Add Feature (and optimization to add it to a fgraph) to prevent computing…

theano/compile/mode.py

@@ -159,6 +159,16 @@ class AddDestroyHandler(gof.Optimizer):
         fgraph.attach_feature(gof.DestroyHandler())
 
 
+class AddNoOutputFromInplace(gof.Optimizer):
+    """This optimizer adds to the fgraph a feature that will prevent outputs
+    of a fgraph to be created by performing inplace operations on intermediary
+    variables.
+    """
+    def add_requirements(self, fgraph):
+        super(AddNoOutputFromInplace, self).add_requirements(fgraph)
+        fgraph.attach_feature(gof.NoOutputFromInplace())
+
+
 class PrintCurrentFunctionGraph(gof.Optimizer):
     """This optimizer is for debugging.
 
@@ -211,6 +221,9 @@ optdb.register('specialize_device', gof.EquilibriumDB(),
 optdb.register('merge2', gof.MergeOptimizer(),
         49, 'fast_run', 'merge')
 
+optdb.register('add_no_output_from_inplace', AddNoOutputFromInplace(),
+        49.4)
+
 optdb.register('add_destroy_handler', AddDestroyHandler(),
         49.5, 'fast_run', 'inplace')
 

theano/compile/tests/test_mode.py

+import theano
+from theano.compile.mode import Mode
+import theano.tensor as T
+
+def test_no_output_from_implace():
+
+    x = T.matrix()
+    y = T.matrix()
+    a = T.dot(x, y)
+    b = T.tanh(a)
+
+    # Ensure that the elemwise op that produces the output is inplace when
+    # using a mode that does not include the optimization
+    fct_no_opt = theano.function([x,y], b, mode="FAST_RUN")
+    op = fct_no_opt.maker.fgraph.outputs[0].owner.op
+    assert (hasattr(op, 'destroy_map') and 0 in op.destroy_map)
+
+    # Ensure that the elemwise op that produces the output is not inplace when
+    # using a mode that includes the optimization
+    mode_opt = Mode(linker="cvm", optimizer="fast_run")
+    mode_opt = mode_opt.including("add_no_output_from_inplace")
+
+    fct_opt = theano.function([x,y], b, mode=mode_opt)
+    op = fct_opt.maker.fgraph.outputs[0].owner.op
+    assert (not hasattr(op, 'destroy_map') or 0 not in op.destroy_map)

theano/gof/__init__.py

@@ -46,7 +46,7 @@ from theano.gof.fg import \
     CachedConstantError, InconsistencyError, MissingInputError, FunctionGraph
 
 from theano.gof.destroyhandler import \
-    DestroyHandler
+    DestroyHandler, NoOutputFromInplace
 
 from theano.gof.graph import \
     Apply, Variable, Constant, view_roots

theano/gof/destroyhandler.py

@@ -7,6 +7,7 @@ import toolbox
 import graph
 from theano.gof.python25 import deque
 from theano.gof.python25 import OrderedDict
+from theano.gof.toolbox import Feature
 from theano.misc.ordered_set import OrderedSet
 
 from fg import InconsistencyError
@@ -1026,3 +1027,23 @@ class DestroyHandler(toolbox.Bookkeeper):
                         rval[app] = root_clients
 
         return rval
+
+
+class NoOutputFromInplace(Feature):
+
+    def __init__(self):
+        pass
+
+    def validate(self, fgraph):
+        if not hasattr(fgraph, 'destroyers'):
+            return True
+        for out in list(fgraph.outputs):
+            # Validate that the node that produces the output does not produce
+            # it by modifying something else inplace.
+            node = out.owner
+            op = node.op
+            out_idx = node.outputs.index(out)
+            if hasattr(op, 'destroy_map') and out_idx in op.destroy_map.keys():
+                raise InconsistencyError(
+                    "Trying to produce an output ", out,
+                     " by modifying another variable inplace")