Unify the cat split storage for CPU. (#7937)

* Unify the cat split storage for CPU.

* Cleanup.

* Workaround.

include/xgboost/tree_model.h

@@ -440,10 +440,10 @@ class RegTree : public Model {
    * \param right_sum         The sum hess of right leaf.
    */
   void ExpandCategorical(bst_node_t nid, unsigned split_index,
-                         common::Span<uint32_t> split_cat, bool default_left,
+                         common::Span<const uint32_t> split_cat, bool default_left,
                          bst_float base_weight, bst_float left_leaf_weight,
-                         bst_float right_leaf_weight, bst_float loss_change,
-                         float sum_hess, float left_sum, float right_sum);
+                         bst_float right_leaf_weight, bst_float loss_change, float sum_hess,
+                         float left_sum, float right_sum);
 
   bool HasCategoricalSplit() const {
     return !split_categories_.empty();

src/tree/hist/evaluate_splits.h

@@ -57,9 +57,9 @@ class HistEvaluator {
   }
 
   /**
-   * \brief Use learned direction with one-hot split. Other implementations (LGB, sklearn)
-   *        create a pseudo-category for missing value but here we just do a complete scan
-   *        to avoid making specialized histogram bin.
+   * \brief Use learned direction with one-hot split. Other implementations (LGB) create a
+   *        pseudo-category for missing value but here we just do a complete scan to avoid
+   *        making specialized histogram bin.
    */
   void EnumerateOneHot(common::HistogramCuts const &cut, const common::GHistRow &hist,
                        bst_feature_t fidx, bst_node_t nidx,
@@ -76,6 +76,7 @@ class HistEvaluator {
     GradStats right_sum;
     // best split so far
     SplitEntry best;
+    best.is_cat = false;  // marker for whether it's updated or not.
 
     auto f_hist = hist.subspan(cut_ptr[fidx], n_bins);
     auto feature_sum = GradStats{
@@ -98,8 +99,8 @@ class HistEvaluator {
       }
 
       // missing on right (treat missing as chosen category)
-      left_sum.SetSubstract(left_sum, missing);
       right_sum.Add(missing);
+      left_sum.SetSubstract(parent.stats, right_sum);
       if (IsValid(left_sum, right_sum)) {
         auto missing_right_chg = static_cast<float>(
             evaluator.CalcSplitGain(param_, nidx, fidx, GradStats{left_sum}, GradStats{right_sum}) -
@@ -108,6 +109,13 @@ class HistEvaluator {
       }
     }
 
+    if (best.is_cat) {
+      auto n = common::CatBitField::ComputeStorageSize(n_bins + 1);
+      best.cat_bits.resize(n, 0);
+      common::CatBitField cat_bits{best.cat_bits};
+      cat_bits.Set(best.split_value);
+    }
+
     p_best->Update(best);
   }
 
@@ -345,25 +353,11 @@ class HistEvaluator {
         evaluator.CalcWeight(candidate.nid, param_, GradStats{candidate.split.right_sum});
 
     if (candidate.split.is_cat) {
-      std::vector<uint32_t> split_cats;
-      if (candidate.split.cat_bits.empty()) {
-        if (common::InvalidCat(candidate.split.split_value)) {
-          common::InvalidCategory();
-        }
-        auto cat = common::AsCat(candidate.split.split_value);
-        split_cats.resize(LBitField32::ComputeStorageSize(std::max(cat + 1, 1)), 0);
-        LBitField32 cat_bits;
-        cat_bits = LBitField32(split_cats);
-        cat_bits.Set(cat);
-      } else {
-        split_cats = candidate.split.cat_bits;
-        common::CatBitField cat_bits{split_cats};
-      }
       tree.ExpandCategorical(
-          candidate.nid, candidate.split.SplitIndex(), split_cats, candidate.split.DefaultLeft(),
-          base_weight, left_weight * param_.learning_rate, right_weight * param_.learning_rate,
-          candidate.split.loss_chg, parent_sum.GetHess(), candidate.split.left_sum.GetHess(),
-          candidate.split.right_sum.GetHess());
+          candidate.nid, candidate.split.SplitIndex(), candidate.split.cat_bits,
+          candidate.split.DefaultLeft(), base_weight, left_weight * param_.learning_rate,
+          right_weight * param_.learning_rate, candidate.split.loss_chg, parent_sum.GetHess(),
+          candidate.split.left_sum.GetHess(), candidate.split.right_sum.GetHess());
     } else {
       tree.ExpandNode(candidate.nid, candidate.split.SplitIndex(), candidate.split.split_value,
                       candidate.split.DefaultLeft(), base_weight,

src/tree/split_evaluator.h

@@ -160,7 +160,7 @@ class TreeEvaluator {
       return;
     }
 
-    auto max_nidx = std::max(leftid, rightid);
+    size_t max_nidx = std::max(leftid, rightid);
     if (lower_bounds_.Size() <= max_nidx) {
       lower_bounds_.Resize(max_nidx * 2 + 1, -std::numeric_limits<float>::max());
     }

src/tree/tree_model.cc

@@ -808,11 +808,9 @@ void RegTree::ExpandNode(bst_node_t nid, unsigned split_index, bst_float split_v
 }
 
 void RegTree::ExpandCategorical(bst_node_t nid, unsigned split_index,
-                                common::Span<uint32_t> split_cat, bool default_left,
-                                bst_float base_weight,
-                                bst_float left_leaf_weight,
-                                bst_float right_leaf_weight,
-                                bst_float loss_change, float sum_hess,
+                                common::Span<const uint32_t> split_cat, bool default_left,
+                                bst_float base_weight, bst_float left_leaf_weight,
+                                bst_float right_leaf_weight, bst_float loss_change, float sum_hess,
                                 float left_sum, float right_sum) {
   this->ExpandNode(nid, split_index, std::numeric_limits<float>::quiet_NaN(),
                    default_left, base_weight,

tests/python/test_updaters.py

@@ -10,10 +10,10 @@
     'nthread': strategies.integers(1, 4),
     'max_depth': strategies.integers(1, 11),
     'min_child_weight': strategies.floats(0.5, 2.0),
-    'alpha': strategies.floats(0.0, 2.0),
+    'alpha': strategies.floats(1e-5, 2.0),
     'lambda': strategies.floats(1e-5, 2.0),
     'eta': strategies.floats(0.01, 0.5),
-    'gamma': strategies.floats(0.0, 2.0),
+    'gamma': strategies.floats(1e-5, 2.0),
     'seed': strategies.integers(0, 10),
     # We cannot enable subsampling as the training loss can increase
     # 'subsample': strategies.floats(0.5, 1.0),