Index: llvm/lib/MC/XCOFFObjectWriter.cpp
===================================================================
--- llvm/lib/MC/XCOFFObjectWriter.cpp
+++ llvm/lib/MC/XCOFFObjectWriter.cpp
@@ -203,7 +203,7 @@
 
   uint64_t writeObject(MCAssembler &, const MCAsmLayout &) override;
 
-  static bool nameShouldBeInStringTable(const StringRef &);
+  bool nameShouldBeInStringTable(const StringRef &);
   void writeSymbolName(const StringRef &);
   void writeSymbolTableEntryForCsectMemberLabel(const Symbol &,
                                                 const ControlSection &, int16_t,
@@ -344,8 +344,9 @@
 
 void XCOFFObjectWriter::executePostLayoutBinding(MCAssembler &Asm,
                                                  const MCAsmLayout &Layout) {
+  // TODO: Add ".file" in the StringTableBuilder implicitly for XCOFF64.
   if (TargetObjectWriter->is64Bit())
-    report_fatal_error("64-bit XCOFF object files are not supported yet.");
+    Strings.add(".file");
 
   for (const auto &S : Asm) {
     const auto *MCSec = cast<const MCSectionXCOFF>(&S);
@@ -559,9 +560,6 @@
   if (Asm.isIncrementalLinkerCompatible())
     report_fatal_error("Incremental linking not supported for XCOFF.");
 
-  if (TargetObjectWriter->is64Bit())
-    report_fatal_error("64-bit XCOFF object files are not supported yet.");
-
   finalizeSectionInfo();
   uint64_t StartOffset = W.OS.tell();
 
@@ -578,29 +576,37 @@
 }
 
 bool XCOFFObjectWriter::nameShouldBeInStringTable(const StringRef &SymbolName) {
-  return SymbolName.size() > XCOFF::NameSize;
+  return SymbolName.size() > XCOFF::NameSize || TargetObjectWriter->is64Bit();
 }
 
 void XCOFFObjectWriter::writeSymbolName(const StringRef &SymbolName) {
-  if (nameShouldBeInStringTable(SymbolName)) {
-    W.write<int32_t>(0);
-    W.write<uint32_t>(Strings.getOffset(SymbolName));
-  } else {
-    char Name[XCOFF::NameSize+1];
-    std::strncpy(Name, SymbolName.data(), XCOFF::NameSize);
-    ArrayRef<char> NameRef(Name, XCOFF::NameSize);
-    W.write(NameRef);
+  if (TargetObjectWriter->is64Bit())
+    W.write<uint32_t>(Strings.getOffset(SymbolName));
+  else {
+    if (nameShouldBeInStringTable(SymbolName)) {
+      W.write<int32_t>(0);
+      W.write<uint32_t>(Strings.getOffset(SymbolName));
+    } else {
+      char Name[XCOFF::NameSize + 1];
+      std::strncpy(Name, SymbolName.data(), XCOFF::NameSize);
+      ArrayRef<char> NameRef(Name, XCOFF::NameSize);
+      W.write(NameRef);
+    }
   }
 }
 
 void XCOFFObjectWriter::writeSymbolTableEntryForCsectMemberLabel(
     const Symbol &SymbolRef, const ControlSection &CSectionRef,
     int16_t SectionIndex, uint64_t SymbolOffset) {
-  // Name or Zeros and string table offset
+  if (TargetObjectWriter->is64Bit())
+    W.write<uint64_t>(CSectionRef.Address + SymbolOffset);
+  // Name or string table offset.
   writeSymbolName(SymbolRef.getSymbolTableName());
-  assert(SymbolOffset <= UINT32_MAX - CSectionRef.Address &&
-         "Symbol address overflows.");
-  W.write<uint32_t>(CSectionRef.Address + SymbolOffset);
+  if (!TargetObjectWriter->is64Bit()) {
+    assert(SymbolOffset <= UINT32_MAX - CSectionRef.Address &&
+           "Symbol address overflows.");
+    W.write<uint32_t>(CSectionRef.Address + SymbolOffset);
+  }
   W.write<int16_t>(SectionIndex);
   // Basic/Derived type. See the description of the n_type field for symbol
   // table entries for a detailed description. Since we don't yet support
@@ -624,19 +630,36 @@
   W.write<uint8_t>(XCOFF::XTY_LD);
   // Storage mapping class.
   W.write<uint8_t>(CSectionRef.MCCsect->getMappingClass());
-  // Reserved (x_stab).
-  W.write<uint32_t>(0);
-  // Reserved (x_snstab).
-  W.write<uint16_t>(0);
+  if (TargetObjectWriter->is64Bit()) {
+    // High 4 bytes length.
+    W.write<uint32_t>((CSectionRef.SymbolTableIndex & 0xFFFFFFFF00000000) >>
+                      32);
+    // Reserved (pad).
+    W.write<uint8_t>(0);
+    // Auxiliary type entry.
+    W.write<uint8_t>(XCOFF::SymbolAuxType::AUX_CSECT);
+  } else {
+    // Reserved (x_stab).
+    W.write<uint32_t>(0);
+    // Reserved (x_snstab).
+    W.write<uint16_t>(0);
+  }
 }
 
 void XCOFFObjectWriter::writeSymbolTableEntryForControlSection(
     const ControlSection &CSectionRef, int16_t SectionIndex,
     XCOFF::StorageClass StorageClass) {
-  // n_name, n_zeros, n_offset
-  writeSymbolName(CSectionRef.getSymbolTableName());
-  // n_value
-  W.write<uint32_t>(CSectionRef.Address);
+  if (TargetObjectWriter->is64Bit()) {
+    // n_value
+    W.write<uint64_t>(CSectionRef.Address);
+    // n_offset
+    writeSymbolName(CSectionRef.getSymbolTableName());
+  } else {
+    // n_name, n_zeros, n_offset
+    writeSymbolName(CSectionRef.getSymbolTableName());
+    // n_value
+    W.write<uint32_t>(CSectionRef.Address);
+  }
   // n_scnum
   W.write<int16_t>(SectionIndex);
   // Basic/Derived type. See the description of the n_type field for symbol
@@ -662,10 +685,19 @@
   W.write<uint8_t>(getEncodedType(CSectionRef.MCCsect));
   // Storage mapping class.
   W.write<uint8_t>(CSectionRef.MCCsect->getMappingClass());
-  // Reserved (x_stab).
-  W.write<uint32_t>(0);
-  // Reserved (x_snstab).
-  W.write<uint16_t>(0);
+  if (TargetObjectWriter->is64Bit()) {
+    // High 4 bytes length.
+    W.write<uint32_t>((CSectionRef.Size & 0xFFFFFFFF00000000) >> 32);
+    // Reserved (pad).
+    W.write<uint8_t>(0);
+    // Auxiliary type entry.
+    W.write<uint8_t>(XCOFF::SymbolAuxType::AUX_CSECT);
+  } else {
+    // Reserved (x_stab).
+    W.write<uint32_t>(0);
+    // Reserved (x_snstab).
+    W.write<uint16_t>(0);
+  }
 }
 
 void XCOFFObjectWriter::writeFileHeader() {
@@ -751,6 +783,9 @@
 
 void XCOFFObjectWriter::writeRelocation(XCOFFRelocation Reloc,
                                         const ControlSection &CSection) {
+  if (TargetObjectWriter->is64Bit())
+    report_fatal_error("64-bit XCOFF object files are not supported yet.");
+
   W.write<uint32_t>(CSection.Address + Reloc.FixupOffsetInCsect);
   W.write<uint32_t>(Reloc.SymbolTableIndex);
   W.write<uint8_t>(Reloc.SignAndSize);
@@ -784,9 +819,15 @@
   // provided by an auxiliary entry, in which case the n_name of the C_FILE
   // symbol is `.file`.
   // FIXME: add the real source filename.
-  writeSymbolName(".file");
-  // n_value. The n_value of a C_FILE symbol is its symbol table index.
-  W.write<uint32_t>(0);
+  if (TargetObjectWriter->is64Bit()) {
+    // n_value. The n_value of a C_FILE symbol is its symbol table index.
+    W.write<uint64_t>(0);
+    writeSymbolName(".file");
+  } else {
+    writeSymbolName(".file");
+    // n_value.
+    W.write<uint32_t>(0);
+  }
   // n_scnum. N_DEBUG is a reserved section number for indicating a special
   // symbolic debugging symbol.
   W.write<int16_t>(XCOFF::ReservedSectionNum::N_DEBUG);