If you're really curious, you can follow some of the breadcrumbs through the "Activate LLVM 3.7" GitHub issue (which was originally started for version 3.5).

https://github.com/JuliaLang/julia/issues/9336

Hmm. It's hard to remember all the issues, but this was the period where LLVM moved from their "old JIT" backend to MCJIT [1]. IIRC, 3.4 broke the old JIT but MCJIT still wasn't developed enough for Julia to use. At some point the old JIT was removed entirely but MCJIT was still not usable (for us) and that didn't resolve itself entirely until 3.7.

[1] http://llvm.org/docs/MCJITDesignAndImplementation.html

I'm guessing that they use LLVM's C++ API, which is not stable between releases. The C++ API changes dramatically from one version to the next, so upgrading LLVM versions typically a big effort for downstream projects. The LLVM C API, however, is meant to be stable. For C++ users, it may make some sense to write a stable C++ API wrapping the LLVM C API.

Disclaimer: I am not affiliated with the LLVM project.

LLVM can bet a very good optimized compiler toolchain, but is complex, hard to use and break easily (your code).

I have tried to use it several times for toy compilers, and each time is a pain for do simple things. I can't imagine how much pain is for depend your project on it.

Certainly worth the effort, but as expected of a C++ tech: You need a army of specialist to make it work.