This year, systemd had a breakup with its bad practice of including unused headers all over the codebase. This resulted in:

• A 33% speedup in from scratch build times
• A 50% reduction in runtime for our build test CI jobs
• Thousands of lines of code removed from the codebase

I'll present how I went about this work, using clang-include-cleaner, clang-tidy and ClangBuildAnalyzer, and including the challenges I faced:

• A scalable way to organize source to minimize unused headers
• Macros
• Different build configurations which change the used headers in a source file due to #ifdef conditionals
• Missing features in clang-tidy and clang-include-cleaner (and my contributions to LLVM to implement those)

https://github.com/systemd/systemd https://github.com/llvm/llvm-project github.com/aras-p/ClangBuildAnalyzer