Use ptr from mmap for GHistIndexMatrix and ColumnMatrix. (#9315)

* Use ptr from mmap for `GHistIndexMatrix` and `ColumnMatrix`.

- Define a resource for holding various types of memory pointers.
- Define ref vector for holding resources.
- Swap the underlying resources for GHist and ColumnM.
- Add documentation for current status.
- s390x support is removed. It should work if you can compile XGBoost, all the old workaround code does is to get GCC to compile.

tests/cpp/common/test_ref_resource_view.cc

@@ -0,0 +1,108 @@
+/**
+ * Copyright 2023, XGBoost Contributors
+ */
+#include <gtest/gtest.h>
+
+#include <cstddef>  // for size_t
+#include <memory>   // for make_shared, make_unique
+#include <numeric>  // for iota
+#include <vector>   // for vector
+
+#include "../../../src/common/ref_resource_view.h"
+#include "dmlc/filesystem.h"  // for TemporaryDirectory
+
+namespace xgboost::common {
+TEST(RefResourceView, Basic) {
+  std::size_t n_bytes = 1024;
+  auto mem = std::make_shared<MallocResource>(n_bytes);
+  {
+    RefResourceView view{reinterpret_cast<float*>(mem->Data()), mem->Size() / sizeof(float), mem};
+
+    RefResourceView kview{reinterpret_cast<float const*>(mem->Data()), mem->Size() / sizeof(float),
+                          mem};
+    ASSERT_EQ(mem.use_count(), 3);
+    ASSERT_EQ(view.size(), n_bytes / sizeof(1024));
+    ASSERT_EQ(kview.size(), n_bytes / sizeof(1024));
+  }
+  {
+    RefResourceView view{reinterpret_cast<float*>(mem->Data()), mem->Size() / sizeof(float), mem,
+                         1.5f};
+    for (auto v : view) {
+      ASSERT_EQ(v, 1.5f);
+    }
+    std::iota(view.begin(), view.end(), 0.0f);
+    ASSERT_EQ(view.front(), 0.0f);
+    ASSERT_EQ(view.back(), static_cast<float>(view.size() - 1));
+
+    view.front() = 1.0f;
+    view.back() = 2.0f;
+    ASSERT_EQ(view.front(), 1.0f);
+    ASSERT_EQ(view.back(), 2.0f);
+  }
+  ASSERT_EQ(mem.use_count(), 1);
+}
+
+TEST(RefResourceView, IO) {
+  dmlc::TemporaryDirectory tmpdir;
+  auto path = tmpdir.path + "/testfile";
+  auto data = MakeFixedVecWithMalloc(123, std::size_t{1});
+
+  {
+    auto fo = std::make_unique<AlignedFileWriteStream>(StringView{path}, "wb");
+    ASSERT_EQ(fo->Write(data.data(), data.size_bytes()), data.size_bytes());
+  }
+  {
+    auto fo = std::make_unique<AlignedFileWriteStream>(StringView{path}, "wb");
+    ASSERT_EQ(WriteVec(fo.get(), data),
+              data.size_bytes() + sizeof(RefResourceView<std::size_t>::size_type));
+  }
+  {
+    auto fi = std::make_unique<PrivateMmapConstStream>(
+        path, 0, data.size_bytes() + sizeof(RefResourceView<std::size_t>::size_type));
+    auto read = MakeFixedVecWithMalloc(123, std::size_t{1});
+    ASSERT_TRUE(ReadVec(fi.get(), &read));
+    for (auto v : read) {
+      ASSERT_EQ(v, 1ul);
+    }
+  }
+}
+
+TEST(RefResourceView, IOAligned) {
+  dmlc::TemporaryDirectory tmpdir;
+  auto path = tmpdir.path + "/testfile";
+  auto data = MakeFixedVecWithMalloc(123, 1.0f);
+
+  {
+    auto fo = std::make_unique<AlignedFileWriteStream>(StringView{path}, "wb");
+    // + sizeof(float) for alignment
+    ASSERT_EQ(WriteVec(fo.get(), data),
+              data.size_bytes() + sizeof(RefResourceView<std::size_t>::size_type) + sizeof(float));
+  }
+  {
+    auto fi = std::make_unique<PrivateMmapConstStream>(
+        path, 0, data.size_bytes() + sizeof(RefResourceView<std::size_t>::size_type));
+    // wrong type, float vs. double
+    auto read = MakeFixedVecWithMalloc(123, 2.0);
+    ASSERT_FALSE(ReadVec(fi.get(), &read));
+  }
+  {
+    auto fi = std::make_unique<PrivateMmapConstStream>(
+        path, 0, data.size_bytes() + sizeof(RefResourceView<std::size_t>::size_type));
+    auto read = MakeFixedVecWithMalloc(123, 2.0f);
+    ASSERT_TRUE(ReadVec(fi.get(), &read));
+    for (auto v : read) {
+      ASSERT_EQ(v, 1ul);
+    }
+  }
+  {
+    // Test std::vector
+    std::vector<float> data(123);
+    std::iota(data.begin(), data.end(), 0.0f);
+    auto fo = std::make_unique<AlignedFileWriteStream>(StringView{path}, "wb");
+    // + sizeof(float) for alignment
+    ASSERT_EQ(WriteVec(fo.get(), data), data.size() * sizeof(float) +
+                                            sizeof(RefResourceView<std::size_t>::size_type) +
+                                            sizeof(float));
+  }
+}
+}  // namespace xgboost::common