Leveraging LLMs for Legacy Code Modernization: Challenges and Opportunities for LLM-Generated Documentation

TL;DR

This study explores the potential of large language models to generate documentation for legacy code in outdated languages, evaluating their effectiveness and limitations through human and automated assessments.

Contribution

It introduces a prompting strategy for LLMs to produce line-wise comments on legacy code and evaluates their quality, revealing current metrics' limitations.

Findings

LLM-generated comments are generally hallucination-free, complete, readable, and useful.

Automated metrics do not strongly correlate with human judgments of comment quality.

A significant challenge remains in developing better evaluation metrics for LLM-generated legacy code documentation.

Abstract

Legacy software systems, written in outdated languages like MUMPS and mainframe assembly, pose challenges in efficiency, maintenance, staffing, and security. While LLMs offer promise for modernizing these systems, their ability to understand legacy languages is largely unknown. This paper investigates the utilization of LLMs to generate documentation for legacy code using two datasets: an electronic health records (EHR) system in MUMPS and open-source applications in IBM mainframe Assembly Language Code (ALC). We propose a prompting strategy for generating line-wise code comments and a rubric to evaluate their completeness, readability, usefulness, and hallucination. Our study assesses the correlation between human evaluations and automated metrics, such as code complexity and reference-based metrics. We find that LLM-generated comments for MUMPS and ALC are generally hallucination-free, complete, readable, and useful compared to ground-truth comments, though ALC poses challenges. However, no automated metrics strongly correlate with comment quality to predict or measure LLM performance. Our findings highlight the limitations of current automated measures and the need for better evaluation metrics for LLM-generated documentation in legacy systems.