I think the problem with that idea is that there's no guarantee that the input to the compiler is going to be ideal. It might be that the programmer write some

I think the problem with that idea is that there's no guarantee that the input to the compiler is going to be ideal. It might be that the programmer write something in a suboptimal or naive way (possibly expecting the compiler to take care of it) or the source was mechanically generated or expanded via a preprocessor. Working strictly backwards from the source seems like it wouldn't be enough. So a production optimizing compiler is going to need all those cleanup passes one way or another. Why not assume they're there and lean on them rather than duplicating that work?