Cleanup to prepare for using mmap pointer in external memory. (#9317)

- Update SparseDMatrix comment.
- Use a pointer in the bitfield. We will replace the `std::vector<bool>` in `ColumnMatrix` with bitfield.
- Clean up the page source. The timer is removed as it's inaccurate once we swap the mmap pointer into the page.

tests/cpp/common/test_bitfield.cc

@@ -1,5 +1,5 @@
-/*!
- * Copyright 2019 XGBoost contributors
+/**
+ * Copyright 2019-2023, XGBoost contributors
  */
 #include <gtest/gtest.h>
 #include "../../../src/common/bitfield.h"
@@ -14,7 +14,7 @@ TEST(BitField, Check) {
                 static_cast<typename common::Span<LBitField64::value_type>::index_type>(
                     storage.size())});
     size_t true_bit = 190;
-    for (size_t i = true_bit + 1; i < bits.Size(); ++i) {
+    for (size_t i = true_bit + 1; i < bits.Capacity(); ++i) {
       ASSERT_FALSE(bits.Check(i));
     }
     ASSERT_TRUE(bits.Check(true_bit));
@@ -34,7 +34,7 @@ TEST(BitField, Check) {
       ASSERT_FALSE(bits.Check(i));
     }
     ASSERT_TRUE(bits.Check(true_bit));
-    for (size_t i = true_bit + 1; i < bits.Size(); ++i) {
+    for (size_t i = true_bit + 1; i < bits.Capacity(); ++i) {
       ASSERT_FALSE(bits.Check(i));
     }
   }

tests/cpp/common/test_bitfield.cu

@@ -1,5 +1,5 @@
-/*!
- * Copyright 2019 XGBoost contributors
+/**
+ * Copyright 2019-2023, XGBoost contributors
  */
 #include <gtest/gtest.h>
 #include <thrust/copy.h>
@@ -12,7 +12,7 @@ namespace xgboost {
 
 __global__ void TestSetKernel(LBitField64 bits) {
   auto tid = threadIdx.x + blockIdx.x * blockDim.x;
-  if (tid < bits.Size()) {
+  if (tid < bits.Capacity()) {
     bits.Set(tid);
   }
 }
@@ -36,20 +36,16 @@ TEST(BitField, GPUSet) {
 
   std::vector<LBitField64::value_type> h_storage(storage.size());
   thrust::copy(storage.begin(), storage.end(), h_storage.begin());
-
-  LBitField64 outputs {
-    common::Span<LBitField64::value_type>{h_storage.data(),
-                                       h_storage.data() + h_storage.size()}};
+  LBitField64 outputs{
+      common::Span<LBitField64::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(LBitField64 lhs, LBitField64 rhs) {
-  lhs |= rhs;
-}
-
-TEST(BitField, GPUAnd) {
+namespace {
+template <bool is_and, typename Op>
+void TestGPULogic(Op op) {
   uint32_t constexpr kBits = 128;
   dh::device_vector<LBitField64::value_type> lhs_storage(kBits);
   dh::device_vector<LBitField64::value_type> rhs_storage(kBits);
@@ -57,13 +53,32 @@ TEST(BitField, GPUAnd) {
   auto rhs = LBitField64(dh::ToSpan(rhs_storage));
   thrust::fill(lhs_storage.begin(), lhs_storage.end(), 0UL);
   thrust::fill(rhs_storage.begin(), rhs_storage.end(), ~static_cast<LBitField64::value_type>(0UL));
-  TestOrKernel<<<1, kBits>>>(lhs, rhs);
+  dh::LaunchN(kBits, [=] __device__(auto) mutable { op(lhs, rhs); });
 
   std::vector<LBitField64::value_type> h_storage(lhs_storage.size());
   thrust::copy(lhs_storage.begin(), lhs_storage.end(), h_storage.begin());
-  LBitField64 outputs {{h_storage.data(), h_storage.data() + h_storage.size()}};
-  for (size_t i = 0; i < kBits; ++i) {
-    ASSERT_TRUE(outputs.Check(i));
+  LBitField64 outputs{{h_storage.data(), h_storage.data() + h_storage.size()}};
+  if (is_and) {
+    for (size_t i = 0; i < kBits; ++i) {
+      ASSERT_FALSE(outputs.Check(i));
+    }
+  } else {
+    for (size_t i = 0; i < kBits; ++i) {
+      ASSERT_TRUE(outputs.Check(i));
+    }
   }
 }
-}  // namespace xgboost
+
+void TestGPUAnd() {
+  TestGPULogic<true>([] XGBOOST_DEVICE(LBitField64 & lhs, LBitField64 const& rhs) { lhs &= rhs; });
+}
+
+void TestGPUOr() {
+  TestGPULogic<false>([] XGBOOST_DEVICE(LBitField64 & lhs, LBitField64 const& rhs) { lhs |= rhs; });
+}
+}  // namespace
+
+TEST(BitField, GPUAnd) { TestGPUAnd(); }
+
+TEST(BitField, GPUOr) { TestGPUOr(); }
+}  // namespace xgboost

tests/cpp/common/test_column_matrix.cc

@@ -83,7 +83,9 @@ template <typename BinIdxType>
 void CheckColumWithMissingValue(const DenseColumnIter<BinIdxType, true>& col,
                                 const GHistIndexMatrix& gmat) {
   for (auto i = 0ull; i < col.Size(); i++) {
-    if (col.IsMissing(i)) continue;
+    if (col.IsMissing(i)) {
+      continue;
+    }
     EXPECT_EQ(gmat.index[gmat.row_ptr[i]], col.GetGlobalBinIdx(i));
   }
 }