Today we will learn how to stop lying to the compiler, and how to get a good night's sleep. Like a sophisticated ahead of time (AOT) compiler, the JIT compiler can apply nontrivial optimizations to the code. But unlike an AOT compiler, the JIT compiler has access to the actual usage patterns at runtime. Once you run the bootstrapping compiler on the code for the bootstrapped compiler, congratulations, you now have a brand new self-hosting compiler for X. And most of what they do all day long is writing compiler code.