lib/MC/WasmObjectWriter.cpp

   1 //===- lib/MC/WasmObjectWriter.cpp - Wasm File Writer ---------------------===//
   2 //
   3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
   4 // See https://llvm.org/LICENSE.txt for license information.
   5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
   6 //
   7 //===----------------------------------------------------------------------===//
   8 //
   9 // This file implements Wasm object file writer information.
  10 //
  11 //===----------------------------------------------------------------------===//
  12
  13 #include "llvm/ADT/STLExtras.h"
  14 #include "llvm/ADT/SmallPtrSet.h"
  15 #include "llvm/BinaryFormat/Wasm.h"
  16 #include "llvm/Config/llvm-config.h"
  17 #include "llvm/MC/MCAsmBackend.h"
  18 #include "llvm/MC/MCAsmLayout.h"
  19 #include "llvm/MC/MCAssembler.h"
  20 #include "llvm/MC/MCContext.h"
  21 #include "llvm/MC/MCExpr.h"
  22 #include "llvm/MC/MCFixupKindInfo.h"
  23 #include "llvm/MC/MCObjectWriter.h"
  24 #include "llvm/MC/MCSectionWasm.h"
  25 #include "llvm/MC/MCSymbolWasm.h"
  26 #include "llvm/MC/MCValue.h"
  27 #include "llvm/MC/MCWasmObjectWriter.h"
  28 #include "llvm/Support/Casting.h"
  29 #include "llvm/Support/Debug.h"
  30 #include "llvm/Support/ErrorHandling.h"
  31 #include "llvm/Support/LEB128.h"
  32 #include "llvm/Support/StringSaver.h"
  33 #include <vector>
  34
  35 using namespace llvm;
  36
  37 #define DEBUG_TYPE "mc"
  38
  39 namespace {
  40
  41 // Went we ceate the indirect function table we start at 1, so that there is
  42 // and emtpy slot at 0 and therefore calling a null function pointer will trap.
  43 static const uint32_t InitialTableOffset = 1;
  44
  45 // For patching purposes, we need to remember where each section starts, both
  46 // for patching up the section size field, and for patching up references to
  47 // locations within the section.
  48 struct SectionBookkeeping {
  49   // Where the size of the section is written.
  50   uint64_t SizeOffset;
  51   // Where the section header ends (without custom section name).
  52   uint64_t PayloadOffset;
  53   // Where the contents of the section starts.
  54   uint64_t ContentsOffset;
  55   uint32_t Index;
  56 };
  57
  58 // The signature of a wasm function or event, in a struct capable of being used
  59 // as a DenseMap key.
  60 // TODO: Consider using wasm::WasmSignature directly instead.
  61 struct WasmSignature {
  62   // Support empty and tombstone instances, needed by DenseMap.
  63   enum { Plain, Empty, Tombstone } State = Plain;
  64
  65   // The return types of the function.
  66   SmallVector<wasm::ValType, 1> Returns;
  67
  68   // The parameter types of the function.
  69   SmallVector<wasm::ValType, 4> Params;
  70
  71   bool operator==(const WasmSignature &Other) const {
  72     return State == Other.State && Returns == Other.Returns &&
  73            Params == Other.Params;
  74   }
  75 };
  76
  77 // Traits for using WasmSignature in a DenseMap.
  78 struct WasmSignatureDenseMapInfo {
  79   static WasmSignature getEmptyKey() {
  80     WasmSignature Sig;
  81     Sig.State = WasmSignature::Empty;
  82     return Sig;
  83   }
  84   static WasmSignature getTombstoneKey() {
  85     WasmSignature Sig;
  86     Sig.State = WasmSignature::Tombstone;
  87     return Sig;
  88   }
  89   static unsigned getHashValue(const WasmSignature &Sig) {
  90     uintptr_t Value = Sig.State;
  91     for (wasm::ValType Ret : Sig.Returns)
  92       Value += DenseMapInfo<uint32_t>::getHashValue(uint32_t(Ret));
  93     for (wasm::ValType Param : Sig.Params)
  94       Value += DenseMapInfo<uint32_t>::getHashValue(uint32_t(Param));
  95     return Value;
  96   }
  97   static bool isEqual(const WasmSignature &LHS, const WasmSignature &RHS) {
  98     return LHS == RHS;
  99   }
 100 };
 101
 102 // A wasm data segment.  A wasm binary contains only a single data section
 103 // but that can contain many segments, each with their own virtual location
 104 // in memory.  Each MCSection data created by llvm is modeled as its own
 105 // wasm data segment.
 106 struct WasmDataSegment {
 107   MCSectionWasm *Section;
 108   StringRef Name;
 109   uint32_t InitFlags;
 110   uint32_t Offset;
 111   uint32_t Alignment;
 112   uint32_t LinkerFlags;
 113   SmallVector<char, 4> Data;
 114 };
 115
 116 // A wasm function to be written into the function section.
 117 struct WasmFunction {
 118   uint32_t SigIndex;
 119   const MCSymbolWasm *Sym;
 120 };
 121
 122 // A wasm global to be written into the global section.
 123 struct WasmGlobal {
 124   wasm::WasmGlobalType Type;
 125   uint64_t InitialValue;
 126 };
 127
 128 // Information about a single item which is part of a COMDAT.  For each data
 129 // segment or function which is in the COMDAT, there is a corresponding
 130 // WasmComdatEntry.
 131 struct WasmComdatEntry {
 132   unsigned Kind;
 133   uint32_t Index;
 134 };
 135
 136 // Information about a single relocation.
 137 struct WasmRelocationEntry {
 138   uint64_t Offset;                   // Where is the relocation.
 139   const MCSymbolWasm *Symbol;        // The symbol to relocate with.
 140   int64_t Addend;                    // A value to add to the symbol.
 141   unsigned Type;                     // The type of the relocation.
 142   const MCSectionWasm *FixupSection; // The section the relocation is targeting.
 143
 144   WasmRelocationEntry(uint64_t Offset, const MCSymbolWasm *Symbol,
 145                       int64_t Addend, unsigned Type,
 146                       const MCSectionWasm *FixupSection)
 147       : Offset(Offset), Symbol(Symbol), Addend(Addend), Type(Type),
 148         FixupSection(FixupSection) {}
 149
 150   bool hasAddend() const { return wasm::relocTypeHasAddend(Type); }
 151
 152   void print(raw_ostream &Out) const {
 153     Out << wasm::relocTypetoString(Type) << " Off=" << Offset
 154         << ", Sym=" << *Symbol << ", Addend=" << Addend
 155         << ", FixupSection=" << FixupSection->getSectionName();
 156   }
 157
 158 #if !defined(NDEBUG) || defined(LLVM_ENABLE_DUMP)
 159   LLVM_DUMP_METHOD void dump() const { print(dbgs()); }
 160 #endif
 161 };
 162
 163 static const uint32_t InvalidIndex = -1;
 164
 165 struct WasmCustomSection {
 166
 167   StringRef Name;
 168   MCSectionWasm *Section;
 169
 170   uint32_t OutputContentsOffset;
 171   uint32_t OutputIndex;
 172
 173   WasmCustomSection(StringRef Name, MCSectionWasm *Section)
 174       : Name(Name), Section(Section), OutputContentsOffset(0),
 175         OutputIndex(InvalidIndex) {}
 176 };
 177
 178 #if !defined(NDEBUG)
 179 raw_ostream &operator<<(raw_ostream &OS, const WasmRelocationEntry &Rel) {
 180   Rel.print(OS);
 181   return OS;
 182 }
 183 #endif
 184
 185 // Write X as an (unsigned) LEB value at offset Offset in Stream, padded
 186 // to allow patching.
 187 static void writePatchableLEB(raw_pwrite_stream &Stream, uint32_t X,
 188                               uint64_t Offset) {
 189   uint8_t Buffer[5];
 190   unsigned SizeLen = encodeULEB128(X, Buffer, 5);
 191   assert(SizeLen == 5);
 192   Stream.pwrite((char *)Buffer, SizeLen, Offset);
 193 }
 194
 195 // Write X as an signed LEB value at offset Offset in Stream, padded
 196 // to allow patching.
 197 static void writePatchableSLEB(raw_pwrite_stream &Stream, int32_t X,
 198                                uint64_t Offset) {
 199   uint8_t Buffer[5];
 200   unsigned SizeLen = encodeSLEB128(X, Buffer, 5);
 201   assert(SizeLen == 5);
 202   Stream.pwrite((char *)Buffer, SizeLen, Offset);
 203 }
 204
 205 // Write X as a plain integer value at offset Offset in Stream.
 206 static void writeI32(raw_pwrite_stream &Stream, uint32_t X, uint64_t Offset) {
 207   uint8_t Buffer[4];
 208   support::endian::write32le(Buffer, X);
 209   Stream.pwrite((char *)Buffer, sizeof(Buffer), Offset);
 210 }
 211
 212 class WasmObjectWriter : public MCObjectWriter {
 213   support::endian::Writer W;
 214
 215   /// The target specific Wasm writer instance.
 216   std::unique_ptr<MCWasmObjectTargetWriter> TargetObjectWriter;
 217
 218   // Relocations for fixing up references in the code section.
 219   std::vector<WasmRelocationEntry> CodeRelocations;
 220   uint32_t CodeSectionIndex;
 221
 222   // Relocations for fixing up references in the data section.
 223   std::vector<WasmRelocationEntry> DataRelocations;
 224   uint32_t DataSectionIndex;
 225
 226   // Index values to use for fixing up call_indirect type indices.
 227   // Maps function symbols to the index of the type of the function
 228   DenseMap<const MCSymbolWasm *, uint32_t> TypeIndices;
 229   // Maps function symbols to the table element index space. Used
 230   // for TABLE_INDEX relocation types (i.e. address taken functions).
 231   DenseMap<const MCSymbolWasm *, uint32_t> TableIndices;
 232   // Maps function/global symbols to the function/global/event/section index
 233   // space.
 234   DenseMap<const MCSymbolWasm *, uint32_t> WasmIndices;
 235   DenseMap<const MCSymbolWasm *, uint32_t> GOTIndices;
 236   // Maps data symbols to the Wasm segment and offset/size with the segment.
 237   DenseMap<const MCSymbolWasm *, wasm::WasmDataReference> DataLocations;
 238
 239   // Stores output data (index, relocations, content offset) for custom
 240   // section.
 241   std::vector<WasmCustomSection> CustomSections;
 242   std::unique_ptr<WasmCustomSection> ProducersSection;
 243   std::unique_ptr<WasmCustomSection> TargetFeaturesSection;
 244   // Relocations for fixing up references in the custom sections.
 245   DenseMap<const MCSectionWasm *, std::vector<WasmRelocationEntry>>
 246       CustomSectionsRelocations;
 247
 248   // Map from section to defining function symbol.
 249   DenseMap<const MCSection *, const MCSymbol *> SectionFunctions;
 250
 251   DenseMap<WasmSignature, uint32_t, WasmSignatureDenseMapInfo> SignatureIndices;
 252   SmallVector<WasmSignature, 4> Signatures;
 253   SmallVector<WasmDataSegment, 4> DataSegments;
 254   unsigned NumFunctionImports = 0;
 255   unsigned NumGlobalImports = 0;
 256   unsigned NumEventImports = 0;
 257   uint32_t SectionCount = 0;
 258
 259   // TargetObjectWriter wrappers.
 260   bool is64Bit() const { return TargetObjectWriter->is64Bit(); }
 261   bool isEmscripten() const { return TargetObjectWriter->isEmscripten(); }
 262
 263   void startSection(SectionBookkeeping &Section, unsigned SectionId);
 264   void startCustomSection(SectionBookkeeping &Section, StringRef Name);
 265   void endSection(SectionBookkeeping &Section);
 266
 267 public:
 268   WasmObjectWriter(std::unique_ptr<MCWasmObjectTargetWriter> MOTW,
 269                    raw_pwrite_stream &OS)
 270       : W(OS, support::little), TargetObjectWriter(std::move(MOTW)) {}
 271
 272 private:
 273   void reset() override {
 274     CodeRelocations.clear();
 275     DataRelocations.clear();
 276     TypeIndices.clear();
 277     WasmIndices.clear();
 278     GOTIndices.clear();
 279     TableIndices.clear();
 280     DataLocations.clear();
 281     CustomSections.clear();
 282     ProducersSection.reset();
 283     TargetFeaturesSection.reset();
 284     CustomSectionsRelocations.clear();
 285     SignatureIndices.clear();
 286     Signatures.clear();
 287     DataSegments.clear();
 288     SectionFunctions.clear();
 289     NumFunctionImports = 0;
 290     NumGlobalImports = 0;
 291     MCObjectWriter::reset();
 292   }
 293
 294   void writeHeader(const MCAssembler &Asm);
 295
 296   void recordRelocation(MCAssembler &Asm, const MCAsmLayout &Layout,
 297                         const MCFragment *Fragment, const MCFixup &Fixup,
 298                         MCValue Target, uint64_t &FixedValue) override;
 299
 300   void executePostLayoutBinding(MCAssembler &Asm,
 301                                 const MCAsmLayout &Layout) override;
 302
 303   uint64_t writeObject(MCAssembler &Asm, const MCAsmLayout &Layout) override;
 304
 305   void writeString(const StringRef Str) {
 306     encodeULEB128(Str.size(), W.OS);
 307     W.OS << Str;
 308   }
 309
 310   void writeValueType(wasm::ValType Ty) { W.OS << static_cast<char>(Ty); }
 311
 312   void writeTypeSection(ArrayRef<WasmSignature> Signatures);
 313   void writeImportSection(ArrayRef<wasm::WasmImport> Imports, uint32_t DataSize,
 314                           uint32_t NumElements);
 315   void writeFunctionSection(ArrayRef<WasmFunction> Functions);
 316   void writeExportSection(ArrayRef<wasm::WasmExport> Exports);
 317   void writeElemSection(ArrayRef<uint32_t> TableElems);
 318   void writeDataCountSection();
 319   void writeCodeSection(const MCAssembler &Asm, const MCAsmLayout &Layout,
 320                         ArrayRef<WasmFunction> Functions);
 321   void writeDataSection();
 322   void writeEventSection(ArrayRef<wasm::WasmEventType> Events);
 323   void writeRelocSection(uint32_t SectionIndex, StringRef Name,
 324                          std::vector<WasmRelocationEntry> &Relocations);
 325   void writeLinkingMetaDataSection(
 326       ArrayRef<wasm::WasmSymbolInfo> SymbolInfos,
 327       ArrayRef<std::pair<uint16_t, uint32_t>> InitFuncs,
 328       const std::map<StringRef, std::vector<WasmComdatEntry>> &Comdats);
 329   void writeCustomSection(WasmCustomSection &CustomSection,
 330                           const MCAssembler &Asm, const MCAsmLayout &Layout);
 331   void writeCustomRelocSections();
 332   void
 333   updateCustomSectionRelocations(const SmallVector<WasmFunction, 4> &Functions,
 334                                  const MCAsmLayout &Layout);
 335
 336   uint32_t getProvisionalValue(const WasmRelocationEntry &RelEntry);
 337   void applyRelocations(ArrayRef<WasmRelocationEntry> Relocations,
 338                         uint64_t ContentsOffset);
 339
 340   uint32_t getRelocationIndexValue(const WasmRelocationEntry &RelEntry);
 341   uint32_t getFunctionType(const MCSymbolWasm &Symbol);
 342   uint32_t getEventType(const MCSymbolWasm &Symbol);
 343   void registerFunctionType(const MCSymbolWasm &Symbol);
 344   void registerEventType(const MCSymbolWasm &Symbol);
 345 };
 346
 347 } // end anonymous namespace
 348
 349 // Write out a section header and a patchable section size field.
 350 void WasmObjectWriter::startSection(SectionBookkeeping &Section,
 351                                     unsigned SectionId) {
 352   LLVM_DEBUG(dbgs() << "startSection " << SectionId << "\n");
 353   W.OS << char(SectionId);
 354
 355   Section.SizeOffset = W.OS.tell();
 356
 357   // The section size. We don't know the size yet, so reserve enough space
 358   // for any 32-bit value; we'll patch it later.
 359   encodeULEB128(0, W.OS, 5);
 360
 361   // The position where the section starts, for measuring its size.
 362   Section.ContentsOffset = W.OS.tell();
 363   Section.PayloadOffset = W.OS.tell();
 364   Section.Index = SectionCount++;
 365 }
 366
 367 void WasmObjectWriter::startCustomSection(SectionBookkeeping &Section,
 368                                           StringRef Name) {
 369   LLVM_DEBUG(dbgs() << "startCustomSection " << Name << "\n");
 370   startSection(Section, wasm::WASM_SEC_CUSTOM);
 371
 372   // The position where the section header ends, for measuring its size.
 373   Section.PayloadOffset = W.OS.tell();
 374
 375   // Custom sections in wasm also have a string identifier.
 376   writeString(Name);
 377
 378   // The position where the custom section starts.
 379   Section.ContentsOffset = W.OS.tell();
 380 }
 381
 382 // Now that the section is complete and we know how big it is, patch up the
 383 // section size field at the start of the section.
 384 void WasmObjectWriter::endSection(SectionBookkeeping &Section) {
 385   uint64_t Size = W.OS.tell();
 386   // /dev/null doesn't support seek/tell and can report offset of 0.
 387   // Simply skip this patching in that case.
 388   if (!Size)
 389     return;
 390
 391   Size -= Section.PayloadOffset;
 392   if (uint32_t(Size) != Size)
 393     report_fatal_error("section size does not fit in a uint32_t");
 394
 395   LLVM_DEBUG(dbgs() << "endSection size=" << Size << "\n");
 396
 397   // Write the final section size to the payload_len field, which follows
 398   // the section id byte.
 399   writePatchableLEB(static_cast<raw_pwrite_stream &>(W.OS), Size,
 400                     Section.SizeOffset);
 401 }
 402
 403 // Emit the Wasm header.
 404 void WasmObjectWriter::writeHeader(const MCAssembler &Asm) {
 405   W.OS.write(wasm::WasmMagic, sizeof(wasm::WasmMagic));
 406   W.write<uint32_t>(wasm::WasmVersion);
 407 }
 408
 409 void WasmObjectWriter::executePostLayoutBinding(MCAssembler &Asm,
 410                                                 const MCAsmLayout &Layout) {
 411   // Build a map of sections to the function that defines them, for use
 412   // in recordRelocation.
 413   for (const MCSymbol &S : Asm.symbols()) {
 414     const auto &WS = static_cast<const MCSymbolWasm &>(S);
 415     if (WS.isDefined() && WS.isFunction() && !WS.isVariable()) {
 416       const auto &Sec = static_cast<const MCSectionWasm &>(S.getSection());
 417       auto Pair = SectionFunctions.insert(std::make_pair(&Sec, &S));
 418       if (!Pair.second)
 419         report_fatal_error("section already has a defining function: " +
 420                            Sec.getSectionName());
 421     }
 422   }
 423 }
 424
 425 void WasmObjectWriter::recordRelocation(MCAssembler &Asm,
 426                                         const MCAsmLayout &Layout,
 427                                         const MCFragment *Fragment,
 428                                         const MCFixup &Fixup, MCValue Target,
 429                                         uint64_t &FixedValue) {
 430   // The WebAssembly backend should never generate FKF_IsPCRel fixups
 431   assert(!(Asm.getBackend().getFixupKindInfo(Fixup.getKind()).Flags &
 432            MCFixupKindInfo::FKF_IsPCRel));
 433
 434   const auto &FixupSection = cast<MCSectionWasm>(*Fragment->getParent());
 435   uint64_t C = Target.getConstant();
 436   uint64_t FixupOffset = Layout.getFragmentOffset(Fragment) + Fixup.getOffset();
 437   MCContext &Ctx = Asm.getContext();
 438
 439   // The .init_array isn't translated as data, so don't do relocations in it.
 440   if (FixupSection.getSectionName().startswith(".init_array"))
 441     return;
 442
 443   if (const MCSymbolRefExpr *RefB = Target.getSymB()) {
 444     // To get here the A - B expression must have failed evaluateAsRelocatable.
 445     // This means either A or B must be undefined and in WebAssembly we can't
 446     // support either of those cases.
 447     const auto &SymB = cast<MCSymbolWasm>(RefB->getSymbol());
 448     Ctx.reportError(
 449         Fixup.getLoc(),
 450         Twine("symbol '") + SymB.getName() +
 451             "': unsupported subtraction expression used in relocation.");
 452     return;
 453   }
 454
 455   // We either rejected the fixup or folded B into C at this point.
 456   const MCSymbolRefExpr *RefA = Target.getSymA();
 457   const auto *SymA = cast<MCSymbolWasm>(&RefA->getSymbol());
 458
 459   if (SymA->isVariable()) {
 460     const MCExpr *Expr = SymA->getVariableValue();
 461     const auto *Inner = cast<MCSymbolRefExpr>(Expr);
 462     if (Inner->getKind() == MCSymbolRefExpr::VK_WEAKREF)
 463       llvm_unreachable("weakref used in reloc not yet implemented");
 464   }
 465
 466   // Put any constant offset in an addend. Offsets can be negative, and
 467   // LLVM expects wrapping, in contrast to wasm's immediates which can't
 468   // be negative and don't wrap.
 469   FixedValue = 0;
 470
 471   unsigned Type = TargetObjectWriter->getRelocType(Target, Fixup);
 472
 473   // Absolute offset within a section or a function.
 474   // Currently only supported for for metadata sections.
 475   // See: test/MC/WebAssembly/blockaddress.ll
 476   if (Type == wasm::R_WASM_FUNCTION_OFFSET_I32 ||
 477       Type == wasm::R_WASM_SECTION_OFFSET_I32) {
 478     if (!FixupSection.getKind().isMetadata())
 479       report_fatal_error("relocations for function or section offsets are "
 480                          "only supported in metadata sections");
 481
 482     const MCSymbol *SectionSymbol = nullptr;
 483     const MCSection &SecA = SymA->getSection();
 484     if (SecA.getKind().isText())
 485       SectionSymbol = SectionFunctions.find(&SecA)->second;
 486     else
 487       SectionSymbol = SecA.getBeginSymbol();
 488     if (!SectionSymbol)
 489       report_fatal_error("section symbol is required for relocation");
 490
 491     C += Layout.getSymbolOffset(*SymA);
 492     SymA = cast<MCSymbolWasm>(SectionSymbol);
 493   }
 494
 495   // Relocation other than R_WASM_TYPE_INDEX_LEB are required to be
 496   // against a named symbol.
 497   if (Type != wasm::R_WASM_TYPE_INDEX_LEB) {
 498     if (SymA->getName().empty())
 499       report_fatal_error("relocations against un-named temporaries are not yet "
 500                          "supported by wasm");
 501
 502     SymA->setUsedInReloc();
 503   }
 504
 505   if (RefA->getKind() == MCSymbolRefExpr::VK_GOT)
 506     SymA->setUsedInGOT();
 507
 508   WasmRelocationEntry Rec(FixupOffset, SymA, C, Type, &FixupSection);
 509   LLVM_DEBUG(dbgs() << "WasmReloc: " << Rec << "\n");
 510
 511   if (FixupSection.isWasmData()) {
 512     DataRelocations.push_back(Rec);
 513   } else if (FixupSection.getKind().isText()) {
 514     CodeRelocations.push_back(Rec);
 515   } else if (FixupSection.getKind().isMetadata()) {
 516     CustomSectionsRelocations[&FixupSection].push_back(Rec);
 517   } else {
 518     llvm_unreachable("unexpected section type");
 519   }
 520 }
 521
 522 static const MCSymbolWasm *resolveSymbol(const MCSymbolWasm &Symbol) {
 523   const MCSymbolWasm* Ret = &Symbol;
 524   while (Ret->isVariable()) {
 525     const MCExpr *Expr = Ret->getVariableValue();
 526     auto *Inner = cast<MCSymbolRefExpr>(Expr);
 527     Ret = cast<MCSymbolWasm>(&Inner->getSymbol());
 528   }
 529   return Ret;
 530 }
 531
 532 // Compute a value to write into the code at the location covered
 533 // by RelEntry. This value isn't used by the static linker; it just serves
 534 // to make the object format more readable and more likely to be directly
 535 // useable.
 536 uint32_t
 537 WasmObjectWriter::getProvisionalValue(const WasmRelocationEntry &RelEntry) {
 538   if (RelEntry.Type == wasm::R_WASM_GLOBAL_INDEX_LEB && !RelEntry.Symbol->isGlobal()) {
 539     assert(GOTIndices.count(RelEntry.Symbol) > 0 && "symbol not found in GOT index space");
 540     return GOTIndices[RelEntry.Symbol];
 541   }
 542
 543   switch (RelEntry.Type) {
 544   case wasm::R_WASM_TABLE_INDEX_REL_SLEB:
 545   case wasm::R_WASM_TABLE_INDEX_SLEB:
 546   case wasm::R_WASM_TABLE_INDEX_I32: {
 547     // Provisional value is table address of the resolved symbol itself
 548     const MCSymbolWasm *Sym = resolveSymbol(*RelEntry.Symbol);
 549     assert(Sym->isFunction());
 550     return TableIndices[Sym];
 551   }
 552   case wasm::R_WASM_TYPE_INDEX_LEB:
 553     // Provisional value is same as the index
 554     return getRelocationIndexValue(RelEntry);
 555   case wasm::R_WASM_FUNCTION_INDEX_LEB:
 556   case wasm::R_WASM_GLOBAL_INDEX_LEB:
 557   case wasm::R_WASM_EVENT_INDEX_LEB:
 558     // Provisional value is function/global/event Wasm index
 559     assert(WasmIndices.count(RelEntry.Symbol) > 0 && "symbol not found in wasm index space");
 560     return WasmIndices[RelEntry.Symbol];
 561   case wasm::R_WASM_FUNCTION_OFFSET_I32:
 562   case wasm::R_WASM_SECTION_OFFSET_I32: {
 563     const auto &Section =
 564         static_cast<const MCSectionWasm &>(RelEntry.Symbol->getSection());
 565     return Section.getSectionOffset() + RelEntry.Addend;
 566   }
 567   case wasm::R_WASM_MEMORY_ADDR_LEB:
 568   case wasm::R_WASM_MEMORY_ADDR_I32:
 569   case wasm::R_WASM_MEMORY_ADDR_REL_SLEB:
 570   case wasm::R_WASM_MEMORY_ADDR_SLEB: {
 571     // Provisional value is address of the global
 572     const MCSymbolWasm *Sym = resolveSymbol(*RelEntry.Symbol);
 573     // For undefined symbols, use zero
 574     if (!Sym->isDefined())
 575       return 0;
 576     const wasm::WasmDataReference &Ref = DataLocations[Sym];
 577     const WasmDataSegment &Segment = DataSegments[Ref.Segment];
 578     // Ignore overflow. LLVM allows address arithmetic to silently wrap.
 579     return Segment.Offset + Ref.Offset + RelEntry.Addend;
 580   }
 581   default:
 582     llvm_unreachable("invalid relocation type");
 583   }
 584 }
 585
 586 static void addData(SmallVectorImpl<char> &DataBytes,
 587                     MCSectionWasm &DataSection) {
 588   LLVM_DEBUG(errs() << "addData: " << DataSection.getSectionName() << "\n");
 589
 590   DataBytes.resize(alignTo(DataBytes.size(), DataSection.getAlignment()));
 591
 592   for (const MCFragment &Frag : DataSection) {
 593     if (Frag.hasInstructions())
 594       report_fatal_error("only data supported in data sections");
 595
 596     if (auto *Align = dyn_cast<MCAlignFragment>(&Frag)) {
 597       if (Align->getValueSize() != 1)
 598         report_fatal_error("only byte values supported for alignment");
 599       // If nops are requested, use zeros, as this is the data section.
 600       uint8_t Value = Align->hasEmitNops() ? 0 : Align->getValue();
 601       uint64_t Size =
 602           std::min<uint64_t>(alignTo(DataBytes.size(), Align->getAlignment()),
 603                              DataBytes.size() + Align->getMaxBytesToEmit());
 604       DataBytes.resize(Size, Value);
 605     } else if (auto *Fill = dyn_cast<MCFillFragment>(&Frag)) {
 606       int64_t NumValues;
 607       if (!Fill->getNumValues().evaluateAsAbsolute(NumValues))
 608         llvm_unreachable("The fill should be an assembler constant");
 609       DataBytes.insert(DataBytes.end(), Fill->getValueSize() * NumValues,
 610                        Fill->getValue());
 611     } else if (auto *LEB = dyn_cast<MCLEBFragment>(&Frag)) {
 612       const SmallVectorImpl<char> &Contents = LEB->getContents();
 613       DataBytes.insert(DataBytes.end(), Contents.begin(), Contents.end());
 614     } else {
 615       const auto &DataFrag = cast<MCDataFragment>(Frag);
 616       const SmallVectorImpl<char> &Contents = DataFrag.getContents();
 617       DataBytes.insert(DataBytes.end(), Contents.begin(), Contents.end());
 618     }
 619   }
 620
 621   LLVM_DEBUG(dbgs() << "addData -> " << DataBytes.size() << "\n");
 622 }
 623
 624 uint32_t
 625 WasmObjectWriter::getRelocationIndexValue(const WasmRelocationEntry &RelEntry) {
 626   if (RelEntry.Type == wasm::R_WASM_TYPE_INDEX_LEB) {
 627     if (!TypeIndices.count(RelEntry.Symbol))
 628       report_fatal_error("symbol not found in type index space: " +
 629                          RelEntry.Symbol->getName());
 630     return TypeIndices[RelEntry.Symbol];
 631   }
 632
 633   return RelEntry.Symbol->getIndex();
 634 }
 635
 636 // Apply the portions of the relocation records that we can handle ourselves
 637 // directly.
 638 void WasmObjectWriter::applyRelocations(
 639     ArrayRef<WasmRelocationEntry> Relocations, uint64_t ContentsOffset) {
 640   auto &Stream = static_cast<raw_pwrite_stream &>(W.OS);
 641   for (const WasmRelocationEntry &RelEntry : Relocations) {
 642     uint64_t Offset = ContentsOffset +
 643                       RelEntry.FixupSection->getSectionOffset() +
 644                       RelEntry.Offset;
 645
 646     LLVM_DEBUG(dbgs() << "applyRelocation: " << RelEntry << "\n");
 647     uint32_t Value = getProvisionalValue(RelEntry);
 648
 649     switch (RelEntry.Type) {
 650     case wasm::R_WASM_FUNCTION_INDEX_LEB:
 651     case wasm::R_WASM_TYPE_INDEX_LEB:
 652     case wasm::R_WASM_GLOBAL_INDEX_LEB:
 653     case wasm::R_WASM_MEMORY_ADDR_LEB:
 654     case wasm::R_WASM_EVENT_INDEX_LEB:
 655       writePatchableLEB(Stream, Value, Offset);
 656       break;
 657     case wasm::R_WASM_TABLE_INDEX_I32:
 658     case wasm::R_WASM_MEMORY_ADDR_I32:
 659     case wasm::R_WASM_FUNCTION_OFFSET_I32:
 660     case wasm::R_WASM_SECTION_OFFSET_I32:
 661       writeI32(Stream, Value, Offset);
 662       break;
 663     case wasm::R_WASM_TABLE_INDEX_SLEB:
 664     case wasm::R_WASM_TABLE_INDEX_REL_SLEB:
 665     case wasm::R_WASM_MEMORY_ADDR_SLEB:
 666     case wasm::R_WASM_MEMORY_ADDR_REL_SLEB:
 667       writePatchableSLEB(Stream, Value, Offset);
 668       break;
 669     default:
 670       llvm_unreachable("invalid relocation type");
 671     }
 672   }
 673 }
 674
 675 void WasmObjectWriter::writeTypeSection(ArrayRef<WasmSignature> Signatures) {
 676   if (Signatures.empty())
 677     return;
 678
 679   SectionBookkeeping Section;
 680   startSection(Section, wasm::WASM_SEC_TYPE);
 681
 682   encodeULEB128(Signatures.size(), W.OS);
 683
 684   for (const WasmSignature &Sig : Signatures) {
 685     W.OS << char(wasm::WASM_TYPE_FUNC);
 686     encodeULEB128(Sig.Params.size(), W.OS);
 687     for (wasm::ValType Ty : Sig.Params)
 688       writeValueType(Ty);
 689     encodeULEB128(Sig.Returns.size(), W.OS);
 690     for (wasm::ValType Ty : Sig.Returns)
 691       writeValueType(Ty);
 692   }
 693
 694   endSection(Section);
 695 }
 696
 697 void WasmObjectWriter::writeImportSection(ArrayRef<wasm::WasmImport> Imports,
 698                                           uint32_t DataSize,
 699                                           uint32_t NumElements) {
 700   if (Imports.empty())
 701     return;
 702
 703   uint32_t NumPages = (DataSize + wasm::WasmPageSize - 1) / wasm::WasmPageSize;
 704
 705   SectionBookkeeping Section;
 706   startSection(Section, wasm::WASM_SEC_IMPORT);
 707
 708   encodeULEB128(Imports.size(), W.OS);
 709   for (const wasm::WasmImport &Import : Imports) {
 710     writeString(Import.Module);
 711     writeString(Import.Field);
 712     W.OS << char(Import.Kind);
 713
 714     switch (Import.Kind) {
 715     case wasm::WASM_EXTERNAL_FUNCTION:
 716       encodeULEB128(Import.SigIndex, W.OS);
 717       break;
 718     case wasm::WASM_EXTERNAL_GLOBAL:
 719       W.OS << char(Import.Global.Type);
 720       W.OS << char(Import.Global.Mutable ? 1 : 0);
 721       break;
 722     case wasm::WASM_EXTERNAL_MEMORY:
 723       encodeULEB128(0, W.OS);        // flags
 724       encodeULEB128(NumPages, W.OS); // initial
 725       break;
 726     case wasm::WASM_EXTERNAL_TABLE:
 727       W.OS << char(Import.Table.ElemType);
 728       encodeULEB128(0, W.OS);           // flags
 729       encodeULEB128(NumElements, W.OS); // initial
 730       break;
 731     case wasm::WASM_EXTERNAL_EVENT:
 732       encodeULEB128(Import.Event.Attribute, W.OS);
 733       encodeULEB128(Import.Event.SigIndex, W.OS);
 734       break;
 735     default:
 736       llvm_unreachable("unsupported import kind");
 737     }
 738   }
 739
 740   endSection(Section);
 741 }
 742
 743 void WasmObjectWriter::writeFunctionSection(ArrayRef<WasmFunction> Functions) {
 744   if (Functions.empty())
 745     return;
 746
 747   SectionBookkeeping Section;
 748   startSection(Section, wasm::WASM_SEC_FUNCTION);
 749
 750   encodeULEB128(Functions.size(), W.OS);
 751   for (const WasmFunction &Func : Functions)
 752     encodeULEB128(Func.SigIndex, W.OS);
 753
 754   endSection(Section);
 755 }
 756
 757 void WasmObjectWriter::writeEventSection(ArrayRef<wasm::WasmEventType> Events) {
 758   if (Events.empty())
 759     return;
 760
 761   SectionBookkeeping Section;
 762   startSection(Section, wasm::WASM_SEC_EVENT);
 763
 764   encodeULEB128(Events.size(), W.OS);
 765   for (const wasm::WasmEventType &Event : Events) {
 766     encodeULEB128(Event.Attribute, W.OS);
 767     encodeULEB128(Event.SigIndex, W.OS);
 768   }
 769
 770   endSection(Section);
 771 }
 772
 773 void WasmObjectWriter::writeExportSection(ArrayRef<wasm::WasmExport> Exports) {
 774   if (Exports.empty())
 775     return;
 776
 777   SectionBookkeeping Section;
 778   startSection(Section, wasm::WASM_SEC_EXPORT);
 779
 780   encodeULEB128(Exports.size(), W.OS);
 781   for (const wasm::WasmExport &Export : Exports) {
 782     writeString(Export.Name);
 783     W.OS << char(Export.Kind);
 784     encodeULEB128(Export.Index, W.OS);
 785   }
 786
 787   endSection(Section);
 788 }
 789
 790 void WasmObjectWriter::writeElemSection(ArrayRef<uint32_t> TableElems) {
 791   if (TableElems.empty())
 792     return;
 793
 794   SectionBookkeeping Section;
 795   startSection(Section, wasm::WASM_SEC_ELEM);
 796
 797   encodeULEB128(1, W.OS); // number of "segments"
 798   encodeULEB128(0, W.OS); // the table index
 799
 800   // init expr for starting offset
 801   W.OS << char(wasm::WASM_OPCODE_I32_CONST);
 802   encodeSLEB128(InitialTableOffset, W.OS);
 803   W.OS << char(wasm::WASM_OPCODE_END);
 804
 805   encodeULEB128(TableElems.size(), W.OS);
 806   for (uint32_t Elem : TableElems)
 807     encodeULEB128(Elem, W.OS);
 808
 809   endSection(Section);
 810 }
 811
 812 void WasmObjectWriter::writeDataCountSection() {
 813   if (DataSegments.empty())
 814     return;
 815
 816   SectionBookkeeping Section;
 817   startSection(Section, wasm::WASM_SEC_DATACOUNT);
 818   encodeULEB128(DataSegments.size(), W.OS);
 819   endSection(Section);
 820 }
 821
 822 void WasmObjectWriter::writeCodeSection(const MCAssembler &Asm,
 823                                         const MCAsmLayout &Layout,
 824                                         ArrayRef<WasmFunction> Functions) {
 825   if (Functions.empty())
 826     return;
 827
 828   SectionBookkeeping Section;
 829   startSection(Section, wasm::WASM_SEC_CODE);
 830   CodeSectionIndex = Section.Index;
 831
 832   encodeULEB128(Functions.size(), W.OS);
 833
 834   for (const WasmFunction &Func : Functions) {
 835     auto &FuncSection = static_cast<MCSectionWasm &>(Func.Sym->getSection());
 836
 837     int64_t Size = 0;
 838     if (!Func.Sym->getSize()->evaluateAsAbsolute(Size, Layout))
 839       report_fatal_error(".size expression must be evaluatable");
 840
 841     encodeULEB128(Size, W.OS);
 842     FuncSection.setSectionOffset(W.OS.tell() - Section.ContentsOffset);
 843     Asm.writeSectionData(W.OS, &FuncSection, Layout);
 844   }
 845
 846   // Apply fixups.
 847   applyRelocations(CodeRelocations, Section.ContentsOffset);
 848
 849   endSection(Section);
 850 }
 851
 852 void WasmObjectWriter::writeDataSection() {
 853   if (DataSegments.empty())
 854     return;
 855
 856   SectionBookkeeping Section;
 857   startSection(Section, wasm::WASM_SEC_DATA);
 858   DataSectionIndex = Section.Index;
 859
 860   encodeULEB128(DataSegments.size(), W.OS); // count
 861
 862   for (const WasmDataSegment &Segment : DataSegments) {
 863     encodeULEB128(Segment.InitFlags, W.OS); // flags
 864     if (Segment.InitFlags & wasm::WASM_SEGMENT_HAS_MEMINDEX)
 865       encodeULEB128(0, W.OS); // memory index
 866     if ((Segment.InitFlags & wasm::WASM_SEGMENT_IS_PASSIVE) == 0) {
 867       W.OS << char(wasm::WASM_OPCODE_I32_CONST);
 868       encodeSLEB128(Segment.Offset, W.OS); // offset
 869       W.OS << char(wasm::WASM_OPCODE_END);
 870     }
 871     encodeULEB128(Segment.Data.size(), W.OS); // size
 872     Segment.Section->setSectionOffset(W.OS.tell() - Section.ContentsOffset);
 873     W.OS << Segment.Data; // data
 874   }
 875
 876   // Apply fixups.
 877   applyRelocations(DataRelocations, Section.ContentsOffset);
 878
 879   endSection(Section);
 880 }
 881
 882 void WasmObjectWriter::writeRelocSection(
 883     uint32_t SectionIndex, StringRef Name,
 884     std::vector<WasmRelocationEntry> &Relocs) {
 885   // See: https://github.com/WebAssembly/tool-conventions/blob/master/Linking.md
 886   // for descriptions of the reloc sections.
 887
 888   if (Relocs.empty())
 889     return;
 890
 891   // First, ensure the relocations are sorted in offset order.  In general they
 892   // should already be sorted since `recordRelocation` is called in offset
 893   // order, but for the code section we combine many MC sections into single
 894   // wasm section, and this order is determined by the order of Asm.Symbols()
 895   // not the sections order.
 896   llvm::stable_sort(
 897       Relocs, [](const WasmRelocationEntry &A, const WasmRelocationEntry &B) {
 898         return (A.Offset + A.FixupSection->getSectionOffset()) <
 899                (B.Offset + B.FixupSection->getSectionOffset());
 900       });
 901
 902   SectionBookkeeping Section;
 903   startCustomSection(Section, std::string("reloc.") + Name.str());
 904
 905   encodeULEB128(SectionIndex, W.OS);
 906   encodeULEB128(Relocs.size(), W.OS);
 907   for (const WasmRelocationEntry &RelEntry : Relocs) {
 908     uint64_t Offset =
 909         RelEntry.Offset + RelEntry.FixupSection->getSectionOffset();
 910     uint32_t Index = getRelocationIndexValue(RelEntry);
 911
 912     W.OS << char(RelEntry.Type);
 913     encodeULEB128(Offset, W.OS);
 914     encodeULEB128(Index, W.OS);
 915     if (RelEntry.hasAddend())
 916       encodeSLEB128(RelEntry.Addend, W.OS);
 917   }
 918
 919   endSection(Section);
 920 }
 921
 922 void WasmObjectWriter::writeCustomRelocSections() {
 923   for (const auto &Sec : CustomSections) {
 924     auto &Relocations = CustomSectionsRelocations[Sec.Section];
 925     writeRelocSection(Sec.OutputIndex, Sec.Name, Relocations);
 926   }
 927 }
 928
 929 void WasmObjectWriter::writeLinkingMetaDataSection(
 930     ArrayRef<wasm::WasmSymbolInfo> SymbolInfos,
 931     ArrayRef<std::pair<uint16_t, uint32_t>> InitFuncs,
 932     const std::map<StringRef, std::vector<WasmComdatEntry>> &Comdats) {
 933   SectionBookkeeping Section;
 934   startCustomSection(Section, "linking");
 935   encodeULEB128(wasm::WasmMetadataVersion, W.OS);
 936
 937   SectionBookkeeping SubSection;
 938   if (SymbolInfos.size() != 0) {
 939     startSection(SubSection, wasm::WASM_SYMBOL_TABLE);
 940     encodeULEB128(SymbolInfos.size(), W.OS);
 941     for (const wasm::WasmSymbolInfo &Sym : SymbolInfos) {
 942       encodeULEB128(Sym.Kind, W.OS);
 943       encodeULEB128(Sym.Flags, W.OS);
 944       switch (Sym.Kind) {
 945       case wasm::WASM_SYMBOL_TYPE_FUNCTION:
 946       case wasm::WASM_SYMBOL_TYPE_GLOBAL:
 947       case wasm::WASM_SYMBOL_TYPE_EVENT:
 948         encodeULEB128(Sym.ElementIndex, W.OS);
 949         if ((Sym.Flags & wasm::WASM_SYMBOL_UNDEFINED) == 0 ||
 950             (Sym.Flags & wasm::WASM_SYMBOL_EXPLICIT_NAME) != 0)
 951           writeString(Sym.Name);
 952         break;
 953       case wasm::WASM_SYMBOL_TYPE_DATA:
 954         writeString(Sym.Name);
 955         if ((Sym.Flags & wasm::WASM_SYMBOL_UNDEFINED) == 0) {
 956           encodeULEB128(Sym.DataRef.Segment, W.OS);
 957           encodeULEB128(Sym.DataRef.Offset, W.OS);
 958           encodeULEB128(Sym.DataRef.Size, W.OS);
 959         }
 960         break;
 961       case wasm::WASM_SYMBOL_TYPE_SECTION: {
 962         const uint32_t SectionIndex =
 963             CustomSections[Sym.ElementIndex].OutputIndex;
 964         encodeULEB128(SectionIndex, W.OS);
 965         break;
 966       }
 967       default:
 968         llvm_unreachable("unexpected kind");
 969       }
 970     }
 971     endSection(SubSection);
 972   }
 973
 974   if (DataSegments.size()) {
 975     startSection(SubSection, wasm::WASM_SEGMENT_INFO);
 976     encodeULEB128(DataSegments.size(), W.OS);
 977     for (const WasmDataSegment &Segment : DataSegments) {
 978       writeString(Segment.Name);
 979       encodeULEB128(Segment.Alignment, W.OS);
 980       encodeULEB128(Segment.LinkerFlags, W.OS);
 981     }
 982     endSection(SubSection);
 983   }
 984
 985   if (!InitFuncs.empty()) {
 986     startSection(SubSection, wasm::WASM_INIT_FUNCS);
 987     encodeULEB128(InitFuncs.size(), W.OS);
 988     for (auto &StartFunc : InitFuncs) {
 989       encodeULEB128(StartFunc.first, W.OS);  // priority
 990       encodeULEB128(StartFunc.second, W.OS); // function index
 991     }
 992     endSection(SubSection);
 993   }
 994
 995   if (Comdats.size()) {
 996     startSection(SubSection, wasm::WASM_COMDAT_INFO);
 997     encodeULEB128(Comdats.size(), W.OS);
 998     for (const auto &C : Comdats) {
 999       writeString(C.first);
1000       encodeULEB128(0, W.OS); // flags for future use
1001       encodeULEB128(C.second.size(), W.OS);
1002       for (const WasmComdatEntry &Entry : C.second) {
1003         encodeULEB128(Entry.Kind, W.OS);
1004         encodeULEB128(Entry.Index, W.OS);
1005       }
1006     }
1007     endSection(SubSection);
1008   }
1009
1010   endSection(Section);
1011 }
1012
1013 void WasmObjectWriter::writeCustomSection(WasmCustomSection &CustomSection,
1014                                           const MCAssembler &Asm,
1015                                           const MCAsmLayout &Layout) {
1016   SectionBookkeeping Section;
1017   auto *Sec = CustomSection.Section;
1018   startCustomSection(Section, CustomSection.Name);
1019
1020   Sec->setSectionOffset(W.OS.tell() - Section.ContentsOffset);
1021   Asm.writeSectionData(W.OS, Sec, Layout);
1022
1023   CustomSection.OutputContentsOffset = Section.ContentsOffset;
1024   CustomSection.OutputIndex = Section.Index;
1025
1026   endSection(Section);
1027
1028   // Apply fixups.
1029   auto &Relocations = CustomSectionsRelocations[CustomSection.Section];
1030   applyRelocations(Relocations, CustomSection.OutputContentsOffset);
1031 }
1032
1033 uint32_t WasmObjectWriter::getFunctionType(const MCSymbolWasm &Symbol) {
1034   assert(Symbol.isFunction());
1035   assert(TypeIndices.count(&Symbol));
1036   return TypeIndices[&Symbol];
1037 }
1038
1039 uint32_t WasmObjectWriter::getEventType(const MCSymbolWasm &Symbol) {
1040   assert(Symbol.isEvent());
1041   assert(TypeIndices.count(&Symbol));
1042   return TypeIndices[&Symbol];
1043 }
1044
1045 void WasmObjectWriter::registerFunctionType(const MCSymbolWasm &Symbol) {
1046   assert(Symbol.isFunction());
1047
1048   WasmSignature S;
1049   const MCSymbolWasm *ResolvedSym = resolveSymbol(Symbol);
1050   if (auto *Sig = ResolvedSym->getSignature()) {
1051     S.Returns = Sig->Returns;
1052     S.Params = Sig->Params;
1053   }
1054
1055   auto Pair = SignatureIndices.insert(std::make_pair(S, Signatures.size()));
1056   if (Pair.second)
1057     Signatures.push_back(S);
1058   TypeIndices[&Symbol] = Pair.first->second;
1059
1060   LLVM_DEBUG(dbgs() << "registerFunctionType: " << Symbol
1061                     << " new:" << Pair.second << "\n");
1062   LLVM_DEBUG(dbgs() << "  -> type index: " << Pair.first->second << "\n");
1063 }
1064
1065 void WasmObjectWriter::registerEventType(const MCSymbolWasm &Symbol) {
1066   assert(Symbol.isEvent());
1067
1068   // TODO Currently we don't generate imported exceptions, but if we do, we
1069   // should have a way of infering types of imported exceptions.
1070   WasmSignature S;
1071   if (auto *Sig = Symbol.getSignature()) {
1072     S.Returns = Sig->Returns;
1073     S.Params = Sig->Params;
1074   }
1075
1076   auto Pair = SignatureIndices.insert(std::make_pair(S, Signatures.size()));
1077   if (Pair.second)
1078     Signatures.push_back(S);
1079   TypeIndices[&Symbol] = Pair.first->second;
1080
1081   LLVM_DEBUG(dbgs() << "registerEventType: " << Symbol << " new:" << Pair.second
1082                     << "\n");
1083   LLVM_DEBUG(dbgs() << "  -> type index: " << Pair.first->second << "\n");
1084 }
1085
1086 static bool isInSymtab(const MCSymbolWasm &Sym) {
1087   if (Sym.isUsedInReloc())
1088     return true;
1089
1090   if (Sym.isComdat() && !Sym.isDefined())
1091     return false;
1092
1093   if (Sym.isTemporary() && Sym.getName().empty())
1094     return false;
1095
1096   if (Sym.isTemporary() && Sym.isData() && !Sym.getSize())
1097     return false;
1098
1099   if (Sym.isSection())
1100     return false;
1101
1102   return true;
1103 }
1104
1105 uint64_t WasmObjectWriter::writeObject(MCAssembler &Asm,
1106                                        const MCAsmLayout &Layout) {
1107   uint64_t StartOffset = W.OS.tell();
1108
1109   LLVM_DEBUG(dbgs() << "WasmObjectWriter::writeObject\n");
1110
1111   // Collect information from the available symbols.
1112   SmallVector<WasmFunction, 4> Functions;
1113   SmallVector<uint32_t, 4> TableElems;
1114   SmallVector<wasm::WasmImport, 4> Imports;
1115   SmallVector<wasm::WasmExport, 4> Exports;
1116   SmallVector<wasm::WasmEventType, 1> Events;
1117   SmallVector<wasm::WasmSymbolInfo, 4> SymbolInfos;
1118   SmallVector<std::pair<uint16_t, uint32_t>, 2> InitFuncs;
1119   std::map<StringRef, std::vector<WasmComdatEntry>> Comdats;
1120   uint32_t DataSize = 0;
1121
1122   // For now, always emit the memory import, since loads and stores are not
1123   // valid without it. In the future, we could perhaps be more clever and omit
1124   // it if there are no loads or stores.
1125   wasm::WasmImport MemImport;
1126   MemImport.Module = "env";
1127   MemImport.Field = "__linear_memory";
1128   MemImport.Kind = wasm::WASM_EXTERNAL_MEMORY;
1129   Imports.push_back(MemImport);
1130
1131   // For now, always emit the table section, since indirect calls are not
1132   // valid without it. In the future, we could perhaps be more clever and omit
1133   // it if there are no indirect calls.
1134   wasm::WasmImport TableImport;
1135   TableImport.Module = "env";
1136   TableImport.Field = "__indirect_function_table";
1137   TableImport.Kind = wasm::WASM_EXTERNAL_TABLE;
1138   TableImport.Table.ElemType = wasm::WASM_TYPE_FUNCREF;
1139   Imports.push_back(TableImport);
1140
1141   // Populate SignatureIndices, and Imports and WasmIndices for undefined
1142   // symbols.  This must be done before populating WasmIndices for defined
1143   // symbols.
1144   for (const MCSymbol &S : Asm.symbols()) {
1145     const auto &WS = static_cast<const MCSymbolWasm &>(S);
1146
1147     // Register types for all functions, including those with private linkage
1148     // (because wasm always needs a type signature).
1149     if (WS.isFunction())
1150       registerFunctionType(WS);
1151
1152     if (WS.isEvent())
1153       registerEventType(WS);
1154
1155     if (WS.isTemporary())
1156       continue;
1157
1158     // If the symbol is not defined in this translation unit, import it.
1159     if (!WS.isDefined() && !WS.isComdat()) {
1160       if (WS.isFunction()) {
1161         wasm::WasmImport Import;
1162         Import.Module = WS.getImportModule();
1163         Import.Field = WS.getImportName();
1164         Import.Kind = wasm::WASM_EXTERNAL_FUNCTION;
1165         Import.SigIndex = getFunctionType(WS);
1166         Imports.push_back(Import);
1167         assert(WasmIndices.count(&WS) == 0);
1168         WasmIndices[&WS] = NumFunctionImports++;
1169       } else if (WS.isGlobal()) {
1170         if (WS.isWeak())
1171           report_fatal_error("undefined global symbol cannot be weak");
1172
1173         wasm::WasmImport Import;
1174         Import.Field = WS.getImportName();
1175         Import.Kind = wasm::WASM_EXTERNAL_GLOBAL;
1176         Import.Module = WS.getImportModule();
1177         Import.Global = WS.getGlobalType();
1178         Imports.push_back(Import);
1179         assert(WasmIndices.count(&WS) == 0);
1180         WasmIndices[&WS] = NumGlobalImports++;
1181       } else if (WS.isEvent()) {
1182         if (WS.isWeak())
1183           report_fatal_error("undefined event symbol cannot be weak");
1184
1185         wasm::WasmImport Import;
1186         Import.Module = WS.getImportModule();
1187         Import.Field = WS.getImportName();
1188         Import.Kind = wasm::WASM_EXTERNAL_EVENT;
1189         Import.Event.Attribute = wasm::WASM_EVENT_ATTRIBUTE_EXCEPTION;
1190         Import.Event.SigIndex = getEventType(WS);
1191         Imports.push_back(Import);
1192         assert(WasmIndices.count(&WS) == 0);
1193         WasmIndices[&WS] = NumEventImports++;
1194       }
1195     }
1196   }
1197
1198   // Add imports for GOT globals
1199   for (const MCSymbol &S : Asm.symbols()) {
1200     const auto &WS = static_cast<const MCSymbolWasm &>(S);
1201     if (WS.isUsedInGOT()) {
1202       wasm::WasmImport Import;
1203       if (WS.isFunction())
1204         Import.Module = "GOT.func";
1205       else
1206         Import.Module = "GOT.mem";
1207       Import.Field = WS.getName();
1208       Import.Kind = wasm::WASM_EXTERNAL_GLOBAL;
1209       Import.Global = {wasm::WASM_TYPE_I32, true};
1210       Imports.push_back(Import);
1211       assert(GOTIndices.count(&WS) == 0);
1212       GOTIndices[&WS] = NumGlobalImports++;
1213     }
1214   }
1215
1216   // Populate DataSegments and CustomSections, which must be done before
1217   // populating DataLocations.
1218   for (MCSection &Sec : Asm) {
1219     auto &Section = static_cast<MCSectionWasm &>(Sec);
1220     StringRef SectionName = Section.getSectionName();
1221
1222     // .init_array sections are handled specially elsewhere.
1223     if (SectionName.startswith(".init_array"))
1224       continue;
1225
1226     // Code is handled separately
1227     if (Section.getKind().isText())
1228       continue;
1229
1230     if (Section.isWasmData()) {
1231       uint32_t SegmentIndex = DataSegments.size();
1232       DataSize = alignTo(DataSize, Section.getAlignment());
1233       DataSegments.emplace_back();
1234       WasmDataSegment &Segment = DataSegments.back();
1235       Segment.Name = SectionName;
1236       Segment.InitFlags =
1237           Section.getPassive() ? (uint32_t)wasm::WASM_SEGMENT_IS_PASSIVE : 0;
1238       Segment.Offset = DataSize;
1239       Segment.Section = &Section;
1240       addData(Segment.Data, Section);
1241       Segment.Alignment = Log2_32(Section.getAlignment());
1242       Segment.LinkerFlags = 0;
1243       DataSize += Segment.Data.size();
1244       Section.setSegmentIndex(SegmentIndex);
1245
1246       if (const MCSymbolWasm *C = Section.getGroup()) {
1247         Comdats[C->getName()].emplace_back(
1248             WasmComdatEntry{wasm::WASM_COMDAT_DATA, SegmentIndex});
1249       }
1250     } else {
1251       // Create custom sections
1252       assert(Sec.getKind().isMetadata());
1253
1254       StringRef Name = SectionName;
1255
1256       // For user-defined custom sections, strip the prefix
1257       if (Name.startswith(".custom_section."))
1258         Name = Name.substr(strlen(".custom_section."));
1259
1260       MCSymbol *Begin = Sec.getBeginSymbol();
1261       if (Begin) {
1262         WasmIndices[cast<MCSymbolWasm>(Begin)] = CustomSections.size();
1263         if (SectionName != Begin->getName())
1264           report_fatal_error("section name and begin symbol should match: " +
1265                              Twine(SectionName));
1266       }
1267
1268       // Separate out the producers and target features sections
1269       if (Name == "producers") {
1270         ProducersSection = std::make_unique<WasmCustomSection>(Name, &Section);
1271         continue;
1272       }
1273       if (Name == "target_features") {
1274         TargetFeaturesSection =
1275             std::make_unique<WasmCustomSection>(Name, &Section);
1276         continue;
1277       }
1278
1279       CustomSections.emplace_back(Name, &Section);
1280     }
1281   }
1282
1283   // Populate WasmIndices and DataLocations for defined symbols.
1284   for (const MCSymbol &S : Asm.symbols()) {
1285     // Ignore unnamed temporary symbols, which aren't ever exported, imported,
1286     // or used in relocations.
1287     if (S.isTemporary() && S.getName().empty())
1288       continue;
1289
1290     const auto &WS = static_cast<const MCSymbolWasm &>(S);
1291     LLVM_DEBUG(
1292         dbgs() << "MCSymbol: " << toString(WS.getType()) << " '" << S << "'"
1293                << " isDefined=" << S.isDefined() << " isExternal="
1294                << S.isExternal() << " isTemporary=" << S.isTemporary()
1295                << " isWeak=" << WS.isWeak() << " isHidden=" << WS.isHidden()
1296                << " isVariable=" << WS.isVariable() << "\n");
1297
1298     if (WS.isVariable())
1299       continue;
1300     if (WS.isComdat() && !WS.isDefined())
1301       continue;
1302
1303     if (WS.isFunction()) {
1304       unsigned Index;
1305       if (WS.isDefined()) {
1306         if (WS.getOffset() != 0)
1307           report_fatal_error(
1308               "function sections must contain one function each");
1309
1310         if (WS.getSize() == nullptr)
1311           report_fatal_error(
1312               "function symbols must have a size set with .size");
1313
1314         // A definition. Write out the function body.
1315         Index = NumFunctionImports + Functions.size();
1316         WasmFunction Func;
1317         Func.SigIndex = getFunctionType(WS);
1318         Func.Sym = &WS;
1319         WasmIndices[&WS] = Index;
1320         Functions.push_back(Func);
1321
1322         auto &Section = static_cast<MCSectionWasm &>(WS.getSection());
1323         if (const MCSymbolWasm *C = Section.getGroup()) {
1324           Comdats[C->getName()].emplace_back(
1325               WasmComdatEntry{wasm::WASM_COMDAT_FUNCTION, Index});
1326         }
1327       } else {
1328         // An import; the index was assigned above.
1329         Index = WasmIndices.find(&WS)->second;
1330       }
1331
1332       LLVM_DEBUG(dbgs() << "  -> function index: " << Index << "\n");
1333
1334     } else if (WS.isData()) {
1335       if (!isInSymtab(WS))
1336         continue;
1337
1338       if (!WS.isDefined()) {
1339         LLVM_DEBUG(dbgs() << "  -> segment index: -1"
1340                           << "\n");
1341         continue;
1342       }
1343
1344       if (!WS.getSize())
1345         report_fatal_error("data symbols must have a size set with .size: " +
1346                            WS.getName());
1347
1348       int64_t Size = 0;
1349       if (!WS.getSize()->evaluateAsAbsolute(Size, Layout))
1350         report_fatal_error(".size expression must be evaluatable");
1351
1352       auto &DataSection = static_cast<MCSectionWasm &>(WS.getSection());
1353       if (!DataSection.isWasmData())
1354         report_fatal_error("data symbols must live in a data section: " +
1355                            WS.getName());
1356
1357       // For each data symbol, export it in the symtab as a reference to the
1358       // corresponding Wasm data segment.
1359       wasm::WasmDataReference Ref = wasm::WasmDataReference{
1360           DataSection.getSegmentIndex(),
1361           static_cast<uint32_t>(Layout.getSymbolOffset(WS)),
1362           static_cast<uint32_t>(Size)};
1363       DataLocations[&WS] = Ref;
1364       LLVM_DEBUG(dbgs() << "  -> segment index: " << Ref.Segment << "\n");
1365
1366     } else if (WS.isGlobal()) {
1367       // A "true" Wasm global (currently just __stack_pointer)
1368       if (WS.isDefined())
1369         report_fatal_error("don't yet support defined globals");
1370
1371       // An import; the index was assigned above
1372       LLVM_DEBUG(dbgs() << "  -> global index: "
1373                         << WasmIndices.find(&WS)->second << "\n");
1374
1375     } else if (WS.isEvent()) {
1376       // C++ exception symbol (__cpp_exception)
1377       unsigned Index;
1378       if (WS.isDefined()) {
1379         Index = NumEventImports + Events.size();
1380         wasm::WasmEventType Event;
1381         Event.SigIndex = getEventType(WS);
1382         Event.Attribute = wasm::WASM_EVENT_ATTRIBUTE_EXCEPTION;
1383         assert(WasmIndices.count(&WS) == 0);
1384         WasmIndices[&WS] = Index;
1385         Events.push_back(Event);
1386       } else {
1387         // An import; the index was assigned above.
1388         assert(WasmIndices.count(&WS) > 0);
1389       }
1390       LLVM_DEBUG(dbgs() << "  -> event index: " << WasmIndices.find(&WS)->second
1391                         << "\n");
1392
1393     } else {
1394       assert(WS.isSection());
1395     }
1396   }
1397
1398   // Populate WasmIndices and DataLocations for aliased symbols.  We need to
1399   // process these in a separate pass because we need to have processed the
1400   // target of the alias before the alias itself and the symbols are not
1401   // necessarily ordered in this way.
1402   for (const MCSymbol &S : Asm.symbols()) {
1403     if (!S.isVariable())
1404       continue;
1405
1406     assert(S.isDefined());
1407
1408     // Find the target symbol of this weak alias and export that index
1409     const auto &WS = static_cast<const MCSymbolWasm &>(S);
1410     const MCSymbolWasm *ResolvedSym = resolveSymbol(WS);
1411     LLVM_DEBUG(dbgs() << WS.getName() << ": weak alias of '" << *ResolvedSym
1412                       << "'\n");
1413
1414     if (ResolvedSym->isFunction()) {
1415       assert(WasmIndices.count(ResolvedSym) > 0);
1416       uint32_t WasmIndex = WasmIndices.find(ResolvedSym)->second;
1417       assert(WasmIndices.count(&WS) == 0);
1418       WasmIndices[&WS] = WasmIndex;
1419       LLVM_DEBUG(dbgs() << "  -> index:" << WasmIndex << "\n");
1420     } else if (ResolvedSym->isData()) {
1421       assert(DataLocations.count(ResolvedSym) > 0);
1422       const wasm::WasmDataReference &Ref =
1423           DataLocations.find(ResolvedSym)->second;
1424       DataLocations[&WS] = Ref;
1425       LLVM_DEBUG(dbgs() << "  -> index:" << Ref.Segment << "\n");
1426     } else {
1427       report_fatal_error("don't yet support global/event aliases");
1428     }
1429   }
1430
1431   // Finally, populate the symbol table itself, in its "natural" order.
1432   for (const MCSymbol &S : Asm.symbols()) {
1433     const auto &WS = static_cast<const MCSymbolWasm &>(S);
1434     if (!isInSymtab(WS)) {
1435       WS.setIndex(InvalidIndex);
1436       continue;
1437     }
1438     LLVM_DEBUG(dbgs() << "adding to symtab: " << WS << "\n");
1439
1440     uint32_t Flags = 0;
1441     if (WS.isWeak())
1442       Flags |= wasm::WASM_SYMBOL_BINDING_WEAK;
1443     if (WS.isHidden())
1444       Flags |= wasm::WASM_SYMBOL_VISIBILITY_HIDDEN;
1445     if (!WS.isExternal() && WS.isDefined())
1446       Flags |= wasm::WASM_SYMBOL_BINDING_LOCAL;
1447     if (WS.isUndefined())
1448       Flags |= wasm::WASM_SYMBOL_UNDEFINED;
1449     if (WS.isNoStrip()) {
1450       Flags |= wasm::WASM_SYMBOL_NO_STRIP;
1451       if (isEmscripten()) {
1452         Flags |= wasm::WASM_SYMBOL_EXPORTED;
1453       }
1454     }
1455     if (WS.getName() != WS.getImportName())
1456       Flags |= wasm::WASM_SYMBOL_EXPLICIT_NAME;
1457
1458     wasm::WasmSymbolInfo Info;
1459     Info.Name = WS.getName();
1460     Info.Kind = WS.getType();
1461     Info.Flags = Flags;
1462     if (!WS.isData()) {
1463       assert(WasmIndices.count(&WS) > 0);
1464       Info.ElementIndex = WasmIndices.find(&WS)->second;
1465     } else if (WS.isDefined()) {
1466       assert(DataLocations.count(&WS) > 0);
1467       Info.DataRef = DataLocations.find(&WS)->second;
1468     }
1469     WS.setIndex(SymbolInfos.size());
1470     SymbolInfos.emplace_back(Info);
1471   }
1472
1473   {
1474     auto HandleReloc = [&](const WasmRelocationEntry &Rel) {
1475       // Functions referenced by a relocation need to put in the table.  This is
1476       // purely to make the object file's provisional values readable, and is
1477       // ignored by the linker, which re-calculates the relocations itself.
1478       if (Rel.Type != wasm::R_WASM_TABLE_INDEX_I32 &&
1479           Rel.Type != wasm::R_WASM_TABLE_INDEX_SLEB)
1480         return;
1481       assert(Rel.Symbol->isFunction());
1482       const MCSymbolWasm &WS = *resolveSymbol(*Rel.Symbol);
1483       uint32_t FunctionIndex = WasmIndices.find(&WS)->second;
1484       uint32_t TableIndex = TableElems.size() + InitialTableOffset;
1485       if (TableIndices.try_emplace(&WS, TableIndex).second) {
1486         LLVM_DEBUG(dbgs() << "  -> adding " << WS.getName()
1487                           << " to table: " << TableIndex << "\n");
1488         TableElems.push_back(FunctionIndex);
1489         registerFunctionType(WS);
1490       }
1491     };
1492
1493     for (const WasmRelocationEntry &RelEntry : CodeRelocations)
1494       HandleReloc(RelEntry);
1495     for (const WasmRelocationEntry &RelEntry : DataRelocations)
1496       HandleReloc(RelEntry);
1497   }
1498
1499   // Translate .init_array section contents into start functions.
1500   for (const MCSection &S : Asm) {
1501     const auto &WS = static_cast<const MCSectionWasm &>(S);
1502     if (WS.getSectionName().startswith(".fini_array"))
1503       report_fatal_error(".fini_array sections are unsupported");
1504     if (!WS.getSectionName().startswith(".init_array"))
1505       continue;
1506     if (WS.getFragmentList().empty())
1507       continue;
1508
1509     // init_array is expected to contain a single non-empty data fragment
1510     if (WS.getFragmentList().size() != 3)
1511       report_fatal_error("only one .init_array section fragment supported");
1512
1513     auto IT = WS.begin();
1514     const MCFragment &EmptyFrag = *IT;
1515     if (EmptyFrag.getKind() != MCFragment::FT_Data)
1516       report_fatal_error(".init_array section should be aligned");
1517
1518     IT = std::next(IT);
1519     const MCFragment &AlignFrag = *IT;
1520     if (AlignFrag.getKind() != MCFragment::FT_Align)
1521       report_fatal_error(".init_array section should be aligned");
1522     if (cast<MCAlignFragment>(AlignFrag).getAlignment() != (is64Bit() ? 8 : 4))
1523       report_fatal_error(".init_array section should be aligned for pointers");
1524
1525     const MCFragment &Frag = *std::next(IT);
1526     if (Frag.hasInstructions() || Frag.getKind() != MCFragment::FT_Data)
1527       report_fatal_error("only data supported in .init_array section");
1528
1529     uint16_t Priority = UINT16_MAX;
1530     unsigned PrefixLength = strlen(".init_array");
1531     if (WS.getSectionName().size() > PrefixLength) {
1532       if (WS.getSectionName()[PrefixLength] != '.')
1533         report_fatal_error(
1534             ".init_array section priority should start with '.'");
1535       if (WS.getSectionName()
1536               .substr(PrefixLength + 1)
1537               .getAsInteger(10, Priority))
1538         report_fatal_error("invalid .init_array section priority");
1539     }
1540     const auto &DataFrag = cast<MCDataFragment>(Frag);
1541     const SmallVectorImpl<char> &Contents = DataFrag.getContents();
1542     for (const uint8_t *
1543              P = (const uint8_t *)Contents.data(),
1544             *End = (const uint8_t *)Contents.data() + Contents.size();
1545          P != End; ++P) {
1546       if (*P != 0)
1547         report_fatal_error("non-symbolic data in .init_array section");
1548     }
1549     for (const MCFixup &Fixup : DataFrag.getFixups()) {
1550       assert(Fixup.getKind() ==
1551              MCFixup::getKindForSize(is64Bit() ? 8 : 4, false));
1552       const MCExpr *Expr = Fixup.getValue();
1553       auto *SymRef = dyn_cast<MCSymbolRefExpr>(Expr);
1554       if (!SymRef)
1555         report_fatal_error("fixups in .init_array should be symbol references");
1556       const auto &TargetSym = cast<const MCSymbolWasm>(SymRef->getSymbol());
1557       if (TargetSym.getIndex() == InvalidIndex)
1558         report_fatal_error("symbols in .init_array should exist in symbtab");
1559       if (!TargetSym.isFunction())
1560         report_fatal_error("symbols in .init_array should be for functions");
1561       InitFuncs.push_back(
1562           std::make_pair(Priority, TargetSym.getIndex()));
1563     }
1564   }
1565
1566   // Write out the Wasm header.
1567   writeHeader(Asm);
1568
1569   writeTypeSection(Signatures);
1570   writeImportSection(Imports, DataSize, TableElems.size());
1571   writeFunctionSection(Functions);
1572   // Skip the "table" section; we import the table instead.
1573   // Skip the "memory" section; we import the memory instead.
1574   writeEventSection(Events);
1575   writeExportSection(Exports);
1576   writeElemSection(TableElems);
1577   writeDataCountSection();
1578   writeCodeSection(Asm, Layout, Functions);
1579   writeDataSection();
1580   for (auto &CustomSection : CustomSections)
1581     writeCustomSection(CustomSection, Asm, Layout);
1582   writeLinkingMetaDataSection(SymbolInfos, InitFuncs, Comdats);
1583   writeRelocSection(CodeSectionIndex, "CODE", CodeRelocations);
1584   writeRelocSection(DataSectionIndex, "DATA", DataRelocations);
1585   writeCustomRelocSections();
1586   if (ProducersSection)
1587     writeCustomSection(*ProducersSection, Asm, Layout);
1588   if (TargetFeaturesSection)
1589     writeCustomSection(*TargetFeaturesSection, Asm, Layout);
1590
1591   // TODO: Translate the .comment section to the output.
1592   return W.OS.tell() - StartOffset;
1593 }
1594
1595 std::unique_ptr<MCObjectWriter>
1596 llvm::createWasmObjectWriter(std::unique_ptr<MCWasmObjectTargetWriter> MOTW,
1597                              raw_pwrite_stream &OS) {
1598   return std::make_unique<WasmObjectWriter>(std::move(MOTW), OS);
1599 }