Use generic dispatching routine for array interface. (#6672)

src/data/array_interface.h

@@ -315,40 +315,50 @@ class ArrayInterface {
     }
   }
 
-  XGBOOST_DEVICE ArrayInterface SliceOffset(size_t offset) const {
-    void* p_values;
+  template <typename Fn>
+  XGBOOST_HOST_DEV_INLINE decltype(auto) DispatchCall(Fn func) const {
     switch (type) {
     case kF4:
-      p_values = reinterpret_cast<float *>(data) + offset;
+      return func(reinterpret_cast<float *>(data));
       break;
     case kF8:
-      p_values = reinterpret_cast<double *>(data) + offset;
+      return func(reinterpret_cast<double *>(data));
       break;
     case kI1:
-      p_values = reinterpret_cast<int8_t *>(data) + offset;
+      return func(reinterpret_cast<int8_t *>(data));
       break;
     case kI2:
-      p_values = reinterpret_cast<int16_t *>(data) + offset;
+      return func(reinterpret_cast<int16_t *>(data));
       break;
     case kI4:
-      p_values = reinterpret_cast<int32_t *>(data) + offset;
+      return func(reinterpret_cast<int32_t *>(data));
       break;
     case kI8:
-      p_values = reinterpret_cast<int64_t *>(data) + offset;
+      return func(reinterpret_cast<int64_t *>(data));
       break;
     case kU1:
-      p_values = reinterpret_cast<uint8_t *>(data) + offset;
+      return func(reinterpret_cast<uint8_t *>(data));
       break;
     case kU2:
-      p_values = reinterpret_cast<uint16_t *>(data) + offset;
+      return func(reinterpret_cast<uint16_t *>(data));
       break;
     case kU4:
-      p_values = reinterpret_cast<uint32_t *>(data) + offset;
+      return func(reinterpret_cast<uint32_t *>(data));
       break;
     case kU8:
-      p_values = reinterpret_cast<uint64_t *>(data) + offset;
+      return func(reinterpret_cast<uint64_t *>(data));
       break;
     }
+    SPAN_CHECK(false);
+    return func(reinterpret_cast<uint64_t *>(data));
+  }
+
+  XGBOOST_DEVICE ArrayInterface SliceOffset(size_t offset) const {
+    void* p_values{nullptr};
+    this->DispatchCall([&p_values, offset](auto *ptr) {
+      p_values = ptr + offset;
+    });
+
     ArrayInterface ret = *this;
     ret.data = p_values;
     return ret;
@@ -390,6 +400,12 @@ class ArrayInterface {
     return reinterpret_cast<float*>(data)[idx];
   }
 
+  XGBOOST_DEVICE size_t ElementSize() {
+    return this->DispatchCall([](auto* p_values) {
+      return sizeof(std::remove_pointer_t<decltype(p_values)>);
+    });
+  }
+
   RBitField8 valid;
   bst_row_t num_rows;
   bst_feature_t num_cols;

tests/cpp/data/test_array_interface.cc

@@ -1,5 +1,5 @@
 /*!
- * Copyright 2020 by XGBoost Contributors
+ * Copyright 2020-2021 by XGBoost Contributors
  */
 #include <gtest/gtest.h>
 #include <xgboost/host_device_vector.h>
@@ -15,6 +15,17 @@ TEST(ArrayInterface, Initialize) {
   ASSERT_EQ(arr_interface.num_rows, kRows);
   ASSERT_EQ(arr_interface.num_cols, kCols);
   ASSERT_EQ(arr_interface.data, storage.ConstHostPointer());
+  ASSERT_EQ(arr_interface.ElementSize(), 4);
+  ASSERT_EQ(arr_interface.type, ArrayInterface::kF4);
+
+  HostDeviceVector<size_t> u64_storage(storage.Size());
+  std::string u64_arr_str;
+  Json::Dump(GetArrayInterface(&u64_storage, kRows, kCols), &u64_arr_str);
+  std::copy(storage.ConstHostVector().cbegin(), storage.ConstHostVector().cend(),
+            u64_storage.HostSpan().begin());
+  auto u64_arr = ArrayInterface{u64_arr_str};
+  ASSERT_EQ(u64_arr.ElementSize(), 8);
+  ASSERT_EQ(u64_arr.type, ArrayInterface::kU8);
 }
 
 TEST(ArrayInterface, Error) {

tests/cpp/helpers.cc

@@ -190,24 +190,7 @@ void RandomDataGenerator::GenerateDense(HostDeviceVector<float> *out) const {
 Json RandomDataGenerator::ArrayInterfaceImpl(HostDeviceVector<float> *storage,
                                              size_t rows, size_t cols) const {
   this->GenerateDense(storage);
-  Json array_interface {Object()};
-  array_interface["data"] = std::vector<Json>(2);
-  if (storage->DeviceCanRead()) {
-    array_interface["data"][0] =
-        Integer(reinterpret_cast<int64_t>(storage->ConstDevicePointer()));
-  } else {
-    array_interface["data"][0] =
-        Integer(reinterpret_cast<int64_t>(storage->ConstHostPointer()));
-  }
-  array_interface["data"][1] = Boolean(false);
-
-  array_interface["shape"] = std::vector<Json>(2);
-  array_interface["shape"][0] = rows;
-  array_interface["shape"][1] = cols;
-
-  array_interface["typestr"] = String("<f4");
-  array_interface["version"] = 1;
-  return array_interface;
+  return GetArrayInterface(storage, rows, cols);
 }
 
 std::string RandomDataGenerator::GenerateArrayInterface(

tests/cpp/helpers.h

@@ -22,6 +22,7 @@
 
 #include "../../src/common/common.h"
 #include "../../src/gbm/gbtree_model.h"
+#include "../../src/data/array_interface.h"
 
 #if defined(__CUDACC__)
 #define DeclareUnifiedTest(name) GPU ## name
@@ -181,6 +182,29 @@ class SimpleRealUniformDistribution {
   }
 };
 
+template <typename T>
+Json GetArrayInterface(HostDeviceVector<T> *storage, size_t rows, size_t cols) {
+  Json array_interface{Object()};
+  array_interface["data"] = std::vector<Json>(2);
+  if (storage->DeviceCanRead()) {
+    array_interface["data"][0] =
+        Integer(reinterpret_cast<int64_t>(storage->ConstDevicePointer()));
+  } else {
+    array_interface["data"][0] =
+        Integer(reinterpret_cast<int64_t>(storage->ConstHostPointer()));
+  }
+  array_interface["data"][1] = Boolean(false);
+
+  array_interface["shape"] = std::vector<Json>(2);
+  array_interface["shape"][0] = rows;
+  array_interface["shape"][1] = cols;
+
+  char t = ArrayInterfaceHandler::TypeChar<T>();
+  array_interface["typestr"] = String(std::string{"<"} + t + std::to_string(sizeof(T)));
+  array_interface["version"] = 1;
+  return array_interface;
+}
+
 // Generate in-memory random data without using DMatrix.
 class RandomDataGenerator {
   bst_row_t rows_;