You can't throw away your interpreter bytecode, because the JIT code needs to be able to bailout if a guard fails (e.g. if you specialized a var as an int and y

You can't throw away your interpreter bytecode, because the JIT code needs to be able to bailout if a guard fails (e.g. if you specialized a var as an int and you get a non-int). So you can always use the size of your bytecode as an inlining heuristic (which is what Chakra does). Of course that wouldn't work before v8 had an interpreter, but they could have used other heuristics. For example, they could have used the size of the AST as a heuristic and saved that even after the AST gets thrown away.