Press the button to see it compile and run a tiny "hello world" program. You can also try modifying the LLVM assembly and pressing the button again - note though that LLVM assembly is typed, so if you add characters to the string for example, you will need to adjust its length, both where it is defined and where it is used (otherwise you will get errors, which will show up in the output area).
This demo was done as a fun hacking project over a holiday vacation, so there are some caveats: The generated code is not optimized at all, so benchmarking is pointless; if you want to benchmark, run emscripten normally with
-O2. Compilation speed has also not been optimized at all. Also, this demo has hardly been tested and glues together several codebases in ways they were not originally intended, there might be things that do not work.