I recommend skimming the PLDI paper linked by asmeurer, it has a good summary. Some of its transforms are pretty C-specific, using the Clang frontend; some are

I recommend skimming the PLDI paper linked by asmeurer, it has a good summary. Some of its transforms are pretty C-specific, using the Clang frontend; some are pretty generic and probably work on any Algol-descended language. It's meant to be a modular tool, so you could add transforms that understand other languages if you like.