diff --git a/include/clang/Basic/DiagnosticParseKinds.td b/include/clang/Basic/DiagnosticParseKinds.td
index ab3bf7e..5dd2fff 100644
--- a/include/clang/Basic/DiagnosticParseKinds.td
+++ b/include/clang/Basic/DiagnosticParseKinds.td
@@ -92,8 +92,8 @@ def err_duplicate_default_assoc : Error<
 def note_previous_default_assoc : Note<
   "previous default generic association is here">;
 
-def ext_c11_alignas : Extension<
-  "_Alignas is a C11-specific feature">, InGroup<C11>;
+def ext_c11_alignment : Extension<
+  "%0 is a C11-specific feature">, InGroup<C11>;
 
 def ext_gnu_indirect_goto : Extension<
   "use of GNU indirect-goto extension">, InGroup<GNU>;
diff --git a/include/clang/Basic/TokenKinds.def b/include/clang/Basic/TokenKinds.def
index 7c800df..d99b7be 100644
--- a/include/clang/Basic/TokenKinds.def
+++ b/include/clang/Basic/TokenKinds.def
@@ -251,6 +251,7 @@ KEYWORD(void                        , KEYALL)
 KEYWORD(volatile                    , KEYALL)
 KEYWORD(while                       , KEYALL)
 KEYWORD(_Alignas                    , KEYALL)
+KEYWORD(_Alignof                    , KEYALL)
 KEYWORD(_Atomic                     , KEYALL)
 KEYWORD(_Bool                       , KEYNOCXX)
 KEYWORD(_Complex                    , KEYALL)
diff --git a/lib/AST/StmtDumper.cpp b/lib/AST/StmtDumper.cpp
index a57cce8..0a7a434 100644
--- a/lib/AST/StmtDumper.cpp
+++ b/lib/AST/StmtDumper.cpp
@@ -462,7 +462,7 @@ void StmtDumper::VisitUnaryExprOrTypeTraitExpr(UnaryExprOrTypeTraitExpr *Node) {
     OS << " sizeof ";
     break;
   case UETT_AlignOf:
-    OS << " __alignof ";
+    OS << " alignof ";
     break;
   case UETT_VecStep:
     OS << " vec_step ";
diff --git a/lib/AST/StmtPrinter.cpp b/lib/AST/StmtPrinter.cpp
index cb757cd..b19883a 100644
--- a/lib/AST/StmtPrinter.cpp
+++ b/lib/AST/StmtPrinter.cpp
@@ -811,7 +811,12 @@ void StmtPrinter::VisitUnaryExprOrTypeTraitExpr(UnaryExprOrTypeTraitExpr *Node){
     OS << "sizeof";
     break;
   case UETT_AlignOf:
-    OS << "__alignof";
+    if (Policy.LangOpts.CPlusPlus)
+      OS << "alignof";
+    else if (Policy.LangOpts.C11)
+      OS << "_Alignof";
+    else
+      OS << "__alignof";
     break;
   case UETT_VecStep:
     OS << "vec_step";
diff --git a/lib/Parse/ParseDecl.cpp b/lib/Parse/ParseDecl.cpp
index 3c1c7e2..39239d2 100644
--- a/lib/Parse/ParseDecl.cpp
+++ b/lib/Parse/ParseDecl.cpp
@@ -2457,7 +2457,7 @@ void Parser::ParseDeclarationSpecifiers(DeclSpec &DS,
     // alignment-specifier
     case tok::kw__Alignas:
       if (!getLangOpts().C11)
-        Diag(Tok, diag::ext_c11_alignas);
+        Diag(Tok, diag::ext_c11_alignment) << Tok.getName();
       ParseAlignmentSpecifier(DS.getAttributes());
       continue;
 
diff --git a/lib/Parse/ParseExpr.cpp b/lib/Parse/ParseExpr.cpp
index d1c5989..9416c54 100644
--- a/lib/Parse/ParseExpr.cpp
+++ b/lib/Parse/ParseExpr.cpp
@@ -507,6 +507,7 @@ class CastExpressionIdValidator : public CorrectionCandidateCallback {
 /// [C++11] 'sizeof' '...' '(' identifier ')'
 /// [GNU]   '__alignof' unary-expression
 /// [GNU]   '__alignof' '(' type-name ')'
+/// [C11]   '_Alignof' '(' type-name ')'
 /// [C++11] 'alignof' '(' type-id ')'
 /// [GNU]   '&&' identifier
 /// [C++11] 'noexcept' '(' expression ')' [C++11 5.3.7]
@@ -920,12 +921,15 @@ ExprResult Parser::ParseCastExpression(bool isUnaryExpression,
       Res = Actions.ActOnUnaryOp(getCurScope(), SavedLoc, SavedKind, Res.get());
     return move(Res);
   }
-  case tok::kw_sizeof:     // unary-expression: 'sizeof' unary-expression
-                           // unary-expression: 'sizeof' '(' type-name ')'
-  case tok::kw_alignof:
+  case tok::kw__Alignof:   // unary-expression: '_Alignof' '(' type-name ')'
+    if (!getLangOpts().C11)
+      Diag(Tok, diag::ext_c11_alignment) << Tok.getName();
+    // fallthrough
+  case tok::kw_alignof:    // unary-expression: 'alignof' '(' type-id ')'
   case tok::kw___alignof:  // unary-expression: '__alignof' unary-expression
                            // unary-expression: '__alignof' '(' type-name ')'
-                           // unary-expression: 'alignof' '(' type-id ')'
+  case tok::kw_sizeof:     // unary-expression: 'sizeof' unary-expression
+                           // unary-expression: 'sizeof' '(' type-name ')'
   case tok::kw_vec_step:   // unary-expression: OpenCL 'vec_step' expression
     return ParseUnaryExprOrTypeTraitExpression();
   case tok::ampamp: {      // unary-expression: '&&' identifier
@@ -1513,6 +1517,7 @@ Parser::ParsePostfixExpressionSuffix(ExprResult LHS) {
 ///         'sizeof' '(' type-name ')'
 /// [GNU]   '__alignof' unary-expression
 /// [GNU]   '__alignof' '(' type-name ')'
+/// [C11]   '_Alignof' '(' type-name ')'
 /// [C++0x] 'alignof' '(' type-id ')'
 ///
 /// [GNU]   typeof-specifier:
@@ -1532,7 +1537,7 @@ Parser::ParseExprAfterUnaryExprOrTypeTrait(const Token &OpTok,
 
   assert((OpTok.is(tok::kw_typeof)    || OpTok.is(tok::kw_sizeof) ||
           OpTok.is(tok::kw___alignof) || OpTok.is(tok::kw_alignof) ||
-          OpTok.is(tok::kw_vec_step)) &&
+          OpTok.is(tok::kw__Alignof)  || OpTok.is(tok::kw_vec_step)) &&
           "Not a typeof/sizeof/alignof/vec_step expression!");
 
   ExprResult Operand;
@@ -1590,11 +1595,13 @@ Parser::ParseExprAfterUnaryExprOrTypeTrait(const Token &OpTok,
 /// [C++0x] 'sizeof' '...' '(' identifier ')'
 /// [GNU]   '__alignof' unary-expression
 /// [GNU]   '__alignof' '(' type-name ')'
+/// [C11]   '_Alignof' '(' type-name ')'
 /// [C++0x] 'alignof' '(' type-id ')'
 /// \endverbatim
 ExprResult Parser::ParseUnaryExprOrTypeTraitExpression() {
-  assert((Tok.is(tok::kw_sizeof) || Tok.is(tok::kw___alignof)
-          || Tok.is(tok::kw_alignof) || Tok.is(tok::kw_vec_step)) &&
+  assert((Tok.is(tok::kw_sizeof) || Tok.is(tok::kw___alignof) ||
+          Tok.is(tok::kw_alignof) || Tok.is(tok::kw__Alignof) ||
+          Tok.is(tok::kw_vec_step)) &&
          "Not a sizeof/alignof/vec_step expression!");
   Token OpTok = Tok;
   ConsumeToken();
@@ -1642,7 +1649,7 @@ ExprResult Parser::ParseUnaryExprOrTypeTraitExpression() {
                                                 RParenLoc);
   }
 
-  if (OpTok.is(tok::kw_alignof))
+  if (OpTok.is(tok::kw_alignof) || OpTok.is(tok::kw__Alignof))
     Diag(OpTok, diag::warn_cxx98_compat_alignof);
 
   EnterExpressionEvaluationContext Unevaluated(Actions, Sema::Unevaluated);
@@ -1656,7 +1663,8 @@ ExprResult Parser::ParseUnaryExprOrTypeTraitExpression() {
                                                           CastRange);
 
   UnaryExprOrTypeTrait ExprKind = UETT_SizeOf;
-  if (OpTok.is(tok::kw_alignof) || OpTok.is(tok::kw___alignof))
+  if (OpTok.is(tok::kw_alignof) || OpTok.is(tok::kw___alignof) ||
+      OpTok.is(tok::kw__Alignof))
     ExprKind = UETT_AlignOf;
   else if (OpTok.is(tok::kw_vec_step))
     ExprKind = UETT_VecStep;
diff --git a/lib/Parse/ParseTentative.cpp b/lib/Parse/ParseTentative.cpp
index c2e539c..fc2ded7 100644
--- a/lib/Parse/ParseTentative.cpp
+++ b/lib/Parse/ParseTentative.cpp
@@ -735,6 +735,7 @@ Parser::isExpressionOrTypeSpecifierSimple(tok::TokenKind Kind) {
   case tok::kw_alignof:
   case tok::kw_noexcept:
   case tok::kw_nullptr:
+  case tok::kw__Alignof:
   case tok::kw___null:
   case tok::kw___alignof:
   case tok::kw___builtin_choose_expr:
diff --git a/lib/Sema/SemaCodeComplete.cpp b/lib/Sema/SemaCodeComplete.cpp
index 5cab990..ff83324 100644
--- a/lib/Sema/SemaCodeComplete.cpp
+++ b/lib/Sema/SemaCodeComplete.cpp
@@ -1955,6 +1955,19 @@ static void AddOrdinaryNameResults(Sema::ParserCompletionContext CCC,
       AddObjCExpressionResults(Results, true);
     }
 
+    if (SemaRef.getLangOpts().C11) {
+      // _Alignof
+      Builder.AddResultTypeChunk("size_t");
+      if (SemaRef.getASTContext().Idents.get("alignof").hasMacroDefinition())
+        Builder.AddTypedTextChunk("alignof");
+      else
+        Builder.AddTypedTextChunk("_Alignof");
+      Builder.AddChunk(CodeCompletionString::CK_LeftParen);
+      Builder.AddPlaceholderChunk("type");
+      Builder.AddChunk(CodeCompletionString::CK_RightParen);
+      Results.AddResult(Result(Builder.TakeString()));
+    }
+
     // sizeof expression
     Builder.AddResultTypeChunk("size_t");
     Builder.AddTypedTextChunk("sizeof");
diff --git a/lib/Sema/SemaLookup.cpp b/lib/Sema/SemaLookup.cpp
index 506a9f8..6b0f9b8 100644
--- a/lib/Sema/SemaLookup.cpp
+++ b/lib/Sema/SemaLookup.cpp
@@ -3617,6 +3617,12 @@ static void AddKeywordsToConsumer(Sema &SemaRef,
         Consumer.addKeywordResult("nullptr");
       }
     }
+
+    if (SemaRef.getLangOpts().C11) {
+      // FIXME: We should not suggest _Alignof if the alignof macro
+      // is present.
+      Consumer.addKeywordResult("_Alignof");
+    }
   }
 
   if (CCC.WantRemainingKeywords) {
diff --git a/test/Sema/alignas.c b/test/Sema/alignas.c
index 5832393..d9a0164 100644
--- a/test/Sema/alignas.c
+++ b/test/Sema/alignas.c
@@ -1,4 +1,5 @@
-// RUN: %clang_cc1 -fsyntax-only -verify -std=c1x %s
+// RUN: %clang_cc1 -fsyntax-only -verify -std=c11 -Dalignof=__alignof %s
+// RUN: %clang_cc1 -fsyntax-only -verify -std=c11 -Dalignof=_Alignof %s
 
 _Alignas(3) int align_illegal; //expected-error {{requested alignment is not a power of 2}}
 _Alignas(int) char align_big;
@@ -11,9 +12,9 @@ struct align_member {
 
 typedef _Alignas(8) char align_typedef; // FIXME: this should be rejected
 
-_Static_assert(__alignof(align_big) == __alignof(int), "k's alignment is wrong");
-_Static_assert(__alignof(align_small) == 1, "j's alignment is wrong");
-_Static_assert(__alignof(align_multiple) == 8, "l's alignment is wrong");
-_Static_assert(__alignof(struct align_member) == 8, "quuux's alignment is wrong");
+_Static_assert(alignof(align_big) == alignof(int), "k's alignment is wrong");
+_Static_assert(alignof(align_small) == 1, "j's alignment is wrong");
+_Static_assert(alignof(align_multiple) == 8, "l's alignment is wrong");
+_Static_assert(alignof(struct align_member) == 8, "quuux's alignment is wrong");
 _Static_assert(sizeof(struct align_member) == 8, "quuux's size is wrong");
-_Static_assert(__alignof(align_typedef) == 8, "typedef's alignment is wrong");
+_Static_assert(alignof(align_typedef) == 8, "typedef's alignment is wrong");