Index: include/clang/Basic/Attr.td
===================================================================
--- include/clang/Basic/Attr.td
+++ include/clang/Basic/Attr.td
@@ -1156,7 +1156,7 @@
 
 def FallThrough : StmtAttr {
   let Spellings = [CXX11<"", "fallthrough", 201603>, C2x<"", "fallthrough">,
-                   CXX11<"clang", "fallthrough">];
+                   CXX11<"clang", "fallthrough">, GCC<"fallthrough">];
 //  let Subjects = [NullStmt];
   let Documentation = [FallthroughDocs];
 }
Index: lib/Parse/ParseStmt.cpp
===================================================================
--- lib/Parse/ParseStmt.cpp
+++ lib/Parse/ParseStmt.cpp
@@ -99,6 +99,7 @@
   ParenBraceBracketBalancer BalancerRAIIObj(*this);
 
   ParsedAttributesWithRange Attrs(AttrFactory);
+  MaybeParseGNUAttributes(Attrs);
   MaybeParseCXX11Attributes(Attrs, nullptr, /*MightBeObjCMessageSend*/ true);
   if (!MaybeParseOpenCLUnrollHintAttribute(Attrs))
     return StmtError();
Index: lib/Sema/AnalysisBasedWarnings.cpp
===================================================================
--- lib/Sema/AnalysisBasedWarnings.cpp
+++ lib/Sema/AnalysisBasedWarnings.cpp
@@ -1215,7 +1215,7 @@
     tok::r_square, tok::r_square
   };
 
-  bool PreferClangAttr = !PP.getLangOpts().CPlusPlus17;
+  bool PreferClangAttr = !PP.getLangOpts().CPlusPlus17 && !PP.getLangOpts().C2x;
 
   StringRef MacroName;
   if (PreferClangAttr)
@@ -1224,24 +1224,19 @@
     MacroName = PP.getLastMacroWithSpelling(Loc, FallthroughTokens);
   if (MacroName.empty() && !PreferClangAttr)
     MacroName = PP.getLastMacroWithSpelling(Loc, ClangFallthroughTokens);
-  if (MacroName.empty())
-    MacroName = PreferClangAttr ? "[[clang::fallthrough]]" : "[[fallthrough]]";
+  if (MacroName.empty()) {
+    if (!PreferClangAttr)
+      MacroName = "[[fallthrough]]";
+    else if (PP.getLangOpts().CPlusPlus)
+      MacroName = "[[clang::fallthrough]]";
+    else
+      MacroName = "__attribute__ ((fallthrough))";
+  }
   return MacroName;
 }
 
 static void DiagnoseSwitchLabelsFallthrough(Sema &S, AnalysisDeclContext &AC,
                                             bool PerFunction) {
-  // Only perform this analysis when using [[]] attributes. There is no good
-  // workflow for this warning when not using C++11. There is no good way to
-  // silence the warning (no attribute is available) unless we are using
-  // [[]] attributes. One could use pragmas to silence the warning, but as a
-  // general solution that is gross and not in the spirit of this warning.
-  //
-  // NOTE: This an intermediate solution. There are on-going discussions on
-  // how to properly support this warning outside of C++11 with an annotation.
-  if (!AC.getASTContext().getLangOpts().DoubleSquareBracketAttributes)
-    return;
-
   FallthroughMapper FM(S);
   FM.TraverseStmt(AC.getBody());
 
@@ -1281,7 +1276,8 @@
       SourceLocation L = Label->getBeginLoc();
       if (L.isMacroID())
         continue;
-      if (S.getLangOpts().CPlusPlus11) {
+      if (S.getLangOpts().CPlusPlus11 || S.getLangOpts().C99 ||
+          S.getLangOpts().GNUMode) {
         const Stmt *Term = B->getTerminatorStmt();
         // Skip empty cases.
         while (B->empty() && !Term && B->succ_size() == 1) {
Index: test/Sema/address_spaces.c
===================================================================
--- test/Sema/address_spaces.c
+++ test/Sema/address_spaces.c
@@ -9,7 +9,7 @@
 void foo(_AS3 float *a,
          _AS1 float b) // expected-error {{parameter may not be qualified with an address space}}
 {
-  _AS2 *x;// expected-warning {{type specifier missing, defaults to 'int'}}
+  _AS2 *x;// expected-error {{use of undeclared identifier 'x'}}
   _AS1 float * _AS2 *B;
 
   int _AS1 _AS2 *Y;   // expected-error {{multiple address spaces specified for type}}
Index: test/Sema/block-literal.c
===================================================================
--- test/Sema/block-literal.c
+++ test/Sema/block-literal.c
@@ -41,8 +41,8 @@
 
   foo:
   takeblock(^{ x = 4; });  // expected-error {{variable is not assignable (missing __block type specifier)}}
-  __block y = 7;    // expected-warning {{type specifier missing, defaults to 'int'}}
-  takeblock(^{ y = 8; });
+  __block y = 7;    // expected-error {{use of undeclared identifier 'y'}}
+  takeblock(^{ x = 8; }); // expected-error {{use of undeclared identifier 'y'}}
 }
 
 
Index: test/Sema/fallthrough-attr.c
===================================================================
--- test/Sema/fallthrough-attr.c
+++ test/Sema/fallthrough-attr.c
@@ -0,0 +1,24 @@
+// RUN: %clang_cc1 -fsyntax-only -std=gnu89 -verify -Wimplicit-fallthrough %s
+// RUN: %clang_cc1 -fsyntax-only -std=gnu99 -verify -Wimplicit-fallthrough %s
+// RUN: %clang_cc1 -fsyntax-only -std=c99 -verify -Wimplicit-fallthrough %s
+// RUN: %clang_cc1 -fsyntax-only -std=c11 -verify -Wimplicit-fallthrough %s
+// RUN: %clang_cc1 -fsyntax-only -std=c2x -DC2X -verify -Wimplicit-fallthrough %s
+
+int fallthrough_attribute_spelling(int n) {
+  switch (n) {
+  case 0:
+    n++;
+  case 1: 
+    #if defined(C2X)
+    // expected-warning@-2{{unannotated fall-through between switch labels}} expected-note@-2{{insert '[[fallthrough]];' to silence this warning}} expected-note@-2{{insert 'break;' to avoid fall-through}}
+    #else
+    // expected-warning@-4{{unannotated fall-through between switch labels}} expected-note@-4{{insert '__attribute__ ((fallthrough));' to silence this warning}} expected-note@-4{{insert 'break;' to avoid fall-through}}
+    #endif
+    n++;
+    __attribute__ ((fallthrough));
+  case 2:
+    n++;
+    break;
+  }
+  return n;
+}
Index: test/SemaCXX/switch-implicit-fallthrough.cpp
===================================================================
--- test/SemaCXX/switch-implicit-fallthrough.cpp
+++ test/SemaCXX/switch-implicit-fallthrough.cpp
@@ -329,3 +329,15 @@
   }
   return n;
 }
+
+int fallthrough_attribute_spelling(int n) {
+  switch (n) {
+  case 0:
+    n++;
+    __attribute__ ((fallthrough));
+  case 1:
+    n++;
+    break;
+  }
+  return n;
+}
Index: test/SemaCXX/warn-unused-label-error.cpp
===================================================================
--- test/SemaCXX/warn-unused-label-error.cpp
+++ test/SemaCXX/warn-unused-label-error.cpp
@@ -18,9 +18,9 @@
   }
 
   void h() {
-    D: // expected-warning {{unused label 'D'}}
+    D:
       #pragma weak unused_local_static
-      __attribute__((unused))  // expected-warning {{declaration does not declare anything}}
+      __attribute__((unused))  // expected-error {{'unused' attribute cannot be applied to a statement}}
       ;
   }
 }