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

[Jason Merrill <jason@redhat.com>]

On 10/5/22 08:04, Jakub Jelinek wrote:
> 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);

Maybe lookup_attribute with a magic namespace argument that matches 
either null or gnu?

>     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)))

And similarly, a new is_attribute_namespace (name, t) function with the 
same behavior, to be used here and in all the below hunks?

>   	{
> -	  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
>