From 54bd64dfd9ab2ab297a23fa3b8247ffca5e14760 Mon Sep 17 00:00:00 2001
From: fis <jm.yuan@outlook.com>
Date: Tue, 31 Mar 2020 21:36:07 +0800
Subject: [PATCH] Implement host span.

---
 include/xgboost/host_device_vector.h        | 3 +++
 tests/cpp/common/test_host_device_vector.cu | 9 +++++++++
 2 files changed, 12 insertions(+)

diff --git a/include/xgboost/host_device_vector.h b/include/xgboost/host_device_vector.h
index b358c78899f9..b3ed36b99366 100644
--- a/include/xgboost/host_device_vector.h
+++ b/include/xgboost/host_device_vector.h
@@ -105,6 +105,9 @@ class HostDeviceVector {
   const T* DevicePointer() const { return ConstDevicePointer(); }
 
   T* HostPointer() { return HostVector().data(); }
+  common::Span<T> HostSpan() { return common::Span<T>{HostVector()}; }
+  common::Span<T const> HostSpan() const { return common::Span<T const>{HostVector()}; }
+  common::Span<T const> ConstHostSpan() const { return HostSpan(); }
   const T* ConstHostPointer() const { return ConstHostVector().data(); }
   const T* HostPointer() const { return ConstHostPointer(); }
 
diff --git a/tests/cpp/common/test_host_device_vector.cu b/tests/cpp/common/test_host_device_vector.cu
index 777f8f7c21de..b483edf3906e 100644
--- a/tests/cpp/common/test_host_device_vector.cu
+++ b/tests/cpp/common/test_host_device_vector.cu
@@ -165,6 +165,15 @@ TEST(HostDeviceVector, Span) {
   auto const_span = vec.ConstDeviceSpan();
   ASSERT_EQ(vec.Size(), const_span.size());
   ASSERT_EQ(vec.ConstDevicePointer(), const_span.data());
+
+  auto h_span = vec.ConstHostSpan();
+  ASSERT_TRUE(vec.HostCanRead());
+  ASSERT_FALSE(vec.HostCanWrite());
+  ASSERT_EQ(h_span.size(), vec.Size());
+  ASSERT_EQ(h_span.data(), vec.ConstHostPointer());
+
+  h_span = vec.HostSpan();
+  ASSERT_TRUE(vec.HostCanWrite());
 }
 
 TEST(HostDeviceVector, MGPU_Basic) {