c++: Improve handling of foreigner namespace attributes

In some cases we want to look up or remove both standard
attributes and attributes from gnu namespace but not others.
This patch arranges for ATTR_NS of "" to stand for ATTR_NS
NULL or "gnu", so that we don't need 2 separate calls, and
introduces is_attribute_namespace_p function which allows
testing the namespace of an attribute similar way.

The patch also 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.

2022-10-07  Jakub Jelinek  <jakub@redhat.com>

gcc/
	* attribs.h (is_attribute_namespace_p): New inline function.
	(lookup_attribute): Document meaning of ATTR_NS equal to "".
	* attribs.cc (remove_attribute): Use is_attribute_namespace_p.
	(private_lookup_attribute): For ATTR_NS "" match either standard
	attribute or "gnu" namespace one.
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.

7 files changed, 123 insertions, 27 deletions

diff --git a/gcc/attribs.cc b/gcc/attribs.cc
index 38f3e926..27dea74 100644
--- a/gcc/attribs.cc
+++ b/gcc/attribs.cc
@@ -1645,7 +1645,8 @@ remove_attribute (const char *attr_name, tree list)
   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)
@@ -1659,15 +1660,11 @@ remove_attribute (const char *attr_ns, const char *attr_name, tree list)
       tree l = *p;
 
       tree attr = get_attribute_name (l);
-      if (is_attribute_p (attr_name, attr))
+      if (is_attribute_p (attr_name, attr)
+	  && is_attribute_namespace_p (attr_ns, l))
 	{
-	  tree ns = get_attribute_namespace (l);
-	  if ((ns == NULL_TREE && attr_ns == NULL)
-	      || (ns && attr_ns && is_attribute_p (attr_ns, ns)))
-	    {
-	      *p = TREE_CHAIN (l);
-	      continue;
-	    }
+	  *p = TREE_CHAIN (l);
+	  continue;
 	}
       p = &TREE_CHAIN (l);
     }
@@ -2088,14 +2085,20 @@ private_lookup_attribute (const char *attr_ns, const char *attr_name,
 	  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;
 	    }
 	}
diff --git a/gcc/attribs.h b/gcc/attribs.h
index 121b9eb..1dc16e4 100644
--- a/gcc/attribs.h
+++ b/gcc/attribs.h
@@ -188,6 +188,22 @@ is_attribute_p (const char *attr_name, const_tree ident)
 		      IDENTIFIER_POINTER (ident), IDENTIFIER_LENGTH (ident));
 }
 
+/* Given an attribute ATTR and a string ATTR_NS, return true
+   if the attribute namespace is valid for the string.  ATTR_NS "" stands
+   for standard attribute (NULL get_attribute_namespace) or "gnu"
+   namespace.  */
+
+static inline bool
+is_attribute_namespace_p (const char *attr_ns, const_tree attr)
+{
+  tree ident = get_attribute_namespace (attr);
+  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 +233,8 @@ lookup_attribute (const char *attr_name, tree list)
     }
 }
 
-/* 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)
diff --git a/gcc/c-family/c-common.cc b/gcc/c-family/c-common.cc
index ffe17ea..9ec9100 100644
--- a/gcc/c-family/c-common.cc
+++ b/gcc/c-family/c-common.cc
@@ -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.  */
@@ -6024,7 +6024,8 @@ 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))
+      if (!is_attribute_p ("fallthrough", name)
+	  || !is_attribute_namespace_p ("", t))
 	{
 	  if (!c_dialect_cxx () && get_attribute_namespace (t) == NULL_TREE)
 	    /* The specifications of standard attributes in C mean
diff --git a/gcc/cp/cp-gimplify.cc b/gcc/cp/cp-gimplify.cc
index 5d26e59..cb8bbd8 100644
--- a/gcc/cp/cp-gimplify.cc
+++ b/gcc/cp/cp-gimplify.cc
@@ -3028,7 +3028,7 @@ 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)
@@ -3036,24 +3036,36 @@ 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 ("", 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 ("", l))
+	{
+	  *p = TREE_CHAIN (l);
+	  continue;
+	}
+      p = &TREE_CHAIN (l);
+    }
   return list;
 }
 
diff --git a/gcc/cp/parser.cc b/gcc/cp/parser.cc
index 90e06f9..555476e 100644
--- a/gcc/cp/parser.cc
+++ b/gcc/cp/parser.cc
@@ -29265,7 +29265,8 @@ cp_parser_check_std_attribute (location_t loc, tree attributes, tree attribute)
   if (attributes)
     for (const auto &a : alist)
       if (is_attribute_p (a, get_attribute_name (attribute))
-	  && lookup_attribute (a, attributes))
+	  && is_attribute_namespace_p ("", attribute)
+	  && lookup_attribute ("", a, attributes))
 	{
 	  if (!from_macro_expansion_at (loc))
 	    warning_at (loc, OPT_Wattributes, "attribute %qs specified "
diff --git a/gcc/testsuite/g++.dg/cpp1z/fallthrough2.C b/gcc/testsuite/g++.dg/cpp1z/fallthrough2.C
new file mode 100644
index 0000000..b74323f
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1z/fallthrough2.C
@@ -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;
+}
diff --git a/gcc/testsuite/g++.dg/cpp2a/attr-likely7.C b/gcc/testsuite/g++.dg/cpp2a/attr-likely7.C
new file mode 100644
index 0000000..638a6d9
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/attr-likely7.C
@@ -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 }
+}