Abstract: This paper reports on a large-scale comparative evaluation of IR-based tools for automatic bug localization. We have divided the tools in our evaluation into the following three generations: (1) The first-generation tools, now over a decade old, that are based purely on the Bag-of-Words (BoW) modeling of software libraries. (2) The somewhat more recent second-generation tools that augment BoW-based modeling with two additional pieces of information: historical data, such as change history, and structured information such as class names, method names, etc. And, finally, (3) The third-generation tools that are currently the focus of much research and that also exploit proximity, order, and semantic relationships between the terms. It is important to realize that the original authors of all these three generations of tools have mostly tested them on relatively small-sized datasets that typically consisted no more than a few thousand bug reports. Additionally, those evaluations only involved Java code libraries. The goal of the present paper is to present a comprehensive large-scale evaluation of all three generations of bug-localization tools with code libraries in multiple languages. Our study involves over 20,000 bug reports drawn from a diverse collection of Java, C/C++, and Python projects. Our results show that the third-generation tools are significantly superior to the older tools. We also show that the word embeddings generated using code files written in one language are effective for retrieval from code libraries in other languages.