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From Bytes to Ideas: Language Modeling with Autoregressive U-Nets (arxiv.org)
2 points by GaggiX on Jun 23, 2025 | hide | past | pdf | discuss on HN

In plain words: Instead of chopping text into fixed chunks beforehand, this model reads raw bytes and pools them into words, then word pairs, then groups of four; deeper layers predict further ahead. With equal training compute, shallow versions matched standard pre-split token models; deeper ones showed promise.

Abstract

Tokenization imposes a fixed granularity on the input text, freezing how a language model operates on data and how far in the future it predicts. Byte Pair Encoding (BPE) and similar schemes split text once, build a static vocabulary, and leave the model stuck with that choice. We relax this rigidity by introducing an autoregressive U-Net that learns to embed its own tokens as it trains. The network reads raw bytes, pools them into words, then pairs of words, then up to 4 words, giving it a multi-scale view of the sequence. At deeper stages, the model must predict further into the future -- anticipating the next few words rather than the next byte -- so deeper stages focus on broader semantic patterns while earlier stages handle fine details. When carefully tuning and controlling pretraining compute, shallow hierarchies tie strong BPE baselines, and deeper hierarchies have a promising trend. Because tokenization now lives inside the model, the same system can handle character-level tasks and carry knowledge across low-resource languages.

Mathurin Videau, Badr Youbi Idrissi, Alessandro Leite, Marc Schoenauer, Olivier Teytaud, David Lopez-Paz
arXiv:2506.14761 · cs.CL, cs.AI · submitted Jun 17, 2025
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