Accuracy Evaluation of Long Short Term Memory Network Based Language Model with Fixed-Point Arithmetic

Long Short Term Memory network based language models are state-of-art techniques in the field of natural language processing. Training LSTM networks is computationally intensive, which naturally results in investigating FPGA acceleration where fixed-point arithmetic is employed. However, previous studies have focused only on accelerators using some fixed bit-widths without thorough accuracy evaluation. The main contribution of this paper is to demonstrate the bit-width effect on the LSTM based language model and the tanh function approximation in a comprehensive way by experimental evaluation. Theoretically, the 12-bit number with 6-bit fractional part is the best choice balancing the accuracy and the storage saving. Gaining similar performance to the software implementation and fitting the bit-widths of FPGA primitives, we further propose a mixed bit-widths solution combing 8-bit numbers and 16-bit numbers. With clear trade-off in accuracy, our results provide a guide to inform the design choices on bit-widths when implementing LSTMs in FPGAs. Additionally, based on our experiments, it is amazing that the scale of the LSTM network is irrelevant to the optimum fixed-point configuration, which indicates that our results are applicable to larger models as well.