--- a/config/config.mk
+++ b/config/config.mk
@@ -262,18 +262,18 @@ HOST_CFLAGS	+= $(HOST_OPTIMIZE_FLAGS)
 HOST_CXXFLAGS	+= $(HOST_OPTIMIZE_FLAGS)
 else
 ifdef MOZ_OPTIMIZE
 HOST_CFLAGS	+= $(MOZ_OPTIMIZE_FLAGS)
 HOST_CXXFLAGS	+= $(MOZ_OPTIMIZE_FLAGS)
 endif # MOZ_OPTIMIZE
 endif # CROSS_COMPILE
 
-COMPILE_CFLAGS	= $(COMPUTED_CFLAGS) $(PGO_CFLAGS) $(MOZBUILD_CFLAGS) $(_DEPEND_CFLAGS) $(MK_COMPILE_DEFINES)
-COMPILE_CXXFLAGS = $(COMPUTED_CXXFLAGS) $(PGO_CFLAGS) $(MOZBUILD_CXXFLAGS) $(_DEPEND_CFLAGS) $(MK_COMPILE_DEFINES)
+COMPILE_CFLAGS	= $(COMPUTED_CFLAGS) $(PGO_CFLAGS) $(_DEPEND_CFLAGS) $(MK_COMPILE_DEFINES)
+COMPILE_CXXFLAGS = $(COMPUTED_CXXFLAGS) $(PGO_CFLAGS) $(_DEPEND_CFLAGS) $(MK_COMPILE_DEFINES)
 COMPILE_CMFLAGS = $(OS_COMPILE_CMFLAGS) $(MOZBUILD_CMFLAGS)
 COMPILE_CMMFLAGS = $(OS_COMPILE_CMMFLAGS) $(MOZBUILD_CMMFLAGS)
 ASFLAGS += $(MOZBUILD_ASFLAGS)
 
 ifndef CROSS_COMPILE
 HOST_CFLAGS += $(RTL_FLAGS)
 endif
 

--- a/python/mozbuild/mozbuild/compilation/database.py
+++ b/python/mozbuild/mozbuild/compilation/database.py
@@ -66,18 +66,17 @@ class CompileDBBackend(CommonBackend):
 
         elif isinstance(obj, (Sources, GeneratedSources)):
             # For other sources, include each source file.
             for f in obj.files:
                 self._build_db_line(obj.objdir, obj.relativedir, obj.config, f,
                                     obj.canonical_suffix)
 
         elif isinstance(obj, VariablePassthru):
-            for var in ('MOZBUILD_CFLAGS', 'MOZBUILD_CXXFLAGS',
-                        'MOZBUILD_CMFLAGS', 'MOZBUILD_CMMFLAGS'):
+            for var in ('MOZBUILD_CMFLAGS', 'MOZBUILD_CMMFLAGS'):
                 if var in obj.variables:
                     self._local_flags[obj.objdir][var] = obj.variables[var]
 
         elif isinstance(obj, PerSourceFlag):
             self._per_source_flags[obj.file_name].extend(obj.flags)
 
         elif isinstance(obj, ComputedFlags):
             for var, flags in obj.get_flags():
@@ -182,15 +181,14 @@ class CompileDBBackend(CommonBackend):
             value = cenv.substs.get(name)
             if not value:
                 return
             if isinstance(value, types.StringTypes):
                 value = value.split()
             db.extend(value)
 
         db.append('$(COMPUTED_%s)' % self.CFLAGS[canonical_suffix])
-        db.append('$(MOZBUILD_%s)' % self.CFLAGS[canonical_suffix])
         if canonical_suffix == '.m':
             append_var('OS_COMPILE_CMFLAGS')
             db.append('$(MOZBUILD_CMFLAGS)')
         elif canonical_suffix == '.mm':
             append_var('OS_COMPILE_CMMFLAGS')
             db.append('$(MOZBUILD_CMMFLAGS)')

--- a/python/mozbuild/mozbuild/frontend/context.py
+++ b/python/mozbuild/mozbuild/frontend/context.py
@@ -340,16 +340,18 @@ class CompileFlags(ContextDerivedValue, 
             ('CLANG_PLUGIN', context.config.substs.get('CLANG_PLUGIN_FLAGS'),
              ('CFLAGS', 'CXXFLAGS', 'CXX_LDFLAGS', 'C_LDFLAGS')),
             ('OPTIMIZE', self._optimize_flags(),
              ('CFLAGS', 'CXXFLAGS', 'CXX_LDFLAGS', 'C_LDFLAGS')),
             ('FRAMEPTR', context.config.substs.get('MOZ_FRAMEPTR_FLAGS'),
              ('CFLAGS', 'CXXFLAGS', 'CXX_LDFLAGS', 'C_LDFLAGS')),
             ('WARNINGS_AS_ERRORS', self._warnings_as_errors(),
              ('CXXFLAGS', 'CFLAGS', 'CXX_LDFLAGS', 'C_LDFLAGS')),
+            ('MOZBUILD_CFLAGS', None, ('CFLAGS',)),
+            ('MOZBUILD_CXXFLAGS', None, ('CXXFLAGS',)),
         )
         self._known_keys = set(k for k, v, _ in self.flag_variables)
 
         # Providing defaults here doesn't play well with multiple templates
         # modifying COMPILE_FLAGS from the same moz.build, because the merge
         # done after the template runs can't tell which values coming from
         # a template were set and which were provided as defaults.
         template_name = getattr(context, 'template', None)

--- a/python/mozbuild/mozbuild/frontend/emitter.py
+++ b/python/mozbuild/mozbuild/frontend/emitter.py
@@ -912,16 +912,18 @@ class TreeMetadataEmitter(LoggingMixin):
 
         # We always emit a directory traversal descriptor. This is needed by
         # the recursive make backend.
         for o in self._emit_directory_traversal_from_context(context): yield o
 
         for obj in self._process_xpidl(context):
             yield obj
 
+        computed_flags = ComputedFlags(context, context['COMPILE_FLAGS'])
+
         # Proxy some variables as-is until we have richer classes to represent
         # them. We should aim to keep this set small because it violates the
         # desired abstraction of the build definition away from makefiles.
         passthru = VariablePassthru(context)
         varlist = [
             'ANDROID_APK_NAME',
             'ANDROID_APK_PACKAGE',
             'ANDROID_GENERATED_RESFILES',
@@ -941,29 +943,31 @@ class TreeMetadataEmitter(LoggingMixin):
                 passthru.variables[v] = context[v]
 
         if context.config.substs.get('OS_TARGET') == 'WINNT' and \
                 context['DELAYLOAD_DLLS']:
             context['LDFLAGS'].extend([('-DELAYLOAD:%s' % dll)
                 for dll in context['DELAYLOAD_DLLS']])
             context['OS_LIBS'].append('delayimp')
 
-        for v in ['CFLAGS', 'CXXFLAGS', 'CMFLAGS', 'CMMFLAGS', 'ASFLAGS',
-                  'LDFLAGS', 'HOST_CFLAGS', 'HOST_CXXFLAGS']:
+        for v in ['CMFLAGS', 'CMMFLAGS', 'ASFLAGS', 'LDFLAGS',
+                  'HOST_CFLAGS', 'HOST_CXXFLAGS']:
             if v in context and context[v]:
                 passthru.variables['MOZBUILD_' + v] = context[v]
 
+        for v in ['CXXFLAGS', 'CFLAGS']:
+            if v in context and context[v]:
+                computed_flags.resolve_flags('MOZBUILD_%s' % v, context[v])
+
         dist_install = context['DIST_INSTALL']
         if dist_install is True:
             passthru.variables['DIST_INSTALL'] = True
         elif dist_install is False:
             passthru.variables['NO_DIST_INSTALL'] = True
 
-        computed_flags = ComputedFlags(context, context['COMPILE_FLAGS'])
-
         # Ideally, this should be done in templates, but this is difficult at
         # the moment because USE_STATIC_LIBS can be set after a template
         # returns. Eventually, with context-based templates, it will be
         # possible.
         if (context.config.substs.get('OS_ARCH') == 'WINNT' and
                 not context.config.substs.get('GNU_CC')):
             use_static_lib = (context.get('USE_STATIC_LIBS') and
                               not context.config.substs.get('MOZ_ASAN'))

--- a/python/mozbuild/mozbuild/test/backend/data/variable_passthru/moz.build
+++ b/python/mozbuild/mozbuild/test/backend/data/variable_passthru/moz.build
@@ -4,14 +4,12 @@
 
 DELAYLOAD_DLLS = ['foo.dll', 'bar.dll']
 
 RCFILE = 'foo.rc'
 RESFILE = 'bar.res'
 RCINCLUDE = 'bar.rc'
 DEFFILE = 'baz.def'
 
-CFLAGS += ['-fno-exceptions', '-w']
-CXXFLAGS += ['-fcxx-exceptions', '-option with spaces']
 LDFLAGS += ['-ld flag with spaces', '-x']
 HOST_CFLAGS += ['-funroll-loops', '-wall']
 HOST_CXXFLAGS += ['-funroll-loops-harder', '-wall-day-everyday']
 WIN32_EXE_LDFLAGS += ['-subsystem:console']

--- a/python/mozbuild/mozbuild/test/backend/test_recursivemake.py
+++ b/python/mozbuild/mozbuild/test/backend/test_recursivemake.py
@@ -323,24 +323,16 @@ class TestRecursiveMakeBackend(BackendTe
                 'RESFILE := bar.res',
             ],
             'RCINCLUDE': [
                 'RCINCLUDE := bar.rc',
             ],
             'DEFFILE': [
                 'DEFFILE := baz.def',
             ],
-            'MOZBUILD_CFLAGS': [
-                'MOZBUILD_CFLAGS += -fno-exceptions',
-                'MOZBUILD_CFLAGS += -w',
-            ],
-            'MOZBUILD_CXXFLAGS': [
-                'MOZBUILD_CXXFLAGS += -fcxx-exceptions',
-                "MOZBUILD_CXXFLAGS += '-option with spaces'",
-            ],
             'MOZBUILD_LDFLAGS': [
                 "MOZBUILD_LDFLAGS += '-ld flag with spaces'",
                 'MOZBUILD_LDFLAGS += -x',
                 'MOZBUILD_LDFLAGS += -DELAYLOAD:foo.dll',
                 'MOZBUILD_LDFLAGS += -DELAYLOAD:bar.dll',
             ],
             'MOZBUILD_HOST_CFLAGS': [
                 'MOZBUILD_HOST_CFLAGS += -funroll-loops',

--- a/python/mozbuild/mozbuild/test/frontend/data/compile-flags/moz.build
+++ b/python/mozbuild/mozbuild/test/frontend/data/compile-flags/moz.build
@@ -8,8 +8,11 @@ def Library(name):
 
 Library('dummy')
 
 @template
 def DisableStlWrapping():
     COMPILE_FLAGS['STL'] = []
 
 UNIFIED_SOURCES += ['test1.c']
+
+CXXFLAGS += ['-funroll-loops', '-Wall']
+CFLAGS += ['-Wall', '-funroll-loops']

--- a/python/mozbuild/mozbuild/test/frontend/data/variable-passthru/moz.build
+++ b/python/mozbuild/mozbuild/test/frontend/data/variable-passthru/moz.build
@@ -6,14 +6,12 @@ DIST_INSTALL = False
 
 DELAYLOAD_DLLS = ['foo.dll', 'bar.dll']
 
 RCFILE = 'foo.rc'
 RESFILE = 'bar.res'
 RCINCLUDE = 'bar.rc'
 DEFFILE = 'baz.def'
 
-CFLAGS += ['-fno-exceptions', '-w']
-CXXFLAGS += ['-fcxx-exceptions', '-include foo.h']
 LDFLAGS += ['-framework Foo', '-x']
 HOST_CFLAGS += ['-funroll-loops', '-wall']
 HOST_CXXFLAGS += ['-funroll-loops-harder', '-wall-day-everyday']
 WIN32_EXE_LDFLAGS += ['-subsystem:console']

--- a/python/mozbuild/mozbuild/test/frontend/test_emitter.py
+++ b/python/mozbuild/mozbuild/test/frontend/test_emitter.py
@@ -183,18 +183,16 @@ class TestEmitterBasic(unittest.TestCase
         self.assertIsInstance(objs[0], VariablePassthru)
 
         wanted = {
             'NO_DIST_INSTALL': True,
             'RCFILE': 'foo.rc',
             'RESFILE': 'bar.res',
             'RCINCLUDE': 'bar.rc',
             'DEFFILE': 'baz.def',
-            'MOZBUILD_CFLAGS': ['-fno-exceptions', '-w'],
-            'MOZBUILD_CXXFLAGS': ['-fcxx-exceptions', '-include foo.h'],
             'MOZBUILD_LDFLAGS': ['-framework Foo', '-x', '-DELAYLOAD:foo.dll',
                                  '-DELAYLOAD:bar.dll'],
             'MOZBUILD_HOST_CFLAGS': ['-funroll-loops', '-wall'],
             'MOZBUILD_HOST_CXXFLAGS': ['-funroll-loops-harder',
                                        '-wall-day-everyday'],
             'WIN32_EXE_LDFLAGS': ['-subsystem:console'],
         }
 
@@ -208,16 +206,18 @@ class TestEmitterBasic(unittest.TestCase
         reader = self.reader('compile-flags', extra_substs={
             'WARNINGS_AS_ERRORS': '-Werror',
         })
         sources, lib, flags = self.read_topsrcdir(reader)
         self.assertIsInstance(flags, ComputedFlags)
         self.assertEqual(flags.flags['STL'], reader.config.substs['STL_FLAGS'])
         self.assertEqual(flags.flags['VISIBILITY'], reader.config.substs['VISIBILITY_FLAGS'])
         self.assertEqual(flags.flags['WARNINGS_AS_ERRORS'], ['-Werror'])
+        self.assertEqual(flags.flags['MOZBUILD_CFLAGS'], ['-Wall', '-funroll-loops'])
+        self.assertEqual(flags.flags['MOZBUILD_CXXFLAGS'], ['-funroll-loops', '-Wall'])
 
     def test_debug_flags(self):
         reader = self.reader('compile-flags', extra_substs={
             'MOZ_DEBUG_FLAGS': '-g',
             'MOZ_DEBUG_SYMBOLS': '1',
         })
         sources, lib, flags = self.read_topsrcdir(reader)
         self.assertIsInstance(flags, ComputedFlags)