Enable EM_JS in side modules

emcc.py

@@ -831,6 +831,10 @@ def process_dynamic_libs(dylibs, lib_dirs):
   for dylib in dylibs:
     exports = webassembly.get_exports(dylib)
     exports = set(e.name for e in exports)
+    # EM_JS function are exports with a special prefix.  We need to strip
+    # this prefix to get the actaul symbol name.  For the main module, this
+    # is handled by extract_metadata.py.
+    exports = [shared.maybe_strip_prefix(e, '__em_js__') for e in exports]
     settings.SIDE_MODULE_EXPORTS.extend(sorted(exports))
 
     imports = webassembly.get_imports(dylib)

emscripten.py

@@ -344,8 +344,6 @@ def emscript(in_wasm, out_wasm, outfile_js, memfile, js_syms):
           diagnostics.warning('em-js-i64', 'using 64-bit arguments in EM_JS function without WASM_BIGINT is not yet fully supported: `%s` (%s, %s)', em_js_func, c_sig, signature.params)
 
   if settings.SIDE_MODULE:
-    if metadata.emJsFuncs:
-      exit_with_error('EM_JS is not supported in side modules')
     logger.debug('emscript: skipping remaining js glue generation')
     return
 

src/library_dylink.js

@@ -178,7 +178,9 @@ var LibraryDylink = {
       '__wasm_call_ctors',
       '__start_em_asm',
       '__stop_em_asm',
-    ].includes(symName)
+      '__start_em_js',
+      '__stop_em_js',
+    ].includes(symName) || symName.startsWith('__em_js__')
 #if SPLIT_MODULE
         // Exports synthesized by wasm-split should be prefixed with '%'
         || symName[0] == '%'
@@ -779,6 +781,39 @@ var LibraryDylink = {
             start = HEAPU8.indexOf(0, start) + 1;
           }
         }
+
+        function addEmJs(name, cSig, body) {
+          // The signature here is a C signature (e.g. "(int foo, char* bar)").
+          // See `create_em_js` in emcc.py` for the build-time version of this
+          // code.
+          var jsArgs = [];
+          cSig = cSig.slice(1, -1)
+          if (cSig != 'void') {
+            cSig = cSig.split(',');
+            for (var i in cSig) {
+              var jsArg = cSig[i].split(' ').pop();
+              jsArgs.push(jsArg.replace('*', ''));
+            }
+          }
+          var func = `(${jsArgs}) => ${body};`;
+#if DYLINK_DEBUG
+          dbg(`adding new EM_JS function: ${jsArgs} = ${func}`);
+#endif
+          {{{ makeEval('moduleExports[name] = eval(func)') }}};
+        }
+
+        for (var name in moduleExports) {
+          if (name.startsWith('__em_js__')) {
+            var start = moduleExports[name]
+            {{{ from64('start') }}}
+            var jsString = UTF8ToString(start);
+            // EM_JS strings are stored in the data section in the form
+            // SIG<::>BODY.
+            var parts = jsString.split('<::>');
+            addEmJs(name.replace('__em_js__', ''), parts[0], parts[1]);
+            delete moduleExports[name];
+          }
+        }
 #endif
 
         // initialize the module

test/other/metadce/test_metadce_hello_dylink.jssize

@@ -1 +1 @@
-14888
+15270

test/other/test_em_js_main.c

@@ -0,0 +1,9 @@
+#include <stdio.h>
+#include <emscripten.h>
+
+int test_side();
+
+int main() {
+  printf("in main\n");
+  return test_side();
+}

test/other/test_em_js_main.out

@@ -0,0 +1,4 @@
+in main
+hello from side module 42 + hello
+hello again
+hello from void func

test/other/test_em_js_side.c

@@ -0,0 +1,21 @@
+#include <emscripten/em_js.h>
+#include <stdio.h>
+
+EM_JS(void*, js_side_func, (int num, char* ptr), {
+  out(`hello from side module ${num} + ${UTF8ToString(ptr)}`);
+  return 99;
+});
+
+EM_JS(void, js_side_func2, (char *ptr), {
+  out(UTF8ToString(ptr));
+});
+
+EM_JS(void, js_side_func_void, (), {
+  out(`hello from void func`);
+});
+
+void test_side() {
+  js_side_func(42, "hello");
+  js_side_func2("hello again");
+  js_side_func_void();
+}

test/test_other.py

@@ -11964,8 +11964,8 @@ def test_runtime_keepalive(self):
 
   @crossplatform
   def test_em_js_side_module(self):
-    err = self.expect_fail([EMXX, '-sSIDE_MODULE', test_file('core/test_em_js.cpp')])
-    self.assertContained('EM_JS is not supported in side modules', err)
+    self.build(test_file('other/test_em_js_side.c'), js_outfile=False, emcc_args=['-sSIDE_MODULE'], output_basename='side')
+    self.do_other_test('test_em_js_main.c', emcc_args=['-sMAIN_MODULE=2', 'side.wasm'])
 
   def test_em_js_main_module(self):
     self.set_setting('MAIN_MODULE', 2)

tools/shared.py

@@ -673,6 +673,13 @@ def strip_prefix(string, prefix):
   return string[len(prefix):]
 
 
+def maybe_strip_prefix(string, prefix):
+  if string.startswith(prefix):
+    return string[len(prefix):]
+  else:
+    return string
+
+
 def make_writable(filename):
   assert os.path.isfile(filename)
   old_mode = stat.S_IMODE(os.stat(filename).st_mode)