Modify tensor argmin/argmax to always return first occurence.

As written, depending on multithreading/gpu, the returned index from
`argmin`/`argmax` is not currently stable.  Here we modify the functors
to always keep the first occurence (i.e. if the value is equal to the
current min/max, then keep the one with the smallest index).

This is otherwise causing unpredictable results in some TF tests.

1 files changed, 12 insertions, 4 deletions

diff --git a/unsupported/Eigen/CXX11/src/Tensor/TensorFunctors.h b/unsupported/Eigen/CXX11/src/Tensor/TensorFunctors.h
index fd8fa00fa..3b2100ab0 100644
--- a/unsupported/Eigen/CXX11/src/Tensor/TensorFunctors.h
+++ b/unsupported/Eigen/CXX11/src/Tensor/TensorFunctors.h
@@ -365,12 +365,16 @@ struct reducer_traits<OrReducer, Device> {
   };
 };
 
-
-// Argmin/Argmax reducers
+// Argmin/Argmax reducers.  Returns the first occurrence if multiple locations
+// contain the same min/max value.
 template <typename T> struct ArgMaxTupleReducer
 {
   EIGEN_DEVICE_FUNC EIGEN_STRONG_INLINE void reduce(const T t, T* accum) const {
-    if (t.second > accum->second) { *accum = t; }
+    if (t.second < accum->second) {
+      return;
+    } else if (t.second > accum->second || t.first < accum->first) {
+      *accum = t;
+    }
   }
   EIGEN_DEVICE_FUNC EIGEN_STRONG_INLINE T initialize() const {
     return T(0, NumTraits<typename T::second_type>::lowest());
@@ -394,7 +398,11 @@ struct reducer_traits<ArgMaxTupleReducer<T>, Device> {
 template <typename T> struct ArgMinTupleReducer
 {
   EIGEN_DEVICE_FUNC EIGEN_STRONG_INLINE void reduce(const T& t, T* accum) const {
-    if (t.second < accum->second) { *accum = t; }
+    if (t.second > accum->second) {
+      return;
+    } else if (t.second < accum->second || t.first < accum->first) {
+      *accum = t;
+    }
   }
   EIGEN_DEVICE_FUNC EIGEN_STRONG_INLINE T initialize() const {
     return T(0, NumTraits<typename T::second_type>::highest());