[DWARFv5] Read and dump multiple .debug_info sections

[probinson]

Summary

Type units go in .debug_info comdats, not .debug_types, in v5.

[probinson]

I reordered the units in the test file, because llvm-dwarfdump will dump all the .debug_info bits before going on to the .debug_types bits. The order of the sections in the file doesn't matter, but the order of the CHECKs does, and this was the simplest way to deal with it.

The lld change is just to match the API change. LLD's only interest in .debug_info is so it can find the .debug_line section for reporting source info in diagnostics. It might need some finagling once we actually start emitting type units in .debug_info, because LLD wants the CU not a TU and currently it will take whatever is first.

[probinson]

Ping. For my next trick I was going to do the patch that emits type units in .debug_info instead of .debug_types, for DWARF v5. Would it help to have that up first?

[aprantl]

llvm/include/llvm/DebugInfo/DWARF/DWARFObject.h

37

A more common pattern in LLVM seems to be an debug_info_sections() accessor that returns an iterator range, such as in http://llvm.org/doxygen/classllvm_1_1object_1_1ObjectFile.html#a7f73649118e365a230be4870d824e7cf

[dblaikie]

Generally looks good to me - I'll leave it to you & Adrian to hash out any concerns he has.

[aprantl]

llvm/include/llvm/DebugInfo/DWARF/DWARFObject.h

37

I don't feel very strongly about it, the iterator range just seems to be slightly more flexible as an interface.

[probinson]

llvm/include/llvm/DebugInfo/DWARF/DWARFObject.h

37

I did it this way to match what was in place for .debug_types; I can take an action to follow up with an iterator range. I'd rather not throw in an NFC refactor of .debug_types along with the functional part.

[aprantl]

Sure.

lld/ELF/DWARF.h

 //===- DWARF.h -----------------------------------------------*- C++ -*-===//
 //
 //                             The LLVM Linker
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===-------------------------------------------------------------------===//
 
 #ifndef LLD_ELF_DWARF_H
 #define LLD_ELF_DWARF_H
 
 #include "InputFiles.h"
+#include "llvm/ADT/STLExtras.h"
 #include "llvm/DebugInfo/DWARF/DWARFContext.h"
 #include "llvm/Object/ELF.h"
 
 namespace lld {
 namespace elf {
 
 class InputSection;
 
 struct LLDDWARFSection final : public llvm::DWARFSection {
   InputSectionBase *Sec = nullptr;
 };
 
 template <class ELFT> class LLDDwarfObj final : public llvm::DWARFObject {
 public:
   explicit LLDDwarfObj(ObjFile<ELFT> *Obj);
 
-  const llvm::DWARFSection &getInfoSection() const override {
-    return InfoSection;
+  void forEachInfoSections(
+      llvm::function_ref<void(const llvm::DWARFSection &)> F) const override {
+    F(InfoSection);
   }
 
   const llvm::DWARFSection &getRangeSection() const override {
     return RangeSection;
   }
 
   const llvm::DWARFSection &getLineSection() const override {
     return LineSection;

llvm/include/llvm/DebugInfo/DWARF/DWARFObject.h

 
 public:
   virtual ~DWARFObject() = default;
   virtual StringRef getFileName() const { llvm_unreachable("unimplemented"); }
   virtual const object::ObjectFile *getFile() const { return nullptr; }
   virtual ArrayRef<SectionName> getSectionNames() const { return {}; }
   virtual bool isLittleEndian() const = 0;
   virtual uint8_t getAddressSize() const { llvm_unreachable("unimplemented"); }
-  virtual const DWARFSection &getInfoSection() const { return Dummy; }
+  virtual void
+  forEachInfoSections(function_ref<void(const DWARFSection &)> F) const {}
   virtual void
   forEachTypesSections(function_ref<void(const DWARFSection &)> F) const {}
   virtual StringRef getAbbrevSection() const { return ""; }
   virtual const DWARFSection &getLocSection() const { return Dummy; }
   virtual const DWARFSection &getLoclistsSection() const { return Dummy; }
   virtual StringRef getARangeSection() const { return ""; }
   virtual StringRef getDebugFrameSection() const { return ""; }
   virtual StringRef getEHFrameSection() const { return ""; }
   virtual const DWARFSection &getLineSection() const { return Dummy; }
   virtual StringRef getLineStringSection() const { return ""; }
   virtual StringRef getStringSection() const { return ""; }
   virtual const DWARFSection &getRangeSection() const { return Dummy; }
   virtual const DWARFSection &getRnglistsSection() const { return Dummy; }
   virtual StringRef getMacinfoSection() const { return ""; }
   virtual StringRef getPubNamesSection() const { return ""; }
   virtual StringRef getPubTypesSection() const { return ""; }
   virtual StringRef getGnuPubNamesSection() const { return ""; }
   virtual StringRef getGnuPubTypesSection() const { return ""; }
   virtual const DWARFSection &getStringOffsetSection() const { return Dummy; }
-  virtual const DWARFSection &getInfoDWOSection() const { return Dummy; }
+  virtual void
+  forEachInfoDWOSections(function_ref<void(const DWARFSection &)> F) const {}
   virtual void
   forEachTypesDWOSections(function_ref<void(const DWARFSection &)> F) const {}
   virtual StringRef getAbbrevDWOSection() const { return ""; }
   virtual const DWARFSection &getLineDWOSection() const { return Dummy; }
   virtual const DWARFSection &getLocDWOSection() const { return Dummy; }
   virtual StringRef getStringDWOSection() const { return ""; }
   virtual const DWARFSection &getStringOffsetDWOSection() const {
     return Dummy;

llvm/lib/DebugInfo/DWARF/DWARFContext.cpp

void DWARFContext::dump(
   // Dump individual sections.
   if (shouldDump(Explicit, ".debug_abbrev", DIDT_ID_DebugAbbrev,
                  DObj->getAbbrevSection()))
     getDebugAbbrev()->dump(OS);
   if (shouldDump(ExplicitDWO, ".debug_abbrev.dwo", DIDT_ID_DebugAbbrev,
                  DObj->getAbbrevDWOSection()))
     getDebugAbbrevDWO()->dump(OS);
 
-  auto dumpDebugInfo = [&](unit_iterator_range Units) {
-    if (DumpOffset)
-      getDIEForOffset(DumpOffset.getValue())
+  auto dumpDebugInfo = [&](const char *Name, unit_iterator_range Units) {
+    OS << '\n' << Name << " contents:\n";
+    if (DumpOffset = DumpOffsets[DIDT_ID_DebugInfo])
+      for (const auto &U : Units)
+        U->getDIEForOffset(DumpOffset.getValue())
             .dump(OS, 0, DumpOpts.noImplicitRecursion());
     else
       for (const auto &U : Units)
         U->dump(OS, DumpOpts);
   };
-  if (shouldDump(Explicit, ".debug_info", DIDT_ID_DebugInfo,
-                 DObj->getInfoSection().Data))
-    dumpDebugInfo(info_section_units());
-  if (shouldDump(ExplicitDWO, ".debug_info.dwo", DIDT_ID_DebugInfo,
-                 DObj->getInfoDWOSection().Data))
-    dumpDebugInfo(dwo_info_section_units());
+  if ((DumpType & DIDT_DebugInfo)) {
+    if (Explicit || getNumCompileUnits())
+      dumpDebugInfo(".debug_info", info_section_units());
+    if (ExplicitDWO || getNumDWOCompileUnits())
+      dumpDebugInfo(".debug_info.dwo", dwo_info_section_units());
+  }
 
   auto dumpDebugType = [&](const char *Name, unit_iterator_range Units) {
     OS << '\n' << Name << " contents:\n";
     DumpOffset = DumpOffsets[DIDT_ID_DebugTypes];
     for (const auto &U : Units)
       if (DumpOffset)
         U->getDIEForOffset(*DumpOffset)
             .dump(OS, 0, DumpOpts.noImplicitRecursion());
DWARFDataExtractor lineData(*DObj, U->getLineSection(), isLittleEndian(),
                               U->getAddressByteSize());
   return Line->getOrParseLineTable(lineData, stmtOffset, *this, U,
                                    RecoverableErrorCallback);
 }
 
 void DWARFContext::parseNormalUnits() {
   if (!NormalUnits.empty())
     return;
-  NormalUnits.addUnitsForSection(*this, DObj->getInfoSection(), DW_SECT_INFO);
+  DObj->forEachInfoSections([&](const DWARFSection &S) {
+    NormalUnits.addUnitsForSection(*this, S, DW_SECT_INFO);
+  });
   NormalUnits.finishedInfoUnits();
   DObj->forEachTypesSections([&](const DWARFSection &S) {
     NormalUnits.addUnitsForSection(*this, S, DW_SECT_TYPES);
   });
 }
 
 void DWARFContext::parseDWOUnits(bool Lazy) {
   if (!DWOUnits.empty())
     return;
-  DWOUnits.addUnitsForDWOSection(*this, DObj->getInfoDWOSection(), DW_SECT_INFO,
-                                 Lazy);
+  DObj->forEachInfoDWOSections([&](const DWARFSection &S) {
+    DWOUnits.addUnitsForDWOSection(*this, S, DW_SECT_INFO, Lazy);
+  });
   DWOUnits.finishedInfoUnits();
   DObj->forEachTypesDWOSections([&](const DWARFSection &S) {
     DWOUnits.addUnitsForDWOSection(*this, S, DW_SECT_TYPES, Lazy);
   });
 }
 
 DWARFCompileUnit *DWARFContext::getCompileUnitForOffset(uint32_t Offset) {
   parseNormalUnits();

 
 class DWARFObjInMemory final : public DWARFObject {
   bool IsLittleEndian;
   uint8_t AddressSize;
   StringRef FileName;
   const object::ObjectFile *Obj = nullptr;
   std::vector<SectionName> SectionNames;
 
-  using TypeSectionMap = MapVector<object::SectionRef, DWARFSectionMap,
+  using InfoSectionMap = MapVector<object::SectionRef, DWARFSectionMap,
                                    std::map<object::SectionRef, unsigned>>;
 
-  TypeSectionMap TypesSections;
-  TypeSectionMap TypesDWOSections;
+  InfoSectionMap InfoSections;
+  InfoSectionMap TypesSections;
+  InfoSectionMap InfoDWOSections;
+  InfoSectionMap TypesDWOSections;
 
-  DWARFSectionMap InfoSection;
   DWARFSectionMap LocSection;
   DWARFSectionMap LocListsSection;
   DWARFSectionMap LineSection;
   DWARFSectionMap RangeSection;
   DWARFSectionMap RnglistsSection;
   DWARFSectionMap StringOffsetSection;
-  DWARFSectionMap InfoDWOSection;
   DWARFSectionMap LineDWOSection;
   DWARFSectionMap LocDWOSection;
   DWARFSectionMap StringOffsetDWOSection;
   DWARFSectionMap RangeDWOSection;
   DWARFSectionMap RnglistsDWOSection;
   DWARFSectionMap AddrSection;
   DWARFSectionMap AppleNamesSection;
   DWARFSectionMap AppleTypesSection;
   DWARFSectionMap AppleNamespacesSection;
   DWARFSectionMap AppleObjCSection;
   DWARFSectionMap DebugNamesSection;
 
   DWARFSectionMap *mapNameToDWARFSection(StringRef Name) {
     return StringSwitch<DWARFSectionMap *>(Name)
-        .Case("debug_info", &InfoSection)
         .Case("debug_loc", &LocSection)
         .Case("debug_loclists", &LocListsSection)
         .Case("debug_line", &LineSection)
         .Case("debug_str_offsets", &StringOffsetSection)
         .Case("debug_ranges", &RangeSection)
         .Case("debug_rnglists", &RnglistsSection)
-        .Case("debug_info.dwo", &InfoDWOSection)
         .Case("debug_loc.dwo", &LocDWOSection)
         .Case("debug_line.dwo", &LineDWOSection)
         .Case("debug_names", &DebugNamesSection)
         .Case("debug_rnglists.dwo", &RnglistsDWOSection)
         .Case("debug_str_offsets.dwo", &StringOffsetDWOSection)
         .Case("debug_addr", &AddrSection)
         .Case("apple_names", &AppleNamesSection)
         .Case("apple_types", &AppleTypesSection)

 
 public:
   DWARFObjInMemory(const StringMap<std::unique_ptr<MemoryBuffer>> &Sections,
                    uint8_t AddrSize, bool IsLittleEndian)
       : IsLittleEndian(IsLittleEndian) {
     for (const auto &SecIt : Sections) {
       if (StringRef *SectionData = mapSectionToMember(SecIt.first()))
         *SectionData = SecIt.second->getBuffer();
+      else if (SecIt.first() == "debug_info")
+        // Find debug_info and debug_types data by section rather than name as
+        // there are multiple, comdat grouped, of these sections.
+        InfoSections[SectionRef()].Data = SecIt.second->getBuffer();
+      else if (SecIt.first() == "debug_info.dwo")
+        InfoDWOSections[SectionRef()].Data = SecIt.second->getBuffer();
+      else if (SecIt.first() == "debug_types")
+        TypesSections[SectionRef()].Data = SecIt.second->getBuffer();
+      else if (SecIt.first() == "debug_types.dwo")
+        TypesDWOSections[SectionRef()].Data = SecIt.second->getBuffer();
     }
   }
   DWARFObjInMemory(const object::ObjectFile &Obj, const LoadedObjectInfo *L,
                    function_ref<ErrorPolicy(Error)> HandleError)
       : IsLittleEndian(Obj.isLittleEndian()),
         AddressSize(Obj.getBytesInAddress()), FileName(Obj.getFileName()),
         Obj(&Obj) {
 
for (const SectionRef &Section : Obj.sections()) {
       Name = Obj.mapDebugSectionName(Name);
 
       if (StringRef *SectionData = mapSectionToMember(Name)) {
         *SectionData = Data;
         if (Name == "debug_ranges") {
           // FIXME: Use the other dwo range section when we emit it.
           RangeDWOSection.Data = Data;
         }
+      } else if (Name == "debug_info") {
+        // Find debug_info and debug_types data by section rather than name as
+        // there are multiple, comdat grouped, of these sections.
+        InfoSections[Section].Data = Data;
+      } else if (Name == "debug_info.dwo") {
+        InfoDWOSections[Section].Data = Data;
       } else if (Name == "debug_types") {
-        // Find debug_types data by section rather than name as there are
-        // multiple, comdat grouped, debug_types sections.
         TypesSections[Section].Data = Data;
       } else if (Name == "debug_types.dwo") {
         TypesDWOSections[Section].Data = Data;
       }
 
       if (RelocatedSection == Obj.section_end())
         continue;
 
for (const SectionRef &Section : Obj.sections()) {
       RelSecName = RelSecName.substr(
           RelSecName.find_first_not_of("._z")); // Skip . and _ prefixes.
 
       // TODO: Add support for relocations in other sections as needed.
       // Record relocations for the debug_info and debug_line sections.
       DWARFSectionMap *Sec = mapNameToDWARFSection(RelSecName);
       RelocAddrMap *Map = Sec ? &Sec->Relocs : nullptr;
       if (!Map) {
-        // Find debug_types relocs by section rather than name as there are
-        // multiple, comdat grouped, debug_types sections.
-        if (RelSecName == "debug_types")
+        // Find debug_info and debug_types relocs by section rather than name
+        // as there are multiple, comdat grouped, of these sections.
+        if (RelSecName == "debug_info")
+          Map = &static_cast<DWARFSectionMap &>(InfoSections[*RelocatedSection])
+                     .Relocs;
+        else if (RelSecName == "debug_info.dwo")
+          Map = &static_cast<DWARFSectionMap &>(
+                     InfoDWOSections[*RelocatedSection])
+                     .Relocs;
+        else if (RelSecName == "debug_types")
           Map =
               &static_cast<DWARFSectionMap &>(TypesSections[*RelocatedSection])
                    .Relocs;
         else if (RelSecName == "debug_types.dwo")
           Map = &static_cast<DWARFSectionMap &>(
                      TypesDWOSections[*RelocatedSection])
                      .Relocs;
         else
public:
 
   // DWARF v5
   const DWARFSection &getStringOffsetSection() const override {
     return StringOffsetSection;
   }
   StringRef getLineStringSection() const override { return LineStringSection; }
 
   // Sections for DWARF5 split dwarf proposal.
-  const DWARFSection &getInfoDWOSection() const override {
-    return InfoDWOSection;
+  void forEachInfoDWOSections(
+      function_ref<void(const DWARFSection &)> F) const override {
+    for (auto &P : InfoDWOSections)
+      F(P.second);
   }
   void forEachTypesDWOSections(
       function_ref<void(const DWARFSection &)> F) const override {
     for (auto &P : TypesDWOSections)
       F(P.second);
   }
 
   StringRef getAbbrevSection() const override { return AbbrevSection; }
const DWARFSection &getAppleObjCSection() const override {
     return AppleObjCSection;
   }
   const DWARFSection &getDebugNamesSection() const override {
     return DebugNamesSection;
   }
 
   StringRef getFileName() const override { return FileName; }
   uint8_t getAddressSize() const override { return AddressSize; }
-  const DWARFSection &getInfoSection() const override { return InfoSection; }
+  void forEachInfoSections(
+      function_ref<void(const DWARFSection &)> F) const override {
+    for (auto &P : InfoSections)
+      F(P.second);
+  }
   void forEachTypesSections(
       function_ref<void(const DWARFSection &)> F) const override {
     for (auto &P : TypesSections)
       F(P.second);
   }
 };
 } // namespace
 

llvm/lib/DebugInfo/DWARF/DWARFVerifier.cpp

unsigned DWARFVerifier::verifyUnitSection(const DWARFSection &S,
   if (!isHeaderChainValid)
     ++NumDebugInfoErrors;
   NumDebugInfoErrors += verifyDebugInfoReferences();
   return NumDebugInfoErrors;
 }
 
 bool DWARFVerifier::handleDebugInfo() {
   const DWARFObject &DObj = DCtx.getDWARFObj();
+  unsigned NumErrors = 0;
 
   OS << "Verifying .debug_info Unit Header Chain...\n";
-  unsigned result = verifyUnitSection(DObj.getInfoSection(), DW_SECT_INFO);
+  DObj.forEachInfoSections([&](const DWARFSection &S) {
+    NumErrors += verifyUnitSection(S, DW_SECT_INFO);
+  });
 
   OS << "Verifying .debug_types Unit Header Chain...\n";
   DObj.forEachTypesSections([&](const DWARFSection &S) {
-    result += verifyUnitSection(S, DW_SECT_TYPES);
+    NumErrors += verifyUnitSection(S, DW_SECT_TYPES);
   });
-  return result == 0;
+  return NumErrors == 0;
 }
 
 unsigned DWARFVerifier::verifyDieRanges(const DWARFDie &Die,
                                         DieRangeInfo &ParentRI) {
   unsigned NumErrors = 0;
 
   if (!Die.isValid())
     return NumErrors;
default:
     break;
   }
   return NumErrors;
 }
 
 unsigned DWARFVerifier::verifyDebugInfoForm(const DWARFDie &Die,
                                             DWARFAttribute &AttrValue) {
   const DWARFObject &DObj = DCtx.getDWARFObj();
+  auto DieCU = Die.getDwarfUnit();
   unsigned NumErrors = 0;
   const auto Form = AttrValue.Value.getForm();
   switch (Form) {
   case DW_FORM_ref1:
   case DW_FORM_ref2:
   case DW_FORM_ref4:
   case DW_FORM_ref8:
   case DW_FORM_ref_udata: {
     // Verify all CU relative references are valid CU offsets.
     Optional<uint64_t> RefVal = AttrValue.Value.getAsReference();
     assert(RefVal);
     if (RefVal) {
-      auto DieCU = Die.getDwarfUnit();
       auto CUSize = DieCU->getNextUnitOffset() - DieCU->getOffset();
       auto CUOffset = AttrValue.Value.getRawUValue();
       if (CUOffset >= CUSize) {
         ++NumErrors;
         error() << FormEncodingString(Form) << " CU offset "
                 << format("0x%08" PRIx64, CUOffset)
                 << " is invalid (must be less than CU size of "
                 << format("0x%08" PRIx32, CUSize) << "):\n";
         Die.dump(OS, 0, DumpOpts);
         dump(Die) << '\n';
       } else {
         // Valid reference, but we will verify it points to an actual
         // DIE later.
         ReferenceToDIEOffsets[*RefVal].insert(Die.getOffset());
       }
     }
     break;
   }
   case DW_FORM_ref_addr: {
     // Verify all absolute DIE references have valid offsets in the
     // .debug_info section.
     Optional<uint64_t> RefVal = AttrValue.Value.getAsReference();
     assert(RefVal);
     if (RefVal) {
-      if (*RefVal >= DObj.getInfoSection().Data.size()) {
+      if (*RefVal >= DieCU->getInfoSection().Data.size()) {
         ++NumErrors;
         error() << "DW_FORM_ref_addr offset beyond .debug_info "
                    "bounds:\n";
         dump(Die) << '\n';
       } else {
         // Valid reference, but we will verify it points to an actual
         // DIE later.
         ReferenceToDIEOffsets[*RefVal].insert(Die.getOffset());

llvm/test/DebugInfo/X86/dwarfdump-header.s

# The split compile-unit DIE, with DW_AT_producer, DW_AT_name, DW_AT_stmt_list.
         .byte 0 # NULL
 CU_split_5_end:
 
 # CHECK: 0x00000000: Compile Unit: length = 0x0000001e version = 0x0005 unit_type = DW_UT_split_compile abbr_offset = 0x0000 addr_size = 0x08 DWO_id = 0x000000000000005a (next unit at 0x00000022)
 # CHECK: 0x00000014: DW_TAG_compile_unit
 # CHECK-NEXT: DW_AT_producer {{.*}} "Handmade DWO producer"
 # CHECK-NEXT: DW_AT_name {{.*}} "V5_dwo_compile_unit"
 
-        .section .debug_types,"",@progbits
-# CHECK-LABEL: .debug_types contents:
-
-# DWARF v4 Type unit header. Normal/split are identical so we do only one.
-TU_4_start:
-        .long  TU_4_end-TU_4_version  # Length of Unit
-TU_4_version:
-        .short 4               # DWARF version number
-        .long .debug_abbrev    # Offset Into Abbrev. Section
-        .byte 8                # Address Size (in bytes)
-        .quad 0x0011223344556677 # Type Signature
-        .long TU_4_type-TU_4_start # Type offset
-# The type-unit DIE, which has a name.
-        .byte 2
-        .long str_TU_4
-# The type DIE, which has a name.
-TU_4_type:
-        .byte 3
-        .long str_TU_4
-        .byte 0 # NULL
-        .byte 0 # NULL
-TU_4_end:
-
-# CHECK: 0x00000000: Type Unit: length = 0x0000001f version = 0x0004 abbr_offset = 0x0000 addr_size = 0x08 name = 'V4_type_unit' type_signature = 0x0011223344556677 type_offset = 0x001c (next unit at 0x00000023)
-# CHECK: 0x00000017: DW_TAG_type_unit
+# Now a DWARF v5 type unit, which goes in a .debug_info.dwo comdat.
+# Note there will not be another ".debug_info.dwo contents:" line, even though
+# there is a separate ELF section header; it's dumped along with the previous
+# unit as if they were in a single section.
 
-        .section .debug_types.dwo,"",@progbits
-# FIXME: DWARF v5 wants type units in .debug_info[.dwo] not .debug_types[.dwo].
-# CHECK: .debug_types.dwo contents:
+        .section .debug_info.dwo,"G",@progbits,5555,comdat
+# CHECK-NOT: .debug_info.dwo
 
 # DWARF v5 split type unit header.
 TU_split_5_start:
         .long  TU_split_5_end-TU_split_5_version  # Length of Unit
 TU_split_5_version:
         .short 5               # DWARF version number
         .byte 6                # DWARF Unit Type
         .byte 8                # Address Size (in bytes)
TU_split_5_type:
         .long dwo_TU_5
         .byte 0 # NULL
         .byte 0 # NULL
 TU_split_5_end:
 
 # CHECK: 0x00000000: Type Unit: length = 0x00000020 version = 0x0005 unit_type = DW_UT_split_type abbr_offset = 0x0000 addr_size = 0x08 name = 'V5_split_type_unit' type_signature = 0x8899aabbccddeeff type_offset = 0x001d (next unit at 0x00000024)
 # CHECK: 0x00000018: DW_TAG_type_unit
 
+        .section .debug_types,"",@progbits
+# CHECK-LABEL: .debug_types contents:
+
+# DWARF v4 Type unit header. Normal/split are identical so we do only one.
+TU_4_start:
+        .long  TU_4_end-TU_4_version  # Length of Unit
+TU_4_version:
+        .short 4               # DWARF version number
+        .long .debug_abbrev    # Offset Into Abbrev. Section
+        .byte 8                # Address Size (in bytes)
+        .quad 0x0011223344556677 # Type Signature
+        .long TU_4_type-TU_4_start # Type offset
+# The type-unit DIE, which has a name.
+        .byte 2
+        .long str_TU_4
+# The type DIE, which has a name.
+TU_4_type:
+        .byte 3
+        .long str_TU_4
+        .byte 0 # NULL
+        .byte 0 # NULL
+TU_4_end:
+
+# CHECK: 0x00000000: Type Unit: length = 0x0000001f version = 0x0004 abbr_offset = 0x0000 addr_size = 0x08 name = 'V4_type_unit' type_signature = 0x0011223344556677 type_offset = 0x001c (next unit at 0x00000023)
+# CHECK: 0x00000017: DW_TAG_type_unit
+
         .section .debug_line,"",@progbits
 # CHECK-LABEL: .debug_line contents:
 
 # DWARF v4 line-table header.
 LH_4_start:
         .long   LH_4_end-LH_4_version   # Length of Unit
 LH_4_version:
         .short  4               # DWARF version number