arXiv Machine Learning By Katherine Tieu, Dongqi Fu, Si Zhang, Hanqing Zeng, Tianxin Wei, Chenglin Wei, Yinglong Xia, Hong Li, Jingrui He

PBPO: Prefix-Block Policy Optimization for Diffusion Language Models

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The paper introduces Prefix-Block Policy Optimization (PBPO), a method that adapts block-level policy optimization to Diffusion Large Language Models (dLLMs). PBPO constructs a prefix-block state and uses a single model evaluation to create a factorized one‑step surrogate for block likelihood, allowing surrogate importance weighting and clipping to be applied separately to each token. Experiments on LLaDA-family models across six benchmarks and three generation lengths show that PBPO achieves competitive performance compared to existing dLLM reinforcement learning methods.

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