Using Base.llvmcall for cross language LTO

[giordano]

@obsidianjulua

This also opens the door for AD across language boundries since traditional ccall stopped the julia jit from following in the IR
[…]
The real payoff is that AD tools can now differentiate through C/C++ code. Since llvmcall makes the IR visible to Julia’s compiler, Enzyme (or any LLVM-level AD) can follow the data
flow straight through what used to be an opaque ccall wall.

To be extremely clear, there’s nothing novel about this. This approach had been demonstrated in the paper Scalable Automatic Differentiation of Multiple Parallel Paradigms through Compiler Augmentation, by @wsmoses @vchuravy and their collaborators, and has been discussed a few times already here on Discourse. As mentioned in Is there an equivalent to cross-language link time optimization via LLVM? - #18 by vchuravy (you even intervened in that thread), the main challenge is an infrastructural one: you need to make sure you use compatible versions of LLVM to compile every piece of code, to be able to merge all bitcodes.