[ELF] Don't use multiple inheritance for OutputSection. NFC

[MaskRay]

Summary

Add an OutputDesc class inheriting from SectionCommand. An OutputDesc wraps an
OutputSection.

With this change, we can inline InputSection::getParent.

My lld executable is 2.3KiB smaller.

[peter.smith]

No objections for the change. I think it makes sense to logically separate OutputSection from SectionCommand. I'd like to see if anyone else has a strong opinion as there are quite a few changes and some more dynamic memory allocations. I'm wondering if there may be a way of avoiding those somehow, for example could an implicit conversion operator to OutputSection* be added?

lld/ELF/OutputSections.h

30

Given that this doesn't use anything about the representation of OutputSection, would it be better to move it to the LinkerScript.h header.

[MaskRay]

Change OutputSection *osec to OutputSection osec

[MaskRay]

In D120650#3349298, @peter.smith wrote:

No objections for the change. I think it makes sense to logically separate OutputSection from SectionCommand. I'd like to see if anyone else has a strong opinion as there are quite a few changes and some more dynamic memory allocations. I'm wondering if there may be a way of avoiding those somehow, for example could an implicit conversion operator to OutputSection* be added?

I have switched to

struct OutputDesc final : SectionCommand {
  OutputSection osec;
...
};

make<OutputDesc> is used in replace of previous make<OutputDescription>. There is no extra allocation.

I'm wondering if there may be a way of avoiding those somehow, for example could an implicit conversion operator to OutputSection* be added?

The majority differences are due to dyn_cast<OutputSection> => dyn_cast<OutputDesc> which need changes.

An conversion operator may help a few osd->osec places but they are typically coupled with dyn_cast<OutputDesc>, so the total number of changes may not decrease a lot.
An explicit osd->osec seems clearer to me.

lld/ELF/OutputSections.h

30

OutputDesc now uses OutputSection as a non-pointer data member, so it needs to stay in this file.

[peter.smith]

Just one small comment on some function names, although that is not a strong opinion. No objections to making the changes, will wait a few days before setting approved to see if there are dissenting opinions.

[ikudrin]

LGTM

lld/ELF/Writer.cpp

1171–1172

curSec -> curSecDesc maybe?

[MaskRay]

curSec => curSecDesc

lld/ELF/Driver.cpp

void LinkerDriver::link(opt::InputArgList &args) {
   {
     llvm::TimeTraceScope timeScope("Merge/finalize input sections");
 
     // Migrate InputSectionDescription::sectionBases to sections. This includes
     // merging MergeInputSections into a single MergeSyntheticSection. From this
     // point onwards InputSectionDescription::sections should be used instead of
     // sectionBases.
     for (SectionCommand *cmd : script->sectionCommands)
-      if (auto *sec = dyn_cast<OutputSection>(cmd))
-        sec->finalizeInputSections();
+      if (auto *osd = dyn_cast<OutputDesc>(cmd))
+        osd->osec.finalizeInputSections();
     llvm::erase_if(inputSections, [](InputSectionBase *s) {
       return isa<MergeInputSection>(s);
     });
   }
 
   // Two input sections with different output sections should not be folded.
   // ICF runs after processSectionCommands() so that we know the output sections.
   if (config->icf != ICFLevel::None) {

lld/ELF/ICF.cpp

template <class ELFT> void ICF<ELFT>::run() {
   parallelForEach(objectFiles, [&](ELFFileBase *file) {
     for (Symbol *sym : file->getLocalSymbols())
       fold(sym);
   });
 
   // InputSectionDescription::sections is populated by processSectionCommands().
   // ICF may fold some input sections assigned to output sections. Remove them.
   for (SectionCommand *cmd : script->sectionCommands)
-    if (auto *sec = dyn_cast<OutputSection>(cmd))
-      for (SectionCommand *subCmd : sec->commands)
+    if (auto *osd = dyn_cast<OutputDesc>(cmd))
+      for (SectionCommand *subCmd : osd->osec.commands)
         if (auto *isd = dyn_cast<InputSectionDescription>(subCmd))
           llvm::erase_if(isd->sections,
                          [](InputSection *isec) { return !isec->isLive(); });
 }
 
 // ICF entry point function.
 template <class ELFT> void elf::doIcf() {
   llvm::TimeTraceScope timeScope("ICF");
   ICF<ELFT>().run();
 }
 
 template void elf::doIcf<ELF32LE>();
 template void elf::doIcf<ELF32BE>();
 template void elf::doIcf<ELF64LE>();
 template void elf::doIcf<ELF64BE>();

lld/ELF/LinkerScript.h

 
 class Defined;
 class InputFile;
 class InputSection;
 class InputSectionBase;
 class OutputSection;
 class SectionBase;
 class ThunkSection;
+struct OutputDesc;
 
 // This represents an r-value in the linker script.
 struct ExprValue {
   ExprValue(SectionBase *sec, bool forceAbsolute, uint64_t val,
             const Twine &loc)
       : sec(sec), val(val), forceAbsolute(forceAbsolute), loc(loc.str()) {}
 
   ExprValue(uint64_t val) : ExprValue(nullptr, false, val, "") {}
struct AddressState {
     AddressState();
     OutputSection *outSec = nullptr;
     MemoryRegion *memRegion = nullptr;
     MemoryRegion *lmaRegion = nullptr;
     uint64_t lmaOffset = 0;
     uint64_t tbssAddr = 0;
   };
 
-  llvm::DenseMap<llvm::CachedHashStringRef, OutputSection *>
-      nameToOutputSection;
+  llvm::DenseMap<llvm::CachedHashStringRef, OutputDesc *> nameToOutputSection;
 
   void addSymbol(SymbolAssignment *cmd);
   void assignSymbol(SymbolAssignment *cmd, bool inSec);
   void setDot(Expr e, const Twine &loc, bool inSec);
   void expandOutputSection(uint64_t size);
   void expandMemoryRegions(uint64_t size);
 
   SmallVector<InputSectionBase *, 0>
class LinkerScript final {
   // LinkerScript.
   AddressState *ctx = nullptr;
 
   OutputSection *aether;
 
   uint64_t dot;
 
 public:
-  OutputSection *createOutputSection(StringRef name, StringRef location);
-  OutputSection *getOrCreateOutputSection(StringRef name);
+  OutputDesc *createOutputSection(StringRef name, StringRef location);
+  OutputDesc *getOrCreateOutputSection(StringRef name);
 
   bool hasPhdrsCommands() { return !phdrsCommands.empty(); }
   uint64_t getDot() { return dot; }
   void discard(InputSectionBase &s);
 
   ExprValue getSymbolValue(StringRef name, const Twine &loc);
 
   void addOrphanSections();
public:
   // A list of symbols referenced by the script.
   SmallVector<llvm::StringRef, 0> referencedSymbols;
 
   // Used to implement INSERT [AFTER|BEFORE]. Contains output sections that need
   // to be reordered.
   SmallVector<InsertCommand, 0> insertCommands;
 
   // OutputSections specified by OVERWRITE_SECTIONS.
-  SmallVector<OutputSection *, 0> overwriteSections;
+  SmallVector<OutputDesc *, 0> overwriteSections;
 
   // Sections that will be warned/errored by --orphan-handling.
   SmallVector<const InputSectionBase *, 0> orphanSections;
 };
 
 extern std::unique_ptr<LinkerScript> script;
 
 } // end namespace elf
 } // end namespace lld
 
 #endif // LLD_ELF_LINKER_SCRIPT_H

lld/ELF/LinkerScript.cpp

uint64_t ExprValue::getSectionOffset() const {
   // If the alignment is trivial, we don't have to compute the full
   // value to know the offset. This allows this function to succeed in
   // cases where the output section is not yet known.
   if (alignment == 1 && !sec)
     return val;
   return getValue() - getSecAddr();
 }
 
-OutputSection *LinkerScript::createOutputSection(StringRef name,
-                                                 StringRef location) {
-  OutputSection *&secRef = nameToOutputSection[CachedHashStringRef(name)];
-  OutputSection *sec;
-  if (secRef && secRef->location.empty()) {
+OutputDesc *LinkerScript::createOutputSection(StringRef name,
+  OutputDesc *&secRef = nameToOutputSection[CachedHashStringRef(name)];
+  OutputDesc *sec;
+  if (secRef && secRef->osec.location.empty()) {
     // There was a forward reference.
     sec = secRef;
   } else {
-    sec = make<OutputSection>(name, SHT_PROGBITS, 0);
+    sec = make<OutputDesc>(name, SHT_PROGBITS, 0);
     if (!secRef)
       secRef = sec;
   }
-  sec->location = std::string(location);
+  sec->osec.location = std::string(location);
   return sec;
 }
 
-OutputSection *LinkerScript::getOrCreateOutputSection(StringRef name) {
-  OutputSection *&cmdRef = nameToOutputSection[CachedHashStringRef(name)];
+OutputDesc *LinkerScript::getOrCreateOutputSection(StringRef name) {
+  OutputDesc *&cmdRef = nameToOutputSection[CachedHashStringRef(name)];
   if (!cmdRef)
-    cmdRef = make<OutputSection>(name, SHT_PROGBITS, 0);
+    cmdRef = make<OutputDesc>(name, SHT_PROGBITS, 0);
   return cmdRef;
 }
 
 // Expands the memory region by the specified size.
 static void expandMemoryRegion(MemoryRegion *memRegion, uint64_t size,
                                StringRef secName) {
   memRegion->curPos += size;
   uint64_t newSize = memRegion->curPos - (memRegion->origin)().getValue();
getSymbolAssignmentValues(ArrayRef<SectionCommand *> sectionCommands) {
   SymbolAssignmentMap ret;
   for (SectionCommand *cmd : sectionCommands) {
     if (auto *assign = dyn_cast<SymbolAssignment>(cmd)) {
       if (assign->sym) // sym is nullptr for dot.
         ret.try_emplace(assign->sym, std::make_pair(assign->sym->section,
                                                     assign->sym->value));
       continue;
     }
-    for (SectionCommand *subCmd : cast<OutputSection>(cmd)->commands)
+    for (SectionCommand *subCmd : cast<OutputDesc>(cmd)->osec.commands)
       if (auto *assign = dyn_cast<SymbolAssignment>(subCmd))
         if (assign->sym)
           ret.try_emplace(assign->sym, std::make_pair(assign->sym->section,
                                                       assign->sym->value));
   }
   return ret;
 }
 
if (std::make_pair(sym->section, sym->value) != it.second &&
       changed = sym;
   }
   return changed;
 }
 
 // Process INSERT [AFTER|BEFORE] commands. For each command, we move the
 // specified output section to the designated place.
 void LinkerScript::processInsertCommands() {
-  SmallVector<OutputSection *, 0> moves;
+  SmallVector<OutputDesc *, 0> moves;
   for (const InsertCommand &cmd : insertCommands) {
     for (StringRef name : cmd.names) {
       // If base is empty, it may have been discarded by
       // adjustOutputSections(). We do not handle such output sections.
       auto from = llvm::find_if(sectionCommands, [&](SectionCommand *subCmd) {
-        return isa<OutputSection>(subCmd) &&
-               cast<OutputSection>(subCmd)->name == name;
+        return isa<OutputDesc>(subCmd) &&
+               cast<OutputDesc>(subCmd)->osec.name == name;
       });
       if (from == sectionCommands.end())
         continue;
-      moves.push_back(cast<OutputSection>(*from));
+      moves.push_back(cast<OutputDesc>(*from));
       sectionCommands.erase(from);
     }
 
     auto insertPos =
         llvm::find_if(sectionCommands, [&cmd](SectionCommand *subCmd) {
-          auto *to = dyn_cast<OutputSection>(subCmd);
-          return to != nullptr && to->name == cmd.where;
+          auto *to = dyn_cast<OutputDesc>(subCmd);
+          return to != nullptr && to->osec.name == cmd.where;
         });
     if (insertPos == sectionCommands.end()) {
       error("unable to insert " + cmd.names[0] +
             (cmd.isAfter ? " after " : " before ") + cmd.where);
     } else {
       if (cmd.isAfter)
         ++insertPos;
       sectionCommands.insert(insertPos, moves.begin(), moves.end());
if (auto *assign = dyn_cast<SymbolAssignment>(cmd)) {
       declareSymbol(assign);
       continue;
     }
 
     // If the output section directive has constraints,
     // we can't say for sure if it is going to be included or not.
     // Skip such sections for now. Improve the checks if we ever
     // need symbols from that sections to be declared early.
-    auto *sec = cast<OutputSection>(cmd);
-    if (sec->constraint != ConstraintKind::NoConstraint)
+    const OutputSection &sec = cast<OutputDesc>(cmd)->osec;
+    if (sec.constraint != ConstraintKind::NoConstraint)
       continue;
-    for (SectionCommand *cmd : sec->commands)
+    for (SectionCommand *cmd : sec.commands)
       if (auto *assign = dyn_cast<SymbolAssignment>(cmd))
         declareSymbol(assign);
   }
 }
 
 // This function is called from assignAddresses, while we are
 // fixing the output section addresses. This function is supposed
 // to set the final value for a given symbol assignment.
auto process = [this](OutputSection *osec) {
     // does. Partitions cannot be used with the SECTIONS command, so this is
     // always 1.
     osec->partition = 1;
     return true;
   };
 
   // Process OVERWRITE_SECTIONS first so that it can overwrite the main script
   // or orphans.
-  DenseMap<CachedHashStringRef, OutputSection *> map;
+  DenseMap<CachedHashStringRef, OutputDesc *> map;
   size_t i = 0;
-  for (OutputSection *osec : overwriteSections)
+  for (OutputDesc *osd : overwriteSections) {
+    OutputSection *osec = &osd->osec;
     if (process(osec) &&
-        !map.try_emplace(CachedHashStringRef(osec->name), osec).second)
+        !map.try_emplace(CachedHashStringRef(osec->name), osd).second)
       warn("OVERWRITE_SECTIONS specifies duplicate " + osec->name);
+  }
   for (SectionCommand *&base : sectionCommands)
-    if (auto *osec = dyn_cast<OutputSection>(base)) {
-      if (OutputSection *overwrite =
-              map.lookup(CachedHashStringRef(osec->name))) {
-        log(overwrite->location + " overwrites " + osec->name);
-        overwrite->sectionIndex = i++;
+    if (auto *osd = dyn_cast<OutputDesc>(base)) {
+      OutputSection *osec = &osd->osec;
+      if (OutputDesc *overwrite = map.lookup(CachedHashStringRef(osec->name))) {
+        log(overwrite->osec.location + " overwrites " + osec->name);
+        overwrite->osec.sectionIndex = i++;
         base = overwrite;
       } else if (process(osec)) {
         osec->sectionIndex = i++;
       }
     }
 
   // If an OVERWRITE_SECTIONS specified output section is not in
   // sectionCommands, append it to the end. The section will be inserted by
   // orphan placement.
-  for (OutputSection *osec : overwriteSections)
-    if (osec->partition == 1 && osec->sectionIndex == UINT32_MAX)
-      sectionCommands.push_back(osec);
+  for (OutputDesc *osd : overwriteSections)
+    if (osd->osec.partition == 1 && osd->osec.sectionIndex == UINT32_MAX)
+      sectionCommands.push_back(osd);
 }
 
 void LinkerScript::processSymbolAssignments() {
   // Dot outside an output section still represents a relative address, whose
   // sh_shndx should not be SHN_UNDEF or SHN_ABS. Create a dummy aether section
   // that fills the void outside a section. It has an index of one, which is
   // indistinguishable from any other regular section index.
   aether = make<OutputSection>("", 0, SHF_ALLOC);
   aether->sectionIndex = 1;
 
   // ctx captures the local AddressState and makes it accessible deliberately.
   // This is needed as there are some cases where we cannot just thread the
   // current state through to a lambda function created by the script parser.
   AddressState state;
   ctx = &state;
   ctx->outSec = aether;
 
   for (SectionCommand *cmd : sectionCommands) {
     if (auto *assign = dyn_cast<SymbolAssignment>(cmd))
       addSymbol(assign);
     else
-      for (SectionCommand *subCmd : cast<OutputSection>(cmd)->commands)
+      for (SectionCommand *subCmd : cast<OutputDesc>(cmd)->osec.commands)
         if (auto *assign = dyn_cast<SymbolAssignment>(subCmd))
           addSymbol(assign);
   }
 
   ctx = nullptr;
 }
 
 static OutputSection *findByName(ArrayRef<SectionCommand *> vec,
                                  StringRef name) {
   for (SectionCommand *cmd : vec)
-    if (auto *sec = dyn_cast<OutputSection>(cmd))
-      if (sec->name == name)
-        return sec;
+    if (auto *osd = dyn_cast<OutputDesc>(cmd))
+      if (osd->osec.name == name)
+        return &osd->osec;
   return nullptr;
 }
 
-static OutputSection *createSection(InputSectionBase *isec,
-                                    StringRef outsecName) {
-  OutputSection *sec = script->createOutputSection(outsecName, "<internal>");
-  sec->recordSection(isec);
-  return sec;
+static OutputDesc *createSection(InputSectionBase *isec, StringRef outsecName) {
+  OutputDesc *osd = script->createOutputSection(outsecName, "<internal>");
+  osd->osec.recordSection(isec);
+  return osd;
 }
 
-static OutputSection *
-addInputSec(StringMap<TinyPtrVector<OutputSection *>> &map,
+static OutputDesc *addInputSec(StringMap<TinyPtrVector<OutputSection *>> &map,
                                InputSectionBase *isec, StringRef outsecName) {
   // Sections with SHT_GROUP or SHF_GROUP attributes reach here only when the -r
   // option is given. A section with SHT_GROUP defines a "section group", and
   // its members have SHF_GROUP attribute. Usually these flags have already been
   // stripped by InputFiles.cpp as section groups are processed and uniquified.
   // However, for the -r option, we want to pass through all section groups
   // as-is because adding/removing members or merging them with other groups
   // change their semantics.
   if (isec->type == SHT_GROUP || (isec->flags & SHF_GROUP))
if (!isa<SyntheticSection>(isec) &&
     auto *sec = cast<InputSection>(isec);
     OutputSection *out = sec->getRelocatedSection()->getOutputSection();
 
     if (out->relocationSection) {
       out->relocationSection->recordSection(sec);
       return nullptr;
     }
 
-    out->relocationSection = createSection(isec, outsecName);
-    return out->relocationSection;
+    OutputDesc *osd = createSection(isec, outsecName);
+    out->relocationSection = &osd->osec;
+    return osd;
   }
 
   //  The ELF spec just says
   // ----------------------------------------------------------------
   // In the first phase, input sections that match in name, type and
   // attribute flags should be concatenated into single sections.
   // ----------------------------------------------------------------
   //
if (config->relocatable && (isec->flags & SHF_LINK_ORDER)) {
       if (firstIsecOut != isec->getLinkOrderDep()->getOutputSection())
         continue;
     }
 
     sec->recordSection(isec);
     return nullptr;
   }
 
-  OutputSection *sec = createSection(isec, outsecName);
-  v.push_back(sec);
-  return sec;
+  OutputDesc *osd = createSection(isec, outsecName);
+  v.push_back(&osd->osec);
+  return osd;
 }
 
 // Add sections that didn't match any sections command.
 void LinkerScript::addOrphanSections() {
   StringMap<TinyPtrVector<OutputSection *>> map;
-  SmallVector<OutputSection *, 0> v;
+  SmallVector<OutputDesc *, 0> v;
 
   auto add = [&](InputSectionBase *s) {
     if (s->isLive() && !s->parent) {
       orphanSections.push_back(s);
 
       StringRef name = getOutputSectionName(s);
       if (config->unique) {
         v.push_back(createSection(s, name));
       } else if (OutputSection *sec = findByName(sectionCommands, name)) {
         sec->recordSection(s);
       } else {
-        if (OutputSection *os = addInputSec(map, s, name))
-          v.push_back(os);
+        if (OutputDesc *osd = addInputSec(map, s, name))
+          v.push_back(osd);
         assert(isa<MergeInputSection>(s) ||
                s->getOutputSection()->sectionIndex == UINT32_MAX);
       }
     }
   };
 
   // For further --emit-reloc handling code we need target output section
   // to be created before we create relocation output section, so we want
void LinkerScript::adjustOutputSections() {
   //   * The address assignment.
   // The other option is to pick flags that minimize the impact the section
   // will have on the rest of the linker. That is why we copy the flags from
   // the previous sections. Only a few flags are needed to keep the impact low.
   uint64_t flags = SHF_ALLOC;
 
   SmallVector<StringRef, 0> defPhdrs;
   for (SectionCommand *&cmd : sectionCommands) {
-    auto *sec = dyn_cast<OutputSection>(cmd);
-    if (!sec)
+    if (!isa<OutputDesc>(cmd))
       continue;
+    auto *sec = &cast<OutputDesc>(cmd)->osec;
 
     // Handle align (e.g. ".foo : ALIGN(16) { ... }").
     if (sec->alignExpr)
       sec->alignment =
           std::max<uint32_t>(sec->alignment, sec->alignExpr().getValue());
 
     bool isEmpty = (getFirstInputSection(sec) == nullptr);
     bool discardable = isEmpty && isDiscardable(*sec);
void LinkerScript::adjustOutputSections() {
   // more aggressive at removing them.
   llvm::erase_if(sectionCommands, [&](SectionCommand *cmd) { return !cmd; });
 }
 
 void LinkerScript::adjustSectionsAfterSorting() {
   // Try and find an appropriate memory region to assign offsets in.
   MemoryRegion *hint = nullptr;
   for (SectionCommand *cmd : sectionCommands) {
-    if (auto *sec = dyn_cast<OutputSection>(cmd)) {
+    if (auto *osd = dyn_cast<OutputDesc>(cmd)) {
+      OutputSection *sec = &osd->osec;
       if (!sec->lmaRegionName.empty()) {
         if (MemoryRegion *m = memoryRegions.lookup(sec->lmaRegionName))
           sec->lmaRegion = m;
         else
           error("memory region '" + sec->lmaRegionName + "' not declared");
       }
       std::tie(sec->memRegion, hint) = findMemoryRegion(sec, hint);
     }
auto firstPtLoad = llvm::find_if(phdrsCommands, [](const PhdrsCommand &cmd) {
     return cmd.type == PT_LOAD;
   });
   if (firstPtLoad != phdrsCommands.end())
     defPhdrs.push_back(firstPtLoad->name);
 
   // Walk the commands and propagate the program headers to commands that don't
   // explicitly specify them.
   for (SectionCommand *cmd : sectionCommands)
-    if (auto *sec = dyn_cast<OutputSection>(cmd))
-      maybePropagatePhdrs(*sec, defPhdrs);
+    if (auto *osd = dyn_cast<OutputDesc>(cmd))
+      maybePropagatePhdrs(osd->osec, defPhdrs);
 }
 
 static uint64_t computeBase(uint64_t min, bool allocateHeaders) {
   // If there is no SECTIONS or if the linkerscript is explicit about program
   // headers, do our best to allocate them.
   if (!script->hasSectionsCommand || allocateHeaders)
     return 0;
   // Otherwise only allocate program headers if that would not add a page.
const Defined *LinkerScript::assignAddresses() {
   SymbolAssignmentMap oldValues = getSymbolAssignmentValues(sectionCommands);
   for (SectionCommand *cmd : sectionCommands) {
     if (auto *assign = dyn_cast<SymbolAssignment>(cmd)) {
       assign->addr = dot;
       assignSymbol(assign, false);
       assign->size = dot - assign->addr;
       continue;
     }
-    assignOffsets(cast<OutputSection>(cmd));
+    assignOffsets(&cast<OutputDesc>(cmd)->osec);
   }
 
   ctx = nullptr;
   return getChangedSymbolAssignment(oldValues);
 }
 
 // Creates program headers as instructed by PHDRS linker script command.
 SmallVector<PhdrEntry *, 0> LinkerScript::createPhdrs() {

lld/ELF/MapFile.cpp

if (auto *assign = dyn_cast<SymbolAssignment>(cmd)) {
       if (assign->provide && !assign->sym)
         continue;
       uint64_t lma = osec ? osec->getLMA() + assign->addr - osec->getVA(0) : 0;
       writeHeader(os, assign->addr, lma, assign->size, 1);
       os << assign->commandString << '\n';
       continue;
     }
 
-    osec = cast<OutputSection>(cmd);
+    osec = &cast<OutputDesc>(cmd)->osec;
     writeHeader(os, osec->addr, osec->getLMA(), osec->size, osec->alignment);
     os << osec->name << '\n';
 
     // Dump symbols for each input section.
     for (SectionCommand *subCmd : osec->commands) {
       if (auto *isd = dyn_cast<InputSectionDescription>(subCmd)) {
         for (InputSection *isec : isd->sections) {
           if (auto *ehSec = dyn_cast<EhFrameSection>(isec)) {

lld/ELF/OutputSections.h

 
 struct CompressedData {
   std::unique_ptr<SmallVector<uint8_t, 0>[]> shards;
   uint32_t numShards = 0;
   uint32_t checksum = 0;
   uint64_t uncompressedSize;
 };
 
 // This represents a section in an output file.
 // It is composed of multiple InputSections.
 // The writer creates multiple OutputSections and assign them unique,
 // non-overlapping file offsets and VAs.
-class OutputSection final : public SectionCommand, public SectionBase {
+class OutputSection final : public SectionBase {
 public:
   OutputSection(StringRef name, uint32_t type, uint64_t flags);
 
   static bool classof(const SectionBase *s) {
     return s->kind() == SectionBase::Output;
   }
 
-  static bool classof(const SectionCommand *c);
-
   uint64_t getLMA() const { return ptLoad ? addr + ptLoad->lmaOffset : addr; }
   template <typename ELFT> void writeHeaderTo(typename ELFT::Shdr *sHdr);
 
   uint32_t sectionIndex = UINT32_MAX;
   unsigned sortRank;
 
   uint32_t getPhdrFlags() const;
 

 
 private:
   // Used for implementation of --compress-debug-sections option.
   CompressedData compressed;
 
   std::array<uint8_t, 4> getFiller();
 };
 
+struct OutputDesc final : SectionCommand {
+  OutputSection osec;
+  OutputDesc(StringRef name, uint32_t type, uint64_t flags)
+      : SectionCommand(OutputSectionKind), osec(name, type, flags) {}
+
+  static bool classof(const SectionCommand *c) {
+    return c->kind == OutputSectionKind;
+  }
+};
+
 int getPriority(StringRef s);
 
 InputSection *getFirstInputSection(const OutputSection *os);
 SmallVector<InputSection *, 0> getInputSections(const OutputSection &os);
 
 // All output sections that are handled by the linker specially are
 // globally accessible. Writer initializes them, so don't use them
 // until Writer is initialized.

lld/ELF/OutputSections.cpp

void OutputSection::writeHeaderTo(typename ELFT::Shdr *shdr) {
   shdr->sh_info = info;
   shdr->sh_link = link;
   shdr->sh_addr = addr;
   shdr->sh_size = size;
   shdr->sh_name = shName;
 }
 
 OutputSection::OutputSection(StringRef name, uint32_t type, uint64_t flags)
-    : SectionCommand(OutputSectionKind),
-      SectionBase(Output, name, flags, /*Entsize*/ 0, /*Alignment*/ 1, type,
+    : SectionBase(Output, name, flags, /*Entsize*/ 0, /*Alignment*/ 1, type,
                   /*Info*/ 0, /*Link*/ 0) {}
 
 // We allow sections of types listed below to merged into a
 // single progbits section. This is typically done by linker
 // scripts. Merging nobits and progbits will force disk space
 // to be allocated for nobits sections. Other ones don't require
 // any special treatment on top of progbits, so there doesn't
 // seem to be a harm in merging them.

 }
 
 uint64_t elf::getHeaderSize() {
   if (config->oFormatBinary)
     return 0;
   return Out::elfHeader->size + Out::programHeaders->size;
 }
 
-bool OutputSection::classof(const SectionCommand *c) {
-  return c->kind == OutputSectionKind;
-}
-
 void OutputSection::sort(llvm::function_ref<int(InputSectionBase *s)> order) {
   assert(isLive());
   for (SectionCommand *b : commands)
     if (auto *isd = dyn_cast<InputSectionDescription>(b))
       sortByOrder(isd->sections, order);
 }
 
 static void nopInstrFill(uint8_t *buf, size_t size) {

lld/ELF/ScriptParser.cpp

private:
   void readVersion();
   void readVersionScriptCommand();
 
   SymbolAssignment *readSymbolAssignment(StringRef name);
   ByteCommand *readByteCommand(StringRef tok);
   std::array<uint8_t, 4> readFill();
   bool readSectionDirective(OutputSection *cmd, StringRef tok1, StringRef tok2);
   void readSectionAddressType(OutputSection *cmd);
-  OutputSection *readOverlaySectionDescription();
-  OutputSection *readOutputSectionDescription(StringRef outSec);
+  OutputDesc *readOverlaySectionDescription();
+  OutputDesc *readOutputSectionDescription(StringRef outSec);
   SmallVector<SectionCommand *, 0> readOverlay();
   SmallVector<StringRef, 0> readOutputSectionPhdrs();
   std::pair<uint64_t, uint64_t> readInputSectionFlags();
   InputSectionDescription *readInputSectionDescription(StringRef tok);
   StringMatcher readFilePatterns();
   SmallVector<SectionPattern, 0> readInputSectionsList();
   InputSectionDescription *readInputSectionRules(StringRef filePattern,
                                                  uint64_t withFlags,
SmallVector<SectionCommand *, 0> ScriptParser::readOverlay() {
   Expr lmaExpr = readParenExpr();
   expect("{");
 
   SmallVector<SectionCommand *, 0> v;
   OutputSection *prev = nullptr;
   while (!errorCount() && !consume("}")) {
     // VA is the same for all sections. The LMAs are consecutive in memory
     // starting from the base load address specified.
-    OutputSection *os = readOverlaySectionDescription();
-    os->addrExpr = addrExpr;
+    OutputDesc *osd = readOverlaySectionDescription();
+    osd->osec.addrExpr = addrExpr;
     if (prev)
-      os->lmaExpr = [=] { return prev->getLMA() + prev->size; };
+      osd->osec.lmaExpr = [=] { return prev->getLMA() + prev->size; };
     else
-      os->lmaExpr = lmaExpr;
-    v.push_back(os);
-    prev = os;
+      osd->osec.lmaExpr = lmaExpr;
+    v.push_back(osd);
+    prev = &osd->osec;
   }
 
   // According to the specification, at the end of the overlay, the location
   // counter should be equal to the overlay base address plus size of the
   // largest section seen in the overlay.
   // Here we want to create the Dot assignment command to achieve that.
   Expr moveDot = [=] {
     uint64_t max = 0;
     for (SectionCommand *cmd : v)
-      max = std::max(max, cast<OutputSection>(cmd)->size);
+      max = std::max(max, cast<OutputDesc>(cmd)->osec.size);
     return addrExpr().getValue() + max;
   };
   v.push_back(make<SymbolAssignment>(".", moveDot, getCurrentLocation()));
   return v;
 }
 
 void ScriptParser::readOverwriteSections() {
   expect("{");
void ScriptParser::readSections() {
   bool isAfter = false;
   if (consume("AFTER"))
     isAfter = true;
   else if (!consume("BEFORE"))
     setError("expected AFTER/BEFORE, but got '" + next() + "'");
   StringRef where = next();
   SmallVector<StringRef, 0> names;
   for (SectionCommand *cmd : v)
-    if (auto *os = dyn_cast<OutputSection>(cmd))
-      names.push_back(os->name);
+    if (auto *os = dyn_cast<OutputDesc>(cmd))
+      names.push_back(os->osec.name);
   if (!names.empty())
     script->insertCommands.push_back({std::move(names), isAfter, where});
 }
 
 void ScriptParser::readTarget() {
   // TARGET(foo) is an alias for "--format foo". Unlike GNU linkers,
   // we accept only a limited set of BFD names (i.e. "elf" or "binary")
   // for --format. We recognize only /^elf/ and "binary" in the linker
return [=] {
     if (!isPowerOf2_64(alignment)) {
       error(loc + ": alignment must be power of 2");
       return (uint64_t)1; // Return a dummy value.
     }
     return alignment;
   };
 }
 
-OutputSection *ScriptParser::readOverlaySectionDescription() {
-  OutputSection *cmd =
-      script->createOutputSection(next(), getCurrentLocation());
-  cmd->inOverlay = true;
+OutputDesc *ScriptParser::readOverlaySectionDescription() {
+  OutputDesc *osd = script->createOutputSection(next(), getCurrentLocation());
+  osd->osec.inOverlay = true;
   expect("{");
   while (!errorCount() && !consume("}")) {
     uint64_t withFlags = 0;
     uint64_t withoutFlags = 0;
     if (consume("INPUT_SECTION_FLAGS"))
       std::tie(withFlags, withoutFlags) = readInputSectionFlags();
-    cmd->commands.push_back(
+    osd->osec.commands.push_back(
         readInputSectionRules(next(), withFlags, withoutFlags));
   }
-  return cmd;
+  return osd;
 }
 
-OutputSection *ScriptParser::readOutputSectionDescription(StringRef outSec) {
-  OutputSection *cmd =
-      script->createOutputSection(outSec, getCurrentLocation());
+OutputDesc *ScriptParser::readOutputSectionDescription(StringRef outSec) {
+  OutputDesc *cmd = script->createOutputSection(outSec, getCurrentLocation());
+  OutputSection *osec = &cmd->osec;
 
   size_t symbolsReferenced = script->referencedSymbols.size();
 
   if (peek() != ":")
-    readSectionAddressType(cmd);
+    readSectionAddressType(osec);
   expect(":");
 
   std::string location = getCurrentLocation();
   if (consume("AT"))
-    cmd->lmaExpr = readParenExpr();
+    osec->lmaExpr = readParenExpr();
   if (consume("ALIGN"))
-    cmd->alignExpr = checkAlignment(readParenExpr(), location);
+    osec->alignExpr = checkAlignment(readParenExpr(), location);
   if (consume("SUBALIGN"))
-    cmd->subalignExpr = checkAlignment(readParenExpr(), location);
+    osec->subalignExpr = checkAlignment(readParenExpr(), location);
 
   // Parse constraints.
   if (consume("ONLY_IF_RO"))
-    cmd->constraint = ConstraintKind::ReadOnly;
+    osec->constraint = ConstraintKind::ReadOnly;
   if (consume("ONLY_IF_RW"))
-    cmd->constraint = ConstraintKind::ReadWrite;
+    osec->constraint = ConstraintKind::ReadWrite;
   expect("{");
 
   while (!errorCount() && !consume("}")) {
     StringRef tok = next();
     if (tok == ";") {
       // Empty commands are allowed. Do nothing here.
     } else if (SymbolAssignment *assign = readAssignment(tok)) {
-      cmd->commands.push_back(assign);
+      osec->commands.push_back(assign);
     } else if (ByteCommand *data = readByteCommand(tok)) {
-      cmd->commands.push_back(data);
+      osec->commands.push_back(data);
     } else if (tok == "CONSTRUCTORS") {
       // CONSTRUCTORS is a keyword to make the linker recognize C++ ctors/dtors
       // by name. This is for very old file formats such as ECOFF/XCOFF.
       // For ELF, we should ignore.
     } else if (tok == "FILL") {
       // We handle the FILL command as an alias for =fillexp section attribute,
       // which is different from what GNU linkers do.
       // https://sourceware.org/binutils/docs/ld/Output-Section-Data.html
       if (peek() != "(")
         setError("( expected, but got " + peek());
-      cmd->filler = readFill();
+      osec->filler = readFill();
     } else if (tok == "SORT") {
       readSort();
     } else if (tok == "INCLUDE") {
       readInclude();
     } else if (peek() == "(") {
-      cmd->commands.push_back(readInputSectionDescription(tok));
+      osec->commands.push_back(readInputSectionDescription(tok));
     } else {
       // We have a file name and no input sections description. It is not a
       // commonly used syntax, but still acceptable. In that case, all sections
       // from the file will be included.
       // FIXME: GNU ld permits INPUT_SECTION_FLAGS to be used here. We do not
       // handle this case here as it will already have been matched by the
       // case above.
       auto *isd = make<InputSectionDescription>(tok);
       isd->sectionPatterns.push_back({{}, StringMatcher("*")});
-      cmd->commands.push_back(isd);
+      osec->commands.push_back(isd);
     }
   }
 
   if (consume(">"))
-    cmd->memoryRegionName = std::string(next());
+    osec->memoryRegionName = std::string(next());
 
   if (consume("AT")) {
     expect(">");
-    cmd->lmaRegionName = std::string(next());
+    osec->lmaRegionName = std::string(next());
   }
 
-  if (cmd->lmaExpr && !cmd->lmaRegionName.empty())
+  if (osec->lmaExpr && !osec->lmaRegionName.empty())
     error("section can't have both LMA and a load region");
 
-  cmd->phdrs = readOutputSectionPhdrs();
+  osec->phdrs = readOutputSectionPhdrs();
 
   if (peek() == "=" || peek().startswith("=")) {
     inExpr = true;
     consume("=");
-    cmd->filler = readFill();
+    osec->filler = readFill();
     inExpr = false;
   }
 
   // Consume optional comma following output section command.
   consume(",");
 
   if (script->referencedSymbols.size() > symbolsReferenced)
-    cmd->expressionsUseSymbols = true;
+    osec->expressionsUseSymbols = true;
   return cmd;
 }
 
 // Reads a `=<fillexp>` expression and returns its value as a big-endian number.
 // https://sourceware.org/binutils/docs/ld/Output-Section-Fill.html
 // We do not support using symbols in such expressions.
 //
 // When reading a hexstring, ld.bfd handles it as a blob of arbitrary
if (tok == "ABSOLUTE") {
     return [=] {
       ExprValue i = inner();
       i.forceAbsolute = true;
       return i;
     };
   }
   if (tok == "ADDR") {
     StringRef name = readParenLiteral();
-    OutputSection *osec = script->getOrCreateOutputSection(name);
+    OutputSection *osec = &script->getOrCreateOutputSection(name)->osec;
     osec->usedInExpression = true;
     return [=]() -> ExprValue {
       checkIfExists(*osec, location);
       return {osec, false, 0, location};
     };
   }
   if (tok == "ALIGN") {
     expect("(");
     Expr e = readExpr();
     if (consume(")")) {
       e = checkAlignment(e, location);
       return [=] { return alignTo(script->getDot(), e().getValue()); };
     }
     expect(",");
     Expr e2 = checkAlignment(readExpr(), location);
     expect(")");
     return [=] {
       ExprValue v = e();
       v.alignment = e2().getValue();
       return v;
     };
   }
   if (tok == "ALIGNOF") {
     StringRef name = readParenLiteral();
-    OutputSection *osec = script->getOrCreateOutputSection(name);
+    OutputSection *osec = &script->getOrCreateOutputSection(name)->osec;
     return [=] {
       checkIfExists(*osec, location);
       return osec->alignment;
     };
   }
   if (tok == "ASSERT")
     return readAssert();
   if (tok == "CONSTANT")
if (tok == "LENGTH") {
     if (script->memoryRegions.count(name) == 0) {
       setError("memory region not defined: " + name);
       return [] { return 0; };
     }
     return script->memoryRegions[name]->length;
   }
   if (tok == "LOADADDR") {
     StringRef name = readParenLiteral();
-    OutputSection *osec = script->getOrCreateOutputSection(name);
+    OutputSection *osec = &script->getOrCreateOutputSection(name)->osec;
     osec->usedInExpression = true;
     return [=] {
       checkIfExists(*osec, location);
       return osec->getLMA();
     };
   }
   if (tok == "LOG2CEIL") {
     expect("(");
if (tok == "SEGMENT_START") {
     skip();
     expect(",");
     Expr e = readExpr();
     expect(")");
     return [=] { return e(); };
   }
   if (tok == "SIZEOF") {
     StringRef name = readParenLiteral();
-    OutputSection *cmd = script->getOrCreateOutputSection(name);
+    OutputSection *cmd = &script->getOrCreateOutputSection(name)->osec;
     // Linker script does not create an output section if its content is empty.
     // We want to allow SIZEOF(.foo) where .foo is a section which happened to
     // be empty.
     return [=] { return cmd->size; };
   }
   if (tok == "SIZEOF_HEADERS")
     return [=] { return elf::getHeaderSize(); };
 

lld/ELF/SyntheticSections.cpp

 bool SymtabShndxSection::isNeeded() const {
   // SHT_SYMTAB can hold symbols with section indices values up to
   // SHN_LORESERVE. If we need more, we want to use extension SHT_SYMTAB_SHNDX
   // section. Problem is that we reveal the final section indices a bit too
   // late, and we do not know them here. For simplicity, we just always create
   // a .symtab_shndx section when the amount of output sections is huge.
   size_t size = 0;
   for (SectionCommand *cmd : script->sectionCommands)
-    if (isa<OutputSection>(cmd))
+    if (isa<OutputDesc>(cmd))
       ++size;
   return size >= SHN_LORESERVE;
 }
 
 void SymtabShndxSection::finalizeContents() {
   getParent()->link = in.symTab->getParent()->sectionIndex;
 }
 

lld/ELF/Writer.cpp

void elf::addReservedSymbols() {
   ElfSym::etext1 = add("etext", -1);
   ElfSym::etext2 = add("_etext", -1);
   ElfSym::edata1 = add("edata", -1);
   ElfSym::edata2 = add("_edata", -1);
 }
 
 static OutputSection *findSection(StringRef name, unsigned partition = 1) {
   for (SectionCommand *cmd : script->sectionCommands)
-    if (auto *sec = dyn_cast<OutputSection>(cmd))
-      if (sec->name == name && sec->partition == partition)
-        return sec;
+    if (auto *osd = dyn_cast<OutputDesc>(cmd))
+      if (osd->osec.name == name && osd->osec.partition == partition)
+        return &osd->osec;
   return nullptr;
 }
 
 template <class ELFT> void elf::createSyntheticSections() {
   // Initialize all pointers with NULL. This is needed because
   // you can call lld::elf::main more than once as a library.
   Out::tlsPhdr = nullptr;
   Out::preinitArray = nullptr;

 }
 
 // Create a section symbol for each output section so that we can represent
 // relocations that point to the section. If we know that no relocation is
 // referring to a section (that happens if the section is a synthetic one), we
 // don't create a section symbol for that section.
 template <class ELFT> void Writer<ELFT>::addSectionSymbols() {
   for (SectionCommand *cmd : script->sectionCommands) {
-    auto *sec = dyn_cast<OutputSection>(cmd);
-    if (!sec)
+    auto *osd = dyn_cast<OutputDesc>(cmd);
+    if (!osd)
       continue;
+    OutputSection *sec = &osd->osec;
     auto i = llvm::find_if(sec->commands, [](SectionCommand *cmd) {
       if (auto *isd = dyn_cast<InputSectionDescription>(cmd))
         return !isd->sections.empty();
       return false;
     });
     if (i == sec->commands.end())
       continue;
     InputSectionBase *isec = cast<InputSectionDescription>(*i)->sections[0];
if (osec.name != ".got")
       rank |= RF_MIPS_NOT_GOT;
   }
 
   return rank;
 }
 
 static bool compareSections(const SectionCommand *aCmd,
                             const SectionCommand *bCmd) {
-  const OutputSection *a = cast<OutputSection>(aCmd);
-  const OutputSection *b = cast<OutputSection>(bCmd);
+  const OutputSection *a = &cast<OutputDesc>(aCmd)->osec;
+  const OutputSection *b = &cast<OutputDesc>(bCmd)->osec;
 
   if (a->sortRank != b->sortRank)
     return a->sortRank < b->sortRank;
 
   if (!(a->sortRank & RF_NOT_ADDR_SET))
     return config->sectionStartMap.lookup(a->name) <
            config->sectionStartMap.lookup(b->name);
   return false;
if (ElfSym::mipsGp) {
     }
   }
 }
 
 // We want to find how similar two ranks are.
 // The more branches in getSectionRank that match, the more similar they are.
 // Since each branch corresponds to a bit flag, we can just use
 // countLeadingZeros.
-static int getRankProximityAux(OutputSection *a, OutputSection *b) {
-  return countLeadingZeros(a->sortRank ^ b->sortRank);
+static int getRankProximityAux(const OutputSection &a, const OutputSection &b) {
+  return countLeadingZeros(a.sortRank ^ b.sortRank);
 }
 
 static int getRankProximity(OutputSection *a, SectionCommand *b) {
-  auto *sec = dyn_cast<OutputSection>(b);
-  return (sec && sec->hasInputSections) ? getRankProximityAux(a, sec) : -1;
+  auto *osd = dyn_cast<OutputDesc>(b);
+  return (osd && osd->osec.hasInputSections)
+             ? getRankProximityAux(*a, osd->osec)
+             : -1;
 }
 
 // When placing orphan sections, we want to place them after symbol assignments
 // so that an orphan after
 //   begin_foo = .;
 //   foo : { *(foo) }
 //   end_foo = .;
 // doesn't break the intended meaning of the begin/end symbols.

 }
 
 // We want to place orphan sections so that they share as much
 // characteristics with their neighbors as possible. For example, if
 // both are rw, or both are tls.
 static SmallVectorImpl<SectionCommand *>::iterator
 findOrphanPos(SmallVectorImpl<SectionCommand *>::iterator b,
               SmallVectorImpl<SectionCommand *>::iterator e) {
-  OutputSection *sec = cast<OutputSection>(*e);
+  OutputSection *sec = &cast<OutputDesc>(*e)->osec;
 
   // Find the first element that has as close a rank as possible.
   auto i = std::max_element(b, e, [=](SectionCommand *a, SectionCommand *b) {
     return getRankProximity(sec, a) < getRankProximity(sec, b);
   });
   if (i == e)
     return e;
-  auto foundSec = dyn_cast<OutputSection>(*i);
-  if (!foundSec)
+  if (!isa<OutputDesc>(*i))
     return e;
+  auto foundSec = &cast<OutputDesc>(*i)->osec;
 
   // Consider all existing sections with the same proximity.
   int proximity = getRankProximity(sec, *i);
   unsigned sortRank = sec->sortRank;
   if (script->hasPhdrsCommands() || !script->memoryRegions.empty())
     // Prevent the orphan section to be placed before the found section. If
     // custom program headers are defined, that helps to avoid adding it to a
     // previous segment and changing flags of that segment, for example, making
     // a read-only segment writable. If memory regions are defined, an orphan
     // section should continue the same region as the found section to better
     // resemble the behavior of GNU ld.
     sortRank = std::max(sortRank, foundSec->sortRank);
   for (; i != e; ++i) {
-    auto *curSec = dyn_cast<OutputSection>(*i);
-    if (!curSec || !curSec->hasInputSections)
+    auto *curSecDesc = dyn_cast<OutputDesc>(*i);
+    if (!curSecDesc || !curSecDesc->osec.hasInputSections)
       continue;
-    if (getRankProximity(sec, curSec) != proximity ||
-        sortRank < curSec->sortRank)
+    if (getRankProximity(sec, curSecDesc) != proximity ||
+        sortRank < curSecDesc->osec.sortRank)
       break;
   }
 
   auto isOutputSecWithInputSections = [](SectionCommand *cmd) {
-    auto *os = dyn_cast<OutputSection>(cmd);
-    return os && os->hasInputSections;
+    auto *osd = dyn_cast<OutputDesc>(cmd);
+    return osd && osd->osec.hasInputSections;
   };
   auto j =
       std::find_if(std::make_reverse_iterator(i), std::make_reverse_iterator(b),
                    isOutputSecWithInputSections);
   i = j.base();
 
   // As a special case, if the orphan section is the last section, put
   // it at the very end, past any other commands.

 
 // If no layout was provided by linker script, we want to apply default
 // sorting for special input sections. This also handles --symbol-ordering-file.
 template <class ELFT> void Writer<ELFT>::sortInputSections() {
   // Build the order once since it is expensive.
   DenseMap<const InputSectionBase *, int> order = buildSectionOrder();
   maybeShuffle(order);
   for (SectionCommand *cmd : script->sectionCommands)
-    if (auto *sec = dyn_cast<OutputSection>(cmd))
-      sortSection(*sec, order);
+    if (auto *osd = dyn_cast<OutputDesc>(cmd))
+      sortSection(osd->osec, order);
 }
 
 template <class ELFT> void Writer<ELFT>::sortSections() {
   llvm::TimeTraceScope timeScope("Sort sections");
 
   // Don't sort if using -r. It is not necessary and we want to preserve the
   // relative order for SHF_LINK_ORDER sections.
   if (config->relocatable) {
     script->adjustOutputSections();
     return;
   }
 
   sortInputSections();
 
   for (SectionCommand *cmd : script->sectionCommands)
-    if (auto *osec = dyn_cast<OutputSection>(cmd))
-      osec->sortRank = getSectionRank(*osec);
+    if (auto *osd = dyn_cast<OutputDesc>(cmd))
+      osd->osec.sortRank = getSectionRank(osd->osec);
   if (!script->hasSectionsCommand) {
     // We know that all the OutputSections are contiguous in this case.
-    auto isSection = [](SectionCommand *cmd) {
-      return isa<OutputSection>(cmd);
-    };
+    auto isSection = [](SectionCommand *cmd) { return isa<OutputDesc>(cmd); };
     std::stable_sort(
         llvm::find_if(script->sectionCommands, isSection),
         llvm::find_if(llvm::reverse(script->sectionCommands), isSection).base(),
         compareSections);
   }
 
   // Process INSERT commands and update output section attributes. From this
   // point onwards the order of script->sectionCommands is fixed.
template <class ELFT> void Writer<ELFT>::sortSections() {
   // formal specification of the linker script. We use heuristics to
   // determine whether a new output command should be added before or
   // after another commands. For the details, look at shouldSkip
   // function.
 
   auto i = script->sectionCommands.begin();
   auto e = script->sectionCommands.end();
   auto nonScriptI = std::find_if(i, e, [](SectionCommand *cmd) {
-    if (auto *sec = dyn_cast<OutputSection>(cmd))
-      return sec->sectionIndex == UINT32_MAX;
+    if (auto *osd = dyn_cast<OutputDesc>(cmd))
+      return osd->osec.sectionIndex == UINT32_MAX;
     return false;
   });
 
   // Sort the orphan sections.
   std::stable_sort(nonScriptI, e, compareSections);
 
   // As a horrible special case, skip the first . assignment if it is before any
   // section. We do this because it is common to set a load address by starting
   // the script with ". = 0xabcd" and the expectation is that every section is
   // after that.
   auto firstSectionOrDotAssignment =
       std::find_if(i, e, [](SectionCommand *cmd) { return !shouldSkip(cmd); });
   if (firstSectionOrDotAssignment != e &&
       isa<SymbolAssignment>(**firstSectionOrDotAssignment))
     ++firstSectionOrDotAssignment;
   i = firstSectionOrDotAssignment;
 
   while (nonScriptI != e) {
     auto pos = findOrphanPos(i, nonScriptI);
-    OutputSection *orphan = cast<OutputSection>(*nonScriptI);
+    OutputSection *orphan = &cast<OutputDesc>(*nonScriptI)->osec;
 
     // As an optimization, find all sections with the same sort rank
     // and insert them with one rotate.
     unsigned rank = orphan->sortRank;
     auto end = std::find_if(nonScriptI + 1, e, [=](SectionCommand *cmd) {
-      return cast<OutputSection>(cmd)->sortRank != rank;
+      return cast<OutputDesc>(cmd)->osec.sortRank != rank;
     });
     std::rotate(pos, nonScriptI, end);
     nonScriptI = end;
   }
 
   script->adjustSectionsAfterSorting();
 }
 
template <class ELFT> void Writer<ELFT>::finalizeAddressDependentContent() {
 
   if (config->relocatable)
     for (OutputSection *sec : outputSections)
       sec->addr = 0;
 
   // If addrExpr is set, the address may not be a multiple of the alignment.
   // Warn because this is error-prone.
   for (SectionCommand *cmd : script->sectionCommands)
-    if (auto *os = dyn_cast<OutputSection>(cmd))
-      if (os->addr % os->alignment != 0)
-        warn("address (0x" + Twine::utohexstr(os->addr) + ") of section " +
-             os->name + " is not a multiple of alignment (" +
-             Twine(os->alignment) + ")");
+    if (auto *osd = dyn_cast<OutputDesc>(cmd)) {
+      OutputSection *osec = &osd->osec;
+      if (osec->addr % osec->alignment != 0)
+        warn("address (0x" + Twine::utohexstr(osec->addr) + ") of section " +
+             osec->name + " is not a multiple of alignment (" +
+             Twine(osec->alignment) + ")");
+    }
 }
 
 // If Input Sections have been shrunk (basic block sections) then
 // update symbol values and sizes associated with these sections.  With basic
 // block sections, input sections can shrink when the jump instructions at
 // the end of the section are relaxed.
 static void fixSymbolsAfterShrinking() {
   for (InputFile *File : objectFiles) {
template <class ELFT> void Writer<ELFT>::finalizeSections() {
   Out::finiArray = findSection(".fini_array");
 
   // The linker needs to define SECNAME_start, SECNAME_end and SECNAME_stop
   // symbols for sections, so that the runtime can get the start and end
   // addresses of each section by section name. Add such symbols.
   if (!config->relocatable) {
     addStartEndSymbols();
     for (SectionCommand *cmd : script->sectionCommands)
-      if (auto *sec = dyn_cast<OutputSection>(cmd))
-        addStartStopSymbols(*sec);
+      if (auto *osd = dyn_cast<OutputDesc>(cmd))
+        addStartStopSymbols(osd->osec);
   }
 
   // Add _DYNAMIC symbol. Unlike GNU gold, our _DYNAMIC symbol has no type.
   // It should be okay as no one seems to care about the type.
   // Even the author of gold doesn't remember why gold behaves that way.
   // https://sourceware.org/ml/binutils/2002-03/msg00360.html
   if (mainPart->dynamic->parent)
     symtab->addSymbol(Defined{/*file=*/nullptr, "_DYNAMIC", STB_WEAK, STV_HIDDEN, STT_NOTYPE,
template <class ELFT> void Writer<ELFT>::finalizeSections() {
   removeUnusedSyntheticSections();
   script->diagnoseOrphanHandling();
 
   sortSections();
 
   // Create a list of OutputSections, assign sectionIndex, and populate
   // in.shStrTab.
   for (SectionCommand *cmd : script->sectionCommands)
-    if (auto *osec = dyn_cast<OutputSection>(cmd)) {
+    if (auto *osd = dyn_cast<OutputDesc>(cmd)) {
+      OutputSection *osec = &osd->osec;
       outputSections.push_back(osec);
       osec->sectionIndex = outputSections.size();
       osec->shName = in.shStrTab->addString(osec->name);
     }
 
   // Prefer command line supplied address over other constraints.
   for (OutputSection *sec : outputSections) {
     auto i = config->sectionStartMap.find(sec->name);