Add ShapeFeature.same_shape, that try to compare 2 var shapes.

theano/tensor/opt.py

@@ -1145,6 +1145,40 @@ class ShapeFeature(object):
                     self.set_shape_i(v, ii, new_r)
         self.shape_of_reverse_index[r] = set()
 
+    def same_shape(self, x, y):
+        """Return True if we are able to assert that x and y have the
+        same shape
+        """
+        sx = self.shape_of[x]
+        sy = self.shape_of[y]
+        if sx is None or sy is None:
+            return False
+        assert len(sx) == len(sy)
+
+        for dx, dy in zip(sx, sy):
+            if dx is dy:
+                continue
+            # Need to try to find that they are the same shape. We
+            # need to compare the full graph. It could be slow. So I
+            # just implement for now the case of Shape_i.
+            if not dx.owner or not dy.owner:
+                return False
+            if (not isinstance(dx.owner.op, Shape_i) or
+                not isinstance(dy.owner.op, Shape_i)):
+                return False
+            opx = dx.owner.op
+            opy = dy.owner.op
+            if not (opx.i == opy.i):
+                return False
+            # FB I'm not sure is this handle correctly constants.
+            if dx.owner.inputs[0] == dy.owner.inputs[0]:
+                return True
+            # To be sure to cover all case, call equal_computation.
+            # Can't use theano.gof.graph.is_same_graph(dx, dy)
+            # As it currently expect that dx and dy aren't in a FunctionGraph
+            from theano.scan_module.scan_utils import equal_computations
+            return equal_computations([dx], [dy])
+
 
 class ShapeOptimizer(Optimizer):
     """Optimizer that serves to add ShapeFeature as an fgraph feature.