xref: /freebsd/contrib/llvm-project/llvm/lib/MC/WasmObjectWriter.cpp (revision f976241773df2260e6170317080761d1c5814fe5)
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 
262   void startSection(SectionBookkeeping &Section, unsigned SectionId);
263   void startCustomSection(SectionBookkeeping &Section, StringRef Name);
264   void endSection(SectionBookkeeping &Section);
265 
266 public:
267   WasmObjectWriter(std::unique_ptr<MCWasmObjectTargetWriter> MOTW,
268                    raw_pwrite_stream &OS)
269       : W(OS, support::little), TargetObjectWriter(std::move(MOTW)) {}
270 
271 private:
272   void reset() override {
273     CodeRelocations.clear();
274     DataRelocations.clear();
275     TypeIndices.clear();
276     WasmIndices.clear();
277     GOTIndices.clear();
278     TableIndices.clear();
279     DataLocations.clear();
280     CustomSections.clear();
281     ProducersSection.reset();
282     TargetFeaturesSection.reset();
283     CustomSectionsRelocations.clear();
284     SignatureIndices.clear();
285     Signatures.clear();
286     DataSegments.clear();
287     SectionFunctions.clear();
288     NumFunctionImports = 0;
289     NumGlobalImports = 0;
290     MCObjectWriter::reset();
291   }
292 
293   void writeHeader(const MCAssembler &Asm);
294 
295   void recordRelocation(MCAssembler &Asm, const MCAsmLayout &Layout,
296                         const MCFragment *Fragment, const MCFixup &Fixup,
297                         MCValue Target, uint64_t &FixedValue) override;
298 
299   void executePostLayoutBinding(MCAssembler &Asm,
300                                 const MCAsmLayout &Layout) override;
301 
302   uint64_t writeObject(MCAssembler &Asm, const MCAsmLayout &Layout) override;
303 
304   void writeString(const StringRef Str) {
305     encodeULEB128(Str.size(), W.OS);
306     W.OS << Str;
307   }
308 
309   void writeValueType(wasm::ValType Ty) { W.OS << static_cast<char>(Ty); }
310 
311   void writeTypeSection(ArrayRef<WasmSignature> Signatures);
312   void writeImportSection(ArrayRef<wasm::WasmImport> Imports, uint32_t DataSize,
313                           uint32_t NumElements);
314   void writeFunctionSection(ArrayRef<WasmFunction> Functions);
315   void writeExportSection(ArrayRef<wasm::WasmExport> Exports);
316   void writeElemSection(ArrayRef<uint32_t> TableElems);
317   void writeDataCountSection();
318   void writeCodeSection(const MCAssembler &Asm, const MCAsmLayout &Layout,
319                         ArrayRef<WasmFunction> Functions);
320   void writeDataSection();
321   void writeEventSection(ArrayRef<wasm::WasmEventType> Events);
322   void writeRelocSection(uint32_t SectionIndex, StringRef Name,
323                          std::vector<WasmRelocationEntry> &Relocations);
324   void writeLinkingMetaDataSection(
325       ArrayRef<wasm::WasmSymbolInfo> SymbolInfos,
326       ArrayRef<std::pair<uint16_t, uint32_t>> InitFuncs,
327       const std::map<StringRef, std::vector<WasmComdatEntry>> &Comdats);
328   void writeCustomSection(WasmCustomSection &CustomSection,
329                           const MCAssembler &Asm, const MCAsmLayout &Layout);
330   void writeCustomRelocSections();
331   void
332   updateCustomSectionRelocations(const SmallVector<WasmFunction, 4> &Functions,
333                                  const MCAsmLayout &Layout);
334 
335   uint32_t getProvisionalValue(const WasmRelocationEntry &RelEntry);
336   void applyRelocations(ArrayRef<WasmRelocationEntry> Relocations,
337                         uint64_t ContentsOffset);
338 
339   uint32_t getRelocationIndexValue(const WasmRelocationEntry &RelEntry);
340   uint32_t getFunctionType(const MCSymbolWasm &Symbol);
341   uint32_t getEventType(const MCSymbolWasm &Symbol);
342   void registerFunctionType(const MCSymbolWasm &Symbol);
343   void registerEventType(const MCSymbolWasm &Symbol);
344 };
345 
346 } // end anonymous namespace
347 
348 // Write out a section header and a patchable section size field.
349 void WasmObjectWriter::startSection(SectionBookkeeping &Section,
350                                     unsigned SectionId) {
351   LLVM_DEBUG(dbgs() << "startSection " << SectionId << "\n");
352   W.OS << char(SectionId);
353 
354   Section.SizeOffset = W.OS.tell();
355 
356   // The section size. We don't know the size yet, so reserve enough space
357   // for any 32-bit value; we'll patch it later.
358   encodeULEB128(0, W.OS, 5);
359 
360   // The position where the section starts, for measuring its size.
361   Section.ContentsOffset = W.OS.tell();
362   Section.PayloadOffset = W.OS.tell();
363   Section.Index = SectionCount++;
364 }
365 
366 void WasmObjectWriter::startCustomSection(SectionBookkeeping &Section,
367                                           StringRef Name) {
368   LLVM_DEBUG(dbgs() << "startCustomSection " << Name << "\n");
369   startSection(Section, wasm::WASM_SEC_CUSTOM);
370 
371   // The position where the section header ends, for measuring its size.
372   Section.PayloadOffset = W.OS.tell();
373 
374   // Custom sections in wasm also have a string identifier.
375   writeString(Name);
376 
377   // The position where the custom section starts.
378   Section.ContentsOffset = W.OS.tell();
379 }
380 
381 // Now that the section is complete and we know how big it is, patch up the
382 // section size field at the start of the section.
383 void WasmObjectWriter::endSection(SectionBookkeeping &Section) {
384   uint64_t Size = W.OS.tell();
385   // /dev/null doesn't support seek/tell and can report offset of 0.
386   // Simply skip this patching in that case.
387   if (!Size)
388     return;
389 
390   Size -= Section.PayloadOffset;
391   if (uint32_t(Size) != Size)
392     report_fatal_error("section size does not fit in a uint32_t");
393 
394   LLVM_DEBUG(dbgs() << "endSection size=" << Size << "\n");
395 
396   // Write the final section size to the payload_len field, which follows
397   // the section id byte.
398   writePatchableLEB(static_cast<raw_pwrite_stream &>(W.OS), Size,
399                     Section.SizeOffset);
400 }
401 
402 // Emit the Wasm header.
403 void WasmObjectWriter::writeHeader(const MCAssembler &Asm) {
404   W.OS.write(wasm::WasmMagic, sizeof(wasm::WasmMagic));
405   W.write<uint32_t>(wasm::WasmVersion);
406 }
407 
408 void WasmObjectWriter::executePostLayoutBinding(MCAssembler &Asm,
409                                                 const MCAsmLayout &Layout) {
410   // Build a map of sections to the function that defines them, for use
411   // in recordRelocation.
412   for (const MCSymbol &S : Asm.symbols()) {
413     const auto &WS = static_cast<const MCSymbolWasm &>(S);
414     if (WS.isDefined() && WS.isFunction() && !WS.isVariable()) {
415       const auto &Sec = static_cast<const MCSectionWasm &>(S.getSection());
416       auto Pair = SectionFunctions.insert(std::make_pair(&Sec, &S));
417       if (!Pair.second)
418         report_fatal_error("section already has a defining function: " +
419                            Sec.getSectionName());
420     }
421   }
422 }
423 
424 void WasmObjectWriter::recordRelocation(MCAssembler &Asm,
425                                         const MCAsmLayout &Layout,
426                                         const MCFragment *Fragment,
427                                         const MCFixup &Fixup, MCValue Target,
428                                         uint64_t &FixedValue) {
429   MCAsmBackend &Backend = Asm.getBackend();
430   bool IsPCRel = Backend.getFixupKindInfo(Fixup.getKind()).Flags &
431                  MCFixupKindInfo::FKF_IsPCRel;
432   const auto &FixupSection = cast<MCSectionWasm>(*Fragment->getParent());
433   uint64_t C = Target.getConstant();
434   uint64_t FixupOffset = Layout.getFragmentOffset(Fragment) + Fixup.getOffset();
435   MCContext &Ctx = Asm.getContext();
436 
437   // The .init_array isn't translated as data, so don't do relocations in it.
438   if (FixupSection.getSectionName().startswith(".init_array"))
439     return;
440 
441   if (const MCSymbolRefExpr *RefB = Target.getSymB()) {
442     assert(RefB->getKind() == MCSymbolRefExpr::VK_None &&
443            "Should not have constructed this");
444 
445     // Let A, B and C being the components of Target and R be the location of
446     // the fixup. If the fixup is not pcrel, we want to compute (A - B + C).
447     // If it is pcrel, we want to compute (A - B + C - R).
448 
449     // In general, Wasm has no relocations for -B. It can only represent (A + C)
450     // or (A + C - R). If B = R + K and the relocation is not pcrel, we can
451     // replace B to implement it: (A - R - K + C)
452     if (IsPCRel) {
453       Ctx.reportError(
454           Fixup.getLoc(),
455           "No relocation available to represent this relative expression");
456       return;
457     }
458 
459     const auto &SymB = cast<MCSymbolWasm>(RefB->getSymbol());
460 
461     if (SymB.isUndefined()) {
462       Ctx.reportError(Fixup.getLoc(),
463                       Twine("symbol '") + SymB.getName() +
464                           "' can not be undefined in a subtraction expression");
465       return;
466     }
467 
468     assert(!SymB.isAbsolute() && "Should have been folded");
469     const MCSection &SecB = SymB.getSection();
470     if (&SecB != &FixupSection) {
471       Ctx.reportError(Fixup.getLoc(),
472                       "Cannot represent a difference across sections");
473       return;
474     }
475 
476     uint64_t SymBOffset = Layout.getSymbolOffset(SymB);
477     uint64_t K = SymBOffset - FixupOffset;
478     IsPCRel = true;
479     C -= K;
480   }
481 
482   // We either rejected the fixup or folded B into C at this point.
483   const MCSymbolRefExpr *RefA = Target.getSymA();
484   const auto *SymA = RefA ? cast<MCSymbolWasm>(&RefA->getSymbol()) : nullptr;
485 
486   if (SymA && SymA->isVariable()) {
487     const MCExpr *Expr = SymA->getVariableValue();
488     const auto *Inner = cast<MCSymbolRefExpr>(Expr);
489     if (Inner->getKind() == MCSymbolRefExpr::VK_WEAKREF)
490       llvm_unreachable("weakref used in reloc not yet implemented");
491   }
492 
493   // Put any constant offset in an addend. Offsets can be negative, and
494   // LLVM expects wrapping, in contrast to wasm's immediates which can't
495   // be negative and don't wrap.
496   FixedValue = 0;
497 
498   unsigned Type = TargetObjectWriter->getRelocType(Target, Fixup);
499   assert(!IsPCRel);
500   assert(SymA);
501 
502   // Absolute offset within a section or a function.
503   // Currently only supported for for metadata sections.
504   // See: test/MC/WebAssembly/blockaddress.ll
505   if (Type == wasm::R_WASM_FUNCTION_OFFSET_I32 ||
506       Type == wasm::R_WASM_SECTION_OFFSET_I32) {
507     if (!FixupSection.getKind().isMetadata())
508       report_fatal_error("relocations for function or section offsets are "
509                          "only supported in metadata sections");
510 
511     const MCSymbol *SectionSymbol = nullptr;
512     const MCSection &SecA = SymA->getSection();
513     if (SecA.getKind().isText())
514       SectionSymbol = SectionFunctions.find(&SecA)->second;
515     else
516       SectionSymbol = SecA.getBeginSymbol();
517     if (!SectionSymbol)
518       report_fatal_error("section symbol is required for relocation");
519 
520     C += Layout.getSymbolOffset(*SymA);
521     SymA = cast<MCSymbolWasm>(SectionSymbol);
522   }
523 
524   // Relocation other than R_WASM_TYPE_INDEX_LEB are required to be
525   // against a named symbol.
526   if (Type != wasm::R_WASM_TYPE_INDEX_LEB) {
527     if (SymA->getName().empty())
528       report_fatal_error("relocations against un-named temporaries are not yet "
529                          "supported by wasm");
530 
531     SymA->setUsedInReloc();
532   }
533 
534   if (RefA->getKind() == MCSymbolRefExpr::VK_GOT)
535     SymA->setUsedInGOT();
536 
537   WasmRelocationEntry Rec(FixupOffset, SymA, C, Type, &FixupSection);
538   LLVM_DEBUG(dbgs() << "WasmReloc: " << Rec << "\n");
539 
540   if (FixupSection.isWasmData()) {
541     DataRelocations.push_back(Rec);
542   } else if (FixupSection.getKind().isText()) {
543     CodeRelocations.push_back(Rec);
544   } else if (FixupSection.getKind().isMetadata()) {
545     CustomSectionsRelocations[&FixupSection].push_back(Rec);
546   } else {
547     llvm_unreachable("unexpected section type");
548   }
549 }
550 
551 static const MCSymbolWasm *resolveSymbol(const MCSymbolWasm &Symbol) {
552   const MCSymbolWasm* Ret = &Symbol;
553   while (Ret->isVariable()) {
554     const MCExpr *Expr = Ret->getVariableValue();
555     auto *Inner = cast<MCSymbolRefExpr>(Expr);
556     Ret = cast<MCSymbolWasm>(&Inner->getSymbol());
557   }
558   return Ret;
559 }
560 
561 // Compute a value to write into the code at the location covered
562 // by RelEntry. This value isn't used by the static linker; it just serves
563 // to make the object format more readable and more likely to be directly
564 // useable.
565 uint32_t
566 WasmObjectWriter::getProvisionalValue(const WasmRelocationEntry &RelEntry) {
567   if (RelEntry.Type == wasm::R_WASM_GLOBAL_INDEX_LEB && !RelEntry.Symbol->isGlobal()) {
568     assert(GOTIndices.count(RelEntry.Symbol) > 0 && "symbol not found in GOT index space");
569     return GOTIndices[RelEntry.Symbol];
570   }
571 
572   switch (RelEntry.Type) {
573   case wasm::R_WASM_TABLE_INDEX_REL_SLEB:
574   case wasm::R_WASM_TABLE_INDEX_SLEB:
575   case wasm::R_WASM_TABLE_INDEX_I32: {
576     // Provisional value is table address of the resolved symbol itself
577     const MCSymbolWasm *Sym = resolveSymbol(*RelEntry.Symbol);
578     assert(Sym->isFunction());
579     return TableIndices[Sym];
580   }
581   case wasm::R_WASM_TYPE_INDEX_LEB:
582     // Provisional value is same as the index
583     return getRelocationIndexValue(RelEntry);
584   case wasm::R_WASM_FUNCTION_INDEX_LEB:
585   case wasm::R_WASM_GLOBAL_INDEX_LEB:
586   case wasm::R_WASM_EVENT_INDEX_LEB:
587     // Provisional value is function/global/event Wasm index
588     assert(WasmIndices.count(RelEntry.Symbol) > 0 && "symbol not found in wasm index space");
589     return WasmIndices[RelEntry.Symbol];
590   case wasm::R_WASM_FUNCTION_OFFSET_I32:
591   case wasm::R_WASM_SECTION_OFFSET_I32: {
592     const auto &Section =
593         static_cast<const MCSectionWasm &>(RelEntry.Symbol->getSection());
594     return Section.getSectionOffset() + RelEntry.Addend;
595   }
596   case wasm::R_WASM_MEMORY_ADDR_LEB:
597   case wasm::R_WASM_MEMORY_ADDR_I32:
598   case wasm::R_WASM_MEMORY_ADDR_REL_SLEB:
599   case wasm::R_WASM_MEMORY_ADDR_SLEB: {
600     // Provisional value is address of the global
601     const MCSymbolWasm *Sym = resolveSymbol(*RelEntry.Symbol);
602     // For undefined symbols, use zero
603     if (!Sym->isDefined())
604       return 0;
605     const wasm::WasmDataReference &Ref = DataLocations[Sym];
606     const WasmDataSegment &Segment = DataSegments[Ref.Segment];
607     // Ignore overflow. LLVM allows address arithmetic to silently wrap.
608     return Segment.Offset + Ref.Offset + RelEntry.Addend;
609   }
610   default:
611     llvm_unreachable("invalid relocation type");
612   }
613 }
614 
615 static void addData(SmallVectorImpl<char> &DataBytes,
616                     MCSectionWasm &DataSection) {
617   LLVM_DEBUG(errs() << "addData: " << DataSection.getSectionName() << "\n");
618 
619   DataBytes.resize(alignTo(DataBytes.size(), DataSection.getAlignment()));
620 
621   for (const MCFragment &Frag : DataSection) {
622     if (Frag.hasInstructions())
623       report_fatal_error("only data supported in data sections");
624 
625     if (auto *Align = dyn_cast<MCAlignFragment>(&Frag)) {
626       if (Align->getValueSize() != 1)
627         report_fatal_error("only byte values supported for alignment");
628       // If nops are requested, use zeros, as this is the data section.
629       uint8_t Value = Align->hasEmitNops() ? 0 : Align->getValue();
630       uint64_t Size =
631           std::min<uint64_t>(alignTo(DataBytes.size(), Align->getAlignment()),
632                              DataBytes.size() + Align->getMaxBytesToEmit());
633       DataBytes.resize(Size, Value);
634     } else if (auto *Fill = dyn_cast<MCFillFragment>(&Frag)) {
635       int64_t NumValues;
636       if (!Fill->getNumValues().evaluateAsAbsolute(NumValues))
637         llvm_unreachable("The fill should be an assembler constant");
638       DataBytes.insert(DataBytes.end(), Fill->getValueSize() * NumValues,
639                        Fill->getValue());
640     } else if (auto *LEB = dyn_cast<MCLEBFragment>(&Frag)) {
641       const SmallVectorImpl<char> &Contents = LEB->getContents();
642       DataBytes.insert(DataBytes.end(), Contents.begin(), Contents.end());
643     } else {
644       const auto &DataFrag = cast<MCDataFragment>(Frag);
645       const SmallVectorImpl<char> &Contents = DataFrag.getContents();
646       DataBytes.insert(DataBytes.end(), Contents.begin(), Contents.end());
647     }
648   }
649 
650   LLVM_DEBUG(dbgs() << "addData -> " << DataBytes.size() << "\n");
651 }
652 
653 uint32_t
654 WasmObjectWriter::getRelocationIndexValue(const WasmRelocationEntry &RelEntry) {
655   if (RelEntry.Type == wasm::R_WASM_TYPE_INDEX_LEB) {
656     if (!TypeIndices.count(RelEntry.Symbol))
657       report_fatal_error("symbol not found in type index space: " +
658                          RelEntry.Symbol->getName());
659     return TypeIndices[RelEntry.Symbol];
660   }
661 
662   return RelEntry.Symbol->getIndex();
663 }
664 
665 // Apply the portions of the relocation records that we can handle ourselves
666 // directly.
667 void WasmObjectWriter::applyRelocations(
668     ArrayRef<WasmRelocationEntry> Relocations, uint64_t ContentsOffset) {
669   auto &Stream = static_cast<raw_pwrite_stream &>(W.OS);
670   for (const WasmRelocationEntry &RelEntry : Relocations) {
671     uint64_t Offset = ContentsOffset +
672                       RelEntry.FixupSection->getSectionOffset() +
673                       RelEntry.Offset;
674 
675     LLVM_DEBUG(dbgs() << "applyRelocation: " << RelEntry << "\n");
676     uint32_t Value = getProvisionalValue(RelEntry);
677 
678     switch (RelEntry.Type) {
679     case wasm::R_WASM_FUNCTION_INDEX_LEB:
680     case wasm::R_WASM_TYPE_INDEX_LEB:
681     case wasm::R_WASM_GLOBAL_INDEX_LEB:
682     case wasm::R_WASM_MEMORY_ADDR_LEB:
683     case wasm::R_WASM_EVENT_INDEX_LEB:
684       writePatchableLEB(Stream, Value, Offset);
685       break;
686     case wasm::R_WASM_TABLE_INDEX_I32:
687     case wasm::R_WASM_MEMORY_ADDR_I32:
688     case wasm::R_WASM_FUNCTION_OFFSET_I32:
689     case wasm::R_WASM_SECTION_OFFSET_I32:
690       writeI32(Stream, Value, Offset);
691       break;
692     case wasm::R_WASM_TABLE_INDEX_SLEB:
693     case wasm::R_WASM_TABLE_INDEX_REL_SLEB:
694     case wasm::R_WASM_MEMORY_ADDR_SLEB:
695     case wasm::R_WASM_MEMORY_ADDR_REL_SLEB:
696       writePatchableSLEB(Stream, Value, Offset);
697       break;
698     default:
699       llvm_unreachable("invalid relocation type");
700     }
701   }
702 }
703 
704 void WasmObjectWriter::writeTypeSection(ArrayRef<WasmSignature> Signatures) {
705   if (Signatures.empty())
706     return;
707 
708   SectionBookkeeping Section;
709   startSection(Section, wasm::WASM_SEC_TYPE);
710 
711   encodeULEB128(Signatures.size(), W.OS);
712 
713   for (const WasmSignature &Sig : Signatures) {
714     W.OS << char(wasm::WASM_TYPE_FUNC);
715     encodeULEB128(Sig.Params.size(), W.OS);
716     for (wasm::ValType Ty : Sig.Params)
717       writeValueType(Ty);
718     encodeULEB128(Sig.Returns.size(), W.OS);
719     for (wasm::ValType Ty : Sig.Returns)
720       writeValueType(Ty);
721   }
722 
723   endSection(Section);
724 }
725 
726 void WasmObjectWriter::writeImportSection(ArrayRef<wasm::WasmImport> Imports,
727                                           uint32_t DataSize,
728                                           uint32_t NumElements) {
729   if (Imports.empty())
730     return;
731 
732   uint32_t NumPages = (DataSize + wasm::WasmPageSize - 1) / wasm::WasmPageSize;
733 
734   SectionBookkeeping Section;
735   startSection(Section, wasm::WASM_SEC_IMPORT);
736 
737   encodeULEB128(Imports.size(), W.OS);
738   for (const wasm::WasmImport &Import : Imports) {
739     writeString(Import.Module);
740     writeString(Import.Field);
741     W.OS << char(Import.Kind);
742 
743     switch (Import.Kind) {
744     case wasm::WASM_EXTERNAL_FUNCTION:
745       encodeULEB128(Import.SigIndex, W.OS);
746       break;
747     case wasm::WASM_EXTERNAL_GLOBAL:
748       W.OS << char(Import.Global.Type);
749       W.OS << char(Import.Global.Mutable ? 1 : 0);
750       break;
751     case wasm::WASM_EXTERNAL_MEMORY:
752       encodeULEB128(0, W.OS);        // flags
753       encodeULEB128(NumPages, W.OS); // initial
754       break;
755     case wasm::WASM_EXTERNAL_TABLE:
756       W.OS << char(Import.Table.ElemType);
757       encodeULEB128(0, W.OS);           // flags
758       encodeULEB128(NumElements, W.OS); // initial
759       break;
760     case wasm::WASM_EXTERNAL_EVENT:
761       encodeULEB128(Import.Event.Attribute, W.OS);
762       encodeULEB128(Import.Event.SigIndex, W.OS);
763       break;
764     default:
765       llvm_unreachable("unsupported import kind");
766     }
767   }
768 
769   endSection(Section);
770 }
771 
772 void WasmObjectWriter::writeFunctionSection(ArrayRef<WasmFunction> Functions) {
773   if (Functions.empty())
774     return;
775 
776   SectionBookkeeping Section;
777   startSection(Section, wasm::WASM_SEC_FUNCTION);
778 
779   encodeULEB128(Functions.size(), W.OS);
780   for (const WasmFunction &Func : Functions)
781     encodeULEB128(Func.SigIndex, W.OS);
782 
783   endSection(Section);
784 }
785 
786 void WasmObjectWriter::writeEventSection(ArrayRef<wasm::WasmEventType> Events) {
787   if (Events.empty())
788     return;
789 
790   SectionBookkeeping Section;
791   startSection(Section, wasm::WASM_SEC_EVENT);
792 
793   encodeULEB128(Events.size(), W.OS);
794   for (const wasm::WasmEventType &Event : Events) {
795     encodeULEB128(Event.Attribute, W.OS);
796     encodeULEB128(Event.SigIndex, W.OS);
797   }
798 
799   endSection(Section);
800 }
801 
802 void WasmObjectWriter::writeExportSection(ArrayRef<wasm::WasmExport> Exports) {
803   if (Exports.empty())
804     return;
805 
806   SectionBookkeeping Section;
807   startSection(Section, wasm::WASM_SEC_EXPORT);
808 
809   encodeULEB128(Exports.size(), W.OS);
810   for (const wasm::WasmExport &Export : Exports) {
811     writeString(Export.Name);
812     W.OS << char(Export.Kind);
813     encodeULEB128(Export.Index, W.OS);
814   }
815 
816   endSection(Section);
817 }
818 
819 void WasmObjectWriter::writeElemSection(ArrayRef<uint32_t> TableElems) {
820   if (TableElems.empty())
821     return;
822 
823   SectionBookkeeping Section;
824   startSection(Section, wasm::WASM_SEC_ELEM);
825 
826   encodeULEB128(1, W.OS); // number of "segments"
827   encodeULEB128(0, W.OS); // the table index
828 
829   // init expr for starting offset
830   W.OS << char(wasm::WASM_OPCODE_I32_CONST);
831   encodeSLEB128(InitialTableOffset, W.OS);
832   W.OS << char(wasm::WASM_OPCODE_END);
833 
834   encodeULEB128(TableElems.size(), W.OS);
835   for (uint32_t Elem : TableElems)
836     encodeULEB128(Elem, W.OS);
837 
838   endSection(Section);
839 }
840 
841 void WasmObjectWriter::writeDataCountSection() {
842   if (DataSegments.empty())
843     return;
844 
845   SectionBookkeeping Section;
846   startSection(Section, wasm::WASM_SEC_DATACOUNT);
847   encodeULEB128(DataSegments.size(), W.OS);
848   endSection(Section);
849 }
850 
851 void WasmObjectWriter::writeCodeSection(const MCAssembler &Asm,
852                                         const MCAsmLayout &Layout,
853                                         ArrayRef<WasmFunction> Functions) {
854   if (Functions.empty())
855     return;
856 
857   SectionBookkeeping Section;
858   startSection(Section, wasm::WASM_SEC_CODE);
859   CodeSectionIndex = Section.Index;
860 
861   encodeULEB128(Functions.size(), W.OS);
862 
863   for (const WasmFunction &Func : Functions) {
864     auto &FuncSection = static_cast<MCSectionWasm &>(Func.Sym->getSection());
865 
866     int64_t Size = 0;
867     if (!Func.Sym->getSize()->evaluateAsAbsolute(Size, Layout))
868       report_fatal_error(".size expression must be evaluatable");
869 
870     encodeULEB128(Size, W.OS);
871     FuncSection.setSectionOffset(W.OS.tell() - Section.ContentsOffset);
872     Asm.writeSectionData(W.OS, &FuncSection, Layout);
873   }
874 
875   // Apply fixups.
876   applyRelocations(CodeRelocations, Section.ContentsOffset);
877 
878   endSection(Section);
879 }
880 
881 void WasmObjectWriter::writeDataSection() {
882   if (DataSegments.empty())
883     return;
884 
885   SectionBookkeeping Section;
886   startSection(Section, wasm::WASM_SEC_DATA);
887   DataSectionIndex = Section.Index;
888 
889   encodeULEB128(DataSegments.size(), W.OS); // count
890 
891   for (const WasmDataSegment &Segment : DataSegments) {
892     encodeULEB128(Segment.InitFlags, W.OS); // flags
893     if (Segment.InitFlags & wasm::WASM_SEGMENT_HAS_MEMINDEX)
894       encodeULEB128(0, W.OS); // memory index
895     if ((Segment.InitFlags & wasm::WASM_SEGMENT_IS_PASSIVE) == 0) {
896       W.OS << char(wasm::WASM_OPCODE_I32_CONST);
897       encodeSLEB128(Segment.Offset, W.OS); // offset
898       W.OS << char(wasm::WASM_OPCODE_END);
899     }
900     encodeULEB128(Segment.Data.size(), W.OS); // size
901     Segment.Section->setSectionOffset(W.OS.tell() - Section.ContentsOffset);
902     W.OS << Segment.Data; // data
903   }
904 
905   // Apply fixups.
906   applyRelocations(DataRelocations, Section.ContentsOffset);
907 
908   endSection(Section);
909 }
910 
911 void WasmObjectWriter::writeRelocSection(
912     uint32_t SectionIndex, StringRef Name,
913     std::vector<WasmRelocationEntry> &Relocs) {
914   // See: https://github.com/WebAssembly/tool-conventions/blob/master/Linking.md
915   // for descriptions of the reloc sections.
916 
917   if (Relocs.empty())
918     return;
919 
920   // First, ensure the relocations are sorted in offset order.  In general they
921   // should already be sorted since `recordRelocation` is called in offset
922   // order, but for the code section we combine many MC sections into single
923   // wasm section, and this order is determined by the order of Asm.Symbols()
924   // not the sections order.
925   llvm::stable_sort(
926       Relocs, [](const WasmRelocationEntry &A, const WasmRelocationEntry &B) {
927         return (A.Offset + A.FixupSection->getSectionOffset()) <
928                (B.Offset + B.FixupSection->getSectionOffset());
929       });
930 
931   SectionBookkeeping Section;
932   startCustomSection(Section, std::string("reloc.") + Name.str());
933 
934   encodeULEB128(SectionIndex, W.OS);
935   encodeULEB128(Relocs.size(), W.OS);
936   for (const WasmRelocationEntry &RelEntry : Relocs) {
937     uint64_t Offset =
938         RelEntry.Offset + RelEntry.FixupSection->getSectionOffset();
939     uint32_t Index = getRelocationIndexValue(RelEntry);
940 
941     W.OS << char(RelEntry.Type);
942     encodeULEB128(Offset, W.OS);
943     encodeULEB128(Index, W.OS);
944     if (RelEntry.hasAddend())
945       encodeSLEB128(RelEntry.Addend, W.OS);
946   }
947 
948   endSection(Section);
949 }
950 
951 void WasmObjectWriter::writeCustomRelocSections() {
952   for (const auto &Sec : CustomSections) {
953     auto &Relocations = CustomSectionsRelocations[Sec.Section];
954     writeRelocSection(Sec.OutputIndex, Sec.Name, Relocations);
955   }
956 }
957 
958 void WasmObjectWriter::writeLinkingMetaDataSection(
959     ArrayRef<wasm::WasmSymbolInfo> SymbolInfos,
960     ArrayRef<std::pair<uint16_t, uint32_t>> InitFuncs,
961     const std::map<StringRef, std::vector<WasmComdatEntry>> &Comdats) {
962   SectionBookkeeping Section;
963   startCustomSection(Section, "linking");
964   encodeULEB128(wasm::WasmMetadataVersion, W.OS);
965 
966   SectionBookkeeping SubSection;
967   if (SymbolInfos.size() != 0) {
968     startSection(SubSection, wasm::WASM_SYMBOL_TABLE);
969     encodeULEB128(SymbolInfos.size(), W.OS);
970     for (const wasm::WasmSymbolInfo &Sym : SymbolInfos) {
971       encodeULEB128(Sym.Kind, W.OS);
972       encodeULEB128(Sym.Flags, W.OS);
973       switch (Sym.Kind) {
974       case wasm::WASM_SYMBOL_TYPE_FUNCTION:
975       case wasm::WASM_SYMBOL_TYPE_GLOBAL:
976       case wasm::WASM_SYMBOL_TYPE_EVENT:
977         encodeULEB128(Sym.ElementIndex, W.OS);
978         if ((Sym.Flags & wasm::WASM_SYMBOL_UNDEFINED) == 0 ||
979             (Sym.Flags & wasm::WASM_SYMBOL_EXPLICIT_NAME) != 0)
980           writeString(Sym.Name);
981         break;
982       case wasm::WASM_SYMBOL_TYPE_DATA:
983         writeString(Sym.Name);
984         if ((Sym.Flags & wasm::WASM_SYMBOL_UNDEFINED) == 0) {
985           encodeULEB128(Sym.DataRef.Segment, W.OS);
986           encodeULEB128(Sym.DataRef.Offset, W.OS);
987           encodeULEB128(Sym.DataRef.Size, W.OS);
988         }
989         break;
990       case wasm::WASM_SYMBOL_TYPE_SECTION: {
991         const uint32_t SectionIndex =
992             CustomSections[Sym.ElementIndex].OutputIndex;
993         encodeULEB128(SectionIndex, W.OS);
994         break;
995       }
996       default:
997         llvm_unreachable("unexpected kind");
998       }
999     }
1000     endSection(SubSection);
1001   }
1002 
1003   if (DataSegments.size()) {
1004     startSection(SubSection, wasm::WASM_SEGMENT_INFO);
1005     encodeULEB128(DataSegments.size(), W.OS);
1006     for (const WasmDataSegment &Segment : DataSegments) {
1007       writeString(Segment.Name);
1008       encodeULEB128(Segment.Alignment, W.OS);
1009       encodeULEB128(Segment.LinkerFlags, W.OS);
1010     }
1011     endSection(SubSection);
1012   }
1013 
1014   if (!InitFuncs.empty()) {
1015     startSection(SubSection, wasm::WASM_INIT_FUNCS);
1016     encodeULEB128(InitFuncs.size(), W.OS);
1017     for (auto &StartFunc : InitFuncs) {
1018       encodeULEB128(StartFunc.first, W.OS);  // priority
1019       encodeULEB128(StartFunc.second, W.OS); // function index
1020     }
1021     endSection(SubSection);
1022   }
1023 
1024   if (Comdats.size()) {
1025     startSection(SubSection, wasm::WASM_COMDAT_INFO);
1026     encodeULEB128(Comdats.size(), W.OS);
1027     for (const auto &C : Comdats) {
1028       writeString(C.first);
1029       encodeULEB128(0, W.OS); // flags for future use
1030       encodeULEB128(C.second.size(), W.OS);
1031       for (const WasmComdatEntry &Entry : C.second) {
1032         encodeULEB128(Entry.Kind, W.OS);
1033         encodeULEB128(Entry.Index, W.OS);
1034       }
1035     }
1036     endSection(SubSection);
1037   }
1038 
1039   endSection(Section);
1040 }
1041 
1042 void WasmObjectWriter::writeCustomSection(WasmCustomSection &CustomSection,
1043                                           const MCAssembler &Asm,
1044                                           const MCAsmLayout &Layout) {
1045   SectionBookkeeping Section;
1046   auto *Sec = CustomSection.Section;
1047   startCustomSection(Section, CustomSection.Name);
1048 
1049   Sec->setSectionOffset(W.OS.tell() - Section.ContentsOffset);
1050   Asm.writeSectionData(W.OS, Sec, Layout);
1051 
1052   CustomSection.OutputContentsOffset = Section.ContentsOffset;
1053   CustomSection.OutputIndex = Section.Index;
1054 
1055   endSection(Section);
1056 
1057   // Apply fixups.
1058   auto &Relocations = CustomSectionsRelocations[CustomSection.Section];
1059   applyRelocations(Relocations, CustomSection.OutputContentsOffset);
1060 }
1061 
1062 uint32_t WasmObjectWriter::getFunctionType(const MCSymbolWasm &Symbol) {
1063   assert(Symbol.isFunction());
1064   assert(TypeIndices.count(&Symbol));
1065   return TypeIndices[&Symbol];
1066 }
1067 
1068 uint32_t WasmObjectWriter::getEventType(const MCSymbolWasm &Symbol) {
1069   assert(Symbol.isEvent());
1070   assert(TypeIndices.count(&Symbol));
1071   return TypeIndices[&Symbol];
1072 }
1073 
1074 void WasmObjectWriter::registerFunctionType(const MCSymbolWasm &Symbol) {
1075   assert(Symbol.isFunction());
1076 
1077   WasmSignature S;
1078   const MCSymbolWasm *ResolvedSym = resolveSymbol(Symbol);
1079   if (auto *Sig = ResolvedSym->getSignature()) {
1080     S.Returns = Sig->Returns;
1081     S.Params = Sig->Params;
1082   }
1083 
1084   auto Pair = SignatureIndices.insert(std::make_pair(S, Signatures.size()));
1085   if (Pair.second)
1086     Signatures.push_back(S);
1087   TypeIndices[&Symbol] = Pair.first->second;
1088 
1089   LLVM_DEBUG(dbgs() << "registerFunctionType: " << Symbol
1090                     << " new:" << Pair.second << "\n");
1091   LLVM_DEBUG(dbgs() << "  -> type index: " << Pair.first->second << "\n");
1092 }
1093 
1094 void WasmObjectWriter::registerEventType(const MCSymbolWasm &Symbol) {
1095   assert(Symbol.isEvent());
1096 
1097   // TODO Currently we don't generate imported exceptions, but if we do, we
1098   // should have a way of infering types of imported exceptions.
1099   WasmSignature S;
1100   if (auto *Sig = Symbol.getSignature()) {
1101     S.Returns = Sig->Returns;
1102     S.Params = Sig->Params;
1103   }
1104 
1105   auto Pair = SignatureIndices.insert(std::make_pair(S, Signatures.size()));
1106   if (Pair.second)
1107     Signatures.push_back(S);
1108   TypeIndices[&Symbol] = Pair.first->second;
1109 
1110   LLVM_DEBUG(dbgs() << "registerEventType: " << Symbol << " new:" << Pair.second
1111                     << "\n");
1112   LLVM_DEBUG(dbgs() << "  -> type index: " << Pair.first->second << "\n");
1113 }
1114 
1115 static bool isInSymtab(const MCSymbolWasm &Sym) {
1116   if (Sym.isUsedInReloc())
1117     return true;
1118 
1119   if (Sym.isComdat() && !Sym.isDefined())
1120     return false;
1121 
1122   if (Sym.isTemporary() && Sym.getName().empty())
1123     return false;
1124 
1125   if (Sym.isTemporary() && Sym.isData() && !Sym.getSize())
1126     return false;
1127 
1128   if (Sym.isSection())
1129     return false;
1130 
1131   return true;
1132 }
1133 
1134 uint64_t WasmObjectWriter::writeObject(MCAssembler &Asm,
1135                                        const MCAsmLayout &Layout) {
1136   uint64_t StartOffset = W.OS.tell();
1137 
1138   LLVM_DEBUG(dbgs() << "WasmObjectWriter::writeObject\n");
1139 
1140   // Collect information from the available symbols.
1141   SmallVector<WasmFunction, 4> Functions;
1142   SmallVector<uint32_t, 4> TableElems;
1143   SmallVector<wasm::WasmImport, 4> Imports;
1144   SmallVector<wasm::WasmExport, 4> Exports;
1145   SmallVector<wasm::WasmEventType, 1> Events;
1146   SmallVector<wasm::WasmSymbolInfo, 4> SymbolInfos;
1147   SmallVector<std::pair<uint16_t, uint32_t>, 2> InitFuncs;
1148   std::map<StringRef, std::vector<WasmComdatEntry>> Comdats;
1149   uint32_t DataSize = 0;
1150 
1151   // For now, always emit the memory import, since loads and stores are not
1152   // valid without it. In the future, we could perhaps be more clever and omit
1153   // it if there are no loads or stores.
1154   wasm::WasmImport MemImport;
1155   MemImport.Module = "env";
1156   MemImport.Field = "__linear_memory";
1157   MemImport.Kind = wasm::WASM_EXTERNAL_MEMORY;
1158   Imports.push_back(MemImport);
1159 
1160   // For now, always emit the table section, since indirect calls are not
1161   // valid without it. In the future, we could perhaps be more clever and omit
1162   // it if there are no indirect calls.
1163   wasm::WasmImport TableImport;
1164   TableImport.Module = "env";
1165   TableImport.Field = "__indirect_function_table";
1166   TableImport.Kind = wasm::WASM_EXTERNAL_TABLE;
1167   TableImport.Table.ElemType = wasm::WASM_TYPE_FUNCREF;
1168   Imports.push_back(TableImport);
1169 
1170   // Populate SignatureIndices, and Imports and WasmIndices for undefined
1171   // symbols.  This must be done before populating WasmIndices for defined
1172   // symbols.
1173   for (const MCSymbol &S : Asm.symbols()) {
1174     const auto &WS = static_cast<const MCSymbolWasm &>(S);
1175 
1176     // Register types for all functions, including those with private linkage
1177     // (because wasm always needs a type signature).
1178     if (WS.isFunction())
1179       registerFunctionType(WS);
1180 
1181     if (WS.isEvent())
1182       registerEventType(WS);
1183 
1184     if (WS.isTemporary())
1185       continue;
1186 
1187     // If the symbol is not defined in this translation unit, import it.
1188     if (!WS.isDefined() && !WS.isComdat()) {
1189       if (WS.isFunction()) {
1190         wasm::WasmImport Import;
1191         Import.Module = WS.getImportModule();
1192         Import.Field = WS.getImportName();
1193         Import.Kind = wasm::WASM_EXTERNAL_FUNCTION;
1194         Import.SigIndex = getFunctionType(WS);
1195         Imports.push_back(Import);
1196         assert(WasmIndices.count(&WS) == 0);
1197         WasmIndices[&WS] = NumFunctionImports++;
1198       } else if (WS.isGlobal()) {
1199         if (WS.isWeak())
1200           report_fatal_error("undefined global symbol cannot be weak");
1201 
1202         wasm::WasmImport Import;
1203         Import.Field = WS.getImportName();
1204         Import.Kind = wasm::WASM_EXTERNAL_GLOBAL;
1205         Import.Module = WS.getImportModule();
1206         Import.Global = WS.getGlobalType();
1207         Imports.push_back(Import);
1208         assert(WasmIndices.count(&WS) == 0);
1209         WasmIndices[&WS] = NumGlobalImports++;
1210       } else if (WS.isEvent()) {
1211         if (WS.isWeak())
1212           report_fatal_error("undefined event symbol cannot be weak");
1213 
1214         wasm::WasmImport Import;
1215         Import.Module = WS.getImportModule();
1216         Import.Field = WS.getImportName();
1217         Import.Kind = wasm::WASM_EXTERNAL_EVENT;
1218         Import.Event.Attribute = wasm::WASM_EVENT_ATTRIBUTE_EXCEPTION;
1219         Import.Event.SigIndex = getEventType(WS);
1220         Imports.push_back(Import);
1221         assert(WasmIndices.count(&WS) == 0);
1222         WasmIndices[&WS] = NumEventImports++;
1223       }
1224     }
1225   }
1226 
1227   // Add imports for GOT globals
1228   for (const MCSymbol &S : Asm.symbols()) {
1229     const auto &WS = static_cast<const MCSymbolWasm &>(S);
1230     if (WS.isUsedInGOT()) {
1231       wasm::WasmImport Import;
1232       if (WS.isFunction())
1233         Import.Module = "GOT.func";
1234       else
1235         Import.Module = "GOT.mem";
1236       Import.Field = WS.getName();
1237       Import.Kind = wasm::WASM_EXTERNAL_GLOBAL;
1238       Import.Global = {wasm::WASM_TYPE_I32, true};
1239       Imports.push_back(Import);
1240       assert(GOTIndices.count(&WS) == 0);
1241       GOTIndices[&WS] = NumGlobalImports++;
1242     }
1243   }
1244 
1245   // Populate DataSegments and CustomSections, which must be done before
1246   // populating DataLocations.
1247   for (MCSection &Sec : Asm) {
1248     auto &Section = static_cast<MCSectionWasm &>(Sec);
1249     StringRef SectionName = Section.getSectionName();
1250 
1251     // .init_array sections are handled specially elsewhere.
1252     if (SectionName.startswith(".init_array"))
1253       continue;
1254 
1255     // Code is handled separately
1256     if (Section.getKind().isText())
1257       continue;
1258 
1259     if (Section.isWasmData()) {
1260       uint32_t SegmentIndex = DataSegments.size();
1261       DataSize = alignTo(DataSize, Section.getAlignment());
1262       DataSegments.emplace_back();
1263       WasmDataSegment &Segment = DataSegments.back();
1264       Segment.Name = SectionName;
1265       Segment.InitFlags =
1266           Section.getPassive() ? (uint32_t)wasm::WASM_SEGMENT_IS_PASSIVE : 0;
1267       Segment.Offset = DataSize;
1268       Segment.Section = &Section;
1269       addData(Segment.Data, Section);
1270       Segment.Alignment = Log2_32(Section.getAlignment());
1271       Segment.LinkerFlags = 0;
1272       DataSize += Segment.Data.size();
1273       Section.setSegmentIndex(SegmentIndex);
1274 
1275       if (const MCSymbolWasm *C = Section.getGroup()) {
1276         Comdats[C->getName()].emplace_back(
1277             WasmComdatEntry{wasm::WASM_COMDAT_DATA, SegmentIndex});
1278       }
1279     } else {
1280       // Create custom sections
1281       assert(Sec.getKind().isMetadata());
1282 
1283       StringRef Name = SectionName;
1284 
1285       // For user-defined custom sections, strip the prefix
1286       if (Name.startswith(".custom_section."))
1287         Name = Name.substr(strlen(".custom_section."));
1288 
1289       MCSymbol *Begin = Sec.getBeginSymbol();
1290       if (Begin) {
1291         WasmIndices[cast<MCSymbolWasm>(Begin)] = CustomSections.size();
1292         if (SectionName != Begin->getName())
1293           report_fatal_error("section name and begin symbol should match: " +
1294                              Twine(SectionName));
1295       }
1296 
1297       // Separate out the producers and target features sections
1298       if (Name == "producers") {
1299         ProducersSection = llvm::make_unique<WasmCustomSection>(Name, &Section);
1300         continue;
1301       }
1302       if (Name == "target_features") {
1303         TargetFeaturesSection =
1304             llvm::make_unique<WasmCustomSection>(Name, &Section);
1305         continue;
1306       }
1307 
1308       CustomSections.emplace_back(Name, &Section);
1309     }
1310   }
1311 
1312   // Populate WasmIndices and DataLocations for defined symbols.
1313   for (const MCSymbol &S : Asm.symbols()) {
1314     // Ignore unnamed temporary symbols, which aren't ever exported, imported,
1315     // or used in relocations.
1316     if (S.isTemporary() && S.getName().empty())
1317       continue;
1318 
1319     const auto &WS = static_cast<const MCSymbolWasm &>(S);
1320     LLVM_DEBUG(
1321         dbgs() << "MCSymbol: " << toString(WS.getType()) << " '" << S << "'"
1322                << " isDefined=" << S.isDefined() << " isExternal="
1323                << S.isExternal() << " isTemporary=" << S.isTemporary()
1324                << " isWeak=" << WS.isWeak() << " isHidden=" << WS.isHidden()
1325                << " isVariable=" << WS.isVariable() << "\n");
1326 
1327     if (WS.isVariable())
1328       continue;
1329     if (WS.isComdat() && !WS.isDefined())
1330       continue;
1331 
1332     if (WS.isFunction()) {
1333       unsigned Index;
1334       if (WS.isDefined()) {
1335         if (WS.getOffset() != 0)
1336           report_fatal_error(
1337               "function sections must contain one function each");
1338 
1339         if (WS.getSize() == nullptr)
1340           report_fatal_error(
1341               "function symbols must have a size set with .size");
1342 
1343         // A definition. Write out the function body.
1344         Index = NumFunctionImports + Functions.size();
1345         WasmFunction Func;
1346         Func.SigIndex = getFunctionType(WS);
1347         Func.Sym = &WS;
1348         WasmIndices[&WS] = Index;
1349         Functions.push_back(Func);
1350 
1351         auto &Section = static_cast<MCSectionWasm &>(WS.getSection());
1352         if (const MCSymbolWasm *C = Section.getGroup()) {
1353           Comdats[C->getName()].emplace_back(
1354               WasmComdatEntry{wasm::WASM_COMDAT_FUNCTION, Index});
1355         }
1356       } else {
1357         // An import; the index was assigned above.
1358         Index = WasmIndices.find(&WS)->second;
1359       }
1360 
1361       LLVM_DEBUG(dbgs() << "  -> function index: " << Index << "\n");
1362 
1363     } else if (WS.isData()) {
1364       if (!isInSymtab(WS))
1365         continue;
1366 
1367       if (!WS.isDefined()) {
1368         LLVM_DEBUG(dbgs() << "  -> segment index: -1"
1369                           << "\n");
1370         continue;
1371       }
1372 
1373       if (!WS.getSize())
1374         report_fatal_error("data symbols must have a size set with .size: " +
1375                            WS.getName());
1376 
1377       int64_t Size = 0;
1378       if (!WS.getSize()->evaluateAsAbsolute(Size, Layout))
1379         report_fatal_error(".size expression must be evaluatable");
1380 
1381       auto &DataSection = static_cast<MCSectionWasm &>(WS.getSection());
1382       assert(DataSection.isWasmData());
1383 
1384       // For each data symbol, export it in the symtab as a reference to the
1385       // corresponding Wasm data segment.
1386       wasm::WasmDataReference Ref = wasm::WasmDataReference{
1387           DataSection.getSegmentIndex(),
1388           static_cast<uint32_t>(Layout.getSymbolOffset(WS)),
1389           static_cast<uint32_t>(Size)};
1390       DataLocations[&WS] = Ref;
1391       LLVM_DEBUG(dbgs() << "  -> segment index: " << Ref.Segment << "\n");
1392 
1393     } else if (WS.isGlobal()) {
1394       // A "true" Wasm global (currently just __stack_pointer)
1395       if (WS.isDefined())
1396         report_fatal_error("don't yet support defined globals");
1397 
1398       // An import; the index was assigned above
1399       LLVM_DEBUG(dbgs() << "  -> global index: "
1400                         << WasmIndices.find(&WS)->second << "\n");
1401 
1402     } else if (WS.isEvent()) {
1403       // C++ exception symbol (__cpp_exception)
1404       unsigned Index;
1405       if (WS.isDefined()) {
1406         Index = NumEventImports + Events.size();
1407         wasm::WasmEventType Event;
1408         Event.SigIndex = getEventType(WS);
1409         Event.Attribute = wasm::WASM_EVENT_ATTRIBUTE_EXCEPTION;
1410         assert(WasmIndices.count(&WS) == 0);
1411         WasmIndices[&WS] = Index;
1412         Events.push_back(Event);
1413       } else {
1414         // An import; the index was assigned above.
1415         assert(WasmIndices.count(&WS) > 0);
1416       }
1417       LLVM_DEBUG(dbgs() << "  -> event index: " << WasmIndices.find(&WS)->second
1418                         << "\n");
1419 
1420     } else {
1421       assert(WS.isSection());
1422     }
1423   }
1424 
1425   // Populate WasmIndices and DataLocations for aliased symbols.  We need to
1426   // process these in a separate pass because we need to have processed the
1427   // target of the alias before the alias itself and the symbols are not
1428   // necessarily ordered in this way.
1429   for (const MCSymbol &S : Asm.symbols()) {
1430     if (!S.isVariable())
1431       continue;
1432 
1433     assert(S.isDefined());
1434 
1435     // Find the target symbol of this weak alias and export that index
1436     const auto &WS = static_cast<const MCSymbolWasm &>(S);
1437     const MCSymbolWasm *ResolvedSym = resolveSymbol(WS);
1438     LLVM_DEBUG(dbgs() << WS.getName() << ": weak alias of '" << *ResolvedSym
1439                       << "'\n");
1440 
1441     if (ResolvedSym->isFunction()) {
1442       assert(WasmIndices.count(ResolvedSym) > 0);
1443       uint32_t WasmIndex = WasmIndices.find(ResolvedSym)->second;
1444       assert(WasmIndices.count(&WS) == 0);
1445       WasmIndices[&WS] = WasmIndex;
1446       LLVM_DEBUG(dbgs() << "  -> index:" << WasmIndex << "\n");
1447     } else if (ResolvedSym->isData()) {
1448       assert(DataLocations.count(ResolvedSym) > 0);
1449       const wasm::WasmDataReference &Ref =
1450           DataLocations.find(ResolvedSym)->second;
1451       DataLocations[&WS] = Ref;
1452       LLVM_DEBUG(dbgs() << "  -> index:" << Ref.Segment << "\n");
1453     } else {
1454       report_fatal_error("don't yet support global/event aliases");
1455     }
1456   }
1457 
1458   // Finally, populate the symbol table itself, in its "natural" order.
1459   for (const MCSymbol &S : Asm.symbols()) {
1460     const auto &WS = static_cast<const MCSymbolWasm &>(S);
1461     if (!isInSymtab(WS)) {
1462       WS.setIndex(InvalidIndex);
1463       continue;
1464     }
1465     LLVM_DEBUG(dbgs() << "adding to symtab: " << WS << "\n");
1466 
1467     uint32_t Flags = 0;
1468     if (WS.isWeak())
1469       Flags |= wasm::WASM_SYMBOL_BINDING_WEAK;
1470     if (WS.isHidden())
1471       Flags |= wasm::WASM_SYMBOL_VISIBILITY_HIDDEN;
1472     if (!WS.isExternal() && WS.isDefined())
1473       Flags |= wasm::WASM_SYMBOL_BINDING_LOCAL;
1474     if (WS.isUndefined())
1475       Flags |= wasm::WASM_SYMBOL_UNDEFINED;
1476     if (WS.isExported())
1477       Flags |= wasm::WASM_SYMBOL_EXPORTED;
1478     if (WS.getName() != WS.getImportName())
1479       Flags |= wasm::WASM_SYMBOL_EXPLICIT_NAME;
1480 
1481     wasm::WasmSymbolInfo Info;
1482     Info.Name = WS.getName();
1483     Info.Kind = WS.getType();
1484     Info.Flags = Flags;
1485     if (!WS.isData()) {
1486       assert(WasmIndices.count(&WS) > 0);
1487       Info.ElementIndex = WasmIndices.find(&WS)->second;
1488     } else if (WS.isDefined()) {
1489       assert(DataLocations.count(&WS) > 0);
1490       Info.DataRef = DataLocations.find(&WS)->second;
1491     }
1492     WS.setIndex(SymbolInfos.size());
1493     SymbolInfos.emplace_back(Info);
1494   }
1495 
1496   {
1497     auto HandleReloc = [&](const WasmRelocationEntry &Rel) {
1498       // Functions referenced by a relocation need to put in the table.  This is
1499       // purely to make the object file's provisional values readable, and is
1500       // ignored by the linker, which re-calculates the relocations itself.
1501       if (Rel.Type != wasm::R_WASM_TABLE_INDEX_I32 &&
1502           Rel.Type != wasm::R_WASM_TABLE_INDEX_SLEB)
1503         return;
1504       assert(Rel.Symbol->isFunction());
1505       const MCSymbolWasm &WS = *resolveSymbol(*Rel.Symbol);
1506       uint32_t FunctionIndex = WasmIndices.find(&WS)->second;
1507       uint32_t TableIndex = TableElems.size() + InitialTableOffset;
1508       if (TableIndices.try_emplace(&WS, TableIndex).second) {
1509         LLVM_DEBUG(dbgs() << "  -> adding " << WS.getName()
1510                           << " to table: " << TableIndex << "\n");
1511         TableElems.push_back(FunctionIndex);
1512         registerFunctionType(WS);
1513       }
1514     };
1515 
1516     for (const WasmRelocationEntry &RelEntry : CodeRelocations)
1517       HandleReloc(RelEntry);
1518     for (const WasmRelocationEntry &RelEntry : DataRelocations)
1519       HandleReloc(RelEntry);
1520   }
1521 
1522   // Translate .init_array section contents into start functions.
1523   for (const MCSection &S : Asm) {
1524     const auto &WS = static_cast<const MCSectionWasm &>(S);
1525     if (WS.getSectionName().startswith(".fini_array"))
1526       report_fatal_error(".fini_array sections are unsupported");
1527     if (!WS.getSectionName().startswith(".init_array"))
1528       continue;
1529     if (WS.getFragmentList().empty())
1530       continue;
1531 
1532     // init_array is expected to contain a single non-empty data fragment
1533     if (WS.getFragmentList().size() != 3)
1534       report_fatal_error("only one .init_array section fragment supported");
1535 
1536     auto IT = WS.begin();
1537     const MCFragment &EmptyFrag = *IT;
1538     if (EmptyFrag.getKind() != MCFragment::FT_Data)
1539       report_fatal_error(".init_array section should be aligned");
1540 
1541     IT = std::next(IT);
1542     const MCFragment &AlignFrag = *IT;
1543     if (AlignFrag.getKind() != MCFragment::FT_Align)
1544       report_fatal_error(".init_array section should be aligned");
1545     if (cast<MCAlignFragment>(AlignFrag).getAlignment() != (is64Bit() ? 8 : 4))
1546       report_fatal_error(".init_array section should be aligned for pointers");
1547 
1548     const MCFragment &Frag = *std::next(IT);
1549     if (Frag.hasInstructions() || Frag.getKind() != MCFragment::FT_Data)
1550       report_fatal_error("only data supported in .init_array section");
1551 
1552     uint16_t Priority = UINT16_MAX;
1553     unsigned PrefixLength = strlen(".init_array");
1554     if (WS.getSectionName().size() > PrefixLength) {
1555       if (WS.getSectionName()[PrefixLength] != '.')
1556         report_fatal_error(
1557             ".init_array section priority should start with '.'");
1558       if (WS.getSectionName()
1559               .substr(PrefixLength + 1)
1560               .getAsInteger(10, Priority))
1561         report_fatal_error("invalid .init_array section priority");
1562     }
1563     const auto &DataFrag = cast<MCDataFragment>(Frag);
1564     const SmallVectorImpl<char> &Contents = DataFrag.getContents();
1565     for (const uint8_t *
1566              P = (const uint8_t *)Contents.data(),
1567             *End = (const uint8_t *)Contents.data() + Contents.size();
1568          P != End; ++P) {
1569       if (*P != 0)
1570         report_fatal_error("non-symbolic data in .init_array section");
1571     }
1572     for (const MCFixup &Fixup : DataFrag.getFixups()) {
1573       assert(Fixup.getKind() ==
1574              MCFixup::getKindForSize(is64Bit() ? 8 : 4, false));
1575       const MCExpr *Expr = Fixup.getValue();
1576       auto *SymRef = dyn_cast<MCSymbolRefExpr>(Expr);
1577       if (!SymRef)
1578         report_fatal_error("fixups in .init_array should be symbol references");
1579       const auto &TargetSym = cast<const MCSymbolWasm>(SymRef->getSymbol());
1580       if (TargetSym.getIndex() == InvalidIndex)
1581         report_fatal_error("symbols in .init_array should exist in symbtab");
1582       if (!TargetSym.isFunction())
1583         report_fatal_error("symbols in .init_array should be for functions");
1584       InitFuncs.push_back(
1585           std::make_pair(Priority, TargetSym.getIndex()));
1586     }
1587   }
1588 
1589   // Write out the Wasm header.
1590   writeHeader(Asm);
1591 
1592   writeTypeSection(Signatures);
1593   writeImportSection(Imports, DataSize, TableElems.size());
1594   writeFunctionSection(Functions);
1595   // Skip the "table" section; we import the table instead.
1596   // Skip the "memory" section; we import the memory instead.
1597   writeEventSection(Events);
1598   writeExportSection(Exports);
1599   writeElemSection(TableElems);
1600   writeDataCountSection();
1601   writeCodeSection(Asm, Layout, Functions);
1602   writeDataSection();
1603   for (auto &CustomSection : CustomSections)
1604     writeCustomSection(CustomSection, Asm, Layout);
1605   writeLinkingMetaDataSection(SymbolInfos, InitFuncs, Comdats);
1606   writeRelocSection(CodeSectionIndex, "CODE", CodeRelocations);
1607   writeRelocSection(DataSectionIndex, "DATA", DataRelocations);
1608   writeCustomRelocSections();
1609   if (ProducersSection)
1610     writeCustomSection(*ProducersSection, Asm, Layout);
1611   if (TargetFeaturesSection)
1612     writeCustomSection(*TargetFeaturesSection, Asm, Layout);
1613 
1614   // TODO: Translate the .comment section to the output.
1615   return W.OS.tell() - StartOffset;
1616 }
1617 
1618 std::unique_ptr<MCObjectWriter>
1619 llvm::createWasmObjectWriter(std::unique_ptr<MCWasmObjectTargetWriter> MOTW,
1620                              raw_pwrite_stream &OS) {
1621   return llvm::make_unique<WasmObjectWriter>(std::move(MOTW), OS);
1622 }
1623