embind: Missing signature __emval_decref with MAIN_MODULE=2 Dead Code Elimination

[Angad Gupta]

Hi All,

I am facing problem when using MAIN_MODULE=2 (dead code elimination). I have explicity added emval_decref in EXPORTED_SYMBOLS list to keep this symbol alive.  Once main module is loading it checked undefined symbols in the module preloadDylibs > reportUndefinedSymbols and tries to add to GOT using addFunctionWasm, but the signature of emval_decref is never provided in main JS file.

JS file exports this only:
Module["__emval_decref"] = __emval_decref;

However it seems signature for this is also required to make WASM/JS binding work. For example as shown below - the exports provide the signature too.
Example: 

  Module["_alBuffer3f"] = _alBuffer3f;

  _alBuffer3f.sig = 'viifff';

My linker flag looks like this:
emcc -std=c++14  -pthread -s DISABLE_EXCEPTION_CATCHING=0 -DDEBUG -D_DEBUG -g -s PTHREADS_DEBUG=1  -error-limit=0 --bind -s DISABLE_EXCEPTION_CATCHING=0 -s USE_SDL=2  -s USE_WEBGL2=1 -s LLD_REPORT_UNDEFINED <input object files> --o <html_path> -s MAIN_MODULE=2 -s EXPORTED_FUNCTIONS=@exports_list.txt

Is this a problem with embind implementation, how to fix it? I am using latest emsdk.

MAIN_MODULE=1 is not an option because it exports millions of symbols and wasm initiation fails because of 1lacs limit.  

[Angad Gupta]

Seems like from sdk 2.0.9, imported  JavaScripts functions need to have `__sig` specified in the js library file. So it looks embind/emval JS libraries still not conforming to this change.

From: ChangeLog.md

2.0.9: 11/16/2020

-----------------

- The ABI used for importing symbol by address in dynamic linking (MAIN_MODULE +

  SIDE_MODULE) is now the same as the ABI used by llvm and wasm-ld.  That is,

  symbol addresses are imported from the 'GOT.mem' and 'GOT.func' pseudo

  modules.  As one side effect of this change it is now required that JavaScript

  functions that are imported by address are now required to have a `__sig`

  specified in the library JavaScript file.

From: library_dylib.js

    if (typeof result === 'function') {

      // Insert the function into the wasm table.  If its a direct wasm function

      // the second argument will not be needed.  If its a JS function we rely

      // on the `sig` attribute being set based on the `<func>__sig` specified

      // in library JS file.

      return addFunctionWasm(result, result.sig);

    } else {

      return result;

    }

  },

[Sam Clegg]

Sorry about that Angad.. we spoke in chat and hopefully you are good to upload a PR to fix this?

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