The rapid rise in the popularity of large language models has highlighted the need for extensive datasets, especially for training on code. However, this growth has also raised important questions about the legal implications of using code in large language model training, particularly regarding the potential infringement of code licenses. At the same time, the availability of clean datasets for evaluating these models is becoming increasingly limited, due to a high risk of contamination which restricts the capacity for reliable research. On top of that, this requires researchers to repeatedly perform data curation steps in order to evaluate their models on downstream tasks, based on previously unseen data. This process is not only time- and resource-intensive but also introduces potential inconsistencies across studies, which can impact their reproducibility.
We address these challenges through a comprehensive licensing analysis and by developing robust datasets to support accurate and reproducible large language model evaluations. We compiled a list of 53 large language models trained on file-level code and analyzed their datasets, discovering pervasive license inconsistencies despite careful selection based on repository licenses. Our analysis, covering 514M code files, reveals 38M exact duplicates of strong copyleft code, and 171M file-leading comments, 16M of which are under copyleft licenses and another 11M discouraging unauthorized copying. To further understand the depth of non-permissive code in public training datasets, we developed StackLessV2, a strong copyleft Java dataset decontaminated against The Stack V2 to facilitate accurate model evaluations. Our results revealed that non-permissive code is also present at the near-duplication level, although, this represents a gray area in terms of legal interpretation, where the boundary between acceptable reuse and license violation is still unclear, emphasizing the need for further legal clarification. Finally, we extend on this and introduce The Heap, a large multilingual copyleft dataset covering 57 programming languages, specifically deduplicated to avoid contamination from existing open training datasets. The Heap offers a solution for conducting fair, reproducible evaluations of large language models without the significant overhead of the data curation process.

In recent years, deep learning techniques, particularly transformer models, have demonstrated remarkable advancements in the accuracy and efficiency of language models. These models provide the foundation for many natural language processing tasks, including code completion. The effectiveness of code completion models has been the subject of a variety of empirical studies. However, none of the existing literature has explicitly investigated the potential impact of common code structures on the performance of large language models during code completion. This paper evaluates the influence of common code structures on the code completion performance of CodeParrot, a state-of-the-art natural language processing model. Using the tuned lens method, we show that typical code structures lead to a higher completion accuracy compared to uncommon code structures, due to their frequent occurrence, consistent syntax, clear semantics, and contextual clues. Finally, we perform an attention investigation to assess the significance of the common code structures and reveal potential data patterns across low- and high-resource languages.