[DWARF] DWARF v5: Rework of string offsets table reader

[wolfgangp]

Summary

The current consumer side implementation of DWARF v5 string offsets tables is
flawed. According to the DWARF standard, contributions to the string offsets table
have their own format - either DWARF32 or DWARF64 - independent of the contributing
unit's (CU or TU) format. The current implementation incorrectly derives the string
offsets table's format from that of its contributing unit.

This patch corrects this problem in the following way: In full compilation units, full type units and
skeleton units we obtain the start of the unit's contribution from the DW_AT_str_offsets_base
attribute as before. Since the standard mandates that a contribution header has to
immediately precede this location, we attempt to detect a well-formed contribution header at
either 16 (DWARF64) or 8 (DWARF32) bytes before it. This establishes the format and length
of the unit's string offsets table contribution and enables us to validate the length
as well as any accesses through the table. Each unit's contribution is captured in a
descriptor that holds all the relevant information about the contribution. If parsing of
the header failed, the descriptor is left in an error state, which prevents extraction
of strings by any consumers. llvm-dwarfdump reports errors based on the state of the
descriptor.

llvm-dwarfdump can now also report gaps in the string offsets table as well as overlapping
contributions because dumping the table is driven by the existing units and their str_offsets_base
attributes and is no longer a simple scan as before.

For split CUs and TUs, the standard says that the DW_AT_str_offsets_table attribute is
not used. The current implementation mistakenly honors it. The patch changes the
implementation to ignore the attribute instead. Additionally, since the standard
mandates that there is only one CU (but possibly multiple TUs) in a .dwo file,
the implementation now assumes that there is a single contribution to the string offsets
table in the .debug_str_offsets.dwo section with a DWARF v5 header at offset 0. It attempts
to derive the format from the header. It also assumes that the CU and all TUs in the .dwo file
share this single contribution. The DWARF5 standard is not completely clear on this
last point but it is the only consistent interpretation I can think of.

In a .dwp file, individual contributions to the string offsets table are identified
by the index tables. There is only one CU per contributing .dwo file, so we can identify
pre-DWARF5 units and correctly parse their contributions to the string offsets table,
which is just a simple array of string offsets without a header.

The tests have been modified to add tests for

1. Units whose string offsets tables have a different format.
2. Recognition of gaps in the table
3. Mixing of DWARF v5 and existing GNU split dwarf units in a dwp file
4. Detection of overlapping contributions to the string offsets table (error)

[aprantl]

include/llvm/DebugInfo/DWARF/DWARFUnit.h

173 ↗

(On Diff #126656)

/// and on line above, so it gets picked up by Doxygen?

lib/DebugInfo/DWARF/DWARFUnit.cpp

570 ↗

(On Diff #126656)

The way that the Valid flag is set as a side-efffect here is a bit difficult to follow. Could it perhaps be a return value of parseDWARF64StringOffsetsTableHeader?

[wolfgangp]

lib/DebugInfo/DWARF/DWARFUnit.cpp

570 ↗

(On Diff #126656)

Sure. I'll bury the resetting of the valid bit inside the parseDWARF* functions as well to localize it a bit more.

[JDevlieghere]

lib/DebugInfo/DWARF/DWARFContext.cpp

97 ↗

(On Diff #126656)

How about just returning the vector by value and relying on copy elision? This signature initially gave me the impression that we might have several calls to this function for the same ContributionCollection.

149 ↗

(On Diff #126656)

I'd add a comment with the stuff you wrote in the patch description here to explain why you're subtracting the 8/16 bytes for DWARFv5 and later.

lib/DebugInfo/DWARF/DWARFUnit.cpp

80 ↗

(On Diff #126656)

I think it would be nice to have this return an Optional<int64_t> rather than this bool/ref combo, but obviously that doesn't have to be part of this patch.

487 ↗

(On Diff #126656)

What do you think about making this a member function of StrOffsetsContributionDescriptor?

571 ↗

(On Diff #126656)

Why do you need to set the value to true here? If I understand correctly the flag is set to false when calling the parseDWARF64StringOffsetsTableHeader and I don't immediately see something that relies on it.

[wolfgangp]

Addressed review comments. Streamlined the parseDWARF??StringOffsetsTableHeader() functions a bit and implemented a couple of other suggested changes.

lib/DebugInfo/DWARF/DWARFContext.cpp

97 ↗

(On Diff #126656)

Hmm, OK. I'm a bit old school on returning larger things from functions, but this is cleaner.

lib/DebugInfo/DWARF/DWARFUnit.cpp

80 ↗

(On Diff #126656)

Right. I think this code originally predates the wider use of Optional<>. Refactoring would seem like a good idea, but not in this patch.

487 ↗

(On Diff #126656)

Makes sense.

570 ↗

(On Diff #126656)

Streamlined this a bit. The parse* routines now expect an invalid descriptor in all cases and return the validity. the parseDWARF32* routine technically doesn't need to do that since its return value is never examined, but it seems more consistent that way.

571 ↗

(On Diff #126656)

Right, it wasn't necessary. This should look a bit more consistent now.

[wolfgangp]

Forgot to use std::move() on return of a vector.

[aprantl]

lib/DebugInfo/DWARF/DWARFContext.cpp

181 ↗

(On Diff #127169)

std::numeric_limits<uint32_t>::max() ? (not sure if this is actually better)

lib/DebugInfo/DWARF/DWARFUnit.cpp

494 ↗

(On Diff #127169)

I think I would prefer a bitwise not ~ over - here.

494 ↗

(On Diff #127169)

Do we need to check for overflow here? Otherwise we'll get a fuzzer bugreport incoming soon.

519 ↗

(On Diff #127169)

It still makes me uneasy that Valid is only set on one path. (Ok, with the assert that makes more sense now).
Would it be even better for this function to return an Expected/Optional<StrOffsetsContributionDescriptor> instead of having an explicit bool Valid member? This would make it less likely that we accidentally operate on an invalid record.

551 ↗

(On Diff #127169)

Do we need to guard against underflow here?

[JDevlieghere]

lib/DebugInfo/DWARF/DWARFContext.cpp

119 ↗

(On Diff #127169)

Moving the ContributionCollection will actually prevent RVO (the compiler should warn you about this).

[wolfgangp]

Refactored the string offsets contribution descriptor using Optional<>, which removes the need for a "valid" bit. Addressed some other review comments.

lib/DebugInfo/DWARF/DWARFContext.cpp

119 ↗

(On Diff #127169)

Looking at the gcc-generated code it doesn't seem to make a difference (probably due to inlining) but yes.

181 ↗

(On Diff #127169)

Looks better anyway.

lib/DebugInfo/DWARF/DWARFUnit.cpp

494 ↗

(On Diff #127169)

This is a real edge case (size > 0xfffffffffffffff8), but fuzzer will probably find it.

519 ↗

(On Diff #127169)

I refactored this using Optional<>. You're right, the valid bit looks so last century...

[aprantl]

Found a few more nitpicks, but otherwise I'm happy now. Thanks!

include/llvm/DebugInfo/DWARF/DWARFUnit.h

211 ↗

(On Diff #127764)

s/It is initially invalid.//

lib/DebugInfo/DWARF/DWARFContext.cpp

113 ↗

(On Diff #127764)

This is clever, but perhaps expanding it makes it more readable?

122 ↗

(On Diff #127764)

Is this intentionally different to the previous comparator? To the casual reader (me) it's non-obvious why. Some comments or more verbose code might help.

lib/DebugInfo/DWARF/DWARFUnit.cpp

494 ↗

(On Diff #127169)

And now with the new operator, this looks really like llvm::alignAddr() :-)

522 ↗

(On Diff #127764)

LLVM style wants to remove extra braces

526 ↗

(On Diff #127764)

same here

[wolfgangp]

Addressed more review comments: Made the logic on how to sort and unique string offsets table contributions a bit more readable when getting ready for dumping.

lib/DebugInfo/DWARF/DWARFContext.cpp

113 ↗

(On Diff #127764)

Let me know if this is clear enough. There are several ways to write this.

122 ↗

(On Diff #127764)

Let me know if this is clearer. Basically, I want to only remove duplicates when they are valid contributions. The dumper quits on the first invalid one anyway and so uniquing invalid contributions seems superfluous.

lib/DebugInfo/DWARF/DWARFUnit.cpp

494 ↗

(On Diff #127169)

Didn't realize that existed...

We are aligning a uint64_t, so it's got to be its cousin alignTo().

[aprantl]

thanks! I'll defer to Jonas to accept this.

lib/DebugInfo/DWARF/DWARFContext.cpp

113 ↗

(On Diff #127790)

Not sure if this matters, but won't this be nondeterministic when both L and R are invalid?

128 ↗

(On Diff #127790)

see above

[probinson]

lib/DebugInfo/DWARF/DWARFContext.cpp

113 ↗

(On Diff #127790)

I think the most compact code would be

if (L && R) return L->base < R->base;
return R;

if both are valid, order depends on the base.
if L is invalid and R valid, L sorts before R.
if L is valid and R invalid, R sorts before L.
if both are invalid, they are equivalent (test(R,L) == test(L,R)).

[wolfgangp]

lib/DebugInfo/DWARF/DWARFContext.cpp

113 ↗

(On Diff #127790)

Not sure if this matters, but won't this be nondeterministic when both L and R are invalid?

That's true. Several invalid contributions in the vector will probably end up in different locations when the initial order is different, but all that matters is that all the invalid ones end up at the beginning. It would only matter if we tried to be more precise about how and why they are invalid (bad start vs. invalid length etc.), but this patch doesn't try to do that. It's not clear to me if it would be worth it.

113 ↗

(On Diff #127790)

I think the most compact code would be

Sleek! I can put that in, if you guys prefer.

[JDevlieghere]

Thanks, Wolfgang. With these few nits addressed this LGTM.

lib/DebugInfo/DWARF/DWARFContext.cpp

113 ↗

(On Diff #127790)

I like Paul's suggestion.

lib/DebugInfo/DWARF/DWARFUnit.cpp

512 ↗

(On Diff #127790)

return StrOffsetsContributionDescriptor(Offset, Size, Version, DWARF64);

527 ↗

(On Diff #127790)

return StrOffsetsContributionDescriptor(Offset, ContributionSize, Version, DWARF32);

567 ↗

(On Diff #127790)

return StrOffsetsContributionDescriptor(Offset, Size, 4, DWARF32);

llvm/trunk/include/llvm/DebugInfo/DWARF/DWARFUnit.h

 };
 
 /// Represents base address of the CU.
 struct BaseAddress {
   uint64_t Address;
   uint64_t SectionIndex;
 };
 
+/// Represents a unit's contribution to the string offsets table.
+struct StrOffsetsContributionDescriptor {
+  uint64_t Base = 0;
+  uint64_t Size = 0;
+  /// Format and version.
+  DWARFFormParams FormParams = {0, 0, dwarf::DwarfFormat::DWARF32};
+
+  StrOffsetsContributionDescriptor(uint64_t Base, uint64_t Size,
+                                   uint8_t Version, dwarf::DwarfFormat Format)
+      : Base(Base), Size(Size), FormParams({Version, 0, Format}) {}
+
+  uint8_t getVersion() const { return FormParams.Version; }
+  dwarf::DwarfFormat getFormat() const { return FormParams.Format; }
+  uint8_t getDwarfOffsetByteSize() const {
+    return FormParams.getDwarfOffsetByteSize();
+  }
+  /// Determine whether a contribution to the string offsets table is
+  /// consistent with the relevant section size and that its length is
+  /// a multiple of the size of one of its entries.
+  Optional<StrOffsetsContributionDescriptor>
+  validateContributionSize(DWARFDataExtractor &DA);
+};
+
 class DWARFUnit {
   DWARFContext &Context;
   /// Section containing this DWARFUnit.
   const DWARFSection &InfoSection;
 
   const DWARFDebugAbbrev *Abbrev;
   const DWARFSection *RangeSection;
   uint32_t RangeSectionBase;
   const DWARFSection &LineSection;
   StringRef StringSection;
   const DWARFSection &StringOffsetSection;
-  uint64_t StringOffsetSectionBase = 0;
   const DWARFSection *AddrOffsetSection;
   uint32_t AddrOffsetSectionBase = 0;
   bool isLittleEndian;
   bool isDWO;
   const DWARFUnitSectionBase &UnitSection;
 
   // Version, address size, and DWARF format.
   DWARFFormParams FormParams;
+  /// Start, length, and DWARF format of the unit's contribution to the string
+  /// offsets table (DWARF v5).
+  Optional<StrOffsetsContributionDescriptor> StringOffsetsTableContribution;
 
   uint32_t Offset;
   uint32_t Length;
   mutable const DWARFAbbreviationDeclarationSet *Abbrevs;
   uint64_t AbbrOffset;
   uint8_t UnitType;
   llvm::Optional<BaseAddress> BaseAddr;
   /// The compile unit debug information entry items.
class DWARFUnit {
   }
 
 protected:
   virtual bool extractImpl(DataExtractor debug_info, uint32_t *offset_ptr);
 
   /// Size in bytes of the unit header.
   virtual uint32_t getHeaderSize() const { return getVersion() <= 4 ? 11 : 12; }
 
+  /// Find the unit's contribution to the string offsets table and determine its
+  /// length and form. The given offset is expected to be derived from the unit
+  /// DIE's DW_AT_str_offsets_base attribute.
+  Optional<StrOffsetsContributionDescriptor>
+  determineStringOffsetsTableContribution(DWARFDataExtractor &DA,
+                                          uint64_t Offset);
+
+  /// Find the unit's contribution to the string offsets table and determine its
+  /// length and form. The given offset is expected to be 0 in a dwo file or,
+  /// in a dwp file, the start of the unit's contribution to the string offsets
+  /// table section (as determined by the index table).
+  Optional<StrOffsetsContributionDescriptor>
+  determineStringOffsetsTableContributionDWO(DWARFDataExtractor &DA,
+                                             uint64_t Offset);
+
 public:
   DWARFUnit(DWARFContext &Context, const DWARFSection &Section,
             const DWARFDebugAbbrev *DA, const DWARFSection *RS, StringRef SS,
             const DWARFSection &SOS, const DWARFSection *AOS,
             const DWARFSection &LS, bool LE, bool IsDWO,
             const DWARFUnitSectionBase &UnitSection,
             const DWARFUnitIndex::Entry *IndexEntry = nullptr);
 
public:
   /// Requires that compile unit is already extracted.
   bool extractRangeList(uint32_t RangeListOffset,
                         DWARFDebugRangeList &RangeList) const;
   void clear();
   uint32_t getOffset() const { return Offset; }
   uint32_t getNextUnitOffset() const { return Offset + Length + 4; }
   uint32_t getLength() const { return Length; }
 
+  const Optional<StrOffsetsContributionDescriptor> &
+  getStringOffsetsTableContribution() const {
+    return StringOffsetsTableContribution;
+  }
   const DWARFFormParams &getFormParams() const { return FormParams; }
   uint16_t getVersion() const { return FormParams.Version; }
   dwarf::DwarfFormat getFormat() const { return FormParams.Format; }
   uint8_t getAddressByteSize() const { return FormParams.AddrSize; }
   uint8_t getRefAddrByteSize() const { return FormParams.getRefAddrByteSize(); }
   uint8_t getDwarfOffsetByteSize() const {
     return FormParams.getDwarfOffsetByteSize();
   }
 
+  uint8_t getDwarfStringOffsetsByteSize() const {
+    assert(StringOffsetsTableContribution);
+    return StringOffsetsTableContribution->getDwarfOffsetByteSize();
+  }
+
+  uint64_t getStringOffsetsBase() const {
+    assert(StringOffsetsTableContribution);
+    return StringOffsetsTableContribution->Base;
+  }
+
   const DWARFAbbreviationDeclarationSet *getAbbreviations() const;
 
   uint8_t getUnitType() const { return UnitType; }
 
   static bool isMatchingUnitTypeAndTag(uint8_t UnitType, dwarf::Tag Tag) {
     switch (UnitType) {
     case dwarf::DW_UT_compile:
       return Tag == dwarf::DW_TAG_compile_unit;

llvm/trunk/lib/DebugInfo/DWARF/DWARFContext.cpp

if (LC.C.cmd == MachO::LC_UUID) {
       memcpy(&UUID, LC.Ptr+sizeof(LC.C), sizeof(UUID));
       OS.write_uuid(UUID);
       OS << ' ' << MachO->getFileFormatName();
       OS << ' ' << MachO->getFileName() << '\n';
     }
   }
 }
 
-static void
-dumpDWARFv5StringOffsetsSection(raw_ostream &OS, StringRef SectionName,
-                                const DWARFObject &Obj,
-                                const DWARFSection &StringOffsetsSection,
-                                StringRef StringSection, bool LittleEndian) {
-  DWARFDataExtractor StrOffsetExt(Obj, StringOffsetsSection, LittleEndian, 0);
-  uint32_t Offset = 0;
-  uint64_t SectionSize = StringOffsetsSection.Data.size();
+using ContributionCollection =
+    std::vector<Optional<StrOffsetsContributionDescriptor>>;
 
-  while (Offset < SectionSize) {
-    unsigned Version = 0;
-    DwarfFormat Format = DWARF32;
-    unsigned EntrySize = 4;
-    // Perform validation and extract the segment size from the header.
-    if (!StrOffsetExt.isValidOffsetForDataOfSize(Offset, 4)) {
-      OS << "error: invalid contribution to string offsets table in section ."
-         << SectionName << ".\n";
-      return;
-    }
-    uint32_t ContributionStart = Offset;
-    uint64_t ContributionSize = StrOffsetExt.getU32(&Offset);
-    // A contribution size of 0xffffffff indicates DWARF64, with the actual size
-    // in the following 8 bytes. Otherwise, the DWARF standard mandates that
-    // the contribution size must be at most 0xfffffff0.
-    if (ContributionSize == 0xffffffff) {
-      if (!StrOffsetExt.isValidOffsetForDataOfSize(Offset, 8)) {
-        OS << "error: invalid contribution to string offsets table in section ."
-           << SectionName << ".\n";
-        return;
-      }
-      Format = DWARF64;
-      EntrySize = 8;
-      ContributionSize = StrOffsetExt.getU64(&Offset);
-    } else if (ContributionSize > 0xfffffff0) {
+// Collect all the contributions to the string offsets table from all units,
+// sort them by their starting offsets and remove duplicates.
+static ContributionCollection
+collectContributionData(DWARFContext::cu_iterator_range CUs,
+                        DWARFContext::tu_section_iterator_range TUSs) {
+  ContributionCollection Contributions;
+  for (const auto &CU : CUs)
+    Contributions.push_back(CU->getStringOffsetsTableContribution());
+  for (const auto &TUS : TUSs)
+    for (const auto &TU : TUS)
+      Contributions.push_back(TU->getStringOffsetsTableContribution());
+
+  // Sort the contributions so that any invalid ones are placed at
+  // the start of the contributions vector. This way they are reported
+  // first.
+  std::sort(Contributions.begin(), Contributions.end(),
+            [](const Optional<StrOffsetsContributionDescriptor> &L,
+               const Optional<StrOffsetsContributionDescriptor> &R) {
+              if (L && R) return L->Base < R->Base;
+              return R.hasValue();
+            });
+
+  // Uniquify contributions, as it is possible that units (specifically
+  // type units in dwo or dwp files) share contributions. We don't want
+  // to report them more than once.
+  Contributions.erase(
+      std::unique(Contributions.begin(), Contributions.end(),
+                  [](const Optional<StrOffsetsContributionDescriptor> &L,
+                     const Optional<StrOffsetsContributionDescriptor> &R) {
+                    if (L && R)
+                      return L->Base == R->Base && L->Size == R->Size;
+                    return false;
+                  }),
+      Contributions.end());
+  return Contributions;
+
+static void dumpDWARFv5StringOffsetsSection(
+    raw_ostream &OS, StringRef SectionName, const DWARFObject &Obj,
+    const DWARFSection &StringOffsetsSection, StringRef StringSection,
+    DWARFContext::cu_iterator_range CUs,
+    DWARFContext::tu_section_iterator_range TUSs, bool LittleEndian) {
+  auto Contributions = collectContributionData(CUs, TUSs);
+  DWARFDataExtractor StrOffsetExt(Obj, StringOffsetsSection, LittleEndian, 0);
+  DataExtractor StrData(StringSection, LittleEndian, 0);
+  uint64_t SectionSize = StringOffsetsSection.Data.size();
+  uint32_t Offset = 0;
+  for (auto &Contribution : Contributions) {
+    // Report an ill-formed contribution.
+    if (!Contribution) {
       OS << "error: invalid contribution to string offsets table in section ."
          << SectionName << ".\n";
       return;
     }
 
-    // We must ensure that we don't read a partial record at the end, so we
-    // validate for a multiple of EntrySize. Also, we're expecting a version
-    // number and padding, which adds an additional 4 bytes.
-    uint64_t ValidationSize =
-        4 + ((ContributionSize + EntrySize - 1) & (-(uint64_t)EntrySize));
-    if (!StrOffsetExt.isValidOffsetForDataOfSize(Offset, ValidationSize)) {
-      OS << "error: contribution to string offsets table in section ."
-         << SectionName << " has invalid length.\n";
+    dwarf::DwarfFormat Format = Contribution->getFormat();
+    uint16_t Version = Contribution->getVersion();
+    uint64_t ContributionHeader = Contribution->Base;
+    // In DWARF v5 there is a contribution header that immediately precedes
+    // the string offsets base (the location we have previously retrieved from
+    // the CU DIE's DW_AT_str_offsets attribute). The header is located either
+    // 8 or 16 bytes before the base, depending on the contribution's format.
+    if (Version >= 5)
+      ContributionHeader -= Format == DWARF32 ? 8 : 16;
+
+    // Detect overlapping contributions.
+    if (Offset > ContributionHeader) {
+      OS << "error: overlapping contributions to string offsets table in "
+            "section ."
+         << SectionName << ".\n";
       return;
     }
-
-    Version = StrOffsetExt.getU16(&Offset);
-    Offset += 2;
-    OS << format("0x%8.8x: ", ContributionStart);
-    OS << "Contribution size = " << ContributionSize
+    // Report a gap in the table.
+    if (Offset < ContributionHeader) {
+      OS << format("0x%8.8x: Gap, length = ", Offset);
+      OS << (ContributionHeader - Offset) << "\n";
+    }
+    OS << format("0x%8.8x: ", ContributionHeader);
+    OS << "Contribution size = " << Contribution->Size
+       << ", Format = " << (Format == DWARF32 ? "DWARF32" : "DWARF64")
        << ", Version = " << Version << "\n";
 
-    uint32_t ContributionBase = Offset;
-    DataExtractor StrData(StringSection, LittleEndian, 0);
-    while (Offset - ContributionBase < ContributionSize) {
+    Offset = Contribution->Base;
+    unsigned EntrySize = Contribution->getDwarfOffsetByteSize();
+    while (Offset - Contribution->Base < Contribution->Size) {
       OS << format("0x%8.8x: ", Offset);
-      // FIXME: We can only extract strings in DWARF32 format at the moment.
+      // FIXME: We can only extract strings if the offset fits in 32 bits.
       uint64_t StringOffset =
           StrOffsetExt.getRelocatedValue(EntrySize, &Offset);
-      if (Format == DWARF32) {
+      // Extract the string if we can and display it. Otherwise just report
+      // the offset.
+      if (StringOffset <= std::numeric_limits<uint32_t>::max()) {
         uint32_t StringOffset32 = (uint32_t)StringOffset;
         OS << format("%8.8x ", StringOffset32);
         const char *S = StrData.getCStr(&StringOffset32);
         if (S)
           OS << format("\"%s\"", S);
       } else
         OS << format("%16.16" PRIx64 " ", StringOffset);
       OS << "\n";
     }
   }
+  // Report a gap at the end of the table.
+  if (Offset < SectionSize) {
+    OS << format("0x%8.8x: Gap, length = ", Offset);
+    OS << (SectionSize - Offset) << "\n";
+  }
 }
 
 // Dump a DWARF string offsets section. This may be a DWARF v5 formatted
 // string offsets section, where each compile or type unit contributes a
 // number of entries (string offsets), with each contribution preceded by
 // a header containing size and version number. Alternatively, it may be a
 // monolithic series of string offsets, as generated by the pre-DWARF v5
 // implementation of split DWARF.
-static void dumpStringOffsetsSection(raw_ostream &OS, StringRef SectionName,
-                                     const DWARFObject &Obj,
-                                     const DWARFSection &StringOffsetsSection,
-                                     StringRef StringSection, bool LittleEndian,
-                                     unsigned MaxVersion) {
+static void dumpStringOffsetsSection(
+    raw_ostream &OS, StringRef SectionName, const DWARFObject &Obj,
+    const DWARFSection &StringOffsetsSection, StringRef StringSection,
+    DWARFContext::cu_iterator_range CUs,
+    DWARFContext::tu_section_iterator_range TUSs, bool LittleEndian,
   // If we have at least one (compile or type) unit with DWARF v5 or greater,
   // we assume that the section is formatted like a DWARF v5 string offsets
   // section.
   if (MaxVersion >= 5)
     dumpDWARFv5StringOffsetsSection(OS, SectionName, Obj, StringOffsetsSection,
-                                    StringSection, LittleEndian);
+                                    StringSection, CUs, TUSs, LittleEndian);
   else {
     DataExtractor strOffsetExt(StringOffsetsSection.Data, LittleEndian, 0);
     uint32_t offset = 0;
     uint64_t size = StringOffsetsSection.Data.size();
     // Ensure that size is a multiple of the size of an entry.
     if (size & ((uint64_t)(sizeof(uint32_t) - 1))) {
       OS << "error: size of ." << SectionName << " is not a multiple of "
          << sizeof(uint32_t) << ".\n";
if (shouldDump(Explicit, ".debug_gnu_pubtypes", DIDT_ID_DebugGnuPubtypes,
     DWARFDebugPubTable(DObj->getGnuPubTypesSection(), isLittleEndian(),
                        true /* GnuStyle */)
         .dump(OS);
 
   if (shouldDump(Explicit, ".debug_str_offsets", DIDT_ID_DebugStrOffsets,
                  DObj->getStringOffsetSection().Data))
     dumpStringOffsetsSection(
         OS, "debug_str_offsets", *DObj, DObj->getStringOffsetSection(),
-        DObj->getStringSection(), isLittleEndian(), getMaxVersion());
+        DObj->getStringSection(), compile_units(), type_unit_sections(),
+        isLittleEndian(), getMaxVersion());
   if (shouldDump(ExplicitDWO, ".debug_str_offsets.dwo", DIDT_ID_DebugStrOffsets,
                  DObj->getStringOffsetDWOSection().Data))
     dumpStringOffsetsSection(
         OS, "debug_str_offsets.dwo", *DObj, DObj->getStringOffsetDWOSection(),
-        DObj->getStringDWOSection(), isLittleEndian(), getMaxVersion());
+        DObj->getStringDWOSection(), dwo_compile_units(),
+        dwo_type_unit_sections(), isLittleEndian(), getMaxVersion());
 
   if (shouldDump(Explicit, ".gnu_index", DIDT_ID_GdbIndex,
                  DObj->getGdbIndexSection())) {
     getGdbIndex().dump(OS);
   }
 
   if (shouldDump(Explicit, ".apple_names", DIDT_ID_AppleNames,
                  DObj->getAppleNamesSection().Data))

llvm/trunk/lib/DebugInfo/DWARF/DWARFUnit.cpp

bool DWARFUnit::getAddrOffsetSectionItem(uint32_t Index,
   DWARFDataExtractor DA(Context.getDWARFObj(), *AddrOffsetSection,
                         isLittleEndian, getAddressByteSize());
   Result = DA.getRelocatedAddress(&Offset);
   return true;
 }
 
 bool DWARFUnit::getStringOffsetSectionItem(uint32_t Index,
                                            uint64_t &Result) const {
-  unsigned ItemSize = getDwarfOffsetByteSize();
-  uint32_t Offset = StringOffsetSectionBase + Index * ItemSize;
+  if (!StringOffsetsTableContribution)
+    return false;
+  unsigned ItemSize = getDwarfStringOffsetsByteSize();
+  uint32_t Offset = getStringOffsetsBase() + Index * ItemSize;
   if (StringOffsetSection.Data.size() < Offset + ItemSize)
     return false;
   DWARFDataExtractor DA(Context.getDWARFObj(), StringOffsetSection,
                         isLittleEndian, 0);
   Result = DA.getRelocatedValue(ItemSize, &Offset);
   return true;
 }
 
if (!HasCUDie) {
     if (!isDWO) {
       assert(AddrOffsetSectionBase == 0);
       assert(RangeSectionBase == 0);
       AddrOffsetSectionBase =
           toSectionOffset(UnitDie.find(DW_AT_GNU_addr_base), 0);
       RangeSectionBase = toSectionOffset(UnitDie.find(DW_AT_rnglists_base), 0);
     }
 
-    // In general, we derive the offset of the unit's contibution to the
-    // debug_str_offsets{.dwo} section from the unit DIE's
-    // DW_AT_str_offsets_base attribute. In dwp files we add to it the offset
-    // we get from the index table.
-    StringOffsetSectionBase =
-        toSectionOffset(UnitDie.find(DW_AT_str_offsets_base), 0);
+    // In general, in DWARF v5 and beyond we derive the start of the unit's
+    // contribution to the string offsets table from the unit DIE's
+    // DW_AT_str_offsets_base attribute. Split DWARF units do not use this
+    // attribute, so we assume that there is a contribution to the string
+    // offsets table starting at offset 0 of the debug_str_offsets.dwo section.
+    // In both cases we need to determine the format of the contribution,
+    // which may differ from the unit's format.
+    uint64_t StringOffsetsContributionBase =
+        isDWO ? 0 : toSectionOffset(UnitDie.find(DW_AT_str_offsets_base), 0);
     if (IndexEntry)
       if (const auto *C = IndexEntry->getOffset(DW_SECT_STR_OFFSETS))
-        StringOffsetSectionBase += C->Offset;
+        StringOffsetsContributionBase += C->Offset;
+
+    DWARFDataExtractor DA(Context.getDWARFObj(), StringOffsetSection,
+                          isLittleEndian, 0);
+    if (isDWO)
+      StringOffsetsTableContribution =
+          determineStringOffsetsTableContributionDWO(
+              DA, StringOffsetsContributionBase);
+    else if (getVersion() >= 5)
+      StringOffsetsTableContribution = determineStringOffsetsTableContribution(
+          DA, StringOffsetsContributionBase);
 
     // Don't fall back to DW_AT_GNU_ranges_base: it should be ignored for
     // skeleton CU DIE, so that DWARF users not aware of it are not broken.
   }
 
   return DieArray.size();
 }
 
DWARFDie DWARFUnit::getFirstChild(const DWARFDebugInfoEntry *Die) {
   return DWARFDie(this, &DieArray[I]);
 }
 
 const DWARFAbbreviationDeclarationSet *DWARFUnit::getAbbreviations() const {
   if (!Abbrevs)
     Abbrevs = Abbrev->getAbbreviationDeclarationSet(AbbrOffset);
   return Abbrevs;
 }
+
+Optional<StrOffsetsContributionDescriptor>
+StrOffsetsContributionDescriptor::validateContributionSize(
+    DWARFDataExtractor &DA) {
+  uint8_t EntrySize = getDwarfOffsetByteSize();
+  // In order to ensure that we don't read a partial record at the end of
+  // the section we validate for a multiple of the entry size.
+  uint64_t ValidationSize = alignTo(Size, EntrySize);
+  // Guard against overflow.
+  if (ValidationSize >= Size)
+    if (DA.isValidOffsetForDataOfSize((uint32_t)Base, ValidationSize))
+      return *this;
+  return Optional<StrOffsetsContributionDescriptor>();
+}
+
+// Look for a DWARF64-formatted contribution to the string offsets table
+// starting at a given offset and record it in a descriptor.
+static Optional<StrOffsetsContributionDescriptor>
+parseDWARF64StringOffsetsTableHeader(DWARFDataExtractor &DA, uint32_t Offset) {
+  if (!DA.isValidOffsetForDataOfSize(Offset, 16))
+    return Optional<StrOffsetsContributionDescriptor>();
+
+  if (DA.getU32(&Offset) != 0xffffffff)
+    return Optional<StrOffsetsContributionDescriptor>();
+
+  uint64_t Size = DA.getU64(&Offset);
+  uint8_t Version = DA.getU16(&Offset);
+  (void)DA.getU16(&Offset); // padding
+  return StrOffsetsContributionDescriptor(Offset, Size, Version, DWARF64);
+  //return Optional<StrOffsetsContributionDescriptor>(Descriptor);
+}
+
+// Look for a DWARF32-formatted contribution to the string offsets table
+// starting at a given offset and record it in a descriptor.
+static Optional<StrOffsetsContributionDescriptor>
+parseDWARF32StringOffsetsTableHeader(DWARFDataExtractor &DA, uint32_t Offset) {
+  if (!DA.isValidOffsetForDataOfSize(Offset, 8))
+    return Optional<StrOffsetsContributionDescriptor>();
+  uint32_t ContributionSize = DA.getU32(&Offset);
+  if (ContributionSize >= 0xfffffff0)
+    return Optional<StrOffsetsContributionDescriptor>();
+  uint8_t Version = DA.getU16(&Offset);
+  (void)DA.getU16(&Offset); // padding
+  return StrOffsetsContributionDescriptor(Offset, ContributionSize, Version, DWARF32);
+  //return Optional<StrOffsetsContributionDescriptor>(Descriptor);
+}
+
+Optional<StrOffsetsContributionDescriptor>
+DWARFUnit::determineStringOffsetsTableContribution(DWARFDataExtractor &DA,
+                                                   uint64_t Offset) {
+  Optional<StrOffsetsContributionDescriptor> Descriptor;
+  // Attempt to find a DWARF64 contribution 16 bytes before the base.
+  if (Offset >= 16)
+    Descriptor =
+        parseDWARF64StringOffsetsTableHeader(DA, (uint32_t)Offset - 16);
+  // Try to find a DWARF32 contribution 8 bytes before the base.
+  if (!Descriptor && Offset >= 8)
+    Descriptor = parseDWARF32StringOffsetsTableHeader(DA, (uint32_t)Offset - 8);
+  return Descriptor ? Descriptor->validateContributionSize(DA) : Descriptor;
+}
+
+Optional<StrOffsetsContributionDescriptor>
+DWARFUnit::determineStringOffsetsTableContributionDWO(DWARFDataExtractor &DA,
+                                                      uint64_t Offset) {
+  if (getVersion() >= 5) {
+    // Look for a valid contribution at the given offset.
+    auto Descriptor =
+        parseDWARF64StringOffsetsTableHeader(DA, (uint32_t)Offset);
+    if (!Descriptor)
+      Descriptor = parseDWARF32StringOffsetsTableHeader(DA, (uint32_t)Offset);
+    return Descriptor ? Descriptor->validateContributionSize(DA) : Descriptor;
+  }
+  // Prior to DWARF v5, we derive the contribution size from the
+  // index table (in a package file). In a .dwo file it is simply
+  // the length of the string offsets section.
+  uint64_t Size = 0;
+  if (!IndexEntry)
+    Size = StringOffsetSection.Data.size();
+  else if (const auto *C = IndexEntry->getOffset(DW_SECT_STR_OFFSETS))
+    Size = C->Length;
+  // Return a descriptor with the given offset as base, version 4 and
+  // DWARF32 format.
+  //return Optional<StrOffsetsContributionDescriptor>(
+      //StrOffsetsContributionDescriptor(Offset, Size, 4, DWARF32));
+  return StrOffsetsContributionDescriptor(Offset, Size, 4, DWARF32);
+}

llvm/trunk/test/DebugInfo/X86/dwarfdump-str-offsets-dwp.s

 # RUN: llvm-mc -triple x86_64-unknown-linux %s -filetype=obj -o %t.o
 # RUN: llvm-dwarfdump -v %t.o | FileCheck %s
 
 # Test object to verify that dwarfdump handles dwp files with DWARF v5 string
-# offset tables. We have 2 CUs and 2 TUs, where it is assumed that 
+# offset tables. We have 3 CUs and 2 TUs, where it is assumed that 
 # CU1 and TU1 came from one object file, CU2 and TU2 from a second object
-# file.
+# file, and CU3 from a third object file that was compiled with 
+# -gdwarf-4.
 #
         .section .debug_str.dwo,"MS",@progbits,1
 str_producer:
         .asciz "Handmade DWARF producer"
 str_CU1:
         .asciz "Compile_Unit_1"
 str_CU1_dir:
         .asciz "/home/test/CU1"
 str_CU2:
         .asciz "Compile_Unit_2"
 str_CU2_dir:
         .asciz "/home/test/CU2"
 str_TU1:
         .asciz "Type_Unit_1"
 str_TU1_type:
         .asciz "MyStruct_1"
 str_TU2:
         .asciz "Type_Unit_2"
 str_TU2_type:
         .asciz "MyStruct_2"
+str_CU3:
+        .asciz "Compile_Unit_3"
+str_CU3_dir:
+        .asciz "/home/test/CU3"
 
         .section .debug_str_offsets.dwo,"",@progbits
 # Object files 1's portion of the .debug_str_offsets.dwo section.
-.debug_str_offsets_object_file1:
-
-# CU1's contribution (from object file 1)
-.debug_str_offsets_start_CU1:
-        .long .debug_str_offsets_end_CU1-.debug_str_offsets_base_CU1
+# CU1 and TU1 share a contribution to the string offsets table.
+.debug_str_offsets_object_file1_start:
+        .long .debug_str_offsets_object_file1_end-.debug_str_offsets_base_1
         .short 5    # DWARF version
         .short 0    # Padding
-.debug_str_offsets_base_CU1:
+.debug_str_offsets_base_1:
         .long str_producer-.debug_str.dwo
         .long str_CU1-.debug_str.dwo
         .long str_CU1_dir-.debug_str.dwo
-.debug_str_offsets_end_CU1:
-
-# TU1's contribution (from object file 1)
-.debug_str_offsets_start_TU1:
-        .long .debug_str_offsets_end_TU1-.debug_str_offsets_base_TU1
-        .short 5    # DWARF version
-        .short 0    # Padding
-.debug_str_offsets_base_TU1:
         .long str_TU1-.debug_str.dwo
         .long str_TU1_type-.debug_str.dwo
-.debug_str_offsets_end_TU1:
+.debug_str_offsets_object_file1_end:
 
 # Object files 2's portion of the .debug_str_offsets.dwo section.
-.debug_str_offsets_object_file2:
-
-# CU2's contribution (from object file 2)
-.debug_str_offsets_start_CU2:
-        .long .debug_str_offsets_end_CU2-.debug_str_offsets_base_CU2
+# CU2 and TU2 share a contribution to the string offsets table.
+.debug_str_offsets_object_file2_start:
+        .long .debug_str_offsets_object_file2_end-.debug_str_offsets_base_2
         .short 5    # DWARF version
         .short 0    # Padding
-.debug_str_offsets_base_CU2:
+.debug_str_offsets_base_2:
         .long str_producer-.debug_str.dwo
         .long str_CU2-.debug_str.dwo
         .long str_CU2_dir-.debug_str.dwo
-.debug_str_offsets_end_CU2:
-
-# TU2's contribution (from object file 2)
-.debug_str_offsets_start_TU2:
-        .long .debug_str_offsets_end_TU2-.debug_str_offsets_base_TU2
-        .short 5    # DWARF version
-        .short 0    # Padding
-.debug_str_offsets_base_TU2:
         .long str_TU2-.debug_str.dwo
         .long str_TU2_type-.debug_str.dwo
-.debug_str_offsets_end_TU2:
+.debug_str_offsets_object_file2_end:
 
+# Object files 3's portion of the .debug_str_offsets.dwo section.
+# This file is assumed to have been compiled with -gdwarf-4 and
+# therefore contains a version 4 CU and a GNU format contribution
+# to the .debug_str_offsets section.
+.debug_str_offsets_object_file3_start:
+.debug_str_offsets_base_3:
+        .long str_producer-.debug_str.dwo
+        .long str_CU3-.debug_str.dwo
+        .long str_CU3_dir-.debug_str.dwo
+.debug_str_offsets_object_file3_end:
 
 # Abbrevs are shared for all compile and type units
         .section .debug_abbrev.dwo,"",@progbits
         .byte 0x01  # Abbrev code
         .byte 0x11  # DW_TAG_compile_unit
         .byte 0x00  # DW_CHILDREN_no
         .byte 0x25  # DW_AT_producer
         .byte 0x1a  # DW_FORM_strx
         .byte 0x03  # DW_AT_name
         .byte 0x1a  # DW_FORM_strx
-        .byte 0x72  # DW_AT_str_offsets_base
-        .byte 0x17  # DW_FORM_sec_offset
         .byte 0x03  # DW_AT_name
         .byte 0x1a  # DW_FORM_strx
         .byte 0x00  # EOM(1)
         .byte 0x00  # EOM(2)
         .byte 0x02  # Abbrev code
         .byte 0x41  # DW_TAG_type_unit
         .byte 0x01  # DW_CHILDREN_yes
         .byte 0x03  # DW_AT_name
         .byte 0x1a  # DW_FORM_strx
-        .byte 0x72  # DW_AT_str_offsets_base
-        .byte 0x17  # DW_FORM_sec_offset
         .byte 0x00  # EOM(1)
         .byte 0x00  # EOM(2)
         .byte 0x03  # Abbrev code
         .byte 0x13  # DW_TAG_structure_type
         .byte 0x00  # DW_CHILDREN_no (no members)
         .byte 0x03  # DW_AT_name
         .byte 0x1a  # DW_FORM_strx
         .byte 0x00  # EOM(1)
         .byte 0x00  # EOM(2)
+        .byte 0x04  # Abbrev code
+        .byte 0x11  # DW_TAG_compile_unit
+        .byte 0x00  # DW_CHILDREN_no
+        .byte 0x25  # DW_AT_producer
+        .short 0x3e82  # DW_FORM_GNU_str_index
+        .byte 0x03  # DW_AT_name
+        .short 0x3e82  # DW_FORM_GNU_str_index
+        .byte 0x03  # DW_AT_name
+        .short 0x3e82  # DW_FORM_GNU_str_index
+        .byte 0x00  # EOM(1)
+        .byte 0x00  # EOM(2)
         .byte 0x00  # EOM(3)
 abbrev_end:
 
         .section .debug_info.dwo,"",@progbits
 
 # DWARF v5 CU header.
 CU1_5_start:
         .long  CU1_5_end-CU1_5_version  # Length of Unit
 CU1_5_version:
         .short 5               # DWARF version number
         .byte 1                # DWARF Unit Type
         .byte 8                # Address Size (in bytes)
         .long .debug_abbrev.dwo # Offset Into Abbrev. Section
-# The compile-unit DIE, which has a DW_AT_producer, DW_AT_name,
-# DW_AT_str_offsets and DW_AT_compdir.
+# The compile-unit DIE, which has a DW_AT_producer, DW_AT_name
+# and DW_AT_compdir.
         .byte 1                # Abbreviation code
         .byte 0                # The index of the producer string
         .byte 1                # The index of the CU name string
-# The DW_AT_str_offsets_base attribute for CU1 contains the offset of CU1's
-# contribution relative to the start of object file 1's portion of the
-# .debug_str_offsets section.
-        .long .debug_str_offsets_base_CU1-.debug_str_offsets_object_file1
         .byte 2                # The index of the comp dir string
         .byte 0 # NULL
 CU1_5_end:
 
 CU2_5_start:
         .long  CU2_5_end-CU2_5_version  # Length of Unit
 CU2_5_version:
         .short 5               # DWARF version number
         .byte 1                # DWARF Unit Type
         .byte 8                # Address Size (in bytes)
         .long .debug_abbrev.dwo # Offset Into Abbrev. Section
-# The compile-unit DIE, which has a DW_AT_producer, DW_AT_name,
-# DW_AT_str_offsets and DW_AT_compdir.
+# The compile-unit DIE, which has a DW_AT_producer, DW_AT_name
+# and DW_AT_compdir.
         .byte 1                # Abbreviation code
         .byte 0                # The index of the producer string
         .byte 1                # The index of the CU name string
-# The DW_AT_str_offsets_base attribute for CU2 contains the offset of CU2's
-# contribution relative to the start of object file 2's portion of the
-# .debug_str_offsets section.
-        .long .debug_str_offsets_base_CU2-.debug_str_offsets_object_file2
         .byte 2                # The index of the comp dir string
         .byte 0 # NULL
 CU2_5_end:
 
+CU3_4_start:
+        .long  CU3_4_end-CU3_4_version  # Length of Unit
+CU3_4_version:
+        .short 4               # DWARF version number
+        .long .debug_abbrev.dwo # Offset Into Abbrev. Section
+        .byte 8                # Address Size (in bytes)
+# The compile-unit DIE, which has a DW_AT_producer, DW_AT_name
+# and DW_AT_compdir.
+        .byte 4                # Abbreviation code
+        .byte 0                # The index of the producer string
+        .byte 1                # The index of the CU name string
+        .byte 2                # The index of the comp dir string
+        .byte 0 # NULL
+CU3_4_end:
+
         .section .debug_types.dwo,"",@progbits
 # DWARF v5 Type unit header.
 TU1_5_start:
         .long  TU1_5_end-TU1_5_version  # Length of Unit
 TU1_5_version:
         .short 5               # DWARF version number
         .byte 2                # DWARF Unit Type
         .byte 8                # Address Size (in bytes)
         .long .debug_abbrev.dwo    # Offset Into Abbrev. Section
         .quad 0x0011223344556677 # Type Signature
         .long TU1_5_type-TU1_5_start # Type offset
 # The type-unit DIE, which has a name.
         .byte 2                # Abbreviation code
-        .byte 0                # Index of the unit type name string
-# The DW_AT_str_offsets_base attribute for TU1 contains the offset of TU1's
-# contribution relative to the start of object file 1's portion of the
-# .debug_str_offsets section.
-        .long .debug_str_offsets_base_TU1-.debug_str_offsets_object_file1
+        .byte 3                # Index of the unit type name string
 # The type DIE, which has a name.
 TU1_5_type:
         .byte 3                # Abbreviation code
-        .byte 1                # Index of the type name string
+        .byte 4                # Index of the type name string
         .byte 0 # NULL
         .byte 0 # NULL
 TU1_5_end:
 
 TU2_5_start:
         .long  TU2_5_end-TU2_5_version  # Length of Unit
 TU2_5_version:
         .short 5               # DWARF version number
         .byte 2                # DWARF Unit Type
         .byte 8                # Address Size (in bytes)
         .long .debug_abbrev.dwo    # Offset Into Abbrev. Section
         .quad 0x00aabbccddeeff99 # Type Signature
         .long TU2_5_type-TU2_5_start # Type offset
 # The type-unit DIE, which has a name.
         .byte 2                # Abbreviation code
-        .byte 0                # Index of the unit type name string
-# The DW_AT_str_offsets_base attribute for TU2 contains the offset of TU2's
-# contribution relative to the start of object file 2's portion of the
-# .debug_str_offsets section.
-        .long .debug_str_offsets_base_TU2-.debug_str_offsets_object_file2
+        .byte 3                # Index of the unit type name string
 # The type DIE, which has a name.
 TU2_5_type:
         .byte 3                # Abbreviation code
-        .byte 1                # Index of the type name string
+        .byte 4                # Index of the type name string
         .byte 0 # NULL
         .byte 0 # NULL
 TU2_5_end:
 
         .section .debug_cu_index,"",@progbits
         # The index header
         .long 2                # Version 
         .long 3                # Columns of contribution matrix
-        .long 2                # number of units
-        .long 2                # number of hash buckets in table
+        .long 3                # number of units
+        .long 3                # number of hash buckets in table
 
-        # The signatures for both CUs.
+        # The signatures for all CUs.
         .quad 0xddeeaaddbbaabbee # signature 1
         .quad 0xff00ffeeffaaff00 # signature 2
+        .quad 0xf00df00df00df00d # signature 2
         # The indexes for both CUs.
         .long 1                # index 1
         .long 2                # index 2
-        # The sections to which both CUs contribute.
+        .long 3                # index 3
+        # The sections to which all CUs contribute.
         .long 1                # DW_SECT_INFO
         .long 3                # DW_SECT_ABBREV
         .long 6                # DW_SECT_STR_OFFSETS
 
-        # The starting offsets of both CU's contributions to info,
+        # The starting offsets of all CU's contributions to info,
         # abbrev and string offsets table.
         .long CU1_5_start-.debug_info.dwo                   
         .long 0
-        .long .debug_str_offsets_object_file1-.debug_str_offsets.dwo
+        .long .debug_str_offsets_object_file1_start-.debug_str_offsets.dwo
         .long CU2_5_start-.debug_info.dwo
         .long 0
-        .long .debug_str_offsets_object_file2-.debug_str_offsets.dwo
+        .long .debug_str_offsets_object_file2_start-.debug_str_offsets.dwo
+        .long CU3_4_start-.debug_info.dwo
+        .long 0
+        .long .debug_str_offsets_object_file3_start-.debug_str_offsets.dwo
 
-        # The lengths of both CU's contributions to info, abbrev and
+        # The lengths of all CU's contributions to info, abbrev and
         # string offsets table.
         .long CU1_5_end-CU1_5_start
         .long abbrev_end-.debug_abbrev.dwo
-        .long .debug_str_offsets_end_CU1-.debug_str_offsets_start_CU1
+        .long .debug_str_offsets_object_file1_end-.debug_str_offsets_object_file1_start
         .long CU2_5_end-CU2_5_start
         .long abbrev_end-.debug_abbrev.dwo
-        .long .debug_str_offsets_end_CU2-.debug_str_offsets_start_CU2
+        .long .debug_str_offsets_object_file2_end-.debug_str_offsets_object_file2_start
+        .long CU3_4_end-CU3_4_start
+        .long abbrev_end-.debug_abbrev.dwo
+        .long .debug_str_offsets_object_file3_end-.debug_str_offsets_object_file3_start
 
         .section .debug_tu_index,"",@progbits
         # The index header
         .long 2                # Version 
         .long 3                # Columns of contribution matrix
         .long 2                # number of units
         .long 2                # number of hash buckets in table
 
         # The signatures for both TUs.
         .quad 0xeeaaddbbaabbeedd # signature 1
         .quad 0x00ffeeffaaff00ff # signature 2
         # The indexes for both TUs.
         .long 1                # index 1
         .long 2                # index 2
         # The sections to which both TUs contribute.
         .long 2                # DW_SECT_TYPES
         .long 3                # DW_SECT_ABBREV
         .long 6                # DW_SECT_STR_OFFSETS
 
         # The starting offsets of both TU's contributions to info,
         # abbrev and string offsets table.
         .long TU1_5_start-.debug_types.dwo
         .long 0
-        .long .debug_str_offsets_object_file1-.debug_str_offsets.dwo
+        .long .debug_str_offsets_object_file1_start-.debug_str_offsets.dwo
         .long TU2_5_start-.debug_types.dwo
         .long 0
-        .long .debug_str_offsets_object_file2-.debug_str_offsets.dwo
+        .long .debug_str_offsets_object_file2_start-.debug_str_offsets.dwo
 
         # The lengths of both TU's contributions to info, abbrev and
         # string offsets table.
         .long TU1_5_end-TU1_5_start
         .long abbrev_end-.debug_abbrev.dwo
-        .long .debug_str_offsets_end_TU1-.debug_str_offsets_start_TU1
+        .long .debug_str_offsets_object_file1_end-.debug_str_offsets_object_file1_start
         .long TU2_5_end-TU2_5_start
         .long abbrev_end-.debug_abbrev.dwo
-        .long .debug_str_offsets_end_TU2-.debug_str_offsets_start_TU2
+        .long .debug_str_offsets_object_file2_end-.debug_str_offsets_object_file2_start
 
 
 # Verify that the correct strings from each unit are displayed and that the
 # index for the .debug_str_offsets section has the right values.
 
 # CHECK:      Compile Unit
 # CHECK-NOT:  NULL
 # CHECK:      DW_TAG_compile_unit
 # CHECK-NEXT: DW_AT_producer [DW_FORM_strx] ( indexed (00000000) string = "Handmade DWARF producer")
 # CHECK-NEXT: DW_AT_name [DW_FORM_strx] ( indexed (00000001) string = "Compile_Unit_1")
-# CHECK-NEXT: DW_AT_str_offsets_base [DW_FORM_sec_offset] (0x00000008)
 # CHECK-NEXT: DW_AT_name [DW_FORM_strx] ( indexed (00000002) string = "/home/test/CU1")
 # CHECK-NOT:  NULL
 
 # CHECK:      Compile Unit
 # CHECK-NOT:  NULL
 # CHECK:      DW_TAG_compile_unit
 # CHECK-NEXT: DW_AT_producer [DW_FORM_strx] ( indexed (00000000) string = "Handmade DWARF producer")
 # CHECK-NEXT: DW_AT_name [DW_FORM_strx] ( indexed (00000001) string = "Compile_Unit_2")
-# CHECK-NEXT: DW_AT_str_offsets_base [DW_FORM_sec_offset] (0x00000008)
 # CHECK-NEXT: DW_AT_name [DW_FORM_strx] ( indexed (00000002) string = "/home/test/CU2")
 # 
 # CHECK:      Type Unit
 # CHECK-NOT:  NULL
 # CHECK:      DW_TAG_type_unit
-# CHECK-NEXT: DW_AT_name [DW_FORM_strx] ( indexed (00000000) string = "Type_Unit_1")
-# CHECK-NEXT: DW_AT_str_offsets_base [DW_FORM_sec_offset] (0x0000001c)
+# CHECK-NEXT: DW_AT_name [DW_FORM_strx] ( indexed (00000003) string = "Type_Unit_1")
 # CHECK-NOT:  NULL
 # CHECK:      DW_TAG_structure_type
-# CHECK-NEXT: DW_AT_name [DW_FORM_strx] ( indexed (00000001) string = "MyStruct_1")
+# CHECK-NEXT: DW_AT_name [DW_FORM_strx] ( indexed (00000004) string = "MyStruct_1")
 #
 # CHECK:      Type Unit
 # CHECK-NOT:  NULL
 # CHECK:      DW_TAG_type_unit
-# CHECK-NEXT: DW_AT_name [DW_FORM_strx] ( indexed (00000000) string = "Type_Unit_2")
-# CHECK-NEXT: DW_AT_str_offsets_base [DW_FORM_sec_offset] (0x0000001c)
+# CHECK-NEXT: DW_AT_name [DW_FORM_strx] ( indexed (00000003) string = "Type_Unit_2")
 # CHECK-NOT:  NULL
 # CHECK:      DW_TAG_structure_type
-# CHECK-NEXT: DW_AT_name [DW_FORM_strx] ( indexed (00000001) string = "MyStruct_2")
+# CHECK-NEXT: DW_AT_name [DW_FORM_strx] ( indexed (00000004) string = "MyStruct_2")
 
 # Verify the correct offets of the compile and type units contributions in the
 # index tables.
 
 # CHECK:      .debug_cu_index contents:
 # CHECK-NOT:  contents:
 # CHECK:      1 0xddeeaaddbbaabbee [{{0x[0-9a-f]*, 0x[0-9a-f]*}}) [{{0x[0-9a-f]*, 0x[0-9a-f]*}})
 # CHECK-SAME: [0x00000000
 # CHECK-NEXT: 2 0xff00ffeeffaaff00 [{{0x[0-9a-f]*, 0x[0-9a-f]*}}) [{{0x[0-9a-f]*, 0x[0-9a-f]*}})
-# CHECK-SAME: [0x00000024
+# CHECK-SAME: [0x0000001c
 
 # CHECK:      .debug_tu_index contents:
 # CHECK-NOT:  contents:
 # CHECK:      1 0xeeaaddbbaabbeedd [{{0x[0-9a-f]*, 0x[0-9a-f]*}}) [{{0x[0-9a-f]*, 0x[0-9a-f]*}})
 # CHECK-SAME: [0x00000000
 # CHECK-NEXT: 2 0x00ffeeffaaff00ff [{{0x[0-9a-f]*, 0x[0-9a-f]*}}) [{{0x[0-9a-f]*, 0x[0-9a-f]*}})
-# CHECK:      [0x00000024
+# CHECK:      [0x0000001c

llvm/trunk/test/DebugInfo/X86/dwarfdump-str-offsets-invalid-3.s

 dwo_str_TU_5_type:
         .asciz "V5_split_Mystruct"
 
 # A rudimentary abbrev section.
         .section .debug_abbrev,"",@progbits
         .byte 0x01  # Abbrev code
         .byte 0x11  # DW_TAG_compile_unit
         .byte 0x00  # DW_CHILDREN_no
+        .byte 0x72  # DW_AT_str_offsets_base
+        .byte 0x17  # DW_FORM_sec_offset
         .byte 0x00  # EOM(1)
         .byte 0x00  # EOM(2)
         .byte 0x00  # EOM(3)
 
 # A rudimentary compile unit to convince dwarfdump that we are dealing with a
 # DWARF v5 string offsets table.
         .section .debug_info,"",@progbits
 
 # DWARF v5 CU header.
         .long  CU1_5_end-CU1_5_version  # Length of Unit
 CU1_5_version:
         .short 5               # DWARF version number
         .byte 1                # DWARF Unit Type
         .byte 8                # Address Size (in bytes)
         .long .debug_abbrev    # Offset Into Abbrev. Section
 # A compile-unit DIE, which has no attributes.
         .byte 1                # Abbreviation code
+        .long .debug_str_offsets_base0
 CU1_5_end:
 
         .section .debug_str_offsets,"",@progbits
 # CU1's contribution
 # Invalid length
         .long 0xfffffffe
-        .long .debug_str_offsets_segment0_end-.debug_str_offsets_base0
         .short 5    # DWARF version
         .short 0    # Padding
 .debug_str_offsets_base0:
         .long str_producer
         .long str_CU1
         .long str_CU1_dir
 .debug_str_offsets_segment0_end:
 # CU2's contribution

llvm/trunk/test/DebugInfo/X86/dwarfdump-str-offsets-invalid-4.s

 str_CU1:
         .asciz "Compile_Unit_1"
 
 # A rudimentary abbrev section.
         .section .debug_abbrev,"",@progbits
         .byte 0x01  # Abbrev code
         .byte 0x11  # DW_TAG_compile_unit
         .byte 0x00  # DW_CHILDREN_no
+        .byte 0x72  # DW_AT_str_offsets_base
+        .byte 0x17  # DW_FORM_sec_offset
         .byte 0x00  # EOM(1)
         .byte 0x00  # EOM(2)
         .byte 0x00  # EOM(3)
 
 # A rudimentary compile unit to convince dwarfdump that we are dealing with a
 # DWARF v5 string offsets table.
         .section .debug_info,"",@progbits
 
 # DWARF v5 CU header.
         .long  CU1_5_end-CU1_5_version  # Length of Unit
 CU1_5_version:
         .short 5               # DWARF version number
         .byte 1                # DWARF Unit Type
         .byte 8                # Address Size (in bytes)
         .long .debug_abbrev    # Offset Into Abbrev. Section
 # A compile-unit DIE, which has no attributes.
         .byte 1                # Abbreviation code
+        .long .debug_str_offsets_base0
 CU1_5_end:
 
 # Every unit contributes to the string_offsets table.
         .section .debug_str_offsets,"",@progbits
 # CU1's contribution
 # The length is not a multiple of 4. Check that we don't read off the
 # end.
         .long .debug_str_offsets_segment0_end-.debug_str_offsets_base0
         .short 5    # DWARF version
         .short 0    # Padding
 .debug_str_offsets_base0:
         .long str_producer
         .long str_CU1
         .byte 0
 .debug_str_offsets_segment0_end:
 
 # INVALIDLENGTH:             .debug_str_offsets contents:
 # INVALIDLENGTH-NOT:         contents:
-# INVALIDLENGTH:             error: contribution to string offsets table in section .debug_str_offsets has invalid length.
+# INVALIDLENGTH:             error: invalid contribution to string offsets table in section .debug_str_offsets.

llvm/trunk/test/DebugInfo/X86/dwarfdump-str-offsets-invalid-6.s

+# RUN: llvm-mc -triple x86_64-unknown-linux %s -filetype=obj -o %t.o
+# RUN: llvm-dwarfdump -v %t.o | FileCheck --check-prefix=OVERLAP %s
+#
+# Test object to verify that llvm-dwarfdump handles an invalid string offsets
+# table with overlapping contributions.
+
+        .section .debug_str,"MS",@progbits,1
+str_producer:
+        .asciz "Handmade DWARF producer"
+str_CU1:
+        .asciz "Compile_Unit_1"
+str_CU1_dir:
+        .asciz "/home/test/CU1"
+str_CU2:
+        .asciz "Compile_Unit_2"
+str_CU2_dir:
+        .asciz "/home/test/CU2"
+str_TU:
+        .asciz "Type_Unit"
+str_TU_type:
+        .asciz "MyStruct"
+
+        .section .debug_str.dwo,"MS",@progbits,1
+dwo_str_CU_5_producer:
+        .asciz "Handmade split DWARF producer"
+dwo_str_CU_5_name:
+        .asciz "V5_split_compile_unit"
+dwo_str_CU_5_comp_dir:
+        .asciz "/home/test/splitCU"
+dwo_str_TU_5:
+        .asciz "V5_split_type_unit"
+dwo_str_TU_5_type:
+        .asciz "V5_split_Mystruct"
+
+# A rudimentary abbrev section.
+        .section .debug_abbrev,"",@progbits
+        .byte 0x01  # Abbrev code
+        .byte 0x11  # DW_TAG_compile_unit
+        .byte 0x00  # DW_CHILDREN_no
+        .byte 0x72  # DW_AT_str_offsets_base
+        .byte 0x17  # DW_FORM_sec_offset
+        .byte 0x00  # EOM(1)
+        .byte 0x00  # EOM(2)
+        .byte 0x00  # EOM(3)
+
+        .section .debug_info,"",@progbits
+# DWARF v5 CU header.
+        .long  CU1_5_end-CU1_5_version  # Length of Unit
+CU1_5_version:
+        .short 5               # DWARF version number
+        .byte 1                # DWARF Unit Type
+        .byte 8                # Address Size (in bytes)
+        .long .debug_abbrev    # Offset Into Abbrev. Section
+# A compile-unit DIE, which has no attributes.
+        .byte 1                # Abbreviation code
+        .long .debug_str_offsets_base0
+CU1_5_end:
+
+# DWARF v5 CU header.
+        .long  CU2_5_end-CU2_5_version  # Length of Unit
+CU2_5_version:
+        .short 5               # DWARF version number
+        .byte 1                # DWARF Unit Type
+        .byte 8                # Address Size (in bytes)
+        .long .debug_abbrev    # Offset Into Abbrev. Section
+# A compile-unit DIE, which has no attributes.
+        .byte 1                # Abbreviation code
+        .long .debug_str_offsets_base1
+CU2_5_end:
+
+        .section .debug_str_offsets,"",@progbits
+# CU1's contribution
+        .long .debug_str_offsets_segment1_end-.debug_str_offsets_base0
+        .short 5    # DWARF version
+        .short 0    # Padding
+.debug_str_offsets_base0:
+        .long str_producer
+        .long str_CU1
+        .long str_CU1_dir
+.debug_str_offsets_segment0_end:
+# CU2's contribution
+# Overlapping with CU1's contribution
+        .long .debug_str_offsets_segment1_end-.debug_str_offsets_base1
+        .short 5    # DWARF version
+        .short 0    # Padding
+.debug_str_offsets_base1:
+        .long str_producer
+        .long str_CU2
+        .long str_CU2_dir
+.debug_str_offsets_segment1_end:
+
+# OVERLAP:            .debug_str_offsets contents:
+# OVERLAP-NOT:        contents:
+# OVERLAP:            error: overlapping contributions to string offsets table in section .debug_str_offsets.

llvm/trunk/test/DebugInfo/X86/dwarfdump-str-offsets-macho.s

Ldebug_str_offsets_base0:
         .long str_producer
         .long str_CU1
         .long str_CU1_dir
         .long str_Subprogram
         .long str_Variable1
         .long str_Variable2
         .long str_Variable3
 Ldebug_str_offsets_segment0_end:
-# CU2's contribution
-        .long Ldebug_str_offsets_segment1_end-Ldebug_str_offsets_base1
+# A 4-byte gap.
+        .long 0
+# CU2's contribution (DWARF64 format)
+        .long 0xffffffff
+        .quad Ldebug_str_offsets_segment1_end-Ldebug_str_offsets_base1
         .short 5    # DWARF version
         .short 0    # Padding
 Ldebug_str_offsets_base1:
-        .long str_producer
-        .long str_CU2
-        .long str_CU2_dir
+        .quad str_producer
+        .quad str_CU2
+        .quad str_CU2_dir
 Ldebug_str_offsets_segment1_end:
 # The TU's contribution
         .long Ldebug_str_offsets_segment2_end-Ldebug_str_offsets_base2
         .short 5    # DWARF version
         .short 0    # Padding
 Ldebug_str_offsets_base2:
         .long str_TU
         .long str_TU_type

 # COMMON-NOT:  NULL
 # COMMON:      DW_TAG_variable
 # COMMON-NEXT: DW_AT_name [DW_FORM_strx4] ( indexed (00000006) string = "MyVar3")
 # 
 # Second compile unit (b.cpp)
 # COMMON:      DW_TAG_compile_unit
 # COMMON-NEXT: DW_AT_producer [DW_FORM_strx] ( indexed (00000000) string = "Handmade DWARF producer")
 # COMMON-NEXT: DW_AT_name [DW_FORM_strx] ( indexed (00000001) string = "Compile_Unit_2")
-# COMMON-NEXT: DW_AT_str_offsets_base [DW_FORM_sec_offset] (0x0000002c)
+# COMMON-NEXT: DW_AT_str_offsets_base [DW_FORM_sec_offset] (0x00000038)
 # COMMON-NEXT: DW_AT_comp_dir [DW_FORM_strx] ( indexed (00000002) string = "/home/test/CU2")
 # 
 # The type unit
 # COMMON:      .debug_types contents:
 # COMMON:      DW_TAG_type_unit
 # COMMON-NEXT: DW_AT_name [DW_FORM_strx] ( indexed (00000000) string = "Type_Unit")
-# COMMON-NEXT: DW_AT_str_offsets_base [DW_FORM_sec_offset]       (0x00000040)
+# COMMON-NEXT: DW_AT_str_offsets_base [DW_FORM_sec_offset]       (0x00000058)
 # COMMON:      DW_TAG_structure_type
 # COMMON-NEXT: DW_AT_name [DW_FORM_strx] ( indexed (00000001) string = "MyStruct")
 # 
 # The .debug_str_offsets section
 # COMMON:      .debug_str_offsets contents:
-# COMMON-NEXT: 0x00000000: Contribution size = 28, Version = 5
+# COMMON-NEXT: 0x00000000: Contribution size = 28, Format = DWARF32, Version = 5
 # COMMON-NEXT: 0x00000008: 00000000 "Handmade DWARF producer"
 # COMMON-NEXT: 0x0000000c: 00000018 "Compile_Unit_1"
 # COMMON-NEXT: 0x00000010: 00000027 "/home/test/CU1"
 # COMMON-NEXT: 0x00000014: 00000067 "MyFunc"
 # COMMON-NEXT: 0x00000018: 0000006e "MyVar1"
 # COMMON-NEXT: 0x0000001c: 00000075 "MyVar2"
 # COMMON-NEXT: 0x00000020: 0000007c "MyVar3"
-# COMMON-NEXT: 0x00000024: Contribution size = 12, Version = 5
-# COMMON-NEXT: 0x0000002c: 00000000 "Handmade DWARF producer"
-# COMMON-NEXT: 0x00000030: 00000036 "Compile_Unit_2"
-# COMMON-NEXT: 0x00000034: 00000045 "/home/test/CU2"
-# COMMON-NEXT: 0x00000038: Contribution size = 8, Version = 5
-# COMMON-NEXT: 0x00000040: 00000054 "Type_Unit"
-# COMMON-NEXT: 0x00000044: 0000005e "MyStruct"
+# COMMON-NEXT: 0x00000024: Gap, length = 4
+# COMMON-NEXT: 0x00000028: Contribution size = 24, Format = DWARF64, Version = 5
+# COMMON-NEXT: 0x00000038: 00000000 "Handmade DWARF producer"
+# COMMON-NEXT: 0x00000040: 00000036 "Compile_Unit_2"
+# COMMON-NEXT: 0x00000048: 00000045 "/home/test/CU2"
+# COMMON-NEXT: 0x00000050: Contribution size = 8, Format = DWARF32, Version = 5
+# COMMON-NEXT: 0x00000058: 00000054 "Type_Unit"
+# COMMON-NEXT: 0x0000005c: 0000005e "MyStruct"

llvm/trunk/test/DebugInfo/X86/dwarfdump-str-offsets.s

.debug_str_offsets_base0:
         .long str_producer
         .long str_CU1
         .long str_CU1_dir
         .long str_Subprogram
         .long str_Variable1
         .long str_Variable2
         .long str_Variable3
 .debug_str_offsets_segment0_end:
-# CU2's contribution
-        .long .debug_str_offsets_segment1_end-.debug_str_offsets_base1
+# A 4-byte gap.
+        .long 0
+# CU2's contribution in DWARF64 format
+        .long 0xffffffff
+        .quad .debug_str_offsets_segment1_end-.debug_str_offsets_base1
         .short 5    # DWARF version
         .short 0    # Padding
 .debug_str_offsets_base1:
-        .long str_producer
-        .long str_CU2
-        .long str_CU2_dir
+        .quad str_producer
+        .quad str_CU2
+        .quad str_CU2_dir
 .debug_str_offsets_segment1_end:
 # The TU's contribution
         .long .debug_str_offsets_segment2_end-.debug_str_offsets_base2
         .short 5    # DWARF version
         .short 0    # Padding
 .debug_str_offsets_base2:
         .long str_TU
         .long str_TU_type
 .debug_str_offsets_segment2_end:
 
         .section .debug_str.dwo,"MS",@progbits,1
 dwo_str_CU_5_producer:
         .asciz "Handmade split DWARF producer"
 dwo_str_CU_5_name:
         .asciz "V5_split_compile_unit"
 dwo_str_CU_5_comp_dir:
         .asciz "/home/test/splitCU"
 dwo_str_TU_5:
         .asciz "V5_split_type_unit"
 dwo_str_TU_5_type:
         .asciz "V5_split_Mystruct"
 
         .section .debug_str_offsets.dwo,"",@progbits
-# The split CU's contribution
+# One contribution only in a .dwo file
         .long .debug_dwo_str_offsets_segment0_end-.debug_dwo_str_offsets_base0
         .short 5    # DWARF version
         .short 0    # Padding
 .debug_dwo_str_offsets_base0:
         .long dwo_str_CU_5_producer-.debug_str.dwo
         .long dwo_str_CU_5_name-.debug_str.dwo
         .long dwo_str_CU_5_comp_dir-.debug_str.dwo
-.debug_dwo_str_offsets_segment0_end:
-# The split TU's contribution
-        .long .debug_dwo_str_offsets_segment1_end-.debug_dwo_str_offsets_base1
-        .short 5    # DWARF version
-        .short 0    # Padding
-.debug_dwo_str_offsets_base1:
         .long dwo_str_TU_5-.debug_str.dwo
         .long dwo_str_TU_5_type-.debug_str.dwo
-.debug_dwo_str_offsets_segment1_end:
+.debug_dwo_str_offsets_segment0_end:
 
 # All CUs/TUs use the same abbrev section for simplicity.
         .section .debug_abbrev,"",@progbits
         .byte 0x01  # Abbrev code
         .byte 0x11  # DW_TAG_compile_unit
         .byte 0x01  # DW_CHILDREN_yes
         .byte 0x25  # DW_AT_producer
         .byte 0x1a  # DW_FORM_strx
# And a .dwo copy of a subset for the .dwo sections.
         .section .debug_abbrev.dwo,"",@progbits
         .byte 0x01  # Abbrev code
         .byte 0x11  # DW_TAG_compile_unit
         .byte 0x00  # DW_CHILDREN_no
         .byte 0x25  # DW_AT_producer
         .byte 0x1a  # DW_FORM_strx
         .byte 0x03  # DW_AT_name
         .byte 0x1a  # DW_FORM_strx
-        .byte 0x72  # DW_AT_str_offsets_base
-        .byte 0x17  # DW_FORM_sec_offset
         .byte 0x1b  # DW_AT_comp_dir
         .byte 0x1a  # DW_FORM_strx
         .byte 0x00  # EOM(1)
         .byte 0x00  # EOM(2)
         .byte 0x02  # Abbrev code
         .byte 0x41  # DW_TAG_type_unit
         .byte 0x01  # DW_CHILDREN_yes
         .byte 0x03  # DW_AT_name
         .byte 0x1a  # DW_FORM_strx
-        .byte 0x72  # DW_AT_str_offsets_base
-        .byte 0x17  # DW_FORM_sec_offset
         .byte 0x00  # EOM(1)
         .byte 0x00  # EOM(2)
         .byte 0x03  # Abbrev code
         .byte 0x13  # DW_TAG_structure_type
         .byte 0x00  # DW_CHILDREN_no (no members)
         .byte 0x03  # DW_AT_name
         .byte 0x1a  # DW_FORM_strx
         .byte 0x00  # EOM(1)
CU_split_5_version:
         .byte 1                # DWARF Unit Type
         .byte 8                # Address Size (in bytes)
         .long .debug_abbrev.dwo  # Offset Into Abbrev Section
 # The compile-unit DIE, which has a DW_AT_producer, DW_AT_name, 
 # DW_AT_str_offsets and DW_AT_compdir.
         .byte 1                # Abbreviation code
         .byte 0                # The index of the producer string
         .byte 1                # The index of the CU name string
-        .long .debug_dwo_str_offsets_base0-.debug_str_offsets.dwo
         .byte 2                # The index of the comp dir string
         .byte 0 # NULL
 CU_split_5_end:
         
         .section .debug_types.dwo,"",@progbits
 
 # 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)
         .long .debug_abbrev.dwo  # Offset Into Abbrev Section
         .quad 0x8899aabbccddeeff # Type Signature
         .long TU_split_5_type-TU_split_5_start  # Type offset
 # The type-unit DIE, which has a name.
         .byte 2                # Abbreviation code
-        .byte 0                # The index of the type unit name string
-        .long .debug_dwo_str_offsets_base1-.debug_str_offsets.dwo 
+        .byte 3                # The index of the type unit name string
 # The type DIE, which has a name.
 TU_split_5_type:
         .byte 3                # Abbreviation code
-        .byte 1                # The index of the type name string
+        .byte 4                # The index of the type name string
         .byte 0 # NULL
         .byte 0 # NULL
 TU_split_5_end:
 
 # We are using a hand-constructed object file and are interest in the correct
 # diplay of the DW_str_offsetsbase attribute, the correct display of strings
 # and the dump of the .debug_str_offsets[.dwo] table.
 

 # COMMON-NOT:  NULL
 # COMMON:      DW_TAG_variable
 # COMMON-NEXT: DW_AT_name [DW_FORM_strx4] ( indexed (00000006) string = "MyVar3")
 # 
 # Second compile unit (b.cpp)
 # COMMON:      DW_TAG_compile_unit
 # COMMON-NEXT: DW_AT_producer [DW_FORM_strx] ( indexed (00000000) string = "Handmade DWARF producer")
 # COMMON-NEXT: DW_AT_name [DW_FORM_strx] ( indexed (00000001) string = "Compile_Unit_2")
-# COMMON-NEXT: DW_AT_str_offsets_base [DW_FORM_sec_offset] (0x0000002c)
+# COMMON-NEXT: DW_AT_str_offsets_base [DW_FORM_sec_offset] (0x00000038)
 # COMMON-NEXT: DW_AT_comp_dir [DW_FORM_strx] ( indexed (00000002) string = "/home/test/CU2")
 # 
 # The split CU
 # SPLIT:       .debug_info.dwo contents:
 # SPLIT-NOT:   contents:
 # SPLIT:       DW_TAG_compile_unit
 # SPLIT-NEXT:  DW_AT_producer [DW_FORM_strx] ( indexed (00000000) string = "Handmade split DWARF producer")
 # SPLIT-NEXT:  DW_AT_name [DW_FORM_strx] ( indexed (00000001) string = "V5_split_compile_unit")
-# SPLIT-NEXT:  DW_AT_str_offsets_base [DW_FORM_sec_offset] (0x00000008)
 # SPLIT-NEXT:  DW_AT_comp_dir [DW_FORM_strx] ( indexed (00000002) string = "/home/test/splitCU")
 # 
 # The type unit
 # COMMON:      .debug_types contents:
 # COMMON:      DW_TAG_type_unit
 # COMMON-NEXT: DW_AT_name [DW_FORM_strx] ( indexed (00000000) string = "Type_Unit")
-# COMMON-NEXT: DW_AT_str_offsets_base [DW_FORM_sec_offset]       (0x00000040)
 # COMMON:      DW_TAG_structure_type
 # COMMON-NEXT: DW_AT_name [DW_FORM_strx] ( indexed (00000001) string = "MyStruct")
 # 
 # The split type unit
 # SPLIT:       .debug_types.dwo contents:
 # SPLIT:       DW_TAG_type_unit
-# SPLIT-NEXT:  DW_AT_name [DW_FORM_strx] ( indexed (00000000) string = "V5_split_type_unit")
-# SPLIT-NEXT:  DW_AT_str_offsets_base [DW_FORM_sec_offset]       (0x0000001c)
+# SPLIT-NEXT:  DW_AT_name [DW_FORM_strx] ( indexed (00000003) string = "V5_split_type_unit")
 # SPLIT:       DW_TAG_structure_type
-# SPLIT-NEXT:  DW_AT_name [DW_FORM_strx] ( indexed (00000001) string = "V5_split_Mystruct")
+# SPLIT-NEXT:  DW_AT_name [DW_FORM_strx] ( indexed (00000004) string = "V5_split_Mystruct")
 # 
 # The .debug_str_offsets section
 # COMMON:      .debug_str_offsets contents:
-# COMMON-NEXT: 0x00000000: Contribution size = 28, Version = 5
+# COMMON-NEXT: 0x00000000: Contribution size = 28, Format = DWARF32, Version = 5
 # COMMON-NEXT: 0x00000008: 00000000 "Handmade DWARF producer"
 # COMMON-NEXT: 0x0000000c: 00000018 "Compile_Unit_1"
 # COMMON-NEXT: 0x00000010: 00000027 "/home/test/CU1"
 # COMMON-NEXT: 0x00000014: 00000067 "MyFunc"
 # COMMON-NEXT: 0x00000018: 0000006e "MyVar1"
 # COMMON-NEXT: 0x0000001c: 00000075 "MyVar2"
 # COMMON-NEXT: 0x00000020: 0000007c "MyVar3"
-# COMMON-NEXT: 0x00000024: Contribution size = 12, Version = 5
-# COMMON-NEXT: 0x0000002c: 00000000 "Handmade DWARF producer"
-# COMMON-NEXT: 0x00000030: 00000036 "Compile_Unit_2"
-# COMMON-NEXT: 0x00000034: 00000045 "/home/test/CU2"
-# COMMON-NEXT: 0x00000038: Contribution size = 8, Version = 5
-# COMMON-NEXT: 0x00000040: 00000054 "Type_Unit"
-# COMMON-NEXT: 0x00000044: 0000005e "MyStruct"
+# COMMON-NEXT: Gap, length = 4
+# COMMON-NEXT: 0x00000028: Contribution size = 24, Format = DWARF64, Version = 5
+# COMMON-NEXT: 0x00000038: 00000000 "Handmade DWARF producer"
+# COMMON-NEXT: 0x00000040: 00000036 "Compile_Unit_2"
+# COMMON-NEXT: 0x00000048: 00000045 "/home/test/CU2"
+# COMMON-NEXT: 0x00000050: Contribution size = 8, Format = DWARF32, Version = 5
+# COMMON-NEXT: 0x00000058: 00000054 "Type_Unit"
+# COMMON-NEXT: 0x0000005c: 0000005e "MyStruct"
 # 
 # SPLIT:       .debug_str_offsets.dwo contents:
-# SPLIT-NEXT:  0x00000000: Contribution size = 12, Version = 5
+# SPLIT-NEXT:  0x00000000: Contribution size = 20, Format = DWARF32, Version = 5
 # SPLIT-NEXT:  0x00000008: 00000000 "Handmade split DWARF producer"
 # SPLIT-NEXT:  0x0000000c: 0000001e "V5_split_compile_unit"
 # SPLIT-NEXT:  0x00000010: 00000034 "/home/test/splitCU"
-# SPLIT-NEXT:  0x00000014: Contribution size = 8, Version = 5
-# SPLIT-NEXT:  0x0000001c: 00000047 "V5_split_type_unit"
-# SPLIT-NEXT:  0x00000020: 0000005a "V5_split_Mystruct"
+# SPLIT-NEXT:  0x00000014: 00000047 "V5_split_type_unit"
+# SPLIT-NEXT:  0x00000018: 0000005a "V5_split_Mystruct"