Optimized reversed-range call when constants are inside of the range.

Cython/Compiler/Optimize.py

@@ -644,9 +644,6 @@ class IterationTransform(Visitor.EnvTransform):
             if step_value == 0:
                 # will lead to an error elsewhere
                 return node
-            if reversed and step_value not in (1, -1):
-                # FIXME: currently broken - requires calculation of the correct bounds
-                return node
             if not isinstance(step, ExprNodes.IntNode):
                 step = ExprNodes.IntNode(step_pos, value=str(step_value),
                                          constant_result=step_value)
@@ -665,6 +662,14 @@ class IterationTransform(Visitor.EnvTransform):
             bound1, bound2 = bound2, bound1
             if step_value < 0:
                 step_value = -step_value
+            if step_value != 1:
+                if isinstance(bound1, ExprNodes.NameNode) or isinstance(bound2, ExprNodes.NameNode): 
+                    # FIXME: Optimize when variable is in range (e.g. reversed(range(x, y, 3)))
+                    return node
+                else:
+                    end_value = int(bound2.value)
+                    begin_value = int(bound1.value)
+                    bound1.value = str(step_value*((begin_value-end_value-1) // step_value) + end_value+1)
         else:
             if step_value < 0:
                 step_value = -step_value

tests/run/reversed_iteration.pyx

@@ -133,9 +133,88 @@ def reversed_range_step_neg(int a, int b):
         result.append(i)
     return result, i
 
-#@cython.test_assert_path_exists('//ForFromStatNode')
+@cython.test_assert_path_exists('//ForFromStatNode')
+def reversed_range_step4():
+    """
+    >>> [ i for i in _reversed(range(-12, -2, 4)) ]
+    [-4, -8, -12]
+    >>> reversed_range_step4()
+    ([-4, -8, -12], -12)
+    """
+    cdef int i = 99
+    result = []
+    for i in reversed(range(1, 1, 4)):
+        result.append(i)
+    assert result == list(reversed(range(1, 1, 4)))
+    assert i == 99
+
+    for i in reversed(range(0, 1, 4)):
+        result.append(i)
+    assert result == list(reversed(range(0, 1, 4)))
+
+    result = []
+    for i in reversed(range(1, 8, 4)):
+        result.append(i)
+    assert result == list(reversed(range(1, 8, 4)))
+
+    result = []
+    for i in reversed(range(1, 9, 4)):
+        result.append(i)
+    assert result == list(reversed(range(1, 9, 4)))
+
+    result = []
+    for i in reversed(range(1, 10, 4)):
+        result.append(i)
+    assert result == list(reversed(range(1, 10, 4)))
+
+    result = []
+    for i in reversed(range(1, 11, 4)):
+        result.append(i)
+    assert result == list(reversed(range(1, 11, 4)))
+
+    result = []
+    for i in reversed(range(1, 12, 4)):
+        result.append(i)
+    assert result == list(reversed(range(1, 12, 4)))
+
+    result = []
+    for i in reversed(range(0, 8, 4)):
+        result.append(i)
+    assert result == list(reversed(range(0, 8, 4)))
+
+    result = []
+    for i in reversed(range(0, 9, 4)):
+        result.append(i)
+    assert result == list(reversed(range(0, 9, 4)))
+
+    result = []
+    for i in reversed(range(0, 10, 4)):
+        result.append(i)
+    assert result == list(reversed(range(0, 10, 4)))
+
+    result = []
+    for i in reversed(range(0, 11, 4)):
+        result.append(i)
+    assert result == list(reversed(range(0, 11, 4)))
+
+    result = []
+    for i in reversed(range(0, 12, 4)):
+        result.append(i)
+    assert result == list(reversed(range(0, 12, 4)))
+
+    result = []
+    for i in reversed(range(-12, -2, 4)):
+        result.append(i)
+    return result, i
+
+#cython.test_assert_path_exists('//ForFromStatNode')
 def reversed_range_step3(int a, int b):
     """
+    >>> [ i for i in _reversed(range(-5, 0, 3)) ]
+    [-2, -5]
+    >>> reversed_range_step3(-5, 0)
+    ([-2, -5], -5)
+
     >>> [ i for i in _reversed(range(0, 5, 3)) ]
     [3, 0]
     >>> reversed_range_step3(0, 5)