COFF: Use a bit in SymbolBody to track which symbols are written to the symbol table.

[pcc]

Summary

Using a set here caused us to take about 1 second longer to write the symbol
table when linking chrome_child.dll. With this I consistently get better
performance on Windows with the new symbol table.

Before r289280 and with r289183 reverted (median of 5 runs): 17.65s
After this change: 17.33s

On Linux things look even better:

Before: 10.700480444s
After: 5.735681610s

[ruiu]

LGTM

lld/COFF/Symbols.h

public:
   const Symbol *symbol() const {
     return const_cast<SymbolBody *>(this)->symbol();
   }
 
 protected:
   friend SymbolTable;
   explicit SymbolBody(Kind K, StringRef N = "")
       : SymbolKind(K), IsExternal(true), IsCOMDAT(false),
-        IsReplaceable(false), Name(N) {}
+        IsReplaceable(false), WrittenToSymtab(false), Name(N) {}
 
   const unsigned SymbolKind : 8;
   unsigned IsExternal : 1;
 
   // This bit is used by the \c DefinedRegular subclass.
   unsigned IsCOMDAT : 1;
 
   // This bit is used by the \c DefinedBitcode subclass.
   unsigned IsReplaceable : 1;
 
+public:
+  // This bit is used by Writer::createSymbolAndStringTable().
+  unsigned WrittenToSymtab : 1;
+
+protected:
   StringRef Name;
 };
 
 // The base class for any defined symbols, including absolute symbols,
 // etc.
 class Defined : public SymbolBody {
 public:
   Defined(Kind K, StringRef N = "") : SymbolBody(K, N) {}

lld/COFF/Writer.cpp

void Writer::createSymbolAndStringTable() {
   // to be written to the string table. First, construct string table.
   for (OutputSection *Sec : OutputSections) {
     StringRef Name = Sec->getName();
     if (Name.size() <= COFF::NameSize)
       continue;
     Sec->setStringTableOff(addEntryToStringTable(Name));
   }
 
-  std::set<SymbolBody *> SeenSymbols;
   for (lld::coff::ObjectFile *File : Symtab->ObjectFiles)
     for (SymbolBody *B : File->getSymbols())
       if (auto *D = dyn_cast<Defined>(B))
-        if (SeenSymbols.insert(D).second)
+        if (!D->WrittenToSymtab) {
+          D->WrittenToSymtab = true;
           if (Optional<coff_symbol16> Sym = createSymbol(D))
             OutputSymtab.push_back(*Sym);
+        }
 
   OutputSection *LastSection = OutputSections.back();
   // We position the symbol table to be adjacent to the end of the last section.
   uint64_t FileOff = LastSection->getFileOff() +
                      alignTo(LastSection->getRawSize(), SectorSize);
   if (!OutputSymtab.empty()) {
     PointerToSymbolTable = FileOff;
     FileOff += OutputSymtab.size() * sizeof(coff_symbol16);