openmp: Initial support for OpenMP directives expressed as C++11 attributes

This is an OpenMP 5.1 feature, but I think it is something very useful for
OpenMP users, so I'm committing it now instead of waiting until all 5.0
work is done.

The support is incomplete, only attributes on statements (or block local
declarations) are supported right now, while for non-executable directives
they should be also supported at namespace scope and at class scope, and
for declarations in all places that appertain to the declarations rather
than e.g. types.

I need to also fix up handling of C++11 non-OpenMP attributes mixed with
OpenMP attributes before block local declarations (currently it throws
them away), probably reject if the directives appertain to labels etc.

In order not to complicate all the OpenMP directive parsing, it is done
by remembering the tokens from the attribute, slightly adjusting them and
feeding them through a temporary new lexer to cp_parse_pragma.

2021-07-02  Jakub Jelinek  <jakub@redhat.com>

gcc/c-family/
	* c-common.h (enum c_omp_directive_kind): New enum.
	(struct c_omp_directive): New type.
	(c_omp_categorize_directive): Declare.
	* c-omp.c (omp_directives): New variable.
	(c_omp_categorize_directive): New function.
gcc/cp/
	* parser.h (struct cp_lexer): Add in_omp_attribute_pragma member.
	(struct cp_omp_declare_simd_data): Likewise.
	* cp-tree.h (enum cp_tree_index): Add CPTI_OMP_IDENTIFIER.
	(omp_identifier): Define.
	* parser.c (cp_parser_skip_to_pragma_eol): Handle
	in_omp_attribute_pragma CPP_PRAGMA_EOL followed by CPP_EOF.
	(cp_parser_require_pragma_eol): Likewise.
	(struct cp_omp_attribute_data): New type.
	(cp_parser_handle_statement_omp_attributes): New function.
	(cp_parser_statement): Handle OpenMP directives in statement's
	attribute-specifier-seq.
	(cp_parser_omp_directive_args, cp_parser_omp_sequence_args): New
	functions.
	(cp_parser_std_attribute): Handle omp::directive and omp::sequence
	attributes.
	(cp_parser_omp_all_clauses): If in_omp_attribute_pragma, allow
	a comma also before the first clause.
	(cp_parser_omp_allocate): Likewise.
	(cp_parser_omp_atomic): Likewise.
	(cp_parser_omp_depobj): Likewise.
	(cp_parser_omp_flush): Likewise.
	(cp_parser_omp_ordered): Likewise.
	(cp_parser_omp_declare_simd): Save in_omp_attribute_pragma
	into struct cp_omp_declare_simd_data.
	(cp_finish_omp_declare_variant): Add in_omp_attribute_pragma
	argument.  If set, allow a comma also before match clause.
	(cp_parser_late_parsing_omp_declare_simd): If in_omp_attribute_pragma,
	allow a comma also before the first clause.  Adjust
	cp_finish_omp_declare_variant caller.
	(cp_parser_omp_declare_target): If in_omp_attribute_pragma, allow
	a comma also before the first clause.
	(cp_parser_omp_declare_reduction_exprs): Likewise.
	(cp_parser_omp_requires): Likewise.
	* decl.c (initialize_predefined_identifiers): Initialize
	omp_identifier.
	* decl2.c (cplus_decl_attributes): Reject omp::directive and
	omp::sequence attributes.
gcc/testsuite/
	* g++.dg/gomp/attrs-1.C: New test.
	* g++.dg/gomp/attrs-2.C: New test.
	* g++.dg/gomp/attrs-3.C: New test.

1 files changed, 25 insertions, 0 deletions

diff --git a/gcc/cp/decl2.c b/gcc/cp/decl2.c
index 090a83b..9564b0d 100644
--- a/gcc/cp/decl2.c
+++ b/gcc/cp/decl2.c
@@ -1584,6 +1584,31 @@ cplus_decl_attributes (tree *decl, tree attributes, int flags)
 
   cp_check_const_attributes (attributes);
 
+  if ((flag_openmp || flag_openmp_simd) && attributes != error_mark_node)
+    {
+      bool diagnosed = false;
+      for (tree *pa = &attributes; *pa; )
+	{
+	  if (get_attribute_namespace (*pa) == omp_identifier)
+	    {
+	      tree name = get_attribute_name (*pa);
+	      if (is_attribute_p ("directive", name)
+		  || is_attribute_p ("sequence", name))
+		{
+		  if (!diagnosed)
+		    {
+		      error ("%<omp::%E%> not allowed to be specified in this "
+			     "context", name);
+		      diagnosed = true;
+		    }
+		  *pa = TREE_CHAIN (*pa);
+		  continue;
+		}
+	    }
+	  pa = &TREE_CHAIN (*pa);
+	}
+    }
+
   if (TREE_CODE (*decl) == TEMPLATE_DECL)
     decl = &DECL_TEMPLATE_RESULT (*decl);