Requires setting leaf stat when expanding tree. (#5501)

* Fix GPU Hist feature importance.

src/tree/tree_model.cc

@@ -607,6 +607,8 @@ XGBOOST_REGISTER_TREE_IO(GraphvizGenerator, "dot")
             return new GraphvizGenerator(fmap, attrs, with_stats);
           });
 
+constexpr bst_node_t RegTree::kRoot;
+
 std::string RegTree::DumpModel(const FeatureMap& fmap,
                                bool with_stats,
                                std::string format) const {
@@ -623,26 +625,40 @@ bool RegTree::Equal(const RegTree& b) const {
   if (NumExtraNodes() != b.NumExtraNodes()) {
     return false;
   }
-
-  std::stack<bst_node_t> nodes;
-  nodes.push(0);
-  auto& self = *this;
-  while (!nodes.empty()) {
-    auto nid = nodes.top();
-    nodes.pop();
-    if (!(self.nodes_.at(nid) == b.nodes_.at(nid))) {
+  auto const& self = *this;
+  bool ret { true };
+  this->WalkTree([&self, &b, &ret](bst_node_t nidx) {
+    if (!(self.nodes_.at(nidx) == b.nodes_.at(nidx))) {
+      ret = false;
       return false;
     }
-    auto left = self[nid].LeftChild();
-    auto right = self[nid].RightChild();
-    if (left != RegTree::kInvalidNodeId) {
-      nodes.push(left);
-    }
-    if (right != RegTree::kInvalidNodeId) {
-      nodes.push(right);
-    }
-  }
-  return true;
+    return true;
+  });
+  return ret;
+}
+
+bst_node_t RegTree::GetNumLeaves() const {
+  bst_node_t leaves { 0 };
+  auto const& self = *this;
+  this->WalkTree([&leaves, &self](bst_node_t nidx) {
+                   if (self[nidx].IsLeaf()) {
+                     leaves++;
+                   }
+                   return true;
+                 });
+  return leaves;
+}
+
+bst_node_t RegTree::GetNumSplitNodes() const {
+  bst_node_t splits { 0 };
+  auto const& self = *this;
+  this->WalkTree([&splits, &self](bst_node_t nidx) {
+                   if (!self[nidx].IsLeaf()) {
+                     splits++;
+                   }
+                   return true;
+                 });
+  return splits;
 }
 
 void RegTree::Load(dmlc::Stream* fi) {

src/tree/updater_colmaker.cc

@@ -499,7 +499,9 @@ class ColMaker: public TreeUpdater {
           p_tree->ExpandNode(nid, e.best.SplitIndex(), e.best.split_value,
                              e.best.DefaultLeft(), e.weight, left_leaf_weight,
                              right_leaf_weight, e.best.loss_chg,
-                             e.stats.sum_hess, 0);
+                             e.stats.sum_hess,
+                             e.best.left_sum.GetHess(), e.best.right_sum.GetHess(),
+                             0);
         } else {
           (*p_tree)[nid].SetLeaf(e.weight * param_.learning_rate);
         }

src/tree/updater_gpu_hist.cu

@@ -814,7 +814,8 @@ struct GPUHistMakerDevice {
     tree.ExpandNode(candidate.nid, candidate.split.findex,
                     candidate.split.fvalue, candidate.split.dir == kLeftDir,
                     base_weight, left_weight, right_weight,
-                    candidate.split.loss_chg, parent_sum.sum_hess);
+                    candidate.split.loss_chg, parent_sum.sum_hess,
+                    left_stats.GetHess(), right_stats.GetHess());
     // Set up child constraints
     node_value_constraints.resize(tree.GetNodes().size());
     node_value_constraints[candidate.nid].SetChild(

src/tree/updater_histmaker.cc

@@ -249,7 +249,8 @@ class HistMaker: public BaseMaker {
         p_tree->ExpandNode(nid, best.SplitIndex(), best.split_value,
                            best.DefaultLeft(), base_weight, left_leaf_weight,
                            right_leaf_weight, best.loss_chg,
-                           node_sum.sum_hess);
+                           node_sum.sum_hess,
+                           best.left_sum.GetHess(), best.right_sum.GetHess());
         GradStats right_sum;
         right_sum.SetSubstract(node_sum, left_sum[wid]);
         auto left_child = (*p_tree)[nid].LeftChild();

src/tree/updater_quantile_hist.cc

@@ -263,7 +263,8 @@ void QuantileHistMaker::Builder::AddSplitsToTree(
           spliteval_->ComputeWeight(nid, e.best.right_sum) * param_.learning_rate;
       p_tree->ExpandNode(nid, e.best.SplitIndex(), e.best.split_value,
                          e.best.DefaultLeft(), e.weight, left_leaf_weight,
-                         right_leaf_weight, e.best.loss_chg, e.stats.sum_hess);
+                         right_leaf_weight, e.best.loss_chg, e.stats.sum_hess,
+                         e.best.left_sum.GetHess(), e.best.right_sum.GetHess());
 
       int left_id = (*p_tree)[nid].LeftChild();
       int right_id = (*p_tree)[nid].RightChild();
@@ -410,7 +411,8 @@ void QuantileHistMaker::Builder::ExpandWithLossGuide(
           spliteval_->ComputeWeight(nid, e.best.right_sum) * param_.learning_rate;
       p_tree->ExpandNode(nid, e.best.SplitIndex(), e.best.split_value,
                          e.best.DefaultLeft(), e.weight, left_leaf_weight,
-                         right_leaf_weight, e.best.loss_chg, e.stats.sum_hess);
+                         right_leaf_weight, e.best.loss_chg, e.stats.sum_hess,
+                         e.best.left_sum.GetHess(), e.best.right_sum.GetHess());
 
       this->ApplySplit({candidate}, gmat, column_matrix, hist_, p_tree);
 

src/tree/updater_skmaker.cc

@@ -289,7 +289,8 @@ class SketchMaker: public BaseMaker {
         p_tree->ExpandNode(nid, best.SplitIndex(), best.split_value,
                            best.DefaultLeft(), base_weight, left_leaf_weight,
                            right_leaf_weight, best.loss_chg,
-                           node_stats_[nid].sum_hess);
+                           node_stats_[nid].sum_hess,
+                           best.left_sum.GetHess(), best.right_sum.GetHess());
       } else {
         (*p_tree)[nid].SetLeaf(p_tree->Stat(nid).base_weight * param_.learning_rate);
       }