Handle '-M' and '-MM' compiler flags similar to '-E'.

emcc

@@ -221,7 +221,7 @@ if CONFIGURE_CONFIG or CMAKE_CONFIG:
     elif arg.endswith('.s'):
       if debug_configure: open(tempout, 'a').write('(compiling .s assembly, must use clang\n')
       if use_js == 1: use_js = 0
-    elif arg == '-E':
+    elif arg == '-E' or arg == '-M' or arg == '-MM':
       if use_js == 1: use_js = 0
 
   if src:
@@ -330,12 +330,6 @@ for i in range(1, len(sys.argv)):
     if arg.endswith(C_ENDINGS + OBJC_ENDINGS):
       use_cxx = False
 
-if '-M' in sys.argv or '-MM' in sys.argv:
-  # Just output dependencies, do not compile. Warning: clang and gcc behave differently with -MF! (clang seems to not recognize it)
-  cmd = [CC] + shared.COMPILER_OPTS + sys.argv[1:]
-  logging.debug('just dependencies: ' + ' '.join(cmd))
-  exit(subprocess.call(cmd))
-
 # Check if a target is specified
 target = None
 for i in range(len(sys.argv)-1):
@@ -819,7 +813,9 @@ try:
     target = target_basename + '.o'
     final_suffix = 'o'
   if '-E' in newargs:
-    final_suffix = 'eout' # not bitcode, not js; something else
+    final_suffix = 'eout' # not bitcode, not js; but just result from preprocessing stage of the input file
+  if '-M' in newargs or '-MM' in newargs:
+    final_suffix = 'mout' # not bitcode, not js; but just dependency rule of the input file
   final_ending = ('.' + final_suffix) if len(final_suffix) > 0 else ''
 
   # Find library files
@@ -1077,12 +1073,13 @@ try:
     return args
 
   # -E preprocessor-only support
-  if '-E' in newargs:
+  if '-E' in newargs or '-M' in newargs or '-MM' in newargs:
     input_files = map(lambda x: x[1], input_files)
     cmd = get_bitcode_args(input_files)
     if specified_target:
       cmd += ['-o', specified_target]
-    logging.debug('just preprocessor ' + ' '.join(cmd))
+    # Do not compile, but just output the result from preprocessing stage or output the dependency rule. Warning: clang and gcc behave differently with -MF! (clang seems to not recognize it)
+    logging.debug(('just preprocessor ' if '-E' in newargs else 'just dependencies: ') + ' '.join(cmd))
     exit(subprocess.call(cmd))
 
   # First, generate LLVM bitcode. For each input file, we get base.o with bitcode

tests/test_other.py

@@ -3323,6 +3323,28 @@ EMSCRIPTEN_KEEPALIVE __EMSCRIPTEN_major__ __EMSCRIPTEN_minor__ __EMSCRIPTEN_tiny
     assert len(with_dash_o) == 0
     assert len(without_dash_o) != 0
 
+  def test_dashM(self):
+    out = Popen([PYTHON, EMXX, path_from_root('tests', 'hello_world.cpp'), '-M'], stdout=PIPE).communicate()[0]
+    self.assertContained('hello_world.o:', out) # Verify output is just a dependency rule instead of bitcode or js
+
+  def test_dashM_consistent(self):
+    normal = Popen([PYTHON, EMXX, '-v', '-Wno-warn-absolute-paths', path_from_root('tests', 'hello_world.cpp'), '-c'], stdout=PIPE, stderr=PIPE).communicate()[1]
+    dash_m = Popen([PYTHON, EMXX, '-v', '-Wno-warn-absolute-paths', path_from_root('tests', 'hello_world.cpp'), '-M'], stdout=PIPE, stderr=PIPE).communicate()[1]
+
+    import difflib
+    diff = [a.rstrip()+'\n' for a in difflib.unified_diff(normal.split('\n'), dash_m.split('\n'), fromfile='normal', tofile='dash_m')]
+    left_std = filter(lambda x: x.startswith('-') and '-std=' in x, diff)
+    right_std = filter(lambda x: x.startswith('+') and '-std=' in x, diff)
+    assert len(left_std) == len(right_std) == 1, '\n\n'.join(diff)
+    bad = filter(lambda x: '-Wno-warn-absolute-paths' in x, diff)
+    assert len(bad) == 0, '\n\n'.join(diff)
+
+  def test_dashM_respect_dashO(self):
+    with_dash_o = Popen([PYTHON, EMXX, path_from_root('tests', 'hello_world.cpp'), '-M', '-o', '/dev/null'], stdout=PIPE, stderr=PIPE).communicate()[0]
+    without_dash_o = Popen([PYTHON, EMXX, path_from_root('tests', 'hello_world.cpp'), '-M'], stdout=PIPE, stderr=PIPE).communicate()[0]
+    assert len(with_dash_o) == 0
+    assert len(without_dash_o) != 0
+
   def test_malloc_implicit(self):
     open('src.cpp', 'w').write(r'''
 #include <stdlib.h>