[BOLT][DWARF] Slice .debug_str from the DWP for each CU (#159540)

Slice .debug_str from the DWP for each CU using .debug_str_offsets and
emit it, instead of directly copying the global .debug_str, in order to
address the bloat issue of DWO after updates. (more details here -
#155766 )

Author: Sockke

bolt/include/bolt/Core/DebugData.h

@@ -471,6 +471,12 @@ class DebugStrOffsetsWriter {
     return std::move(StrOffsetsBuffer);
   }
 
+  /// Returns strings of .debug_str_offsets.
+  StringRef getBufferStr() {
+    return StringRef(reinterpret_cast<const char *>(StrOffsetsBuffer->data()),
+                     StrOffsetsBuffer->size());
+  }
+
   /// Initializes Buffer and Stream.
   void initialize(DWARFUnit &Unit);
 
@@ -507,6 +513,12 @@ class DebugStrWriter {
     return std::move(StrBuffer);
   }
 
+  /// Returns strings of .debug_str.
+  StringRef getBufferStr() {
+    return StringRef(reinterpret_cast<const char *>(StrBuffer->data()),
+                     StrBuffer->size());
+  }
+
   /// Adds string to .debug_str.
   /// On first invocation it initializes internal data structures.
   uint32_t addString(StringRef Str);

bolt/lib/Rewrite/DWARFRewriter.cpp

@@ -1723,7 +1723,76 @@ StringRef getSectionName(const SectionRef &Section) {
   return Name;
 }
 
-// Extracts an appropriate slice if input is DWP.
+/// Extracts the slice of the .debug_str.dwo section for a given CU from a DWP
+/// file, based on the .debug_str_offsets.dwo section. This helps address DWO
+/// bloat that may occur after updates.
+///
+/// A slice of .debug_str.dwo may be composed of several non-contiguous
+/// fragments. These non-contiguous string views will be written out
+/// sequentially, avoiding the copying overhead caused by assembling them.
+///
+/// The .debug_str_offsets for the first CU often does not need to be updated,
+/// so copying is only performed when .debug_str_offsets requires updating.
+static void UpdateStrAndStrOffsets(StringRef StrDWOContent,
+                                   StringRef StrOffsetsContent,
+                                   SmallVectorImpl<StringRef> &StrDWOOutData,
+                                   std::string &StrOffsetsOutData,
+                                   unsigned DwarfVersion, bool IsLittleEndian) {
+  const llvm::endianness Endian =
+      IsLittleEndian ? llvm::endianness::little : llvm::endianness::big;
+  const uint64_t HeaderOffset = (DwarfVersion >= 5) ? 8 : 0;
+  constexpr size_t SizeOfOffset = sizeof(int32_t);
+  const uint64_t NumOffsets =
+      (StrOffsetsContent.size() - HeaderOffset) / SizeOfOffset;
+
+  DataExtractor Extractor(StrOffsetsContent, IsLittleEndian, 0);
+  uint64_t ExtractionOffset = HeaderOffset;
+
+  using StringFragment = DWARFUnitIndex::Entry::SectionContribution;
+  const auto getStringLength = [](StringRef Content,
+                                  uint64_t Offset) -> uint64_t {
+    size_t NullPos = Content.find('\0', Offset);
+    return (NullPos != StringRef::npos) ? (NullPos - Offset + 1) : 0;
+  };
+  const auto isContiguous = [](const StringFragment &Fragment,
+                               uint64_t NextOffset) -> bool {
+    return NextOffset == Fragment.getOffset() + Fragment.getLength();
+  };
+  std::optional<StringFragment> CurrentFragment;
+  uint64_t AccumulatedStrLen = 0;
+  for (uint64_t I = 0; I < NumOffsets; ++I) {
+    const uint64_t StrOffset = Extractor.getU32(&ExtractionOffset);
+    const uint64_t StringLength = getStringLength(StrDWOContent, StrOffset);
+    if (!CurrentFragment) {
+      // First init.
+      CurrentFragment = StringFragment(StrOffset, StringLength);
+    } else {
+      if (isContiguous(*CurrentFragment, StrOffset)) {
+        // Expanding the current fragment.
+        CurrentFragment->setLength(CurrentFragment->getLength() + StringLength);
+      } else {
+        // Saving the current fragment and start a new one.
+        StrDWOOutData.push_back(StrDWOContent.substr(
+            CurrentFragment->getOffset(), CurrentFragment->getLength()));
+        CurrentFragment = StringFragment(StrOffset, StringLength);
+      }
+    }
+    if (AccumulatedStrLen != StrOffset) {
+      // Updating str offsets.
+      if (StrOffsetsOutData.empty())
+        StrOffsetsOutData = StrOffsetsContent.str();
+      llvm::support::endian::write32(
+          &StrOffsetsOutData[HeaderOffset + I * SizeOfOffset],
+          static_cast<uint32_t>(AccumulatedStrLen), Endian);
+    }
+    AccumulatedStrLen += StringLength;
+  }
+  if (CurrentFragment)
+    StrDWOOutData.push_back(StrDWOContent.substr(CurrentFragment->getOffset(),
+                                                 CurrentFragment->getLength()));
+}
+
+// Exctracts an appropriate slice if input is DWP.
 // Applies patches or overwrites the section.
 std::optional<StringRef> updateDebugData(
     DWARFContext &DWCtx, StringRef SectionName, StringRef SectionContents,
@@ -1772,6 +1841,8 @@ std::optional<StringRef> updateDebugData(
       errs() << "BOLT-WARNING: unsupported debug section: " << SectionName
              << "\n";
     if (StrWriter.isInitialized()) {
+      if (CUDWOEntry)
+        return StrWriter.getBufferStr();
       OutputBuffer = StrWriter.releaseBuffer();
       return StringRef(reinterpret_cast<const char *>(OutputBuffer->data()),
                        OutputBuffer->size());
@@ -1786,6 +1857,8 @@ std::optional<StringRef> updateDebugData(
   }
   case DWARFSectionKind::DW_SECT_STR_OFFSETS: {
     if (StrOffstsWriter.isFinalized()) {
+      if (CUDWOEntry)
+        return StrOffstsWriter.getBufferStr();
       OutputBuffer = StrOffstsWriter.releaseBuffer();
       return StringRef(reinterpret_cast<const char *>(OutputBuffer->data()),
                        OutputBuffer->size());
@@ -1888,18 +1961,61 @@ void DWARFRewriter::writeDWOFiles(
     }
   }
 
+  StringRef StrDWOContent;
+  StringRef StrOffsetsContent;
+  llvm::SmallVector<StringRef, 3> StrDWOOutData;
+  std::string StrOffsetsOutData;
   for (const SectionRef &Section : File->sections()) {
     std::unique_ptr<DebugBufferVector> OutputData;
     StringRef SectionName = getSectionName(Section);
     if (SectionName == "debug_rnglists.dwo")
       continue;
     Expected<StringRef> ContentsExp = Section.getContents();
     assert(ContentsExp && "Invalid contents.");
+    if (IsDWP && SectionName == "debug_str.dwo") {
+      if (StrWriter.isInitialized())
+        StrDWOContent = StrWriter.getBufferStr();
+      else
+        StrDWOContent = *ContentsExp;
+      continue;
+    }
     if (std::optional<StringRef> OutData = updateDebugData(
             (*DWOCU)->getContext(), SectionName, *ContentsExp, KnownSections,
             *Streamer, *this, CUDWOEntry, DWOId, OutputData, RangeListssWriter,
-            LocWriter, StrOffstsWriter, StrWriter, OverridenSections))
+            LocWriter, StrOffstsWriter, StrWriter, OverridenSections)) {
+      if (IsDWP && SectionName == "debug_str_offsets.dwo") {
+        StrOffsetsContent = *OutData;
+        continue;
+      }
       Streamer->emitBytes(*OutData);
+    }
+  }
+
+  if (IsDWP) {
+    // Handling both .debug_str.dwo and .debug_str_offsets.dwo concurrently. In
+    // the original DWP, .debug_str is a deduplicated global table, and the
+    // .debug_str.dwo slice for a single CU needs to be extracted according to
+    // .debug_str_offsets.dwo.
+    UpdateStrAndStrOffsets(StrDWOContent, StrOffsetsContent, StrDWOOutData,
+                           StrOffsetsOutData, CU.getVersion(),
+                           (*DWOCU)->getContext().isLittleEndian());
+    auto SectionIter = KnownSections.find("debug_str.dwo");
+    if (SectionIter != KnownSections.end()) {
+      Streamer->switchSection(SectionIter->second.first);
+      for (size_t i = 0; i < StrDWOOutData.size(); ++i) {
+        StringRef OutData = StrDWOOutData[i];
+        if (!OutData.empty())
+          Streamer->emitBytes(OutData);
+      }
+    }
+    SectionIter = KnownSections.find("debug_str_offsets.dwo");
+    if (SectionIter != KnownSections.end()) {
+      Streamer->switchSection(SectionIter->second.first);
+      if (!StrOffsetsOutData.empty())
+        Streamer->emitBytes(StrOffsetsOutData);
+      else
+        Streamer->emitBytes(StrOffsetsContent);
+    }
   }
   Streamer->finish();
   TempOut->keep();