Add example of using externref in assembly code (#15913)

See #15878

tests/core/test_externref.c

@@ -0,0 +1,15 @@
+#include <stdio.h>
+
+void log_externref();
+void get_externref();
+
+int main() {
+  printf("in main\n");
+  log_externref();
+  get_externref();
+  printf("extenref stored\n");
+  log_externref();
+  printf("extenref logged\n");
+  return 0;
+}
+

tests/core/test_externref.js

@@ -0,0 +1,10 @@
+mergeInto(LibraryManager.library, {
+  get_externref_js: function () {
+    var test_object = { foo: 1 };
+    return test_object;
+  },
+
+  log_externref_js: function (ref) {
+    console.log('log_externref_js: ' +  JSON.stringify(ref));
+  },
+});

tests/core/test_externref.out

@@ -0,0 +1,5 @@
+in main
+log_externref_js: null
+extenref stored
+log_externref_js: {"foo":1}
+extenref logged

tests/core/test_externref.s

@@ -0,0 +1,28 @@
+# Define a global of type `externref`
+
+.globaltype my_global, externref
+my_global:
+
+# Define a function that will call out to JS, passing the
+# externref to the JS function
+
+.functype log_externref_js (externref) -> ()
+
+.globl log_externref
+log_externref:
+  .functype log_externref () -> ()
+  global.get my_global
+  call log_externref_js
+  end_function
+
+# Define a function that will call out to JS to grab an
+# externref
+
+.functype get_externref_js () -> (externref)
+
+.globl get_externref
+get_externref:
+  .functype get_externref () -> ()
+  call get_externref_js
+  global.set my_global
+  end_function

tests/test_core.py

@@ -8835,6 +8835,14 @@ def test_em_async_js(self):
     self.maybe_closure()
     self.do_core_test('test_em_async_js.c')
 
+  @require_v8
+  @no_wasm2js('wasm2js does not support reference types')
+  def test_externref(self):
+    self.run_process([EMCC, '-c', test_file('core/test_externref.s'), '-o', 'asm.o'])
+    self.emcc_args += ['--js-library', test_file('core/test_externref.js')]
+    self.emcc_args += ['-mreference-types']
+    self.do_core_test('test_externref.c', libraries=['asm.o'])
+
 
 # Generate tests for everything
 def make_run(name, emcc_args, settings=None, env=None, node_args=None):