Index: test/tools/llvm-objcopy/COFF/bigobj.test
===================================================================
--- /dev/null
+++ test/tools/llvm-objcopy/COFF/bigobj.test
@@ -0,0 +1,59 @@
+# RUN: yaml2obj %s > %t.in.o
+#
+# RUN: llvm-objdump -t %t.in.o | FileCheck %s --check-prefixes=SYMBOLS,SYMBOLS-SMALL
+#
+# RUN: llvm-objcopy --force-bigobj %t.in.o %t.big.o
+# RUN: llvm-objdump -t %t.big.o | FileCheck %s --check-prefixes=SYMBOLS,SYMBOLS-BIG
+# RUN: llvm-objcopy %t.big.o %t.small.o
+# RUN: llvm-objdump -t %t.small.o | FileCheck %s --check-prefixes=SYMBOLS,SYMBOLS-SMALL
+
+# SYMBOLS:            SYMBOL TABLE:
+# SYMBOLS-NEXT:       [ 0]{{.*}} (nx 1) {{.*}} .text
+# SYMBOLS-NEXT:       AUX scnlen
+# SYMBOLS-SMALL-NEXT: [ 2]{{.*}} (nx 2) {{.*}} .file
+# SYMBOLS-BIG-NEXT:   [ 2]{{.*}} (nx 1) {{.*}} .file
+# SYMBOLS-NEXT:       AUX abcdefghijklmnopqrs
+# SYMBOLS-SMALL-NEXT: [ 5]{{.*}} (nx 0) {{.*}} foo
+# SYMBOLS-BIG-NEXT:   [ 4]{{.*}} (nx 0) {{.*}} foo
+# SYMBOLS-EMPTY:
+
+--- !COFF
+header:          
+  Machine:         IMAGE_FILE_MACHINE_AMD64
+  Characteristics: [  ]
+sections:        
+  - Name:            .text
+    Characteristics: [  ]
+    Alignment:       4
+    SectionData:     488B0500000000C3
+    Relocations:     
+      - VirtualAddress:  3
+        SymbolName:      foo
+        Type:            IMAGE_REL_AMD64_REL32
+symbols:         
+  - Name:            .text
+    Value:           0
+    SectionNumber:   1
+    SimpleType:      IMAGE_SYM_TYPE_NULL
+    ComplexType:     IMAGE_SYM_DTYPE_NULL
+    StorageClass:    IMAGE_SYM_CLASS_STATIC
+    SectionDefinition: 
+      Length:          1
+      NumberOfRelocations: 1
+      NumberOfLinenumbers: 0
+      CheckSum:        583624169
+      Number:          1
+  - Name:            .file
+    Value:           0
+    SectionNumber:   -2
+    SimpleType:      IMAGE_SYM_TYPE_NULL
+    ComplexType:     IMAGE_SYM_DTYPE_NULL
+    StorageClass:    IMAGE_SYM_CLASS_FILE
+    File:            abcdefghijklmnopqrs
+  - Name:            foo
+    Value:           0
+    SectionNumber:   1
+    SimpleType:      IMAGE_SYM_TYPE_NULL
+    ComplexType:     IMAGE_SYM_DTYPE_NULL
+    StorageClass:    IMAGE_SYM_CLASS_EXTERNAL
+...
Index: tools/llvm-objcopy/COFF/COFFObjcopy.cpp
===================================================================
--- tools/llvm-objcopy/COFF/COFFObjcopy.cpp
+++ tools/llvm-objcopy/COFF/COFFObjcopy.cpp
@@ -183,7 +183,7 @@
   assert(Obj && "Unable to deserialize COFF object");
   if (Error E = handleArgs(Config, *Obj))
     reportError(Config.InputFilename, std::move(E));
-  COFFWriter Writer(*Obj, Out);
+  COFFWriter Writer(*Obj, Out, Config.ForceBigObj);
   if (Error E = Writer.write())
     reportError(Config.OutputFilename, std::move(E));
 }
Index: tools/llvm-objcopy/COFF/Object.h
===================================================================
--- tools/llvm-objcopy/COFF/Object.h
+++ tools/llvm-objcopy/COFF/Object.h
@@ -66,10 +66,24 @@
   std::vector<uint8_t> OwnedContents;
 };
 
+struct AuxSymbol {
+  AuxSymbol(ArrayRef<uint8_t> In) {
+    assert(In.size() == sizeof(Opaque));
+    std::copy(In.begin(), In.end(), Opaque);
+  }
+
+  ArrayRef<uint8_t> getRef() const {
+    return ArrayRef<uint8_t>(Opaque, sizeof(Opaque));
+  }
+
+  uint8_t Opaque[sizeof(object::coff_symbol16)];
+};
+
 struct Symbol {
   object::coff_symbol32 Sym;
   StringRef Name;
-  std::vector<uint8_t> AuxData;
+  std::vector<AuxSymbol> AuxData;
+  StringRef AuxFile;
   ssize_t TargetSectionId;
   ssize_t AssociativeComdatTargetSectionId = 0;
   Optional<size_t> WeakTargetSymbolId;
@@ -132,7 +146,7 @@
 
   ssize_t NextSectionUniqueId = 1; // Allow a UniqueId 0 to mean undefined.
 
-  // Update SymbolMap and RawIndex in each Symbol.
+  // Update SymbolMap.
   void updateSymbols();
 
   // Update SectionMap and Index in each Section.
Index: tools/llvm-objcopy/COFF/Object.cpp
===================================================================
--- tools/llvm-objcopy/COFF/Object.cpp
+++ tools/llvm-objcopy/COFF/Object.cpp
@@ -26,12 +26,8 @@
 
 void Object::updateSymbols() {
   SymbolMap = DenseMap<size_t, Symbol *>(Symbols.size());
-  size_t RawSymIndex = 0;
-  for (Symbol &Sym : Symbols) {
+  for (Symbol &Sym : Symbols)
     SymbolMap[Sym.UniqueId] = &Sym;
-    Sym.RawIndex = RawSymIndex;
-    RawSymIndex += 1 + Sym.Sym.NumberOfAuxSymbols;
-  }
 }
 
 const Symbol *Object::findSymbol(size_t UniqueId) const {
Index: tools/llvm-objcopy/COFF/Reader.cpp
===================================================================
--- tools/llvm-objcopy/COFF/Reader.cpp
+++ tools/llvm-objcopy/COFF/Reader.cpp
@@ -107,9 +107,17 @@
                  *reinterpret_cast<const coff_symbol16 *>(SymRef.getRawPtr()));
     if (auto EC = COFFObj.getSymbolName(SymRef, Sym.Name))
       return errorCodeToError(EC);
-    Sym.AuxData = COFFObj.getSymbolAuxData(SymRef);
-    assert((Sym.AuxData.size() %
-            (IsBigObj ? sizeof(coff_symbol32) : sizeof(coff_symbol16))) == 0);
+    ArrayRef<uint8_t> AuxData = COFFObj.getSymbolAuxData(SymRef);
+    size_t SymSize = IsBigObj ? sizeof(coff_symbol32) : sizeof(coff_symbol16);
+    assert(AuxData.size() == SymSize * SymRef.getNumberOfAuxSymbols());
+    if (SymRef.isFileRecord()) {
+      Sym.AuxFile = StringRef(reinterpret_cast<const char *>(AuxData.data()),
+                              AuxData.size())
+                        .rtrim('\0');
+    } else {
+      for (size_t I = 0; I < SymRef.getNumberOfAuxSymbols(); I++)
+        Sym.AuxData.push_back(AuxData.slice(I * SymSize, sizeof(AuxSymbol)));
+    }
     // Find the unique id of the section
     if (SymRef.getSectionNumber() <=
         0) // Special symbol (undefined/absolute/debug)
Index: tools/llvm-objcopy/COFF/Writer.h
===================================================================
--- tools/llvm-objcopy/COFF/Writer.h
+++ tools/llvm-objcopy/COFF/Writer.h
@@ -24,17 +24,18 @@
 class COFFWriter {
   Object &Obj;
   Buffer &Buf;
+  bool ForceBigObj;
 
   size_t FileSize;
   size_t FileAlignment;
   size_t SizeOfInitializedData;
   StringTableBuilder StrTabBuilder;
 
+  template <class SymbolTy> std::pair<size_t, size_t> finalizeSymbolTable();
   Error finalizeRelocTargets();
   Error finalizeSymbolContents();
   void layoutSections();
   size_t finalizeStringTable();
-  template <class SymbolTy> std::pair<size_t, size_t> finalizeSymbolTable();
 
   Error finalize(bool IsBigObj);
 
@@ -50,8 +51,9 @@
   virtual ~COFFWriter() {}
   Error write();
 
-  COFFWriter(Object &Obj, Buffer &Buf)
-      : Obj(Obj), Buf(Buf), StrTabBuilder(StringTableBuilder::WinCOFF) {}
+  COFFWriter(Object &Obj, Buffer &Buf, bool ForceBigObj = false)
+      : Obj(Obj), Buf(Buf), ForceBigObj(ForceBigObj),
+        StrTabBuilder(StringTableBuilder::WinCOFF) {}
 };
 
 } // end namespace coff
Index: tools/llvm-objcopy/COFF/Writer.cpp
===================================================================
--- tools/llvm-objcopy/COFF/Writer.cpp
+++ tools/llvm-objcopy/COFF/Writer.cpp
@@ -55,7 +55,8 @@
       if (Sym.Sym.NumberOfAuxSymbols == 1 &&
           Sym.Sym.StorageClass == IMAGE_SYM_CLASS_STATIC) {
         coff_aux_section_definition *SD =
-            reinterpret_cast<coff_aux_section_definition *>(Sym.AuxData.data());
+            reinterpret_cast<coff_aux_section_definition *>(
+                Sym.AuxData[0].Opaque);
         uint32_t SDSectionNumber;
         if (Sym.AssociativeComdatTargetSectionId == 0) {
           // Not a comdat associative section; just set the Number field to
@@ -79,7 +80,7 @@
     // we want to set. Only >= 1 would be required, but only == 1 makes sense.
     if (Sym.WeakTargetSymbolId && Sym.Sym.NumberOfAuxSymbols == 1) {
       coff_aux_weak_external *WE =
-          reinterpret_cast<coff_aux_weak_external *>(Sym.AuxData.data());
+          reinterpret_cast<coff_aux_weak_external *>(Sym.AuxData[0].Opaque);
       const Symbol *Target = Obj.findSymbol(*Sym.WeakTargetSymbolId);
       if (Target == nullptr)
         return createStringError(object_error::invalid_symbol_index,
@@ -140,13 +141,23 @@
 
 template <class SymbolTy>
 std::pair<size_t, size_t> COFFWriter::finalizeSymbolTable() {
-  size_t SymTabSize = Obj.getSymbols().size() * sizeof(SymbolTy);
-  for (const auto &S : Obj.getSymbols())
-    SymTabSize += S.AuxData.size();
-  return std::make_pair(SymTabSize, sizeof(SymbolTy));
+  size_t RawSymIndex = 0;
+  for (auto &S : Obj.getMutableSymbols()) {
+    if (!S.AuxFile.empty())
+      S.Sym.NumberOfAuxSymbols =
+          alignTo(S.AuxFile.size(), sizeof(SymbolTy)) / sizeof(SymbolTy);
+    S.RawIndex = RawSymIndex;
+    RawSymIndex += 1 + S.Sym.NumberOfAuxSymbols;
+  }
+  return std::make_pair(RawSymIndex * sizeof(SymbolTy), sizeof(SymbolTy));
 }
 
 Error COFFWriter::finalize(bool IsBigObj) {
+  size_t SymTabSize, SymbolSize;
+  std::tie(SymTabSize, SymbolSize) = IsBigObj
+                                         ? finalizeSymbolTable<coff_symbol32>()
+                                         : finalizeSymbolTable<coff_symbol16>();
+
   if (Error E = finalizeRelocTargets())
     return E;
   if (Error E = finalizeSymbolContents())
@@ -198,10 +209,6 @@
   }
 
   size_t StrTabSize = finalizeStringTable();
-  size_t SymTabSize, SymbolSize;
-  std::tie(SymTabSize, SymbolSize) = IsBigObj
-                                         ? finalizeSymbolTable<coff_symbol32>()
-                                         : finalizeSymbolTable<coff_symbol16>();
 
   size_t PointerToSymbolTable = FileSize;
   // StrTabSize <= 4 is the size of an empty string table, only consisting
@@ -311,8 +318,18 @@
     copySymbol<SymbolTy, coff_symbol32>(*reinterpret_cast<SymbolTy *>(Ptr),
                                         S.Sym);
     Ptr += sizeof(SymbolTy);
-    std::copy(S.AuxData.begin(), S.AuxData.end(), Ptr);
-    Ptr += S.AuxData.size();
+    if (!S.AuxFile.empty()) {
+      std::copy(S.AuxFile.begin(), S.AuxFile.end(), Ptr);
+      // This assumes that unwritten parts of the memory mapped file
+      // are initialized to zero.
+      Ptr += S.Sym.NumberOfAuxSymbols * sizeof(SymbolTy);
+    } else {
+      for (const AuxSymbol &AuxSym : S.AuxData) {
+        ArrayRef<uint8_t> Ref = AuxSym.getRef();
+        std::copy(Ref.begin(), Ref.end(), Ptr);
+        Ptr += sizeof(SymbolTy);
+      }
+    }
   }
   if (StrTabBuilder.getSize() > 4 || !Obj.IsPE) {
     // Always write a string table in object files, even an empty one.
@@ -378,7 +395,8 @@
 }
 
 Error COFFWriter::write() {
-  bool IsBigObj = Obj.getSections().size() > MaxNumberOfSections16;
+  bool IsBigObj =
+      Obj.getSections().size() > MaxNumberOfSections16 || ForceBigObj;
   if (IsBigObj && Obj.IsPE)
     return createStringError(object_error::parse_failed,
                              "Too many sections for executable");
Index: tools/llvm-objcopy/CopyConfig.h
===================================================================
--- tools/llvm-objcopy/CopyConfig.h
+++ tools/llvm-objcopy/CopyConfig.h
@@ -93,6 +93,9 @@
   bool Weaken = false;
   bool DecompressDebugSections = false;
   DebugCompressionType CompressionType = DebugCompressionType::None;
+
+  // Options for testing only
+  bool ForceBigObj = false;
 };
 
 // Configuration for the overall invocation of this tool. When invoked as
Index: tools/llvm-objcopy/CopyConfig.cpp
===================================================================
--- tools/llvm-objcopy/CopyConfig.cpp
+++ tools/llvm-objcopy/CopyConfig.cpp
@@ -374,6 +374,8 @@
 
   Config.PreserveDates = InputArgs.hasArg(OBJCOPY_preserve_dates);
 
+  Config.ForceBigObj = InputArgs.hasArg(OBJCOPY_force_bigobj);
+
   if (Config.DecompressDebugSections &&
       Config.CompressionType != DebugCompressionType::None) {
     error("Cannot specify --compress-debug-sections at the same time as "
Index: tools/llvm-objcopy/ObjcopyOpts.td
===================================================================
--- tools/llvm-objcopy/ObjcopyOpts.td
+++ tools/llvm-objcopy/ObjcopyOpts.td
@@ -178,3 +178,6 @@
     : Eq<"build-id-link-output", "Hard-link the output to <dir>/xx/xxx<suffix> "
                                  "name derived from hex build ID">,
       MetaVarName<"suffix">;
+
+// Undocumented options only used for testing.
+def force_bigobj : Flag<["--"], "force-bigobj">;