This paper introduces ECO, a system that automatically modifies source code to improve performance at scale. ECO overcomes the localization problem by combining fleet-wide continuous profiling to identify performance-critical code with an embedding-based search to pinpoint specific optimization candidates, guided by a mined dictionary of performance anti-patterns. It overcomes the reliability problem through a multi-stage verification approach that uses automated testing, LLM-based self-review, and post-deployment monitoring to ensure changes are both correct and effective. Fully productionized and deployed within Google's hyperscale production fleet, ECO has successfully landed over 6,400 commits, changing more than 25,000 lines of production code. Incorrect changes are caught before they are submitted to production, and 99.5% of the submitted commits did not cause any rollbacks. These optimizations have resulted in savings equivalent to several hundred thousand normalized CPU cores, showing that ECO makes LLM-based optimization both practical at scale and highly impactful in real-world settings.