This talk recounts the experience of migrating a proprietary, closed-source vendor codebase to the standard Clang toolchain, a significant departure from previous projects where full source code was available. The lack of source access necessitated a heavy reliance on automated solutions and the development of new features within the LLVM ecosystem. Key strategies included building minimal baremetal ports of the **UBSan** and **PGO** runtimes to detect undefined behavior and guide performance optimizations, respectively. The team also utilized **FatLTO** for aggressive optimizations under tight resource constraints, implemented the `-fseparate-named-section` option to manage memory layout, and proposed a **DWARF CFI validation** feature to aid in debugging hand-written assembly. The presentation will cover the lessons learned, the new features developed during this process, and ideas to simplify similar migrations in the future.