COFF: Use a DenseSet instead of a map to atomic_flag to track which archive members have been read.

[ruiu]

LGTM

lld/COFF/InputFiles.h

 //===- InputFiles.h ---------------------------------------------*- C++ -*-===//
 //
 //                             The LLVM Linker
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 
 #ifndef LLD_COFF_INPUT_FILES_H
 #define LLD_COFF_INPUT_FILES_H
 
 #include "lld/Core/LLVM.h"
 #include "llvm/ADT/ArrayRef.h"
+#include "llvm/ADT/DenseSet.h"
 #include "llvm/IR/LLVMContext.h"
 #include "llvm/LTO/legacy/LTOModule.h"
 #include "llvm/Object/Archive.h"
 #include "llvm/Object/COFF.h"
 #include "llvm/Support/StringSaver.h"
 #include <memory>
 #include <mutex>
 #include <set>
public:
   // Returns an input file for a given symbol. A null pointer is returned if we
   // have already returned the same input file. (So that we don't instantiate
   // the same member more than once.)
   InputFile *getMember(const Archive::Symbol *Sym);
 
 private:
   std::unique_ptr<Archive> File;
   std::string Filename;
-  std::map<uint64_t, std::atomic_flag> Seen;
+  llvm::DenseSet<uint64_t> Seen;
 };
 
 // .obj or .o file. This may be a member of an archive file.
 class ObjectFile : public InputFile {
 public:
   explicit ObjectFile(MemoryBufferRef M) : InputFile(ObjectKind, M) {}
   static bool classof(const InputFile *F) { return F->kind() == ObjectKind; }
   void parse() override;

lld/COFF/InputFiles.cpp

 LLVMContext BitcodeFile::Context;
 
 ArchiveFile::ArchiveFile(MemoryBufferRef M) : InputFile(ArchiveKind, M) {}
 
 void ArchiveFile::parse() {
   // Parse a MemoryBufferRef as an archive file.
   File = check(Archive::create(MB), toString(this));
 
-  // Seen is a map from member files to boolean values. Initially
-  // all members are mapped to false, which indicates all these files
-  // are not read yet.
-  Error Err = Error::success();
-  for (auto &Child : File->children(Err))
-    Seen[Child.getChildOffset()].clear();
-  if (Err)
-    fatal(Err, toString(this));
-
   // Read the symbol table to construct Lazy objects.
   for (const Archive::Symbol &Sym : File->symbols())
     Symtab->addLazy(this, Sym);
 }
 
 // Returns a buffer pointing to a member file containing a given symbol.
-// This function is thread-safe.
 InputFile *ArchiveFile::getMember(const Archive::Symbol *Sym) {
   const Archive::Child &C =
       check(Sym->getMember(),
             "could not get the member for symbol " + Sym->getName());
 
   // Return an empty buffer if we have already returned the same buffer.
-  if (Seen[C.getChildOffset()].test_and_set())
+  if (!Seen.insert(C.getChildOffset()).second)
     return nullptr;
 
   MemoryBufferRef MB =
       check(C.getMemoryBufferRef(),
             "could not get the buffer for the member defining symbol " +
                 Sym->getName());
   if (C.getParent()->isThin() && Driver->Cpio)
     Driver->Cpio->append(relativeToRoot(check(C.getFullName())),