Index: clang/include/clang/Basic/Attr.td
===================================================================
--- clang/include/clang/Basic/Attr.td
+++ clang/include/clang/Basic/Attr.td
@@ -257,7 +257,6 @@
 class Spelling<string name, string variety> {
   string Name = name;
   string Variety = variety;
-  bit KnownToGCC;
 }
 
 class GNU<string name> : Spelling<name, "GNU">;
@@ -277,11 +276,11 @@
   string Namespace = namespace;
 }
 
-// The GCC spelling implies GNU<name> and CXX11<"gnu", name> and also sets
-// KnownToGCC to 1. This spelling should be used for any GCC-compatible
+// The GCC spelling implies GNU<name>, CXX11<"gnu", name>, and optionally,
+// C2x<"gnu", name>. This spelling should be used for any GCC-compatible
 // attributes.
-class GCC<string name> : Spelling<name, "GCC"> {
-  let KnownToGCC = 1;
+class GCC<string name, bit allowInC = 1> : Spelling<name, "GCC"> {
+  bit AllowInC = allowInC;
 }
 
 // The Clang spelling implies GNU<name>, CXX11<"clang", name>, and optionally,
@@ -605,7 +604,7 @@
 //
 
 def AbiTag : Attr {
-  let Spellings = [GCC<"abi_tag">];
+  let Spellings = [GCC<"abi_tag", /*AllowInC*/0>];
   let Args = [VariadicStringArgument<"Tags">];
   let Subjects = SubjectList<[Struct, Var, Function, Namespace], ErrorDiag>;
   let MeaningfulToClassTemplateDefinition = 1;
@@ -2113,7 +2112,7 @@
 }
 
 def InitPriority : InheritableAttr {
-  let Spellings = [GCC<"init_priority">];
+  let Spellings = [GCC<"init_priority", /*AllowInC*/0>];
   let Args = [UnsignedArgument<"Priority">];
   let Subjects = SubjectList<[Var], ErrorDiag>;
   let Documentation = [Undocumented];
Index: clang/test/Sema/attr-c2x.c
===================================================================
--- clang/test/Sema/attr-c2x.c
+++ clang/test/Sema/attr-c2x.c
@@ -27,3 +27,15 @@
 
 [[nodiscard]] int without_underscores(void);
 [[__nodiscard__]] int underscores(void);
+
+// Match GCC's behavior for C attributes as well.
+[[gnu::constructor]] void ctor_func(void);
+[[gnu::destructor]] void dtor_func(void);
+[[gnu::hot]] void hot_func(void);
+[[__gnu__::hot]] void hot_func2(void);
+[[gnu::__hot__]] void hot_func3(void);
+[[__gnu__::__hot__]] void hot_func4(void);
+
+// Note how not all GCC attributes are supported in C.
+[[gnu::abi_tag("")]] void abi_func(void); // expected-warning {{unknown attribute 'abi_tag' ignored}}
+struct S s [[gnu::init_priority(1)]]; // expected-warning {{unknown attribute 'init_priority' ignored}}
Index: clang/utils/TableGen/ClangAttrEmitter.cpp
===================================================================
--- clang/utils/TableGen/ClangAttrEmitter.cpp
+++ clang/utils/TableGen/ClangAttrEmitter.cpp
@@ -48,7 +48,7 @@
 
 class FlattenedSpelling {
   std::string V, N, NS;
-  bool K;
+  bool K = false;
 
 public:
   FlattenedSpelling(const std::string &Variety, const std::string &Name,
@@ -61,8 +61,6 @@
            "Given a GCC spelling, which means this hasn't been flattened!");
     if (V == "CXX11" || V == "C2x" || V == "Pragma")
       NS = std::string(Spelling.getValueAsString("Namespace"));
-    bool Unset;
-    K = Spelling.getValueAsBitOrUnset("KnownToGCC", Unset);
   }
 
   const std::string &variety() const { return V; }
@@ -82,9 +80,10 @@
     StringRef Variety = Spelling->getValueAsString("Variety");
     StringRef Name = Spelling->getValueAsString("Name");
     if (Variety == "GCC") {
-      // Gin up two new spelling objects to add into the list.
       Ret.emplace_back("GNU", std::string(Name), "", true);
       Ret.emplace_back("CXX11", std::string(Name), "gnu", true);
+      if (Spelling->getValueAsBit("AllowInC"))
+        Ret.emplace_back("C2x", std::string(Name), "gnu", true);
     } else if (Variety == "Clang") {
       Ret.emplace_back("GNU", std::string(Name), "", false);
       Ret.emplace_back("CXX11", std::string(Name), "clang", false);