hdl.ir: call back from Fragment.prepare if a clock domain is missing.

See #57.

nmigen/hdl/ir.py

@@ -351,14 +351,19 @@ class Fragment:
 
             subfrag._propagate_domains_down()
 
-    def _propagate_domains(self, ensure_sync_exists):
+    def _propagate_domains(self, missing_domain):
+        from .xfrm import DomainCollector
+
         self._propagate_domains_up()
-        if ensure_sync_exists and not self.domains:
-            cd_sync = ClockDomain()
-            self.add_domains(cd_sync)
-            new_domains = (cd_sync,)
-        else:
-            new_domains = ()
+        new_domains = []
+        for domain_name in DomainCollector()(self):
+            if domain_name is None:
+                continue
+            if domain_name not in self.domains:
+                domain = missing_domain(domain_name)
+                if domain is not None:
+                    self.add_domains(domain)
+                    new_domains.append(domain)
         self._propagate_domains_down()
         return new_domains
 
@@ -513,11 +518,11 @@ class Fragment:
             else:
                 self.add_ports(sig, dir="i")
 
-    def prepare(self, ports=None, ensure_sync_exists=True):
+    def prepare(self, ports=None, missing_domain=lambda name: ClockDomain(name)):
         from .xfrm import SampleLowerer
 
         fragment = SampleLowerer()(self)
-        new_domains = fragment._propagate_domains(ensure_sync_exists)
+        new_domains = fragment._propagate_domains(missing_domain)
         fragment._resolve_hierarchy_conflicts()
         fragment = fragment._insert_domain_resets()
         fragment = fragment._lower_domain_signals()

nmigen/hdl/xfrm.py

@@ -15,7 +15,7 @@ __all__ = ["ValueVisitor", "ValueTransformer",
            "StatementVisitor", "StatementTransformer",
            "FragmentTransformer",
            "TransformedElaboratable",
-           "DomainRenamer", "DomainLowerer",
+           "DomainCollector", "DomainRenamer", "DomainLowerer",
            "SampleDomainInjector", "SampleLowerer",
            "SwitchCleaner", "LHSGroupAnalyzer", "LHSGroupFilter",
            "ResetInserter", "CEInserter"]
@@ -325,6 +325,85 @@ class TransformedElaboratable(Elaboratable):
         return fragment
 
 
+class DomainCollector(ValueVisitor, StatementVisitor):
+    def __init__(self):
+        self.domains = set()
+
+    def on_ignore(self, value):
+        pass
+
+    on_Const = on_ignore
+    on_AnyConst = on_ignore
+    on_AnySeq = on_ignore
+    on_Signal = on_ignore
+
+    def on_ClockSignal(self, value):
+        self.domains.add(value.domain)
+
+    def on_ResetSignal(self, value):
+        self.domains.add(value.domain)
+
+    on_Record = on_ignore
+
+    def on_Operator(self, value):
+        for o in value.operands:
+            self.on_value(o)
+
+    def on_Slice(self, value):
+        self.on_value(value.value)
+
+    def on_Part(self, value):
+        self.on_value(value.value)
+        self.on_value(value.offset)
+
+    def on_Cat(self, value):
+        for o in value.parts:
+            self.on_value(o)
+
+    def on_Repl(self, value):
+        self.on_value(value.value)
+
+    def on_ArrayProxy(self, value):
+        for elem in value._iter_as_values():
+            self.on_value(elem)
+        self.on_value(value.index)
+
+    def on_Sample(self, value):
+        self.on_value(value.value)
+
+    def on_Assign(self, stmt):
+        self.on_value(stmt.lhs)
+        self.on_value(stmt.rhs)
+
+    def on_Assert(self, stmt):
+        self.on_value(stmt.test)
+
+    def on_Assume(self, stmt):
+        self.on_value(stmt.test)
+
+    def on_Switch(self, stmt):
+        self.on_value(stmt.test)
+        for stmts in stmt.cases.values():
+            self.on_statement(stmts)
+
+    def on_statements(self, stmts):
+        for stmt in stmts:
+            self.on_statement(stmt)
+
+    def on_fragment(self, fragment):
+        if isinstance(fragment, Instance):
+            for name, (value, dir) in fragment.named_ports.items():
+                self.on_value(value)
+        self.on_statements(fragment.statements)
+        self.domains.update(fragment.drivers.keys())
+        for subfragment, name in fragment.subfragments:
+            self.on_fragment(subfragment)
+
+    def __call__(self, fragment):
+        self.on_fragment(fragment)
+        return self.domains
+
+
 class DomainRenamer(FragmentTransformer, ValueTransformer, StatementTransformer):
     def __init__(self, domain_map):
         if isinstance(domain_map, str):