Skip final JS compiler step when only building wasm. NFC (#20970)

The early return was originally delayed in #12729 since we wanted to
be able to report undefined symbols based on JS library contents, but
since we now pass JS library information into wasm-ld all the undefined
symbol reporting is done there.

See #21026

Author: sbc100

test/test_other.py

@@ -2371,25 +2371,43 @@ def test_link_memcpy(self):
 ''', output)
     self.assertNotContained('warning: library.js memcpy should not be running, it is only for testing!', output)
 
-  def test_undefined_exported_function(self):
-    cmd = [EMCC, test_file('hello_world.c')]
+  @parameterized({
+    '': ('out.js',),
+    'standalone': ('out.wasm',)
+  })
+  def test_undefined_exported_function(self, outfile):
+    cmd = [EMCC, test_file('hello_world.c'), '-o', outfile]
     self.run_process(cmd)
 
-    # adding a missing symbol to EXPORTED_FUNCTIONS should cause failure
+    # Adding a missing symbol to EXPORTED_FUNCTIONS should cause a link failure
     cmd += ['-sEXPORTED_FUNCTIONS=_foobar']
     err = self.expect_fail(cmd)
     self.assertContained('wasm-ld: error: symbol exported via --export not found: foobar', err)
 
-  def test_undefined_exported_js_function(self):
-    cmd = [EMXX, test_file('hello_world.cpp')]
+    # Adding -sERROR_ON_UNDEFINED_SYMBOLS=0 means the error gets reported later
+    # by emscripten.py.
+    cmd += ['-sERROR_ON_UNDEFINED_SYMBOLS=0']
+    err = self.expect_fail(cmd)
+    self.assertContained('undefined exported symbol: "_foobar"', err)
+
+    # setting `-Wno-undefined` should suppress the error
+    cmd += ['-Wno-undefined']
+    self.run_process(cmd)
+
+  @parameterized({
+    '': ('out.js',),
+    'standalone': ('out.wasm',)
+  })
+  def test_undefined_exported_js_function(self, outfile):
+    cmd = [EMXX, test_file('hello_world.cpp'), '-o', outfile]
     self.run_process(cmd)
 
     # adding a missing symbol to EXPORTED_FUNCTIONS should cause failure
     cmd += ['-sEXPORTED_FUNCTIONS=foobar']
     err = self.expect_fail(cmd)
     self.assertContained('undefined exported symbol: "foobar"', err)
 
-    # setting `-Wno-undefined` should suppress error
+    # setting `-Wno-undefined` should suppress the error
     cmd += ['-Wno-undefined']
     self.run_process(cmd)
 

tools/diagnostics.py

@@ -242,6 +242,10 @@ def disable_warning(name):
   manager.warnings[name]['enabled'] = False
 
 
+def is_enabled(name):
+  return manager.warnings[name]['enabled']
+
+
 def warning(warning_type, message, *args):
   manager.warning(warning_type, message, *args)
 

tools/emscripten.py

@@ -210,13 +210,22 @@ def set_memory(static_bump):
   settings.HEAP_BASE = align_memory(stack_high)
 
 
-def report_missing_symbols(js_symbols):
-  # Report any symbol that was explicitly exported but is present neither
-  # as a native function nor as a JS library function.
-  defined_symbols = set(asmjs_mangle(e) for e in settings.WASM_EXPORTS).union(js_symbols)
-  missing = set(settings.USER_EXPORTED_FUNCTIONS) - defined_symbols
-  for symbol in sorted(missing):
-    diagnostics.warning('undefined', f'undefined exported symbol: "{symbol}"')
+def report_missing_exports_wasm_only(metadata):
+  if diagnostics.is_enabled('undefined'):
+    defined_symbols = set(asmjs_mangle(e) for e in metadata.all_exports)
+    missing = set(settings.USER_EXPORTED_FUNCTIONS) - defined_symbols
+    for symbol in sorted(missing):
+      diagnostics.warning('undefined', f'undefined exported symbol: "{symbol}"')
+
+
+def report_missing_exports(js_symbols):
+  if diagnostics.is_enabled('undefined'):
+    # Report any symbol that was explicitly exported but is present neither
+    # as a native function nor as a JS library function.
+    defined_symbols = set(asmjs_mangle(e) for e in settings.WASM_EXPORTS).union(js_symbols)
+    missing = set(settings.USER_EXPORTED_FUNCTIONS) - defined_symbols
+    for symbol in sorted(missing):
+      diagnostics.warning('undefined', f'undefined exported symbol: "{symbol}"')
 
   # Special hanlding for the `_main` symbol
 
@@ -359,7 +368,9 @@ def emscript(in_wasm, out_wasm, outfile_js, js_syms, finalize=True):
       if proc.returncode:
         exit_with_error(f'EM_JS function validation failed (node returned {proc.returncode})')
 
-    logger.debug('emscript: skipping remaining js glue generation')
+  if not outfile_js:
+    report_missing_exports_wasm_only(metadata)
+    logger.debug('emscript: skipping js glue generation')
     return
 
   # memory and global initializers
@@ -398,11 +409,7 @@ def emscript(in_wasm, out_wasm, outfile_js, js_syms, finalize=True):
         pre += f"  ignoredModuleProp('{sym}');\n"
     pre += "}\n"
 
-  report_missing_symbols(forwarded_json['librarySymbols'])
-
-  if not outfile_js:
-    logger.debug('emscript: skipping remaining js glue generation')
-    return
+  report_missing_exports(forwarded_json['librarySymbols'])
 
   if settings.MINIMAL_RUNTIME:
     # In MINIMAL_RUNTIME, atinit exists in the postamble part