enable binaryen async compilation only in wasm-only mode

emcc.py

@@ -1291,9 +1291,16 @@ def check(input_file):
         if opt_level > 0:
           if 'BINARYEN_ASYNC_COMPILATION=0' not in settings_changes:
             shared.Settings.BINARYEN_ASYNC_COMPILATION = 1
-        # async compilation requires a swappable module - we swap it in when it's ready
         if shared.Settings.BINARYEN_ASYNC_COMPILATION == 1:
-          shared.Settings.SWAPPABLE_ASM_MODULE = 1
+          if shared.Building.is_wasm_only():
+            # async compilation requires a swappable module - we swap it in when it's ready
+            shared.Settings.SWAPPABLE_ASM_MODULE = 1
+          else:
+            # if not wasm-only, we can't do async compilation as the build can run in other
+            # modes than wasm (like asm.js) which may not support an async step
+            shared.Settings.BINARYEN_ASYNC_COMPILATION = 0
+            if 'BINARYEN_ASYNC_COMPILATION=1' in settings_changes:
+              logging.warning('BINARYEN_ASYNC_COMPILATION requested, but disabled since not in wasm-only mode')
 
       # wasm outputs are only possible with a side wasm
       if target.endswith(WASM_ENDINGS):

tests/test_browser.py

@@ -3307,6 +3307,7 @@ def test_binaryen_async(self):
       (['-O3'], 1),
       (['-s', 'BINARYEN_ASYNC_COMPILATION=1'], 1), # force it on
       (['-O1', '-s', 'BINARYEN_ASYNC_COMPILATION=0'], 0), # force it off
+      (['-s', 'BINARYEN_ASYNC_COMPILATION=1', '-s', 'BINARYEN_METHOD="native-wasm,asmjs"'], 0), # try to force it on, but have it disabled
     ]:
       print opts, expect
       self.btest('binaryen_async.c', expected=str(expect), args=['-s', 'BINARYEN=1', '--shell-file', 'shell.html'] + opts)