Simplify sparse and dense CPU hist kernels (#7029)

* Simplify sparse and dense kernels
* Extract row partitioner.

Co-authored-by: Kirill Shvets <kirill.shvets@intel.com>

src/common/row_set.h

@@ -126,130 +126,6 @@ class RowSetCollection {
   std::vector<Elem> elem_of_each_node_;
 };
 
-
-// The builder is required for samples partition to left and rights children for set of nodes
-// Responsible for:
-// 1) Effective memory allocation for intermediate results for multi-thread work
-// 2) Merging partial results produced by threads into original row set (row_set_collection_)
-// BlockSize is template to enable memory alignment easily with C++11 'alignas()' feature
-template<size_t BlockSize>
-class PartitionBuilder {
- public:
-  template<typename Func>
-  void Init(const size_t n_tasks, size_t n_nodes, Func funcNTaks) {
-    left_right_nodes_sizes_.resize(n_nodes);
-    blocks_offsets_.resize(n_nodes+1);
-
-    blocks_offsets_[0] = 0;
-    for (size_t i = 1; i < n_nodes+1; ++i) {
-      blocks_offsets_[i] = blocks_offsets_[i-1] + funcNTaks(i-1);
-    }
-
-    if (n_tasks > max_n_tasks_) {
-      mem_blocks_.resize(n_tasks);
-      max_n_tasks_ = n_tasks;
-    }
-  }
-
-  // allocate thread local memory, should be called for each specific task
-  void AllocateForTask(size_t id) {
-    if (mem_blocks_[id].get() == nullptr) {
-      BlockInfo* local_block_ptr = new BlockInfo;
-      CHECK_NE(local_block_ptr, (BlockInfo*)nullptr);
-      mem_blocks_[id].reset(local_block_ptr);
-    }
-  }
-
-  common::Span<size_t> GetLeftBuffer(int nid, size_t begin, size_t end) {
-    const size_t task_idx = GetTaskIdx(nid, begin);
-    return { mem_blocks_.at(task_idx)->Left(), end - begin };
-  }
-
-  common::Span<size_t> GetRightBuffer(int nid, size_t begin, size_t end) {
-    const size_t task_idx = GetTaskIdx(nid, begin);
-    return { mem_blocks_.at(task_idx)->Right(), end - begin };
-  }
-
-  void SetNLeftElems(int nid, size_t begin, size_t end, size_t n_left) {
-    size_t task_idx = GetTaskIdx(nid, begin);
-    mem_blocks_.at(task_idx)->n_left = n_left;
-  }
-
-  void SetNRightElems(int nid, size_t begin, size_t end, size_t n_right) {
-    size_t task_idx = GetTaskIdx(nid, begin);
-    mem_blocks_.at(task_idx)->n_right = n_right;
-  }
-
-
-  size_t GetNLeftElems(int nid) const {
-    return left_right_nodes_sizes_[nid].first;
-  }
-
-  size_t GetNRightElems(int nid) const {
-    return left_right_nodes_sizes_[nid].second;
-  }
-
-  // Each thread has partial results for some set of tree-nodes
-  // The function decides order of merging partial results into final row set
-  void CalculateRowOffsets() {
-    for (size_t i = 0; i < blocks_offsets_.size()-1; ++i) {
-      size_t n_left = 0;
-      for (size_t j = blocks_offsets_[i]; j < blocks_offsets_[i+1]; ++j) {
-        mem_blocks_[j]->n_offset_left = n_left;
-        n_left += mem_blocks_[j]->n_left;
-      }
-      size_t n_right = 0;
-      for (size_t j = blocks_offsets_[i]; j < blocks_offsets_[i+1]; ++j) {
-        mem_blocks_[j]->n_offset_right = n_left + n_right;
-        n_right += mem_blocks_[j]->n_right;
-      }
-      left_right_nodes_sizes_[i] = {n_left, n_right};
-    }
-  }
-
-  void MergeToArray(int nid, size_t begin, size_t* rows_indexes) {
-    size_t task_idx = GetTaskIdx(nid, begin);
-
-    size_t* left_result  = rows_indexes + mem_blocks_[task_idx]->n_offset_left;
-    size_t* right_result = rows_indexes + mem_blocks_[task_idx]->n_offset_right;
-
-    const size_t* left = mem_blocks_[task_idx]->Left();
-    const size_t* right = mem_blocks_[task_idx]->Right();
-
-    std::copy_n(left, mem_blocks_[task_idx]->n_left, left_result);
-    std::copy_n(right, mem_blocks_[task_idx]->n_right, right_result);
-  }
-
-  size_t GetTaskIdx(int nid, size_t begin) {
-    return blocks_offsets_[nid] + begin / BlockSize;
-  }
-
- protected:
-  struct BlockInfo{
-    size_t n_left;
-    size_t n_right;
-
-    size_t n_offset_left;
-    size_t n_offset_right;
-
-    size_t* Left() {
-      return &left_data_[0];
-    }
-
-    size_t* Right() {
-      return &right_data_[0];
-    }
-   private:
-    size_t left_data_[BlockSize];
-    size_t right_data_[BlockSize];
-  };
-  std::vector<std::pair<size_t, size_t>> left_right_nodes_sizes_;
-  std::vector<size_t> blocks_offsets_;
-  std::vector<std::shared_ptr<BlockInfo>> mem_blocks_;
-  size_t max_n_tasks_ = 0;
-};
-
-
 }  // namespace common
 }  // namespace xgboost