Index: llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp
===================================================================
--- llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp
+++ llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp
@@ -373,6 +373,11 @@
   DwarfVersion =
       TT.isNVPTX() ? 2 : (DwarfVersion ? DwarfVersion : dwarf::DWARF_VERSION);
 
+  bool Dwarf64 = Asm->TM.Options.MCOptions.Dwarf64 &&
+                 DwarfVersion >= 3 &&   // DWARF64 was introduced in DWARFv3.
+                 TT.isArch64Bit() &&    // DWARF64 requires 64-bit relocations.
+                 TT.isOSBinFormatELF(); // Support only ELF for now.
+
   UseRangesSection = !NoDwarfRangesSection && !TT.isNVPTX();
 
   // Use sections as references. Force for NVPTX.
@@ -414,6 +419,8 @@
       DwarfVersion >= 5 || (UseGNUDebugMacro && !useSplitDwarf());
 
   Asm->OutStreamer->getContext().setDwarfVersion(DwarfVersion);
+  Asm->OutStreamer->getContext().setDwarfFormat(Dwarf64 ? dwarf::DWARF64
+                                                        : dwarf::DWARF32);
 }
 
 // Define out of line so we don't have to include DwarfUnit.h in DwarfDebug.h.
Index: llvm/test/DebugInfo/X86/debug-frame-dwarf64.ll
===================================================================
--- /dev/null
+++ llvm/test/DebugInfo/X86/debug-frame-dwarf64.ll
@@ -0,0 +1,25 @@
+; This checks that .debug_frame can be generated in the DWARF64 format.
+
+; RUN: llc -mtriple=x86_64 -dwarf64 -filetype=obj %s -o %t
+; RUN: llvm-dwarfdump -debug-frame %t | FileCheck %s
+
+; CHECK:      .debug_frame contents:
+; CHECK:      00000000 {{.+}} ffffffffffffffff CIE
+; CHECK-NEXT:   Format:                DWARF64
+; CHECK:      {{.+}} 0000000000000000 FDE cie=00000000 pc=
+; CHECK-NEXT:   Format:       DWARF64
+
+define void @f() nounwind !dbg !4 {
+entry:
+  ret void
+}
+
+!llvm.dbg.cu = !{!2}
+!llvm.module.flags = !{!0}
+
+!0 = !{i32 1, !"Debug Info Version", i32 3}
+!1 = !DIFile(filename: "foo.c", directory: "/tmp")
+!2 = distinct !DICompileUnit(language: DW_LANG_C99, file: !1, producer: "Manual", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug, enums: !3, globals: !3)
+!3 = !{}
+!4 = distinct !DISubprogram(name: "foo", line: 1, isLocal: false, isDefinition: true, virtualIndex: 6, flags: DIFlagPrototyped, isOptimized: false, unit: !2, scopeLine: 1, file: !1, scope: !1, type: !5)
+!5 = !DISubroutineType(types: !3)
Index: llvm/test/DebugInfo/X86/debug-line-dwarf64.ll
===================================================================
--- /dev/null
+++ llvm/test/DebugInfo/X86/debug-line-dwarf64.ll
@@ -0,0 +1,24 @@
+; This checks that .debug_line can be generated in the DWARF64 format.
+
+; RUN: llc -mtriple=x86_64 -dwarf-version=3 -dwarf64 -filetype=obj %s -o %t3
+; RUN: llvm-dwarfdump -debug-line %t3 | FileCheck %s
+
+; CHECK:        .debug_line contents:
+; CHECK-NEXT:   debug_line[0x00000000]
+; CHECK-NEXT:   Line table prologue:
+; CHECK-NEXT:       total_length:
+; CHECK-NEXT:         format: DWARF64
+
+@foo = common dso_local global i32 0, align 4, !dbg !5
+
+!llvm.dbg.cu = !{!2}
+!llvm.module.flags = !{!0}
+
+!0 = !{i32 2, !"Debug Info Version", i32 3}
+!1 = !DIFile(filename: "foo.c", directory: "/tmp")
+!2 = distinct !DICompileUnit(language: DW_LANG_C99, file: !1, producer: "Manual", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug, enums: !3, globals: !4, splitDebugInlining: false, nameTableKind: None)
+!3 = !{}
+!4 = !{!5}
+!5 = !DIGlobalVariableExpression(var: !6, expr: !DIExpression())
+!6 = distinct !DIGlobalVariable(name: "foo", scope: !2, file: !1, line: 1, type: !7, isLocal: false, isDefinition: true)
+!7 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
Index: llvm/test/DebugInfo/X86/dwarf64-support.ll
===================================================================
--- /dev/null
+++ llvm/test/DebugInfo/X86/dwarf64-support.ll
@@ -0,0 +1,48 @@
+; This checks cases when the 64-bit DWARF debug info should not be generated
+; even if '-dwarf64' is specified.
+
+; The 64-bit DWARF format was introduced in DWARFv3, so the '-dwarf64' switch
+; should be ignored for earlier versions.
+; RUN: llc -mtriple=x86_64 -dwarf-version=2 -dwarf64 -filetype=obj %s -o - | \
+; RUN:   llvm-dwarfdump -debug-line - | \
+; RUN:   FileCheck %s --check-prefixes=ELF64,CHECK
+
+; DWARF64 requires 64-bit relocations, so it is not produced for 32-bit targets.
+; RUN: llc -mtriple=i386 -dwarf-version=5 -dwarf64 -filetype=obj %s -o - | \
+; RUN:   llvm-dwarfdump -debug-line - | \
+; RUN:   FileCheck %s --check-prefixes=ELF32,CHECK
+
+; DWARF64 is enabled only for ELF targets. The switch should be ignored for COFF.
+; RUN: llc -mtriple=x86_64-windows-gnu -dwarf-version=5 -dwarf64 -filetype=obj %s -o - | \
+; RUN:   llvm-dwarfdump -debug-line - | \
+; RUN:   FileCheck %s --check-prefixes=COFF,CHECK
+
+; DWARF64 is enabled only for ELF targets. The switch should be ignored for Mach-O.
+; RUN: llc -mtriple=x86_64-apple-darwin -dwarf-version=5 -dwarf64 -filetype=obj %s -o - | \
+; RUN:   llvm-dwarfdump -debug-line - | \
+; RUN:   FileCheck %s --check-prefixes=MACHO,CHECK
+
+; ELF64:    file format elf64-x86-64
+; ELF32:    file format elf32-i386
+; COFF:     file format COFF-x86-64
+; MACHO:    file format Mach-O 64-bit x86-64
+
+; CHECK:      .debug_line contents:
+; CHECK-NEXT: debug_line[0x00000000]
+; CHECK-NEXT: Line table prologue:
+; CHECK-NEXT:     total_length:
+; CHECK-NEXT:         format: DWARF32
+
+@foo = common dso_local global i32 0, align 4, !dbg !5
+
+!llvm.dbg.cu = !{!2}
+!llvm.module.flags = !{!0}
+
+!0 = !{i32 2, !"Debug Info Version", i32 3}
+!1 = !DIFile(filename: "foo.c", directory: "/tmp")
+!2 = distinct !DICompileUnit(language: DW_LANG_C99, file: !1, producer: "Manual", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug, enums: !3, globals: !4, splitDebugInlining: false, nameTableKind: None)
+!3 = !{}
+!4 = !{!5}
+!5 = !DIGlobalVariableExpression(var: !6, expr: !DIExpression())
+!6 = distinct !DIGlobalVariable(name: "foo", scope: !2, file: !1, line: 1, type: !7, isLocal: false, isDefinition: true)
+!7 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)