Combining GNAT with LLVM

Presenting the GNAT LLVM project

At AdaCore labs, we have been working for some time now on combining the GNAT Ada front-end with a different code generator than GCC.

The GNAT front-end is particularly well suited for this kind of exercise.  Indeed, we've already plugged many different code generators into GNAT in the past, including a Java byte code generator (the old "JGNAT" product for those who remember it), a .NET byte code generator derived from the Java one, a Why3 generator, used at the heart of the SPARK formal verification technology, a SCIL (Statically Checkable Intermediate Language) generator used in our advanced static analyzer CodePeer, and a C back-end used in GNAT CCG (Common Code Generator).

This time, we're looking at another general purpose code generator, called LLVM, in order to expand the outreach of Ada to the LLVM ecosystem (be it the compiler itself or other components such as static analysis tools).

This work-in-progress research project is called "GNAT LLVM" and is meant to show the feasibility of generating LLVM bytecode for Ada and to open the LLVM ecosystem to Ada, including tools such as KLEE , that we are also planning to work with and add Ada support for. Note that we are not planning on replacing any existing GNAT port based on GCC, so this project goes in addition rather than in replacement.

Technical Approach

We decided on a "pure" LLVM approach that's as easy to integrate and fit into the LLVM ecosystem as possible, using the existing LLVM API directly, while at the same time doing what we do best: write Ada code! So we are using the LLVM "C" API and generate automatically Ada bindings via the GCC -fdump-ada-spec switch and a bit of postprocessing done in a python script, that we can then call directly from Ada, which allows us to both easily traverse the GNAT tree and generate LLVM instructions, all in Ada.

By the way, if you know about the DragonEgg project then a natural question would be "why are you starting a GNAT LLVM project from scratch instead of building on top of DragonEgg?". If you want to know the answer, check the file README.dragonegg in the repository!

Next Steps

We have just published the GNAT LLVM tool sources licensed under GPLv3 on GitHub for hobbyists and researchers to experiment with it and give us their feedback.

If you are interested, give it a try, and let us know via Issues and Pull Requests on GitHub how it works for you, or by leaving comments at the bottom of this page! One exciting experiment would be to compile Ada code using the webassembly LLVM back-end for instance!