Prevent CSE of placeholder ops. (#86)

The bug was introduced in #84.

tensorflow-ops/src/TensorFlow/Ops.hs

@@ -203,9 +203,15 @@ matTranspose' params = flip (CoreOps.transpose' params) (vector [1, 0 :: Int32])
 placeholder :: (MonadBuild m, TensorType a) => Shape -> m (Tensor Value a)
 placeholder = placeholder' id
 
-placeholder' :: (MonadBuild m, TensorType a) => OpParams -> Shape -> m (Tensor Value a)
+placeholder' :: forall m a . (MonadBuild m, TensorType a)
+             => OpParams -> Shape -> m (Tensor Value a)
 placeholder' params pShape
-    = render $ CoreOps.placeholder' (params . (opAttr "shape" .~ pShape))
+    -- Note: we don't use CoreOps.placeholder' since that op isn't stateful,
+    -- and thus would be CSE'd.
+    = build $ buildOp $ opDef "Placeholder"
+                & opAttr "dtype" .~ tensorType (undefined :: a)
+                & opAttr "shape" .~ pShape
+                & params
 
 -- | Creates a variable initialized to the given value.
 -- Initialization happens next time session runs.

tensorflow-ops/tests/OpsTest.hs

@@ -12,6 +12,7 @@
 -- See the License for the specific language governing permissions and
 -- limitations under the License.
 
+{-# LANGUAGE OverloadedLists #-}
 {-# LANGUAGE OverloadedStrings #-}
 
 module Main where
@@ -68,9 +69,20 @@ testSaveRestore = testCase "testSaveRestore" $
             TF.run v
         liftIO $ TF.Scalar 134 @=? result
 
+-- | Test that 'placeholder' is not CSE'd.
+testPlaceholderCse :: Test
+testPlaceholderCse = testCase "testPlaceholderCse" $ TF.runSession $ do
+    p1 <- TF.placeholder []
+    p2 <- TF.placeholder []
+    let enc :: Float -> TF.TensorData Float
+        enc n = TF.encodeTensorData [] (V.fromList [n])
+    result <- TF.runWithFeeds [TF.feed p1 (enc 2), TF.feed p2 (enc 3)] $ p1 + p2
+    liftIO $ result @=? TF.Scalar 5
+
 
 main :: IO ()
 main = googleTest [ testSaveRestore
                   , testSize
                   , testReducedShape
+                  , testPlaceholderCse
                   ]