Title: PSC: Efficient Grammar-Constrained Decoding via Parser Stack Classification
Primary area: applications to computer vision, audio, language, and other modalities
Keywords: constrained decoding, structured generation, LLM inference, context-free grammars

Abstract: LLMs are widely used to generate structured output like source code or JSON. Grammar-constrained decoding (GCD) can guarantee the syntactic validity of the generated output, by masking out tokens that violate rules specified by a context-free grammar. However, the online computational overhead of existing GCD methods, with latency typically scaling linearly with vocabulary size, limits the throughput of LLMs, especially for models with large vocabularies. To address this issue, we propose PSC, a novel grammar-constrained decoding method. By combining acceptance conditions of all vocabulary tokens into a single classifier of the parser stack during preprocessing, PSC can compute the complete vocabulary mask by checking the parser stack exactly once per decoding step, with time complexity independent of the vocabulary size. Experiments show that PSC computes masks up to 770× faster than baselines on complex programming language grammars, and up to 30× faster for schema-conformant JSON; end-to-end LLM throughput with PSC approaches that of unconstrained decoding.