The plan for emscripten is actually to replace its own wasm compiler with LLVM, and possibly drop its asm.js compiler in favor of running wasm2asm on LLVM's wasm output: https://github.com/kripken/emscripten/issues/5827

Aside from keeping emscripten's code smaller / more maintainable and allowing the team to focus more on its role as high-level tooling, this should improve the size and performance of emscripten output since IIRC it's currently missing out on a lot of optimization opportunity by producing wasm as transpiled asm.js.

> IIRC it's currently missing out on a lot of optimization opportunity by producing wasm as transpiled asm.js.

That's actually not true: the asm.js => wasm path emits better code (smaller, faster) than the wasm backend path currently.

However, the wasm backend path is being improved, and should eventually get to parity.

Ah okay, interesting. Is it because wasm doesn't yet add any new functionality over asm.js that using asm.js as an intermediary step isn't inherently worse?

In that case, it sounds like the LLVM backend will only yield clear user-facing benefits when new features like pthreads are introduced?

Well, the "asm.js to wasm" path actually isn't pure asm.js anymore. We added i64 support and other things a while back, as intrinsics. So the asm2wasm path isn't limited by asm.js. It's weird ;) but it produces good code...

The wasm backend does have other benefits, which is why we'd like to move emscripten to use it by default:

* It uses LLVM's default legalization code, so it can handle LLVM IR from more sources (i.e. not just C and C++ from clang).

* We can stop maintaining the out-of-tree LLVM that asm2wasm depends on.

The LLVM wasm backend isn't ready yet (larger output code, slower compile times, a few missing features) but it's getting there.

Incidentally, that's how this target works in Rust; we also have an emscripten-based target.