Lookup Table Refactoring: Towards Efficient Logarithmic Number System Addition for Large Language Models

Compared to integer quantization, logarithmic quantization aligns more effectively with the long-tailed distribution of data in large language models (LLMs), resulting in lower quantization errors. Moreover, the logarithmic number system (LNS) employs a fixed-point adder to perform multiplication, indicating a potential reduction in computational complexity for LLM accelerators that require extensive multiply-accumulate (MAC) operations. However, a key bottleneck is that LNS addition requires complex nonlinear functions, which are typically approximated using lookup tables (LUTs). This study aims to reduce the hardware resources needed for LUTs in LNS addition while maintaining high precision. Specifically, we investigate the specific nature of addition operations within LLMs; the relationship between the hardware parameters of the LUT and the computing errors is then mathematically derived. Based on these insights, we propose LUT refactoring to optimize the LUT for enhanced efficiency in LNS addition. With 10.93% and 19.78% reductions in area-delay product (ADP) and power-delay product (PDP), respectively, LUT refactoring results in an accuracy improvement of up to 33.5% in LLM benchmarks compared to the naive design. When compared to integer quantization, our method achieves higher accuracy while reducing area by 18.27% and power by 42.61%.