> * Writing systems that never reboot[1] and are efficient is very hard. Dynamic variables usually require inserting checks at runtime that can't be optimized away by an AOT compiler. This is where a speculatively-optimizing JIT comes in handy, but JITs aren't appropriate for all uses, and even where they are, good optimizing JITs are notoriously hard to write.

But special case JITs that essentially build a version of a function out of predefined blocks, are not that hard to implement. That's enough to remove current (constant) state dependant branches and do constant folding.

Useful for parsing, some generic data structures and graphics. Anything that is executed very many times, has configurable (but constant) state that controls conditional branching or a part of computation that could be constant folded.