Just-In-Time (JIT) Compiler with LLVM
JIT compilation is a combination of Ahead-Of-Time (AOT) compilation and interpretation. As we saw previously, our Calc interpreter evaluates AST to values (actual integer i32 values) but a JIT compiler differs from an interpreter in what it outputs. Intuitively, JIT outputs are like AOT outputs but generated at runtime when traversing the AST.
LLVM
LLVM (which is not an acronym) is a mature compiler backend (code generator) infrastructure powering many languages such as Clang, Rust, Swift, etc. It has its own IR and Virtual Machine Bytecode abstracting away the underlying platform-specific differences.
We will use inkwell which provides a safe Rust wrapper around LLVM.
Note: The JIT examples in this chapter require nightly Rust and LLVM installed. The interpreter and VM examples work with stable Rust.
Alternatives
Other code generators that you can use (see the exercises) in this book (not at mature as LLVM) are cratelift-simpljit and gcc-jit.