JIT (just-in-time) compilation is when source code (or bytecode) gets compiled on-demand at runtime as it executes rather than prior to being executed. JIT compilation is a feature of an interpreter. Traditionally, source code is compiled to machine code directly by an AOT compiler like gcc for C and C++, meaning no further compilation is done during execution.
JIT vs. AOT
- JIT tends to be better during development. AOT tends to be better during production, but not necessarily always since a JIT compiler works at runtime, it can make optimisations like inlining functions that are invoked frequently (which minimises the overhead in the low level operation of setting up a function context).
- In languages like Dart and TypeScript, it’s common to use a JIT compiler during development and then compile everything for production using an AOT compiler.
- JIT tends to be less secure than AOT because of JIT spraying exploits.