Whether code is written to a file or just executed in memory is irrelevant. JIT compilers compile code at run time, and may also recompile the already compiled code again to optimize it based on run-time analysis. SBCL however has distinct compile-time and run-time.

For example, you have a function that does a bunch of math without knowing the types of the arguments. A JIT compiler can first compile it with generic math, but later observe that it's almost always called with fixnums, and choose to create a specialized implementation for fixnums. SBCL relies on the programmer to implement and call the optimized version when appropriate.