Rethinking Layer Redundancy in Large Language Models: Calibration Objectives and Search for Depth Pruning

Depth pruning improves the inference efficiency of large language models by removing Transformer blocks. Prior work has largely treated layer redundancy as an inherent structural property of pretrained networks, emphasizing importance criteria and search algorithms for identifying removable layers. In contrast, we adopt a \emph{functional perspective}, where redundancy depends jointly on the model and the calibration objective, suggesting that a universal layer ranking may not exist. Through an empirical study across three LLM families, two calibration objectives, and seven search algorithms, we find that different objectives produce qualitatively different pruning patterns, while perplexity and downstream reasoning accuracy rankings often fail to align. In contrast, under a fixed objective, different search algorithms tend to converge to similar pruning solutions. Overall, our results suggest that the calibration objective may play a larger role than the particular search algorithm in determining which layers appear redundant.