--cflags option (emscripten-core#4776)

Allows getting the flags emcc passes to clang.

emcc.py

@@ -115,6 +115,8 @@ def run():
   if len(sys.argv) <= 1 or ('--help' not in sys.argv and len(sys.argv) >= 2 and sys.argv[1] != '--version'):
     shared.check_sanity(force=DEBUG)
 
+  misc_temp_files = shared.configuration.get_temp_files()
+
   # Handle some global flags
 
   if len(sys.argv) == 1:
@@ -181,6 +183,22 @@ def run():
     print shared.get_llvm_target()
     exit(0)
 
+  elif '--cflags' in sys.argv:
+    # fake running the command, to see the full args we pass to clang
+    debug_env = os.environ.copy()
+    debug_env['EMCC_DEBUG'] = '1'
+    args = filter(lambda x: x != '--cflags', sys.argv)
+    with misc_temp_files.get_file(suffix='.o') as temp_target:
+      input_file = 'hello_world.c'
+      out, err = subprocess.Popen([shared.PYTHON] + args + [shared.path_from_root('tests', input_file), '-c', '-o', temp_target], stderr=subprocess.PIPE, env=debug_env).communicate()
+      lines = filter(lambda x: shared.CLANG_CC in x and input_file in x, err.split(os.linesep))
+      line = lines[0]
+      assert 'running:' in line
+      parts = line.split(' ')[2:]
+      parts = filter(lambda x: x != '-c' and x != '-o' and input_file not in x and temp_target not in x and '-emit-llvm' not in x, parts)
+      print ' '.join(parts)
+    exit(0)
+
   def is_minus_s_for_emcc(newargs, i):
     assert newargs[i] == '-s'
     if i+1 < len(newargs) and '=' in newargs[i+1] and not newargs[i+1].startswith('-'): # -s OPT=VALUE is for us, -s by itself is a linker option
@@ -397,8 +415,6 @@ def log_time(name):
 
   use_cxx = True
 
-  misc_temp_files = shared.configuration.get_temp_files()
-
   try:
     with ToolchainProfiler.profile_block('parse arguments and setup'):
       ## Parse args

site/source/docs/tools_reference/emcc.rst

@@ -450,6 +450,9 @@ Options that are modified or new in *emcc* are listed below:
 ``--output-eol windows|linux``
 	Specifies the line ending to generate for the text files that are outputted. If "--output-eol windows" is passed, the final output files will have Windows \r\n line endings in them. With "--output-eol linux", the final generated files will be written with Unix \n line endings.
 
+``--cflags``
+	Prints out the flags ``emcc`` would pass to ``clang`` to compile source code to object/bitcode form. You can use this to invoke clang yourself, and then run ``emcc`` on those outputs just for the final linking+conversion to JS.
+
 .. _emcc-environment-variables:
 
 Environment variables

tests/test_other.py

@@ -398,6 +398,17 @@ def test_emcc_cache_flag(self):
       os.chdir(path_from_root('tests')) # Move away from the directory we are about to remove.
       shutil.rmtree(tempdirname)
 
+  def test_emcc_cflags(self):
+    # see we print them out
+    output = Popen([PYTHON, EMCC, '--cflags'], stdout=PIPE, stderr=PIPE).communicate()
+    flags = output[0].strip()
+    self.assertContained(' '.join(COMPILER_OPTS), flags)
+    # check they work
+    cmd = [CLANG, path_from_root('tests', 'hello_world.cpp')] + flags.split(' ') + ['-c', '-emit-llvm', '-o', 'a.bc']
+    subprocess.check_call(cmd)
+    subprocess.check_call([PYTHON, EMCC, 'a.bc'])
+    self.assertContained('hello, world!', run_js(self.in_dir('a.out.js')))
+
   def test_emar_em_config_flag(self):
     # We expand this in case the EM_CONFIG is ~/.emscripten (default)
     config = os.path.expanduser(EM_CONFIG)