[NFC][COFF] Use COFFSection.MCSection when writeSection

[HaohaiWen]

Summary

Each COFFSection bind MCSection when created. No need to iterate
throught MCAssembler when writeSection.

[skan]

Add a NFC or NFCI in the title?

[skan]

LGTM with minor suggestion

llvm/lib/MC/WinCOFFObjectWriter.cpp

619

From line 620, we can see that Sec is never a NULL pointer. So the [] operator should have no side effect here.

llvm/lib/MC/WinCOFFObjectWriter.cpp

public:
   void WriteFileHeader(const COFF::header &Header);
   void WriteSymbol(const COFFSymbol &S);
   void WriteAuxiliarySymbols(const COFFSymbol::AuxiliarySymbols &S);
   void writeSectionHeaders();
   void WriteRelocation(const COFF::relocation &R);
   uint32_t writeSectionContents(MCAssembler &Asm, const MCAsmLayout &Layout,
                                 const MCSection &MCSec);
   void writeSection(MCAssembler &Asm, const MCAsmLayout &Layout,
-                    const COFFSection &Sec, const MCSection &MCSec);
+                    const COFFSection &Sec);
 
   // MCObjectWriter interface implementation.
 
   void executePostLayoutBinding(MCAssembler &Asm,
                                 const MCAsmLayout &Layout) override;
 
   bool isSymbolRefDifferenceFullyResolvedImpl(const MCAssembler &Asm,
                                               const MCSymbol &SymA,
uint32_t WinCOFFObjectWriter::writeSectionContents(MCAssembler &Asm,
   // JamCRC is specified but it aligns with the expected output.
   JamCRC JC(/*Init=*/0);
   JC.update(ArrayRef(reinterpret_cast<uint8_t *>(Buf.data()), Buf.size()));
   return JC.getCRC();
 }
 
 void WinCOFFObjectWriter::writeSection(MCAssembler &Asm,
                                        const MCAsmLayout &Layout,
-                                       const COFFSection &Sec,
-                                       const MCSection &MCSec) {
+                                       const COFFSection &Sec) {
   if (Sec.Number == -1)
     return;
 
   // Write the section contents.
   if (Sec.Header.PointerToRawData != 0) {
     assert(W.OS.tell() == Sec.Header.PointerToRawData &&
            "Section::PointerToRawData is insane!");
 
-    uint32_t CRC = writeSectionContents(Asm, Layout, MCSec);
+    uint32_t CRC = writeSectionContents(Asm, Layout, *Sec.MCSection);
 
     // Update the section definition auxiliary symbol to record the CRC.
-    COFFSection *Sec = SectionMap[&MCSec];
-    COFFSymbol::AuxiliarySymbols &AuxSyms = Sec->Symbol->Aux;
+    COFFSymbol::AuxiliarySymbols &AuxSyms = Sec.Symbol->Aux;
     assert(AuxSyms.size() == 1 && AuxSyms[0].AuxType == ATSectionDefinition);
     AuxSymbol &SecDef = AuxSyms[0];
     SecDef.Aux.SectionDefinition.CheckSum = CRC;
   }
 
   // Write relocations for this section.
   if (Sec.Relocations.empty()) {
     assert(Sec.Header.PointerToRelocations == 0 &&
if (Asm.isIncrementalLinkerCompatible()) {
     // Have deterministic output if /INCREMENTAL isn't needed. Also matches GNU.
     Header.TimeDateStamp = 0;
   }
 
   // Write it all to disk...
   WriteFileHeader(Header);
   writeSectionHeaders();
 
-  // Write section contents.
+#ifndef NDEBUG
   sections::iterator I = Sections.begin();
   sections::iterator IE = Sections.end();
   MCAssembler::iterator J = Asm.begin();
   MCAssembler::iterator JE = Asm.end();
   for (; I != IE && J != JE; ++I, ++J)
-    writeSection(Asm, Layout, **I, *J);
+    assert((**I).MCSection == &*J && "Wrong bound MCSection");
+#endif
+
+  // Write section contents.
+  for (std::unique_ptr<COFFSection> &Sec : Sections)
+    writeSection(Asm, Layout, *Sec);
 
   assert(W.OS.tell() == Header.PointerToSymbolTable &&
          "Header::PointerToSymbolTable is insane!");
 
   // Write a symbol table.
   for (auto &Symbol : Symbols)
     if (Symbol->getIndex() != -1)
       WriteSymbol(*Symbol);