Serialize expand entry for allgather. (#9702)

src/common/bitfield.h

@@ -9,7 +9,7 @@
 #include <bitset>       // for bitset
 #include <cstdint>      // for uint32_t, uint64_t, uint8_t
 #include <ostream>      // for ostream
-#include <type_traits>  // for conditional_t, is_signed_v
+#include <type_traits>  // for conditional_t, is_signed_v, add_const_t
 
 #if defined(__CUDACC__)
 #include <thrust/copy.h>

src/common/type.h

@@ -0,0 +1,24 @@
+/**
+ * Copyright 2023, XGBoost Contributors
+ */
+#pragma once
+#include <cstdint>      // for int8_t
+#include <type_traits>  // for is_const_v, add_const_t, conditional_t, add_pointer_t
+
+#include "xgboost/span.h"  // for Span
+namespace xgboost::common {
+template <typename T, typename U = std::conditional_t<std::is_const_v<T>,
+                                                      std::add_const_t<std::int8_t>, std::int8_t>>
+common::Span<U> EraseType(common::Span<T> data) {
+  auto n_total_bytes = data.size_bytes();
+  auto erased = common::Span{reinterpret_cast<std::add_pointer_t<U>>(data.data()), n_total_bytes};
+  return erased;
+}
+
+template <typename T, typename U>
+common::Span<T> RestoreType(common::Span<U> data) {
+  auto n_total_bytes = data.size_bytes();
+  auto restored = common::Span{reinterpret_cast<T*>(data.data()), n_total_bytes / sizeof(T)};
+  return restored;
+}
+}  // namespace xgboost::common