From 2451679951cf6befb69204c211e4d84902dd86e4 Mon Sep 17 00:00:00 2001
From: Benoit Steiner <benoit.steiner.goog@gmail.com>
Date: Tue, 19 May 2015 15:19:01 -0700
Subject: [PATCH] Avoid using the cuda memcpy for small tensor slices since the
 memcpy kernel is very expensive to launch

---
 .../Eigen/CXX11/src/Tensor/TensorMorphing.h   | 23 ++++++++++++++++++-
 1 file changed, 22 insertions(+), 1 deletion(-)

diff --git a/unsupported/Eigen/CXX11/src/Tensor/TensorMorphing.h b/unsupported/Eigen/CXX11/src/Tensor/TensorMorphing.h
index d15055727..fa1e6931c 100644
--- a/unsupported/Eigen/CXX11/src/Tensor/TensorMorphing.h
+++ b/unsupported/Eigen/CXX11/src/Tensor/TensorMorphing.h
@@ -283,6 +283,26 @@ class TensorSlicingOp : public TensorBase<TensorSlicingOp<StartIndices, Sizes, X
 };
 
 
+// Fixme: figure out the exact threshold
+namespace {
+template <typename Index, typename Device> struct MemcpyTriggerForSlicing {
+  EIGEN_DEVICE_FUNC MemcpyTriggerForSlicing(const Device& device) : threshold_(2 * device.numThreads()) { }
+  EIGEN_DEVICE_FUNC bool operator ()(Index val) const { return val > threshold_; }
+
+ private:
+  Index threshold_;
+};
+
+// It is very expensive to start the memcpy kernel on GPU: we therefore only
+// use it for large copies.
+#ifdef EIGEN_USE_GPU
+template <typename Index> struct MemcpyTriggerForSlicing<Index, GpuDevice>  {
+  EIGEN_DEVICE_FUNC MemcpyTriggerForSlicing(const GpuDevice&) { }
+  EIGEN_DEVICE_FUNC bool operator ()(Index val) const { return val > 4*1024*1024; }
+};
+#endif
+}
+
 // Eval as rvalue
 template<typename StartIndices, typename Sizes, typename ArgType, typename Device>
 struct TensorEvaluator<const TensorSlicingOp<StartIndices, Sizes, ArgType>, Device>
@@ -364,7 +384,8 @@ struct TensorEvaluator<const TensorSlicingOp<StartIndices, Sizes, ArgType>, Devi
         }
       }
       // Use memcpy if it's going to be faster than using the regular evaluation.
-      if (contiguous_values > static_cast<Index>(2 * m_device.numThreads())) {
+      const MemcpyTriggerForSlicing<Index, Device> trigger(m_device);
+      if (trigger(contiguous_values)) {
         Scalar* src = m_impl.data();
         for (int i = 0; i < internal::array_prod(dimensions()); i += contiguous_values) {
           Index offset = srcCoeff(i);