[Option] Store arg strings in a set backed by a BumpPtrAllocator

No functionality change.

This is preparing to move response file parsing into lib/Option so it
can be shared between clang and lld.  This change isn't just a
micro-optimization.  Clang's driver uses a std::set<std::string> to
unique arguments while parsing response files, so this matches that.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@186319 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/Option/ArgList.h

@@ -12,9 +12,10 @@
 
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/ADT/StringRef.h"
+#include "llvm/ADT/StringSet.h"
 #include "llvm/Option/OptSpecifier.h"
 #include "llvm/Option/Option.h"
-#include <list>
+#include "llvm/Support/Allocator.h"
 #include <string>
 #include <vector>
 
@@ -298,7 +299,7 @@ private:
   /// This is mutable since we treat the ArgList as being the list
   /// of Args, and allow routines to add new strings (to have a
   /// convenient place to store the memory) via MakeIndex.
-  mutable std::list<std::string> SynthesizedStrings;
+  mutable StringSet<BumpPtrAllocator> SynthesizedStrings;
 
   /// The number of original input argument strings.
   unsigned NumInputArgStrings;

lib/Option/ArgList.cpp

@@ -323,9 +323,18 @@ InputArgList::~InputArgList() {
 unsigned InputArgList::MakeIndex(StringRef String0) const {
   unsigned Index = ArgStrings.size();
 
+  // If necessary, make a copy so we can null terminate it.
+  std::string NullTerminated;
+  if (String0.back() != '\0') {
+    NullTerminated.append(String0.data(), String0.size());
+    NullTerminated.push_back('\0');
+    String0 = StringRef(&NullTerminated[0], NullTerminated.size());
+  }
+
   // Tuck away so we have a reliable const char *.
-  SynthesizedStrings.push_back(String0);
-  ArgStrings.push_back(SynthesizedStrings.back().c_str());
+  String0 = SynthesizedStrings.GetOrCreateValue(String0).getKey();
+  assert(String0.back() == '\0');
+  ArgStrings.push_back(String0.data());
 
   return Index;
 }