ToEx: Accelerating Generation Stage of Transformer-Based Language Models via Token-Adaptive Early Exit

Transformer-based language models have recently gained popularity in numerous natural language processing (NLP) applications due to their superior performance compared to traditional algorithms. These models involve two execution stages: summarization and generation. The generation stage accounts for a significant portion of the total execution time due to its auto-regressive property, which necessitates considerable and repetitive off-chip accesses. Consequently, our objective is to minimize off-chip accesses during the generation stage to expedite transformer execution. To achieve the goal, we propose a token-adaptive early exit (ToEx) that generates output tokens using fewer decoders, thereby reducing off-chip accesses for loading weight parameters. Although our approach has the potential to minimize data communication, it brings two challenges: 1) inaccurate self-attention computation, and 2) significant overhead for exit decision. To overcome these challenges, we introduce a methodology that facilitates accurate self-attention by lazily performing computations for previously exited tokens. Moreover, we mitigate the overhead of exit decision by incorporating a lightweight output embedding layer. We also present a hardware design to efficiently support the proposed work. Evaluation results demonstrate that our work can reduce the number of decoders by 2.6$\times$ on average. Accordingly, it achieves 3.2$\times$ speedup on average compared to transformer execution without our work.