Export/import tables

[pepyakin]

I did two experiments with wasm runtimes (1, 2) and both of them requires access to tables.

Today, in wasm32-unknown-unknown tables are defined inside the wasm but not exported. To workaround this, I have to disassemble wasm, add export for table and then assemble back before I can use the modules (script).

So I think, we need a way to communicate to linker that we need either to import or export tables from binary. I understand, this might be not feasible to do with our binaryen 'linking'.
I just want we make sure that this is not problem for upcoming proper linking with LLD.

[koute]

Yep. This is absolutely essential for rich interop between Rust and JS. I also automatically export the table when compiling with cargo-web.

[pepyakin]

Seems like the patch with LLD won't help with this. At least, I can't find any table related options in the gist posted by Alex.

[mgattozzi]

@alexcrichton do you know if LLD supports/will support this or are going to have to do something else here?

[alexcrichton]

Heh I don't think there's event fully functional import directives yet so I'd imagine that tables are probably still "on the TODO list". I'd imagine though that they'd be receptive to an implementation!

[mgattozzi]

@pepyakin linked this in #29 but it looks like that landed upstream here!

[alexcrichton]

Er yes, but that was the support I was testing out which didn't actually work with LLD, it only worked with LLVM.

[mgattozzi]

Oh that's unfortunate -_-

[alexcrichton]

LLD recently gained support for specifying what to do about the function table, whether it's imported or exported

[pepyakin]

can confirm that it works for me on LLVM 7