Emscripten emulation layer for llvm-based compiler

[Jacek Sieka]

Hi!

I'm experimenting a little with wasm for a llvm-based compiler I'm working on - https://github.com/arnetheduck/nlvm - for the Nim language.

Nim typically is compiled to C and then the C compiler takes over meaning that it's fairly straightforward get it to compile with emcc to wasm instead. In `nlvm`, Nim is compiled straight to llvm IR and consequently, nlvm is able to output wasm code as well.

The next step that I think would make sense to get the Nim code to run in a browser would be to use the emscripten runtime, but from what I can tell, when emcc runs it customizes the runtime for the specific case.

How does that work? IFor example, is there some tooling in emcc that I can give an llvm IR file to (which imports various C functions like clang-compiled C code would), and have emcc output the corresponding html/js/wasm files?

Thanks!

Jacek

[Alon Zakai]

Hi Jacek,

Yes, basically that should work - run

emcc input.bc -o output.js

where input.bc is LLVM IR, and then you'll get a js+wasm combination that you can run in node.js or on the Web. And with -o output.html you'd also get html etc.

Things to be careful of:

 * The LLVM version that generated the IR should be close enough to emscripten's LLVM version. It's best to use the wasm backend (https://v8.dev/blog/emscripten-llvm-wasm#testing) with emscripten - the default there is a very recent build of LLVM from master. If that's too new for you, you can use an older emscripten version perhaps.

 * An alternative to using LLVM IR is to use wasm object files, which are a much more stable format. You can emit those using your compiler's LLVM by telling it to use the wasm backend, and then run emcc on that .o file.

 * Emscripten's libc etc. assume the code was compiled with the emscripten headers. If not, the ABI may be incompatible for things like syscalls.

- Alon

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