[lld-macho][nfc] rename parsed-section types & variables

[gkm]

Summary

This is an NFC diff that prepares for pruning & relocating __eh_frame.

Along the way, I made the following changes to ...

• clarify usage of section vs. subsection
• remove map & vec from type names
• disambiguate class Section from template parameter SectionHeader.

[gkm]

I missed a rename ...

• s/Subsection/ParsedSubsection/

[gkm]

lld/MachO/Writer.cpp

506

Gotta revert this

[int3]

The Map instead of Vec thing was intentional... a vector of pairs is semantically a map.

Not a huge fan of the Parsed stuff either, I don't think there's much confusion about when we are working with parsed vs serialized structures...

As for the offset member... seems kind of hard to evaluate it without the context of the __eh_frame.

[gkm]

In D113241#3113825, @int3 wrote:

The Map instead of Vec thing was intentional... a vector of pairs is semantically a map.

I was thinking of "map" operationally, i.e., used for associative lookups.

Not a huge fan of the Parsed stuff either, I don't think there's much confusion about when we are working with parsed vs serialized structures...

That sounds like mitigated speech for "veto". Correct me if I misinterpret.

As for the offset member... seems kind of hard to evaluate it without the context of the __eh_frame.

FYI, it morphed into an address member. I'll put-up a new __eh_frame diff.

[gkm]

In D113241#3113825, @int3 wrote:

Not a huge fan of the Parsed stuff either, I don't think there's much confusion about when we are working with parsed vs serialized structures...

Prepending Parsed was a late change and I'm OK with reversing it. My concern was that Section is misleading, since it is not a base class of InputSection or OutputSection as the name might suggest. No big deal though. The more important change was s/Section/SectionHeader/ for the template arg.

[thevinster]

I'm a big fan of the variable names switching from s/subsections/sections, s/subsectionMap/subsections. It's clearer to me that there are layers whereas the original subsections->subsectionMap layer is not as easily decipherable. Section->SectionHeader makes sense to me, but I would like it if we had some consistency, which means also renaming SegmentCommand->SegmentCommandHeader, NList->NListHeader, etc.

[gkm]

In D113241#3114368, @thevinster wrote:

I'm a big fan of the variable names switching from s/subsections/sections, s/subsectionMap/subsections. It's clearer to me that there are layers whereas the original subsections->subsectionMap layer is not as easily decipherable.

Yes, that really is the crux of this effort. It is good to hear that this change helps newcomers to the code.

Section->SectionHeader makes sense to me, but I would like it if we had some consistency, which means also renaming SegmentCommand->SegmentCommandHeader, NList->NListHeader, etc.

In InputFiles.cpp, sections are front & center, and disambiguating the several varieties of them is important. Section->SectionHeader has value because it offloads the overloaded name Section.

OTOH, SegmentCommand is not overloaded, and used only once. Nlist is not overloaded, and isn't even a header but rather an element in a vector.

[thevinster]

OTOH, SegmentCommand is not overloaded, and used only once. Nlist is not overloaded, and isn't even a header but rather an element in a vector.

Makes sense. I guess what I was getting at is whether we can make it more obvious that Nlist or SegmentCommand are structures defined from MachOStructs.h. If this is the best we can do, then it's fine as is, but assuming we could find such a pattern, then it could also apply to Section to make it consistent.

[gkm]

Major FUBAR revision -- ignore this one.

[gkm]

Revert previous revision

[oontvoo]

also +1 to the s/section/subsection and SectionHeader renaming(much clearer now)

I've no strong opinion on Map vs Vec but generally I'm not a fan of putting the type (ie., vect, list, map, etc) into the variable name anyway. Seems redundant.
Perhaps if we could make the name clear enough, then it wouldn't be necessary to add the extra clarification.

lld/MachO/InputSection.cpp

80 ↗

(On Diff #385406)

IMHO, it is not particularly clearer to say ParsedSection here

[int3]

That sounds like mitigated speech for "veto". Correct me if I misinterpret.

It's vote, not veto ;)

Yeah the s/subsections/sections/ and s/subsecMap/subsections/ thing looks reasonable. +1 to Vy's opinion that types should ideally be left out of the name anyway; let's drop both the Map and Vec.

I agree with Vincent's opinion that Section -> SectionHeader breaks consistency with the other MachOStructs.h types. The convention I was going for was to have all non-serialized section/segment types be prefixed by "Input" or "Output". But I don't feel super strongly about this, so if you prefer SectionHeader, go for it.

[gkm]

• revert Parsed prefix to shorten class names and lower impact of this diff

[thevinster]

Gucci.

lld/MachO/InputSection.cpp

80 ↗

(On Diff #387341)

Missed this part?

[oontvoo]

In D113241#3132610, @thevinster wrote:

Gucci.

👜 ???

[oontvoo]

lld/MachO/InputSection.cpp

80 ↗

(On Diff #387341)

right :) maybe just print the invalid type here

[thevinster]

In D113241#3132635, @oontvoo wrote:

In D113241#3132610, @thevinster wrote:

Gucci.

👜 ???

Haha. https://slangit.com/meaning/gucci

[int3]

Thanks!

[gkm]

• fix one remaining s/ParsedSection/Section/

lld/MachO/Driver.cpp

for (const Arg *arg : args) {
     }
   }
 }
 
 static void gatherInputSections() {
   TimeTraceScope timeScope("Gathering input sections");
   int inputOrder = 0;
   for (const InputFile *file : inputFiles) {
-    for (const SubsectionMap &map : file->subsections) {
+    for (const Section &section : file->sections) {
       ConcatOutputSection *osec = nullptr;
-      for (const SubsectionEntry &entry : map) {
-        if (auto *isec = dyn_cast<ConcatInputSection>(entry.isec)) {
+      for (const Subsection &subsection : section.subsections) {
+        if (auto *isec = dyn_cast<ConcatInputSection>(subsection.isec)) {
           if (isec->isCoalescedWeak())
             continue;
           if (isec->getSegName() == segment_names::ld) {
             assert(isec->getName() == section_names::compactUnwind);
             continue;
           }
           isec->outSecOff = inputOrder++;
           if (!osec)
             osec = ConcatOutputSection::getOrCreateForInput(isec);
           isec->parent = osec;
           inputSections.push_back(isec);
-        } else if (auto *isec = dyn_cast<CStringInputSection>(entry.isec)) {
+        } else if (auto *isec =
+                       dyn_cast<CStringInputSection>(subsection.isec)) {
           if (in.cStringSection->inputOrder == UnspecifiedInputOrder)
             in.cStringSection->inputOrder = inputOrder++;
           in.cStringSection->addInput(isec);
-        } else if (auto *isec = dyn_cast<WordLiteralInputSection>(entry.isec)) {
+        } else if (auto *isec =
+                       dyn_cast<WordLiteralInputSection>(subsection.isec)) {
           if (in.wordLiteralSection->inputOrder == UnspecifiedInputOrder)
             in.wordLiteralSection->inputOrder = inputOrder++;
           in.wordLiteralSection->addInput(isec);
         } else {
           llvm_unreachable("unexpected input section kind");
         }
       }
     }

lld/MachO/InputFiles.h

 
 // If --reproduce option is given, all input files are written
 // to this tar archive.
 extern std::unique_ptr<llvm::TarWriter> tar;
 
 // If .subsections_via_symbols is set, each InputSection will be split along
 // symbol boundaries. The field offset represents the offset of the subsection
 // from the start of the original pre-split InputSection.
-struct SubsectionEntry {
-  uint64_t offset;
-  InputSection *isec;
+struct Subsection {
+  uint64_t offset = 0;
+  InputSection *isec = nullptr;
+};
+
+using Subsections = std::vector<Subsection>;
+
+struct Section {
+  // uint64_t address = 0; // TODO(gkm): this will debut with __eh_frame handler
+  Subsections subsections;
 };
-using SubsectionMap = std::vector<SubsectionEntry>;
 
 class InputFile {
 public:
   enum Kind {
     ObjKind,
     OpaqueKind,
     DylibKind,
     ArchiveKind,
     BitcodeKind,
   };
 
   virtual ~InputFile() = default;
   Kind kind() const { return fileKind; }
   StringRef getName() const { return name; }
   static void resetIdCount() { idCount = 0; }
 
   MemoryBufferRef mb;
 
   std::vector<Symbol *> symbols;
-  std::vector<SubsectionMap> subsections;
+  std::vector<Section> sections;
   // Provides an easy way to sort InputFiles deterministically.
   const int id;
 
   // If not empty, this stores the name of the archive containing this file.
   // We use this string for creating error messages.
   std::string archiveName;
 
 protected:
public:
 
   llvm::DWARFUnit *compileUnit = nullptr;
   const uint32_t modTime;
   std::vector<ConcatInputSection *> debugSections;
   ArrayRef<llvm::MachO::data_in_code_entry> dataInCodeEntries;
 
 private:
   template <class LP> void parse();
-  template <class Section> void parseSections(ArrayRef<Section>);
+  template <class SectionHeader> void parseSections(ArrayRef<SectionHeader>);
   template <class LP>
   void parseSymbols(ArrayRef<typename LP::section> sectionHeaders,
                     ArrayRef<typename LP::nlist> nList, const char *strtab,
                     bool subsectionsViaSymbols);
   template <class NList>
   Symbol *parseNonSectionSymbol(const NList &sym, StringRef name);
-  template <class Section>
-  void parseRelocations(ArrayRef<Section> sectionHeaders, const Section &,
-                        SubsectionMap &);
+  template <class SectionHeader>
+  void parseRelocations(ArrayRef<SectionHeader> sectionHeaders,
+                        const SectionHeader &, Subsections &);
   void parseDebugInfo();
   void parseDataInCode();
   void registerCompactUnwind();
 };
 
 // command-line -sectcreate file
 class OpaqueFile final : public InputFile {
 public:

lld/MachO/InputFiles.cpp

if (config->icfLevel != ICFLevel::none && segname == segment_names::data)
       return target->wordSize == 8 ? 32 : 16;
   } else if (name == section_names::compactUnwind) {
     if (segname == segment_names::ld)
       return target->wordSize == 8 ? 32 : 20;
   }
   return {};
 }
 
-template <class Section>
-void ObjFile::parseSections(ArrayRef<Section> sections) {
-  subsections.reserve(sections.size());
+// Parse the sequence of sections within a single LC_SEGMENT(_64).
+// Split each section into subsections.
+template <class SectionHeader>
+void ObjFile::parseSections(ArrayRef<SectionHeader> sectionHeaders) {
+  sections.reserve(sectionHeaders.size());
   auto *buf = reinterpret_cast<const uint8_t *>(mb.getBufferStart());
 
-  for (const Section &sec : sections) {
+  for (const SectionHeader &sec : sectionHeaders) {
     StringRef name =
         StringRef(sec.sectname, strnlen(sec.sectname, sizeof(sec.sectname)));
     StringRef segname =
         StringRef(sec.segname, strnlen(sec.segname, sizeof(sec.segname)));
     ArrayRef<uint8_t> data = {isZeroFill(sec.flags) ? nullptr
                                                     : buf + sec.offset,
                               static_cast<size_t>(sec.size)};
     if (sec.align >= 32) {
       error("alignment " + std::to_string(sec.align) + " of section " + name +
             " is too large");
-      subsections.push_back({});
+      sections.push_back({});
       continue;
     }
     uint32_t align = 1 << sec.align;
     uint32_t flags = sec.flags;
 
     auto splitRecords = [&](int recordSize) -> void {
-      subsections.push_back({});
+      sections.push_back({});
       if (data.empty())
         return;
 
-      SubsectionMap &subsecMap = subsections.back();
-      subsecMap.reserve(data.size() / recordSize);
+      Subsections &subsections = sections.back().subsections;
+      subsections.reserve(data.size() / recordSize);
       auto *isec = make<ConcatInputSection>(
           segname, name, this, data.slice(0, recordSize), align, flags);
-      subsecMap.push_back({0, isec});
+      subsections.push_back({0, isec});
       for (uint64_t off = recordSize; off < data.size(); off += recordSize) {
         // Copying requires less memory than constructing a fresh InputSection.
         auto *copy = make<ConcatInputSection>(*isec);
         copy->data = data.slice(off, recordSize);
-        subsecMap.push_back({off, copy});
+        subsections.push_back({off, copy});
       }
     };
 
     if (sectionType(sec.flags) == S_CSTRING_LITERALS ||
         (config->dedupLiterals && isWordLiteralSection(sec.flags))) {
       if (sec.nreloc && config->dedupLiterals)
         fatal(toString(this) + " contains relocations in " + sec.segname + "," +
               sec.sectname +
               ", so LLD cannot deduplicate literals. Try re-running without "
               "--deduplicate-literals.");
 
       InputSection *isec;
       if (sectionType(sec.flags) == S_CSTRING_LITERALS) {
         isec =
             make<CStringInputSection>(segname, name, this, data, align, flags);
         // FIXME: parallelize this?
         cast<CStringInputSection>(isec)->splitIntoPieces();
       } else {
         isec = make<WordLiteralInputSection>(segname, name, this, data, align,
                                              flags);
       }
-      subsections.push_back({{0, isec}});
+      sections.push_back({});
+      sections.back().subsections.push_back({0, isec});
     } else if (auto recordSize = getRecordSize(segname, name)) {
       splitRecords(*recordSize);
     } else if (segname == segment_names::llvm) {
       // ld64 does not appear to emit contents from sections within the __LLVM
       // segment. Symbols within those sections point to bitcode metadata
       // instead of actual symbols. Global symbols within those sections could
       // have the same name without causing duplicate symbol errors. Push an
-      // empty map to ensure indices line up for the remaining sections.
+      // empty entry to ensure indices line up for the remaining sections.
       // TODO: Evaluate whether the bitcode metadata is needed.
-      subsections.push_back({});
+      sections.push_back({});
     } else {
       auto *isec =
           make<ConcatInputSection>(segname, name, this, data, align, flags);
       if (isDebugSection(isec->getFlags()) &&
           isec->getSegName() == segment_names::dwarf) {
         // Instead of emitting DWARF sections, we emit STABS symbols to the
         // object files that contain them. We filter them out early to avoid
         // parsing their relocations unnecessarily. But we must still push an
-        // empty map to ensure the indices line up for the remaining sections.
-        subsections.push_back({});
+        // empty entry to ensure the indices line up for the remaining sections.
+        sections.push_back({});
         debugSections.push_back(isec);
       } else {
-        subsections.push_back({{0, isec}});
+        sections.push_back({});
+        sections.back().subsections.push_back({0, isec});
       }
     }
   }
 }
 
 // Find the subsection corresponding to the greatest section offset that is <=
 // that of the given offset.
 //
 // offset: an offset relative to the start of the original InputSection (before
 // any subsection splitting has occurred). It will be updated to represent the
 // same location as an offset relative to the start of the containing
 // subsection.
-static InputSection *findContainingSubsection(SubsectionMap &map,
+static InputSection *findContainingSubsection(Subsections &subsections,
                                               uint64_t *offset) {
   auto it = std::prev(llvm::upper_bound(
-      map, *offset, [](uint64_t value, SubsectionEntry subsecEntry) {
-        return value < subsecEntry.offset;
-      }));
+      subsections, *offset,
+      [](uint64_t value, Subsection subsec) { return value < subsec.offset; }));
   *offset -= it->offset;
   return it->isec;
 }
 
-template <class Section>
-static bool validateRelocationInfo(InputFile *file, const Section &sec,
+template <class SectionHeader>
+static bool validateRelocationInfo(InputFile *file, const SectionHeader &sec,
                                    relocation_info rel) {
   const RelocAttrs &relocAttrs = target->getRelocAttrs(rel.r_type);
   bool valid = true;
   auto message = [relocAttrs, file, sec, rel, &valid](const Twine &diagnostic) {
     valid = false;
     return (relocAttrs.name + " relocation " + diagnostic + " at offset " +
             std::to_string(rel.r_address) + " of " + sec.segname + "," +
             sec.sectname + " in " + toString(file))
if (rel.r_length < 2 || rel.r_length > 3 ||
     error(message("has width " + std::to_string(1 << rel.r_length) +
                   " bytes, but must be " +
                   widths[(static_cast<int>(relocAttrs.bits) >> 2) & 3] +
                   " bytes"));
   }
   return valid;
 }
 
-template <class Section>
-void ObjFile::parseRelocations(ArrayRef<Section> sectionHeaders,
-                               const Section &sec, SubsectionMap &subsecMap) {
+template <class SectionHeader>
+void ObjFile::parseRelocations(ArrayRef<SectionHeader> sectionHeaders,
+                               const SectionHeader &sec,
+                               Subsections &subsections) {
   auto *buf = reinterpret_cast<const uint8_t *>(mb.getBufferStart());
   ArrayRef<relocation_info> relInfos(
       reinterpret_cast<const relocation_info *>(buf + sec.reloff), sec.nreloc);
 
-  auto subsecIt = subsecMap.rbegin();
+  auto subsecIt = subsections.rbegin();
   for (size_t i = 0; i < relInfos.size(); i++) {
     // Paired relocations serve as Mach-O's method for attaching a
     // supplemental datum to a primary relocation record. ELF does not
     // need them because the *_RELOC_RELA records contain the extra
     // addend field, vs. *_RELOC_REL which omit the addend.
     //
     // The {X86_64,ARM64}_RELOC_SUBTRACTOR record holds the subtrahend,
     // and the paired *_RELOC_UNSIGNED record holds the minuend. The
for (size_t i = 0; i < relInfos.size(); i++) {
     r.pcrel = relInfo.r_pcrel;
     r.length = relInfo.r_length;
     r.offset = relInfo.r_address;
     if (relInfo.r_extern) {
       r.referent = symbols[relInfo.r_symbolnum];
       r.addend = isSubtrahend ? 0 : totalAddend;
     } else {
       assert(!isSubtrahend);
-      const Section &referentSec = sectionHeaders[relInfo.r_symbolnum - 1];
+      const SectionHeader &referentSecHead =
+          sectionHeaders[relInfo.r_symbolnum - 1];
       uint64_t referentOffset;
       if (relInfo.r_pcrel) {
         // The implicit addend for pcrel section relocations is the pcrel offset
         // in terms of the addresses in the input file. Here we adjust it so
         // that it describes the offset from the start of the referent section.
         // FIXME This logic was written around x86_64 behavior -- ARM64 doesn't
         // have pcrel section relocations. We may want to factor this out into
         // the arch-specific .cpp file.
         assert(target->hasAttr(r.type, RelocAttrBits::BYTE4));
-        referentOffset =
-            sec.addr + relInfo.r_address + 4 + totalAddend - referentSec.addr;
+        referentOffset = sec.addr + relInfo.r_address + 4 + totalAddend -
+                         referentSecHead.addr;
       } else {
         // The addend for a non-pcrel relocation is its absolute address.
-        referentOffset = totalAddend - referentSec.addr;
+        referentOffset = totalAddend - referentSecHead.addr;
       }
-      SubsectionMap &referentSubsecMap = subsections[relInfo.r_symbolnum - 1];
-      r.referent = findContainingSubsection(referentSubsecMap, &referentOffset);
+      Subsections &referentSubsections =
+          sections[relInfo.r_symbolnum - 1].subsections;
+      r.referent =
+          findContainingSubsection(referentSubsections, &referentOffset);
       r.addend = referentOffset;
     }
 
     // Find the subsection that this relocation belongs to.
     // Though not required by the Mach-O format, clang and gcc seem to emit
     // relocations in order, so let's take advantage of it. However, ld64 emits
     // unsorted relocations (in `-r` mode), so we have a fallback for that
     // uncommon case.
     InputSection *subsec;
-    while (subsecIt != subsecMap.rend() && subsecIt->offset > r.offset)
+    while (subsecIt != subsections.rend() && subsecIt->offset > r.offset)
       ++subsecIt;
-    if (subsecIt == subsecMap.rend() ||
+    if (subsecIt == subsections.rend() ||
         subsecIt->offset + subsecIt->isec->getSize() <= r.offset) {
-      subsec = findContainingSubsection(subsecMap, &r.offset);
+      subsec = findContainingSubsection(subsections, &r.offset);
       // Now that we know the relocs are unsorted, avoid trying the 'fast path'
       // for the other relocations.
-      subsecIt = subsecMap.rend();
+      subsecIt = subsections.rend();
     } else {
       subsec = subsecIt->isec;
       r.offset -= subsecIt->offset;
     }
     subsec->relocs.push_back(r);
 
     if (isSubtrahend) {
       relocation_info minuendInfo = relInfos[++i];
       // SUBTRACTOR relocations should always be followed by an UNSIGNED one
       // attached to the same address.
       assert(target->hasAttr(minuendInfo.r_type, RelocAttrBits::UNSIGNED) &&
              relInfo.r_address == minuendInfo.r_address);
       Reloc p;
       p.type = minuendInfo.r_type;
       if (minuendInfo.r_extern) {
         p.referent = symbols[minuendInfo.r_symbolnum];
         p.addend = totalAddend;
       } else {
         uint64_t referentOffset =
             totalAddend - sectionHeaders[minuendInfo.r_symbolnum - 1].addr;
-        SubsectionMap &referentSubsecMap =
-            subsections[minuendInfo.r_symbolnum - 1];
+        Subsections &referentSubsectVec =
+            sections[minuendInfo.r_symbolnum - 1].subsections;
         p.referent =
-            findContainingSubsection(referentSubsecMap, &referentOffset);
+            findContainingSubsection(referentSubsectVec, &referentOffset);
         p.addend = referentOffset;
       }
       subsec->relocs.push_back(p);
     }
   }
 }
 
 template <class NList>

 
 template <class LP>
 void ObjFile::parseSymbols(ArrayRef<typename LP::section> sectionHeaders,
                            ArrayRef<typename LP::nlist> nList,
                            const char *strtab, bool subsectionsViaSymbols) {
   using NList = typename LP::nlist;
 
   // Groups indices of the symbols by the sections that contain them.
-  std::vector<std::vector<uint32_t>> symbolsBySection(subsections.size());
+  std::vector<std::vector<uint32_t>> symbolsBySection(sections.size());
   symbols.resize(nList.size());
   SmallVector<unsigned, 32> undefineds;
   for (uint32_t i = 0; i < nList.size(); ++i) {
     const NList &sym = nList[i];
 
     // Ignore debug symbols for now.
     // FIXME: may need special handling.
     if (sym.n_type & N_STAB)
       continue;
 
     StringRef name = strtab + sym.n_strx;
     if ((sym.n_type & N_TYPE) == N_SECT) {
-      SubsectionMap &subsecMap = subsections[sym.n_sect - 1];
+      Subsections &subsections = sections[sym.n_sect - 1].subsections;
       // parseSections() may have chosen not to parse this section.
-      if (subsecMap.empty())
+      if (subsections.empty())
         continue;
       symbolsBySection[sym.n_sect - 1].push_back(i);
     } else if (isUndef(sym)) {
       undefineds.push_back(i);
     } else {
       symbols[i] = parseNonSectionSymbol(sym, name);
     }
   }
 
-  for (size_t i = 0; i < subsections.size(); ++i) {
-    SubsectionMap &subsecMap = subsections[i];
-    if (subsecMap.empty())
+  for (size_t i = 0; i < sections.size(); ++i) {
+    Subsections &subsections = sections[i].subsections;
+    if (subsections.empty())
       continue;
 
     std::vector<uint32_t> &symbolIndices = symbolsBySection[i];
     uint64_t sectionAddr = sectionHeaders[i].addr;
     uint32_t sectionAlign = 1u << sectionHeaders[i].align;
 
-    InputSection *lastIsec = subsecMap.back().isec;
+    InputSection *lastIsec = subsections.back().isec;
     // Record-based sections have already been split into subsections during
     // parseSections(), so we simply need to match Symbols to the corresponding
     // subsection here.
     if (getRecordSize(lastIsec->getSegName(), lastIsec->getName())) {
       for (size_t j = 0; j < symbolIndices.size(); ++j) {
         uint32_t symIndex = symbolIndices[j];
         const NList &sym = nList[symIndex];
         StringRef name = strtab + sym.n_strx;
         uint64_t symbolOffset = sym.n_value - sectionAddr;
-        InputSection *isec = findContainingSubsection(subsecMap, &symbolOffset);
+        InputSection *isec =
+            findContainingSubsection(subsections, &symbolOffset);
         if (symbolOffset != 0) {
           error(toString(lastIsec) + ":  symbol " + name +
                 " at misaligned offset");
           continue;
         }
         symbols[symIndex] = createDefined(sym, name, isec, 0, isec->getSize());
       }
       continue;
     }
 
     // Calculate symbol sizes and create subsections by splitting the sections
     // along symbol boundaries.
-    // We populate subsecMap by repeatedly splitting the last (highest address)
-    // subsection.
+    // We populate subsections by repeatedly splitting the last (highest
+    // address) subsection.
     llvm::stable_sort(symbolIndices, [&](uint32_t lhs, uint32_t rhs) {
       return nList[lhs].n_value < nList[rhs].n_value;
     });
-    SubsectionEntry subsecEntry = subsecMap.back();
+    Subsection subsec = subsections.back();
     for (size_t j = 0; j < symbolIndices.size(); ++j) {
       uint32_t symIndex = symbolIndices[j];
       const NList &sym = nList[symIndex];
       StringRef name = strtab + sym.n_strx;
-      InputSection *isec = subsecEntry.isec;
+      InputSection *isec = subsec.isec;
 
-      uint64_t subsecAddr = sectionAddr + subsecEntry.offset;
+      uint64_t subsecAddr = sectionAddr + subsec.offset;
       size_t symbolOffset = sym.n_value - subsecAddr;
       uint64_t symbolSize =
           j + 1 < symbolIndices.size()
               ? nList[symbolIndices[j + 1]].n_value - sym.n_value
               : isec->data.size() - symbolOffset;
       // There are 4 cases where we do not need to create a new subsection:
       //   1. If the input file does not use subsections-via-symbols.
       //   2. Multiple symbols at the same address only induce one subsection.
for (size_t j = 0; j < symbolIndices.size(); ++j) {
       // By construction, the symbol will be at offset zero in the new
       // subsection.
       symbols[symIndex] =
           createDefined(sym, name, nextIsec, /*value=*/0, symbolSize);
       // TODO: ld64 appears to preserve the original alignment as well as each
       // subsection's offset from the last aligned address. We should consider
       // emulating that behavior.
       nextIsec->align = MinAlign(sectionAlign, sym.n_value);
-      subsecMap.push_back({sym.n_value - sectionAddr, nextIsec});
-      subsecEntry = subsecMap.back();
+      subsections.push_back({sym.n_value - sectionAddr, nextIsec});
+      subsec = subsections.back();
     }
   }
 
   // Undefined symbols can trigger recursive fetch from Archives due to
   // LazySymbols. Process defined symbols first so that the relative order
   // between a defined symbol and an undefined symbol does not change the
   // symbol resolution behavior. In addition, a set of interconnected symbols
   // will all be resolved to the same file, instead of being resolved to
OpaqueFile::OpaqueFile(MemoryBufferRef mb, StringRef segName,
                        StringRef sectName)
     : InputFile(OpaqueKind, mb) {
   const auto *buf = reinterpret_cast<const uint8_t *>(mb.getBufferStart());
   ArrayRef<uint8_t> data = {buf, mb.getBufferSize()};
   ConcatInputSection *isec =
       make<ConcatInputSection>(segName.take_front(16), sectName.take_front(16),
                                /*file=*/this, data);
   isec->live = true;
-  subsections.push_back({{0, isec}});
+  sections.push_back({});
+  sections.back().subsections.push_back({0, isec});
 }
 
 ObjFile::ObjFile(MemoryBufferRef mb, uint32_t modTime, StringRef archiveName)
     : InputFile(ObjKind, mb), modTime(modTime) {
   this->archiveName = std::string(archiveName);
   if (target->wordSize == 8)
     parse<LP64>();
   else
     parse<ILP32>();
 }
 
 template <class LP> void ObjFile::parse() {
   using Header = typename LP::mach_header;
   using SegmentCommand = typename LP::segment_command;
-  using Section = typename LP::section;
+  using SectionHeader = typename LP::section;
   using NList = typename LP::nlist;
 
   auto *buf = reinterpret_cast<const uint8_t *>(mb.getBufferStart());
   auto *hdr = reinterpret_cast<const Header *>(mb.getBufferStart());
 
   Architecture arch = getArchitectureFromCpuType(hdr->cputype, hdr->cpusubtype);
   if (arch != config->arch()) {
     auto msg = config->errorForArchMismatch
if (!checkCompatibility(this))
     return;
 
   for (auto *cmd : findCommands<linker_option_command>(hdr, LC_LINKER_OPTION)) {
     StringRef data{reinterpret_cast<const char *>(cmd + 1),
                    cmd->cmdsize - sizeof(linker_option_command)};
     parseLCLinkerOption(this, cmd->count, data);
   }
 
-  ArrayRef<Section> sectionHeaders;
+  ArrayRef<SectionHeader> sectionHeaders;
   if (const load_command *cmd = findCommand(hdr, LP::segmentLCType)) {
     auto *c = reinterpret_cast<const SegmentCommand *>(cmd);
-    sectionHeaders =
-        ArrayRef<Section>{reinterpret_cast<const Section *>(c + 1), c->nsects};
+    sectionHeaders = ArrayRef<SectionHeader>{
+        reinterpret_cast<const SectionHeader *>(c + 1), c->nsects};
     parseSections(sectionHeaders);
   }
 
   // TODO: Error on missing LC_SYMTAB?
   if (const load_command *cmd = findCommand(hdr, LC_SYMTAB)) {
     auto *c = reinterpret_cast<const symtab_command *>(cmd);
     ArrayRef<NList> nList(reinterpret_cast<const NList *>(buf + c->symoff),
                           c->nsyms);
     const char *strtab = reinterpret_cast<const char *>(buf) + c->stroff;
     bool subsectionsViaSymbols = hdr->flags & MH_SUBSECTIONS_VIA_SYMBOLS;
     parseSymbols<LP>(sectionHeaders, nList, strtab, subsectionsViaSymbols);
   }
 
   // The relocations may refer to the symbols, so we parse them after we have
   // parsed all the symbols.
-  for (size_t i = 0, n = subsections.size(); i < n; ++i)
-    if (!subsections[i].empty())
-      parseRelocations(sectionHeaders, sectionHeaders[i], subsections[i]);
+  for (size_t i = 0, n = sections.size(); i < n; ++i)
+    if (!sections[i].subsections.empty())
+      parseRelocations(sectionHeaders, sectionHeaders[i],
+                       sections[i].subsections);
 
   parseDebugInfo();
   if (config->emitDataInCodeInfo)
     parseDataInCode();
   registerCompactUnwind();
 }
 
 void ObjFile::parseDebugInfo() {
assert(is_sorted(dataInCodeEntries, [](const data_in_code_entry &lhs,
                                          const data_in_code_entry &rhs) {
     return lhs.offset < rhs.offset;
   }));
 }
 
 // Create pointers from symbols to their associated compact unwind entries.
 void ObjFile::registerCompactUnwind() {
   // First, locate the __compact_unwind section.
-  SubsectionMap *cuSubsecMap = nullptr;
-  for (SubsectionMap &map : subsections) {
-    if (map.empty())
+  Section *cuSection = nullptr;
+  for (Section &section : sections) {
+    if (section.subsections.empty())
       continue;
-    if (map[0].isec->getSegName() != segment_names::ld)
+    if (section.subsections[0].isec->getSegName() != segment_names::ld)
       continue;
-    cuSubsecMap = &map;
+    cuSection = &section;
     break;
   }
-  if (!cuSubsecMap)
+  if (!cuSection)
     return;
 
-  for (SubsectionEntry &entry : *cuSubsecMap) {
-    ConcatInputSection *isec = cast<ConcatInputSection>(entry.isec);
+  for (Subsection &subsection : cuSection->subsections) {
+    ConcatInputSection *isec = cast<ConcatInputSection>(subsection.isec);
     // Hack!! Since each CUE contains a different function address, if ICF
     // operated naively and compared the entire contents of each CUE, entries
     // with identical unwind info but belonging to different functions would
     // never be considered equivalent. To work around this problem, we slice
     // away the function address here. (Note that we do not adjust the offsets
     // of the corresponding relocations.) We rely on `relocateCompactUnwind()`
     // to correctly handle these truncated input sections.
     isec->data = isec->data.slice(target->wordSize);

lld/MachO/ObjC.cpp

 #include "llvm/Bitcode/BitcodeReader.h"
 
 using namespace llvm;
 using namespace llvm::MachO;
 using namespace lld;
 using namespace lld::macho;
 
 template <class LP> static bool objectHasObjCSection(MemoryBufferRef mb) {
-  using Section = typename LP::section;
+  using SectionHeader = typename LP::section;
 
   auto *hdr =
       reinterpret_cast<const typename LP::mach_header *>(mb.getBufferStart());
   if (hdr->magic != LP::magic)
     return false;
 
   if (const auto *c =
           findCommand<typename LP::segment_command>(hdr, LP::segmentLCType)) {
-    auto sectionHeaders =
-        ArrayRef<Section>{reinterpret_cast<const Section *>(c + 1), c->nsects};
-    for (const Section &sec : sectionHeaders) {
-      StringRef sectname(sec.sectname,
-                         strnlen(sec.sectname, sizeof(sec.sectname)));
-      StringRef segname(sec.segname, strnlen(sec.segname, sizeof(sec.segname)));
+    auto sectionHeaders = ArrayRef<SectionHeader>{
+        reinterpret_cast<const SectionHeader *>(c + 1), c->nsects};
+    for (const SectionHeader &secHead : sectionHeaders) {
+      StringRef sectname(secHead.sectname,
+                         strnlen(secHead.sectname, sizeof(secHead.sectname)));
+      StringRef segname(secHead.segname,
+                        strnlen(secHead.segname, sizeof(secHead.segname)));
       if ((segname == segment_names::data &&
            sectname == section_names::objcCatList) ||
           (segname == segment_names::text &&
            sectname == section_names::swift)) {
         return true;
       }
     }
   }

lld/MachO/SyntheticSections.cpp

 void ExportSection::writeTo(uint8_t *buf) const { trieBuilder.writeTo(buf); }
 
 DataInCodeSection::DataInCodeSection()
     : LinkEditSection(segment_names::linkEdit, section_names::dataInCode) {}
 
 template <class LP>
 static std::vector<MachO::data_in_code_entry> collectDataInCodeEntries() {
   using SegmentCommand = typename LP::segment_command;
-  using Section = typename LP::section;
+  using SectionHeader = typename LP::section;
 
   std::vector<MachO::data_in_code_entry> dataInCodeEntries;
   for (const InputFile *inputFile : inputFiles) {
     if (!isa<ObjFile>(inputFile))
       continue;
     const ObjFile *objFile = cast<ObjFile>(inputFile);
     const auto *c = reinterpret_cast<const SegmentCommand *>(
         findCommand(objFile->mb.getBufferStart(), LP::segmentLCType));
     if (!c)
       continue;
-    ArrayRef<Section> sections{reinterpret_cast<const Section *>(c + 1),
-                               c->nsects};
+    ArrayRef<SectionHeader> sectionHeaders{
+        reinterpret_cast<const SectionHeader *>(c + 1), c->nsects};
 
     ArrayRef<MachO::data_in_code_entry> entries = objFile->dataInCodeEntries;
     if (entries.empty())
       continue;
     // For each code subsection find 'data in code' entries residing in it.
     // Compute the new offset values as
     // <offset within subsection> + <subsection address> - <__TEXT address>.
-    for (size_t i = 0, n = sections.size(); i < n; ++i) {
-      const SubsectionMap &subsecMap = objFile->subsections[i];
-      for (const SubsectionEntry &subsecEntry : subsecMap) {
-        const InputSection *isec = subsecEntry.isec;
+    for (size_t i = 0, n = sectionHeaders.size(); i < n; ++i) {
+      for (const Subsection &subsec : objFile->sections[i].subsections) {
+        const InputSection *isec = subsec.isec;
         if (!isCodeSection(isec))
           continue;
         if (cast<ConcatInputSection>(isec)->shouldOmitFromOutput())
           continue;
-        const uint64_t beginAddr = sections[i].addr + subsecEntry.offset;
+        const uint64_t beginAddr = sectionHeaders[i].addr + subsec.offset;
         auto it = llvm::lower_bound(
             entries, beginAddr,
             [](const MachO::data_in_code_entry &entry, uint64_t addr) {
               return entry.offset < addr;
             });
         const uint64_t endAddr = beginAddr + isec->getFileSize();
         for (const auto end = entries.end();
              it != end && it->offset + it->length <= endAddr; ++it)

lld/MachO/Writer.cpp

public:
 
   uint32_t getSize() const override {
     return sizeof(typename LP::segment_command) +
            seg->numNonHiddenSections() * sizeof(typename LP::section);
   }
 
   void writeTo(uint8_t *buf) const override {
     using SegmentCommand = typename LP::segment_command;
-    using Section = typename LP::section;
+    using SectionHeader = typename LP::section;
 
     auto *c = reinterpret_cast<SegmentCommand *>(buf);
     buf += sizeof(SegmentCommand);
 
     c->cmd = LP::segmentLCType;
     c->cmdsize = getSize();
     memcpy(c->segname, name.data(), name.size());
     c->fileoff = seg->fileOff;
     c->maxprot = seg->maxProt;
     c->initprot = seg->initProt;
 
     c->vmaddr = seg->addr;
     c->vmsize = seg->vmSize;
     c->filesize = seg->fileSize;
     c->nsects = seg->numNonHiddenSections();
 
     for (const OutputSection *osec : seg->getSections()) {
       if (osec->isHidden())
         continue;
 
-      auto *sectHdr = reinterpret_cast<Section *>(buf);
-      buf += sizeof(Section);
+      auto *sectHdr = reinterpret_cast<SectionHeader *>(buf);
+      buf += sizeof(SectionHeader);
 
       memcpy(sectHdr->sectname, osec->name.data(), osec->name.size());
       memcpy(sectHdr->segname, name.data(), name.size());
 
       sectHdr->addr = osec->addr;
       sectHdr->offset = osec->fileOff;
       sectHdr->align = Log2_32(osec->align);
       sectHdr->flags = osec->flags;
void writeUuid(uint64_t digest) const {
     // want to lose bits of the digest in byte 8, so swap that with a byte of
     // fixed data that happens to have the right bits set.
     std::swap(uuidBuf[3], uuidBuf[8]);
 
     // Claim that this is an MD5-based hash. It isn't, but this signals that
     // this is not a time-based and not a random hash. MD5 seems like the least
     // bad lie we can put here.
     assert((uuidBuf[6] & 0xf0) == 0x30 && "See RFC 4122 Sections 4.2.2, 4.1.3");
     assert((uuidBuf[8] & 0xc0) == 0x80 && "See RFC 4122 Section 4.2.2");
   }
 
   mutable uint8_t *uuidBuf;
 };
 
 template <class LP> class LCEncryptionInfo final : public LoadCommand {
 public:
   uint32_t getSize() const override {