LLVM use in Faasm

LLVM is a key dependency in Faasm. At the time of the writing, the Faasm ecosystem uses LLVM 13.

LLVM is used all across the project:

• In faasm/cpp, LLVM is used to cross-compile C and C++ code to WebAssembly. Different LLVM versions ship with different WASM backends, which may alter the cross-compilation step.

• All across the project, we use clang++ to build C++ code.

• In particular, in faasm/faasm, we build different WASM runtimes, namely WAVM and WAMR. WASM runtimes do several optimizations to WASM code during code generation, they therefore make heavy use of the LLVM libraries.

We could potentially use different versions of LLVM across the project. In the past, LLVM 10 and LLVM 13 have co-existed in the project: the former to cross-compile to WebAssembly, and the latter to compile C++ code. This is why we install them differently: the LLVM toolkit to build C++ code is installed in faasm/faabric from APT, whereas the LLVM toolkit to cross-compile to WebAssembly is build from our LLVM fork in faasm/cpp. It is preferable to maintain the same version throughout, but the former is easier to update than the latter, so they can have different upgrade life-cycles.

Upgrading LLVM

Given the above, upgrading the LLVM version is a delicate procedure, and things are likely to break.

The best advice is to replicate the steps followed in another PR to upgrade LLVM. See here for the PR that upgraded to LLVM 13.