--- gcc/attribs.h.jj	2022-10-05 16:15:29.834889059 +0200
+++ gcc/attribs.h	2022-10-06 17:59:56.918430468 +0200
@@ -188,6 +188,21 @@ is_attribute_p (const char *attr_name, c
 		      IDENTIFIER_POINTER (ident), IDENTIFIER_LENGTH (ident));
 }
 
+/* Given an identifier node IDENT or NULL and a string ATTR_NS, return true
+   if the identifier node is a valid attribute namespace for the string.
+   ATTR_NS "" stands for standard attribute (NULL IDENT) or "gnu"
+   namespace.  */
+
+static inline bool
+is_attribute_namespace_p (const char *attr_ns, const_tree ident)
+{
+  if (attr_ns == NULL)
+    return ident == NULL_TREE;
+  if (attr_ns[0])
+    return ident && is_attribute_p (attr_ns, ident);
+  return ident == NULL_TREE || is_attribute_p ("gnu", ident);
+}
+
 /* Given an attribute name ATTR_NAME and a list of attributes LIST,
    return a pointer to the attribute's list element if the attribute
    is part of the list, or NULL_TREE if not found.  If the attribute
@@ -217,7 +232,8 @@ lookup_attribute (const char *attr_name,
     }
 }
 
-/* Similar to lookup_attribute, but also match the attribute namespace.  */
+/* Similar to lookup_attribute, but also match the attribute namespace.
+   ATTR_NS "" stands for either standard attribute or "gnu" namespace.  */
 
 static inline tree
 lookup_attribute (const char *attr_ns, const char *attr_name, tree list)
--- gcc/attribs.cc.jj	2022-10-04 23:13:10.289090796 +0200
+++ gcc/attribs.cc	2022-10-06 18:01:32.096146195 +0200
@@ -1645,7 +1645,8 @@ remove_attribute (const char *attr_name,
   return list;
 }
 
-/* Similarly but also match namespace on the removed attributes.  */
+/* Similarly but also match namespace on the removed attributes.
+   ATTR_NS "" stands for NULL or "gnu" namespace.  */
 
 tree
 remove_attribute (const char *attr_ns, const char *attr_name, tree list)
@@ -1662,8 +1663,7 @@ remove_attribute (const char *attr_ns, c
       if (is_attribute_p (attr_name, attr))
 	{
 	  tree ns = get_attribute_namespace (l);
-	  if ((ns == NULL_TREE && attr_ns == NULL)
-	      || (ns && attr_ns && is_attribute_p (attr_ns, ns)))
+	  if (is_attribute_namespace_p (attr_ns, ns))
 	    {
 	      *p = TREE_CHAIN (l);
 	      continue;
@@ -2088,14 +2088,20 @@ private_lookup_attribute (const char *at
 	  tree ns = get_attribute_namespace (list);
 	  if (ns == NULL_TREE)
 	    {
-	      if (attr_ns == NULL)
+	      if (attr_ns_len == 0)
 		break;
 	    }
 	  else if (attr_ns)
 	    {
 	      ident_len = IDENTIFIER_LENGTH (ns);
-	      if (cmp_attribs (attr_ns, attr_ns_len, IDENTIFIER_POINTER (ns),
-			       ident_len))
+	      if (attr_ns_len == 0)
+		{
+		  if (cmp_attribs ("gnu", strlen ("gnu"),
+				   IDENTIFIER_POINTER (ns), ident_len))
+		    break;
+		}
+	      else if (cmp_attribs (attr_ns, attr_ns_len,
+				    IDENTIFIER_POINTER (ns), ident_len))
 		break;
 	    }
 	}
--- gcc/c-family/c-common.cc.jj	2022-10-06 17:43:47.880502941 +0200
+++ gcc/c-family/c-common.cc	2022-10-06 18:56:05.432888180 +0200
@@ -6008,12 +6008,12 @@ attribute_fallthrough_p (tree attr)
 {
   if (attr == error_mark_node)
    return false;
-  tree t = lookup_attribute ("fallthrough", attr);
+  tree t = lookup_attribute ("", "fallthrough", attr);
   if (t == NULL_TREE)
     return false;
   /* It is no longer true that "this attribute shall appear at most once in
      each attribute-list", but we still give a warning.  */
-  if (lookup_attribute ("fallthrough", TREE_CHAIN (t)))
+  if (lookup_attribute ("", "fallthrough", TREE_CHAIN (t)))
     warning (OPT_Wattributes, "attribute %<fallthrough%> specified multiple "
 	     "times");
   /* No attribute-argument-clause shall be present.  */
@@ -6024,9 +6024,11 @@ attribute_fallthrough_p (tree attr)
   for (t = attr; t != NULL_TREE; t = TREE_CHAIN (t))
     {
       tree name = get_attribute_name (t);
-      if (!is_attribute_p ("fallthrough", name))
+      tree ns = get_attribute_namespace (t);
+      if (!is_attribute_p ("fallthrough", name)
+	  || !is_attribute_namespace_p ("", ns))
 	{
-	  if (!c_dialect_cxx () && get_attribute_namespace (t) == NULL_TREE)
+	  if (!c_dialect_cxx () && ns == NULL_TREE)
 	    /* The specifications of standard attributes in C mean
 	       this is a constraint violation.  */
 	    pedwarn (input_location, OPT_Wattributes, "%qE attribute ignored",
--- gcc/cp/parser.cc.jj	2022-10-06 17:43:42.418576616 +0200
+++ gcc/cp/parser.cc	2022-10-06 18:57:14.019956324 +0200
@@ -29265,7 +29265,8 @@ cp_parser_check_std_attribute (location_
   if (attributes)
     for (const auto &a : alist)
       if (is_attribute_p (a, get_attribute_name (attribute))
-	  && lookup_attribute (a, attributes))
+	  && is_attribute_namespace_p ("", get_attribute_namespace (attribute))
+	  && lookup_attribute ("", a, attributes))
 	{
 	  if (!from_macro_expansion_at (loc))
 	    warning_at (loc, OPT_Wattributes, "attribute %qs specified "
--- gcc/cp/cp-gimplify.cc.jj	2022-10-06 17:43:42.413576684 +0200
+++ gcc/cp/cp-gimplify.cc	2022-10-06 18:59:20.763234354 +0200
@@ -3027,7 +3027,7 @@ cp_fold (tree x)
   return x;
 }
 
-/* Look up either "hot" or "cold" in attribute list LIST.  */
+/* Look up "hot", "cold", "likely" or "unlikely" in attribute list LIST.  */
 
 tree
 lookup_hotness_attribute (tree list)
@@ -3035,24 +3035,36 @@ lookup_hotness_attribute (tree list)
   for (; list; list = TREE_CHAIN (list))
     {
       tree name = get_attribute_name (list);
-      if (is_attribute_p ("hot", name)
-	  || is_attribute_p ("cold", name)
-	  || is_attribute_p ("likely", name)
-	  || is_attribute_p ("unlikely", name))
+      if ((is_attribute_p ("hot", name)
+	   || is_attribute_p ("cold", name)
+	   || is_attribute_p ("likely", name)
+	   || is_attribute_p ("unlikely", name))
+	  && is_attribute_namespace_p ("", get_attribute_namespace (list)))
 	break;
     }
   return list;
 }
 
-/* Remove both "hot" and "cold" attributes from LIST.  */
+/* Remove "hot", "cold", "likely" and "unlikely" attributes from LIST.  */
 
 static tree
 remove_hotness_attribute (tree list)
 {
-  list = remove_attribute ("hot", list);
-  list = remove_attribute ("cold", list);
-  list = remove_attribute ("likely", list);
-  list = remove_attribute ("unlikely", list);
+  for (tree *p = &list; *p; )
+    {
+      tree l = *p;
+      tree name = get_attribute_name (l);
+      if ((is_attribute_p ("hot", name)
+	   || is_attribute_p ("cold", name)
+	   || is_attribute_p ("likely", name)
+	   || is_attribute_p ("unlikely", name))
+	  && is_attribute_namespace_p ("", get_attribute_namespace (l)))
+	{
+	  *p = TREE_CHAIN (l);
+	  continue;
+	}
+      p = &TREE_CHAIN (l);
+    }
   return list;
 }
 
--- gcc/testsuite/g++.dg/cpp1z/fallthrough2.C.jj	2022-10-06 17:45:55.452782142 +0200
+++ gcc/testsuite/g++.dg/cpp1z/fallthrough2.C	2022-10-06 17:45:55.452782142 +0200
@@ -0,0 +1,24 @@
+// { dg-do compile { target c++17 } }
+// { dg-options "-Wextra -Wall -Wpedantic" }
+
+int
+foo (int i)
+{
+  switch (i)
+    {
+    case 2:
+      ++i;
+      [[fallthrough, whatever::fallthrough]];		// { dg-bogus "attribute 'fallthrough' specified multiple times" }
+    case 3:						// { dg-warning "'fallthrough' attribute ignored" "" { target *-*-* } .-1 }
+      ++i;
+      [[fallthrough, whatever2::fallthrough(1, 2, 3)]];	// { dg-bogus "attribute 'fallthrough' specified multiple times" }
+    case 4:						// { dg-warning "'fallthrough' attribute ignored" "" { target *-*-* } .-1 }
+      [[whatever3::fallthrough("abcd")]];		// { dg-warning "attributes at the beginning of statement are ignored" }
+    case 5:
+      [[whatever4::fallthrough]];			// { dg-bogus "attribute 'fallthrough' not preceding a case label or default label" }
+      ++i;						// { dg-warning "attributes at the beginning of statement are ignored" "" { target *-*-* } .-1 }
+    default:
+      break;
+    }
+  return i;
+}
--- gcc/testsuite/g++.dg/cpp2a/attr-likely7.C.jj	2022-10-06 17:45:55.452782142 +0200
+++ gcc/testsuite/g++.dg/cpp2a/attr-likely7.C	2022-10-06 17:45:55.452782142 +0200
@@ -0,0 +1,38 @@
+// { dg-do compile { target c++20 } }
+// { dg-additional-options -fdump-tree-gimple }
+// { dg-final { scan-tree-dump-times "hot label" 5 "gimple" } }
+// { dg-final { scan-tree-dump-times "cold label" 3 "gimple" } }
+
+bool b;
+
+template <class T> int f()
+{
+  if (b)
+    [[likely, whatever::unlikely ("abcd")]] return 0;		// { dg-bogus "ignoring attribute 'unlikely' after earlier 'likely'" }
+  else								// { dg-warning "attributes at the beginning of statement are ignored" "" { target *-*-* } .-1 }
+    [[unlikely, whatever2::hot]] flabel: return 1;		// { dg-warning "'whatever2::hot' scoped attribute directive ignored" }
+  switch (b)
+    {
+      [[likely, whatever3::cold (1, 2, 3)]] case true: break;	// { dg-warning "'whatever3::cold' scoped attribute directive ignored" }
+    };
+  return 1;
+}
+
+int main()
+{
+  if (b)
+    [[whatever4::unlikely (1), likely]] return 0;		// { dg-bogus "ignoring attribute 'likely' after earlier 'unlikely'" }
+  else if (b)							// { dg-warning "attributes at the beginning of statement are ignored" "" { target *-*-* } .-1 }
+    [[whatever5::hot, unlikely]] elabel:			// { dg-warning "'whatever5::hot' scoped attribute directive ignored" }
+      return 1;
+  else
+    [[whatever6::cold, likely]] b = false;			// { dg-bogus "ignoring attribute 'likely' after earlier 'cold'" }
+								// { dg-warning "attributes at the beginning of statement are ignored" "" { target *-*-* } .-1 }
+  f<int>();
+
+  switch (b)
+    {
+      [[whatever7::unlikely (1), likely]] case true: break;	// { dg-warning "'whatever7::unlikely' scoped attribute directive ignored" }
+      [[whatever8::unlikely, unlikely]] case false: break;	// { dg-bogus "attribute 'unlikely' specified multiple times" }
+    };								// { dg-warning "'whatever8::unlikely' scoped attribute directive ignored" "" { target *-*-* } .-1 }
+}
