Clean up training code. (#3825)

* Remove GHistRow, GHistEntry, GHistIndexRow.
* Remove kSimpleStats.
* Remove CheckInfo, SetLeafVec in GradStats and in SKStats.
* Clean up the GradStats.
* Cleanup calcgain.
* Move LossChangeMissing out of common.
* Remove [] operator from GHistIndexBlock.

src/common/hist_util.h

@@ -16,45 +16,8 @@
 #include "../include/rabit/rabit.h"
 
 namespace xgboost {
-
 namespace common {
 
-/*! \brief sums of gradient statistics corresponding to a histogram bin */
-struct GHistEntry {
-  /*! \brief sum of first-order gradient statistics */
-  double sum_grad{0};
-  /*! \brief sum of second-order gradient statistics */
-  double sum_hess{0};
-
-  GHistEntry()  = default;
-
-  inline void Clear() {
-    sum_grad = sum_hess = 0;
-  }
-
-  /*! \brief add a GradientPair to the sum */
-  inline void Add(const GradientPair& e) {
-    sum_grad += e.GetGrad();
-    sum_hess += e.GetHess();
-  }
-
-  /*! \brief add a GHistEntry to the sum */
-  inline void Add(const GHistEntry& e) {
-    sum_grad += e.sum_grad;
-    sum_hess += e.sum_hess;
-  }
-
-  inline static void Reduce(GHistEntry& a, const GHistEntry& b) { // NOLINT(*)
-    a.Add(b);
-  }
-
-  /*! \brief set sum to be difference of two GHistEntry's */
-  inline void SetSubtract(const GHistEntry& a, const GHistEntry& b) {
-    sum_grad = a.sum_grad - b.sum_grad;
-    sum_hess = a.sum_hess - b.sum_hess;
-  }
-};
-
 /*! \brief Cut configuration for all the features. */
 struct HistCutMatrix {
   /*! \brief Unit pointer to rows by element position */
@@ -83,15 +46,7 @@ void DeviceSketch
  * \brief A single row in global histogram index.
  *  Directly represent the global index in the histogram entry.
  */
-struct GHistIndexRow {
-  /*! \brief The index of the histogram */
-  const uint32_t* index;
-  /*! \brief The size of the histogram */
-  size_t size;
-  GHistIndexRow() = default;
-  GHistIndexRow(const uint32_t* index, size_t size)
-      : index(index), size(size) {}
-};
+using GHistIndexRow = Span<uint32_t const>;
 
 /*!
  * \brief preprocessed global index matrix, in CSR format
@@ -111,7 +66,9 @@ struct GHistIndexMatrix {
   void Init(DMatrix* p_fmat, int max_num_bins);
   // get i-th row
   inline GHistIndexRow operator[](size_t i) const {
-    return {&index[0] + row_ptr[i], row_ptr[i + 1] - row_ptr[i]};
+    return {&index[0] + row_ptr[i],
+            static_cast<GHistIndexRow::index_type>(
+                row_ptr[i + 1] - row_ptr[i])};
   }
   inline void GetFeatureCounts(size_t* counts) const {
     auto nfeature = cut.row_ptr.size() - 1;
@@ -134,11 +91,6 @@ struct GHistIndexBlock {
 
   inline GHistIndexBlock(const size_t* row_ptr, const uint32_t* index)
     : row_ptr(row_ptr), index(index) {}
-
-  // get i-th row
-  inline GHistIndexRow operator[](size_t i) const {
-    return {&index[0] + row_ptr[i], row_ptr[i + 1] - row_ptr[i]};
-  }
 };
 
 class ColumnMatrix;
@@ -171,21 +123,12 @@ class GHistIndexBlockMatrix {
 };
 
 /*!
- * \brief histogram of gradient statistics for a single node.
- *  Consists of multiple GHistEntry's, each entry showing total graident statistics
+ * \brief histogram of graident statistics for a single node.
+ *  Consists of multiple GradStats, each entry showing total graident statistics
  *     for that particular bin
  *  Uses global bin id so as to represent all features simultaneously
  */
-struct GHistRow {
-  /*! \brief base pointer to first entry */
-  GHistEntry* begin;
-  /*! \brief number of entries */
-  uint32_t size;
-
-  GHistRow() = default;
-  GHistRow(GHistEntry* begin, uint32_t size)
-    : begin(begin), size(size) {}
-};
+using GHistRow = Span<tree::GradStats>;
 
 /*!
  * \brief histogram of gradient statistics for multiple nodes
@@ -193,27 +136,29 @@ struct GHistRow {
 class HistCollection {
  public:
   // access histogram for i-th node
-  inline GHistRow operator[](bst_uint nid) const {
+  GHistRow operator[](bst_uint nid) const {
     constexpr uint32_t kMax = std::numeric_limits<uint32_t>::max();
     CHECK_NE(row_ptr_[nid], kMax);
-    return {const_cast<GHistEntry*>(dmlc::BeginPtr(data_) + row_ptr_[nid]), nbins_};
+    tree::GradStats* ptr =
+        const_cast<tree::GradStats*>(dmlc::BeginPtr(data_) + row_ptr_[nid]);
+    return {ptr, nbins_};
   }
 
   // have we computed a histogram for i-th node?
-  inline bool RowExists(bst_uint nid) const {
+  bool RowExists(bst_uint nid) const {
     const uint32_t k_max = std::numeric_limits<uint32_t>::max();
     return (nid < row_ptr_.size() && row_ptr_[nid] != k_max);
   }
 
   // initialize histogram collection
-  inline void Init(uint32_t nbins) {
+  void Init(uint32_t nbins) {
     nbins_ = nbins;
     row_ptr_.clear();
     data_.clear();
   }
 
   // create an empty histogram for i-th node
-  inline void AddHistRow(bst_uint nid) {
+  void AddHistRow(bst_uint nid) {
     constexpr uint32_t kMax = std::numeric_limits<uint32_t>::max();
     if (nid >= row_ptr_.size()) {
       row_ptr_.resize(nid + 1, kMax);
@@ -228,7 +173,7 @@ class HistCollection {
   /*! \brief number of all bins over all features */
   uint32_t nbins_;
 
-  std::vector<GHistEntry> data_;
+  std::vector<tree::GradStats> data_;
 
   /*! \brief row_ptr_[nid] locates bin for historgram of node nid */
   std::vector<size_t> row_ptr_;
@@ -268,8 +213,8 @@ class GHistBuilder {
   size_t nthread_;
   /*! \brief number of all bins over all features */
   uint32_t nbins_;
-  std::vector<GHistEntry> data_;
   std::vector<size_t> thread_init_;
+  std::vector<tree::GradStats> data_;
 };