Index: include/clang/Basic/Attr.td
===================================================================
--- include/clang/Basic/Attr.td	(revision 189821)
+++ include/clang/Basic/Attr.td	(working copy)
@@ -382,8 +382,11 @@
 
 def Format : InheritableAttr {
   let Spellings = [GNU<"format">, CXX11<"gnu", "format">];
-  let Args = [StringArgument<"Type">, IntArgument<"FormatIdx">,
+  let Args = [IdentifierArgument<"TypeIdent">, IntArgument<"FormatIdx">,
               IntArgument<"FirstArg">];
+  let AdditionalMembers = [{
+    llvm::StringRef getType() const { return getTypeIdent()->getName(); }
+  }];
 }
 
 def FormatArg : InheritableAttr {
Index: include/clang/Sema/Sema.h
===================================================================
--- include/clang/Sema/Sema.h	(revision 189821)
+++ include/clang/Sema/Sema.h	(working copy)
@@ -1840,9 +1840,9 @@
                                     unsigned AttrSpellingListIndex);
   DLLExportAttr *mergeDLLExportAttr(Decl *D, SourceRange Range,
                                     unsigned AttrSpellingListIndex);
-  FormatAttr *mergeFormatAttr(Decl *D, SourceRange Range, StringRef Format,
-                              int FormatIdx, int FirstArg,
-                              unsigned AttrSpellingListIndex);
+  FormatAttr *mergeFormatAttr(Decl *D, SourceRange Range,
+                              IdentifierInfo *Format, int FormatIdx,
+                              int FirstArg, unsigned AttrSpellingListIndex);
   SectionAttr *mergeSectionAttr(Decl *D, SourceRange Range, StringRef Name,
                                 unsigned AttrSpellingListIndex);
 
Index: lib/Sema/SemaDecl.cpp
===================================================================
--- lib/Sema/SemaDecl.cpp	(revision 189821)
+++ lib/Sema/SemaDecl.cpp	(working copy)
@@ -1964,7 +1964,7 @@
     NewAttr = S.mergeDLLExportAttr(D, ExportA->getRange(),
                                    AttrSpellingListIndex);
   else if (FormatAttr *FA = dyn_cast<FormatAttr>(Attr))
-    NewAttr = S.mergeFormatAttr(D, FA->getRange(), FA->getType(),
+    NewAttr = S.mergeFormatAttr(D, FA->getRange(), FA->getTypeIdent(),
                                 FA->getFormatIdx(), FA->getFirstArg(),
                                 AttrSpellingListIndex);
   else if (SectionAttr *SA = dyn_cast<SectionAttr>(Attr))
@@ -9759,7 +9759,8 @@
             FD->getParamDecl(FormatIdx)->getType()->isObjCObjectPointerType())
           fmt = "NSString";
         FD->addAttr(::new (Context) FormatAttr(FD->getLocation(), Context,
-                                               fmt, FormatIdx+1,
+                                               &Context.Idents.get(fmt),
+                                               FormatIdx+1,
                                                HasVAListArg ? 0 : FormatIdx+2));
       }
     }
@@ -9767,7 +9768,8 @@
                                              HasVAListArg)) {
      if (!FD->getAttr<FormatAttr>())
        FD->addAttr(::new (Context) FormatAttr(FD->getLocation(), Context,
-                                              "scanf", FormatIdx+1,
+                                              &Context.Idents.get("scanf"),
+                                              FormatIdx+1,
                                               HasVAListArg ? 0 : FormatIdx+2));
     }
 
@@ -9807,7 +9809,7 @@
     // target-specific builtins, perhaps?
     if (!FD->getAttr<FormatAttr>())
       FD->addAttr(::new (Context) FormatAttr(FD->getLocation(), Context,
-                                             "printf", 2,
+                                             &Context.Idents.get("printf"), 2,
                                              Name->isStr("vasprintf") ? 0 : 3));
   }
 
Index: lib/Sema/SemaDeclAttr.cpp
===================================================================
--- lib/Sema/SemaDeclAttr.cpp	(revision 189821)
+++ lib/Sema/SemaDeclAttr.cpp	(working copy)
@@ -3119,8 +3119,9 @@
                               Attr.getAttributeSpellingListIndex()));
 }
 
-FormatAttr *Sema::mergeFormatAttr(Decl *D, SourceRange Range, StringRef Format,
-                                  int FormatIdx, int FirstArg,
+FormatAttr *Sema::mergeFormatAttr(Decl *D, SourceRange Range,
+                                  IdentifierInfo *Format, int FormatIdx,
+                                  int FirstArg,
                                   unsigned AttrSpellingListIndex) {
   // Check whether we already have an equivalent format attribute.
   for (specific_attr_iterator<FormatAttr>
@@ -3128,7 +3129,7 @@
          e = D->specific_attr_end<FormatAttr>();
        i != e ; ++i) {
     FormatAttr *f = *i;
-    if (f->getType() == Format &&
+    if (f->getType() == Format->getName() &&
         f->getFormatIdx() == FormatIdx &&
         f->getFirstArg() == FirstArg) {
       // If we don't have a valid location for this attribute, adopt the
@@ -3139,8 +3140,8 @@
     }
   }
 
-  return ::new (Context) FormatAttr(Range, Context, Format, FormatIdx, FirstArg,
-                                    AttrSpellingListIndex);
+  return ::new (Context) FormatAttr(Range, Context, Format, FormatIdx,
+                                    FirstArg, AttrSpellingListIndex);
 }
 
 /// Handle __attribute__((format(type,idx,firstarg))) attributes based on
@@ -3171,8 +3172,11 @@
   StringRef Format = II->getName();
 
   // Normalize the argument, __foo__ becomes foo.
-  if (Format.startswith("__") && Format.endswith("__"))
+  if (Format.startswith("__") && Format.endswith("__")) {
     Format = Format.substr(2, Format.size() - 4);
+    // If we've modified the string name, we need a new identifier for it.
+    II = &S.Context.Idents.get(Format);
+  }
 
   // Check for supported formats.
   FormatAttrKind Kind = getFormatAttrKind(Format);
@@ -3278,7 +3282,7 @@
     return;
   }
 
-  FormatAttr *NewAttr = S.mergeFormatAttr(D, Attr.getRange(), Format,
+  FormatAttr *NewAttr = S.mergeFormatAttr(D, Attr.getRange(), II,
                                           Idx.getZExtValue(),
                                           FirstArg.getZExtValue(),
                                           Attr.getAttributeSpellingListIndex());
Index: test/SemaCXX/cxx11-attr-print.cpp
===================================================================
--- test/SemaCXX/cxx11-attr-print.cpp	(revision 189821)
+++ test/SemaCXX/cxx11-attr-print.cpp	(working copy)
@@ -52,7 +52,7 @@
 inline void f7 [[gnu::gnu_inline]] ();
 
 // arguments printing
-// CHECK: __attribute__((format("printf", 2, 3)));
+// CHECK: __attribute__((format(printf, 2, 3)));
 void f8 (void *, const char *, ...) __attribute__ ((format (printf, 2, 3)));
 
 // CHECK: int m __attribute__((aligned(4
Index: utils/TableGen/ClangAttrEmitter.cpp
===================================================================
--- utils/TableGen/ClangAttrEmitter.cpp	(revision 189821)
+++ utils/TableGen/ClangAttrEmitter.cpp	(working copy)
@@ -1005,6 +1005,7 @@
           .Case("DefaultIntArgument", true)
           .Case("IntArgument", true)
           .Case("ExprArgument", true)
+          .Case("StringArgument", true)
           .Case("UnsignedArgument", true)
           .Case("VariadicUnsignedArgument", true)
           .Case("VariadicExprArgument", true)