Java Hello World, LLVM Edition

Summary

After exploring Java bytecode in previous years (2022, 2023, 2024), this year we’ll take an unexpected detour for a Java advent: instead of generating Java bytecode, we’ll use Java to build and execute LLVM IR, the intermediate language behind compilers like clang. Using Java’s Foreign Function & Memory (FFM) API, we’ll call the LLVM C API, generate a “Hello, World!” program, and even JIT-compile it to native code – all from Java. The task is simple: create a program that simply prints “Hello, World!”. But we must do this from Java via LLVM. What is LLVM? The LLVM Project, a collection of modular compiler and toolchain technologies, began as a research project over 20 years ago at the University of Illinois. It has grown significantly, underpinning many compilers and tools like clang. The core libraries provide a source & target independent optimizer along with code generation for a multitude of target machines. They are built around the LLVM IR, an intermediate representation, which we’ll generate & execute from Java. Installing LLVM To use the LLVM C API from Java, we’ll need LLVM’s shared libraries and headers installed locally. There is an automatic installation script available to easily install LLVM on Ubuntu/Debian systems, for example to install LLVM 20: $ wget https://apt.llvm.org/llvm.sh $ chmod +x llvm.sh $ ./llvm.sh 20 Once we have LLVM installed we can use the LLVM tooling to execute textual-form LLVM IR and we’ll also be able to use the LLVM C API in Java via the FFM API. LLVM IR LLVM IR is a strongly-typed, SSA-based intermediate language. It abstracts away most machine-specific details, making it easier to represent high-level constructs in a compiler-friendly format. There are three equivalent representations of the IR: an in-memory format, a bitcode format for serialisation and a human readable assembly language representation. The textual form of the LLVM IR for our “Hello, World!” looks like this: @str = private constant [14 x i8] c"Hello, World!...