[PATCH] c++: Improve handling of foreigner namespace attributes

[Jakub Jelinek <jakub@redhat.com>]

Hi!

The following patch uses the new lookup_attribute overload and extra
tests to avoid emitting weird warnings on foreign namespace attributes
which we should just ignore (perhaps with a warning), but shouldn't
imply any meaning to them just because they have a name matching some
standard or gnu attribute name.

Lightly tested so far, ok for trunk if full bootstrap/regtest succeeds?

2022-10-05  Jakub Jelinek  <jakub@redhat.com>

gcc/c-family/
	* c-common.cc (attribute_fallthrough_p): Lookup fallthrough attribute
	only in gnu namespace or as standard attribute, treat fallthrough
	attributes in other namespaces like any other unknown attribute.
gcc/cp/
	* parser.cc (cp_parser_check_std_attribute): Only do checks if
	attribute is a standard attribute or in gnu namespace and only
	lookup other attributes in those namespaces.
	* cp-gimplify.cc (lookup_hotness_attribute): Adjust function comment.
	Only return true for standard attribute or gnu namespace attribute.
	(remove_hotness_attribute): Only remove hotness attributes when
	they are standard or in gnu namespace, implement it in a single
	loop rather than former 4 now 8 remove_attribute calls.
gcc/testsuite/
	* g++.dg/cpp1z/fallthrough2.C: New test.
	* g++.dg/cpp2a/attr-likely7.C: New test.

--- gcc/c-family/c-common.cc.jj	2022-10-03 18:00:58.548661062 +0200
+++ gcc/c-family/c-common.cc	2022-10-05 12:58:08.396043609 +0200
@@ -6007,12 +6007,15 @@ attribute_fallthrough_p (tree attr)
 {
   if (attr == error_mark_node)
    return false;
-  tree t = lookup_attribute ("fallthrough", attr);
+  tree t = lookup_attribute (NULL, "fallthrough", attr);
+  if (t == NULL_TREE)
+    t = lookup_attribute ("gnu", "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 (NULL, "fallthrough", TREE_CHAIN (t))
+      || lookup_attribute ("gnu", "fallthrough", TREE_CHAIN (t)))
     warning (OPT_Wattributes, "attribute %<fallthrough%> specified multiple "
 	     "times");
   /* No attribute-argument-clause shall be present.  */
@@ -6023,9 +6026,11 @@ 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)
+	  || (ns && !is_attribute_p ("gnu", 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	2022-10-05 11:28:28.292141301 +0200
+++ gcc/cp/parser.cc	2022-10-05 13:29:38.203433190 +0200
@@ -29265,7 +29265,10 @@ 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))
+	  && (get_attribute_namespace (attribute) == NULL_TREE
+	      || get_attribute_namespace (attribute) == gnu_identifier)
+	  && (lookup_attribute (NULL, a, attributes)
+	      || lookup_attribute ("gnu", a, attributes)))
 	{
 	  if (!from_macro_expansion_at (loc))
 	    warning_at (loc, OPT_Wattributes, "attribute %qs specified "
--- gcc/cp/cp-gimplify.cc.jj	2022-10-05 11:19:19.160573401 +0200
+++ gcc/cp/cp-gimplify.cc	2022-10-05 12:53:08.367115180 +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)
@@ -3039,20 +3039,38 @@ lookup_hotness_attribute (tree list)
 	  || is_attribute_p ("cold", name)
 	  || is_attribute_p ("likely", name)
 	  || is_attribute_p ("unlikely", name))
-	break;
+	{
+	  tree ns = get_attribute_namespace (list);
+	  if (ns == NULL_TREE || ns == gnu_identifier)
+	    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))
+	{
+	  tree ns = get_attribute_namespace (list);
+	  if (ns == NULL_TREE || ns == gnu_identifier)
+	    {
+	      *p = TREE_CHAIN (l);
+	      continue;
+	    }
+	}
+      p = &TREE_CHAIN (l);
+    }
   return list;
 }
 
--- gcc/testsuite/g++.dg/cpp1z/fallthrough2.C.jj	2022-10-05 13:11:31.622152911 +0200
+++ gcc/testsuite/g++.dg/cpp1z/fallthrough2.C	2022-10-05 13:31:48.175672763 +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	2022-10-05 13:12:21.910471659 +0200
+++ gcc/testsuite/g++.dg/cpp2a/attr-likely7.C	2022-10-05 13:35:01.318056722 +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 }
+}

	Jakub