From 139ccc9902e7845da4de4dd8931b5e5151de09e5 Mon Sep 17 00:00:00 2001
From: Jiaming Yuan <jm.yuan@outlook.com>
Date: Mon, 30 Dec 2019 02:18:45 +0800
Subject: [PATCH] Fix num_roots to be 1. (#5165)

---
 include/xgboost/tree_model.h | 1 +
 src/gbm/gbtree_model.h       | 1 +
 src/tree/tree_model.cc       | 1 +
 3 files changed, 3 insertions(+)

diff --git a/include/xgboost/tree_model.h b/include/xgboost/tree_model.h
index fa65ebb64..3bcb1cc3b 100644
--- a/include/xgboost/tree_model.h
+++ b/include/xgboost/tree_model.h
@@ -56,6 +56,7 @@ struct TreeParam : public dmlc::Parameter<TreeParam> {
                   "TreeParam: 64 bit align");
     std::memset(this, 0, sizeof(TreeParam));
     num_nodes = 1;
+    deprecated_num_roots = 1;
   }
   // declare the parameters
   DMLC_DECLARE_PARAMETER(TreeParam) {
diff --git a/src/gbm/gbtree_model.h b/src/gbm/gbtree_model.h
index 627f950dd..a03f163af 100644
--- a/src/gbm/gbtree_model.h
+++ b/src/gbm/gbtree_model.h
@@ -47,6 +47,7 @@ struct GBTreeModelParam : public dmlc::Parameter<GBTreeModelParam> {
     std::memset(this, 0, sizeof(GBTreeModelParam));  // FIXME(trivialfis): Why?
     static_assert(sizeof(GBTreeModelParam) == (4 + 2 + 2 + 32) * sizeof(int32_t),
                   "64/32 bit compatibility issue");
+    deprecated_num_roots = 1;
   }
 
   // declare parameters, only declare those that need to be set.
diff --git a/src/tree/tree_model.cc b/src/tree/tree_model.cc
index c3752f337..f8d24a6ce 100644
--- a/src/tree/tree_model.cc
+++ b/src/tree/tree_model.cc
@@ -640,6 +640,7 @@ void RegTree::Save(dmlc::Stream* fo) const {
   CHECK_EQ(param.num_nodes, static_cast<int>(nodes_.size()));
   CHECK_EQ(param.num_nodes, static_cast<int>(stats_.size()));
   fo->Write(&param, sizeof(TreeParam));
+  CHECK_EQ(param.deprecated_num_roots, 1);
   CHECK_NE(param.num_nodes, 0);
   fo->Write(dmlc::BeginPtr(nodes_), sizeof(Node) * nodes_.size());
   fo->Write(dmlc::BeginPtr(stats_), sizeof(RTreeNodeStat) * nodes_.size());