Index: ELF/InputSection.h
===================================================================
--- ELF/InputSection.h
+++ ELF/InputSection.h
@@ -18,6 +18,7 @@
 #include "llvm/ADT/DenseSet.h"
 #include "llvm/ADT/TinyPtrVector.h"
 #include "llvm/Object/ELF.h"
+#include "llvm/Support/Threading.h"
 #include <mutex>
 
 namespace lld {
@@ -248,7 +249,7 @@
   std::vector<uint32_t> Hashes;
 
   mutable llvm::DenseMap<uint64_t, uint64_t> OffsetMap;
-  mutable std::once_flag InitOffsetMap;
+  mutable llvm::once_flag InitOffsetMap;
 
   llvm::DenseSet<uint64_t> LiveOffsets;
 };
Index: ELF/InputSection.cpp
===================================================================
--- ELF/InputSection.cpp
+++ ELF/InputSection.cpp
@@ -23,6 +23,7 @@
 #include "llvm/Support/Compression.h"
 #include "llvm/Support/Endian.h"
 #include "llvm/Support/Path.h"
+#include "llvm/Support/Threading.h"
 #include <mutex>
 
 using namespace llvm;
@@ -866,7 +867,7 @@
 // it is not just an addition to a base output offset.
 uint64_t MergeInputSection::getOffset(uint64_t Offset) const {
   // Initialize OffsetMap lazily.
-  std::call_once(InitOffsetMap, [&] {
+  llvm::call_once(InitOffsetMap, [&] {
     OffsetMap.reserve(Pieces.size());
     for (const SectionPiece &Piece : Pieces)
       OffsetMap[Piece.InputOff] = Piece.OutputOff;