if your compiler took a day (or even an hour) to compile a small program, you simply wouldn’t use it, no matter how good the resulting code was
Some research has been moving in that direction, since there does seem to be a demand for it. After all, if someone is willing to wait for you to spend a week hand-tuning a function, maybe they'd also be willing to run a compiler in a "take 10 hours to crunch on this" mode. Example: http://blog.regehr.org/archives/923