1 files changed, 1622 insertions, 0 deletions

diff --git a/contrib/llvm-project/llvm/lib/MC/WasmObjectWriter.cpp b/contrib/llvm-project/llvm/lib/MC/WasmObjectWriter.cpp
new file mode 100644
index 000000000000..098343cd0107
--- /dev/null
+++ b/contrib/llvm-project/llvm/lib/MC/WasmObjectWriter.cpp
@@ -0,0 +1,1622 @@
+//===- lib/MC/WasmObjectWriter.cpp - Wasm File Writer ---------------------===//
+//
+// 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 implements Wasm object file writer information.
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/ADT/STLExtras.h"
+#include "llvm/ADT/SmallPtrSet.h"
+#include "llvm/BinaryFormat/Wasm.h"
+#include "llvm/Config/llvm-config.h"
+#include "llvm/MC/MCAsmBackend.h"
+#include "llvm/MC/MCAsmLayout.h"
+#include "llvm/MC/MCAssembler.h"
+#include "llvm/MC/MCContext.h"
+#include "llvm/MC/MCExpr.h"
+#include "llvm/MC/MCFixupKindInfo.h"
+#include "llvm/MC/MCObjectWriter.h"
+#include "llvm/MC/MCSectionWasm.h"
+#include "llvm/MC/MCSymbolWasm.h"
+#include "llvm/MC/MCValue.h"
+#include "llvm/MC/MCWasmObjectWriter.h"
+#include "llvm/Support/Casting.h"
+#include "llvm/Support/Debug.h"
+#include "llvm/Support/ErrorHandling.h"
+#include "llvm/Support/LEB128.h"
+#include "llvm/Support/StringSaver.h"
+#include <vector>
+
+using namespace llvm;
+
+#define DEBUG_TYPE "mc"
+
+namespace {
+
+// Went we ceate the indirect function table we start at 1, so that there is
+// and emtpy slot at 0 and therefore calling a null function pointer will trap.
+static const uint32_t InitialTableOffset = 1;
+
+// For patching purposes, we need to remember where each section starts, both
+// for patching up the section size field, and for patching up references to
+// locations within the section.
+struct SectionBookkeeping {
+  // Where the size of the section is written.
+  uint64_t SizeOffset;
+  // Where the section header ends (without custom section name).
+  uint64_t PayloadOffset;
+  // Where the contents of the section starts.
+  uint64_t ContentsOffset;
+  uint32_t Index;
+};
+
+// The signature of a wasm function or event, in a struct capable of being used
+// as a DenseMap key.
+// TODO: Consider using wasm::WasmSignature directly instead.
+struct WasmSignature {
+  // Support empty and tombstone instances, needed by DenseMap.
+  enum { Plain, Empty, Tombstone } State = Plain;
+
+  // The return types of the function.
+  SmallVector<wasm::ValType, 1> Returns;
+
+  // The parameter types of the function.
+  SmallVector<wasm::ValType, 4> Params;
+
+  bool operator==(const WasmSignature &Other) const {
+    return State == Other.State && Returns == Other.Returns &&
+           Params == Other.Params;
+  }
+};
+
+// Traits for using WasmSignature in a DenseMap.
+struct WasmSignatureDenseMapInfo {
+  static WasmSignature getEmptyKey() {
+    WasmSignature Sig;
+    Sig.State = WasmSignature::Empty;
+    return Sig;
+  }
+  static WasmSignature getTombstoneKey() {
+    WasmSignature Sig;
+    Sig.State = WasmSignature::Tombstone;
+    return Sig;
+  }
+  static unsigned getHashValue(const WasmSignature &Sig) {
+    uintptr_t Value = Sig.State;
+    for (wasm::ValType Ret : Sig.Returns)
+      Value += DenseMapInfo<uint32_t>::getHashValue(uint32_t(Ret));
+    for (wasm::ValType Param : Sig.Params)
+      Value += DenseMapInfo<uint32_t>::getHashValue(uint32_t(Param));
+    return Value;
+  }
+  static bool isEqual(const WasmSignature &LHS, const WasmSignature &RHS) {
+    return LHS == RHS;
+  }
+};
+
+// A wasm data segment.  A wasm binary contains only a single data section
+// but that can contain many segments, each with their own virtual location
+// in memory.  Each MCSection data created by llvm is modeled as its own
+// wasm data segment.
+struct WasmDataSegment {
+  MCSectionWasm *Section;
+  StringRef Name;
+  uint32_t InitFlags;
+  uint32_t Offset;
+  uint32_t Alignment;
+  uint32_t LinkerFlags;
+  SmallVector<char, 4> Data;
+};
+
+// A wasm function to be written into the function section.
+struct WasmFunction {
+  uint32_t SigIndex;
+  const MCSymbolWasm *Sym;
+};
+
+// A wasm global to be written into the global section.
+struct WasmGlobal {
+  wasm::WasmGlobalType Type;
+  uint64_t InitialValue;
+};
+
+// Information about a single item which is part of a COMDAT.  For each data
+// segment or function which is in the COMDAT, there is a corresponding
+// WasmComdatEntry.
+struct WasmComdatEntry {
+  unsigned Kind;
+  uint32_t Index;
+};
+
+// Information about a single relocation.
+struct WasmRelocationEntry {
+  uint64_t Offset;                   // Where is the relocation.
+  const MCSymbolWasm *Symbol;        // The symbol to relocate with.
+  int64_t Addend;                    // A value to add to the symbol.
+  unsigned Type;                     // The type of the relocation.
+  const MCSectionWasm *FixupSection; // The section the relocation is targeting.
+
+  WasmRelocationEntry(uint64_t Offset, const MCSymbolWasm *Symbol,
+                      int64_t Addend, unsigned Type,
+                      const MCSectionWasm *FixupSection)
+      : Offset(Offset), Symbol(Symbol), Addend(Addend), Type(Type),
+        FixupSection(FixupSection) {}
+
+  bool hasAddend() const { return wasm::relocTypeHasAddend(Type); }
+
+  void print(raw_ostream &Out) const {
+    Out << wasm::relocTypetoString(Type) << " Off=" << Offset
+        << ", Sym=" << *Symbol << ", Addend=" << Addend
+        << ", FixupSection=" << FixupSection->getSectionName();
+  }
+
+#if !defined(NDEBUG) || defined(LLVM_ENABLE_DUMP)
+  LLVM_DUMP_METHOD void dump() const { print(dbgs()); }
+#endif
+};
+
+static const uint32_t InvalidIndex = -1;
+
+struct WasmCustomSection {
+
+  StringRef Name;
+  MCSectionWasm *Section;
+
+  uint32_t OutputContentsOffset;
+  uint32_t OutputIndex;
+
+  WasmCustomSection(StringRef Name, MCSectionWasm *Section)
+      : Name(Name), Section(Section), OutputContentsOffset(0),
+        OutputIndex(InvalidIndex) {}
+};
+
+#if !defined(NDEBUG)
+raw_ostream &operator<<(raw_ostream &OS, const WasmRelocationEntry &Rel) {
+  Rel.print(OS);
+  return OS;
+}
+#endif
+
+// Write X as an (unsigned) LEB value at offset Offset in Stream, padded
+// to allow patching.
+static void writePatchableLEB(raw_pwrite_stream &Stream, uint32_t X,
+                              uint64_t Offset) {
+  uint8_t Buffer[5];
+  unsigned SizeLen = encodeULEB128(X, Buffer, 5);
+  assert(SizeLen == 5);
+  Stream.pwrite((char *)Buffer, SizeLen, Offset);
+}
+
+// Write X as an signed LEB value at offset Offset in Stream, padded
+// to allow patching.
+static void writePatchableSLEB(raw_pwrite_stream &Stream, int32_t X,
+                               uint64_t Offset) {
+  uint8_t Buffer[5];
+  unsigned SizeLen = encodeSLEB128(X, Buffer, 5);
+  assert(SizeLen == 5);
+  Stream.pwrite((char *)Buffer, SizeLen, Offset);
+}
+
+// Write X as a plain integer value at offset Offset in Stream.
+static void writeI32(raw_pwrite_stream &Stream, uint32_t X, uint64_t Offset) {
+  uint8_t Buffer[4];
+  support::endian::write32le(Buffer, X);
+  Stream.pwrite((char *)Buffer, sizeof(Buffer), Offset);
+}
+
+class WasmObjectWriter : public MCObjectWriter {
+  support::endian::Writer W;
+
+  /// The target specific Wasm writer instance.
+  std::unique_ptr<MCWasmObjectTargetWriter> TargetObjectWriter;
+
+  // Relocations for fixing up references in the code section.
+  std::vector<WasmRelocationEntry> CodeRelocations;
+  uint32_t CodeSectionIndex;
+
+  // Relocations for fixing up references in the data section.
+  std::vector<WasmRelocationEntry> DataRelocations;
+  uint32_t DataSectionIndex;
+
+  // Index values to use for fixing up call_indirect type indices.
+  // Maps function symbols to the index of the type of the function
+  DenseMap<const MCSymbolWasm *, uint32_t> TypeIndices;
+  // Maps function symbols to the table element index space. Used
+  // for TABLE_INDEX relocation types (i.e. address taken functions).
+  DenseMap<const MCSymbolWasm *, uint32_t> TableIndices;
+  // Maps function/global symbols to the function/global/event/section index
+  // space.
+  DenseMap<const MCSymbolWasm *, uint32_t> WasmIndices;
+  DenseMap<const MCSymbolWasm *, uint32_t> GOTIndices;
+  // Maps data symbols to the Wasm segment and offset/size with the segment.
+  DenseMap<const MCSymbolWasm *, wasm::WasmDataReference> DataLocations;
+
+  // Stores output data (index, relocations, content offset) for custom
+  // section.
+  std::vector<WasmCustomSection> CustomSections;
+  std::unique_ptr<WasmCustomSection> ProducersSection;
+  std::unique_ptr<WasmCustomSection> TargetFeaturesSection;
+  // Relocations for fixing up references in the custom sections.
+  DenseMap<const MCSectionWasm *, std::vector<WasmRelocationEntry>>
+      CustomSectionsRelocations;
+
+  // Map from section to defining function symbol.
+  DenseMap<const MCSection *, const MCSymbol *> SectionFunctions;
+
+  DenseMap<WasmSignature, uint32_t, WasmSignatureDenseMapInfo> SignatureIndices;
+  SmallVector<WasmSignature, 4> Signatures;
+  SmallVector<WasmDataSegment, 4> DataSegments;
+  unsigned NumFunctionImports = 0;
+  unsigned NumGlobalImports = 0;
+  unsigned NumEventImports = 0;
+  uint32_t SectionCount = 0;
+
+  // TargetObjectWriter wrappers.
+  bool is64Bit() const { return TargetObjectWriter->is64Bit(); }
+
+  void startSection(SectionBookkeeping &Section, unsigned SectionId);
+  void startCustomSection(SectionBookkeeping &Section, StringRef Name);
+  void endSection(SectionBookkeeping &Section);
+
+public:
+  WasmObjectWriter(std::unique_ptr<MCWasmObjectTargetWriter> MOTW,
+                   raw_pwrite_stream &OS)
+      : W(OS, support::little), TargetObjectWriter(std::move(MOTW)) {}
+
+private:
+  void reset() override {
+    CodeRelocations.clear();
+    DataRelocations.clear();
+    TypeIndices.clear();
+    WasmIndices.clear();
+    GOTIndices.clear();
+    TableIndices.clear();
+    DataLocations.clear();
+    CustomSections.clear();
+    ProducersSection.reset();
+    TargetFeaturesSection.reset();
+    CustomSectionsRelocations.clear();
+    SignatureIndices.clear();
+    Signatures.clear();
+    DataSegments.clear();
+    SectionFunctions.clear();
+    NumFunctionImports = 0;
+    NumGlobalImports = 0;
+    MCObjectWriter::reset();
+  }
+
+  void writeHeader(const MCAssembler &Asm);
+
+  void recordRelocation(MCAssembler &Asm, const MCAsmLayout &Layout,
+                        const MCFragment *Fragment, const MCFixup &Fixup,
+                        MCValue Target, uint64_t &FixedValue) override;
+
+  void executePostLayoutBinding(MCAssembler &Asm,
+                                const MCAsmLayout &Layout) override;
+
+  uint64_t writeObject(MCAssembler &Asm, const MCAsmLayout &Layout) override;
+
+  void writeString(const StringRef Str) {
+    encodeULEB128(Str.size(), W.OS);
+    W.OS << Str;
+  }
+
+  void writeValueType(wasm::ValType Ty) { W.OS << static_cast<char>(Ty); }
+
+  void writeTypeSection(ArrayRef<WasmSignature> Signatures);
+  void writeImportSection(ArrayRef<wasm::WasmImport> Imports, uint32_t DataSize,
+                          uint32_t NumElements);
+  void writeFunctionSection(ArrayRef<WasmFunction> Functions);
+  void writeExportSection(ArrayRef<wasm::WasmExport> Exports);
+  void writeElemSection(ArrayRef<uint32_t> TableElems);
+  void writeDataCountSection();
+  void writeCodeSection(const MCAssembler &Asm, const MCAsmLayout &Layout,
+                        ArrayRef<WasmFunction> Functions);
+  void writeDataSection();
+  void writeEventSection(ArrayRef<wasm::WasmEventType> Events);
+  void writeRelocSection(uint32_t SectionIndex, StringRef Name,
+                         std::vector<WasmRelocationEntry> &Relocations);
+  void writeLinkingMetaDataSection(
+      ArrayRef<wasm::WasmSymbolInfo> SymbolInfos,
+      ArrayRef<std::pair<uint16_t, uint32_t>> InitFuncs,
+      const std::map<StringRef, std::vector<WasmComdatEntry>> &Comdats);
+  void writeCustomSection(WasmCustomSection &CustomSection,
+                          const MCAssembler &Asm, const MCAsmLayout &Layout);
+  void writeCustomRelocSections();
+  void
+  updateCustomSectionRelocations(const SmallVector<WasmFunction, 4> &Functions,
+                                 const MCAsmLayout &Layout);
+
+  uint32_t getProvisionalValue(const WasmRelocationEntry &RelEntry);
+  void applyRelocations(ArrayRef<WasmRelocationEntry> Relocations,
+                        uint64_t ContentsOffset);
+
+  uint32_t getRelocationIndexValue(const WasmRelocationEntry &RelEntry);
+  uint32_t getFunctionType(const MCSymbolWasm &Symbol);
+  uint32_t getEventType(const MCSymbolWasm &Symbol);
+  void registerFunctionType(const MCSymbolWasm &Symbol);
+  void registerEventType(const MCSymbolWasm &Symbol);
+};
+
+} // end anonymous namespace
+
+// Write out a section header and a patchable section size field.
+void WasmObjectWriter::startSection(SectionBookkeeping &Section,
+                                    unsigned SectionId) {
+  LLVM_DEBUG(dbgs() << "startSection " << SectionId << "\n");
+  W.OS << char(SectionId);
+
+  Section.SizeOffset = W.OS.tell();
+
+  // The section size. We don't know the size yet, so reserve enough space
+  // for any 32-bit value; we'll patch it later.
+  encodeULEB128(0, W.OS, 5);
+
+  // The position where the section starts, for measuring its size.
+  Section.ContentsOffset = W.OS.tell();
+  Section.PayloadOffset = W.OS.tell();
+  Section.Index = SectionCount++;
+}
+
+void WasmObjectWriter::startCustomSection(SectionBookkeeping &Section,
+                                          StringRef Name) {
+  LLVM_DEBUG(dbgs() << "startCustomSection " << Name << "\n");
+  startSection(Section, wasm::WASM_SEC_CUSTOM);
+
+  // The position where the section header ends, for measuring its size.
+  Section.PayloadOffset = W.OS.tell();
+
+  // Custom sections in wasm also have a string identifier.
+  writeString(Name);
+
+  // The position where the custom section starts.
+  Section.ContentsOffset = W.OS.tell();
+}
+
+// Now that the section is complete and we know how big it is, patch up the
+// section size field at the start of the section.
+void WasmObjectWriter::endSection(SectionBookkeeping &Section) {
+  uint64_t Size = W.OS.tell();
+  // /dev/null doesn't support seek/tell and can report offset of 0.
+  // Simply skip this patching in that case.
+  if (!Size)
+    return;
+
+  Size -= Section.PayloadOffset;
+  if (uint32_t(Size) != Size)
+    report_fatal_error("section size does not fit in a uint32_t");
+
+  LLVM_DEBUG(dbgs() << "endSection size=" << Size << "\n");
+
+  // Write the final section size to the payload_len field, which follows
+  // the section id byte.
+  writePatchableLEB(static_cast<raw_pwrite_stream &>(W.OS), Size,
+                    Section.SizeOffset);
+}
+
+// Emit the Wasm header.
+void WasmObjectWriter::writeHeader(const MCAssembler &Asm) {
+  W.OS.write(wasm::WasmMagic, sizeof(wasm::WasmMagic));
+  W.write<uint32_t>(wasm::WasmVersion);
+}
+
+void WasmObjectWriter::executePostLayoutBinding(MCAssembler &Asm,
+                                                const MCAsmLayout &Layout) {
+  // Build a map of sections to the function that defines them, for use
+  // in recordRelocation.
+  for (const MCSymbol &S : Asm.symbols()) {
+    const auto &WS = static_cast<const MCSymbolWasm &>(S);
+    if (WS.isDefined() && WS.isFunction() && !WS.isVariable()) {
+      const auto &Sec = static_cast<const MCSectionWasm &>(S.getSection());
+      auto Pair = SectionFunctions.insert(std::make_pair(&Sec, &S));
+      if (!Pair.second)
+        report_fatal_error("section already has a defining function: " +
+                           Sec.getSectionName());
+    }
+  }
+}
+
+void WasmObjectWriter::recordRelocation(MCAssembler &Asm,
+                                        const MCAsmLayout &Layout,
+                                        const MCFragment *Fragment,
+                                        const MCFixup &Fixup, MCValue Target,
+                                        uint64_t &FixedValue) {
+  MCAsmBackend &Backend = Asm.getBackend();
+  bool IsPCRel = Backend.getFixupKindInfo(Fixup.getKind()).Flags &
+                 MCFixupKindInfo::FKF_IsPCRel;
+  const auto &FixupSection = cast<MCSectionWasm>(*Fragment->getParent());
+  uint64_t C = Target.getConstant();
+  uint64_t FixupOffset = Layout.getFragmentOffset(Fragment) + Fixup.getOffset();
+  MCContext &Ctx = Asm.getContext();
+
+  // The .init_array isn't translated as data, so don't do relocations in it.
+  if (FixupSection.getSectionName().startswith(".init_array"))
+    return;
+
+  if (const MCSymbolRefExpr *RefB = Target.getSymB()) {
+    assert(RefB->getKind() == MCSymbolRefExpr::VK_None &&
+           "Should not have constructed this");
+
+    // Let A, B and C being the components of Target and R be the location of
+    // the fixup. If the fixup is not pcrel, we want to compute (A - B + C).
+    // If it is pcrel, we want to compute (A - B + C - R).
+
+    // In general, Wasm has no relocations for -B. It can only represent (A + C)
+    // or (A + C - R). If B = R + K and the relocation is not pcrel, we can
+    // replace B to implement it: (A - R - K + C)
+    if (IsPCRel) {
+      Ctx.reportError(
+          Fixup.getLoc(),
+          "No relocation available to represent this relative expression");
+      return;
+    }
+
+    const auto &SymB = cast<MCSymbolWasm>(RefB->getSymbol());
+
+    if (SymB.isUndefined()) {
+      Ctx.reportError(Fixup.getLoc(),
+                      Twine("symbol '") + SymB.getName() +
+                          "' can not be undefined in a subtraction expression");
+      return;
+    }
+
+    assert(!SymB.isAbsolute() && "Should have been folded");
+    const MCSection &SecB = SymB.getSection();
+    if (&SecB != &FixupSection) {
+      Ctx.reportError(Fixup.getLoc(),
+                      "Cannot represent a difference across sections");
+      return;
+    }
+
+    uint64_t SymBOffset = Layout.getSymbolOffset(SymB);
+    uint64_t K = SymBOffset - FixupOffset;
+    IsPCRel = true;
+    C -= K;
+  }
+
+  // We either rejected the fixup or folded B into C at this point.
+  const MCSymbolRefExpr *RefA = Target.getSymA();
+  const auto *SymA = RefA ? cast<MCSymbolWasm>(&RefA->getSymbol()) : nullptr;
+
+  if (SymA && SymA->isVariable()) {
+    const MCExpr *Expr = SymA->getVariableValue();
+    const auto *Inner = cast<MCSymbolRefExpr>(Expr);
+    if (Inner->getKind() == MCSymbolRefExpr::VK_WEAKREF)
+      llvm_unreachable("weakref used in reloc not yet implemented");
+  }
+
+  // Put any constant offset in an addend. Offsets can be negative, and
+  // LLVM expects wrapping, in contrast to wasm's immediates which can't
+  // be negative and don't wrap.
+  FixedValue = 0;
+
+  unsigned Type = TargetObjectWriter->getRelocType(Target, Fixup);
+  assert(!IsPCRel);
+  assert(SymA);
+
+  // Absolute offset within a section or a function.
+  // Currently only supported for for metadata sections.
+  // See: test/MC/WebAssembly/blockaddress.ll
+  if (Type == wasm::R_WASM_FUNCTION_OFFSET_I32 ||
+      Type == wasm::R_WASM_SECTION_OFFSET_I32) {
+    if (!FixupSection.getKind().isMetadata())
+      report_fatal_error("relocations for function or section offsets are "
+                         "only supported in metadata sections");
+
+    const MCSymbol *SectionSymbol = nullptr;
+    const MCSection &SecA = SymA->getSection();
+    if (SecA.getKind().isText())
+      SectionSymbol = SectionFunctions.find(&SecA)->second;
+    else
+      SectionSymbol = SecA.getBeginSymbol();
+    if (!SectionSymbol)
+      report_fatal_error("section symbol is required for relocation");
+
+    C += Layout.getSymbolOffset(*SymA);
+    SymA = cast<MCSymbolWasm>(SectionSymbol);
+  }
+
+  // Relocation other than R_WASM_TYPE_INDEX_LEB are required to be
+  // against a named symbol.
+  if (Type != wasm::R_WASM_TYPE_INDEX_LEB) {
+    if (SymA->getName().empty())
+      report_fatal_error("relocations against un-named temporaries are not yet "
+                         "supported by wasm");
+
+    SymA->setUsedInReloc();
+  }
+
+  if (RefA->getKind() == MCSymbolRefExpr::VK_GOT)
+    SymA->setUsedInGOT();
+
+  WasmRelocationEntry Rec(FixupOffset, SymA, C, Type, &FixupSection);
+  LLVM_DEBUG(dbgs() << "WasmReloc: " << Rec << "\n");
+
+  if (FixupSection.isWasmData()) {
+    DataRelocations.push_back(Rec);
+  } else if (FixupSection.getKind().isText()) {
+    CodeRelocations.push_back(Rec);
+  } else if (FixupSection.getKind().isMetadata()) {
+    CustomSectionsRelocations[&FixupSection].push_back(Rec);
+  } else {
+    llvm_unreachable("unexpected section type");
+  }
+}
+
+static const MCSymbolWasm *resolveSymbol(const MCSymbolWasm &Symbol) {
+  const MCSymbolWasm* Ret = &Symbol;
+  while (Ret->isVariable()) {
+    const MCExpr *Expr = Ret->getVariableValue();
+    auto *Inner = cast<MCSymbolRefExpr>(Expr);
+    Ret = cast<MCSymbolWasm>(&Inner->getSymbol());
+  }
+  return Ret;
+}
+
+// Compute a value to write into the code at the location covered
+// by RelEntry. This value isn't used by the static linker; it just serves
+// to make the object format more readable and more likely to be directly
+// useable.
+uint32_t
+WasmObjectWriter::getProvisionalValue(const WasmRelocationEntry &RelEntry) {
+  if (RelEntry.Type == wasm::R_WASM_GLOBAL_INDEX_LEB && !RelEntry.Symbol->isGlobal()) {
+    assert(GOTIndices.count(RelEntry.Symbol) > 0 && "symbol not found in GOT index space");
+    return GOTIndices[RelEntry.Symbol];
+  }
+
+  switch (RelEntry.Type) {
+  case wasm::R_WASM_TABLE_INDEX_REL_SLEB:
+  case wasm::R_WASM_TABLE_INDEX_SLEB:
+  case wasm::R_WASM_TABLE_INDEX_I32: {
+    // Provisional value is table address of the resolved symbol itself
+    const MCSymbolWasm *Sym = resolveSymbol(*RelEntry.Symbol);
+    assert(Sym->isFunction());
+    return TableIndices[Sym];
+  }
+  case wasm::R_WASM_TYPE_INDEX_LEB:
+    // Provisional value is same as the index
+    return getRelocationIndexValue(RelEntry);
+  case wasm::R_WASM_FUNCTION_INDEX_LEB:
+  case wasm::R_WASM_GLOBAL_INDEX_LEB:
+  case wasm::R_WASM_EVENT_INDEX_LEB:
+    // Provisional value is function/global/event Wasm index
+    assert(WasmIndices.count(RelEntry.Symbol) > 0 && "symbol not found in wasm index space");
+    return WasmIndices[RelEntry.Symbol];
+  case wasm::R_WASM_FUNCTION_OFFSET_I32:
+  case wasm::R_WASM_SECTION_OFFSET_I32: {
+    const auto &Section =
+        static_cast<const MCSectionWasm &>(RelEntry.Symbol->getSection());
+    return Section.getSectionOffset() + RelEntry.Addend;
+  }
+  case wasm::R_WASM_MEMORY_ADDR_LEB:
+  case wasm::R_WASM_MEMORY_ADDR_I32:
+  case wasm::R_WASM_MEMORY_ADDR_REL_SLEB:
+  case wasm::R_WASM_MEMORY_ADDR_SLEB: {
+    // Provisional value is address of the global
+    const MCSymbolWasm *Sym = resolveSymbol(*RelEntry.Symbol);
+    // For undefined symbols, use zero
+    if (!Sym->isDefined())
+      return 0;
+    const wasm::WasmDataReference &Ref = DataLocations[Sym];
+    const WasmDataSegment &Segment = DataSegments[Ref.Segment];
+    // Ignore overflow. LLVM allows address arithmetic to silently wrap.
+    return Segment.Offset + Ref.Offset + RelEntry.Addend;
+  }
+  default:
+    llvm_unreachable("invalid relocation type");
+  }
+}
+
+static void addData(SmallVectorImpl<char> &DataBytes,
+                    MCSectionWasm &DataSection) {
+  LLVM_DEBUG(errs() << "addData: " << DataSection.getSectionName() << "\n");
+
+  DataBytes.resize(alignTo(DataBytes.size(), DataSection.getAlignment()));
+
+  for (const MCFragment &Frag : DataSection) {
+    if (Frag.hasInstructions())
+      report_fatal_error("only data supported in data sections");
+
+    if (auto *Align = dyn_cast<MCAlignFragment>(&Frag)) {
+      if (Align->getValueSize() != 1)
+        report_fatal_error("only byte values supported for alignment");
+      // If nops are requested, use zeros, as this is the data section.
+      uint8_t Value = Align->hasEmitNops() ? 0 : Align->getValue();
+      uint64_t Size =
+          std::min<uint64_t>(alignTo(DataBytes.size(), Align->getAlignment()),
+                             DataBytes.size() + Align->getMaxBytesToEmit());
+      DataBytes.resize(Size, Value);
+    } else if (auto *Fill = dyn_cast<MCFillFragment>(&Frag)) {
+      int64_t NumValues;
+      if (!Fill->getNumValues().evaluateAsAbsolute(NumValues))
+        llvm_unreachable("The fill should be an assembler constant");
+      DataBytes.insert(DataBytes.end(), Fill->getValueSize() * NumValues,
+                       Fill->getValue());
+    } else if (auto *LEB = dyn_cast<MCLEBFragment>(&Frag)) {
+      const SmallVectorImpl<char> &Contents = LEB->getContents();
+      DataBytes.insert(DataBytes.end(), Contents.begin(), Contents.end());
+    } else {
+      const auto &DataFrag = cast<MCDataFragment>(Frag);
+      const SmallVectorImpl<char> &Contents = DataFrag.getContents();
+      DataBytes.insert(DataBytes.end(), Contents.begin(), Contents.end());
+    }
+  }
+
+  LLVM_DEBUG(dbgs() << "addData -> " << DataBytes.size() << "\n");
+}
+
+uint32_t
+WasmObjectWriter::getRelocationIndexValue(const WasmRelocationEntry &RelEntry) {
+  if (RelEntry.Type == wasm::R_WASM_TYPE_INDEX_LEB) {
+    if (!TypeIndices.count(RelEntry.Symbol))
+      report_fatal_error("symbol not found in type index space: " +
+                         RelEntry.Symbol->getName());
+    return TypeIndices[RelEntry.Symbol];
+  }
+
+  return RelEntry.Symbol->getIndex();
+}
+
+// Apply the portions of the relocation records that we can handle ourselves
+// directly.
+void WasmObjectWriter::applyRelocations(
+    ArrayRef<WasmRelocationEntry> Relocations, uint64_t ContentsOffset) {
+  auto &Stream = static_cast<raw_pwrite_stream &>(W.OS);
+  for (const WasmRelocationEntry &RelEntry : Relocations) {
+    uint64_t Offset = ContentsOffset +
+                      RelEntry.FixupSection->getSectionOffset() +
+                      RelEntry.Offset;
+
+    LLVM_DEBUG(dbgs() << "applyRelocation: " << RelEntry << "\n");
+    uint32_t Value = getProvisionalValue(RelEntry);
+
+    switch (RelEntry.Type) {
+    case wasm::R_WASM_FUNCTION_INDEX_LEB:
+    case wasm::R_WASM_TYPE_INDEX_LEB:
+    case wasm::R_WASM_GLOBAL_INDEX_LEB:
+    case wasm::R_WASM_MEMORY_ADDR_LEB:
+    case wasm::R_WASM_EVENT_INDEX_LEB:
+      writePatchableLEB(Stream, Value, Offset);
+      break;
+    case wasm::R_WASM_TABLE_INDEX_I32:
+    case wasm::R_WASM_MEMORY_ADDR_I32:
+    case wasm::R_WASM_FUNCTION_OFFSET_I32:
+    case wasm::R_WASM_SECTION_OFFSET_I32:
+      writeI32(Stream, Value, Offset);
+      break;
+    case wasm::R_WASM_TABLE_INDEX_SLEB:
+    case wasm::R_WASM_TABLE_INDEX_REL_SLEB:
+    case wasm::R_WASM_MEMORY_ADDR_SLEB:
+    case wasm::R_WASM_MEMORY_ADDR_REL_SLEB:
+      writePatchableSLEB(Stream, Value, Offset);
+      break;
+    default:
+      llvm_unreachable("invalid relocation type");
+    }
+  }
+}
+
+void WasmObjectWriter::writeTypeSection(ArrayRef<WasmSignature> Signatures) {
+  if (Signatures.empty())
+    return;
+
+  SectionBookkeeping Section;
+  startSection(Section, wasm::WASM_SEC_TYPE);
+
+  encodeULEB128(Signatures.size(), W.OS);
+
+  for (const WasmSignature &Sig : Signatures) {
+    W.OS << char(wasm::WASM_TYPE_FUNC);
+    encodeULEB128(Sig.Params.size(), W.OS);
+    for (wasm::ValType Ty : Sig.Params)
+      writeValueType(Ty);
+    encodeULEB128(Sig.Returns.size(), W.OS);
+    for (wasm::ValType Ty : Sig.Returns)
+      writeValueType(Ty);
+  }
+
+  endSection(Section);
+}
+
+void WasmObjectWriter::writeImportSection(ArrayRef<wasm::WasmImport> Imports,
+                                          uint32_t DataSize,
+                                          uint32_t NumElements) {
+  if (Imports.empty())
+    return;
+
+  uint32_t NumPages = (DataSize + wasm::WasmPageSize - 1) / wasm::WasmPageSize;
+
+  SectionBookkeeping Section;
+  startSection(Section, wasm::WASM_SEC_IMPORT);
+
+  encodeULEB128(Imports.size(), W.OS);
+  for (const wasm::WasmImport &Import : Imports) {
+    writeString(Import.Module);
+    writeString(Import.Field);
+    W.OS << char(Import.Kind);
+
+    switch (Import.Kind) {
+    case wasm::WASM_EXTERNAL_FUNCTION:
+      encodeULEB128(Import.SigIndex, W.OS);
+      break;
+    case wasm::WASM_EXTERNAL_GLOBAL:
+      W.OS << char(Import.Global.Type);
+      W.OS << char(Import.Global.Mutable ? 1 : 0);
+      break;
+    case wasm::WASM_EXTERNAL_MEMORY:
+      encodeULEB128(0, W.OS);        // flags
+      encodeULEB128(NumPages, W.OS); // initial
+      break;
+    case wasm::WASM_EXTERNAL_TABLE:
+      W.OS << char(Import.Table.ElemType);
+      encodeULEB128(0, W.OS);           // flags
+      encodeULEB128(NumElements, W.OS); // initial
+      break;
+    case wasm::WASM_EXTERNAL_EVENT:
+      encodeULEB128(Import.Event.Attribute, W.OS);
+      encodeULEB128(Import.Event.SigIndex, W.OS);
+      break;
+    default:
+      llvm_unreachable("unsupported import kind");
+    }
+  }
+
+  endSection(Section);
+}
+
+void WasmObjectWriter::writeFunctionSection(ArrayRef<WasmFunction> Functions) {
+  if (Functions.empty())
+    return;
+
+  SectionBookkeeping Section;
+  startSection(Section, wasm::WASM_SEC_FUNCTION);
+
+  encodeULEB128(Functions.size(), W.OS);
+  for (const WasmFunction &Func : Functions)
+    encodeULEB128(Func.SigIndex, W.OS);
+
+  endSection(Section);
+}
+
+void WasmObjectWriter::writeEventSection(ArrayRef<wasm::WasmEventType> Events) {
+  if (Events.empty())
+    return;
+
+  SectionBookkeeping Section;
+  startSection(Section, wasm::WASM_SEC_EVENT);
+
+  encodeULEB128(Events.size(), W.OS);
+  for (const wasm::WasmEventType &Event : Events) {
+    encodeULEB128(Event.Attribute, W.OS);
+    encodeULEB128(Event.SigIndex, W.OS);
+  }
+
+  endSection(Section);
+}
+
+void WasmObjectWriter::writeExportSection(ArrayRef<wasm::WasmExport> Exports) {
+  if (Exports.empty())
+    return;
+
+  SectionBookkeeping Section;
+  startSection(Section, wasm::WASM_SEC_EXPORT);
+
+  encodeULEB128(Exports.size(), W.OS);
+  for (const wasm::WasmExport &Export : Exports) {
+    writeString(Export.Name);
+    W.OS << char(Export.Kind);
+    encodeULEB128(Export.Index, W.OS);
+  }
+
+  endSection(Section);
+}
+
+void WasmObjectWriter::writeElemSection(ArrayRef<uint32_t> TableElems) {
+  if (TableElems.empty())
+    return;
+
+  SectionBookkeeping Section;
+  startSection(Section, wasm::WASM_SEC_ELEM);
+
+  encodeULEB128(1, W.OS); // number of "segments"
+  encodeULEB128(0, W.OS); // the table index
+
+  // init expr for starting offset
+  W.OS << char(wasm::WASM_OPCODE_I32_CONST);
+  encodeSLEB128(InitialTableOffset, W.OS);
+  W.OS << char(wasm::WASM_OPCODE_END);
+
+  encodeULEB128(TableElems.size(), W.OS);
+  for (uint32_t Elem : TableElems)
+    encodeULEB128(Elem, W.OS);
+
+  endSection(Section);
+}
+
+void WasmObjectWriter::writeDataCountSection() {
+  if (DataSegments.empty())
+    return;
+
+  SectionBookkeeping Section;
+  startSection(Section, wasm::WASM_SEC_DATACOUNT);
+  encodeULEB128(DataSegments.size(), W.OS);
+  endSection(Section);
+}
+
+void WasmObjectWriter::writeCodeSection(const MCAssembler &Asm,
+                                        const MCAsmLayout &Layout,
+                                        ArrayRef<WasmFunction> Functions) {
+  if (Functions.empty())
+    return;
+
+  SectionBookkeeping Section;
+  startSection(Section, wasm::WASM_SEC_CODE);
+  CodeSectionIndex = Section.Index;
+
+  encodeULEB128(Functions.size(), W.OS);
+
+  for (const WasmFunction &Func : Functions) {
+    auto &FuncSection = static_cast<MCSectionWasm &>(Func.Sym->getSection());
+
+    int64_t Size = 0;
+    if (!Func.Sym->getSize()->evaluateAsAbsolute(Size, Layout))
+      report_fatal_error(".size expression must be evaluatable");
+
+    encodeULEB128(Size, W.OS);
+    FuncSection.setSectionOffset(W.OS.tell() - Section.ContentsOffset);
+    Asm.writeSectionData(W.OS, &FuncSection, Layout);
+  }
+
+  // Apply fixups.
+  applyRelocations(CodeRelocations, Section.ContentsOffset);
+
+  endSection(Section);
+}
+
+void WasmObjectWriter::writeDataSection() {
+  if (DataSegments.empty())
+    return;
+
+  SectionBookkeeping Section;
+  startSection(Section, wasm::WASM_SEC_DATA);
+  DataSectionIndex = Section.Index;
+
+  encodeULEB128(DataSegments.size(), W.OS); // count
+
+  for (const WasmDataSegment &Segment : DataSegments) {
+    encodeULEB128(Segment.InitFlags, W.OS); // flags
+    if (Segment.InitFlags & wasm::WASM_SEGMENT_HAS_MEMINDEX)
+      encodeULEB128(0, W.OS); // memory index
+    if ((Segment.InitFlags & wasm::WASM_SEGMENT_IS_PASSIVE) == 0) {
+      W.OS << char(wasm::WASM_OPCODE_I32_CONST);
+      encodeSLEB128(Segment.Offset, W.OS); // offset
+      W.OS << char(wasm::WASM_OPCODE_END);
+    }
+    encodeULEB128(Segment.Data.size(), W.OS); // size
+    Segment.Section->setSectionOffset(W.OS.tell() - Section.ContentsOffset);
+    W.OS << Segment.Data; // data
+  }
+
+  // Apply fixups.
+  applyRelocations(DataRelocations, Section.ContentsOffset);
+
+  endSection(Section);
+}
+
+void WasmObjectWriter::writeRelocSection(
+    uint32_t SectionIndex, StringRef Name,
+    std::vector<WasmRelocationEntry> &Relocs) {
+  // See: https://github.com/WebAssembly/tool-conventions/blob/master/Linking.md
+  // for descriptions of the reloc sections.
+
+  if (Relocs.empty())
+    return;
+
+  // First, ensure the relocations are sorted in offset order.  In general they
+  // should already be sorted since `recordRelocation` is called in offset
+  // order, but for the code section we combine many MC sections into single
+  // wasm section, and this order is determined by the order of Asm.Symbols()
+  // not the sections order.
+  llvm::stable_sort(
+      Relocs, [](const WasmRelocationEntry &A, const WasmRelocationEntry &B) {
+        return (A.Offset + A.FixupSection->getSectionOffset()) <
+               (B.Offset + B.FixupSection->getSectionOffset());
+      });
+
+  SectionBookkeeping Section;
+  startCustomSection(Section, std::string("reloc.") + Name.str());
+
+  encodeULEB128(SectionIndex, W.OS);
+  encodeULEB128(Relocs.size(), W.OS);
+  for (const WasmRelocationEntry &RelEntry : Relocs) {
+    uint64_t Offset =
+        RelEntry.Offset + RelEntry.FixupSection->getSectionOffset();
+    uint32_t Index = getRelocationIndexValue(RelEntry);
+
+    W.OS << char(RelEntry.Type);
+    encodeULEB128(Offset, W.OS);
+    encodeULEB128(Index, W.OS);
+    if (RelEntry.hasAddend())
+      encodeSLEB128(RelEntry.Addend, W.OS);
+  }
+
+  endSection(Section);
+}
+
+void WasmObjectWriter::writeCustomRelocSections() {
+  for (const auto &Sec : CustomSections) {
+    auto &Relocations = CustomSectionsRelocations[Sec.Section];
+    writeRelocSection(Sec.OutputIndex, Sec.Name, Relocations);
+  }
+}
+
+void WasmObjectWriter::writeLinkingMetaDataSection(
+    ArrayRef<wasm::WasmSymbolInfo> SymbolInfos,
+    ArrayRef<std::pair<uint16_t, uint32_t>> InitFuncs,
+    const std::map<StringRef, std::vector<WasmComdatEntry>> &Comdats) {
+  SectionBookkeeping Section;
+  startCustomSection(Section, "linking");
+  encodeULEB128(wasm::WasmMetadataVersion, W.OS);
+
+  SectionBookkeeping SubSection;
+  if (SymbolInfos.size() != 0) {
+    startSection(SubSection, wasm::WASM_SYMBOL_TABLE);
+    encodeULEB128(SymbolInfos.size(), W.OS);
+    for (const wasm::WasmSymbolInfo &Sym : SymbolInfos) {
+      encodeULEB128(Sym.Kind, W.OS);
+      encodeULEB128(Sym.Flags, W.OS);
+      switch (Sym.Kind) {
+      case wasm::WASM_SYMBOL_TYPE_FUNCTION:
+      case wasm::WASM_SYMBOL_TYPE_GLOBAL:
+      case wasm::WASM_SYMBOL_TYPE_EVENT:
+        encodeULEB128(Sym.ElementIndex, W.OS);
+        if ((Sym.Flags & wasm::WASM_SYMBOL_UNDEFINED) == 0 ||
+            (Sym.Flags & wasm::WASM_SYMBOL_EXPLICIT_NAME) != 0)
+          writeString(Sym.Name);
+        break;
+      case wasm::WASM_SYMBOL_TYPE_DATA:
+        writeString(Sym.Name);
+        if ((Sym.Flags & wasm::WASM_SYMBOL_UNDEFINED) == 0) {
+          encodeULEB128(Sym.DataRef.Segment, W.OS);
+          encodeULEB128(Sym.DataRef.Offset, W.OS);
+          encodeULEB128(Sym.DataRef.Size, W.OS);
+        }
+        break;
+      case wasm::WASM_SYMBOL_TYPE_SECTION: {
+        const uint32_t SectionIndex =
+            CustomSections[Sym.ElementIndex].OutputIndex;
+        encodeULEB128(SectionIndex, W.OS);
+        break;
+      }
+      default:
+        llvm_unreachable("unexpected kind");
+      }
+    }
+    endSection(SubSection);
+  }
+
+  if (DataSegments.size()) {
+    startSection(SubSection, wasm::WASM_SEGMENT_INFO);
+    encodeULEB128(DataSegments.size(), W.OS);
+    for (const WasmDataSegment &Segment : DataSegments) {
+      writeString(Segment.Name);
+      encodeULEB128(Segment.Alignment, W.OS);
+      encodeULEB128(Segment.LinkerFlags, W.OS);
+    }
+    endSection(SubSection);
+  }
+
+  if (!InitFuncs.empty()) {
+    startSection(SubSection, wasm::WASM_INIT_FUNCS);
+    encodeULEB128(InitFuncs.size(), W.OS);
+    for (auto &StartFunc : InitFuncs) {
+      encodeULEB128(StartFunc.first, W.OS);  // priority
+      encodeULEB128(StartFunc.second, W.OS); // function index
+    }
+    endSection(SubSection);
+  }
+
+  if (Comdats.size()) {
+    startSection(SubSection, wasm::WASM_COMDAT_INFO);
+    encodeULEB128(Comdats.size(), W.OS);
+    for (const auto &C : Comdats) {
+      writeString(C.first);
+      encodeULEB128(0, W.OS); // flags for future use
+      encodeULEB128(C.second.size(), W.OS);
+      for (const WasmComdatEntry &Entry : C.second) {
+        encodeULEB128(Entry.Kind, W.OS);
+        encodeULEB128(Entry.Index, W.OS);
+      }
+    }
+    endSection(SubSection);
+  }
+
+  endSection(Section);
+}
+
+void WasmObjectWriter::writeCustomSection(WasmCustomSection &CustomSection,
+                                          const MCAssembler &Asm,
+                                          const MCAsmLayout &Layout) {
+  SectionBookkeeping Section;
+  auto *Sec = CustomSection.Section;
+  startCustomSection(Section, CustomSection.Name);
+
+  Sec->setSectionOffset(W.OS.tell() - Section.ContentsOffset);
+  Asm.writeSectionData(W.OS, Sec, Layout);
+
+  CustomSection.OutputContentsOffset = Section.ContentsOffset;
+  CustomSection.OutputIndex = Section.Index;
+
+  endSection(Section);
+
+  // Apply fixups.
+  auto &Relocations = CustomSectionsRelocations[CustomSection.Section];
+  applyRelocations(Relocations, CustomSection.OutputContentsOffset);
+}
+
+uint32_t WasmObjectWriter::getFunctionType(const MCSymbolWasm &Symbol) {
+  assert(Symbol.isFunction());
+  assert(TypeIndices.count(&Symbol));
+  return TypeIndices[&Symbol];
+}
+
+uint32_t WasmObjectWriter::getEventType(const MCSymbolWasm &Symbol) {
+  assert(Symbol.isEvent());
+  assert(TypeIndices.count(&Symbol));
+  return TypeIndices[&Symbol];
+}
+
+void WasmObjectWriter::registerFunctionType(const MCSymbolWasm &Symbol) {
+  assert(Symbol.isFunction());
+
+  WasmSignature S;
+  const MCSymbolWasm *ResolvedSym = resolveSymbol(Symbol);
+  if (auto *Sig = ResolvedSym->getSignature()) {
+    S.Returns = Sig->Returns;
+    S.Params = Sig->Params;
+  }
+
+  auto Pair = SignatureIndices.insert(std::make_pair(S, Signatures.size()));
+  if (Pair.second)
+    Signatures.push_back(S);
+  TypeIndices[&Symbol] = Pair.first->second;
+
+  LLVM_DEBUG(dbgs() << "registerFunctionType: " << Symbol
+                    << " new:" << Pair.second << "\n");
+  LLVM_DEBUG(dbgs() << "  -> type index: " << Pair.first->second << "\n");
+}
+
+void WasmObjectWriter::registerEventType(const MCSymbolWasm &Symbol) {
+  assert(Symbol.isEvent());
+
+  // TODO Currently we don't generate imported exceptions, but if we do, we
+  // should have a way of infering types of imported exceptions.
+  WasmSignature S;
+  if (auto *Sig = Symbol.getSignature()) {
+    S.Returns = Sig->Returns;
+    S.Params = Sig->Params;
+  }
+
+  auto Pair = SignatureIndices.insert(std::make_pair(S, Signatures.size()));
+  if (Pair.second)
+    Signatures.push_back(S);
+  TypeIndices[&Symbol] = Pair.first->second;
+
+  LLVM_DEBUG(dbgs() << "registerEventType: " << Symbol << " new:" << Pair.second
+                    << "\n");
+  LLVM_DEBUG(dbgs() << "  -> type index: " << Pair.first->second << "\n");
+}
+
+static bool isInSymtab(const MCSymbolWasm &Sym) {
+  if (Sym.isUsedInReloc())
+    return true;
+
+  if (Sym.isComdat() && !Sym.isDefined())
+    return false;
+
+  if (Sym.isTemporary() && Sym.getName().empty())
+    return false;
+
+  if (Sym.isTemporary() && Sym.isData() && !Sym.getSize())
+    return false;
+
+  if (Sym.isSection())
+    return false;
+
+  return true;
+}
+
+uint64_t WasmObjectWriter::writeObject(MCAssembler &Asm,
+                                       const MCAsmLayout &Layout) {
+  uint64_t StartOffset = W.OS.tell();
+
+  LLVM_DEBUG(dbgs() << "WasmObjectWriter::writeObject\n");
+
+  // Collect information from the available symbols.
+  SmallVector<WasmFunction, 4> Functions;
+  SmallVector<uint32_t, 4> TableElems;
+  SmallVector<wasm::WasmImport, 4> Imports;
+  SmallVector<wasm::WasmExport, 4> Exports;
+  SmallVector<wasm::WasmEventType, 1> Events;
+  SmallVector<wasm::WasmSymbolInfo, 4> SymbolInfos;
+  SmallVector<std::pair<uint16_t, uint32_t>, 2> InitFuncs;
+  std::map<StringRef, std::vector<WasmComdatEntry>> Comdats;
+  uint32_t DataSize = 0;
+
+  // For now, always emit the memory import, since loads and stores are not
+  // valid without it. In the future, we could perhaps be more clever and omit
+  // it if there are no loads or stores.
+  wasm::WasmImport MemImport;
+  MemImport.Module = "env";
+  MemImport.Field = "__linear_memory";
+  MemImport.Kind = wasm::WASM_EXTERNAL_MEMORY;
+  Imports.push_back(MemImport);
+
+  // For now, always emit the table section, since indirect calls are not
+  // valid without it. In the future, we could perhaps be more clever and omit
+  // it if there are no indirect calls.
+  wasm::WasmImport TableImport;
+  TableImport.Module = "env";
+  TableImport.Field = "__indirect_function_table";
+  TableImport.Kind = wasm::WASM_EXTERNAL_TABLE;
+  TableImport.Table.ElemType = wasm::WASM_TYPE_FUNCREF;
+  Imports.push_back(TableImport);
+
+  // Populate SignatureIndices, and Imports and WasmIndices for undefined
+  // symbols.  This must be done before populating WasmIndices for defined
+  // symbols.
+  for (const MCSymbol &S : Asm.symbols()) {
+    const auto &WS = static_cast<const MCSymbolWasm &>(S);
+
+    // Register types for all functions, including those with private linkage
+    // (because wasm always needs a type signature).
+    if (WS.isFunction())
+      registerFunctionType(WS);
+
+    if (WS.isEvent())
+      registerEventType(WS);
+
+    if (WS.isTemporary())
+      continue;
+
+    // If the symbol is not defined in this translation unit, import it.
+    if (!WS.isDefined() && !WS.isComdat()) {
+      if (WS.isFunction()) {
+        wasm::WasmImport Import;
+        Import.Module = WS.getImportModule();
+        Import.Field = WS.getImportName();
+        Import.Kind = wasm::WASM_EXTERNAL_FUNCTION;
+        Import.SigIndex = getFunctionType(WS);
+        Imports.push_back(Import);
+        assert(WasmIndices.count(&WS) == 0);
+        WasmIndices[&WS] = NumFunctionImports++;
+      } else if (WS.isGlobal()) {
+        if (WS.isWeak())
+          report_fatal_error("undefined global symbol cannot be weak");
+
+        wasm::WasmImport Import;
+        Import.Field = WS.getImportName();
+        Import.Kind = wasm::WASM_EXTERNAL_GLOBAL;
+        Import.Module = WS.getImportModule();
+        Import.Global = WS.getGlobalType();
+        Imports.push_back(Import);
+        assert(WasmIndices.count(&WS) == 0);
+        WasmIndices[&WS] = NumGlobalImports++;
+      } else if (WS.isEvent()) {
+        if (WS.isWeak())
+          report_fatal_error("undefined event symbol cannot be weak");
+
+        wasm::WasmImport Import;
+        Import.Module = WS.getImportModule();
+        Import.Field = WS.getImportName();
+        Import.Kind = wasm::WASM_EXTERNAL_EVENT;
+        Import.Event.Attribute = wasm::WASM_EVENT_ATTRIBUTE_EXCEPTION;
+        Import.Event.SigIndex = getEventType(WS);
+        Imports.push_back(Import);
+        assert(WasmIndices.count(&WS) == 0);
+        WasmIndices[&WS] = NumEventImports++;
+      }
+    }
+  }
+
+  // Add imports for GOT globals
+  for (const MCSymbol &S : Asm.symbols()) {
+    const auto &WS = static_cast<const MCSymbolWasm &>(S);
+    if (WS.isUsedInGOT()) {
+      wasm::WasmImport Import;
+      if (WS.isFunction())
+        Import.Module = "GOT.func";
+      else
+        Import.Module = "GOT.mem";
+      Import.Field = WS.getName();
+      Import.Kind = wasm::WASM_EXTERNAL_GLOBAL;
+      Import.Global = {wasm::WASM_TYPE_I32, true};
+      Imports.push_back(Import);
+      assert(GOTIndices.count(&WS) == 0);
+      GOTIndices[&WS] = NumGlobalImports++;
+    }
+  }
+
+  // Populate DataSegments and CustomSections, which must be done before
+  // populating DataLocations.
+  for (MCSection &Sec : Asm) {
+    auto &Section = static_cast<MCSectionWasm &>(Sec);
+    StringRef SectionName = Section.getSectionName();
+
+    // .init_array sections are handled specially elsewhere.
+    if (SectionName.startswith(".init_array"))
+      continue;
+
+    // Code is handled separately
+    if (Section.getKind().isText())
+      continue;
+
+    if (Section.isWasmData()) {
+      uint32_t SegmentIndex = DataSegments.size();
+      DataSize = alignTo(DataSize, Section.getAlignment());
+      DataSegments.emplace_back();
+      WasmDataSegment &Segment = DataSegments.back();
+      Segment.Name = SectionName;
+      Segment.InitFlags =
+          Section.getPassive() ? (uint32_t)wasm::WASM_SEGMENT_IS_PASSIVE : 0;
+      Segment.Offset = DataSize;
+      Segment.Section = &Section;
+      addData(Segment.Data, Section);
+      Segment.Alignment = Log2_32(Section.getAlignment());
+      Segment.LinkerFlags = 0;
+      DataSize += Segment.Data.size();
+      Section.setSegmentIndex(SegmentIndex);
+
+      if (const MCSymbolWasm *C = Section.getGroup()) {
+        Comdats[C->getName()].emplace_back(
+            WasmComdatEntry{wasm::WASM_COMDAT_DATA, SegmentIndex});
+      }
+    } else {
+      // Create custom sections
+      assert(Sec.getKind().isMetadata());
+
+      StringRef Name = SectionName;
+
+      // For user-defined custom sections, strip the prefix
+      if (Name.startswith(".custom_section."))
+        Name = Name.substr(strlen(".custom_section."));
+
+      MCSymbol *Begin = Sec.getBeginSymbol();
+      if (Begin) {
+        WasmIndices[cast<MCSymbolWasm>(Begin)] = CustomSections.size();
+        if (SectionName != Begin->getName())
+          report_fatal_error("section name and begin symbol should match: " +
+                             Twine(SectionName));
+      }
+
+      // Separate out the producers and target features sections
+      if (Name == "producers") {
+        ProducersSection = llvm::make_unique<WasmCustomSection>(Name, &Section);
+        continue;
+      }
+      if (Name == "target_features") {
+        TargetFeaturesSection =
+            llvm::make_unique<WasmCustomSection>(Name, &Section);
+        continue;
+      }
+
+      CustomSections.emplace_back(Name, &Section);
+    }
+  }
+
+  // Populate WasmIndices and DataLocations for defined symbols.
+  for (const MCSymbol &S : Asm.symbols()) {
+    // Ignore unnamed temporary symbols, which aren't ever exported, imported,
+    // or used in relocations.
+    if (S.isTemporary() && S.getName().empty())
+      continue;
+
+    const auto &WS = static_cast<const MCSymbolWasm &>(S);
+    LLVM_DEBUG(
+        dbgs() << "MCSymbol: " << toString(WS.getType()) << " '" << S << "'"
+               << " isDefined=" << S.isDefined() << " isExternal="
+               << S.isExternal() << " isTemporary=" << S.isTemporary()
+               << " isWeak=" << WS.isWeak() << " isHidden=" << WS.isHidden()
+               << " isVariable=" << WS.isVariable() << "\n");
+
+    if (WS.isVariable())
+      continue;
+    if (WS.isComdat() && !WS.isDefined())
+      continue;
+
+    if (WS.isFunction()) {
+      unsigned Index;
+      if (WS.isDefined()) {
+        if (WS.getOffset() != 0)
+          report_fatal_error(
+              "function sections must contain one function each");
+
+        if (WS.getSize() == nullptr)
+          report_fatal_error(
+              "function symbols must have a size set with .size");
+
+        // A definition. Write out the function body.
+        Index = NumFunctionImports + Functions.size();
+        WasmFunction Func;
+        Func.SigIndex = getFunctionType(WS);
+        Func.Sym = &WS;
+        WasmIndices[&WS] = Index;
+        Functions.push_back(Func);
+
+        auto &Section = static_cast<MCSectionWasm &>(WS.getSection());
+        if (const MCSymbolWasm *C = Section.getGroup()) {
+          Comdats[C->getName()].emplace_back(
+              WasmComdatEntry{wasm::WASM_COMDAT_FUNCTION, Index});
+        }
+      } else {
+        // An import; the index was assigned above.
+        Index = WasmIndices.find(&WS)->second;
+      }
+
+      LLVM_DEBUG(dbgs() << "  -> function index: " << Index << "\n");
+
+    } else if (WS.isData()) {
+      if (!isInSymtab(WS))
+        continue;
+
+      if (!WS.isDefined()) {
+        LLVM_DEBUG(dbgs() << "  -> segment index: -1"
+                          << "\n");
+        continue;
+      }
+
+      if (!WS.getSize())
+        report_fatal_error("data symbols must have a size set with .size: " +
+                           WS.getName());
+
+      int64_t Size = 0;
+      if (!WS.getSize()->evaluateAsAbsolute(Size, Layout))
+        report_fatal_error(".size expression must be evaluatable");
+
+      auto &DataSection = static_cast<MCSectionWasm &>(WS.getSection());
+      assert(DataSection.isWasmData());
+
+      // For each data symbol, export it in the symtab as a reference to the
+      // corresponding Wasm data segment.
+      wasm::WasmDataReference Ref = wasm::WasmDataReference{
+          DataSection.getSegmentIndex(),
+          static_cast<uint32_t>(Layout.getSymbolOffset(WS)),
+          static_cast<uint32_t>(Size)};
+      DataLocations[&WS] = Ref;
+      LLVM_DEBUG(dbgs() << "  -> segment index: " << Ref.Segment << "\n");
+
+    } else if (WS.isGlobal()) {
+      // A "true" Wasm global (currently just __stack_pointer)
+      if (WS.isDefined())
+        report_fatal_error("don't yet support defined globals");
+
+      // An import; the index was assigned above
+      LLVM_DEBUG(dbgs() << "  -> global index: "
+                        << WasmIndices.find(&WS)->second << "\n");
+
+    } else if (WS.isEvent()) {
+      // C++ exception symbol (__cpp_exception)
+      unsigned Index;
+      if (WS.isDefined()) {
+        Index = NumEventImports + Events.size();
+        wasm::WasmEventType Event;
+        Event.SigIndex = getEventType(WS);
+        Event.Attribute = wasm::WASM_EVENT_ATTRIBUTE_EXCEPTION;
+        assert(WasmIndices.count(&WS) == 0);
+        WasmIndices[&WS] = Index;
+        Events.push_back(Event);
+      } else {
+        // An import; the index was assigned above.
+        assert(WasmIndices.count(&WS) > 0);
+      }
+      LLVM_DEBUG(dbgs() << "  -> event index: " << WasmIndices.find(&WS)->second
+                        << "\n");
+
+    } else {
+      assert(WS.isSection());
+    }
+  }
+
+  // Populate WasmIndices and DataLocations for aliased symbols.  We need to
+  // process these in a separate pass because we need to have processed the
+  // target of the alias before the alias itself and the symbols are not
+  // necessarily ordered in this way.
+  for (const MCSymbol &S : Asm.symbols()) {
+    if (!S.isVariable())
+      continue;
+
+    assert(S.isDefined());
+
+    // Find the target symbol of this weak alias and export that index
+    const auto &WS = static_cast<const MCSymbolWasm &>(S);
+    const MCSymbolWasm *ResolvedSym = resolveSymbol(WS);
+    LLVM_DEBUG(dbgs() << WS.getName() << ": weak alias of '" << *ResolvedSym
+                      << "'\n");
+
+    if (ResolvedSym->isFunction()) {
+      assert(WasmIndices.count(ResolvedSym) > 0);
+      uint32_t WasmIndex = WasmIndices.find(ResolvedSym)->second;
+      assert(WasmIndices.count(&WS) == 0);
+      WasmIndices[&WS] = WasmIndex;
+      LLVM_DEBUG(dbgs() << "  -> index:" << WasmIndex << "\n");
+    } else if (ResolvedSym->isData()) {
+      assert(DataLocations.count(ResolvedSym) > 0);
+      const wasm::WasmDataReference &Ref =
+          DataLocations.find(ResolvedSym)->second;
+      DataLocations[&WS] = Ref;
+      LLVM_DEBUG(dbgs() << "  -> index:" << Ref.Segment << "\n");
+    } else {
+      report_fatal_error("don't yet support global/event aliases");
+    }
+  }
+
+  // Finally, populate the symbol table itself, in its "natural" order.
+  for (const MCSymbol &S : Asm.symbols()) {
+    const auto &WS = static_cast<const MCSymbolWasm &>(S);
+    if (!isInSymtab(WS)) {
+      WS.setIndex(InvalidIndex);
+      continue;
+    }
+    LLVM_DEBUG(dbgs() << "adding to symtab: " << WS << "\n");
+
+    uint32_t Flags = 0;
+    if (WS.isWeak())
+      Flags |= wasm::WASM_SYMBOL_BINDING_WEAK;
+    if (WS.isHidden())
+      Flags |= wasm::WASM_SYMBOL_VISIBILITY_HIDDEN;
+    if (!WS.isExternal() && WS.isDefined())
+      Flags |= wasm::WASM_SYMBOL_BINDING_LOCAL;
+    if (WS.isUndefined())
+      Flags |= wasm::WASM_SYMBOL_UNDEFINED;
+    if (WS.isExported())
+      Flags |= wasm::WASM_SYMBOL_EXPORTED;
+    if (WS.getName() != WS.getImportName())
+      Flags |= wasm::WASM_SYMBOL_EXPLICIT_NAME;
+
+    wasm::WasmSymbolInfo Info;
+    Info.Name = WS.getName();
+    Info.Kind = WS.getType();
+    Info.Flags = Flags;
+    if (!WS.isData()) {
+      assert(WasmIndices.count(&WS) > 0);
+      Info.ElementIndex = WasmIndices.find(&WS)->second;
+    } else if (WS.isDefined()) {
+      assert(DataLocations.count(&WS) > 0);
+      Info.DataRef = DataLocations.find(&WS)->second;
+    }
+    WS.setIndex(SymbolInfos.size());
+    SymbolInfos.emplace_back(Info);
+  }
+
+  {
+    auto HandleReloc = [&](const WasmRelocationEntry &Rel) {
+      // Functions referenced by a relocation need to put in the table.  This is
+      // purely to make the object file's provisional values readable, and is
+      // ignored by the linker, which re-calculates the relocations itself.
+      if (Rel.Type != wasm::R_WASM_TABLE_INDEX_I32 &&
+          Rel.Type != wasm::R_WASM_TABLE_INDEX_SLEB)
+        return;
+      assert(Rel.Symbol->isFunction());
+      const MCSymbolWasm &WS = *resolveSymbol(*Rel.Symbol);
+      uint32_t FunctionIndex = WasmIndices.find(&WS)->second;
+      uint32_t TableIndex = TableElems.size() + InitialTableOffset;
+      if (TableIndices.try_emplace(&WS, TableIndex).second) {
+        LLVM_DEBUG(dbgs() << "  -> adding " << WS.getName()
+                          << " to table: " << TableIndex << "\n");
+        TableElems.push_back(FunctionIndex);
+        registerFunctionType(WS);
+      }
+    };
+
+    for (const WasmRelocationEntry &RelEntry : CodeRelocations)
+      HandleReloc(RelEntry);
+    for (const WasmRelocationEntry &RelEntry : DataRelocations)
+      HandleReloc(RelEntry);
+  }
+
+  // Translate .init_array section contents into start functions.
+  for (const MCSection &S : Asm) {
+    const auto &WS = static_cast<const MCSectionWasm &>(S);
+    if (WS.getSectionName().startswith(".fini_array"))
+      report_fatal_error(".fini_array sections are unsupported");
+    if (!WS.getSectionName().startswith(".init_array"))
+      continue;
+    if (WS.getFragmentList().empty())
+      continue;
+
+    // init_array is expected to contain a single non-empty data fragment
+    if (WS.getFragmentList().size() != 3)
+      report_fatal_error("only one .init_array section fragment supported");
+
+    auto IT = WS.begin();
+    const MCFragment &EmptyFrag = *IT;
+    if (EmptyFrag.getKind() != MCFragment::FT_Data)
+      report_fatal_error(".init_array section should be aligned");
+
+    IT = std::next(IT);
+    const MCFragment &AlignFrag = *IT;
+    if (AlignFrag.getKind() != MCFragment::FT_Align)
+      report_fatal_error(".init_array section should be aligned");
+    if (cast<MCAlignFragment>(AlignFrag).getAlignment() != (is64Bit() ? 8 : 4))
+      report_fatal_error(".init_array section should be aligned for pointers");
+
+    const MCFragment &Frag = *std::next(IT);
+    if (Frag.hasInstructions() || Frag.getKind() != MCFragment::FT_Data)
+      report_fatal_error("only data supported in .init_array section");
+
+    uint16_t Priority = UINT16_MAX;
+    unsigned PrefixLength = strlen(".init_array");
+    if (WS.getSectionName().size() > PrefixLength) {
+      if (WS.getSectionName()[PrefixLength] != '.')
+        report_fatal_error(
+            ".init_array section priority should start with '.'");
+      if (WS.getSectionName()
+              .substr(PrefixLength + 1)
+              .getAsInteger(10, Priority))
+        report_fatal_error("invalid .init_array section priority");
+    }
+    const auto &DataFrag = cast<MCDataFragment>(Frag);
+    const SmallVectorImpl<char> &Contents = DataFrag.getContents();
+    for (const uint8_t *
+             P = (const uint8_t *)Contents.data(),
+            *End = (const uint8_t *)Contents.data() + Contents.size();
+         P != End; ++P) {
+      if (*P != 0)
+        report_fatal_error("non-symbolic data in .init_array section");
+    }
+    for (const MCFixup &Fixup : DataFrag.getFixups()) {
+      assert(Fixup.getKind() ==
+             MCFixup::getKindForSize(is64Bit() ? 8 : 4, false));
+      const MCExpr *Expr = Fixup.getValue();
+      auto *SymRef = dyn_cast<MCSymbolRefExpr>(Expr);
+      if (!SymRef)
+        report_fatal_error("fixups in .init_array should be symbol references");
+      const auto &TargetSym = cast<const MCSymbolWasm>(SymRef->getSymbol());
+      if (TargetSym.getIndex() == InvalidIndex)
+        report_fatal_error("symbols in .init_array should exist in symbtab");
+      if (!TargetSym.isFunction())
+        report_fatal_error("symbols in .init_array should be for functions");
+      InitFuncs.push_back(
+          std::make_pair(Priority, TargetSym.getIndex()));
+    }
+  }
+
+  // Write out the Wasm header.
+  writeHeader(Asm);
+
+  writeTypeSection(Signatures);
+  writeImportSection(Imports, DataSize, TableElems.size());
+  writeFunctionSection(Functions);
+  // Skip the "table" section; we import the table instead.
+  // Skip the "memory" section; we import the memory instead.
+  writeEventSection(Events);
+  writeExportSection(Exports);
+  writeElemSection(TableElems);
+  writeDataCountSection();
+  writeCodeSection(Asm, Layout, Functions);
+  writeDataSection();
+  for (auto &CustomSection : CustomSections)
+    writeCustomSection(CustomSection, Asm, Layout);
+  writeLinkingMetaDataSection(SymbolInfos, InitFuncs, Comdats);
+  writeRelocSection(CodeSectionIndex, "CODE", CodeRelocations);
+  writeRelocSection(DataSectionIndex, "DATA", DataRelocations);
+  writeCustomRelocSections();
+  if (ProducersSection)
+    writeCustomSection(*ProducersSection, Asm, Layout);
+  if (TargetFeaturesSection)
+    writeCustomSection(*TargetFeaturesSection, Asm, Layout);
+
+  // TODO: Translate the .comment section to the output.
+  return W.OS.tell() - StartOffset;
+}
+
+std::unique_ptr<MCObjectWriter>
+llvm::createWasmObjectWriter(std::unique_ptr<MCWasmObjectTargetWriter> MOTW,
+                             raw_pwrite_stream &OS) {
+  return llvm::make_unique<WasmObjectWriter>(std::move(MOTW), OS);
+}