This is true but what is really happening is that the frontend and backend cannot communicate intent effectively because of they way they are separated. The frontend doesn’t know what is important for optimisation because that’s not its job and the backend only sees the code the front gives it (never a wholistic view). So the easiest (and slowest) approach is to do everything over and over again.

Increasing Codegen units (multi unit compilation) is just the user taking a risk that splitting things up will not affect performance optimisations. Nothing smart about it.

If you had tiny compilation units and the frontend understood their significance to the backend then it would be able to build a graph of dirty code to be recompiled when a small piece of it changes.