hdl.rec: make Record inherit from UserValue.

Closes #354.

nmigen/back/pysim.py

@@ -374,9 +374,6 @@ class _ValueCompiler(ValueVisitor, _Compiler):
     def on_ResetSignal(self, value):
         raise NotImplementedError # :nocov:
 
-    def on_Record(self, value):
-        return self(Cat(value.fields.values()))
-
     def on_AnyConst(self, value):
         raise NotImplementedError # :nocov:
 

nmigen/back/rtlil.py

@@ -365,9 +365,6 @@ class _ValueCompiler(xfrm.ValueVisitor):
     def on_Initial(self, value):
         raise NotImplementedError # :nocov:
 
-    def on_Record(self, value):
-        return self(ast.Cat(value.fields.values()))
-
     def on_Cat(self, value):
         return "{{ {} }}".format(" ".join(reversed([self(o) for o in value.parts])))
 
@@ -378,7 +375,11 @@ class _ValueCompiler(xfrm.ValueVisitor):
         if value.start == 0 and value.stop == len(value.value):
             return self(value.value)
 
-        sigspec = self._prepare_value_for_Slice(value.value)
+        if isinstance(value.value, ast.UserValue):
+            sigspec = self._prepare_value_for_Slice(value.value._lazy_lower())
+        else:
+            sigspec = self._prepare_value_for_Slice(value.value)
+
         if value.start == value.stop:
             return "{}"
         elif value.start + 1 == value.stop:
@@ -644,7 +645,7 @@ class _LHSValueCompiler(_ValueCompiler):
         return wire_next or wire_curr
 
     def _prepare_value_for_Slice(self, value):
-        assert isinstance(value, (ast.Signal, ast.Slice, ast.Cat, rec.Record))
+        assert isinstance(value, (ast.Signal, ast.Slice, ast.Cat))
         return self(value)
 
     def on_Part(self, value):

nmigen/hdl/ast.py

@@ -1188,7 +1188,10 @@ class UserValue(Value):
 
     def _lazy_lower(self):
         if self.__lowered is None:
-            self.__lowered = Value.cast(self.lower())
+            lowered = self.lower()
+            if isinstance(lowered, UserValue):
+                lowered = lowered._lazy_lower()
+            self.__lowered = Value.cast(lowered)
         return self.__lowered
 
     def shape(self):

nmigen/hdl/rec.py

@@ -85,7 +85,7 @@ class Layout:
 
 
 # Unlike most Values, Record *can* be subclassed.
-class Record(Value):
+class Record(UserValue):
     @staticmethod
     def like(other, *, name=None, name_suffix=None, src_loc_at=0):
         if name is not None:
@@ -113,6 +113,8 @@ class Record(Value):
         return Record(other.layout, name=new_name, fields=fields, src_loc_at=1)
 
     def __init__(self, layout, *, name=None, fields=None, src_loc_at=0):
+        super().__init__(src_loc_at=src_loc_at)
+
         if name is None:
             name = tracer.get_var_name(depth=2 + src_loc_at, default=None)
 
@@ -165,8 +167,8 @@ class Record(Value):
         else:
             return super().__getitem__(item)
 
-    def shape(self):
-        return Shape(sum(len(f) for f in self.fields.values()))
+    def lower(self):
+        return Cat(self.fields.values())
 
     def _lhs_signals(self):
         return union((f._lhs_signals() for f in self.fields.values()), start=SignalSet())

nmigen/hdl/xfrm.py

@@ -38,10 +38,6 @@ class ValueVisitor(metaclass=ABCMeta):
     def on_Signal(self, value):
         pass # :nocov:
 
-    @abstractmethod
-    def on_Record(self, value):
-        pass # :nocov:
-
     @abstractmethod
     def on_ClockSignal(self, value):
         pass # :nocov:
@@ -98,9 +94,6 @@ class ValueVisitor(metaclass=ABCMeta):
         elif isinstance(value, Signal):
             # Uses `isinstance()` and not `type() is` because nmigen.compat requires it.
             new_value = self.on_Signal(value)
-        elif isinstance(value, Record):
-            # Uses `isinstance()` and not `type() is` to allow inheriting from Record.
-            new_value = self.on_Record(value)
         elif type(value) is ClockSignal:
             new_value = self.on_ClockSignal(value)
         elif type(value) is ResetSignal:
@@ -147,9 +140,6 @@ class ValueTransformer(ValueVisitor):
     def on_Signal(self, value):
         return value
 
-    def on_Record(self, value):
-        return value
-
     def on_ClockSignal(self, value):
         return value
 
@@ -372,8 +362,6 @@ class DomainCollector(ValueVisitor, StatementVisitor):
     def on_ResetSignal(self, value):
         self._add_used_domain(value.domain)
 
-    on_Record = on_ignore
-
     def on_Operator(self, value):
         for o in value.operands:
             self.on_value(o)

nmigen/test/test_hdl_ast.py

@@ -916,6 +916,14 @@ class UserValueTestCase(FHDLTestCase):
         self.assertEqual(uv.shape(), unsigned(1))
         self.assertEqual(uv.lower_count, 1)
 
+    def test_lower_to_user_value(self):
+        uv = MockUserValue(MockUserValue(1))
+        self.assertEqual(uv.shape(), unsigned(1))
+        self.assertIsInstance(uv.shape(), Shape)
+        uv.lowered = MockUserValue(2)
+        self.assertEqual(uv.shape(), unsigned(1))
+        self.assertEqual(uv.lower_count, 1)
+
 
 class SampleTestCase(FHDLTestCase):
     def test_const(self):

nmigen/test/test_hdl_xfrm.py

@@ -620,3 +620,12 @@ class UserValueTestCase(FHDLTestCase):
             )
         )
         """)
+
+
+class UserValueRecursiveTestCase(UserValueTestCase):
+    def setUp(self):
+        self.s = Signal()
+        self.c = Signal()
+        self.uv = MockUserValue(MockUserValue(self.s))
+
+    # inherit the test_lower method from UserValueTestCase because the checks are the same