* Recoded portions of _check_view_map (is much simpler now)

theano/compile/debugmode.py

@@ -286,7 +286,8 @@ def _check_viewmap(node, storage_map):
     """
 
     for oi, onode in enumerate(node.outputs):
-        input_alias = None
+        
+        good_alias, bad_alias = {}, {}
         outstorage = storage_map[onode][0]
         instorage_id = [id(storage_map[i][0]) for i in node.inputs]
 
@@ -295,32 +296,38 @@ def _check_viewmap(node, storage_map):
         danger_flag = id(outstorage) in instorage_id or\
                       (type(outstorage)==numpy.ndarray and 
                        outstorage.flags['OWNDATA']==False)
+
         if danger_flag:
             # first find out which input it aliases
+            view_map = getattr(node.op, 'view_map', {})
+            destroy_map = getattr(node.op, 'destroy_map', {})
 
             # In theory, theano's view_map only allows for 1 output to alias 1 input
             # Checking for multiple aliases just in case...
-            alias = {}
+
             for ii, inode in enumerate(node.inputs):
+
                 if _may_share_memory(outstorage, storage_map[inode][0]):
-                    alias[ii] = (ii,inode)
 
-            # if its aliased but its declared in the view/destroy map = OK
-            viewmapped = False
-            view_map = getattr(node.op, 'view_map', {})
-            destroy_map = getattr(node.op, 'destroy_map', {})
-            for key,val in view_map.items()+destroy_map.items():
-                val = val[0] # view_map stores a list with single-entries
-                if key==oi and val in alias.keys():
-                    # pfeew, its viewmapped. we're good
-                    input_alias = alias.pop(val)
-
-            # if there's anything left in alias, there's a problem
-            if len(alias):
-                raise BadViewMap(node, oi, outstorage, alias.keys())
-            
-        #need to check output->output aliasing as well
-        if not input_alias and _is_used_in_graph(onode):
+                    nodeid = id(inode)
+                    bad_alias[nodeid] = ii
+
+                    # check that the aliasing was declared in [view|destroy]_map
+                    if ([ii]==view_map.get(oi,None) or\
+                        [ii]==destroy_map.get(oi,None)):
+
+                        good_alias[nodeid] = bad_alias.pop(nodeid)
+      
+            #TODO: make sure this is correct
+            # According to OB, duplicate inputs are rejected on build graph time
+            # if they cause problems. So if they are here it should be ok. 
+            for key,val in good_alias.iteritems():
+                bad_alias.pop(key, None)
+            if bad_alias:
+                raise BadViewMap(node, oi, outstorage, bad_alias.values())
+            
+        #if its not aliased to input, check output->output aliasing
+        if not good_alias and _is_used_in_graph(onode):
             for other_oi, other_onode in enumerate(node.outputs):
                 if other_oi==oi: continue
 

theano/gof/utils.py

@@ -89,6 +89,9 @@ def uniq(seq):
     return [x for i, x in enumerate(seq) if seq.index(x) == i]
 
 def difference(seq1, seq2):
+    """
+    Returns all elements in seq1 which are not in seq2: i.e seq1\seq2 
+    """
     try: 
         # try to use O(const * len(seq1)) algo
         if len(seq2) < 4: # I'm guessing this threshold -JB