COMPACT: Co-processor for Multi-mode Precision-adjustable Non-linear Activation Functions

Non-linear activation functions imitating neuron behaviors are ubiquitous in machine learning algorithms for time series signals while also demonstrating significant gain in precision for conventional vision-based deep learning networks. State-of-the-art implementation of such functions on GPU-like devices incurs a large physical cost, whereas edge devices adopt either linear interpolation or simplified linear functions leading to degraded precision. In this work, we design COMPACT, a co-processor with adjustable precision for multiple non-linear activation functions including but not limited to exponent, sigmoid, tangent, logarithm, and mish. Benchmarking with state-of-the-arts, COMPACT achieves a 26% reduction in the absolute error on a 1.6x widen approximation range taking advantage of the triple decomposition technique inspired by Hajduk's formula of Padé approximation. A SIMD-ISA-based vector co-processor has been implemented on FPGA which leads to a 30% reduction in execution latency but the area overhead nearly remains the same with related designs. Furthermore, COMPACT is adjustable to 46% latency improvement when the maximum absolute error is tolerant to the order of 1E-3.