Simplify optimisation code for cascaded comparisons and improve its test coverage.

Cython/Compiler/Optimize.py

@@ -4527,22 +4527,20 @@ class ConstantFolding(Visitor.VisitorTransform, SkipDeclarations):
         cascades = [[node.operand1]]
         final_false_result = []
 
-        def split_cascades(cmp_node):
+        cmp_node = node
+        while cmp_node is not None:
             if cmp_node.has_constant_result():
                 if not cmp_node.constant_result:
                     # False => short-circuit
                     final_false_result.append(self._bool_node(cmp_node, False))
-                    return
+                    break
                 else:
                     # True => discard and start new cascade
                     cascades.append([cmp_node.operand2])
             else:
                 # not constant => append to current cascade
                 cascades[-1].append(cmp_node)
-            if cmp_node.cascade:
-                split_cascades(cmp_node.cascade)
-
-        split_cascades(node)
+            cmp_node = cmp_node.cascade
 
         cmp_nodes = []
         for cascade in cascades:

tests/compile/cascmp.pyx

-# mode: compile
-
-cdef void foo():
-    cdef int bool, int1=0, int2=0, int3=0, int4=0
-    cdef object obj1, obj2, obj3, obj4
-    obj1 = 1
-    obj2 = 2
-    obj3 = 3
-    obj4 = 4
-    bool = int1 < int2 < int3
-    bool = obj1 < obj2 < obj3
-    bool = int1 < int2 < obj3
-    bool = obj1 < 2 < 3
-    bool = obj1 < 2 < 3 < 4
-    bool = int1 < (int2 == int3) < int4
-
-foo()

tests/run/cascmp.pyx

+# mode: run
+# tag: cascade, compare
+
+def ints_and_objects():
+    """
+    >>> ints_and_objects()
+    (0, 1, 0, 1, 1, 0)
+    """
+    cdef int int1=0, int2=0, int3=0, int4=0
+    cdef int r1, r2, r3, r4, r5, r6
+    cdef object obj1, obj2, obj3, obj4
+    obj1 = 1
+    obj2 = 2
+    obj3 = 3
+    obj4 = 4
+    r1 = int1 < int2 < int3
+    r2 = obj1 < obj2 < obj3
+    r3 = int1 < int2 < obj3
+    r4 = obj1 < 2 < 3
+    r5 = obj1 < 2 < 3 < 4
+    r6 = int1 < (int2 == int3) < int4
+    return r1, r2, r3, r4, r5, r6
+
+
+def const_cascade(x):
+    """
+    >>> const_cascade(2)
+    (True, False, True, False, False, True, False)
+    """
+    return (
+        0 <= 1,
+        1 <= 0,
+        1 <= 1 <= 2,
+        1 <= 0 < 1,
+        1 <= 1 <= 0,
+        1 <= 1 <= x <= 2 <= 3 > x <= 2 <= 2,
+        1 <= 1 <= x <= 1 <= 1 <= x <= 2,
+    )