Less about calcifying the specific optimization and more like “this loop is expected to be vectorized”. That way if someone changes something subtle that prevents it from being vectorized it’s a compiler error. Striking that balance and how to express those constraints in a flexible way is the hard bit.

As you say register and inline were wrong, but we have force inline and force inline so clearly the pendulum swung back a little bit because the compiler completely ignoring is also not good. We have ways to force the compiler to do an unconditional move because source level heuristics are completely incorrect for making such a decision. The die is already cast, we just keep living with a shitty status quo instead of something a bit more robust.