[ScottPJones]

Julia's JIT compilation is rather different than what is referred to as JIT compilation in other languages, such as Java or JavaScript, where the language is interpreted (which may be interpreting instructions from a virtual machine such as the JVM), and the run-time decides if some code is being hit frequently enough to warrant compilation to native code. Julia first compiles to an AST representation (also expanding macros, etc), performs type inference, etc. When a method is called with types that haven't been used before to call that method, that's when Julia does it's magic and compiles a version of that method specialized for those types, using LLVM to generate the final machine code (just like most C and C++ implementations these days, as well as Rust and others). That also means that it's rare for Julia to have to dynamically dispatch methods based on the type of the arguments, which is one of the things that can really slow down other languages with dynamic types.