back.pysim: undriven comb signals should return to reset value.

nmigen/back/pysim.py

@@ -326,8 +326,13 @@ class Simulator:
                     self._sync_signals.update(signals)
                     self._domain_signals[domain].update(signals)
 
+            statements = []
+            for signal in fragment.iter_comb():
+                statements.append(signal.eq(signal.reset))
+            statements += fragment.statements
+
             compiler = _StatementCompiler()
-            funclet  = compiler(fragment.statements)
+            funclet  = compiler(statements)
             for signal in compiler.sensitivity:
                 self._add_funclet(signal, funclet)
             for domain, cd in fragment.domains.items():

nmigen/fhdl/ir.py

@@ -39,7 +39,8 @@ class Fragment:
                 yield domain, signal
 
     def iter_comb(self):
-        yield from self.drivers[None]
+        if None in self.drivers:
+            yield from self.drivers[None]
 
     def iter_sync(self):
         for domain, signals in self.drivers.items():