MAIN_MODULE=3 to Whitelist Exported Functions

[awtcode]

@kripken , one of the reasons why the number of exported functions is large for a main module is because we cannot differentiate between a system module and a user-implemented module. Thus, we end up exporting everything.

Would it make sense to introduce a MAIN_MODULE=3 whereby all functions are hidden by default and only those that are exported explicitly will be exposed? We can always get the list of C++ std functions by building a dummy main module with MAIN_MODULE=1 and add that list to the whitelist. This would be similar to DISABLE_EXCEPTION_CATCHING where we have a mode 2 to specify functions that should have exceptions enabled.

Can you let me have your thoughts on this proposal? Thanks.

[kripken]

we cannot differentiate between a system module and a user-implemented module

What do you mean here specifically?

Would it make sense to introduce a MAIN_MODULE=3 whereby all functions are hidden by default and only those that are exported explicitly will be exposed?

How does that differ from the currently-existing mode 2?

[awtcode]

I was thinking of the following code:

def get_exported_implemented_functions(all_exported_functions, all_implemented, metadata):
  funcs = set(metadata['exports'])
  export_bindings = shared.Settings.EXPORT_BINDINGS
  export_all = shared.Settings.EXPORT_ALL
  for key in all_implemented:
    if key in all_exported_functions or export_all or (export_bindings and key.startswith('_emscripten_bind')):
      funcs.add(key)

whereby both std C++ functions and user-implemented functions are exported by default when using MAIN_MODULE because shared.Settings.EXPORT_ALL is always set to True for MAIN_MODULE. The reason being that we cannot differentiate between std C++ and user functions at this point of the compilation.

We could set shared.Settings.EXPORT_ALL to False even for MAIN_MODULE and use EXPORTED_FUNCTIONS to specify the std C++ functions we want to export. The list could be obtained by building a program with MAIN_MODULE=1 if necessary.

The difference with MAIN_MODULE=2 is that user-implemented functions will not be exported by default.

@kripken , please let me know if the above needs more clarifications. Thanks.

[awtcode]

@kripken , by the way, the motivation for having MAIN_MODULE=3 is to reduce the inter-wasm calls between the main module (which contains only the std c++ functions) and the side module because the performance is rather slow and it would also help to reduce the code size if we did not have to export all the std C++ functions as well.

[awtcode]

@kripken , sorry to trouble you again. did you get the chance to look at this?

[kripken]

I think I see what you mean. So in the current model, we assume other modules loaded later may want to call any of the functions in the main module, so we export them. And you're suggesting a mode in which we don't export them by default. So the user would need to specify what is callable from outside the module.

Sounds good to me. Can maybe call that EXPORT_SHARED_MODULE_CONTENTS which defaults to 1, but the user can set to 0.

[kripken]

Would also be good to remove the "mode 2" stuff - can call that DCE_SHARED_MODULE_CONTENTS, default to 0, maybe.