Another thing to keep in mind is that the compiler is still pretty much the same compiler you think it is. This is the case because it is very difficult to codify all of the information in the Intel Optimization Manual [1]. It's a harder thing to use that information and then a harder thing still to codify, use and use correctly all that information for multiple targets and architectures.

LLVM Tablegen is an approach for representation but it still requires the compiler writer to write everything down and then use it effectively and correctly. This is really hard and it doesn't get done. Instead someone ports a backend and then kinda sorta improves things until it seems better than it was.

Intel has a fine compiler which goes further. It sure isn't open source and it costs a few bucks. They're also contributing LLVM now.

The reason don't even try makes sense both for compiler writer and assembly writer is that this stuff is really hard. Also CPUs are damn fast and Intel (and ARM and ...) work really hard to solve problems dynamically (branch prediction, speculation, register renaming, caching, speculation, ...). In fact, you shouldn't even try this unless you know a lot. Agner Fog and Peter Cordes should be familiar names. But if you do know a lot, beating compilers is not difficult at all. Finding the right place to beat them? That's tough.

[1] https://www.intel.com/content/dam/www/public/us/en/documents...