Memorization Dynamics of Fill-in-the-Middle Pretraining
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Computer Science > Computation and Language
Title:Memorization Dynamics of Fill-in-the-Middle Pretraining
Abstract:Fill-in-the-middle (FIM) is a pretraining objective widely used to equip causal language models with infilling ability, yet its effect on verbatim memorization remains underexplored. We study the memorization dynamics of FIM in a controlled setting by pretraining matched Llama 3.2 models with FIM and standard left-to-right (LTR) objectives on a FineWeb-Gutenberg corpus containing repeated Gutenberg excerpts. With prefix-based probes, FIM more often recovers short or partially matching spans, while LTR more often assigns high confidence to long exact continuations. We observe that verbatim extraction under FIM-training grows approximately linearly with repetitions over the tested range. Evaluating native FIM-format probes reveals that suffix context is not sufficient: verbatim recall under FIM-training remains strongly anchored in prefix context. Our results also show that evaluating only one span length or probing format can miss important nuances in memorization behavior.
| Comments: | MemFM @ ICML 2026 |
| Subjects: | Computation and Language (cs.CL); Artificial Intelligence (cs.AI); Machine Learning (cs.LG) |
| Cite as: | arXiv:2605.22981 [cs.CL] |
| (or arXiv:2605.22981v1 [cs.CL] for this version) | |
| https://doi.org/10.48550/arXiv.2605.22981
arXiv-issued DOI via DataCite (pending registration)
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