diff --git a/emcc.py b/emcc.py
index 985ab6c78bdd..f551c64fa2cc 100644
--- a/emcc.py
+++ b/emcc.py
@@ -1147,6 +1147,12 @@ def consume_arg_file():
         requested_level = 1
         settings.SHRINK_LEVEL = 0
         settings.DEBUG_LEVEL = max(settings.DEBUG_LEVEL, 1)
+      elif requested_level == 'fast':
+        # TODO(https://github.com/emscripten-core/emscripten/issues/21497):
+        # If we ever map `-ffast-math` to `wasm-opt --fast-math` then
+        # then we should enable that too here.
+        requested_level = 3
+        settings.SHRINK_LEVEL = 0
       else:
         settings.SHRINK_LEVEL = 0
       settings.OPT_LEVEL = validate_arg_level(requested_level, 3, 'invalid optimization level: ' + arg, clamp=True)
diff --git a/test/test_other.py b/test/test_other.py
index 483ed2475f96..506d87f35e42 100644
--- a/test/test_other.py
+++ b/test/test_other.py
@@ -490,7 +490,8 @@ def test_dumpmachine(self):
 
   @parameterized({
     'c': [EMCC, '.c'],
-    'cxx': [EMXX, '.cpp']})
+    'cxx': [EMXX, '.cpp']
+  })
   def test_emcc_2(self, compiler, suffix):
     # emcc src.cpp -c    and   emcc -c src.cpp -o src.[o|foo|so] ==> should always give an object file
     for args in [[], ['-o', 'src.o'], ['-o', 'src.foo'], ['-o', 'src.so']]:
@@ -513,7 +514,8 @@ def test_bc_output_warning(self):
 
   @parameterized({
     'c': [EMCC, '.c'],
-    'cxx': [EMXX, '.cpp']})
+    'cxx': [EMXX, '.cpp']
+  })
   def test_emcc_3(self, compiler, suffix):
     # handle singleton archives
     self.run_process([compiler, '-c', test_file('hello_world' + suffix), '-o', 'a.o'])
@@ -538,9 +540,7 @@ def test_emcc_3(self, compiler, suffix):
       delete_file(path)
 
   @is_slow_test
-  @parameterized({
-    'c': [EMCC],
-    'cxx': [EMXX]})
+  @with_both_compilers
   def test_emcc_4(self, compiler):
     # Optimization: emcc src.cpp -o something.js [-Ox]. -O0 is the same as not specifying any optimization setting
     # link_param are used after compiling first
@@ -4687,8 +4687,8 @@ def test_fs_after_main(self):
       self.run_process([EMXX, test_file('fs_after_main.cpp')])
       self.assertContained('Test passed.', self.run_js('a.out.js'))
 
-  def test_os_oz(self):
-    for opt in ['-O1', '-O2', '-Os', '-Oz', '-O3', '-Og']:
+  def test_opt_levels(self):
+    for opt in ['-O1', '-O2', '-Os', '-Oz', '-O3', '-Og', '-Ofast']:
       print(opt)
       proc = self.run_process([EMCC, '-v', test_file('hello_world.c'), opt], stderr=PIPE)
       self.assertContained(opt, proc.stderr)