hdl.ast: accept Signals with identical min/max bounds.

And produce a 0-bit signal.

Fixes #58.

diff --git a/nmigen/hdl/ast.py b/nmigen/hdl/ast.py
index 3a4222094fc80a194c9b2d700dda062c3a55072d..37816cecc75706f323eda027f87ee6c8112c78f3 100644 (file)
--- a/nmigen/hdl/ast.py
+++ b/nmigen/hdl/ast.py
@@ -593,11 +593,15 @@ class Signal(Value, DUID):
             if max is None:
                 max = 2
             max -= 1  # make both bounds inclusive
-            if not min < max:
-                raise ValueError("Lower bound {} should be less than higher bound {}"
-                                 .format(min, max))
+            if min > max:
+                raise ValueError("Lower bound {} should be less or equal to higher bound {}"
+                                 .format(min, max + 1))
             self.signed = min < 0 or max < 0
-            self.nbits  = builtins.max(bits_for(min, self.signed), bits_for(max, self.signed))
+            if min == max:
+                self.nbits = 0
+            else:
+                self.nbits = builtins.max(bits_for(min, self.signed),
+                                          bits_for(max, self.signed))
 
         else:
             if not (min is None and max is None):

diff --git a/nmigen/test/test_hdl_ast.py b/nmigen/test/test_hdl_ast.py
index 608870fdc4f1db4ad88ce73837dcb309fdbc9f11..d2cc4cd39dfb51ff407ab07f20801570c721a14b 100644 (file)
--- a/nmigen/test/test_hdl_ast.py
+++ b/nmigen/test/test_hdl_ast.py
@@ -414,13 +414,18 @@ class SignalTestCase(FHDLTestCase):
         self.assertEqual(s8.shape(), (6, True))
         s9 = Signal(0)
         self.assertEqual(s9.shape(), (0, False))
+        s10 = Signal(max=1)
+        self.assertEqual(s10.shape(), (0, False))
 
     def test_shape_bad(self):
-        with self.assertRaises(ValueError):
+        with self.assertRaises(ValueError,
+                msg="Lower bound 10 should be less or equal to higher bound 4"):
             Signal(min=10, max=4)
-        with self.assertRaises(ValueError):
+        with self.assertRaises(ValueError,
+                msg="Only one of bits/signedness or bounds may be specified"):
             Signal(2, min=10)
-        with self.assertRaises(TypeError):
+        with self.assertRaises(TypeError,
+                msg="Width must be a non-negative integer, not '-10'"):
             Signal(-10)
 
     def test_name(self):