sim: support clock domains without sync

migen/sim.py

@@ -148,7 +148,6 @@ class Simulator:
             modified = self.evaluator.commit()
 
     def _process_generators(self, cd):
-        if cd in self.generators:
         exhausted = []
         for generator in self.generators[cd]:
             reply = None
@@ -178,7 +177,9 @@ class Simulator:
         while True:
             cds = self.time.tick()
             for cd in cds:
+                if cd in self.fragment.sync:
                     self.evaluator.execute(self.fragment.sync[cd])
+                if cd in self.generators:
                     self._process_generators(cd)
             self._comb_propagate(self.evaluator.commit())