Index: ELF/InputSection.h
===================================================================
--- ELF/InputSection.h
+++ ELF/InputSection.h
@@ -254,13 +254,13 @@
   const SectionPiece *getSectionPiece(uint64_t Offset) const;
 
   SyntheticSection *getParent() const;
+  void initOffsetMap();
 
 private:
   void splitStrings(ArrayRef<uint8_t> A, size_t Size);
   void splitNonStrings(ArrayRef<uint8_t> A, size_t Size);
 
-  mutable llvm::DenseMap<uint32_t, uint32_t> OffsetMap;
-  mutable llvm::once_flag InitOffsetMap;
+  llvm::DenseMap<uint32_t, uint32_t> OffsetMap;
 
   llvm::DenseSet<uint64_t> LiveOffsets;
 };
Index: ELF/InputSection.cpp
===================================================================
--- ELF/InputSection.cpp
+++ ELF/InputSection.cpp
@@ -963,13 +963,6 @@
   if (!Live)
     return 0;
 
-  // Initialize OffsetMap lazily.
-  llvm::call_once(InitOffsetMap, [&] {
-    OffsetMap.reserve(Pieces.size());
-    for (size_t I = 0; I < Pieces.size(); ++I)
-      OffsetMap[Pieces[I].InputOff] = I;
-  });
-
   // Find a string starting at a given offset.
   auto It = OffsetMap.find(Offset);
   if (It != OffsetMap.end())
@@ -985,6 +978,12 @@
   return Piece.OutputOff + Addend;
 }
 
+void MergeInputSection::initOffsetMap() {
+  OffsetMap.reserve(Pieces.size());
+  for (size_t I = 0; I < Pieces.size(); ++I)
+    OffsetMap[Pieces[I].InputOff] = I;
+}
+
 template InputSection::InputSection(ObjFile<ELF32LE> &, const ELF32LE::Shdr &,
                                     StringRef);
 template InputSection::InputSection(ObjFile<ELF32BE> &, const ELF32BE::Shdr &,
Index: ELF/SyntheticSections.h
===================================================================
--- ELF/SyntheticSections.h
+++ ELF/SyntheticSections.h
@@ -684,13 +684,12 @@
 class MergeSyntheticSection : public SyntheticSection {
 public:
   void addSection(MergeInputSection *MS);
+  std::vector<MergeInputSection *> Sections;
 
 protected:
   MergeSyntheticSection(StringRef Name, uint32_t Type, uint64_t Flags,
                         uint32_t Alignment)
       : SyntheticSection(Flags, Type, Alignment, Name) {}
-
-  std::vector<MergeInputSection *> Sections;
 };
 
 class MergeTailSection final : public MergeSyntheticSection {
Index: ELF/SyntheticSections.cpp
===================================================================
--- ELF/SyntheticSections.cpp
+++ ELF/SyntheticSections.cpp
@@ -2573,8 +2573,11 @@
     }
     (*I)->addSection(MS);
   }
-  for (auto *MS : MergeSections)
+  for (auto *MS : MergeSections) {
     MS->finalizeContents();
+    parallelForEach(MS->Sections,
+                    [](MergeInputSection *Sec) { Sec->initOffsetMap(); });
+  }
 
   std::vector<InputSectionBase *> &V = InputSections;
   V.erase(std::remove(V.begin(), V.end(), nullptr), V.end());