commit 3a087ccb99b454dc34484333e608e836e7032213
author Antonio Sanchez <cantonios@google.com> Fri Jun 25 14:22:19 2021 -0700
committer Antonio Sanchez <cantonios@google.com> Tue Jun 29 10:36:20 2021 -0700
tree c14d2a34f8582ead6d7e9b24ceb3ba980bba3d4f
parent 2d132d17365ffc84c0cc7a7da9b8f7090e94b476

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.

unsupported/Eigen/CXX11/src/Tensor/TensorFunctors.h

diff --git a/unsupported/Eigen/CXX11/src/Tensor/TensorFunctors.h b/unsupported/Eigen/CXX11/src/Tensor/TensorFunctors.h
index fd8fa00..3b2100a 100644
--- a/unsupported/Eigen/CXX11/src/Tensor/TensorFunctors.h
+++ b/unsupported/Eigen/CXX11/src/Tensor/TensorFunctors.h
@@ -365,12 +365,16 @@
   };
 };
 
-
-// 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 @@
 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());