[WebAssembly] Create section start symbols automatically for all sections

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.

Differential Revision: https://reviews.llvm.org/D46561

llvm-svn: 332005

llvm/lib/MC/MCContext.cpp

@@ -515,15 +515,18 @@ MCSectionWasm *MCContext::getWasmSection(const Twine &Section, SectionKind Kind,
 
   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);
+  cast<MCSymbolWasm>(Begin)->setType(wasm::WASM_SYMBOL_TYPE_SECTION);
 
   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;
 }
 

llvm/lib/MC/MCObjectFileInfo.cpp

@@ -861,27 +861,29 @@ void MCObjectFileInfo::initCOFFMCObjectFileInfo(const Triple &T) {
 }
 
 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());

llvm/lib/MC/MCWasmStreamer.cpp

@@ -66,6 +66,7 @@ void MCWasmStreamer::ChangeSection(MCSection *Section,
     Asm.registerSymbol(*Grp);
 
   this->MCObjectStreamer::ChangeSection(Section, Subsection);
+  Asm.registerSymbol(*Section->getBeginSymbol());
 }
 
 void MCWasmStreamer::EmitWeakReference(MCSymbol *Alias,

llvm/lib/MC/WasmObjectWriter.cpp

@@ -1204,9 +1204,9 @@ void WasmObjectWriter::writeObject(MCAssembler &Asm,
       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);
     }
@@ -1405,16 +1405,27 @@ void WasmObjectWriter::writeObject(MCAssembler &Asm,
       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) {