Which is why we don't write entire programs in raw assembly, but that doesn't mean it's actually difficult to do better than a compiler. You focus your efforts on where benchmarking and profiling tell you your bottlenecks are in your hot paths. If you actually engage in this in a regular basis, you will recognise that despite the myth-building and fear around writing low-level code, compilers do "stupid" inefficient stuff all the time, and it's not hard to do better than them with the specific context of your project in mind.