--- a/python/mozbuild/mozbuild/backend/tup.py
+++ b/python/mozbuild/mozbuild/backend/tup.py
@@ -30,16 +30,17 @@ from ..frontend.data import (
     GeneratedFile,
     GeneratedSources,
     HostDefines,
     HostSources,
     JARManifest,
     ObjdirFiles,
     PerSourceFlag,
     Sources,
+    VariablePassthru,
 )
 from ..util import (
     FileAvoidWrite,
     expand_variables,
 )
 from ..frontend.context import (
     AbsolutePath,
     ObjDirPath,
@@ -60,16 +61,17 @@ class BackendTupfile(object):
         self.shell_exported = False
         self.defines = []
         self.host_defines = []
         self.delayed_generated_files = []
         self.per_source_flags = defaultdict(list)
         self.local_flags = defaultdict(list)
         self.sources = defaultdict(list)
         self.host_sources = defaultdict(list)
+        self.variables = {}
 
         self.fh = FileAvoidWrite(self.name, capture_diff=True)
         self.fh.write('# THIS FILE WAS AUTOMATICALLY GENERATED. DO NOT EDIT.\n')
         self.fh.write('\n')
 
     def write(self, buf):
         self.fh.write(buf)
 
@@ -116,31 +118,34 @@ class BackendTupfile(object):
             cmd=['!tup_ln'],
             inputs=[source],
             outputs=outputs,
         )
 
     def gen_sources_rules(self, extra_inputs):
         sources = self.sources
         host_sources = self.host_sources
+        as_dash_c = self.variables.get('AS_DASH_C_FLAG', self.environment.substs['AS_DASH_C_FLAG'])
         compilers = [
-            (sources['.S'], 'AS', 'ASFLAGS', ''),
-            (sources['.cpp'], 'CXX', 'CXXFLAGS', ''),
-            (sources['.c'], 'CC', 'CFLAGS', ''),
-            (host_sources['.cpp'], 'HOST_CXX', 'HOST_CXXFLAGS', 'host_'),
-            (host_sources['.c'], 'HOST_CC', 'HOST_CFLAGS', 'host_'),
+            (sources['.S'], 'AS', 'SFLAGS', '-c', ''),
+            (sources['.s'], 'AS', 'ASFLAGS', as_dash_c, ''),
+            (sources['.cpp'], 'CXX', 'CXXFLAGS', '-c', ''),
+            (sources['.c'], 'CC', 'CFLAGS', '-c', ''),
+            (host_sources['.cpp'], 'HOST_CXX', 'HOST_CXXFLAGS', '-c', 'host_'),
+            (host_sources['.c'], 'HOST_CC', 'HOST_CFLAGS', '-c', 'host_'),
         ]
-        for srcs, compiler, flags, prefix in compilers:
+        for srcs, compiler, flags, dash_c, prefix in compilers:
             for src in sorted(srcs):
                 # AS can be set to $(CC), so we need to call expand_variables on
                 # the compiler to get the real value.
-                cmd = [expand_variables(self.environment.substs[compiler], self.environment.substs)]
+                compiler_value = self.variables.get(compiler, self.environment.substs[compiler])
+                cmd = [expand_variables(compiler_value, self.environment.substs)]
                 cmd.extend(shell_quote(f) for f in self.local_flags[flags])
                 cmd.extend(shell_quote(f) for f in self.per_source_flags[src])
-                cmd.extend(['-c', '%f', '-o', '%o'])
+                cmd.extend([dash_c, '%f', '-o', '%o'])
                 self.rule(
                     cmd=cmd,
                     inputs=[src],
                     extra_inputs=extra_inputs,
                     outputs=[prefix + '%B.o'],
                     display='%s %%f' % compiler,
                 )
 
@@ -275,16 +280,18 @@ class TupOnly(CommonBackend, PartialBack
             backend_file.per_source_flags[obj.file_name].extend(obj.flags)
         elif isinstance(obj, ComputedFlags):
             self._process_computed_flags(obj, backend_file)
         elif isinstance(obj, (Sources, GeneratedSources)):
             if obj.relobjdir.startswith(self._supported_dirs):
                 backend_file.sources[obj.canonical_suffix].extend(obj.files)
         elif isinstance(obj, HostSources):
             backend_file.host_sources[obj.canonical_suffix].extend(obj.files)
+        elif isinstance(obj, VariablePassthru):
+            backend_file.variables = obj.variables
 
         return True
 
     def consume_finished(self):
         CommonBackend.consume_finished(self)
 
         # The approach here is similar to fastermake.py, but we
         # simply write out the resulting files here.