[WebAssembly] Refactor synthetic sections and relocation processing. NFC.

[sbc100]

Summary

Major refactor to better match the structure of the ELF linker.

• Split out relocation processing into scanRelocations
• Split out synthetic sections into their own classes.

This is a major refactor but is mostly just moving code around. The
only non-trivial change is that to the way in which SectionSymbols
are generated. In order output section symbols we now track the
output section from each input section, and each output section can
have an options SectionSymbol.

[sbc100]

• remove comments

[sbc100]

• move work from OutputSection constructors to finalizeContent

[sbc100]

• refactor all the things

[sbc100]

lld/wasm/SyntheticSections.h

318 ↗

(On Diff #199345)

I was wondering about the naming of this struct and global.

In ELF I think "In" might mean "Input" since there the synthetic sections are input sections. However we currently use OutputSections, so this name make less sense.

Perhaps I should call these "OutStruct" and "Out" or "Synth" perhaps?

[sbc100]

Let me know if this is too much to review. I could try to break it down, but it is mostly a pure refactor.

[sbc100]

Split out part of this change into a smaller CL: https://reviews.llvm.org/D61971

[sbc100]

• rebase

[sbc100]

• rebase

[ruiu]

lld/wasm/OutputSections.h

51 ↗

(On Diff #199947)

nit: OutputSectionSymbol* -> OutputSectionSymbol *

lld/wasm/Relocations.cpp

38 ↗

(On Diff #199947)

auto -> Symbol

58–59 ↗

(On Diff #199947)

nit: you can merge the two ifs into one if.

75 ↗

(On Diff #199947)

Isn't this the same as line 38?

86 ↗

(On Diff #199947)

Ditto

lld/wasm/Symbols.h

20–21 ↗

(On Diff #199947)

It would be nice to explain them as comment.

45 ↗

(On Diff #199947)

It seems like a domain error that we have "output section kind" of a symbol, because symbols are not sections and vice versa. What is this?

201 ↗

(On Diff #199947)

Ah, okay, so this class represents a section symbol... but it is not clear why we have two different section symbol types. Could you add comments?

[sbc100]

• clang-format
• feedback

[sbc100]

• rename InStruct

[sbc100]

lld/wasm/Symbols.h

20–21 ↗

(On Diff #199947)

Done.

These are shared constants.. can you think of a better place to store them? Maybe Config?

[ruiu]

Overall looking good. I didn't read each line that seems to be just a code move, though.

lld/wasm/Symbols.h

20–21 ↗

(On Diff #199947)

I think just global variables are fine. Config are mainly for command line parameters.

lld/wasm/SyntheticSections.cpp

509 ↗

(On Diff #200242)

nit: remove the empty line.

lld/wasm/SyntheticSections.h

318 ↗

(On Diff #199345)

Either Out and Synth are fine. It's up to you.

lld/wasm/Writer.cpp

149–150 ↗

(On Diff #200242)

Instead of skipping many sections that are not needed, does it make sense to add another loop to remove !isNeeded() sections from OutputSections?

532 ↗

(On Diff #200242)

auto -> InputChunk?

[sbc100]

• feedback

[sbc100]

• cleanup

[ruiu]

LGTM

wasm/CMakeLists.txt

 set(LLVM_TARGET_DEFINITIONS Options.td)
 tablegen(LLVM Options.inc -gen-opt-parser-defs)
 add_public_tablegen_target(WasmOptionsTableGen)
 
 if(NOT LLD_BUILT_STANDALONE)
   set(tablegen_deps intrinsics_gen)
 endif()
 
 add_lld_library(lldWasm
   Driver.cpp
   InputChunks.cpp
   InputFiles.cpp
   LTO.cpp
   MarkLive.cpp
   OutputSections.cpp
+  Relocations.cpp
   SymbolTable.cpp
   Symbols.cpp
+  SyntheticSections.cpp
   Writer.cpp
   WriterUtils.cpp
 
   LINK_COMPONENTS
   ${LLVM_TARGETS_TO_BUILD}
   BinaryFormat
   Core
   Demangle
   LTO
   MC
   Object
   Option
   Support
 
   LINK_LIBS
   lldCommon
 
   DEPENDS
   WasmOptionsTableGen
   ${tablegen_deps}
-  )
+  )
 

wasm/InputChunks.h

 #include "lld/Common/LLVM.h"
 #include "llvm/Object/Wasm.h"
 
 namespace lld {
 namespace wasm {
 
 class ObjFile;
 class OutputSegment;
+class OutputSection;
 
 class InputChunk {
 public:
   enum Kind { DataSegment, Function, SyntheticFunction, Section };
 
   Kind kind() const { return SectionKind; }
 
   virtual uint32_t getSize() const { return data().size(); }
InputSection(const WasmSection &S, ObjFile *F)
       : InputChunk(F, InputChunk::Section), Section(S) {
     assert(Section.Type == llvm::wasm::WASM_SEC_CUSTOM);
   }
 
   StringRef getName() const override { return Section.Name; }
   StringRef getDebugName() const override { return StringRef(); }
   uint32_t getComdat() const override { return UINT32_MAX; }
 
+  OutputSection *OutputSec = nullptr;
+
 protected:
   ArrayRef<uint8_t> data() const override { return Section.Content; }
 
   // Offset within the input section.  This is only zero since this chunk
   // type represents an entire input section, not part of one.
   uint32_t getInputSectionOffset() const override { return 0; }
 
   const WasmSection &Section;

wasm/InputFiles.cpp

 // Relocations contain either symbol or type indices.  This function takes a
 // relocation and returns relocated index (i.e. translates from the input
 // symbol/type space to the output symbol/type space).
 uint32_t ObjFile::calcNewIndex(const WasmRelocation &Reloc) const {
   if (Reloc.Type == R_WASM_TYPE_INDEX_LEB) {
     assert(TypeIsUsed[Reloc.Index]);
     return TypeMap[Reloc.Index];
   }
-  return Symbols[Reloc.Index]->getOutputSymbolIndex();
+  const Symbol *Sym = Symbols[Reloc.Index];
+  if (auto *SS = dyn_cast<SectionSymbol>(Sym))
+    Sym = SS->getOutputSectionSymbol();
+  return Sym->getOutputSymbolIndex();
 }
 
 // Relocations can contain addend for combined sections. This function takes a
 // relocation and returns updated addend by offset in the output section.
 uint32_t ObjFile::calcNewAddend(const WasmRelocation &Reloc) const {
   switch (Reloc.Type) {
   case R_WASM_MEMORY_ADDR_LEB:
   case R_WASM_MEMORY_ADDR_SLEB:
InputGlobal *Global =
         Globals[Sym.Info.ElementIndex - WasmObj->getNumImportedGlobals()];
     if (Sym.isBindingLocal())
       return make<DefinedGlobal>(Name, Flags, this, Global);
     return Symtab->addDefinedGlobal(Name, Flags, this, Global);
   }
   case WASM_SYMBOL_TYPE_SECTION: {
     InputSection *Section = CustomSectionsByIndex[Sym.Info.ElementIndex];
     assert(Sym.isBindingLocal());
-    return make<SectionSymbol>(Name, Flags, Section, this);
+    return make<SectionSymbol>(Flags, Section, this);
   }
   case WASM_SYMBOL_TYPE_EVENT: {
     InputEvent *Event =
         Events[Sym.Info.ElementIndex - WasmObj->getNumImportedEvents()];
     if (Sym.isBindingLocal())
       return make<DefinedEvent>(Name, Flags, this, Event);
     return Symtab->addDefinedEvent(Name, Flags, this, Event);
   }

wasm/OutputSections.h

public:
   virtual ~OutputSection() = default;
 
   StringRef getSectionName() const;
   void setOffset(size_t NewOffset) {
     log("setOffset: " + toString(*this) + ": " + Twine(NewOffset));
     Offset = NewOffset;
   }
   void createHeader(size_t BodySize);
+  virtual bool isNeeded() const { return true; }
   virtual size_t getSize() const = 0;
   virtual void writeTo(uint8_t *Buf) = 0;
   virtual void finalizeContents() = 0;
   virtual uint32_t numRelocations() const { return 0; }
   virtual void writeRelocations(raw_ostream &OS) const {}
 
   std::string Header;
   uint32_t Type;
+  uint32_t SectionIndex = UINT32_MAX;
   std::string Name;
+  OutputSectionSymbol *SectionSym = nullptr;
 
 protected:
   size_t Offset = 0;
 };
 
-class SyntheticSection : public OutputSection {
-public:
-  SyntheticSection(uint32_t Type, std::string Name = "")
-      : OutputSection(Type, Name), BodyOutputStream(Body) {
-    if (!Name.empty())
-      writeStr(BodyOutputStream, Name, "section name");
-  }
-
-  void writeTo(uint8_t *Buf) override {
-    assert(Offset);
-    log("writing " + toString(*this));
-    memcpy(Buf + Offset, Header.data(), Header.size());
-    memcpy(Buf + Offset + Header.size(), Body.data(), Body.size());
-  }
-
-  size_t getSize() const override { return Header.size() + Body.size(); }
-
-  virtual void writeBody() {}
-
-  void finalizeContents() override {
-    writeBody();
-    BodyOutputStream.flush();
-    createHeader(Body.size());
-  }
-
-  raw_ostream &getStream() { return BodyOutputStream; }
-
-  std::string Body;
-
-protected:
-  llvm::raw_string_ostream BodyOutputStream;
-};
-
 class CodeSection : public OutputSection {
 public:
   explicit CodeSection(ArrayRef<InputFunction *> Functions)
       : OutputSection(llvm::wasm::WASM_SEC_CODE), Functions(Functions) {}
 
-  size_t getSize() const override { assert(BodySize); return Header.size() + BodySize; }
+  size_t getSize() const override { return Header.size() + BodySize; }
   void writeTo(uint8_t *Buf) override;
   uint32_t numRelocations() const override;
   void writeRelocations(raw_ostream &OS) const override;
+  bool isNeeded() const override { return Functions.size() > 0; }
   void finalizeContents() override;
 
 protected:
   ArrayRef<InputFunction *> Functions;
   std::string CodeSectionHeader;
   size_t BodySize = 0;
 };
 
 class DataSection : public OutputSection {
 public:
   explicit DataSection(ArrayRef<OutputSegment *> Segments)
       : OutputSection(llvm::wasm::WASM_SEC_DATA), Segments(Segments) {}
 
   size_t getSize() const override { return Header.size() + BodySize; }
   void writeTo(uint8_t *Buf) override;
   uint32_t numRelocations() const override;
   void writeRelocations(raw_ostream &OS) const override;
+  bool isNeeded() const override { return Segments.size() > 0; }
   void finalizeContents() override;
 
 protected:
   ArrayRef<OutputSegment *> Segments;
   std::string DataSectionHeader;
   size_t BodySize = 0;
 };
 
public:
   void finalizeContents() override;
 
 protected:
   size_t PayloadSize = 0;
   ArrayRef<InputSection *> InputSections;
   std::string NameData;
 };
 
-class RelocSection : public SyntheticSection {
-public:
-  RelocSection(StringRef Name, OutputSection *Sec, uint32_t SectionIndex)
-      : SyntheticSection(llvm::wasm::WASM_SEC_CUSTOM, Name), Sec(Sec),
-        SectionIndex(SectionIndex) {}
-  void writeBody() override;
-
-protected:
-  OutputSection* Sec;
-  uint32_t SectionIndex;
-};
-
-
 } // namespace wasm
 } // namespace lld
 
 #endif // LLD_WASM_OUTPUT_SECTIONS_H

wasm/OutputSections.cpp

 void CustomSection::finalizeContents() {
   raw_string_ostream OS(NameData);
   encodeULEB128(Name.size(), OS);
   OS << Name;
   OS.flush();
 
   for (InputSection *Section : InputSections) {
     Section->OutputOffset = PayloadSize;
+    Section->OutputSec = this;
     PayloadSize += Section->getSize();
   }
 
   createHeader(PayloadSize + NameData.size());
 }
 
 void CustomSection::writeTo(uint8_t *Buf) {
   log("writing " + toString(*this) + " size=" + Twine(getSize()) +
for (const InputSection *InputSect : InputSections)
     Count += InputSect->NumRelocations();
   return Count;
 }
 
 void CustomSection::writeRelocations(raw_ostream &OS) const {
   for (const InputSection *S : InputSections)
     S->writeRelocations(OS);
 }
-
-void RelocSection::writeBody() {
-  writeUleb128(BodyOutputStream, SectionIndex, "reloc section");
-  writeUleb128(BodyOutputStream, Sec->numRelocations(), "reloc count");
-  Sec->writeRelocations(BodyOutputStream);
-}

wasm/Relocations.h

+//===- Relocations.h -------------------------------------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLD_WASM_RELOCATIONS_H
+#define LLD_WASM_RELOCATIONS_H
+
+namespace lld {
+namespace wasm {
+
+class InputChunk;
+
+void scanRelocations(InputChunk *Chunk);
+
+} // namespace wasm
+} // namespace lld
+
+#endif

wasm/Relocations.cpp

+//===- Relocations.cpp ----------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "Relocations.h"
+
+#include "InputChunks.h"
+#include "SyntheticSections.h"
+
+using namespace llvm;
+using namespace llvm::wasm;
+
+using namespace lld;
+using namespace lld::wasm;
+
+static bool requiresGOTAccess(const Symbol *Sym) {
+  return Config->Pic && !Sym->isHidden() && !Sym->isLocal();
+}
+
+void lld::wasm::scanRelocations(InputChunk *Chunk) {
+  if (!Chunk->Live)
+    return;
+  ObjFile *File = Chunk->File;
+  ArrayRef<WasmSignature> Types = File->getWasmObj()->types();
+  for (const WasmRelocation &Reloc : Chunk->getRelocations()) {
+    if (Reloc.Type == R_WASM_TYPE_INDEX_LEB) {
+      // Mark target type as live
+      File->TypeMap[Reloc.Index] =
+          Out.TypeSec->registerType(Types[Reloc.Index]);
+      File->TypeIsUsed[Reloc.Index] = true;
+      continue;
+    }
+
+    // Other relocation types all have a corresponding symbol
+    Symbol *Sym = File->getSymbols()[Reloc.Index];
+
+    switch (Reloc.Type) {
+    case R_WASM_TABLE_INDEX_I32:
+    case R_WASM_TABLE_INDEX_SLEB:
+    case R_WASM_TABLE_INDEX_REL_SLEB:
+      if (requiresGOTAccess(Sym))
+        break;
+      Out.ElemSec->addEntry(cast<FunctionSymbol>(Sym));
+      break;
+    case R_WASM_GLOBAL_INDEX_LEB:
+      if (!isa<GlobalSymbol>(Sym))
+        Out.ImportSec->addGOTEntry(Sym);
+      break;
+    case R_WASM_MEMORY_ADDR_SLEB:
+    case R_WASM_MEMORY_ADDR_LEB:
+    case R_WASM_MEMORY_ADDR_REL_SLEB:
+      if (!Config->Relocatable && Sym->isUndefined() && !Sym->isWeak()) {
+        error(toString(File) + ": cannot resolve relocation of type " +
+              relocTypeToString(Reloc.Type) +
+              " against undefined (non-weak) data symbol: " + toString(*Sym));
+      }
+      break;
+    }
+
+    if (Config->Pic) {
+      switch (Reloc.Type) {
+      case R_WASM_TABLE_INDEX_SLEB:
+      case R_WASM_MEMORY_ADDR_SLEB:
+      case R_WASM_MEMORY_ADDR_LEB:
+        // Certain relocation types can't be used when building PIC output,
+        // since they would require absolute symbol addresses at link time.
+        error(toString(File) + ": relocation " + relocTypeToString(Reloc.Type) +
+              " cannot be used againt symbol " + toString(*Sym) +
+              "; recompile with -fPIC");
+        break;
+      case R_WASM_TABLE_INDEX_I32:
+      case R_WASM_MEMORY_ADDR_I32:
+        // These relocation types are only present in the data section and
+        // will be converted into code by `generateRelocationCode`.  This code
+        // requires the symbols to have GOT entires.
+        if (requiresGOTAccess(Sym))
+          Out.ImportSec->addGOTEntry(Sym);
+        break;
+      }
+    }
+  }
+}

wasm/Symbols.h

 #include "Config.h"
 #include "lld/Common/LLVM.h"
 #include "llvm/Object/Archive.h"
 #include "llvm/Object/Wasm.h"
 
 namespace lld {
 namespace wasm {
 
+// Shared string constants
+
+// The default module name to use for symbol imports.
+extern const char *DefaultModule;
+
+// The name under which to import or export the wasm table.
+extern const char *FunctionTableName;
+
 using llvm::wasm::WasmSymbolType;
 
 class InputFile;
 class InputChunk;
 class InputSegment;
 class InputFunction;
 class InputGlobal;
 class InputEvent;
 class InputSection;
+class OutputSection;
 
 #define INVALID_INDEX UINT32_MAX
 
 // The base class for real symbol classes.
 class Symbol {
 public:
   enum Kind {
     DefinedFunctionKind,
     DefinedDataKind,
     DefinedGlobalKind,
     DefinedEventKind,
     SectionKind,
+    OutputSectionKind,
     UndefinedFunctionKind,
     UndefinedDataKind,
     UndefinedGlobalKind,
     LazyKind,
   };
 
   Kind kind() const { return SymbolKind; }
 
-  bool isDefined() const {
-    return SymbolKind == DefinedFunctionKind || SymbolKind == DefinedDataKind ||
-           SymbolKind == DefinedGlobalKind || SymbolKind == DefinedEventKind ||
-           SymbolKind == SectionKind;
-  }
+  bool isDefined() const { return !isLazy() && !isUndefined(); }
 
   bool isUndefined() const {
     return SymbolKind == UndefinedFunctionKind ||
            SymbolKind == UndefinedDataKind || SymbolKind == UndefinedGlobalKind;
   }
 
   bool isLazy() const { return SymbolKind == LazyKind; }
 
public:
   static bool classof(const Symbol *S) {
     return S->kind() == UndefinedFunctionKind;
   }
 
   StringRef ImportName;
   StringRef ImportModule;
 };
 
+// Section symbols for output sections are different from those for input
+// section.  These are generated by the linker and point the OutputSection
+// rather than an InputSection.
+class OutputSectionSymbol : public Symbol {
+public:
+  OutputSectionSymbol(const OutputSection *S)
+      : Symbol("", OutputSectionKind, llvm::wasm::WASM_SYMBOL_BINDING_LOCAL,
+               nullptr),
+        Section(S) {}
+
+  static bool classof(const Symbol *S) {
+    return S->kind() == OutputSectionKind;
+  }
+
+  const OutputSection *Section;
+};
+
 class SectionSymbol : public Symbol {
 public:
+  SectionSymbol(uint32_t Flags, const InputSection *S, InputFile *F = nullptr)
+      : Symbol("", SectionKind, Flags, F), Section(S) {}
+
   static bool classof(const Symbol *S) { return S->kind() == SectionKind; }
 
-  SectionSymbol(StringRef Name, uint32_t Flags, const InputSection *S,
-                InputFile *F = nullptr)
-      : Symbol(Name, SectionKind, Flags, F), Section(S) {}
+  const OutputSectionSymbol *getOutputSectionSymbol() const;
 
   const InputSection *Section;
-
-  uint32_t getOutputSectionIndex() const;
-  void setOutputSectionIndex(uint32_t Index);
-
-protected:
-  uint32_t OutputSectionIndex = INVALID_INDEX;
 };
 
 class DataSymbol : public Symbol {
 public:
   static bool classof(const Symbol *S) {
     return S->kind() == DefinedDataKind || S->kind() == UndefinedDataKind;
   }
 

wasm/Symbols.cpp

 //===- Symbols.cpp --------------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 #include "Symbols.h"
 #include "Config.h"
 #include "InputChunks.h"
 #include "InputEvent.h"
 #include "InputFiles.h"
 #include "InputGlobal.h"
+#include "OutputSections.h"
 #include "OutputSegment.h"
 #include "lld/Common/ErrorHandler.h"
 #include "lld/Common/Strings.h"
 
 #define DEBUG_TYPE "lld"
 
 using namespace llvm;
 using namespace llvm::wasm;
WasmSymbolType Symbol::getWasmType() const {
   if (isa<FunctionSymbol>(this))
     return WASM_SYMBOL_TYPE_FUNCTION;
   if (isa<DataSymbol>(this))
     return WASM_SYMBOL_TYPE_DATA;
   if (isa<GlobalSymbol>(this))
     return WASM_SYMBOL_TYPE_GLOBAL;
   if (isa<EventSymbol>(this))
     return WASM_SYMBOL_TYPE_EVENT;
-  if (isa<SectionSymbol>(this))
+  if (isa<SectionSymbol>(this) || isa<OutputSectionSymbol>(this))
     return WASM_SYMBOL_TYPE_SECTION;
   llvm_unreachable("invalid symbol kind");
 }
 
 const WasmSignature *Symbol::getSignature() const {
   if (auto* F = dyn_cast<FunctionSymbol>(this))
     return F->Signature;
   if (auto *L = dyn_cast<LazySymbol>(this))

 
 DefinedEvent::DefinedEvent(StringRef Name, uint32_t Flags, InputFile *File,
                            InputEvent *Event)
     : EventSymbol(Name, DefinedEventKind, Flags, File,
                   Event ? &Event->getType() : nullptr,
                   Event ? &Event->Signature : nullptr),
       Event(Event) {}
 
-uint32_t SectionSymbol::getOutputSectionIndex() const {
-  LLVM_DEBUG(dbgs() << "getOutputSectionIndex: " << getName() << "\n");
-  assert(OutputSectionIndex != INVALID_INDEX);
-  return OutputSectionIndex;
-}
-
-void SectionSymbol::setOutputSectionIndex(uint32_t Index) {
-  LLVM_DEBUG(dbgs() << "setOutputSectionIndex: " << getName() << " -> " << Index
-                    << "\n");
-  assert(Index != INVALID_INDEX);
-  OutputSectionIndex = Index;
+const OutputSectionSymbol *SectionSymbol::getOutputSectionSymbol() const {
+  assert(Section->OutputSec && Section->OutputSec->SectionSym);
+  return Section->OutputSec->SectionSym;
 }
 
 void LazySymbol::fetch() { cast<ArchiveFile>(File)->addMember(&ArchiveSymbol); }
 
 std::string lld::toString(const wasm::Symbol &Sym) {
   return lld::maybeDemangleSymbol(Sym.getName());
 }
 
std::string lld::toString(wasm::Symbol::Kind Kind) {
   case wasm::Symbol::UndefinedDataKind:
     return "UndefinedData";
   case wasm::Symbol::UndefinedGlobalKind:
     return "UndefinedGlobal";
   case wasm::Symbol::LazyKind:
     return "LazyKind";
   case wasm::Symbol::SectionKind:
     return "SectionKind";
+  case wasm::Symbol::OutputSectionKind:
+    return "OutputSectionKind";
   }
   llvm_unreachable("invalid symbol kind");
 }
 
 // Print out a log message for --trace-symbol.
 void lld::wasm::printTraceSymbol(Symbol *Sym) {
   std::string S;
   if (Sym->isUndefined())
     S = ": reference to ";
   else if (Sym->isLazy())
     S = ": lazy definition of ";
   else
     S = ": definition of ";
 
   message(toString(Sym->getFile()) + S + Sym->getName());
 }
+
+const char *lld::wasm::DefaultModule = "env";
+const char *lld::wasm::FunctionTableName = "__indirect_function_table";

wasm/SyntheticSections.h

+//===- SyntheticSection.h ---------------------------------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// Synthetic sections represent chunks of linker-created data. If you
+// need to create a chunk of data that to be included in some section
+// in the result, you probably want to create that as a synthetic section.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLD_WASM_SYNTHETIC_SECTIONS_H
+#define LLD_WASM_SYNTHETIC_SECTIONS_H
+
+#include "OutputSections.h"
+
+#include "llvm/ADT/SmallSet.h"
+#include "llvm/ADT/StringMap.h"
+#include "llvm/Object/WasmTraits.h"
+
+#define DEBUG_TYPE "lld"
+
+namespace lld {
+namespace wasm {
+
+// An init entry to be written to either the synthetic init func or the
+// linking metadata.
+struct WasmInitEntry {
+  const FunctionSymbol *Sym;
+  uint32_t Priority;
+};
+
+class SyntheticSection : public OutputSection {
+public:
+  SyntheticSection(uint32_t Type, std::string Name = "")
+      : OutputSection(Type, Name), BodyOutputStream(Body) {
+    if (!Name.empty())
+      writeStr(BodyOutputStream, Name, "section name");
+  }
+
+  void writeTo(uint8_t *Buf) override {
+    assert(Offset);
+    log("writing " + toString(*this));
+    memcpy(Buf + Offset, Header.data(), Header.size());
+    memcpy(Buf + Offset + Header.size(), Body.data(), Body.size());
+  }
+
+  size_t getSize() const override { return Header.size() + Body.size(); }
+
+  virtual void writeBody() {}
+
+  void finalizeContents() override {
+    writeBody();
+    BodyOutputStream.flush();
+    createHeader(Body.size());
+  }
+
+  raw_ostream &getStream() { return BodyOutputStream; }
+
+  std::string Body;
+
+protected:
+  llvm::raw_string_ostream BodyOutputStream;
+};
+
+// Create the custom "dylink" section containing information for the dynamic
+// linker.
+// See
+// https://github.com/WebAssembly/tool-conventions/blob/master/DynamicLinking.md
+class DylinkSection : public SyntheticSection {
+public:
+  DylinkSection() : SyntheticSection(llvm::wasm::WASM_SEC_CUSTOM, "dylink") {}
+  bool isNeeded() const override { return Config->Pic; }
+  void writeBody() override;
+
+  uint32_t MemAlign = 0;
+  uint32_t MemSize = 0;
+};
+
+class TypeSection : public SyntheticSection {
+public:
+  TypeSection() : SyntheticSection(llvm::wasm::WASM_SEC_TYPE) {}
+
+  bool isNeeded() const override { return Types.size() > 0; };
+  void writeBody() override;
+  uint32_t registerType(const WasmSignature &Sig);
+  uint32_t lookupType(const WasmSignature &Sig);
+
+protected:
+  std::vector<const WasmSignature *> Types;
+  llvm::DenseMap<WasmSignature, int32_t> TypeIndices;
+};
+
+class ImportSection : public SyntheticSection {
+public:
+  ImportSection() : SyntheticSection(llvm::wasm::WASM_SEC_IMPORT) {}
+  bool isNeeded() const override { return numImports() > 0; }
+  void writeBody() override;
+  void addImport(Symbol *Sym);
+  void addGOTEntry(Symbol *Sym);
+  uint32_t numImports() const;
+
+  unsigned NumImportedGlobals = 0;
+  unsigned NumImportedFunctions = 0;
+  unsigned NumImportedEvents = 0;
+  std::vector<const Symbol *> ImportedSymbols;
+
+protected:
+  std::vector<const Symbol *> GOTSymbols;
+};
+
+class FunctionSection : public SyntheticSection {
+public:
+  FunctionSection() : SyntheticSection(llvm::wasm::WASM_SEC_FUNCTION) {}
+
+  bool isNeeded() const override { return InputFunctions.size() > 0; };
+  void writeBody() override;
+  void addFunction(InputFunction *Func);
+
+  std::vector<InputFunction *> InputFunctions;
+
+protected:
+};
+
+class MemorySection : public SyntheticSection {
+public:
+  MemorySection() : SyntheticSection(llvm::wasm::WASM_SEC_MEMORY) {}
+
+  bool isNeeded() const override { return !Config->ImportMemory; }
+  void writeBody() override;
+
+  uint32_t NumMemoryPages = 0;
+  uint32_t MaxMemoryPages = 0;
+};
+
+class TableSection : public SyntheticSection {
+public:
+  TableSection() : SyntheticSection(llvm::wasm::WASM_SEC_TABLE) {}
+
+  bool isNeeded() const override {
+    // Always output a table section (or table import), even if there are no
+    // indirect calls.  There are two reasons for this:
+    //  1. For executables it is useful to have an empty table slot at 0
+    //     which can be filled with a null function call handler.
+    //  2. If we don't do this, any program that contains a call_indirect but
+    //     no address-taken function will fail at validation time since it is
+    //     a validation error to include a call_indirect instruction if there
+    //     is not table.
+    return !Config->ImportTable;
+  }
+
+  void writeBody() override;
+};
+
+class GlobalSection : public SyntheticSection {
+public:
+  GlobalSection() : SyntheticSection(llvm::wasm::WASM_SEC_GLOBAL) {}
+  uint32_t numGlobals() const {
+    return InputGlobals.size() + DefinedFakeGlobals.size();
+  }
+  bool isNeeded() const override { return numGlobals() > 0; }
+  void writeBody() override;
+  void addGlobal(InputGlobal *Global);
+
+  std::vector<const DefinedData *> DefinedFakeGlobals;
+  std::vector<InputGlobal *> InputGlobals;
+};
+
+// The event section contains a list of declared wasm events associated with the
+// module. Currently the only supported event kind is exceptions. A single event
+// entry represents a single event with an event tag. All C++ exceptions are
+// represented by a single event. An event entry in this section contains
+// information on what kind of event it is (e.g. exception) and the type of
+// values contained in a single event object. (In wasm, an event can contain
+// multiple values of primitive types. But for C++ exceptions, we just throw a
+// pointer which is an i32 value (for wasm32 architecture), so the signature of
+// C++ exception is (i32)->(void), because all event types are assumed to have
+// void return type to share WasmSignature with functions.)
+class EventSection : public SyntheticSection {
+public:
+  EventSection() : SyntheticSection(llvm::wasm::WASM_SEC_EVENT) {}
+  void writeBody() override;
+  bool isNeeded() const override { return InputEvents.size() > 0; }
+  void addEvent(InputEvent *Event);
+
+  std::vector<InputEvent *> InputEvents;
+};
+
+class ExportSection : public SyntheticSection {
+public:
+  ExportSection() : SyntheticSection(llvm::wasm::WASM_SEC_EXPORT) {}
+  bool isNeeded() const override { return Exports.size() > 0; }
+  void writeBody() override;
+
+  std::vector<llvm::wasm::WasmExport> Exports;
+};
+
+class ElemSection : public SyntheticSection {
+public:
+  ElemSection(uint32_t Offset)
+      : SyntheticSection(llvm::wasm::WASM_SEC_ELEM), ElemOffset(Offset) {}
+  bool isNeeded() const override { return IndirectFunctions.size() > 0; };
+  void writeBody() override;
+  void addEntry(FunctionSymbol *Sym);
+  uint32_t numEntries() const { return IndirectFunctions.size(); }
+  uint32_t ElemOffset;
+
+protected:
+  std::vector<const FunctionSymbol *> IndirectFunctions;
+};
+
+class DataCountSection : public SyntheticSection {
+public:
+  DataCountSection(uint32_t NumSegments)
+      : SyntheticSection(llvm::wasm::WASM_SEC_DATACOUNT),
+        NumSegments(NumSegments) {}
+  bool isNeeded() const override;
+  void writeBody() override;
+
+protected:
+  uint32_t NumSegments;
+};
+
+// Create the custom "linking" section containing linker metadata.
+// This is only created when relocatable output is requested.
+class LinkingSection : public SyntheticSection {
+public:
+  LinkingSection(const std::vector<WasmInitEntry> &InitFunctions,
+                 const std::vector<OutputSegment *> &DataSegments)
+      : SyntheticSection(llvm::wasm::WASM_SEC_CUSTOM, "linking"),
+        InitFunctions(InitFunctions), DataSegments(DataSegments) {}
+  bool isNeeded() const override { return Config->Relocatable; }
+  void writeBody() override;
+  void addToSymtab(Symbol *Sym);
+
+protected:
+  std::vector<const Symbol *> SymtabEntries;
+  llvm::StringMap<uint32_t> SectionSymbolIndices;
+  const std::vector<WasmInitEntry> &InitFunctions;
+  const std::vector<OutputSegment *> &DataSegments;
+};
+
+// Create the custom "name" section containing debug symbol names.
+class NameSection : public SyntheticSection {
+public:
+  NameSection() : SyntheticSection(llvm::wasm::WASM_SEC_CUSTOM, "name") {}
+  bool isNeeded() const override {
+    return !Config->StripDebug && !Config->StripAll && numNames() > 0;
+  }
+  void writeBody() override;
+  unsigned numNames() const;
+};
+
+class ProducersSection : public SyntheticSection {
+public:
+  ProducersSection()
+      : SyntheticSection(llvm::wasm::WASM_SEC_CUSTOM, "producers") {}
+  bool isNeeded() const override {
+    return !Config->StripAll && fieldCount() > 0;
+  }
+  void writeBody() override;
+  void addInfo(const llvm::wasm::WasmProducerInfo &Info);
+
+protected:
+  int fieldCount() const {
+    return int(!Languages.empty()) + int(!Tools.empty()) + int(!SDKs.empty());
+  }
+  SmallVector<std::pair<std::string, std::string>, 8> Languages;
+  SmallVector<std::pair<std::string, std::string>, 8> Tools;
+  SmallVector<std::pair<std::string, std::string>, 8> SDKs;
+};
+
+class TargetFeaturesSection : public SyntheticSection {
+public:
+  TargetFeaturesSection()
+      : SyntheticSection(llvm::wasm::WASM_SEC_CUSTOM, "target_features") {}
+  bool isNeeded() const override {
+    return !Config->StripAll && Features.size() > 0;
+  }
+  void writeBody() override;
+
+  llvm::SmallSet<std::string, 8> Features;
+};
+
+class RelocSection : public SyntheticSection {
+public:
+  RelocSection(StringRef Name, OutputSection *Sec)
+      : SyntheticSection(llvm::wasm::WASM_SEC_CUSTOM, Name), Sec(Sec) {}
+  void writeBody() override;
+  bool isNeeded() const override { return Sec->numRelocations() > 0; };
+
+protected:
+  OutputSection *Sec;
+};
+
+// Linker generated output sections
+struct OutStruct {
+  DylinkSection *DylinkSec;
+  TypeSection *TypeSec;
+  FunctionSection *FunctionSec;
+  ImportSection *ImportSec;
+  TableSection *TableSec;
+  MemorySection *MemorySec;
+  GlobalSection *GlobalSec;
+  EventSection *EventSec;
+  ExportSection *ExportSec;
+  ElemSection *ElemSec;
+  DataCountSection *DataCountSec;
+  LinkingSection *LinkingSec;
+  NameSection *NameSec;
+  ProducersSection *ProducersSec;
+  TargetFeaturesSection *TargetFeaturesSec;
+};
+
+extern OutStruct Out;
+
+} // namespace wasm
+} // namespace lld
+
+#endif

wasm/SyntheticSections.cpp

+//===- SyntheticSections.cpp ----------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// This file contains linker-synthesized sections.
+//
+//===----------------------------------------------------------------------===//
+
+#include "SyntheticSections.h"
+
+#include "InputChunks.h"
+#include "InputEvent.h"
+#include "InputGlobal.h"
+#include "OutputSegment.h"
+#include "SymbolTable.h"
+#include "llvm/Support/Path.h"
+
+using namespace llvm;
+using namespace llvm::wasm;
+
+using namespace lld;
+using namespace lld::wasm;
+
+OutStruct lld::wasm::Out;
+
+namespace {
+
+// Some synthetic sections (e.g. "name" and "linking") have subsections.
+// Just like the synthetic sections themselves these need to be created before
+// they can be written out (since they are preceded by their length). This
+// class is used to create subsections and then write them into the stream
+// of the parent section.
+class SubSection {
+public:
+  explicit SubSection(uint32_t Type) : Type(Type) {}
+
+  void writeTo(raw_ostream &To) {
+    OS.flush();
+    writeUleb128(To, Type, "subsection type");
+    writeUleb128(To, Body.size(), "subsection size");
+    To.write(Body.data(), Body.size());
+  }
+
+private:
+  uint32_t Type;
+  std::string Body;
+
+public:
+  raw_string_ostream OS{Body};
+};
+
+} // namespace
+
+void DylinkSection::writeBody() {
+  raw_ostream &OS = BodyOutputStream;
+
+  writeUleb128(OS, MemSize, "MemSize");
+  writeUleb128(OS, MemAlign, "MemAlign");
+  writeUleb128(OS, Out.ElemSec->numEntries(), "TableSize");
+  writeUleb128(OS, 0, "TableAlign");
+  writeUleb128(OS, Symtab->SharedFiles.size(), "Needed");
+  for (auto *SO : Symtab->SharedFiles)
+    writeStr(OS, llvm::sys::path::filename(SO->getName()), "so name");
+}
+
+uint32_t TypeSection::registerType(const WasmSignature &Sig) {
+  auto Pair = TypeIndices.insert(std::make_pair(Sig, Types.size()));
+  if (Pair.second) {
+    LLVM_DEBUG(llvm::dbgs() << "type " << toString(Sig) << "\n");
+    Types.push_back(&Sig);
+  }
+  return Pair.first->second;
+}
+
+uint32_t TypeSection::lookupType(const WasmSignature &Sig) {
+  auto It = TypeIndices.find(Sig);
+  if (It == TypeIndices.end()) {
+    error("type not found: " + toString(Sig));
+    return 0;
+  }
+  return It->second;
+}
+
+void TypeSection::writeBody() {
+  writeUleb128(BodyOutputStream, Types.size(), "type count");
+  for (const WasmSignature *Sig : Types)
+    writeSig(BodyOutputStream, *Sig);
+}
+
+uint32_t ImportSection::numImports() const {
+  uint32_t NumImports = ImportedSymbols.size() + GOTSymbols.size();
+  if (Config->ImportMemory)
+    ++NumImports;
+  if (Config->ImportTable)
+    ++NumImports;
+  return NumImports;
+}
+
+void ImportSection::addGOTEntry(Symbol *Sym) {
+  if (Sym->hasGOTIndex())
+    return;
+  Sym->setGOTIndex(NumImportedGlobals++);
+  GOTSymbols.push_back(Sym);
+}
+
+void ImportSection::addImport(Symbol *Sym) {
+  ImportedSymbols.emplace_back(Sym);
+  if (auto *F = dyn_cast<FunctionSymbol>(Sym))
+    F->setFunctionIndex(NumImportedFunctions++);
+  else if (auto *G = dyn_cast<GlobalSymbol>(Sym))
+    G->setGlobalIndex(NumImportedGlobals++);
+  else
+    cast<EventSymbol>(Sym)->setEventIndex(NumImportedEvents++);
+}
+
+void ImportSection::writeBody() {
+  raw_ostream &OS = BodyOutputStream;
+
+  writeUleb128(OS, numImports(), "import count");
+
+  if (Config->ImportMemory) {
+    WasmImport Import;
+    Import.Module = DefaultModule;
+    Import.Field = "memory";
+    Import.Kind = WASM_EXTERNAL_MEMORY;
+    Import.Memory.Flags = 0;
+    Import.Memory.Initial = Out.MemorySec->NumMemoryPages;
+    if (Out.MemorySec->MaxMemoryPages != 0 || Config->SharedMemory) {
+      Import.Memory.Flags |= WASM_LIMITS_FLAG_HAS_MAX;
+      Import.Memory.Maximum = Out.MemorySec->MaxMemoryPages;
+    }
+    if (Config->SharedMemory)
+      Import.Memory.Flags |= WASM_LIMITS_FLAG_IS_SHARED;
+    writeImport(OS, Import);
+  }
+
+  if (Config->ImportTable) {
+    uint32_t TableSize = Out.ElemSec->ElemOffset + Out.ElemSec->numEntries();
+    WasmImport Import;
+    Import.Module = DefaultModule;
+    Import.Field = FunctionTableName;
+    Import.Kind = WASM_EXTERNAL_TABLE;
+    Import.Table.ElemType = WASM_TYPE_FUNCREF;
+    Import.Table.Limits = {0, TableSize, 0};
+    writeImport(OS, Import);
+  }
+
+  for (const Symbol *Sym : ImportedSymbols) {
+    WasmImport Import;
+    if (auto *F = dyn_cast<UndefinedFunction>(Sym)) {
+      Import.Field = F->ImportName;
+      Import.Module = F->ImportModule;
+    } else if (auto *G = dyn_cast<UndefinedGlobal>(Sym)) {
+      Import.Field = G->ImportName;
+      Import.Module = G->ImportModule;
+    } else {
+      Import.Field = Sym->getName();
+      Import.Module = DefaultModule;
+    }
+
+    if (auto *FunctionSym = dyn_cast<FunctionSymbol>(Sym)) {
+      Import.Kind = WASM_EXTERNAL_FUNCTION;
+      Import.SigIndex = Out.TypeSec->lookupType(*FunctionSym->Signature);
+    } else if (auto *GlobalSym = dyn_cast<GlobalSymbol>(Sym)) {
+      Import.Kind = WASM_EXTERNAL_GLOBAL;
+      Import.Global = *GlobalSym->getGlobalType();
+    } else {
+      auto *EventSym = cast<EventSymbol>(Sym);
+      Import.Kind = WASM_EXTERNAL_EVENT;
+      Import.Event.Attribute = EventSym->getEventType()->Attribute;
+      Import.Event.SigIndex = Out.TypeSec->lookupType(*EventSym->Signature);
+    }
+    writeImport(OS, Import);
+  }
+
+  for (const Symbol *Sym : GOTSymbols) {
+    WasmImport Import;
+    Import.Kind = WASM_EXTERNAL_GLOBAL;
+    Import.Global = {WASM_TYPE_I32, true};
+    if (isa<DataSymbol>(Sym))
+      Import.Module = "GOT.mem";
+    else
+      Import.Module = "GOT.func";
+    Import.Field = Sym->getName();
+    writeImport(OS, Import);
+  }
+}
+
+void FunctionSection::writeBody() {
+  raw_ostream &OS = BodyOutputStream;
+
+  writeUleb128(OS, InputFunctions.size(), "function count");
+  for (const InputFunction *Func : InputFunctions)
+    writeUleb128(OS, Out.TypeSec->lookupType(Func->Signature), "sig index");
+}
+
+void FunctionSection::addFunction(InputFunction *Func) {
+  if (!Func->Live)
+    return;
+  uint32_t FunctionIndex =
+      Out.ImportSec->NumImportedFunctions + InputFunctions.size();
+  InputFunctions.emplace_back(Func);
+  Func->setFunctionIndex(FunctionIndex);
+}
+
+void TableSection::writeBody() {
+  uint32_t TableSize = Out.ElemSec->ElemOffset + Out.ElemSec->numEntries();
+
+  raw_ostream &OS = BodyOutputStream;
+  writeUleb128(OS, 1, "table count");
+  WasmLimits Limits = {WASM_LIMITS_FLAG_HAS_MAX, TableSize, TableSize};
+  writeTableType(OS, WasmTable{WASM_TYPE_FUNCREF, Limits});
+}
+
+void MemorySection::writeBody() {
+  raw_ostream &OS = BodyOutputStream;
+
+  bool HasMax = MaxMemoryPages != 0 || Config->SharedMemory;
+  writeUleb128(OS, 1, "memory count");
+  unsigned Flags = 0;
+  if (HasMax)
+    Flags |= WASM_LIMITS_FLAG_HAS_MAX;
+  if (Config->SharedMemory)
+    Flags |= WASM_LIMITS_FLAG_IS_SHARED;
+  writeUleb128(OS, Flags, "memory limits flags");
+  writeUleb128(OS, NumMemoryPages, "initial pages");
+  if (HasMax)
+    writeUleb128(OS, MaxMemoryPages, "max pages");
+}
+
+void GlobalSection::writeBody() {
+  raw_ostream &OS = BodyOutputStream;
+
+  writeUleb128(OS, numGlobals(), "global count");
+  for (const InputGlobal *G : InputGlobals)
+    writeGlobal(OS, G->Global);
+  for (const DefinedData *Sym : DefinedFakeGlobals) {
+    WasmGlobal Global;
+    Global.Type = {WASM_TYPE_I32, false};
+    Global.InitExpr.Opcode = WASM_OPCODE_I32_CONST;
+    Global.InitExpr.Value.Int32 = Sym->getVirtualAddress();
+    writeGlobal(OS, Global);
+  }
+}
+
+void GlobalSection::addGlobal(InputGlobal *Global) {
+  if (!Global->Live)
+    return;
+  uint32_t GlobalIndex =
+      Out.ImportSec->NumImportedGlobals + InputGlobals.size();
+  LLVM_DEBUG(dbgs() << "addGlobal: " << GlobalIndex << "\n");
+  Global->setGlobalIndex(GlobalIndex);
+  Out.GlobalSec->InputGlobals.push_back(Global);
+}
+
+void EventSection::writeBody() {
+  raw_ostream &OS = BodyOutputStream;
+
+  writeUleb128(OS, InputEvents.size(), "event count");
+  for (InputEvent *E : InputEvents) {
+    E->Event.Type.SigIndex = Out.TypeSec->lookupType(E->Signature);
+    writeEvent(OS, E->Event);
+  }
+}
+
+void EventSection::addEvent(InputEvent *Event) {
+  if (!Event->Live)
+    return;
+  uint32_t EventIndex = Out.ImportSec->NumImportedEvents + InputEvents.size();
+  LLVM_DEBUG(dbgs() << "addEvent: " << EventIndex << "\n");
+  Event->setEventIndex(EventIndex);
+  InputEvents.push_back(Event);
+}
+
+void ExportSection::writeBody() {
+  raw_ostream &OS = BodyOutputStream;
+
+  writeUleb128(OS, Exports.size(), "export count");
+  for (const WasmExport &Export : Exports)
+    writeExport(OS, Export);
+}
+
+void ElemSection::addEntry(FunctionSymbol *Sym) {
+  if (Sym->hasTableIndex())
+    return;
+  Sym->setTableIndex(ElemOffset + IndirectFunctions.size());
+  IndirectFunctions.emplace_back(Sym);
+}
+
+void ElemSection::writeBody() {
+  raw_ostream &OS = BodyOutputStream;
+
+  writeUleb128(OS, 1, "segment count");
+  writeUleb128(OS, 0, "table index");
+  WasmInitExpr InitExpr;
+  if (Config->Pic) {
+    InitExpr.Opcode = WASM_OPCODE_GLOBAL_GET;
+    InitExpr.Value.Global = WasmSym::TableBase->getGlobalIndex();
+  } else {
+    InitExpr.Opcode = WASM_OPCODE_I32_CONST;
+    InitExpr.Value.Int32 = ElemOffset;
+  }
+  writeInitExpr(OS, InitExpr);
+  writeUleb128(OS, IndirectFunctions.size(), "elem count");
+
+  uint32_t TableIndex = ElemOffset;
+  for (const FunctionSymbol *Sym : IndirectFunctions) {
+    assert(Sym->getTableIndex() == TableIndex);
+    writeUleb128(OS, Sym->getFunctionIndex(), "function index");
+    ++TableIndex;
+  }
+}
+
+void DataCountSection::writeBody() {
+  writeUleb128(BodyOutputStream, NumSegments, "data count");
+}
+
+bool DataCountSection::isNeeded() const {
+  return NumSegments && Out.TargetFeaturesSec->Features.count("bulk-memory");
+}
+
+static uint32_t getWasmFlags(const Symbol *Sym) {
+  uint32_t Flags = 0;
+  if (Sym->isLocal())
+    Flags |= WASM_SYMBOL_BINDING_LOCAL;
+  if (Sym->isWeak())
+    Flags |= WASM_SYMBOL_BINDING_WEAK;
+  if (Sym->isHidden())
+    Flags |= WASM_SYMBOL_VISIBILITY_HIDDEN;
+  if (Sym->isUndefined())
+    Flags |= WASM_SYMBOL_UNDEFINED;
+  if (auto *F = dyn_cast<UndefinedFunction>(Sym)) {
+    if (F->getName() != F->ImportName)
+      Flags |= WASM_SYMBOL_EXPLICIT_NAME;
+  } else if (auto *G = dyn_cast<UndefinedGlobal>(Sym)) {
+    if (G->getName() != G->ImportName)
+      Flags |= WASM_SYMBOL_EXPLICIT_NAME;
+  }
+  return Flags;
+}
+
+void LinkingSection::writeBody() {
+  raw_ostream &OS = BodyOutputStream;
+
+  writeUleb128(OS, WasmMetadataVersion, "Version");
+
+  if (!SymtabEntries.empty()) {
+    SubSection Sub(WASM_SYMBOL_TABLE);
+    writeUleb128(Sub.OS, SymtabEntries.size(), "num symbols");
+
+    for (const Symbol *Sym : SymtabEntries) {
+      assert(Sym->isDefined() || Sym->isUndefined());
+      WasmSymbolType Kind = Sym->getWasmType();
+      uint32_t Flags = getWasmFlags(Sym);
+
+      writeU8(Sub.OS, Kind, "sym kind");
+      writeUleb128(Sub.OS, Flags, "sym flags");
+
+      if (auto *F = dyn_cast<FunctionSymbol>(Sym)) {
+        writeUleb128(Sub.OS, F->getFunctionIndex(), "index");
+        if (Sym->isDefined() || (Flags & WASM_SYMBOL_EXPLICIT_NAME) != 0)
+          writeStr(Sub.OS, Sym->getName(), "sym name");
+      } else if (auto *G = dyn_cast<GlobalSymbol>(Sym)) {
+        writeUleb128(Sub.OS, G->getGlobalIndex(), "index");
+        if (Sym->isDefined() || (Flags & WASM_SYMBOL_EXPLICIT_NAME) != 0)
+          writeStr(Sub.OS, Sym->getName(), "sym name");
+      } else if (auto *E = dyn_cast<EventSymbol>(Sym)) {
+        writeUleb128(Sub.OS, E->getEventIndex(), "index");
+        if (Sym->isDefined() || (Flags & WASM_SYMBOL_EXPLICIT_NAME) != 0)
+          writeStr(Sub.OS, Sym->getName(), "sym name");
+      } else if (isa<DataSymbol>(Sym)) {
+        writeStr(Sub.OS, Sym->getName(), "sym name");
+        if (auto *DataSym = dyn_cast<DefinedData>(Sym)) {
+          writeUleb128(Sub.OS, DataSym->getOutputSegmentIndex(), "index");
+          writeUleb128(Sub.OS, DataSym->getOutputSegmentOffset(),
+                       "data offset");
+          writeUleb128(Sub.OS, DataSym->getSize(), "data size");
+        }
+      } else {
+        auto *S = cast<OutputSectionSymbol>(Sym);
+        writeUleb128(Sub.OS, S->Section->SectionIndex, "sym section index");
+      }
+    }
+
+    Sub.writeTo(OS);
+  }
+
+  if (DataSegments.size()) {
+    SubSection Sub(WASM_SEGMENT_INFO);
+    writeUleb128(Sub.OS, DataSegments.size(), "num data segments");
+    for (const OutputSegment *S : DataSegments) {
+      writeStr(Sub.OS, S->Name, "segment name");
+      writeUleb128(Sub.OS, S->Alignment, "alignment");
+      writeUleb128(Sub.OS, 0, "flags");
+    }
+    Sub.writeTo(OS);
+  }
+
+  if (!InitFunctions.empty()) {
+    SubSection Sub(WASM_INIT_FUNCS);
+    writeUleb128(Sub.OS, InitFunctions.size(), "num init functions");
+    for (const WasmInitEntry &F : InitFunctions) {
+      writeUleb128(Sub.OS, F.Priority, "priority");
+      writeUleb128(Sub.OS, F.Sym->getOutputSymbolIndex(), "function index");
+    }
+    Sub.writeTo(OS);
+  }
+
+  struct ComdatEntry {
+    unsigned Kind;
+    uint32_t Index;
+  };
+  std::map<StringRef, std::vector<ComdatEntry>> Comdats;
+
+  for (const InputFunction *F : Out.FunctionSec->InputFunctions) {
+    StringRef Comdat = F->getComdatName();
+    if (!Comdat.empty())
+      Comdats[Comdat].emplace_back(
+          ComdatEntry{WASM_COMDAT_FUNCTION, F->getFunctionIndex()});
+  }
+  for (uint32_t I = 0; I < DataSegments.size(); ++I) {
+    const auto &InputSegments = DataSegments[I]->InputSegments;
+    if (InputSegments.empty())
+      continue;
+    StringRef Comdat = InputSegments[0]->getComdatName();
+#ifndef NDEBUG
+    for (const InputSegment *IS : InputSegments)
+      assert(IS->getComdatName() == Comdat);
+#endif
+    if (!Comdat.empty())
+      Comdats[Comdat].emplace_back(ComdatEntry{WASM_COMDAT_DATA, I});
+  }
+
+  if (!Comdats.empty()) {
+    SubSection Sub(WASM_COMDAT_INFO);
+    writeUleb128(Sub.OS, Comdats.size(), "num comdats");
+    for (const auto &C : Comdats) {
+      writeStr(Sub.OS, C.first, "comdat name");
+      writeUleb128(Sub.OS, 0, "comdat flags"); // flags for future use
+      writeUleb128(Sub.OS, C.second.size(), "num entries");
+      for (const ComdatEntry &Entry : C.second) {
+        writeU8(Sub.OS, Entry.Kind, "entry kind");
+        writeUleb128(Sub.OS, Entry.Index, "entry index");
+      }
+    }
+    Sub.writeTo(OS);
+  }
+}
+
+void LinkingSection::addToSymtab(Symbol *Sym) {
+  Sym->setOutputSymbolIndex(SymtabEntries.size());
+  SymtabEntries.emplace_back(Sym);
+}
+
+unsigned NameSection::numNames() const {
+  unsigned NumNames = Out.ImportSec->NumImportedFunctions;
+  for (const InputFunction *F : Out.FunctionSec->InputFunctions)
+    if (!F->getName().empty() || !F->getDebugName().empty())
+      ++NumNames;
+
+  return NumNames;
+}
+
+// Create the custom "name" section containing debug symbol names.
+void NameSection::writeBody() {
+  SubSection Sub(WASM_NAMES_FUNCTION);
+  writeUleb128(Sub.OS, numNames(), "name count");
+
+  // Names must appear in function index order.  As it happens ImportedSymbols
+  // and InputFunctions are numbered in order with imported functions coming
+  // first.
+  for (const Symbol *S : Out.ImportSec->ImportedSymbols) {
+    if (auto *F = dyn_cast<FunctionSymbol>(S)) {
+      writeUleb128(Sub.OS, F->getFunctionIndex(), "func index");
+      writeStr(Sub.OS, toString(*S), "symbol name");
+    }
+  }
+  for (const InputFunction *F : Out.FunctionSec->InputFunctions) {
+    if (!F->getName().empty()) {
+      writeUleb128(Sub.OS, F->getFunctionIndex(), "func index");
+      if (!F->getDebugName().empty()) {
+        writeStr(Sub.OS, F->getDebugName(), "symbol name");
+      } else {
+        writeStr(Sub.OS, maybeDemangleSymbol(F->getName()), "symbol name");
+      }
+    }
+  }
+
+  Sub.writeTo(BodyOutputStream);
+}
+
+void ProducersSection::addInfo(const WasmProducerInfo &Info) {
+  for (auto &Producers :
+       {std::make_pair(&Info.Languages, &Languages),
+        std::make_pair(&Info.Tools, &Tools), std::make_pair(&Info.SDKs, &SDKs)})
+    for (auto &Producer : *Producers.first)
+      if (Producers.second->end() ==
+          llvm::find_if(*Producers.second,
+                        [&](std::pair<std::string, std::string> Seen) {
+                          return Seen.first == Producer.first;
+                        }))
+        Producers.second->push_back(Producer);
+}
+
+void ProducersSection::writeBody() {
+  auto &OS = BodyOutputStream;
+  writeUleb128(OS, fieldCount(), "field count");
+  for (auto &Field :
+       {std::make_pair("language", Languages),
+        std::make_pair("processed-by", Tools), std::make_pair("sdk", SDKs)}) {
+    if (Field.second.empty())
+      continue;
+    writeStr(OS, Field.first, "field name");
+    writeUleb128(OS, Field.second.size(), "number of entries");
+    for (auto &Entry : Field.second) {
+      writeStr(OS, Entry.first, "producer name");
+      writeStr(OS, Entry.second, "producer version");
+    }
+  }
+}
+
+void TargetFeaturesSection::writeBody() {
+  SmallVector<std::string, 8> Emitted(Features.begin(), Features.end());
+  llvm::sort(Emitted);
+  auto &OS = BodyOutputStream;
+  writeUleb128(OS, Emitted.size(), "feature count");
+  for (auto &Feature : Emitted) {
+    writeU8(OS, WASM_FEATURE_PREFIX_USED, "feature used prefix");
+    writeStr(OS, Feature, "feature name");
+  }
+}
+
+void RelocSection::writeBody() {
+  uint32_t Count = Sec->numRelocations();
+  assert(Sec->SectionIndex != UINT32_MAX);
+  writeUleb128(BodyOutputStream, Sec->SectionIndex, "reloc section");
+  writeUleb128(BodyOutputStream, Count, "reloc count");
+  Sec->writeRelocations(BodyOutputStream);
+}

wasm/Writer.h

 //===- Writer.h -------------------------------------------------*- C++ -*-===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 #ifndef LLD_WASM_WRITER_H
 #define LLD_WASM_WRITER_H
 
 namespace lld {
 namespace wasm {
 
 void writeResult();
 
-extern const char *DefaultModule;
-
 } // namespace wasm
 } // namespace lld
 
 #endif

wasm/Writer.cpp

 //===- Writer.cpp ---------------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 #include "Writer.h"
 #include "Config.h"
 #include "InputChunks.h"
 #include "InputEvent.h"
 #include "InputGlobal.h"
 #include "OutputSections.h"
 #include "OutputSegment.h"
+#include "Relocations.h"
 #include "SymbolTable.h"
+#include "SyntheticSections.h"
 #include "WriterUtils.h"
 #include "lld/Common/ErrorHandler.h"
 #include "lld/Common/Memory.h"
 #include "lld/Common/Strings.h"
 #include "lld/Common/Threads.h"
 #include "llvm/ADT/DenseSet.h"
 #include "llvm/ADT/SmallSet.h"
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/ADT/StringMap.h"
 #include "llvm/BinaryFormat/Wasm.h"
 #include "llvm/Object/WasmTraits.h"
 #include "llvm/Support/FileOutputBuffer.h"
 #include "llvm/Support/Format.h"
 #include "llvm/Support/FormatVariadic.h"
 #include "llvm/Support/LEB128.h"
-#include "llvm/Support/Path.h"
 
 #include <cstdarg>
 #include <map>
 
 #define DEBUG_TYPE "lld"
 
 using namespace llvm;
 using namespace llvm::wasm;
 using namespace lld;
 using namespace lld::wasm;
 
 static constexpr int StackAlignment = 16;
-static constexpr const char *FunctionTableName = "__indirect_function_table";
-const char *lld::wasm::DefaultModule = "env";
 
 namespace {
 
-// An init entry to be written to either the synthetic init func or the
-// linking metadata.
-struct WasmInitEntry {
-  const FunctionSymbol *Sym;
-  uint32_t Priority;
-};
-
 // The writer writes a SymbolTable result to a file.
 class Writer {
 public:
   void run();
 
 private:
   void openFile();
 
-  uint32_t lookupType(const WasmSignature &Sig);
-  uint32_t registerType(const WasmSignature &Sig);
-
   void createApplyRelocationsFunction();
   void createCallCtorsFunction();
 
-  void calculateInitFunctions();
-  void processRelocations(InputChunk *Chunk);
   void assignIndexes();
-  void calculateTargetFeatures();
+  void populateSymtab();
+  void populateProducers();
+  void populateTargetFeatures();
+  void calculateInitFunctions();
   void calculateImports();
   void calculateExports();
   void calculateCustomSections();
-  void assignSymtab();
   void calculateTypes();
   void createOutputSegments();
   void layoutMemory();
   void createHeader();
-  void createSections();
-  SyntheticSection *createSyntheticSection(uint32_t Type, StringRef Name = "");
 
-  // Builtin sections
-  void createTypeSection();
-  void createFunctionSection();
-  void createTableSection();
-  void createGlobalSection();
-  void createEventSection();
-  void createExportSection();
-  void createImportSection();
-  void createMemorySection();
-  void createElemSection();
-  void createDataCountSection();
-  void createCodeSection();
-  void createDataSection();
+  void addSection(OutputSection *Sec);
+
+  void addSections();
   void createCustomSections();
+  void createSyntheticSections();
+  void finalizeSections();
 
   // Custom sections
-  void createDylinkSection();
   void createRelocSections();
-  void createLinkingSection();
-  void createNameSection();
-  void createProducersSection();
-  void createTargetFeaturesSection();
 
   void writeHeader();
   void writeSections();
 
   uint64_t FileSize = 0;
   uint32_t TableBase = 0;
-  uint32_t NumMemoryPages = 0;
-  uint32_t MaxMemoryPages = 0;
-  // Memory size and aligment. Written to the "dylink" section
-  // when build with -shared or -pie.
-  uint32_t MemAlign = 0;
-  uint32_t MemSize = 0;
-
-  std::vector<const WasmSignature *> Types;
-  DenseMap<WasmSignature, int32_t> TypeIndices;
-  std::vector<const Symbol *> ImportedSymbols;
-  std::vector<const Symbol *> GOTSymbols;
-  unsigned NumImportedFunctions = 0;
-  unsigned NumImportedGlobals = 0;
-  unsigned NumImportedEvents = 0;
-  std::vector<WasmExport> Exports;
-  std::vector<const DefinedData *> DefinedFakeGlobals;
-  std::vector<InputGlobal *> InputGlobals;
-  std::vector<InputFunction *> InputFunctions;
-  std::vector<InputEvent *> InputEvents;
-  std::vector<const FunctionSymbol *> IndirectFunctions;
-  std::vector<const Symbol *> SymtabEntries;
-  std::vector<WasmInitEntry> InitFunctions;
 
+  std::vector<WasmInitEntry> InitFunctions;
   llvm::StringMap<std::vector<InputSection *>> CustomSectionMapping;
-  llvm::StringMap<SectionSymbol *> CustomSectionSymbols;
-  llvm::SmallSet<std::string, 8> TargetFeatures;
 
   // Elements that are used to construct the final output
   std::string Header;
   std::vector<OutputSection *> OutputSections;
 
   std::unique_ptr<FileOutputBuffer> Buffer;
 
   std::vector<OutputSegment *> Segments;
   llvm::SmallDenseMap<StringRef, OutputSegment *> SegmentMap;
 };
 
 } // anonymous namespace
 
-void Writer::createImportSection() {
-  uint32_t NumImports = ImportedSymbols.size() + GOTSymbols.size();
-  if (Config->ImportMemory)
-    ++NumImports;
-  if (Config->ImportTable)
-    ++NumImports;
-
-  if (NumImports == 0)
-    return;
-
-  SyntheticSection *Section = createSyntheticSection(WASM_SEC_IMPORT);
-  raw_ostream &OS = Section->getStream();
-
-  writeUleb128(OS, NumImports, "import count");
-
-  if (Config->ImportMemory) {
-    WasmImport Import;
-    Import.Module = DefaultModule;
-    Import.Field = "memory";
-    Import.Kind = WASM_EXTERNAL_MEMORY;
-    Import.Memory.Flags = 0;
-    Import.Memory.Initial = NumMemoryPages;
-    if (MaxMemoryPages != 0 || Config->SharedMemory) {
-      Import.Memory.Flags |= WASM_LIMITS_FLAG_HAS_MAX;
-      Import.Memory.Maximum = MaxMemoryPages;
-    }
-    if (Config->SharedMemory)
-      Import.Memory.Flags |= WASM_LIMITS_FLAG_IS_SHARED;
-    writeImport(OS, Import);
-  }
-
-  if (Config->ImportTable) {
-    uint32_t TableSize = TableBase + IndirectFunctions.size();
-    WasmImport Import;
-    Import.Module = DefaultModule;
-    Import.Field = FunctionTableName;
-    Import.Kind = WASM_EXTERNAL_TABLE;
-    Import.Table.ElemType = WASM_TYPE_FUNCREF;
-    Import.Table.Limits = {0, TableSize, 0};
-    writeImport(OS, Import);
-  }
-
-  for (const Symbol *Sym : ImportedSymbols) {
-    WasmImport Import;
-    if (auto *F = dyn_cast<UndefinedFunction>(Sym)) {
-      Import.Field = F->ImportName;
-      Import.Module = F->ImportModule;
-    } else if (auto *G = dyn_cast<UndefinedGlobal>(Sym)) {
-      Import.Field = G->ImportName;
-      Import.Module = G->ImportModule;
-    } else {
-      Import.Field = Sym->getName();
-      Import.Module = DefaultModule;
-    }
-
-    if (auto *FunctionSym = dyn_cast<FunctionSymbol>(Sym)) {
-      Import.Kind = WASM_EXTERNAL_FUNCTION;
-      Import.SigIndex = lookupType(*FunctionSym->Signature);
-    } else if (auto *GlobalSym = dyn_cast<GlobalSymbol>(Sym)) {
-      Import.Kind = WASM_EXTERNAL_GLOBAL;
-      Import.Global = *GlobalSym->getGlobalType();
-    } else {
-      auto *EventSym = cast<EventSymbol>(Sym);
-      Import.Kind = WASM_EXTERNAL_EVENT;
-      Import.Event.Attribute = EventSym->getEventType()->Attribute;
-      Import.Event.SigIndex = lookupType(*EventSym->Signature);
-    }
-    writeImport(OS, Import);
-  }
-
-  for (const Symbol *Sym : GOTSymbols) {
-    WasmImport Import;
-    Import.Kind = WASM_EXTERNAL_GLOBAL;
-    Import.Global = {WASM_TYPE_I32, true};
-    if (isa<DataSymbol>(Sym))
-      Import.Module = "GOT.mem";
-    else
-      Import.Module = "GOT.func";
-    Import.Field = Sym->getName();
-    writeImport(OS, Import);
-  }
-}
-
-void Writer::createTypeSection() {
-  if (!Types.size())
-    return;
-  SyntheticSection *Section = createSyntheticSection(WASM_SEC_TYPE);
-  raw_ostream &OS = Section->getStream();
-  writeUleb128(OS, Types.size(), "type count");
-  for (const WasmSignature *Sig : Types)
-    writeSig(OS, *Sig);
-}
-
-void Writer::createFunctionSection() {
-  if (InputFunctions.empty())
-    return;
-
-  SyntheticSection *Section = createSyntheticSection(WASM_SEC_FUNCTION);
-  raw_ostream &OS = Section->getStream();
-
-  writeUleb128(OS, InputFunctions.size(), "function count");
-  for (const InputFunction *Func : InputFunctions)
-    writeUleb128(OS, lookupType(Func->Signature), "sig index");
-}
-
-void Writer::createMemorySection() {
-  if (Config->ImportMemory)
-    return;
-
-  SyntheticSection *Section = createSyntheticSection(WASM_SEC_MEMORY);
-  raw_ostream &OS = Section->getStream();
-
-  bool HasMax = MaxMemoryPages != 0 || Config->SharedMemory;
-  writeUleb128(OS, 1, "memory count");
-  unsigned Flags = 0;
-  if (HasMax)
-    Flags |= WASM_LIMITS_FLAG_HAS_MAX;
-  if (Config->SharedMemory)
-    Flags |= WASM_LIMITS_FLAG_IS_SHARED;
-  writeUleb128(OS, Flags, "memory limits flags");
-  writeUleb128(OS, NumMemoryPages, "initial pages");
-  if (HasMax)
-    writeUleb128(OS, MaxMemoryPages, "max pages");
-}
-
-void Writer::createGlobalSection() {
-  unsigned NumGlobals = InputGlobals.size() + DefinedFakeGlobals.size();
-  if (NumGlobals == 0)
-    return;
-
-  SyntheticSection *Section = createSyntheticSection(WASM_SEC_GLOBAL);
-  raw_ostream &OS = Section->getStream();
-
-  writeUleb128(OS, NumGlobals, "global count");
-  for (const InputGlobal *G : InputGlobals)
-    writeGlobal(OS, G->Global);
-  for (const DefinedData *Sym : DefinedFakeGlobals) {
-    WasmGlobal Global;
-    Global.Type = {WASM_TYPE_I32, false};
-    Global.InitExpr.Opcode = WASM_OPCODE_I32_CONST;
-    Global.InitExpr.Value.Int32 = Sym->getVirtualAddress();
-    writeGlobal(OS, Global);
-  }
-}
-
-// The event section contains a list of declared wasm events associated with the
-// module. Currently the only supported event kind is exceptions. A single event
-// entry represents a single event with an event tag. All C++ exceptions are
-// represented by a single event. An event entry in this section contains
-// information on what kind of event it is (e.g. exception) and the type of
-// values contained in a single event object. (In wasm, an event can contain
-// multiple values of primitive types. But for C++ exceptions, we just throw a
-// pointer which is an i32 value (for wasm32 architecture), so the signature of
-// C++ exception is (i32)->(void), because all event types are assumed to have
-// void return type to share WasmSignature with functions.)
-void Writer::createEventSection() {
-  unsigned NumEvents = InputEvents.size();
-  if (NumEvents == 0)
-    return;
-
-  SyntheticSection *Section = createSyntheticSection(WASM_SEC_EVENT);
-  raw_ostream &OS = Section->getStream();
-
-  writeUleb128(OS, NumEvents, "event count");
-  for (InputEvent *E : InputEvents) {
-    E->Event.Type.SigIndex = lookupType(E->Signature);
-    writeEvent(OS, E->Event);
-  }
-}
-
-void Writer::createTableSection() {
-  if (Config->ImportTable)
-    return;
-
-  // Always output a table section (or table import), even if there are no
-  // indirect calls.  There are two reasons for this:
-  //  1. For executables it is useful to have an empty table slot at 0
-  //     which can be filled with a null function call handler.
-  //  2. If we don't do this, any program that contains a call_indirect but
-  //     no address-taken function will fail at validation time since it is
-  //     a validation error to include a call_indirect instruction if there
-  //     is not table.
-  uint32_t TableSize = TableBase + IndirectFunctions.size();
-
-  SyntheticSection *Section = createSyntheticSection(WASM_SEC_TABLE);
-  raw_ostream &OS = Section->getStream();
-
-  writeUleb128(OS, 1, "table count");
-  WasmLimits Limits = {WASM_LIMITS_FLAG_HAS_MAX, TableSize, TableSize};
-  writeTableType(OS, WasmTable{WASM_TYPE_FUNCREF, Limits});
-}
-
-void Writer::createExportSection() {
-  if (!Exports.size())
-    return;
-
-  SyntheticSection *Section = createSyntheticSection(WASM_SEC_EXPORT);
-  raw_ostream &OS = Section->getStream();
-
-  writeUleb128(OS, Exports.size(), "export count");
-  for (const WasmExport &Export : Exports)
-    writeExport(OS, Export);
-}
-
 void Writer::calculateCustomSections() {
   log("calculateCustomSections");
   bool StripDebug = Config->StripDebug || Config->StripAll;
   for (ObjFile *File : Symtab->ObjectFiles) {
     for (InputSection *Section : File->CustomSections) {
       StringRef Name = Section->getName();
       // These custom sections are known the linker and synthesized rather than
       // blindly copied
       if (Name == "linking" || Name == "name" || Name == "producers" ||
           Name == "target_features" || Name.startswith("reloc."))
         continue;
       // .. or it is a debug section
       if (StripDebug && Name.startswith(".debug_"))
         continue;
       CustomSectionMapping[Name].push_back(Section);
     }
   }
 }
 
 void Writer::createCustomSections() {
   log("createCustomSections");
   for (auto &Pair : CustomSectionMapping) {
     StringRef Name = Pair.first();
-
-    auto P = CustomSectionSymbols.find(Name);
-    if (P != CustomSectionSymbols.end()) {
-      uint32_t SectionIndex = OutputSections.size();
-      P->second->setOutputSectionIndex(SectionIndex);
-    }
-
     LLVM_DEBUG(dbgs() << "createCustomSection: " << Name << "\n");
-    OutputSections.push_back(make<CustomSection>(Name, Pair.second));
-  }
-}
-
-void Writer::createElemSection() {
-  if (IndirectFunctions.empty())
-    return;
-
-  SyntheticSection *Section = createSyntheticSection(WASM_SEC_ELEM);
-  raw_ostream &OS = Section->getStream();
-
-  writeUleb128(OS, 1, "segment count");
-  writeUleb128(OS, 0, "table index");
-  WasmInitExpr InitExpr;
-  if (Config->Pic) {
-    InitExpr.Opcode = WASM_OPCODE_GLOBAL_GET;
-    InitExpr.Value.Global = WasmSym::TableBase->getGlobalIndex();
-  } else {
-    InitExpr.Opcode = WASM_OPCODE_I32_CONST;
-    InitExpr.Value.Int32 = TableBase;
-  }
-  writeInitExpr(OS, InitExpr);
-  writeUleb128(OS, IndirectFunctions.size(), "elem count");
 
-  uint32_t TableIndex = TableBase;
-  for (const FunctionSymbol *Sym : IndirectFunctions) {
-    assert(Sym->getTableIndex() == TableIndex);
-    writeUleb128(OS, Sym->getFunctionIndex(), "function index");
-    ++TableIndex;
-  }
-}
-
-void Writer::createDataCountSection() {
-  if (!Segments.size() || !TargetFeatures.count("bulk-memory"))
-    return;
-
-  log("createDataCountSection");
-  SyntheticSection *Section = createSyntheticSection(WASM_SEC_DATACOUNT);
-  raw_ostream &OS = Section->getStream();
-  writeUleb128(OS, Segments.size(), "data count");
+    OutputSection *Sec = make<CustomSection>(Name, Pair.second);
+    if (Config->Relocatable) {
+      auto *Sym = make<OutputSectionSymbol>(Sec);
+      Out.LinkingSec->addToSymtab(Sym);
+      Sec->SectionSym = Sym;
     }
-
-void Writer::createCodeSection() {
-  if (InputFunctions.empty())
-    return;
-
-  log("createCodeSection");
-
-  auto Section = make<CodeSection>(InputFunctions);
-  OutputSections.push_back(Section);
+    addSection(Sec);
   }
-
-void Writer::createDataSection() {
-  if (!Segments.size())
-    return;
-
-  log("createDataSection");
-  auto Section = make<DataSection>(Segments);
-  OutputSections.push_back(Section);
 }
 
 // Create relocations sections in the final output.
 // These are only created when relocatable output is requested.
 void Writer::createRelocSections() {
   log("createRelocSections");
   // Don't use iterator here since we are adding to OutputSection
   size_t OrigSize = OutputSections.size();
   for (size_t I = 0; I < OrigSize; I++) {
-    OutputSection *OSec = OutputSections[I];
-    uint32_t Count = OSec->numRelocations();
+    LLVM_DEBUG(dbgs() << "check section " << I << "\n");
+    OutputSection *Sec = OutputSections[I];
+
+    // Count the number of needed sections.
+    uint32_t Count = Sec->numRelocations();
     if (!Count)
       continue;
 
     StringRef Name;
-    if (OSec->Type == WASM_SEC_DATA)
+    if (Sec->Type == WASM_SEC_DATA)
       Name = "reloc.DATA";
-    else if (OSec->Type == WASM_SEC_CODE)
+    else if (Sec->Type == WASM_SEC_CODE)
       Name = "reloc.CODE";
-    else if (OSec->Type == WASM_SEC_CUSTOM)
-      Name = Saver.save("reloc." + OSec->Name);
+    else if (Sec->Type == WASM_SEC_CUSTOM)
+      Name = Saver.save("reloc." + Sec->Name);
     else
       llvm_unreachable(
           "relocations only supported for code, data, or custom sections");
 
-    OutputSections.push_back(make<RelocSection>(Name, OSec, I));
-  }
-}
-
-static uint32_t getWasmFlags(const Symbol *Sym) {
-  uint32_t Flags = 0;
-  if (Sym->isLocal())
-    Flags |= WASM_SYMBOL_BINDING_LOCAL;
-  if (Sym->isWeak())
-    Flags |= WASM_SYMBOL_BINDING_WEAK;
-  if (Sym->isHidden())
-    Flags |= WASM_SYMBOL_VISIBILITY_HIDDEN;
-  if (Sym->isUndefined())
-    Flags |= WASM_SYMBOL_UNDEFINED;
-  if (auto *F = dyn_cast<UndefinedFunction>(Sym)) {
-    if (F->getName() != F->ImportName)
-      Flags |= WASM_SYMBOL_EXPLICIT_NAME;
-  } else if (auto *G = dyn_cast<UndefinedGlobal>(Sym)) {
-    if (G->getName() != G->ImportName)
-      Flags |= WASM_SYMBOL_EXPLICIT_NAME;
-  }
-  return Flags;
-}
-
-// Some synthetic sections (e.g. "name" and "linking") have subsections.
-// Just like the synthetic sections themselves these need to be created before
-// they can be written out (since they are preceded by their length). This
-// class is used to create subsections and then write them into the stream
-// of the parent section.
-class SubSection {
-public:
-  explicit SubSection(uint32_t Type) : Type(Type) {}
-
-  void writeTo(raw_ostream &To) {
-    OS.flush();
-    writeUleb128(To, Type, "subsection type");
-    writeUleb128(To, Body.size(), "subsection size");
-    To.write(Body.data(), Body.size());
-  }
-
-private:
-  uint32_t Type;
-  std::string Body;
-
-public:
-  raw_string_ostream OS{Body};
-};
-
-// Create the custom "dylink" section containing information for the dynamic
-// linker.
-// See
-// https://github.com/WebAssembly/tool-conventions/blob/master/DynamicLinking.md
-void Writer::createDylinkSection() {
-  SyntheticSection *Section = createSyntheticSection(WASM_SEC_CUSTOM, "dylink");
-  raw_ostream &OS = Section->getStream();
-
-  writeUleb128(OS, MemSize, "MemSize");
-  writeUleb128(OS, MemAlign, "MemAlign");
-  writeUleb128(OS, IndirectFunctions.size(), "TableSize");
-  writeUleb128(OS, 0, "TableAlign");
-  writeUleb128(OS, Symtab->SharedFiles.size(), "Needed");
-  for (auto *SO : Symtab->SharedFiles)
-    writeStr(OS, llvm::sys::path::filename(SO->getName()), "so name");
-}
-
-// Create the custom "linking" section containing linker metadata.
-// This is only created when relocatable output is requested.
-void Writer::createLinkingSection() {
-  SyntheticSection *Section =
-      createSyntheticSection(WASM_SEC_CUSTOM, "linking");
-  raw_ostream &OS = Section->getStream();
-
-  writeUleb128(OS, WasmMetadataVersion, "Version");
-
-  if (!SymtabEntries.empty()) {
-    SubSection Sub(WASM_SYMBOL_TABLE);
-    writeUleb128(Sub.OS, SymtabEntries.size(), "num symbols");
-
-    for (const Symbol *Sym : SymtabEntries) {
-      assert(Sym->isDefined() || Sym->isUndefined());
-      WasmSymbolType Kind = Sym->getWasmType();
-      uint32_t Flags = getWasmFlags(Sym);
-
-      writeU8(Sub.OS, Kind, "sym kind");
-      writeUleb128(Sub.OS, Flags, "sym flags");
-
-      if (auto *F = dyn_cast<FunctionSymbol>(Sym)) {
-        writeUleb128(Sub.OS, F->getFunctionIndex(), "index");
-        if (Sym->isDefined() ||
-            (Flags & WASM_SYMBOL_EXPLICIT_NAME) != 0)
-          writeStr(Sub.OS, Sym->getName(), "sym name");
-      } else if (auto *G = dyn_cast<GlobalSymbol>(Sym)) {
-        writeUleb128(Sub.OS, G->getGlobalIndex(), "index");
-        if (Sym->isDefined() ||
-            (Flags & WASM_SYMBOL_EXPLICIT_NAME) != 0)
-          writeStr(Sub.OS, Sym->getName(), "sym name");
-      } else if (auto *E = dyn_cast<EventSymbol>(Sym)) {
-        writeUleb128(Sub.OS, E->getEventIndex(), "index");
-        if (Sym->isDefined() ||
-            (Flags & WASM_SYMBOL_EXPLICIT_NAME) != 0)
-          writeStr(Sub.OS, Sym->getName(), "sym name");
-      } else if (isa<DataSymbol>(Sym)) {
-        writeStr(Sub.OS, Sym->getName(), "sym name");
-        if (auto *DataSym = dyn_cast<DefinedData>(Sym)) {
-          writeUleb128(Sub.OS, DataSym->getOutputSegmentIndex(), "index");
-          writeUleb128(Sub.OS, DataSym->getOutputSegmentOffset(),
-                       "data offset");
-          writeUleb128(Sub.OS, DataSym->getSize(), "data size");
-        }
-      } else {
-        auto *S = cast<SectionSymbol>(Sym);
-        writeUleb128(Sub.OS, S->getOutputSectionIndex(), "sym section index");
-      }
-    }
+    addSection(make<RelocSection>(Name, Sec));
 
-    Sub.writeTo(OS);
-  }
-
-  if (Segments.size()) {
-    SubSection Sub(WASM_SEGMENT_INFO);
-    writeUleb128(Sub.OS, Segments.size(), "num data segments");
-    for (const OutputSegment *S : Segments) {
-      writeStr(Sub.OS, S->Name, "segment name");
-      writeUleb128(Sub.OS, S->Alignment, "alignment");
-      writeUleb128(Sub.OS, 0, "flags");
-    }
-    Sub.writeTo(OS);
-  }
-
-  if (!InitFunctions.empty()) {
-    SubSection Sub(WASM_INIT_FUNCS);
-    writeUleb128(Sub.OS, InitFunctions.size(), "num init functions");
-    for (const WasmInitEntry &F : InitFunctions) {
-      writeUleb128(Sub.OS, F.Priority, "priority");
-      writeUleb128(Sub.OS, F.Sym->getOutputSymbolIndex(), "function index");
-    }
-    Sub.writeTo(OS);
-  }
-
-  struct ComdatEntry {
-    unsigned Kind;
-    uint32_t Index;
-  };
-  std::map<StringRef, std::vector<ComdatEntry>> Comdats;
-
-  for (const InputFunction *F : InputFunctions) {
-    StringRef Comdat = F->getComdatName();
-    if (!Comdat.empty())
-      Comdats[Comdat].emplace_back(
-          ComdatEntry{WASM_COMDAT_FUNCTION, F->getFunctionIndex()});
-  }
-  for (uint32_t I = 0; I < Segments.size(); ++I) {
-    const auto &InputSegments = Segments[I]->InputSegments;
-    if (InputSegments.empty())
-      continue;
-    StringRef Comdat = InputSegments[0]->getComdatName();
-#ifndef NDEBUG
-    for (const InputSegment *IS : InputSegments)
-      assert(IS->getComdatName() == Comdat);
-#endif
-    if (!Comdat.empty())
-      Comdats[Comdat].emplace_back(ComdatEntry{WASM_COMDAT_DATA, I});
-  }
-
-  if (!Comdats.empty()) {
-    SubSection Sub(WASM_COMDAT_INFO);
-    writeUleb128(Sub.OS, Comdats.size(), "num comdats");
-    for (const auto &C : Comdats) {
-      writeStr(Sub.OS, C.first, "comdat name");
-      writeUleb128(Sub.OS, 0, "comdat flags"); // flags for future use
-      writeUleb128(Sub.OS, C.second.size(), "num entries");
-      for (const ComdatEntry &Entry : C.second) {
-        writeU8(Sub.OS, Entry.Kind, "entry kind");
-        writeUleb128(Sub.OS, Entry.Index, "entry index");
-      }
-    }
-    Sub.writeTo(OS);
-  }
-}
-
-// Create the custom "name" section containing debug symbol names.
-void Writer::createNameSection() {
-  unsigned NumNames = NumImportedFunctions;
-  for (const InputFunction *F : InputFunctions)
-    if (!F->getName().empty() || !F->getDebugName().empty())
-      ++NumNames;
-
-  if (NumNames == 0)
-    return;
-
-  SyntheticSection *Section = createSyntheticSection(WASM_SEC_CUSTOM, "name");
-
-  SubSection Sub(WASM_NAMES_FUNCTION);
-  writeUleb128(Sub.OS, NumNames, "name count");
-
-  // Names must appear in function index order.  As it happens ImportedSymbols
-  // and InputFunctions are numbered in order with imported functions coming
-  // first.
-  for (const Symbol *S : ImportedSymbols) {
-    if (auto *F = dyn_cast<FunctionSymbol>(S)) {
-      writeUleb128(Sub.OS, F->getFunctionIndex(), "func index");
-      writeStr(Sub.OS, toString(*S), "symbol name");
-    }
-  }
-  for (const InputFunction *F : InputFunctions) {
-    if (!F->getName().empty()) {
-      writeUleb128(Sub.OS, F->getFunctionIndex(), "func index");
-      if (!F->getDebugName().empty()) {
-        writeStr(Sub.OS, F->getDebugName(), "symbol name");
-      } else {
-        writeStr(Sub.OS, maybeDemangleSymbol(F->getName()), "symbol name");
-      }
-    }
-  }
-
-  Sub.writeTo(Section->getStream());
-}
-
-void Writer::createProducersSection() {
-  SmallVector<std::pair<std::string, std::string>, 8> Languages;
-  SmallVector<std::pair<std::string, std::string>, 8> Tools;
-  SmallVector<std::pair<std::string, std::string>, 8> SDKs;
+void Writer::populateProducers() {
   for (ObjFile *File : Symtab->ObjectFiles) {
     const WasmProducerInfo &Info = File->getWasmObj()->getProducerInfo();
-    for (auto &Producers : {std::make_pair(&Info.Languages, &Languages),
-                            std::make_pair(&Info.Tools, &Tools),
-                            std::make_pair(&Info.SDKs, &SDKs)})
-      for (auto &Producer : *Producers.first)
-        if (Producers.second->end() ==
-            llvm::find_if(*Producers.second,
-                          [&](std::pair<std::string, std::string> Seen) {
-                            return Seen.first == Producer.first;
-                          }))
-          Producers.second->push_back(Producer);
-  }
-  int FieldCount =
-      int(!Languages.empty()) + int(!Tools.empty()) + int(!SDKs.empty());
-  if (FieldCount == 0)
-    return;
-  SyntheticSection *Section =
-      createSyntheticSection(WASM_SEC_CUSTOM, "producers");
-  auto &OS = Section->getStream();
-  writeUleb128(OS, FieldCount, "field count");
-  for (auto &Field :
-       {std::make_pair("language", Languages),
-        std::make_pair("processed-by", Tools), std::make_pair("sdk", SDKs)}) {
-    if (Field.second.empty())
-      continue;
-    writeStr(OS, Field.first, "field name");
-    writeUleb128(OS, Field.second.size(), "number of entries");
-    for (auto &Entry : Field.second) {
-      writeStr(OS, Entry.first, "producer name");
-      writeStr(OS, Entry.second, "producer version");
-    }
-  }
-}
-
-void Writer::createTargetFeaturesSection() {
-  if (TargetFeatures.empty())
-    return;
-
-  SmallVector<std::string, 8> Emitted(TargetFeatures.begin(),
-                                      TargetFeatures.end());
-  llvm::sort(Emitted);
-  SyntheticSection *Section =
-      createSyntheticSection(WASM_SEC_CUSTOM, "target_features");
-  auto &OS = Section->getStream();
-  writeUleb128(OS, Emitted.size(), "feature count");
-  for (auto &Feature : Emitted) {
-    writeU8(OS, WASM_FEATURE_PREFIX_USED, "feature used prefix");
-    writeStr(OS, Feature, "feature name");
+    Out.ProducersSec->addInfo(Info);
   }
 }
 
 void Writer::writeHeader() {
   memcpy(Buffer->getBufferStart(), Header.data(), Header.size());
 }
 
 void Writer::writeSections() {
   uint8_t *Buf = Buffer->getBufferStart();
-  parallelForEach(OutputSections, [Buf](OutputSection *S) { S->writeTo(Buf); });
+  parallelForEach(OutputSections, [Buf](OutputSection *S) {
+    assert(S->isNeeded());
+    S->writeTo(Buf);
+  });
 }
 
 // Fix the memory layout of the output binary.  This assigns memory offsets
 // to each of the input data sections as well as the explicit stack region.
 // The default memory layout is as follows, from low to high.
 //
 //  - initialized data (starting at Config->GlobalBase)
 //  - BSS data (not currently implemented in llvm)
 //  - explicit stack (Config->ZStackSize)
 //  - heap start / unallocated
 //
 // The --stack-first option means that stack is placed before any static data.
 // This can be useful since it means that stack overflow traps immediately
 // rather than overwriting global data, but also increases code size since all
 // static data loads and stores requires larger offsets.
 void Writer::layoutMemory() {
-  createOutputSegments();
-
   uint32_t MemoryPtr = 0;
 
   auto PlaceStack = [&]() {
     if (Config->Relocatable || Config->Shared)
       return;
     MemoryPtr = alignTo(MemoryPtr, StackAlignment);
     if (Config->ZStackSize != alignTo(Config->ZStackSize, StackAlignment))
       error("stack size must be " + Twine(StackAlignment) + "-byte aligned");
void Writer::layoutMemory() {
 
   uint32_t DataStart = MemoryPtr;
 
   // Arbitrarily set __dso_handle handle to point to the start of the data
   // segments.
   if (WasmSym::DsoHandle)
     WasmSym::DsoHandle->setVirtualAddress(DataStart);
 
-  MemAlign = 0;
+  Out.DylinkSec->MemAlign = 0;
   for (OutputSegment *Seg : Segments) {
-    MemAlign = std::max(MemAlign, Seg->Alignment);
+    Out.DylinkSec->MemAlign = std::max(Out.DylinkSec->MemAlign, Seg->Alignment);
     MemoryPtr = alignTo(MemoryPtr, 1ULL << Seg->Alignment);
     Seg->StartVA = MemoryPtr;
     log(formatv("mem: {0,-15} offset={1,-8} size={2,-8} align={3}", Seg->Name,
                 MemoryPtr, Seg->Size, Seg->Alignment));
     MemoryPtr += Seg->Size;
   }
 
   // TODO: Add .bss space here.
   if (WasmSym::DataEnd)
     WasmSym::DataEnd->setVirtualAddress(MemoryPtr);
 
   log("mem: static data = " + Twine(MemoryPtr - DataStart));
 
   if (Config->Shared) {
-    MemSize = MemoryPtr;
+    Out.DylinkSec->MemSize = MemoryPtr;
     return;
   }
 
   if (!Config->StackFirst)
     PlaceStack();
 
   // Set `__heap_base` to directly follow the end of the stack or global data.
   // The fact that this comes last means that a malloc/brk implementation
   // can grow the heap at runtime.
   if (!Config->Relocatable) {
     WasmSym::HeapBase->setVirtualAddress(MemoryPtr);
     log("mem: heap base   = " + Twine(MemoryPtr));
   }
 
   if (Config->InitialMemory != 0) {
     if (Config->InitialMemory != alignTo(Config->InitialMemory, WasmPageSize))
       error("initial memory must be " + Twine(WasmPageSize) + "-byte aligned");
     if (MemoryPtr > Config->InitialMemory)
       error("initial memory too small, " + Twine(MemoryPtr) + " bytes needed");
     else
       MemoryPtr = Config->InitialMemory;
   }
-  MemSize = MemoryPtr;
-  NumMemoryPages = alignTo(MemoryPtr, WasmPageSize) / WasmPageSize;
-  log("mem: total pages = " + Twine(NumMemoryPages));
+  Out.DylinkSec->MemSize = MemoryPtr;
+  Out.MemorySec->NumMemoryPages =
+      alignTo(MemoryPtr, WasmPageSize) / WasmPageSize;
+  log("mem: total pages = " + Twine(Out.MemorySec->NumMemoryPages));
 
   // Check max if explicitly supplied or required by shared memory
   if (Config->MaxMemory != 0 || Config->SharedMemory) {
     if (Config->MaxMemory != alignTo(Config->MaxMemory, WasmPageSize))
       error("maximum memory must be " + Twine(WasmPageSize) + "-byte aligned");
     if (MemoryPtr > Config->MaxMemory)
       error("maximum memory too small, " + Twine(MemoryPtr) + " bytes needed");
-    MaxMemoryPages = Config->MaxMemory / WasmPageSize;
-    log("mem: max pages   = " + Twine(MaxMemoryPages));
+    Out.MemorySec->MaxMemoryPages = Config->MaxMemory / WasmPageSize;
+    log("mem: max pages   = " + Twine(Out.MemorySec->MaxMemoryPages));
   }
 }
 
-SyntheticSection *Writer::createSyntheticSection(uint32_t Type,
-                                                 StringRef Name) {
-  auto Sec = make<SyntheticSection>(Type, Name);
-  log("createSection: " + toString(*Sec));
+void Writer::addSection(OutputSection *Sec) {
+  if (!Sec->isNeeded())
+    return;
+  log("addSection: " + toString(*Sec));
+  Sec->SectionIndex = OutputSections.size();
   OutputSections.push_back(Sec);
-  return Sec;
 }
 
-void Writer::createSections() {
-  // Known sections
-  if (Config->Pic)
-    createDylinkSection();
-  createTypeSection();
-  createImportSection();
-  createFunctionSection();
-  createTableSection();
-  createMemorySection();
-  createGlobalSection();
-  createEventSection();
-  createExportSection();
-  createElemSection();
-  createDataCountSection();
-  createCodeSection();
-  createDataSection();
+void Writer::addSections() {
+  addSection(Out.DylinkSec);
+  addSection(Out.TypeSec);
+  addSection(Out.ImportSec);
+  addSection(Out.FunctionSec);
+  addSection(Out.TableSec);
+  addSection(Out.MemorySec);
+  addSection(Out.GlobalSec);
+  addSection(Out.EventSec);
+  addSection(Out.ExportSec);
+  addSection(Out.ElemSec);
+  addSection(Out.DataCountSec);
+
+  addSection(make<CodeSection>(Out.FunctionSec->InputFunctions));
+  addSection(make<DataSection>(Segments));
+
   createCustomSections();
 
-  // Custom sections
+  addSection(Out.LinkingSec);
   if (Config->Relocatable) {
-    createLinkingSection();
     createRelocSections();
   }
 
-  if (!Config->StripDebug && !Config->StripAll)
-    createNameSection();
-
-  if (!Config->StripAll) {
-    createProducersSection();
-    createTargetFeaturesSection();
-  }
+  addSection(Out.NameSec);
+  addSection(Out.ProducersSec);
+  addSection(Out.TargetFeaturesSec);
 
+void Writer::finalizeSections() {
   for (OutputSection *S : OutputSections) {
     S->setOffset(FileSize);
     S->finalizeContents();
     FileSize += S->getSize();
   }
 }
 
-void Writer::calculateTargetFeatures() {
+void Writer::populateTargetFeatures() {
   SmallSet<std::string, 8> Used;
   SmallSet<std::string, 8> Required;
   SmallSet<std::string, 8> Disallowed;
 
   // Only infer used features if user did not specify features
   bool InferFeatures = !Config->Features.hasValue();
 
   if (!InferFeatures) {
     for (auto &Feature : Config->Features.getValue())
-      TargetFeatures.insert(Feature);
+      Out.TargetFeaturesSec->Features.insert(Feature);
     // No need to read or check features
     if (!Config->CheckFeatures)
       return;
   }
 
   // Find the sets of used, required, and disallowed features
   for (ObjFile *File : Symtab->ObjectFiles) {
     for (auto &Feature : File->getWasmObj()->getTargetFeatures()) {
for (auto &Feature : File->getWasmObj()->getTargetFeatures()) {
       default:
         error("Unrecognized feature policy prefix " +
               std::to_string(Feature.Prefix));
       }
     }
   }
 
   if (InferFeatures)
-    TargetFeatures.insert(Used.begin(), Used.end());
+    Out.TargetFeaturesSec->Features.insert(Used.begin(), Used.end());
 
-  if (TargetFeatures.count("atomics") && !Config->SharedMemory)
+  if (Out.TargetFeaturesSec->Features.count("atomics") && !Config->SharedMemory)
     error("'atomics' feature is used, so --shared-memory must be used");
 
   if (!Config->CheckFeatures)
     return;
 
   if (Disallowed.count("atomics") && Config->SharedMemory)
     error(
         "'atomics' feature is disallowed, so --shared-memory must not be used");
 
   // Validate that used features are allowed in output
   if (!InferFeatures) {
     for (auto &Feature : Used) {
-      if (!TargetFeatures.count(Feature))
+      if (!Out.TargetFeaturesSec->Features.count(Feature))
         error(Twine("Target feature '") + Feature + "' is not allowed.");
     }
   }
 
   // Validate the required and disallowed constraints for each file
   for (ObjFile *File : Symtab->ObjectFiles) {
     SmallSet<std::string, 8> ObjectFeatures;
     for (auto &Feature : File->getWasmObj()->getTargetFeatures()) {
for (Symbol *Sym : Symtab->getSymbols()) {
     if (!Sym->IsUsedInRegularObj)
       continue;
     // We don't generate imports for data symbols. They however can be imported
     // as GOT entries.
     if (isa<DataSymbol>(Sym))
       continue;
 
     LLVM_DEBUG(dbgs() << "import: " << Sym->getName() << "\n");
-    ImportedSymbols.emplace_back(Sym);
-    if (auto *F = dyn_cast<FunctionSymbol>(Sym))
-      F->setFunctionIndex(NumImportedFunctions++);
-    else if (auto *G = dyn_cast<GlobalSymbol>(Sym))
-      G->setGlobalIndex(NumImportedGlobals++);
-    else
-      cast<EventSymbol>(Sym)->setEventIndex(NumImportedEvents++);
+    Out.ImportSec->addImport(Sym);
   }
 }
 
 void Writer::calculateExports() {
   if (Config->Relocatable)
     return;
 
   if (!Config->Relocatable && !Config->ImportMemory)
-    Exports.push_back(WasmExport{"memory", WASM_EXTERNAL_MEMORY, 0});
+    Out.ExportSec->Exports.push_back(
+        WasmExport{"memory", WASM_EXTERNAL_MEMORY, 0});
 
   if (!Config->Relocatable && Config->ExportTable)
-    Exports.push_back(WasmExport{FunctionTableName, WASM_EXTERNAL_TABLE, 0});
+    Out.ExportSec->Exports.push_back(
+        WasmExport{FunctionTableName, WASM_EXTERNAL_TABLE, 0});
 
-  unsigned FakeGlobalIndex = NumImportedGlobals + InputGlobals.size();
+  unsigned FakeGlobalIndex =
+      Out.ImportSec->NumImportedGlobals + Out.GlobalSec->InputGlobals.size();
 
   for (Symbol *Sym : Symtab->getSymbols()) {
     if (!Sym->isExported())
       continue;
     if (!Sym->isLive())
       continue;
 
     StringRef Name = Sym->getName();
if (auto *F = dyn_cast<DefinedFunction>(Sym)) {
         assert(G == WasmSym::StackPointer);
         continue;
       }
       Export = {Name, WASM_EXTERNAL_GLOBAL, G->getGlobalIndex()};
     } else if (auto *E = dyn_cast<DefinedEvent>(Sym)) {
       Export = {Name, WASM_EXTERNAL_EVENT, E->getEventIndex()};
     } else {
       auto *D = cast<DefinedData>(Sym);
-      DefinedFakeGlobals.emplace_back(D);
+      Out.GlobalSec->DefinedFakeGlobals.emplace_back(D);
       Export = {Name, WASM_EXTERNAL_GLOBAL, FakeGlobalIndex++};
     }
 
     LLVM_DEBUG(dbgs() << "Export: " << Name << "\n");
-    Exports.push_back(Export);
+    Out.ExportSec->Exports.push_back(Export);
   }
 }
 
-void Writer::assignSymtab() {
+void Writer::populateSymtab() {
   if (!Config->Relocatable)
     return;
 
-  StringMap<uint32_t> SectionSymbolIndices;
-
-  unsigned SymbolIndex = SymtabEntries.size();
-
-  auto AddSymbol = [&](Symbol *Sym) {
-    if (auto *S = dyn_cast<SectionSymbol>(Sym)) {
-      StringRef Name = S->getName();
-      if (CustomSectionMapping.count(Name) == 0)
-        return;
-
-      auto SSI = SectionSymbolIndices.find(Name);
-      if (SSI != SectionSymbolIndices.end()) {
-        Sym->setOutputSymbolIndex(SSI->second);
-        return;
-      }
-
-      SectionSymbolIndices[Name] = SymbolIndex;
-      CustomSectionSymbols[Name] = cast<SectionSymbol>(Sym);
-
-      Sym->markLive();
-    }
-
-    // (Since this is relocatable output, GC is not performed so symbols must
-    // be live.)
-    assert(Sym->isLive());
-    Sym->setOutputSymbolIndex(SymbolIndex++);
-    SymtabEntries.emplace_back(Sym);
-  };
-
   for (Symbol *Sym : Symtab->getSymbols())
     if (Sym->IsUsedInRegularObj)
-      AddSymbol(Sym);
+      Out.LinkingSec->addToSymtab(Sym);
 
   for (ObjFile *File : Symtab->ObjectFiles) {
     LLVM_DEBUG(dbgs() << "Local symtab entries: " << File->getName() << "\n");
     for (Symbol *Sym : File->getSymbols())
-      if (Sym->isLocal())
-        AddSymbol(Sym);
+      if (Sym->isLocal() && !isa<SectionSymbol>(Sym))
+        Out.LinkingSec->addToSymtab(Sym);
   }
 }
 
-uint32_t Writer::lookupType(const WasmSignature &Sig) {
-  auto It = TypeIndices.find(Sig);
-  if (It == TypeIndices.end()) {
-    error("type not found: " + toString(Sig));
-    return 0;
-  }
-  return It->second;
-}
-
-uint32_t Writer::registerType(const WasmSignature &Sig) {
-  auto Pair = TypeIndices.insert(std::make_pair(Sig, Types.size()));
-  if (Pair.second) {
-    LLVM_DEBUG(dbgs() << "type " << toString(Sig) << "\n");
-    Types.push_back(&Sig);
-  }
-  return Pair.first->second;
-}
-
 void Writer::calculateTypes() {
   // The output type section is the union of the following sets:
   // 1. Any signature used in the TYPE relocation
   // 2. The signatures of all imported functions
   // 3. The signatures of all defined functions
   // 4. The signatures of all imported events
   // 5. The signatures of all defined events
 
   for (ObjFile *File : Symtab->ObjectFiles) {
     ArrayRef<WasmSignature> Types = File->getWasmObj()->types();
     for (uint32_t I = 0; I < Types.size(); I++)
       if (File->TypeIsUsed[I])
-        File->TypeMap[I] = registerType(Types[I]);
+        File->TypeMap[I] = Out.TypeSec->registerType(Types[I]);
   }
 
-  for (const Symbol *Sym : ImportedSymbols) {
+  for (const Symbol *Sym : Out.ImportSec->ImportedSymbols) {
     if (auto *F = dyn_cast<FunctionSymbol>(Sym))
-      registerType(*F->Signature);
+      Out.TypeSec->registerType(*F->Signature);
     else if (auto *E = dyn_cast<EventSymbol>(Sym))
-      registerType(*E->Signature);
+      Out.TypeSec->registerType(*E->Signature);
   }
 
-  for (const InputFunction *F : InputFunctions)
-    registerType(F->Signature);
-
-  for (const InputEvent *E : InputEvents)
-    registerType(E->Signature);
-}
-
-static bool requiresGOTAccess(const Symbol* Sym) {
-  return Config->Pic && !Sym->isHidden() && !Sym->isLocal();
-}
-
-void Writer::processRelocations(InputChunk *Chunk) {
-  if (!Chunk->Live)
-    return;
-  ObjFile *File = Chunk->File;
-  ArrayRef<WasmSignature> Types = File->getWasmObj()->types();
-  for (const WasmRelocation &Reloc : Chunk->getRelocations()) {
-    if (Reloc.Type == R_WASM_TYPE_INDEX_LEB) {
-      // Mark target type as live
-      File->TypeMap[Reloc.Index] = registerType(Types[Reloc.Index]);
-      File->TypeIsUsed[Reloc.Index] = true;
-      continue;
-    }
+  for (const InputFunction *F : Out.FunctionSec->InputFunctions)
+    Out.TypeSec->registerType(F->Signature);
 
-    // Other relocation types all have a corresponding symbol
-    auto *Sym = File->getSymbols()[Reloc.Index];
-    switch (Reloc.Type) {
-    case R_WASM_TABLE_INDEX_I32:
-    case R_WASM_TABLE_INDEX_SLEB:
-    case R_WASM_TABLE_INDEX_REL_SLEB: {
-      auto *F = cast<FunctionSymbol>(Sym);
-      if (F->hasTableIndex() || !F->hasFunctionIndex() || requiresGOTAccess(F))
-        break;
-      F->setTableIndex(TableBase + IndirectFunctions.size());
-      IndirectFunctions.emplace_back(F);
-      break;
-    }
-    case R_WASM_TYPE_INDEX_LEB:
-      break;
-    case R_WASM_GLOBAL_INDEX_LEB:
-      if (!isa<GlobalSymbol>(Sym) && !Sym->isInGOT()) {
-        Sym->setGOTIndex(NumImportedGlobals++);
-        GOTSymbols.push_back(Sym);
-      }
-      break;
-    case R_WASM_MEMORY_ADDR_SLEB:
-    case R_WASM_MEMORY_ADDR_LEB:
-    case R_WASM_MEMORY_ADDR_REL_SLEB:
-      if (!Config->Relocatable) {
-        if (Sym->isUndefined() && !Sym->isWeak()) {
-          error(toString(File) + ": cannot resolve relocation of type " +
-                relocTypeToString(Reloc.Type) +
-                " against undefined (non-weak) data symbol: " + toString(*Sym));
-        }
-      }
-      break;
-    }
+  for (const InputEvent *E : Out.EventSec->InputEvents)
+    Out.TypeSec->registerType(E->Signature);
 
-    if (Config->Pic) {
-      switch (Reloc.Type) {
-      case R_WASM_TABLE_INDEX_SLEB:
-      case R_WASM_MEMORY_ADDR_SLEB:
-      case R_WASM_MEMORY_ADDR_LEB:
-        // Certain relocation types can't be used when building PIC output, since
-        // they would require absolute symbol addresses at link time.
-        error(toString(File) + ": relocation " +
-              relocTypeToString(Reloc.Type) + " cannot be used againt symbol " +
-              toString(*Sym) + "; recompile with -fPIC");
-        break;
-      case R_WASM_TABLE_INDEX_I32:
-      case R_WASM_MEMORY_ADDR_I32:
-        // These relocation types are only present in the data section and
-        // will be converted into code by `generateRelocationCode`.  This code
-        // requires the symbols to have GOT entires.
-        if (requiresGOTAccess(Sym) && !Sym->isInGOT()) {
-          Sym->setGOTIndex(NumImportedGlobals++);
-          GOTSymbols.push_back(Sym);
-        }
-        break;
-      }
-    }
+static void scanRelocations() {
+  for (ObjFile *File : Symtab->ObjectFiles) {
+    LLVM_DEBUG(dbgs() << "scanRelocations: " << File->getName() << "\n");
+    for (InputChunk *Chunk : File->Functions)
+      scanRelocations(Chunk);
+    for (InputChunk *Chunk : File->Segments)
+      scanRelocations(Chunk);
+    for (auto &P : File->CustomSections)
+      scanRelocations(P);
   }
 }
 
 void Writer::assignIndexes() {
-  assert(InputFunctions.empty());
-  uint32_t FunctionIndex = NumImportedFunctions;
-  auto AddDefinedFunction = [&](InputFunction *Func) {
-    if (!Func->Live)
-      return;
-    InputFunctions.emplace_back(Func);
-    Func->setFunctionIndex(FunctionIndex++);
-  };
+  assert(Out.FunctionSec->InputFunctions.empty());
 
   for (InputFunction *Func : Symtab->SyntheticFunctions)
-    AddDefinedFunction(Func);
+    Out.FunctionSec->addFunction(Func);
 
   for (ObjFile *File : Symtab->ObjectFiles) {
     LLVM_DEBUG(dbgs() << "Functions: " << File->getName() << "\n");
     for (InputFunction *Func : File->Functions)
-      AddDefinedFunction(Func);
+      Out.FunctionSec->addFunction(Func);
   }
 
-  for (ObjFile *File : Symtab->ObjectFiles) {
-    LLVM_DEBUG(dbgs() << "Handle relocs: " << File->getName() << "\n");
-    for (InputChunk *Chunk : File->Functions)
-      processRelocations(Chunk);
-    for (InputChunk *Chunk : File->Segments)
-      processRelocations(Chunk);
-    for (auto &P : File->CustomSections)
-      processRelocations(P);
-  }
-
-  assert(InputGlobals.empty());
-  uint32_t GlobalIndex = NumImportedGlobals;
-  auto AddDefinedGlobal = [&](InputGlobal *Global) {
-    if (Global->Live) {
-      LLVM_DEBUG(dbgs() << "AddDefinedGlobal: " << GlobalIndex << "\n");
-      Global->setGlobalIndex(GlobalIndex++);
-      InputGlobals.push_back(Global);
-    }
-  };
+  scanRelocations();
 
   for (InputGlobal *Global : Symtab->SyntheticGlobals)
-    AddDefinedGlobal(Global);
+    Out.GlobalSec->addGlobal(Global);
 
   for (ObjFile *File : Symtab->ObjectFiles) {
     LLVM_DEBUG(dbgs() << "Globals: " << File->getName() << "\n");
     for (InputGlobal *Global : File->Globals)
-      AddDefinedGlobal(Global);
+      Out.GlobalSec->addGlobal(Global);
   }
 
-  assert(InputEvents.empty());
-  uint32_t EventIndex = NumImportedEvents;
-  auto AddDefinedEvent = [&](InputEvent *Event) {
-    if (Event->Live) {
-      LLVM_DEBUG(dbgs() << "AddDefinedEvent: " << EventIndex << "\n");
-      Event->setEventIndex(EventIndex++);
-      InputEvents.push_back(Event);
-    }
-  };
-
   for (ObjFile *File : Symtab->ObjectFiles) {
     LLVM_DEBUG(dbgs() << "Events: " << File->getName() << "\n");
     for (InputEvent *Event : File->Events)
-      AddDefinedEvent(Event);
+      Out.EventSec->addEvent(Event);
   }
 }
 
 static StringRef getOutputDataSegmentName(StringRef Name) {
   // With PIC code we currently only support a single data segment since
   // we only have a single __memory_base to use as our base address.
   if (Config->Pic)
     return "data";
void Writer::calculateInitFunctions() {
   // Sort in order of priority (lowest first) so that they are called
   // in the correct order.
   llvm::stable_sort(InitFunctions,
                     [](const WasmInitEntry &L, const WasmInitEntry &R) {
                       return L.Priority < R.Priority;
                     });
 }
 
+void Writer::createSyntheticSections() {
+  Out.DylinkSec = make<DylinkSection>();
+  Out.TypeSec = make<TypeSection>();
+  Out.ImportSec = make<ImportSection>();
+  Out.FunctionSec = make<FunctionSection>();
+  Out.TableSec = make<TableSection>();
+  Out.MemorySec = make<MemorySection>();
+  Out.GlobalSec = make<GlobalSection>();
+  Out.EventSec = make<EventSection>();
+  Out.ExportSec = make<ExportSection>();
+  Out.ElemSec = make<ElemSection>(TableBase);
+  Out.DataCountSec = make<DataCountSection>(Segments.size());
+  Out.LinkingSec = make<LinkingSection>(InitFunctions, Segments);
+  Out.NameSec = make<NameSection>();
+  Out.ProducersSec = make<ProducersSection>();
+  Out.TargetFeaturesSec = make<TargetFeaturesSection>();
+}
+
 void Writer::run() {
   if (Config->Relocatable || Config->Pic)
     Config->GlobalBase = 0;
 
   // For PIC code the table base is assigned dynamically by the loader.
   // For non-PIC, we start at 1 so that accessing table index 0 always traps.
   if (!Config->Pic)
     TableBase = 1;
 
-  log("-- calculateTargetFeatures");
-  calculateTargetFeatures();
+  log("-- createOutputSegments");
+  createOutputSegments();
+  log("-- createSyntheticSections");
+  createSyntheticSections();
+  log("-- populateProducers");
+  populateProducers();
+  log("-- populateTargetFeatures");
+  populateTargetFeatures();
   log("-- calculateImports");
   calculateImports();
   log("-- assignIndexes");
   assignIndexes();
   log("-- calculateInitFunctions");
   calculateInitFunctions();
   log("-- calculateTypes");
   calculateTypes();
   log("-- layoutMemory");
   layoutMemory();
   if (!Config->Relocatable) {
     if (Config->Pic)
       createApplyRelocationsFunction();
     createCallCtorsFunction();
   }
   log("-- calculateExports");
   calculateExports();
   log("-- calculateCustomSections");
   calculateCustomSections();
-  log("-- assignSymtab");
-  assignSymtab();
+  log("-- populateSymtab");
+  populateSymtab();
+  log("-- addSections");
+  addSections();
 
   if (errorHandler().Verbose) {
-    log("Defined Functions: " + Twine(InputFunctions.size()));
-    log("Defined Globals  : " + Twine(InputGlobals.size()));
-    log("Defined Events   : " + Twine(InputEvents.size()));
-    log("Function Imports : " + Twine(NumImportedFunctions));
-    log("Global Imports   : " + Twine(NumImportedGlobals));
-    log("Event Imports    : " + Twine(NumImportedEvents));
+    log("Defined Functions: " + Twine(Out.FunctionSec->InputFunctions.size()));
+    log("Defined Globals  : " + Twine(Out.GlobalSec->InputGlobals.size()));
+    log("Defined Events   : " + Twine(Out.EventSec->InputEvents.size()));
+    log("Function Imports : " + Twine(Out.ImportSec->NumImportedFunctions));
+    log("Global Imports   : " + Twine(Out.ImportSec->NumImportedGlobals));
+    log("Event Imports    : " + Twine(Out.ImportSec->NumImportedEvents));
     for (ObjFile *File : Symtab->ObjectFiles)
       File->dumpInfo();
   }
 
   createHeader();
-  log("-- createSections");
-  createSections();
+  log("-- finalizeSections");
+  finalizeSections();
 
   log("-- openFile");
   openFile();
   if (errorCount())
     return;
 
   writeHeader();