diff --git a/src/common/hist_util.h b/src/common/hist_util.h
index be48bdbbb49a..442bddfcdaab 100644
--- a/src/common/hist_util.h
+++ b/src/common/hist_util.h
@@ -642,39 +642,6 @@ class GHistBuilder {
   /*! \brief number of all bins over all features */
   uint32_t nbins_ { 0 };
 };
-
-/*!
- * \brief A C-style array with in-stack allocation. As long as the array is smaller than
- * MaxStackSize, it will be allocated inside the stack. Otherwise, it will be
- * heap-allocated.
- */
-template <typename T, size_t MaxStackSize>
-class MemStackAllocator {
- public:
-  explicit MemStackAllocator(size_t required_size) : required_size_(required_size) {
-    if (MaxStackSize >= required_size_) {
-      ptr_ = stack_mem_;
-    } else {
-      ptr_ = reinterpret_cast<T*>(malloc(required_size_ * sizeof(T)));
-    }
-    if (!ptr_) {
-      throw std::bad_alloc{};
-    }
-  }
-
-  ~MemStackAllocator() {
-    if (required_size_ > MaxStackSize) {
-      free(ptr_);
-    }
-  }
-  T& operator[](size_t i) { return ptr_[i]; }
-  T const& operator[](size_t i) const { return ptr_[i]; }
-
- private:
-  T* ptr_ = nullptr;
-  size_t required_size_;
-  T stack_mem_[MaxStackSize];
-};
 }  // namespace common
 }  // namespace xgboost
 #endif  // XGBOOST_COMMON_HIST_UTIL_H_
diff --git a/src/common/threading_utils.h b/src/common/threading_utils.h
index 5c8dfc3ff05f..95357ff6316f 100644
--- a/src/common/threading_utils.h
+++ b/src/common/threading_utils.h
@@ -246,6 +246,40 @@ inline int32_t OmpGetNumThreads(int32_t n_threads) {
   n_threads = std::max(n_threads, 1);
   return n_threads;
 }
+
+
+/*!
+ * \brief A C-style array with in-stack allocation. As long as the array is smaller than
+ * MaxStackSize, it will be allocated inside the stack. Otherwise, it will be
+ * heap-allocated.
+ */
+template <typename T, size_t MaxStackSize>
+class MemStackAllocator {
+ public:
+  explicit MemStackAllocator(size_t required_size) : required_size_(required_size) {
+    if (MaxStackSize >= required_size_) {
+      ptr_ = stack_mem_;
+    } else {
+      ptr_ = reinterpret_cast<T*>(malloc(required_size_ * sizeof(T)));
+    }
+    if (!ptr_) {
+      throw std::bad_alloc{};
+    }
+  }
+
+  ~MemStackAllocator() {
+    if (required_size_ > MaxStackSize) {
+      free(ptr_);
+    }
+  }
+  T& operator[](size_t i) { return ptr_[i]; }
+  T const& operator[](size_t i) const { return ptr_[i]; }
+
+ private:
+  T* ptr_ = nullptr;
+  size_t required_size_;
+  T stack_mem_[MaxStackSize];
+};
 }  // namespace common
 }  // namespace xgboost
 
diff --git a/src/data/gradient_index.cc b/src/data/gradient_index.cc
index a56232360791..eef8f9519807 100644
--- a/src/data/gradient_index.cc
+++ b/src/data/gradient_index.cc
@@ -10,6 +10,7 @@
 
 #include "../common/column_matrix.h"
 #include "../common/hist_util.h"
+#include "../common/threading_utils.h"
 
 namespace xgboost {