[FEATURE REQUEST] Pre-compiled Dynlink for .cma files without linking compiler-libs/js_of_ocaml-compiler

[ejgallego]

Is your feature request related to a problem? Please describe.

jsCoq usually requires to Dynlink.loadfile of some heavy .cma files (plugins in Coq terminology to work.

Loading such files with JSOO Dynlink.loadfile support works fine, but it is usually quite slow, and moreover increases the output .js file by almost 3 MiB (50%) as it will link js_of_ocaml-compiler.

Describe the solution you'd like

I'd be great if we could precompile the .cma files to .js files, and load them without having to link the compiler.

Describe alternatives you've considered

For jsCoq and JSOO 4.0.0 , this used to work. Updating to JSOO 5.8.3 is done here https://github.com/jscoq/jscoq/pull/372/files#diff-89191e3161815dcdaf5594e6d98bfaa5ab81f742ccbeb9f5a657655c27504cdcR16, that also works, however it increases the binary size by almost 50% as we need to link the compiler.

I tried to provide my own version of toplevelReloc , which seems to be the only missing primitive for this to work, but without success.

[hhugo]

There is a tests checking that one can load precompiled cmo successfully. It lives in compiler/tests-dynlink-js.

It seems that there is an issue if one compile the main program with the jsoo --dynlink flags but without linking js_of_ocaml-compiler.dynlink.

Do you really need --dynlink ?

Alternatively, the following patch/hack should fix your issue.

 //Provides: caml_register_global (const, shallow, const)
 //Requires: caml_global_data, caml_callback, caml_build_symbols
 //Requires: caml_failwith
 function caml_register_global(n, v, name_opt) {
   if (name_opt) {
     var name = name_opt;
     if (globalThis.toplevelReloc) {
       n = caml_callback(globalThis.toplevelReloc, [name]);
     } else if (caml_global_data.symbols) {
       if (!caml_global_data.symidx) {
         caml_global_data.symidx = caml_build_symbols(caml_global_data.symbols);
       }
       var nid = caml_global_data.symidx[name];
       if (nid >= 0) n = nid;
       else {
-        caml_failwith("caml_register_global: cannot locate " + name);
+        n = caml_global_data.length - 1;
       }
     }

[ejgallego]

Thanks @hhugo , I confirm that with #1676 this problem is solved.

I don't fully understand all the details, but both using --linkall and --dynlink work now. I guess the first option is the preferred one, right?

[hhugo]

The thing is that lhe ocamlc link flag -linkall is supposed to be propagated by dune to js_of_ocaml (but it is sometime not).
Ideally, you don't need to pass linkall to jsoo explicitly .

[hhugo]

fixed by #1676

[ejgallego]

Thanks a lot @hhugo !