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Yeah, I expect the real advantage of a JIT is that you can perform proper register allocation and avoid a lot of stack and/or virtual register manipulation.

I wrote a toy copy-patch JIT before and I don't remember being impressed with the performance, even compared to a naive dispatch loop, even on my ~11 year old processor.

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The difference between interpreters and simple JITs has narrowed partly due to two factors: better indirect branch predictors with global history, and wider execution bandwidth to absorb the additional dispatch instructions. Intel CPUs starting with Haswell, for instance, show less branch misprediction impact due to better ability to predict jump path patterns through the interpreter. A basic jump table no longer suffers as much compared to tail-calling/dispatch or a simple splicing JIT.

Exactly, and it's not just register allocatio: but for many languages also addign proper typing, some basic data flow optimization, some constant folding, and a few other things that can be done fairly quickly, without the full set of trees and progressive lowering of the operators down to instruactions.

What's odd about the "JIT vs interpreter" debate is that it keeps coming up, given that it is fairly easy to see even in toy examples.

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