hdl.ast,back.rtlil: implement Cover.

Fixes #194.

nmigen/hdl/ast.py

@@ -15,7 +15,7 @@ __all__ = [
     "Signal", "ClockSignal", "ResetSignal",
     "UserValue",
     "Sample", "Past", "Stable", "Rose", "Fell", "Initial",
-    "Statement", "Assign", "Assert", "Assume", "Switch", "Delay", "Tick",
+    "Statement", "Assign", "Assert", "Assume", "Cover", "Switch", "Delay", "Tick",
     "Passive", "ValueKey", "ValueDict", "ValueSet", "SignalKey", "SignalDict",
     "SignalSet",
 ]
@@ -1080,6 +1080,11 @@ class Assume(Property):
     _kind = "assume"
 
 
+@final
+class Cover(Property):
+    _kind = "cover"
+
+
 # @final
 class Switch(Statement):
     def __init__(self, test, cases, *, src_loc=None, src_loc_at=0, case_src_locs={}):

nmigen/hdl/dsl.py

@@ -417,9 +417,9 @@ class Module(_ModuleBuilderRoot, Elaboratable):
             self._pop_ctrl()
 
         for assign in Statement.wrap(assigns):
-            if not compat_mode and not isinstance(assign, (Assign, Assert, Assume)):
+            if not compat_mode and not isinstance(assign, (Assign, Assert, Assume, Cover)):
                 raise SyntaxError(
-                    "Only assignments, asserts, and assumes may be appended to d.{}"
+                    "Only assignments and property checks may be appended to d.{}"
                     .format(domain_name(domain)))
 
             assign = SampleDomainInjector(domain)(assign)

nmigen/hdl/xfrm.py

@@ -196,6 +196,10 @@ class StatementVisitor(metaclass=ABCMeta):
     def on_Assume(self, stmt):
         pass # :nocov:
 
+    @abstractmethod
+    def on_Cover(self, stmt):
+        pass # :nocov:
+
     @abstractmethod
     def on_Switch(self, stmt):
         pass # :nocov:
@@ -217,6 +221,8 @@ class StatementVisitor(metaclass=ABCMeta):
             new_stmt = self.on_Assert(stmt)
         elif type(stmt) is Assume:
             new_stmt = self.on_Assume(stmt)
+        elif type(stmt) is Cover:
+            new_stmt = self.on_Cover(stmt)
         elif isinstance(stmt, Switch):
             # Uses `isinstance()` and not `type() is` because nmigen.compat requires it.
             new_stmt = self.on_Switch(stmt)
@@ -247,6 +253,9 @@ class StatementTransformer(StatementVisitor):
     def on_Assume(self, stmt):
         return Assume(self.on_value(stmt.test), _check=stmt._check, _en=stmt._en)
 
+    def on_Cover(self, stmt):
+        return Cover(self.on_value(stmt.test), _check=stmt._check, _en=stmt._en)
+
     def on_Switch(self, stmt):
         cases = OrderedDict((k, self.on_statement(s)) for k, s in stmt.cases.items())
         return Switch(self.on_value(stmt.test), cases)
@@ -396,11 +405,12 @@ class DomainCollector(ValueVisitor, StatementVisitor):
         self.on_value(stmt.lhs)
         self.on_value(stmt.rhs)
 
-    def on_Assert(self, stmt):
+    def on_property(self, stmt):
         self.on_value(stmt.test)
 
-    def on_Assume(self, stmt):
-        self.on_value(stmt.test)
+    on_Assert = on_property
+    on_Assume = on_property
+    on_Cover  = on_property
 
     def on_Switch(self, stmt):
         self.on_value(stmt.test)
@@ -598,12 +608,13 @@ class SampleLowerer(FragmentTransformer, ValueTransformer, StatementTransformer)
 
 
 class SwitchCleaner(StatementVisitor):
-    def on_Assign(self, stmt):
+    def on_ignore(self, stmt):
         return stmt
 
-    on_Assert = on_Assign
-
-    on_Assume = on_Assign
+    on_Assign = on_ignore
+    on_Assert = on_ignore
+    on_Assume = on_ignore
+    on_Cover  = on_ignore
 
     def on_Switch(self, stmt):
         cases = OrderedDict((k, self.on_statement(s)) for k, s in stmt.cases.items())
@@ -651,9 +662,14 @@ class LHSGroupAnalyzer(StatementVisitor):
         if lhs_signals:
             self.unify(*stmt._lhs_signals())
 
-    on_Assert = on_Assign
+    def on_property(self, stmt):
+        lhs_signals = stmt._lhs_signals()
+        if lhs_signals:
+            self.unify(*stmt._lhs_signals())
 
-    on_Assume = on_Assign
+    on_Assert = on_property
+    on_Assume = on_property
+    on_Cover  = on_property
 
     def on_Switch(self, stmt):
         for case_stmts in stmt.cases.values():
@@ -681,12 +697,14 @@ class LHSGroupFilter(SwitchCleaner):
             if any_lhs_signal in self.signals:
                 return stmt
 
-    def on_Assert(self, stmt):
+    def on_property(self, stmt):
         any_lhs_signal = next(iter(stmt._lhs_signals()))
         if any_lhs_signal in self.signals:
             return stmt
 
-    on_Assume = on_Assert
+    on_Assert = on_property
+    on_Assume = on_property
+    on_Cover  = on_property
 
 
 class _ControlInserter(FragmentTransformer):