hdl.ast: add Past, Stable, Rose, Fell.

nmigen/hdl/ast.py

@@ -10,7 +10,8 @@ from ..tools import *
 
 __all__ = [
     "Value", "Const", "C", "AnyConst", "AnySeq", "Operator", "Mux", "Part", "Slice", "Cat", "Repl",
-    "Array", "ArrayProxy", "Sample",
+    "Array", "ArrayProxy",
+    "Sample", "Past", "Stable", "Rose", "Fell",
     "Signal", "ClockSignal", "ResetSignal",
     "Statement", "Assign", "Assert", "Assume", "Switch", "Delay", "Tick",
     "Passive", "ValueKey", "ValueDict", "ValueSet", "SignalKey", "SignalDict",
@@ -471,10 +472,10 @@ class Cat(Value):
         return sum(len(part) for part in self.parts), False
 
     def _lhs_signals(self):
-        return union(part._lhs_signals() for part in self.parts)
+        return union((part._lhs_signals() for part in self.parts), start=ValueSet())
 
     def _rhs_signals(self):
-        return union(part._rhs_signals() for part in self.parts)
+        return union((part._rhs_signals() for part in self.parts), start=ValueSet())
 
     def _as_const(self):
         value = 0
@@ -832,9 +833,15 @@ class ArrayProxy(Value):
 
 
 class Sample(Value):
-    def __init__(self, value, clocks, domain):
+    """Value from the past.
+
+    A ``Sample`` of an expression is equal to the value of the expression ``clocks`` clock edges
+    of the ``domain`` clock back. If that moment is before the beginning of time, it is equal
+    to the value of the expression calculated as if each signal had its reset value.
+    """
+    def __init__(self, expr, clocks, domain):
         super().__init__(src_loc_at=1)
-        self.value  = Value.wrap(value)
+        self.value  = Value.wrap(expr)
         self.clocks = int(clocks)
         self.domain = domain
         if not isinstance(self.value, (Const, Signal)):
@@ -855,6 +862,22 @@ class Sample(Value):
             self.value, "<default>" if self.domain is None else self.domain, self.clocks)
 
 
+def Past(expr, clocks=1, domain=None):
+    return Sample(expr, clocks, domain)
+
+
+def Stable(expr, clocks=0, domain=None):
+    return Sample(expr, clocks + 1, domain) == Sample(expr, clocks, domain)
+
+
+def Rose(expr, clocks=0, domain=None):
+    return ~Sample(expr, clocks + 1, domain) & Sample(expr, clocks, domain)
+
+
+def Fell(expr, clocks=0, domain=None):
+    return Sample(expr, clocks + 1, domain) & ~Sample(expr, clocks, domain)
+
+
 class _StatementList(list):
     def __repr__(self):
         return "({})".format(" ".join(map(repr, self)))

nmigen/hdl/dsl.py

@@ -353,6 +353,7 @@ class Module(_ModuleBuilderRoot):
                     "Only assignments, asserts, and assumes may be appended to d.{}"
                     .format(domain_name(domain)))
 
+            assign = SampleDomainInjector(domain)(assign)
             for signal in assign._lhs_signals():
                 if signal not in self._driving:
                     self._driving[signal] = domain
@@ -384,7 +385,8 @@ class Module(_ModuleBuilderRoot):
         fragment = Fragment()
         for submodule, name in self._submodules:
             fragment.add_subfragment(submodule.get_fragment(platform), name)
-        fragment.add_statements(self._statements)
+        statements = SampleDomainInjector("sync")(self._statements)
+        fragment.add_statements(statements)
         for signal, domain in self._driving.items():
             fragment.add_driver(signal, domain)
         fragment.add_domains(self._domains)

nmigen/hdl/ir.py

@@ -363,9 +363,9 @@ class Fragment:
                 SignalSet(self.iter_ports("io")))
 
     def prepare(self, ports=(), ensure_sync_exists=True):
-        from .xfrm import FragmentTransformer
+        from .xfrm import SampleLowerer
 
-        fragment = FragmentTransformer()(self)
+        fragment = SampleLowerer()(self)
         fragment._propagate_domains(ensure_sync_exists)
         fragment._resolve_hierarchy_conflicts()
         fragment = fragment._insert_domain_resets()

nmigen/hdl/xfrm.py

@@ -13,7 +13,8 @@ from .rec import *
 __all__ = ["ValueVisitor", "ValueTransformer",
            "StatementVisitor", "StatementTransformer",
            "FragmentTransformer",
-           "DomainRenamer", "DomainLowerer", "SampleLowerer",
+           "DomainRenamer", "DomainLowerer",
+           "SampleDomainInjector", "SampleLowerer",
            "SwitchCleaner", "LHSGroupAnalyzer", "LHSGroupFilter",
            "ResetInserter", "CEInserter"]
 
@@ -340,6 +341,19 @@ class DomainLowerer(FragmentTransformer, ValueTransformer, StatementTransformer)
         return cd.rst
 
 
+class SampleDomainInjector(ValueTransformer, StatementTransformer):
+    def __init__(self, domain):
+        self.domain = domain
+
+    def on_Sample(self, value):
+        if value.domain is not None:
+            return value
+        return Sample(value.value, value.clocks, self.domain)
+
+    def __call__(self, stmts):
+        return self.on_statement(stmts)
+
+
 class SampleLowerer(FragmentTransformer, ValueTransformer, StatementTransformer):
     def __init__(self):
         self.sample_cache = ValueDict()