Does em++ able to generate standalone wasm with C++ exception?

[harry900831]

Hi all,

I'm currently working on adding wasm exception handling proposal support for a runtime. According to my test, it seems that if we generate the wasm with following code and command: em++ -fwasm-exceptions a.cpp -o a.wasm
, it still depends on an import object called (import "env" "__throw_exception_with_stack_trace" (func (;2;) (type 3))).

#include <stdio.h>

int main() {
  try {
    puts("throw...");
    throw 1;
    puts("(never reached)");
  } catch(...) {
    puts("catch!");
  }
  return 0;
}

According to this page (last update on 2019), if I understand correctly, it states that emscripten still can't generate standalone wasm with C++ exceptions.

Many APIs that emscripten uses depend on JS or Web APIs, though, like C++ exceptions as mentioned earlier, things like WebGL or WebAudio, etc. 
Calls to those APIs will still be emitted in standalone mode because we have no better alternative; this also gives you the option to add necessary APIs to a custom wasm embedding.

Moreover, If I generate the wasm with the command: em++ -O1 -fwasm-exceptions a.cpp -o a.wasm, it eliminate the import object (import "env" "__throw_exception_with_stack_trace" (func (;2;) (type 3))), but the output after running it is "throw..." only.

I would like to ask whether there is any update for standalone wasm that support C++ exception handling. And if capable, how to generate one?

[kripken]

__throw_exception_with_stack_trace is a debugging helper. That shouldn't be emitted in standalone mode, so we should add an if to remove that. But right now you are getting that since at -O0 we enable ASSERTIONS by default. Building with -O1 or above (or -O0 -sASSERTIONS=0) will avoid that issue.

Aside from that, I'm not aware of any issues. emcc a.cpp -O1 --profiling -fwasm-exceptions -sSTANDALONE_WASM on your cpp file emits a standalone wasm with no odd imports, and when I run it using the JS shim in node 16.5 with --experimental-wasm-eh it emits

throw...
catch!

That should work in a standalone wasm VM as well, but I'm not sure which of them support wasm exceptions atm.

[harry900831]

Thanks for your reply! My problem is that I misunderstood something related to run the standalone wasm on node. It's all clear now!

[vchigrin]

That shouldn't be emitted in standalone mode, so we should add an if to remove that.

I attempt to fix issue here
#24680