Index: llvm/lib/CodeGen/AsmPrinter/DwarfStringPool.cpp
===================================================================
--- llvm/lib/CodeGen/AsmPrinter/DwarfStringPool.cpp
+++ llvm/lib/CodeGen/AsmPrinter/DwarfStringPool.cpp
@@ -120,7 +120,7 @@
     }
 
     Asm.OutStreamer->SwitchSection(OffsetSection);
-    unsigned size = 4; // FIXME: DWARF64 is 8.
+    unsigned size = Asm.getDwarfOffsetByteSize();
     for (const auto &Entry : Entries)
       if (UseRelativeOffsets)
         Asm.emitDwarfStringOffset(Entry->getValue());
Index: llvm/lib/CodeGen/AsmPrinter/DwarfUnit.cpp
===================================================================
--- llvm/lib/CodeGen/AsmPrinter/DwarfUnit.cpp
+++ llvm/lib/CodeGen/AsmPrinter/DwarfUnit.cpp
@@ -1718,7 +1718,7 @@
   Asm->OutStreamer->AddComment("Offset Into Abbrev. Section");
   const TargetLoweringObjectFile &TLOF = Asm->getObjFileLowering();
   if (UseOffsets)
-    Asm->emitInt32(0);
+    Asm->emitDwarfLengthOrOffset(0);
   else
     Asm->emitDwarfSymbolReference(
         TLOF.getDwarfAbbrevSection()->getBeginSymbol(), false);
Index: llvm/test/DebugInfo/X86/debug-info-dwo-dwarf64.ll
===================================================================
--- /dev/null
+++ llvm/test/DebugInfo/X86/debug-info-dwo-dwarf64.ll
@@ -0,0 +1,21 @@
+; This checks that .debug_info.dwo can be generated in the DWARF64 format.
+
+; RUN: llc -mtriple=x86_64 -dwarf-version=5 -dwarf64 -split-dwarf-file=foo.dwo -filetype=obj %s -o %t
+; RUN: llvm-dwarfdump -debug-info %t | FileCheck %s
+
+; CHECK:      .debug_info.dwo contents:
+; CHECK-NEXT: Compile Unit: length = 0x{{([[:xdigit:]]{16})}}, format = DWARF64, version = 0x0005, unit_type = DW_UT_split_compile, abbr_offset = 0x0000, addr_size = 0x08
+
+@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/debug-str-offsets-dwo-dwarf64.ll
===================================================================
--- /dev/null
+++ llvm/test/DebugInfo/X86/debug-str-offsets-dwo-dwarf64.ll
@@ -0,0 +1,45 @@
+; This checks that .debug_str_offsets.dwo can be generated in the DWARF64 format.
+
+; RUN: llc -mtriple=x86_64 -dwarf-version=5 -dwarf64 -split-dwarf-file=foo.dwo -filetype=obj %s -o %t
+; RUN: llvm-dwarfdump -debug-info -debug-str -debug-str-offsets -v %t | \
+; RUN:   FileCheck %s
+
+; CHECK:      .debug_info.dwo contents:
+; CHECK-NEXT: Compile Unit: {{.*}}, format = DWARF64,
+; CHECK:      DW_TAG_compile_unit [1] *
+; CHECK:        DW_AT_producer [DW_FORM_strx1] (indexed (00000002) string = "Manual")
+; CHECK:        DW_AT_name [DW_FORM_strx1] (indexed (00000003) string = "foo.c")
+; CHECK:        DW_AT_dwo_name [DW_FORM_strx1] (indexed (00000004) string = "foo.dwo")
+; CHECK:      DW_TAG_variable [2]
+; CHECK:        DW_AT_name [DW_FORM_strx1] (indexed (00000000) string = "foo")
+; CHECK:      DW_TAG_base_type [3]
+; CHECK:        DW_AT_name [DW_FORM_strx1] (indexed (00000001) string = "int")
+
+; CHECK:      .debug_str.dwo contents:
+; CHECK-NEXT: 0x00000000: "foo"
+; CHECK-NEXT: 0x00000004: "int"
+; CHECK-NEXT: 0x00000008: "Manual"
+; CHECK-NEXT: 0x0000000f: "foo.c"
+; CHECK-NEXT: 0x00000015: "foo.dwo"
+
+; CHECK:      .debug_str_offsets.dwo contents:
+; CHECK-NEXT: 0x00000000: Contribution size = 44, Format = DWARF64, Version = 5
+; CHECK-NEXT: 0x00000010: 0000000000000000 "foo"
+; CHECK-NEXT: 0x00000018: 0000000000000004 "int"
+; CHECK-NEXT: 0x00000020: 0000000000000008 "Manual"
+; CHECK-NEXT: 0x00000028: 000000000000000f "foo.c"
+; CHECK-NEXT: 0x00000030: 0000000000000015 "foo.dwo"
+
+@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)