machine learning algorithms across a wide range
of applications. However, their computational complexity makes it necessary to
design efficient hardware accelerators. Most CNN accelerators focus on
exploring dataflow styles that exploit computational parallelism. However,
potential performance speedup from sparsity has not been adequately addressed.
The computation and memory footprint of CNNs can be significantly reduced if
sparsity is exploited in network evaluations. To take advantage of sparsity,
some accelerator designs explore sparsity encoding and evaluation on CNN
accelerators. However, sparsity encoding is just performed on activation or
weight and only in inference. It has been shown that activation and weight also
have high sparsity levels during training. Hence, sparsity-aware computation
should also be considered in training. To further improve performance and
energy efficiency, some accelerators evaluate CNNs with limited precision.
However, this is limited to the inference since reduced precision sacrifices
network accuracy if used in training. In addition, CNN evaluation is usually
memory-intensive, especially in training. In this paper, we propose SPRING, a
SParsity-aware Reduced-precision Monolithic 3D CNN accelerator for trainING and
inference. SPRING supports both CNN training and inference. It uses a binary
mask scheme to encode sparsities in activation and weight. It uses the
stochastic rounding algorithm to train CNNs with reduced precision without
accuracy loss. To alleviate the memory bottleneck in CNN evaluation, especially
in training, SPRING uses an efficient monolithic 3D NVM interface to increase
memory bandwidth. Compared to GTX 1080 Ti, SPRING achieves 15.6X, 4.2X and
66.0X improvements in performance, power reduction, and energy efficiency,
respectively, for CNN training, and 15.5X, 4.5X and 69.1X improvements for
inference.
