Title: Task-Agnostic and Adaptive-Size BERT Compression. Abstract: While pre-trained language models such as BERT and RoBERTa have achieved impressive results on various natural language processing tasks, they have huge numbers of parameters and suffer from huge computational and memory costs, which make them difficult for real-world deployment. Hence, model compression should be performed in order to reduce the computation and memory cost of pre-trained models. In this work, we aim to compress BERT and address the following two challenging practical issues: (1) The compression algorithm should be able to output multiple compressed models with different sizes and latencies, so as to support devices with different kinds of memory and latency limitations; (2) the algorithm should be downstream task agnostic, so that the compressed models are generally applicable for different downstream tasks.  We leverage techniques in neural architecture search (NAS) and propose NAS-BERT, an efficient method for BERT compression. NAS-BERT trains a big supernet on a carefully designed search space containing various architectures and outputs multiple compressed models with adaptive sizes and latency. Furthermore, the training of NAS-BERT is conducted on standard self-supervised pre-training tasks (e.g., masked language model) and does not depend on specific downstream tasks. Thus, the models it produces can be used across various downstream tasks. The technical challenge of NAS-BERT is that training a big supernet on the pre-training task is extremely costly. We employ several techniques including block-wise search, search space pruning, and performance approximation to improve search efficiency and accuracy. Extensive experiments on GLUE benchmark datasets demonstrate that NAS-BERT can find lightweight models with better accuracy than previous approaches, and can be directly applied to different downstream tasks with adaptive model sizes for different requirements of memory or latency.