Move bitfield into common. (#4737)

* Prepare for columnar format support.

tests/cpp/common/test_bitfield.cu

@@ -0,0 +1,60 @@
+/*!
+ * Copyright 2019 XGBoost contributors
+ */
+#include <gtest/gtest.h>
+#include <thrust/copy.h>
+#include <thrust/device_vector.h>
+#include <vector>
+#include "../../../src/common/bitfield.cuh"
+#include "../../../src/common/device_helpers.cuh"
+
+namespace xgboost {
+
+__global__ void TestSetKernel(BitField bits) {
+  auto tid = threadIdx.x + blockIdx.x * blockDim.x;
+  if (tid < bits.Size()) {
+    bits.Set(tid);
+  }
+}
+
+TEST(BitField, Set) {
+  dh::device_vector<BitField::value_type> storage;
+  uint32_t constexpr kBits = 128;
+  storage.resize(128);
+  auto bits = BitField(dh::ToSpan(storage));
+  TestSetKernel<<<1, kBits>>>(bits);
+
+  std::vector<BitField::value_type> h_storage(storage.size());
+  thrust::copy(storage.begin(), storage.end(), h_storage.begin());
+
+  BitField outputs {
+    common::Span<BitField::value_type>{h_storage.data(),
+                                       h_storage.data() + h_storage.size()}};
+  for (size_t i = 0; i < kBits; ++i) {
+    ASSERT_TRUE(outputs.Check(i));
+  }
+}
+
+__global__ void TestOrKernel(BitField lhs, BitField rhs) {
+  lhs |= rhs;
+}
+
+TEST(BitField, And) {
+  uint32_t constexpr kBits = 128;
+  dh::device_vector<BitField::value_type> lhs_storage(kBits);
+  dh::device_vector<BitField::value_type> rhs_storage(kBits);
+  auto lhs = BitField(dh::ToSpan(lhs_storage));
+  auto rhs = BitField(dh::ToSpan(rhs_storage));
+  thrust::fill(lhs_storage.begin(), lhs_storage.end(), 0UL);
+  thrust::fill(rhs_storage.begin(), rhs_storage.end(), ~static_cast<BitField::value_type>(0UL));
+  TestOrKernel<<<1, kBits>>>(lhs, rhs);
+
+  std::vector<BitField::value_type> h_storage(lhs_storage.size());
+  thrust::copy(lhs_storage.begin(), lhs_storage.end(), h_storage.begin());
+  BitField outputs {{h_storage.data(), h_storage.data() + h_storage.size()}};
+  for (size_t i = 0; i < kBits; ++i) {
+    ASSERT_TRUE(outputs.Check(i));
+  }
+}
+
+}  // namespace xgboost