DiffuCoder: Understanding and Improving Masked Diffusion Models for Code
  Generation
Authors: name: Shansan Gong; name: Ruixiang Zhang; name: Huangjie Zheng; name: Jiatao Gu; name: Navdeep Jaitly; name: Lingpeng Kong; name: Yizhe Zhang
Published: 2025-06-25 17:35:47

Abstract: Diffusion large language models (dLLMs) are compelling alternatives to
autoregressive (AR) models because their denoising models operate over the
entire sequence. The global planning and iterative refinement features of dLLMs
are particularly useful for code generation. However, current training and
inference mechanisms for dLLMs in coding are still under-explored. To demystify
the decoding behavior of dLLMs and unlock their potential for coding, we
systematically investigate their denoising processes and reinforcement learning
(RL) methods. We train a 7B dLLM, DiffuCoder, on 130B tokens of code.
Using this model as a testbed, we analyze its decoding behavior, revealing how
it differs from that of AR models: (1) dLLMs can decide how causal their
generation should be without relying on semi-AR decoding, and (2) increasing
the sampling temperature diversifies not only token choices but also their
generation order. This diversity creates a rich search space for RL rollouts.
For RL training, to reduce the variance of token log-likelihood estimates and
maintain training efficiency, we propose coupled-GRPO, a novel
sampling scheme that constructs complementary mask noise for completions used
in training. In our experiments, coupled-GRPO significantly improves
DiffuCoder's performance on code generation benchmarks (+4.4\% on EvalPlus) and
reduces reliance on AR causal during decoding. Our work provides deeper insight
into the machinery of dLLM generation and offers an effective, diffusion-native
RL training framework. https://github.com/apple/ml-diffucoder.