Index: llvm/include/llvm/MC/MCContext.h
===================================================================
--- llvm/include/llvm/MC/MCContext.h
+++ llvm/include/llvm/MC/MCContext.h
@@ -185,6 +185,9 @@
     /// The maximum version of dwarf that we should emit.
     uint16_t DwarfVersion = 4;
 
+    /// The format of dwarf that we emit.
+    dwarf::DwarfFormat DwarfFormat = dwarf::DWARF32;
+
     /// Honor temporary labels, this is useful for debugging semantic
     /// differences between temporary and non-temporary labels (primarily on
     /// Darwin).
@@ -694,10 +697,8 @@
     void setDwarfDebugProducer(StringRef S) { DwarfDebugProducer = S; }
     StringRef getDwarfDebugProducer() { return DwarfDebugProducer; }
 
-    dwarf::DwarfFormat getDwarfFormat() const {
-      // TODO: Support DWARF64
-      return dwarf::DWARF32;
-    }
+    void setDwarfFormat(dwarf::DwarfFormat f) { DwarfFormat = f; }
+    dwarf::DwarfFormat getDwarfFormat() const { return DwarfFormat; }
 
     void setDwarfVersion(uint16_t v) { DwarfVersion = v; }
     uint16_t getDwarfVersion() const { return DwarfVersion; }
Index: llvm/include/llvm/MC/MCTargetOptions.h
===================================================================
--- llvm/include/llvm/MC/MCTargetOptions.h
+++ llvm/include/llvm/MC/MCTargetOptions.h
@@ -53,6 +53,7 @@
   /// Preserve Comments in Assembly.
   bool PreserveAsmComments : 1;
 
+  bool Dwarf64 : 1;
   int DwarfVersion = 0;
 
   std::string ABIName;
Index: llvm/include/llvm/MC/MCTargetOptionsCommandFlags.h
===================================================================
--- llvm/include/llvm/MC/MCTargetOptionsCommandFlags.h
+++ llvm/include/llvm/MC/MCTargetOptionsCommandFlags.h
@@ -30,6 +30,8 @@
 
 int getDwarfVersion();
 
+bool getDwarf64();
+
 bool getShowMCInst();
 
 bool getFatalWarnings();
Index: llvm/lib/MC/MCTargetOptions.cpp
===================================================================
--- llvm/lib/MC/MCTargetOptions.cpp
+++ llvm/lib/MC/MCTargetOptions.cpp
@@ -16,7 +16,7 @@
       MCNoWarn(false), MCNoDeprecatedWarn(false), MCSaveTempLabels(false),
       MCUseDwarfDirectory(false), MCIncrementalLinkerCompatible(false),
       ShowMCEncoding(false), ShowMCInst(false), AsmVerbose(false),
-      PreserveAsmComments(true) {}
+      PreserveAsmComments(true), Dwarf64(false) {}
 
 StringRef MCTargetOptions::getABIName() const {
   return ABIName;
Index: llvm/lib/MC/MCTargetOptionsCommandFlags.cpp
===================================================================
--- llvm/lib/MC/MCTargetOptionsCommandFlags.cpp
+++ llvm/lib/MC/MCTargetOptionsCommandFlags.cpp
@@ -38,6 +38,7 @@
 MCOPT_EXP(bool, RelaxAll)
 MCOPT(bool, IncrementalLinkerCompatible)
 MCOPT(int, DwarfVersion)
+MCOPT(bool, Dwarf64)
 MCOPT(bool, ShowMCInst)
 MCOPT(bool, FatalWarnings)
 MCOPT(bool, NoWarn)
@@ -66,6 +67,11 @@
                                    cl::init(0));
   MCBINDOPT(DwarfVersion);
 
+  static cl::opt<bool> Dwarf64(
+      "dwarf64",
+      cl::desc("Generate debugging info in the 64-bit DWARF format"));
+  MCBINDOPT(Dwarf64);
+
   static cl::opt<bool> ShowMCInst(
       "asm-show-inst",
       cl::desc("Emit internal instruction representation to assembly file"));
@@ -97,6 +103,7 @@
   MCTargetOptions Options;
   Options.MCRelaxAll = getRelaxAll();
   Options.MCIncrementalLinkerCompatible = getIncrementalLinkerCompatible();
+  Options.Dwarf64 = getDwarf64();
   Options.DwarfVersion = getDwarfVersion();
   Options.ShowMCInst = getShowMCInst();
   Options.ABIName = getABIName();
Index: llvm/test/MC/COFF/dwarf64-err.s
===================================================================
--- /dev/null
+++ llvm/test/MC/COFF/dwarf64-err.s
@@ -0,0 +1,3 @@
+# RUN: not llvm-mc -dwarf64 -triple x86_64-unknown-windows-gnu %s -o - 2>&1 | FileCheck %s
+
+# CHECK: the 64-bit DWARF format is not supported for x86_64-unknown-windows-gnu
Index: llvm/test/MC/ELF/dwarf64-err.s
===================================================================
--- /dev/null
+++ llvm/test/MC/ELF/dwarf64-err.s
@@ -0,0 +1,5 @@
+# RUN: not llvm-mc -g -dwarf-version 2 -dwarf64 -triple x86_64 %s -filetype=asm -o - 2>&1 | FileCheck --check-prefix=DWARF2 %s
+# RUN: not llvm-mc -g -dwarf-version 5 -dwarf64 -triple i686 %s -filetype=asm -o - 2>&1 | FileCheck --check-prefix=I686 %s
+
+# DWARF2: the 64-bit DWARF format is not supported for DWARF versions prior to 3
+# I686: the 64-bit DWARF format is only supported for 64-bit targets
Index: llvm/tools/llvm-mc/llvm-mc.cpp
===================================================================
--- llvm/tools/llvm-mc/llvm-mc.cpp
+++ llvm/tools/llvm-mc/llvm-mc.cpp
@@ -387,6 +387,31 @@
     return 1;
   }
   Ctx.setDwarfVersion(DwarfVersion);
+  if (MCOptions.Dwarf64) {
+    // The 64-bit DWARF format was introduced in DWARFv3.
+    if (DwarfVersion < 3) {
+      errs() << ProgName
+             << ": the 64-bit DWARF format is not supported for DWARF versions "
+                "prior to 3\n";
+      return 1;
+    }
+    // 32-bit targets don't support DWARF64, which requires 64-bit relocations.
+    if (MAI->getCodePointerSize() < 8) {
+      errs() << ProgName
+             << ": the 64-bit DWARF format is only supported for 64-bit "
+                "targets\n";
+      return 1;
+    }
+    // If needsDwarfSectionOffsetDirective is true, we would eventually call
+    // MCStreamer::emitSymbolValue() with IsSectionRelative = true, but that
+    // is supported only for 4-byte long references.
+    if (MAI->needsDwarfSectionOffsetDirective()) {
+      errs() << ProgName << ": the 64-bit DWARF format is not supported for "
+             << TheTriple.normalize() << "\n";
+      return 1;
+    }
+    Ctx.setDwarfFormat(dwarf::DWARF64);
+  }
   if (!DwarfDebugFlags.empty())
     Ctx.setDwarfDebugFlags(StringRef(DwarfDebugFlags));
   if (!DwarfDebugProducer.empty())