Comment by stefncb

This is all in theory. Everyone says this like it's already here but it's really not (in the sense that these fast jits are still mostly worse than well-written C).

But what it is is mostly choosing optimizations based on runtime characteristics. It's a dynamic analogue to profile-guided optimization. Like you might have an optimization that trades code size for CPU cycles, which you could choose not to do at runtime if you're low on memory bandwidth instead of CPU time. Stuff like that.