Add same_as to NodeBase (#550)

* Add same_as to NodeBase

1. Most class inherited from NodeBase(Schedule, Stage, etc) still have
the convenience of using '==' for object identity. And this is the right
behavior for non-Expr classes.
2. subclasses of ExprOp now create EQ expression when '==' is used.

`__nonzero__` and `__bool__` in EQ and NE is a comprise that in some cases
object identity semantics is still useful, like in unit test. For instance:
````
assert a == b
````

"a == b" will create EQ expression, assert then calls `__nonzero__` of the
result expression. `Expr.__nonzero__` throws exception since it prohibits
evaluating IR expression.

More complex case like:
````
assert a in b # b is dict
````

it will call `__eq__` on a and all keys of b, then `__bool__` on the result
expression. This could not easily be done by same_as.

* Retain __hash__ from NodeBase in Python3

python/tvm/_ffi/node.py

@@ -40,9 +40,7 @@ class NodeBase(_NodeBase):
         return _api_internal._raw_ptr(self)
 
     def __eq__(self, other):
-        if not isinstance(other, NodeBase):
-            return False
-        return self.__hash__() == other.__hash__()
+        return self.same_as(other)
 
     def __ne__(self, other):
         return not self.__eq__(other)
@@ -67,6 +65,12 @@ class NodeBase(_NodeBase):
         else:
             self.handle = None
 
+    def same_as(self, other):
+        """check object identity equality"""
+        if not isinstance(other, NodeBase):
+            return False
+        return self.__hash__() == other.__hash__()
+
 
 def register_node(type_key=None):
     """register node type

python/tvm/expr.py

@@ -138,9 +138,11 @@ class ExprOp(object):
         return _make.static_cast(dtype, self)
 
 
-class Expr(NodeBase, ExprOp):
+class Expr(ExprOp, NodeBase):
     """Base class of all tvm Expressions"""
-    pass
+    # In Python3, We have to explicity tell interpreter to retain __hash__ if we overide __eq__
+    # https://docs.python.org/3.1/reference/datamodel.html#object.__hash__
+    __hash__ = NodeBase.__hash__
 
 class ConstExpr(Expr):
     pass
@@ -213,11 +215,19 @@ class Max(BinaryOpExpr):
 
 @register_node
 class EQ(CmpExpr):
-    pass
+    def __nonzero__(self):
+        return self.a.same_as(self.b)
+
+    def __bool__(self):
+        return self.__nonzero__()
 
 @register_node
 class NE(CmpExpr):
-    pass
+    def __nonzero__(self):
+        return not self.a.same_as(self.b)
+
+    def __bool__(self):
+        return self.__nonzero__()
 
 @register_node
 class LT(CmpExpr):

python/tvm/ir_builder.py

@@ -90,7 +90,7 @@ class IRBuilder(object):
         n = tvm.var("n")
         A = ib.allocate("float32", n, name="A")
         with ib.for_range(0, n, name="i") as i:
-            with ib.if_scope((i % 2).equal(0)):
+            with ib.if_scope((i % 2) == 0):
                 A[i] = A[i] + 1
         # The result stmt.
         stmt = ib.get()

tests/python/unittest/test_ir_builder.py

@@ -25,7 +25,7 @@ def test_if():
     n = tvm.var("n")
     A = ib.pointer("float32", name="A")
     with ib.for_range(0, n, name="i") as i:
-        with ib.if_scope((i % 2).equal(0)):
+        with ib.if_scope((i % 2) == 0):
             A[i] = A[i] + 1
         with ib.else_scope():
             A[0] = A[i] + 2