Small cleanup to hist tree method. (#7735)

* Remove special optimization using number of bins.
* Remove 1-based index for column sampling.
* Remove data layout.
* Unify update prediction cache.

src/common/row_set.h

@@ -55,8 +55,6 @@ class RowSetCollection {
   /*! \brief return corresponding element set given the node_id */
   inline const Elem& operator[](unsigned node_id) const {
     const Elem& e = elem_of_each_node_[node_id];
-    CHECK(e.begin != nullptr)
-        << "access element that is not in the set";
     return e;
   }
 
@@ -75,14 +73,10 @@ class RowSetCollection {
     CHECK_EQ(elem_of_each_node_.size(), 0U);
 
     if (row_indices_.empty()) {  // edge case: empty instance set
-      // assign arbitrary address here, to bypass nullptr check
-      // (nullptr usually indicates a nonexistent rowset, but we want to
-      //  indicate a valid rowset that happens to have zero length and occupies
-      //  the whole instance set)
-      // this is okay, as BuildHist will compute (end-begin) as the set size
-      const size_t* begin = reinterpret_cast<size_t*>(20);
-      const size_t* end = begin;
-      elem_of_each_node_.emplace_back(Elem(begin, end, 0));
+      constexpr size_t* kBegin = nullptr;
+      constexpr size_t* kEnd = nullptr;
+      static_assert(kEnd - kBegin == 0, "");
+      elem_of_each_node_.emplace_back(Elem(kBegin, kEnd, 0));
       return;
     }
 
@@ -93,15 +87,19 @@ class RowSetCollection {
 
   std::vector<size_t>* Data() { return &row_indices_; }
   // split rowset into two
-  inline void AddSplit(unsigned node_id,
-                       unsigned left_node_id,
-                       unsigned right_node_id,
-                       size_t n_left,
-                       size_t n_right) {
+  inline void AddSplit(unsigned node_id, unsigned left_node_id, unsigned right_node_id,
+                       size_t n_left, size_t n_right) {
     const Elem e = elem_of_each_node_[node_id];
-    CHECK(e.begin != nullptr);
-    size_t* all_begin = dmlc::BeginPtr(row_indices_);
-    size_t* begin = all_begin + (e.begin - all_begin);
+
+    size_t* all_begin{nullptr};
+    size_t* begin{nullptr};
+    if (e.begin == nullptr) {
+      CHECK_EQ(n_left, 0);
+      CHECK_EQ(n_right, 0);
+    } else {
+      all_begin = dmlc::BeginPtr(row_indices_);
+      begin = all_begin + (e.begin - all_begin);
+    }
 
     CHECK_EQ(n_left + n_right, e.Size());
     CHECK_LE(begin + n_left, e.end);