Index: lld/trunk/ELF/InputFiles.h
===================================================================
--- lld/trunk/ELF/InputFiles.h
+++ lld/trunk/ELF/InputFiles.h
@@ -24,6 +24,7 @@
 #include "llvm/Object/Archive.h"
 #include "llvm/Object/ELF.h"
 #include "llvm/Object/IRObjectFile.h"
+#include "llvm/Support/Threading.h"
 
 #include <map>
 
@@ -212,6 +213,7 @@
   // single object file, so we cache debugging information in order to
   // parse it only once for each object file we link.
   std::unique_ptr<llvm::DWARFDebugLine> DwarfLine;
+  llvm::once_flag InitDwarfLine;
 };
 
 // LazyObjectFile is analogous to ArchiveFile in the sense that
Index: lld/trunk/ELF/InputFiles.cpp
===================================================================
--- lld/trunk/ELF/InputFiles.cpp
+++ lld/trunk/ELF/InputFiles.cpp
@@ -80,8 +80,7 @@
 template <class ELFT>
 Optional<DILineInfo> elf::ObjectFile<ELFT>::getDILineInfo(InputSectionBase *S,
                                                           uint64_t Offset) {
-  if (!DwarfLine)
-    initializeDwarfLine();
+  llvm::call_once(InitDwarfLine, [this]() { initializeDwarfLine(); });
 
   // The offset to CU is 0.
   const DWARFDebugLine::LineTable *Tbl = DwarfLine->getLineTable(0);