JIT and LLVM

Julia is a Just In Time (JIT) compiled language, rather than an interpreted one. This allows Julia to be dynamic, without having the overhead of interpretation. This compilation infrastructure is build on top of Low Level Virtual Machine (LLVM) (http://llvm.org).

The LLVM compiler without infrastructure project originated at University of Illinois. It now has contributions from a very large number of corporate as well as independent developers. As a result of all this work, it is now a very high-quality, yet modular, system for many different compilation and code generation activities.

Julia uses LLVM for its JIT compilation needs. The Julia runtime generates LLVM Intermediate Representation (IR) and hands it over to LLVM's JIT compiler, which in turn generates machine code that is executed on the CPU. As a result, sophisticated compilation techniques that are built into LLVM are ready and available to Julia, from the simple (such as Loop Unrolling or Loop Deletion) to state-of-the-art (such as SIMD Vectorization) ones. These compiler optimizations form a very large body of work, and in this sense, the existence is of LLVM is very much a pre-requisite to the existence of Julia. It would have been an almost impossible task for a small team of developers to build this infrastructure from scratch.

Types

We will have much more to say about types in Julia throughout this book. At this stage, suffice it to say that Julia's concept of types is a key ingredient in its performance.

The Julia compiler tries to infer the type of all data used in a program, and compiles different versions of functions specialized to particular types of its arguments. To take a simple example, consider the sqrt function. This function can be called with integer or floating-point arguments. Julia will compile two versions of the code, one for integer arguments, and one for floating point arguments. This means that, at runtime, fast, straight-line code without any type checks will be executed on the CPU.

The ability of the compiler to reason about types is due to the combination of a sophisticated dataflow-based algorithm, and careful language design that allows this information to be inferred from most programs before execution begins. Put in another way, the language is designed to make it easy to statically analyze.

If there is a single reason for Julia is being such a high-performance language, this is it. This is why Julia is able to run at C-like speeds while still being a dynamic language. Type inference and code specialization are as close to a secret sauce as Julia gets. It is notable that, outside this type inference mechanism, the Julia compiler is quite simple. It does not include many advanced Just in Time optimizations that Java and JavaScript compilers are known to use. When the compiler has enough information about the types within the code, it can generate optimized, straight-line, code without many of these advanced techniques.

It is useful to note here that unlike some other optionally typed dynamic languages, simply adding type annotations to your code does not usually make Julia go any faster. Type inference means that the compiler is, in most cases, able to figure out the types of variables when necessary. Hence you can usually write high-level code without fighting with the compiler about types, and still achieve superior performance.