[sycl] add loss guided hist building (#10251)

Co-authored-by: Dmitry Razdoburdin <>

plugin/sycl/common/hist_util.h

@@ -46,6 +46,93 @@ template<typename GradientSumT>
                               const GHistRow<GradientSumT, MemoryType::on_device>& src2,
                               size_t size, ::sycl::event event_priv);
 
+/*!
+ * \brief Histograms of gradient statistics for multiple nodes
+ */
+template<typename GradientSumT, MemoryType memory_type = MemoryType::shared>
+class HistCollection {
+ public:
+  using GHistRowT = GHistRow<GradientSumT, memory_type>;
+
+  // Access histogram for i-th node
+  GHistRowT& operator[](bst_uint nid) {
+    return *(data_.at(nid));
+  }
+
+  const GHistRowT& operator[](bst_uint nid) const {
+    return *(data_.at(nid));
+  }
+
+  // Initialize histogram collection
+  void Init(::sycl::queue qu, uint32_t nbins) {
+    qu_ = qu;
+    if (nbins_ != nbins) {
+      nbins_ = nbins;
+      data_.clear();
+    }
+  }
+
+  // Create an empty histogram for i-th node
+  ::sycl::event AddHistRow(bst_uint nid) {
+    ::sycl::event event;
+    if (data_.count(nid) == 0) {
+      data_[nid] =
+        std::make_shared<GHistRowT>(&qu_, nbins_,
+                                    xgboost::detail::GradientPairInternal<GradientSumT>(0, 0),
+                                    &event);
+    } else {
+      data_[nid]->Resize(&qu_, nbins_,
+                         xgboost::detail::GradientPairInternal<GradientSumT>(0, 0),
+                         &event);
+    }
+    return event;
+  }
+
+ private:
+  /*! \brief Number of all bins over all features */
+  uint32_t nbins_ = 0;
+
+  std::unordered_map<uint32_t, std::shared_ptr<GHistRowT>> data_;
+
+  ::sycl::queue qu_;
+};
+
+/*!
+ * \brief Stores temporary histograms to compute them in parallel
+ */
+template<typename GradientSumT>
+class ParallelGHistBuilder {
+ public:
+  using GHistRowT = GHistRow<GradientSumT, MemoryType::on_device>;
+
+  void Init(::sycl::queue qu, size_t nbins) {
+    qu_ = qu;
+    if (nbins != nbins_) {
+      hist_buffer_.Init(qu_, nbins);
+      nbins_ = nbins;
+    }
+  }
+
+  void Reset(size_t nblocks) {
+    hist_device_buffer_.Resize(&qu_, nblocks * nbins_ * 2);
+  }
+
+  GHistRowT& GetDeviceBuffer() {
+    return hist_device_buffer_;
+  }
+
+ protected:
+  /*! \brief Number of bins in each histogram */
+  size_t nbins_ = 0;
+  /*! \brief Buffers for histograms for all nodes processed */
+  HistCollection<GradientSumT> hist_buffer_;
+
+  /*! \brief Buffer for additional histograms for Parallel processing  */
+  GHistRowT hist_device_buffer_;
+
+  ::sycl::queue qu_;
+};
+
 /*!
  * \brief Builder for histograms of gradient statistics
  */