I never do this kind of work so I can’t say. But if I did, I’d imagine I want more control. I mean, perf improvements are welcome to all code, but if I
need a piece of code to have a specific optimization I’d rather opt-in
through language constructs, so that the compiler (or other tooling) can tell me when it breaks. A well designed API with adapters from and to regular code would be better, no?
For instance, imagine I have auto-perf something and I check (manually mind you) the asm and all is good. Then someone changes the algorithm slightly, or another engineer adds a layer of indirection for some unrelated purpose, or maybe the compiler updates its code paths which misses some cases that were previously supported. And the optimization goes away silently.