C23 added in N2956 ( https://open-std.org/JTC1/SC22/WG14/www/docs/n2956.htm ) two new attributes, which are described as similar to GCC const and pure attributes, but they aren't really same and it seems that even the paper is missing some of the differences. The paper says unsequenced is the same as const on functions without pointer arguments and reproducible is the same as pure on such functions (except that they are function type attributes rather than function declaration ones), but it seems the paper doesn't consider the finiteness GCC relies on (aka non-DECL_LOOPING_CONST_OR_PURE_P) - the paper only talks about using the attributes for CSE etc., not for DCE. The following patch introduces (for now limited) support for those attributes, both as standard C23 attributes and as GNU extensions (the difference is that the patch is then less strict on where it allows them, like other function type attributes they can be specified on function declarations as well and apply to the type, while C23 standard ones must go on the function declarators (i.e. after closing paren after function parameters) or in type specifiers of function type. If function doesn't have any pointer/reference arguments, the patch adds additional internal attribute with " noptr" suffix which then is used by flags_from_decl_or_type to handle those easy cases as ECF_CONST|ECF_LOOPING_CONST_OR_PURE or ECF_PURE|ECF_LOOPING_CONST_OR_PURE The harder cases aren't handled right now, I'd hope they can be handled incrementally. I wonder whether we shouldn't emit a warning for the gcc.dg/c23-attr-{reproducible,unsequenced}-5.c cases, while the standard clearly specifies that composite types should union the attributes and it is what GCC implements for decades, for ?: that feels dangerous for the new attributes, it would be much better to be conservative on say (cond ? unsequenced_function : normal_function) (args) There is no diagnostics on incorrect [[unsequenced]] or [[reproducible]] function definitions, while I think diagnosing non-const static/TLS declarations in the former could be easy, the rest feels hard. E.g. the const/pure discovery can just punt on everything it doesn't understand, but complete diagnostics would need to understand it. 2024-08-31 Jakub Jelinek <jakub@redhat.com> PR c/116130 gcc/ * doc/extend.texi (unsequenced, reproducible): Document new function type attributes. * calls.cc (flags_from_decl_or_type): Handle "unsequenced noptr" and "reproducible noptr" attributes. gcc/c-family/ * c-attribs.cc (c_common_gnu_attributes): Add entries for "unsequenced", "reproducible", "unsequenced noptr" and "reproducible noptr" attributes. (handle_unsequenced_attribute): New function. (handle_reproducible_attribute): Likewise. * c-common.h (handle_unsequenced_attribute): Declare. (handle_reproducible_attribute): Likewise. * c-lex.cc (c_common_has_attribute): Return 202311 for standard unsequenced and reproducible attributes. gcc/c/ * c-decl.cc (handle_std_unsequenced_attribute): New function. (handle_std_reproducible_attribute): Likewise. (std_attributes): Add entries for "unsequenced" and "reproducible" attributes. (c_warn_type_attributes): Add TYPE argument. Allow unsequenced or reproducible attributes if it is FUNCTION_TYPE. (groktypename): Adjust c_warn_type_attributes caller. (grokdeclarator): Likewise. (finish_declspecs): Likewise. * c-parser.cc (c_parser_declaration_or_fndef): Likewise. * c-tree.h (c_warn_type_attributes): Add TYPE argument. gcc/testsuite/ * c-c++-common/attr-reproducible-1.c: New test. * c-c++-common/attr-reproducible-2.c: New test. * c-c++-common/attr-unsequenced-1.c: New test. * c-c++-common/attr-unsequenced-2.c: New test. * gcc.dg/c23-attr-reproducible-1.c: New test. * gcc.dg/c23-attr-reproducible-2.c: New test. * gcc.dg/c23-attr-reproducible-3.c: New test. * gcc.dg/c23-attr-reproducible-4.c: New test. * gcc.dg/c23-attr-reproducible-5.c: New test. * gcc.dg/c23-attr-reproducible-5-aux.c: New file. * gcc.dg/c23-attr-unsequenced-1.c: New test. * gcc.dg/c23-attr-unsequenced-2.c: New test. * gcc.dg/c23-attr-unsequenced-3.c: New test. * gcc.dg/c23-attr-unsequenced-4.c: New test. * gcc.dg/c23-attr-unsequenced-5.c: New test. * gcc.dg/c23-attr-unsequenced-5-aux.c: New file. * gcc.dg/c23-has-c-attribute-2.c: Add tests for unsequenced and reproducible attributes.