Do not parse the same /export string more than once.

[ruiu]

Summary

Do not parse the same /export string more than once.

[takuto.ikuta]

LGTM.
This attains the same performance improvement with https://reviews.llvm.org/D41591

lld/COFF/Driver.h

 #define LLD_COFF_DRIVER_H
 
 #include "Config.h"
 #include "SymbolTable.h"
 #include "lld/Common/LLVM.h"
 #include "lld/Common/Reproduce.h"
 #include "llvm/ADT/Optional.h"
 #include "llvm/ADT/StringRef.h"
+#include "llvm/ADT/StringSet.h"
 #include "llvm/Object/Archive.h"
 #include "llvm/Object/COFF.h"
 #include "llvm/Option/Arg.h"
 #include "llvm/Option/ArgList.h"
 #include "llvm/Support/TarWriter.h"
 #include <memory>
 #include <set>
 #include <vector>
private:
   void enqueuePath(StringRef Path, bool WholeArchive);
 
   void enqueueTask(std::function<void()> Task);
   bool run();
 
   std::list<std::function<void()>> TaskQueue;
   std::vector<StringRef> FilePaths;
   std::vector<MemoryBufferRef> Resources;
+
+  llvm::StringSet<> DirectivesExports;
 };
 
 // Functions below this line are defined in DriverUtils.cpp.
 
 void printHelp(const char *Argv0);
 
 // For /machine option.
 MachineTypes getMachineType(StringRef Arg);

lld/COFF/Driver.cpp

for (auto *Arg : Args) {
     case OPT_defaultlib:
       if (Optional<StringRef> Path = findLib(Arg->getValue()))
         enqueuePath(*Path, false);
       break;
     case OPT_entry:
       Config->Entry = addUndefined(mangle(Arg->getValue()));
       break;
     case OPT_export: {
+      // If a common header file contains dllexported function
+      // declarations, many object files may end up with having the
+      // same /EXPORT options. In order to save cost of parsing them,
+      // we dedup them first.
+      if (!DirectivesExports.insert(Arg->getValue()).second)
+        break;
+
       Export E = parseExport(Arg->getValue());
       if (Config->Machine == I386 && Config->MinGW) {
         if (!isDecorated(E.Name))
           E.Name = Saver.save("_" + E.Name);
         if (!E.ExtName.empty() && !isDecorated(E.ExtName))
           E.ExtName = Saver.save("_" + E.ExtName);
       }
       E.Directives = true;