Calling Dex code from Julia via sharing LLVM bitcode

[oxinabox]

From #625 (review)

Also, doesn't Julia use LLVM for compilation too? I wonder if it would be possible to lower Dex functions to LLVM modules and link those together with the larger Julia programs. That would give you essentially no FFI boundary but would all get compiled into a single binary blob!

I asked @vchuravy about how realistic that was and they said:

It is how Cxx.jl works
There is some version dependency
I.e. you want to use the same LLVM version
Otherwise once you have the bitcode it's straightforward

Apparently, this is not a reason Cxx.jl is incredibly fragile.
So this seems possible.

Normal builds of Julia 1.6 is LLVM v11, Julia 1.7 is LLVM v12.
I think there is a long of swing possible when compiling from source but its pretty rare for people to do that and so don't want to depend on it.

[vchuravy]

We could start with adding dex to Yggdrasil, building against our specific LLVM there.

[MasonProtter]

Julia needs a patched LLVM. Are those patches compatible with Dex? Does it have its own patches?

[oxinabox]

I think Julia's LLVM is not very patched?
Just a little bit.
People on some distributions run it without the patchs.
Because the distribution maintainers hate that.

I would expect that they still can talk same bytecode?
@vchuravy is the expert here though.

Dex does not use any patches on llvm afaik.
But I would expect it would run happily enough on llvm with Julia's patches

[vchuravy]

We should still be bitcode compatible with an unpatched version, as long as the optimizations are run by the Julia LLVM, (and we are upstreaming our patch list)

[apaszke]

Yeah we don't need any LLVM changes. If the Julia patches are minor then I don't expect any significant difficulties. Right now we can support LLVM 9 or LLVM HEAD, but it shouldn't be hard to add LLVM 12 support as well.