Fix for HDRNet producing nondeterministic results.

Eigen reads and writes volatile scalars to allow thread A to communicate
to thread B that data written by thread A is available for thread B to
read.  However, volatile on Android doesn't guarantee ordered accesses,
so it's not certain that after thread B sees the volatile write from
thread A that data written by thread A is actually visible to thread B.

The solution is to replace those volatile scalars with std::atomic<>.
There might be an alternative with better performance (such as imposing
looser ordering constraints than the default std::atomic<> ordering
constraints).

Bug: 109953668
Test: NeuralNetworksTest_static
Test: adb shell am instrument -e class \
      "com.example.android.nn.benchmark.NNTest#testNNAPI11Seconds[hdrnet_float]" \
      -w com.example.android.nn.benchmark/android.support.test.runner.AndroidJUnitRunner
      # test device (including neural networks runtime) is aosp_walleye-userdebug
      # bench is from goog/master
      # also ran with mobilenet_float in place of hdrnet_float
      # could not reproduce problem in this environment; reproduced on
      #   an internal branch on unreleased hardware

Change-Id: Idea6e4b0a3db168dce5915c1beacb643267f3ec6

2 files changed, 4 insertions, 3 deletions

diff --git a/Eigen/src/Core/products/GeneralMatrixMatrix.h b/Eigen/src/Core/products/GeneralMatrixMatrix.h
index 6440e1d09..41cfb0e03 100644
--- a/Eigen/src/Core/products/GeneralMatrixMatrix.h
+++ b/Eigen/src/Core/products/GeneralMatrixMatrix.h
@@ -146,7 +146,6 @@ static void run(Index rows, Index cols, Index depth,
       // Release all the sub blocks A'_i of A' for the current thread,
       // i.e., we simply decrement the number of users by 1
       for(Index i=0; i<threads; ++i)
-        #pragma omp atomic
         info[i].users -= 1;
     }
   }
diff --git a/Eigen/src/Core/products/Parallelizer.h b/Eigen/src/Core/products/Parallelizer.h
index c2f084c82..c0ddc0c06 100644
--- a/Eigen/src/Core/products/Parallelizer.h
+++ b/Eigen/src/Core/products/Parallelizer.h
@@ -10,6 +10,8 @@
 #ifndef EIGEN_PARALLELIZER_H
 #define EIGEN_PARALLELIZER_H
 
+#include <atomic>
+
 namespace Eigen {
 
 namespace internal {
@@ -75,8 +77,8 @@ template<typename Index> struct GemmParallelInfo
 {
   GemmParallelInfo() : sync(-1), users(0), lhs_start(0), lhs_length(0) {}
 
-  Index volatile sync;
-  int volatile users;
+  std::atomic<Index> sync;
+  std::atomic<int> users;
 
   Index lhs_start;
   Index lhs_length;