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/random.h

@@ -156,9 +156,8 @@ class ColumnSampler {
    * \param colsample_bytree
    * \param skip_index_0      (Optional) True to skip index 0.
    */
-  void Init(int64_t num_col, std::vector<float> feature_weights,
-            float colsample_bynode, float colsample_bylevel,
-            float colsample_bytree, bool skip_index_0 = false) {
+  void Init(int64_t num_col, std::vector<float> feature_weights, float colsample_bynode,
+            float colsample_bylevel, float colsample_bytree) {
     feature_weights_ = std::move(feature_weights);
     colsample_bylevel_ = colsample_bylevel;
     colsample_bytree_ = colsample_bytree;
@@ -169,10 +168,8 @@ class ColumnSampler {
     }
     Reset();
 
-    int begin_idx = skip_index_0 ? 1 : 0;
-    feature_set_tree_->Resize(num_col - begin_idx);
-    std::iota(feature_set_tree_->HostVector().begin(),
-              feature_set_tree_->HostVector().end(), begin_idx);
+    feature_set_tree_->Resize(num_col);
+    std::iota(feature_set_tree_->HostVector().begin(), feature_set_tree_->HostVector().end(), 0);
 
     feature_set_tree_ = ColSample(feature_set_tree_, colsample_bytree_);
   }

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);

src/common/threading_utils.h

@@ -266,6 +266,9 @@ class MemStackAllocator {
       throw std::bad_alloc{};
     }
   }
+  MemStackAllocator(size_t required_size, T init) : MemStackAllocator{required_size} {
+    std::fill_n(ptr_, required_size_, init);
+  }
 
   ~MemStackAllocator() {
     if (required_size_ > MaxStackSize) {