[WebAssembly] Create section start symbols automatically for all sections

[sbc100]

Summary

These symbols only get included in the output symbols table if
they are used in a relocation.

This behaviour matches more closely the ELF object writer.

[sbc100]

• clang-format

[sbc100]

rebase

[sbc100]

• rebase

lib/MC/MCContext.cpp

MCSectionWasm *MCContext::getWasmSection(const Twine &Section, SectionKind Kind,
   auto IterBool = WasmUniquingMap.insert(
       std::make_pair(WasmSectionKey{Section.str(), Group, UniqueID}, nullptr));
   auto &Entry = *IterBool.first;
   if (!IterBool.second)
     return Entry.second;
 
   StringRef CachedName = Entry.first.SectionName;
 
-  MCSymbol *Begin = nullptr;
-  if (BeginSymName) {
-    Begin = createSymbol(BeginSymName, false, false);
-    cast<MCSymbolWasm>(Begin)->setType(wasm::WASM_SYMBOL_TYPE_SECTION);
-  }
+  MCSymbol *Begin = createSymbol(CachedName, false, false);
 
   MCSectionWasm *Result = new (WasmAllocator.Allocate())
       MCSectionWasm(CachedName, Kind, GroupSym, UniqueID, Begin);
   Entry.second = Result;
+
+  auto *F = new MCDataFragment();
+  Result->getFragmentList().insert(Result->begin(), F);
+  F->setParent(Result);
+  Begin->setFragment(F);
+
   return Result;
 }
 
 MCSubtargetInfo &MCContext::getSubtargetCopy(const MCSubtargetInfo &STI) {
   return *new (MCSubtargetAllocator.Allocate()) MCSubtargetInfo(STI);
 }
 
 //===----------------------------------------------------------------------===//

lib/MC/MCObjectFileInfo.cpp

void MCObjectFileInfo::initCOFFMCObjectFileInfo(const Triple &T) {
 
   StackMapSection = Ctx->getCOFFSection(".llvm_stackmaps",
                                         COFF::IMAGE_SCN_CNT_INITIALIZED_DATA |
                                             COFF::IMAGE_SCN_MEM_READ,
                                         SectionKind::getReadOnly());
 }
 
 void MCObjectFileInfo::initWasmMCObjectFileInfo(const Triple &T) {
-  // TODO: Set the section types and flags.
   TextSection = Ctx->getWasmSection(".text", SectionKind::getText());
   DataSection = Ctx->getWasmSection(".data", SectionKind::getData());
 
-  // TODO: Set the section types and flags.
-  DwarfLineSection = Ctx->getWasmSection(".debug_line", SectionKind::getMetadata(), ".debug_line");
+  DwarfLineSection =
+      Ctx->getWasmSection(".debug_line", SectionKind::getMetadata());
   DwarfLineStrSection =
       Ctx->getWasmSection(".debug_line_str", SectionKind::getMetadata());
-  DwarfStrSection = Ctx->getWasmSection(".debug_str", SectionKind::getMetadata(), ".debug_str");
-  DwarfLocSection = Ctx->getWasmSection(
-      ".debug_loc", SectionKind::getMetadata(), ".debug_loc");
-  DwarfAbbrevSection = Ctx->getWasmSection(
-      ".debug_abbrev", SectionKind::getMetadata(), ".debug_abbrev");
+  DwarfStrSection =
+      Ctx->getWasmSection(".debug_str", SectionKind::getMetadata());
+  DwarfLocSection =
+      Ctx->getWasmSection(".debug_loc", SectionKind::getMetadata());
+  DwarfAbbrevSection =
+      Ctx->getWasmSection(".debug_abbrev", SectionKind::getMetadata());
   DwarfARangesSection = Ctx->getWasmSection(".debug_aranges", SectionKind::getMetadata());
-  DwarfRangesSection = Ctx->getWasmSection(
-      ".debug_ranges", SectionKind::getMetadata(), ".debug_ranges");
-  DwarfMacinfoSection = Ctx->getWasmSection(".debug_macinfo", SectionKind::getMetadata(), ".debug_macinfo");
+  DwarfRangesSection =
+      Ctx->getWasmSection(".debug_ranges", SectionKind::getMetadata());
+  DwarfMacinfoSection =
+      Ctx->getWasmSection(".debug_macinfo", SectionKind::getMetadata());
   DwarfAddrSection = Ctx->getWasmSection(".debug_addr", SectionKind::getMetadata());
   DwarfCUIndexSection = Ctx->getWasmSection(".debug_cu_index", SectionKind::getMetadata());
   DwarfTUIndexSection = Ctx->getWasmSection(".debug_tu_index", SectionKind::getMetadata());
-  DwarfInfoSection = Ctx->getWasmSection(".debug_info", SectionKind::getMetadata(), ".debug_info");
+  DwarfInfoSection =
+      Ctx->getWasmSection(".debug_info", SectionKind::getMetadata());
   DwarfFrameSection = Ctx->getWasmSection(".debug_frame", SectionKind::getMetadata());
   DwarfPubNamesSection = Ctx->getWasmSection(".debug_pubnames", SectionKind::getMetadata());
   DwarfPubTypesSection = Ctx->getWasmSection(".debug_pubtypes", SectionKind::getMetadata());
 
   // TODO: Define more sections.
 }
 
 void MCObjectFileInfo::InitMCObjectFileInfo(const Triple &TheTriple, bool PIC,

lib/MC/MCWasmStreamer.cpp

void MCWasmStreamer::ChangeSection(MCSection *Section,
                                    const MCExpr *Subsection) {
   MCAssembler &Asm = getAssembler();
   auto *SectionWasm = static_cast<const MCSectionWasm *>(Section);
   const MCSymbol *Grp = SectionWasm->getGroup();
   if (Grp)
     Asm.registerSymbol(*Grp);
 
   this->MCObjectStreamer::ChangeSection(Section, Subsection);
+  Asm.registerSymbol(*Section->getBeginSymbol());
 }
 
 void MCWasmStreamer::EmitWeakReference(MCSymbol *Alias,
                                        const MCSymbol *Symbol) {
   getAssembler().registerSymbol(*Symbol);
   const MCExpr *Value = MCSymbolRefExpr::create(
       Symbol, MCSymbolRefExpr::VK_WEAKREF, getContext());
   Alias->setVariableValue(Value);

lib/MC/WasmObjectWriter.cpp

if (Section.isWasmData()) {
 
       // For user-defined custom sections, strip the prefix
       if (Name.startswith(".custom_section."))
         Name = Name.substr(strlen(".custom_section."));
 
       MCSymbol* Begin = Sec.getBeginSymbol();
       if (Begin) {
         WasmIndices[cast<MCSymbolWasm>(Begin)] = CustomSections.size();
-        if (Name != Begin->getName())
+        if (SectionName != Begin->getName())
           report_fatal_error("section name and begin symbol should match: " +
-                             Twine(Name));
+                             Twine(SectionName));
       }
       CustomSections.emplace_back(Name, &Section);
     }
   }
 
   // Populate WasmIndices and DataLocations for defined symbols.
   for (const MCSymbol &S : Asm.symbols()) {
     // Ignore unnamed temporary symbols, which aren't ever exported, imported,
for (const MCSymbol &S : Asm.symbols()) {
     }
   }
 
   // Finally, populate the symbol table itself, in its "natural" order.
   for (const MCSymbol &S : Asm.symbols()) {
     const auto &WS = static_cast<const MCSymbolWasm &>(S);
     if (!isInSymtab(WS)) {
       WS.setIndex(INVALID_INDEX);
+      DEBUG(dbgs() << "skipping symtab: " << WS << "\n");
       continue;
     }
     DEBUG(dbgs() << "adding to symtab: " << WS << "\n");
 
     uint32_t Flags = 0;
     if (WS.isWeak())
       Flags |= wasm::WASM_SYMBOL_BINDING_WEAK;
     if (WS.isHidden())
void WasmObjectWriter::writeObject(MCAssembler &Asm,
   for (const MCSection &S : Asm) {
     const auto &WS = static_cast<const MCSectionWasm &>(S);
     if (WS.getSectionName().startswith(".fini_array"))
       report_fatal_error(".fini_array sections are unsupported");
     if (!WS.getSectionName().startswith(".init_array"))
       continue;
     if (WS.getFragmentList().empty())
       continue;
-    if (WS.getFragmentList().size() != 2)
+
+    // init_array is expected to contain a single non-empty data fragment
+    if (WS.getFragmentList().size() != 3)
       report_fatal_error("only one .init_array section fragment supported");
-    const MCFragment &AlignFrag = *WS.begin();
+
+    auto IT = WS.begin();
+    const MCFragment &EmptyFrag = *IT;
+    if (EmptyFrag.getKind() != MCFragment::FT_Data)
+      report_fatal_error(".init_array section should be aligned");
+
+    IT = std::next(IT);
+    const MCFragment &AlignFrag = *IT;
     if (AlignFrag.getKind() != MCFragment::FT_Align)
       report_fatal_error(".init_array section should be aligned");
     if (cast<MCAlignFragment>(AlignFrag).getAlignment() != (is64Bit() ? 8 : 4))
       report_fatal_error(".init_array section should be aligned for pointers");
-    const MCFragment &Frag = *std::next(WS.begin());
+
+    const MCFragment &Frag = *std::next(IT);
     if (Frag.hasInstructions() || Frag.getKind() != MCFragment::FT_Data)
       report_fatal_error("only data supported in .init_array section");
+
     uint16_t Priority = UINT16_MAX;
     unsigned PrefixLength = strlen(".init_array");
     if (WS.getSectionName().size() > PrefixLength) {
       if (WS.getSectionName()[PrefixLength] != '.')
         report_fatal_error(".init_array section priority should start with '.'");
       if (WS.getSectionName()
               .substr(PrefixLength + 1)
               .getAsInteger(10, Priority))
for (const MCFixup &Fixup : DataFrag.getFixups()) {
       const MCExpr *Expr = Fixup.getValue();
       auto *Sym = dyn_cast<MCSymbolRefExpr>(Expr);
       if (!Sym)
         report_fatal_error("fixups in .init_array should be symbol references");
       if (Sym->getKind() != MCSymbolRefExpr::VK_WebAssembly_FUNCTION)
         report_fatal_error("symbols in .init_array should be for functions");
       if (Sym->getSymbol().getIndex() == INVALID_INDEX)
         report_fatal_error("symbols in .init_array should exist in symbtab");
-      InitFuncs.push_back(std::make_pair(Priority, Sym->getSymbol().getIndex()));
+      InitFuncs.push_back(
+          std::make_pair(Priority, Sym->getSymbol().getIndex()));
     }
   }
 
   // Write out the Wasm header.
   writeHeader(Asm);
 
   writeTypeSection(FunctionTypes);
   writeImportSection(Imports, DataSize, TableElems.size());