try to use a system compiler if using our clang fails to build the native optimizer; fixes #2997

tests/test_other.py

@@ -4407,7 +4407,7 @@ int main(void) {
     output = Popen(NODE_JS + ['-e', 'require("./a.out.js")'], stdout=PIPE, stderr=PIPE).communicate()
     assert output == ('hello, world!\n \n', ''), 'expected no output, got\n===\nSTDOUT\n%s\n===\nSTDERR\n%s\n===\n' % output
 
-  def zzztest_native_optimizer(self):
+  def test_native_optimizer(self):
     old_debug = os.environ.get('EMCC_DEBUG')
     old_native = os.environ.get('EMCC_NATIVE_OPTIMIZER')
     try:

tools/js_optimizer.py

@@ -32,9 +32,14 @@ def get_native_optimizer():
     shared.logging.debug('building native optimizer')
     output = shared.Cache.get_path('optimizer.exe')
     shared.try_delete(output)
-    subprocess.Popen([shared.CLANG, shared.path_from_root('tools', 'optimizer', 'optimizer.cpp'), '-O3', '-std=c++11', '-fno-exceptions', '-fno-rtti', '-o', output]).communicate() # , '-g', '-fno-omit-frame-pointer'
-    assert os.path.exists(output)
-    return output
+    errs = []
+    for compiler in [shared.CLANG, 'g++', 'clang++']: # try our clang first, otherwise hope for a system compiler in the path
+      shared.logging.debug('  using ' + compiler)
+      out, err = subprocess.Popen([shared.CLANG, shared.path_from_root('tools', 'optimizer', 'optimizer.cpp'), '-O3', '-std=c++11', '-fno-exceptions', '-fno-rtti', '-o', output], stderr=subprocess.PIPE).communicate()
+      # for profiling/debugging: '-g', '-fno-omit-frame-pointer'
+      if os.path.exists(output): return output
+      errs.append(err)
+    raise Exception('failed to build native optimizer, errors from each attempt: ' + '\n=================\n'.join(errs))
   return shared.Cache.get('optimizer.exe', create_optimizer, extension='exe')
 
 # Check if we should run a pass or set of passes natively. if a set of passes, they must all be valid to run in the native optimizer at once.