Consider cross-compilation in TH mode

[edsko]

We need to consider what the workflow will look like; how does the user start a build on their host machine for a particular target machine, and importantly, how do those flags then make it to the TH splice? We could require users to do set this up themselves (as long as we accept clang args, #341, that's at least possible), but it would be good to provide some out-of-the-box support.

[phadej]

Cross compilation doesn't really make sense for the current TH setup. TH is always run on the target architecture, so for now the TH story is actually quite straight forward.

[edsko]

@phadej I don't understand what you mean. Surely TH is always run on the host, not the target? We must mean different things. Here's the kind of thing I had in mind. Suppose we are on platform A, and are compiling for platform B, and we're using hs-bindgen in TH mode. The important thing is that when we invoke libclang from TH, we pass the appropriate flags. Perhaps we do this through setting an environment variable which we read from TH

export TARGET=B

We then read this environment variable, set appropriate libclang flags, and generate splices as normal, for platform B. The code is running on platform A, but that's fine: the code that's running is code that generates code; it doesn't matter what platform that runs on, as long as it generates code for B.

Do you see a problem with that setup?

[phadej]

Surely TH is always run on the host, not the target?

TH is always run on the target. GHCJS runs node, new JS backend does something among those lines too, I guess WASM too.

Suppose we are on platform A, and are compiling for platform B, and we're using hs-bindgen in TH mode.

The TH has to be run on platform B. I found this post https://medium.com/@zw3rk/cross-compiling-template-haskell-7e38c00c2914 explaining -fexternal-interpreter and as far as I know nothing has considerably changed since.

E.g. https://www.tweag.io/blog/2020-11-25-asterius-th/ says

Asterius uses the external interpreter approach to support Template Haskell. The execution of compiled WebAssembly code and the JavaScript runtime is done in a node process.

[edsko]

I see. Thank you for that clarification, that certainly connected a few dots for me!