In the Ethereum ecosystem, smart contract execution is governed by a gas model where every computational operation incurs a direct economic cost. Consequently, gas efficiency is a critical requirement for the economic sustainability and widespread adoption of decentralized applications. Despite the importance of cost-effective development, there remains a significant gap between theoretical optimization patterns and the actual practices applied by developers in real-world projects. This paper presents an empirical study investigating gas optimization practices in open-source Solidity repositories created on GitHub between 2018 and 2025. Adopting a Mining Software Repositories approach, we analyzed a dataset of gas-related commits identified through a multi-stage pipeline involving NLP-based filtering and manual qualitative inspection. Based on the analysis of 400 representative commits, we derive an empirical taxonomy of gas optimization strategies consisting of 8 macro-categories and 36 subcategories. Our findings reveal that optimization efforts span multiple levels of abstraction, from low-level arithmetic refinements and bytecode-size reductions to broader architectural changes such as logic internalization and external call reduction. Furthermore, we observe that developers often manage complex trade-offs between execution frequency, safety checks, and computational costs, highlighting that effective optimization is inherently context-dependent. This study provides a practical knowledge base for the design of future developer support tools and advances the understanding of cost-aware software engineering in the blockchain domain.

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