Extract CPUExpandEntry and HistParam. (#7321)

* Remove kRootNid.
* Check for empty hessian.

src/tree/hist/expand_entry.h

@@ -0,0 +1,64 @@
+/*!
+ * Copyright 2021 XGBoost contributors
+ */
+#ifndef XGBOOST_TREE_HIST_EXPAND_ENTRY_H_
+#define XGBOOST_TREE_HIST_EXPAND_ENTRY_H_
+
+#include <utility>
+#include "../param.h"
+
+namespace xgboost {
+namespace tree {
+
+struct CPUExpandEntry {
+  int nid;
+  int depth;
+  SplitEntry split;
+  CPUExpandEntry() = default;
+  XGBOOST_DEVICE
+  CPUExpandEntry(int nid, int depth, SplitEntry split)
+      : nid(nid), depth(depth), split(std::move(split)) {}
+  CPUExpandEntry(int nid, int depth, float loss_chg)
+      : nid(nid), depth(depth)  {
+    split.loss_chg = loss_chg;
+  }
+
+  bool IsValid(const TrainParam& param, int num_leaves) const {
+    if (split.loss_chg <= kRtEps) return false;
+    if (split.left_sum.GetHess() == 0 || split.right_sum.GetHess() == 0) {
+      return false;
+    }
+    if (split.loss_chg < param.min_split_loss) {
+      return false;
+    }
+    if (param.max_depth > 0 && depth == param.max_depth) {
+      return false;
+    }
+    if (param.max_leaves > 0 && num_leaves == param.max_leaves) {
+      return false;
+    }
+    return true;
+  }
+
+  float GetLossChange() const { return split.loss_chg; }
+  bst_node_t GetNodeId() const { return nid; }
+
+  static bool ChildIsValid(const TrainParam& param, int depth, int num_leaves) {
+    if (param.max_depth > 0 && depth >= param.max_depth) return false;
+    if (param.max_leaves > 0 && num_leaves >= param.max_leaves) return false;
+    return true;
+  }
+
+  friend std::ostream& operator<<(std::ostream& os, const CPUExpandEntry& e) {
+    os << "ExpandEntry: \n";
+    os << "nidx: " << e.nid << "\n";
+    os << "depth: " << e.depth << "\n";
+    os << "loss: " << e.split.loss_chg << "\n";
+    os << "left_sum: " << e.split.left_sum << "\n";
+    os << "right_sum: " << e.split.right_sum << "\n";
+    return os;
+  }
+};
+}  // namespace tree
+}  // namespace xgboost
+#endif  // XGBOOST_TREE_HIST_EXPAND_ENTRY_H_

src/tree/hist/param.h

@@ -0,0 +1,23 @@
+/*!
+ * Copyright 2021 XGBoost contributors
+ */
+#ifndef XGBOOST_TREE_HIST_PARAM_H_
+#define XGBOOST_TREE_HIST_PARAM_H_
+#include "xgboost/parameter.h"
+
+namespace xgboost {
+namespace tree {
+// training parameters specific to this algorithm
+struct CPUHistMakerTrainParam
+    : public XGBoostParameter<CPUHistMakerTrainParam> {
+  bool single_precision_histogram;
+  // declare parameters
+  DMLC_DECLARE_PARAMETER(CPUHistMakerTrainParam) {
+    DMLC_DECLARE_FIELD(single_precision_histogram).set_default(false).describe(
+        "Use single precision to build histograms.");
+  }
+};
+}  // namespace tree
+}  // namespace xgboost
+
+#endif  // XGBOOST_TREE_HIST_PARAM_H_

src/tree/updater_quantile_hist.cc

@@ -124,7 +124,7 @@ template <bool any_missing>
 void QuantileHistMaker::Builder<GradientSumT>::InitRoot(
     DMatrix *p_fmat, RegTree *p_tree, const std::vector<GradientPair> &gpair_h,
     int *num_leaves, std::vector<CPUExpandEntry> *expand) {
-  CPUExpandEntry node(CPUExpandEntry::kRootNid, p_tree->GetDepth(0), 0.0f);
+  CPUExpandEntry node(RegTree::kRoot, p_tree->GetDepth(0), 0.0f);
 
   nodes_for_explicit_hist_build_.clear();
   nodes_for_subtraction_trick_.clear();
@@ -135,7 +135,7 @@ void QuantileHistMaker::Builder<GradientSumT>::InitRoot(
                                       nodes_for_subtraction_trick_, gpair_h);
 
   {
-    auto nid = CPUExpandEntry::kRootNid;
+    auto nid = RegTree::kRoot;
     GHistRowT hist = this->histogram_builder_->Histogram()[nid];
     GradientPairT grad_stat;
     if (data_layout_ == DataLayout::kDenseDataZeroBased ||

src/tree/updater_quantile_hist.h

@@ -23,6 +23,9 @@
 
 #include "hist/evaluate_splits.h"
 #include "hist/histogram.h"
+#include "hist/expand_entry.h"
+#include "hist/param.h"
+
 #include "constraints.h"
 #include "./param.h"
 #include "./driver.h"
@@ -89,51 +92,6 @@ using xgboost::common::GHistBuilder;
 using xgboost::common::ColumnMatrix;
 using xgboost::common::Column;
 
-// training parameters specific to this algorithm
-struct CPUHistMakerTrainParam
-    : public XGBoostParameter<CPUHistMakerTrainParam> {
-  bool single_precision_histogram = false;
-  // declare parameters
-  DMLC_DECLARE_PARAMETER(CPUHistMakerTrainParam) {
-    DMLC_DECLARE_FIELD(single_precision_histogram).set_default(false).describe(
-        "Use single precision to build histograms.");
-  }
-};
-
-/* tree growing policies */
-struct CPUExpandEntry {
-  static const int kRootNid  = 0;
-  static const int kEmptyNid = -1;
-  int nid;
-  int depth;
-  SplitEntry split;
-
-  CPUExpandEntry() = default;
-  CPUExpandEntry(int nid, int depth, bst_float loss_chg)
-      : nid(nid), depth(depth) {
-    split.loss_chg = loss_chg;
-  }
-
-  bool IsValid(TrainParam const &param, int32_t num_leaves) const {
-    bool invalid = split.loss_chg <= kRtEps ||
-                   (param.max_depth > 0 && this->depth == param.max_depth) ||
-                   (param.max_leaves > 0 && num_leaves == param.max_leaves);
-    return !invalid;
-  }
-
-  bst_float GetLossChange() const {
-    return split.loss_chg;
-  }
-
-  int GetNodeId() const {
-    return nid;
-  }
-
-  int GetDepth() const {
-    return depth;
-  }
-};
-
 /*! \brief construct a tree using quantized feature values */
 class QuantileHistMaker: public TreeUpdater {
  public:

tests/cpp/tree/hist/test_histogram.cc

@@ -258,7 +258,7 @@ void TestBuildHistogram(bool is_distributed) {
   std::iota(row_indices.begin(), row_indices.end(), 0);
   row_set_collection_.Init();
 
-  CPUExpandEntry node(CPUExpandEntry::kRootNid, tree.GetDepth(0), 0.0f);
+  CPUExpandEntry node(RegTree::kRoot, tree.GetDepth(0), 0.0f);
   std::vector<CPUExpandEntry> nodes_for_explicit_hist_build_;
   nodes_for_explicit_hist_build_.push_back(node);
   histogram.BuildHist(p_fmat.get(), &tree, row_set_collection_,