Building projects with emscripten – stuck on acorn-optimizer

  asm.js, c++, emscripten

I’m writing a medium-sized C++ program (around 900 lines; quite a few templates, idiomatic C++, only RAII and collections, no raw arrays, no pointers, no manual memory allocation). Native build for my application takes around 5 minutes, but problems start with emscripten build.

Webassembly is emitted relatively quickly, in a few seconds. The build stalls on acorn-optimizer trying to optimize the libstdc++ runtime (?). It’s been running for 20 minutes, and i tried to re-run and kill it multiple times to get it to build my application.

I use the following commandline:

em++ -std=c++20 -o tau.html lexer.cpp symtab.cpp parser.cpp decl.cpp tau.cpp -O3 -s WASM=1

em++ launches a process with the following commandline:

/usr/bin/node /usr/share/emscripten/tools/acorn-optimizer.js /tmp/emscripten_temp_gwg74gkp/tau.js AJSDCE minifyWhitespace

which is constantly using a single core of my processor and ~50MB (fluctuating) of RAM, and due to it, the compilation doesn’t finish.

enter image description here

tau.js contents follow: link. I managed to (somewhat) track down the issue – a 800-line C++ project seems to be too big for Emscripten to handle. asm.js output is well over 6 megabytes on -g3 -O0 build (the one which doesn’t run the minifier)

enter image description here

I was surprised at the inefficiency and horribleness of the Emscripten ABI.

My question follows – how do I get this project to build and optimize? Do i have to leave the build overnight every time I want to compile something? Is there a better tool for compiling C++ and targetting webasm or asm.js?

I gave up on -O3 and decided that I’ll try -O1, without success – compiler crashes:

em++ -std=c++20 -o tau.html lexer.cpp symtab.cpp parser.cpp decl.cpp tau.cpp -O1 -s WASM=0
Traceback (most recent call last):
  File "/usr/share/emscripten/", line 14, in <module>
  File "/usr/share/emscripten/", line 2156, in run
    post_link(options, wasm_target, wasm_target, target)
  File "/usr/share/emscripten/", line 2320, in post_link
    generate_html(target, options, js_target, target_basename,
  File "/usr/share/emscripten/", line 3075, in generate_html
  File "/usr/share/emscripten/", line 3049, in minify_html
    shared.check_call(['htmlmin', opts, '--', filename, filename])
  File "/usr/share/emscripten/tools/", line 104, in check_call
    return run_process(cmd, *args, **kw)
  File "/usr/share/emscripten/tools/", line 94, in run_process
    ret =, check=check, input=input, *args, **kw)
  File "/usr/lib/python3.9/", line 505, in run
    with Popen(*popenargs, **kwargs) as process:
  File "/usr/lib/python3.9/", line 951, in __init__
    self._execute_child(args, executable, preexec_fn, close_fds,
  File "/usr/lib/python3.9/", line 1756, in _execute_child = _posixsubprocess.fork_exec(
TypeError: expected str, bytes or os.PathLike object, not list
make: *** [Makefile:44: webasm] Error 1

I also tried -g3 -O0, but the output code doesn’t work:

enter image description here

7zipped output of emscripten for -g3 -O0 follows: link. I don’t think my source code is significant here – the issue clearly happens within emscripten runtime.

For the record, I’m using emscripten built using emsdk on 24 January this year, that is, 2021.

Source: Windows Questions C++