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.
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