'Merge more subtensor of subtensor(the case var[::-1][int])'

theano/tensor/opt.py

@@ -1113,20 +1113,21 @@ def local_subtensor_lift(node):
 @gof.local_optimizer([])
 def local_subtensor_merge(node):
     """
-    var[int:][-1] -> var[-1]
+    1) var[int:][-1] -> var[-1]
+    2) var[::-1][int] -> var[-int-1]
 
-    The optimization is valid for any int be it constant or not.
     """
     if (isinstance(node.op, T.Subtensor) and
-        len(node.inputs)==1 and
-        len(node.op.idx_list)==1 and
-        node.op.idx_list[0]==-1):
+        len(node.op.idx_list)==1):
 
         u = node.inputs[0]
-        if not u.owner or len(u.clients) > 1:
+        if (not u.owner or len(u.clients) > 1 or
+            not isinstance(u.owner.op, T.Subtensor)):
             return False
 
-        if (isinstance(u.owner.op, T.Subtensor) and
+        # var[int:][-1] -> var[-1]
+        if (len(node.inputs)==1 and
+            node.op.idx_list[0]==-1 and
             len(u.owner.op.idx_list)==1 and
             isinstance(u.owner.op.idx_list[0], slice) and
             u.owner.op.idx_list[0].stop is None and
@@ -1161,6 +1162,25 @@ def local_subtensor_merge(node):
             new_index = T.scalar_from_tensor(new_index)
             return [u.owner.inputs[0][new_index]]
 
+        # var[::-1][int] -> var[-int-1]
+        if (len(node.inputs) in [1,2] and
+            len(u.owner.op.idx_list)==1 and
+            isinstance(u.owner.op.idx_list[0], slice) and
+            u.owner.op.idx_list[0].start is None and
+            u.owner.op.idx_list[0].stop is None and
+            u.owner.op.idx_list[0].step == -1
+            ):
+            idx = node.op.idx_list[0]
+            if len(node.inputs) == 1 and isinstance(idx, int):
+                pass
+            elif (len(node.inputs) == 2 and
+                  isinstance (idx, scalar.basic.Scalar)):
+                idx = T.tensor_from_scalar(node.inputs[1])
+            else:
+                return False
+
+            return [u.owner.inputs[0][-idx-1]]
+
 
 @register_canonicalize
 @gof.local_optimizer([None])

theano/tensor/tests/test_opt.py

@@ -1322,6 +1322,65 @@ class test_local_subtensor_merge(unittest.TestCase):
         assert isinstance(topo[2].op, theano.compile.function_module.DeepCopyOp)
         f([[0,1],[2,3]]) # let debugmode test something
 
+    def test_const2(self):
+        # var[::-1][const] -> var[-1]
+        x = TT.matrix('x')
+        for idx in range(-5,4):
+            f = function([x], x[::-1][idx], mode=mode_opt)
+
+            #theano.printing.debugprint(f, print_type=True)
+            topo=f.maker.env.toposort()
+            #print [t for t in topo if isinstance(t.op, TT.Subtensor)]
+            assert len([t for t in topo if isinstance(t.op, TT.Subtensor)]) == 1
+            #print topo[-1].op
+            assert isinstance(topo[-1].op, theano.compile.function_module.DeepCopyOp)
+
+            x_val = [[0,1],[2,3]]
+            if idx<2 and idx>=-2:
+                # The first subtensor is non-empty, so it makes sense
+                f(x_val) # let debugmode test something
+            else:
+                # A non-empty subtensor of an empty one should be an IndexError
+                self.assertRaises(IndexError, f, x_val)
+                f2 = function([x], x[::-1][idx], mode=mode_opt.excluding('local_subtensor_merge'))
+                self.assertRaises(IndexError, f2, x_val)
+
+    def test_scalar2(self):
+        # var[::-1][const] -> var[-1]
+        x = TT.matrix('x')
+        y = TT.iscalar('y')
+        f = function([x,y], x[::-1][y], mode=mode_opt)
+        #theano.printing.debugprint(f, print_type=True)
+
+        topo=f.maker.env.toposort()
+        #print [t for t in topo if isinstance(t.op, TT.Subtensor)]
+        assert len([t for t in topo if isinstance(t.op, TT.Subtensor)]) == 1
+        #print topo[-1].op
+        assert isinstance(topo[-1].op, theano.compile.function_module.DeepCopyOp)
+
+        x_val = [[0,1],[2,3]]
+        for idx in range(-2,2):
+            f(x_val, idx) # let debugmode test something
+        for idx in range(2,5)+range(-5,-2):
+            self.assertRaises(IndexError, f, x_val, idx)
+            f = function([x,y], x[::-1][y], mode=mode_opt.excluding('local_subtensor_merge'))
+            self.assertRaises(IndexError, f, x_val, idx)
+
+    def test_dont_opt2(self):
+        # Test that we don't optimize some case
+        # var[::-1][const] should be optimized but not
+        # x[::other int][const]
+        x = TT.matrix('x')
+        f = function([x], x[::-2][0], mode=mode_opt)
+        #theano.printing.debugprint(f)
+
+        topo=f.maker.env.toposort()
+        assert len(topo)==3
+        assert isinstance(topo[0].op, TT.Subtensor)
+        assert isinstance(topo[1].op, TT.Subtensor)
+        assert isinstance(topo[2].op, theano.compile.function_module.DeepCopyOp)
+        f([[0,1],[2,3]]) # let debugmode test something
+
 def test_local_fill_useless():
     m = theano.config.mode
     if m == 'FAST_COMPILE':