speed up subset of 4bits_bin.

src/boosting/gbdt.cpp

@@ -347,7 +347,7 @@ bool GBDT::TrainOneIter(const score_t* gradient, const score_t* hessian, bool is
 #endif
     // if cannot learn a new tree, then stop
     if (new_tree->num_leaves() <= 1) {
-      Log::Info("Stopped training because there are no more leafs that meet the split requirements.");
+      Log::Info("Stopped training because there are no more leaves that meet the split requirements.");
       return true;
     }
 

src/io/dense_nbits_bin.hpp

@@ -206,15 +206,19 @@ public:
   void LoadFromMemory(const void* memory, const std::vector<data_size_t>& local_used_indices) override {
     const uint8_t* mem_data = reinterpret_cast<const uint8_t*>(memory);
     if (!local_used_indices.empty()) {
-      for (int i = 0; i < num_data_; ++i) {
+      for (int i = 0; i < num_data_; i += 2) {
         // get old bins
-        const data_size_t idx = local_used_indices[i];
-        const auto bin = (mem_data[idx >> 1] >> ((idx & 1) << 2)) & 0xf;
+        data_size_t idx = local_used_indices[i];
+        const auto bin1 = static_cast<uint8_t>((mem_data[idx >> 1] >> ((idx & 1) << 2)) & 0xf);
+        idx = local_used_indices[i + 1];
+        const auto bin2 = static_cast<uint8_t>((mem_data[idx >> 1] >> ((idx & 1) << 2)) & 0xf);
         // add
         const int i1 = i >> 1;
-        const int i2 = (i & 1) << 2;
-        const uint8_t val = static_cast<uint8_t>(bin) << i2;
-        data_[i1] |= val;
+        data_[i1] = (bin1 | (bin2 << 4));
+      }
+      if ((num_data_ & 1) == 1) {
+        data_size_t idx = local_used_indices[num_data_ - 1];
+        data_[num_data_ / 2 + 1] = (mem_data[idx >> 1] >> ((idx & 1) << 2)) & 0xf;
       }
     } else {
       for (size_t i = 0; i < data_.size(); ++i) {
@@ -225,13 +229,17 @@ public:
 
   void CopySubset(const Bin* full_bin, const data_size_t* used_indices, data_size_t num_used_indices) override {
     auto other_bin = reinterpret_cast<const Dense4bitsBin*>(full_bin);
-    for (int i = 0; i < num_used_indices; ++i) {
-      const data_size_t idx = used_indices[i];
-      const auto bin = (other_bin->data_[idx >> 1] >> ((idx & 1) << 2)) & 0xf;
+    for (int i = 0; i < num_used_indices; i += 2) {
+      data_size_t idx = used_indices[i];
+      const auto bin1 = static_cast<uint8_t>((other_bin->data_[idx >> 1] >> ((idx & 1) << 2)) & 0xf);
+      idx = used_indices[i + 1];
+      const auto bin2 = static_cast<uint8_t>((other_bin->data_[idx >> 1] >> ((idx & 1) << 2)) & 0xf);
       const int i1 = i >> 1;
-      const int i2 = (i & 1) << 2;
-      const uint8_t val = static_cast<uint8_t>(bin) << i2;
-      data_[i1] |= val;
+      data_[i1] = (bin1 | (bin2 << 4));
+    }
+    if ((num_used_indices & 1) == 1) {
+      data_size_t idx = used_indices[num_used_indices - 1];
+      data_[num_used_indices / 2 + 1] = (other_bin->data_[idx >> 1] >> ((idx & 1) << 2)) & 0xf;
     }
   }