Re: [PATCH RFC] c++: streamline process for adding new builtin trait

[Patrick Palka <ppalka@redhat.com>]

On Thu, 29 Sep 2022, Patrick Palka wrote:

> Adding a new builtin trait currently involves some boilerplate (as can
> be seen in r13-2956-g9ca147154074a0) of defining corresponding RID_ and
> CPTK_ enumerators and adding them to various switch statements across
> many files.  The exact switch statements we need to change is determined
> by whether the proposed trait yields a type or an expression.
> 
> This RFC patch attempts to streamline this process via a centralized
> cp-trait.def file for declaring the important parts about a builtin trait
> (whether it yields a type or an expression, its code, its spelling and
> its arity) and using this file to automate away the switch statement
> addition boilerplate.  It also converts 9 traits to use this approach
> by way of example (we can convert all the traits once the design is
> settled).
> 
> After this change, the process of adding a new builtin trait is just
> (modulo tests): declare it in cp-trait.def, define its behavior in
> finish_trait_type/expr, and handle it in diagnose_trait_expr if it's
> an expression-yielding trait (this last step is unfortunate but since
> the switch has no default case, we'll at least get a diagnostic if we
> forget to do it).

Here's an example of adding e.g. __remove_const using this framework:

 gcc/cp/cp-trait.def | 1 +
 gcc/cp/semantics.cc | 3 +++
 2 files changed, 4 insertions(+)

diff --git a/gcc/cp/cp-trait.def b/gcc/cp/cp-trait.def
index 817951f3e42..f6ad16e38cf 100644
--- a/gcc/cp/cp-trait.def
+++ b/gcc/cp/cp-trait.def
@@ -25,6 +25,7 @@ DEFTRAIT_EXPR (IS_NOTHROW_CONVERTIBLE, "__is_nothrow_convertible", 2)
 DEFTRAIT_TYPE (REMOVE_CV, "__remove_cv", 1)
 DEFTRAIT_TYPE (REMOVE_REFERENCE, "__remove_reference", 1)
 DEFTRAIT_TYPE (REMOVE_CVREF, "__remove_cvref", 1)
+DEFTRAIT_TYPE (REMOVE_CONST, "__remove_const", 1)
 
 #ifdef DEFTRAIT_EXPR_DEFAULTED
 #undef DEFTRAIT_EXPR
diff --git a/gcc/cp/semantics.cc b/gcc/cp/semantics.cc
index 66ee2186a84..eaf608085ae 100644
--- a/gcc/cp/semantics.cc
+++ b/gcc/cp/semantics.cc
@@ -12250,6 +12250,9 @@ finish_trait_type (cp_trait_kind kind, tree type1, tree type2)
       if (TYPE_REF_P (type1))
 	type1 = TREE_TYPE (type1);
       return cv_unqualified (type1);
+    case CPTK_REMOVE_CONST:
+      return cp_build_qualified_type (type1,
+				      cp_type_quals (type1) & ~TYPE_QUAL_CONST);
     default:
       gcc_unreachable ();
     }

That's it!

> 
> Does this look like a good approach?
> 
> gcc/c-family/ChangeLog:
> 
> 	* c-common.cc (c_common_reswords): Use cp/cp-trait.def
> 	to handle C++ traits.
> 	* c-common.h (enum rid): Likewise.
> 
> gcc/cp/ChangeLog:
> 
> 	* constraint.cc (diagnose_trait_expr): Likewise.
> 	* cp-objcp-common.cc (names_builtin_p): Likewise.
> 	* cp-tree.h (enum cp_trait_kind): Likewise.
> 	* cxx-pretty-print (pp_cxx_trait): Likewise.
> 	* parser.cc (cp_keyword_starts_decl_specifier_p): Likewise.
> 	(cp_parser_primary_expression): Likewise.
> 	(cp_parser_trait): Likewise.
> 	(cp_parser_simple_type_specifier): Likewise.
> 	* cp-trait.def: New file.
> ---
>  gcc/c-family/c-common.cc   | 13 +++-----
>  gcc/c-family/c-common.h    |  8 ++---
>  gcc/cp/constraint.cc       |  7 ++--
>  gcc/cp/cp-objcp-common.cc  | 13 +++-----
>  gcc/cp/cp-trait.def        | 37 +++++++++++++++++++++
>  gcc/cp/cp-tree.h           | 13 +++-----
>  gcc/cp/cxx-pretty-print.cc | 31 +++---------------
>  gcc/cp/parser.cc           | 67 ++++++++++++--------------------------
>  8 files changed, 82 insertions(+), 107 deletions(-)
>  create mode 100644 gcc/cp/cp-trait.def
> 
> diff --git a/gcc/c-family/c-common.cc b/gcc/c-family/c-common.cc
> index 6e0af863a49..1b2fd37c583 100644
> --- a/gcc/c-family/c-common.cc
> +++ b/gcc/c-family/c-common.cc
> @@ -537,19 +537,14 @@ const struct c_common_resword c_common_reswords[] =
>    { "volatile",		RID_VOLATILE,	0 },
>    { "wchar_t",		RID_WCHAR,	D_CXXONLY },
>    { "while",		RID_WHILE,	0 },
> -  { "__is_assignable", RID_IS_ASSIGNABLE, D_CXXONLY },
> -  { "__is_constructible", RID_IS_CONSTRUCTIBLE, D_CXXONLY },
> -  { "__is_nothrow_assignable", RID_IS_NOTHROW_ASSIGNABLE, D_CXXONLY },
> -  { "__is_nothrow_constructible", RID_IS_NOTHROW_CONSTRUCTIBLE, D_CXXONLY },
> -  { "__is_convertible", RID_IS_CONVERTIBLE, D_CXXONLY },
> -  { "__is_nothrow_convertible", RID_IS_NOTHROW_CONVERTIBLE, D_CXXONLY },
>    { "__reference_constructs_from_temporary", RID_REF_CONSTRUCTS_FROM_TEMPORARY,
>  					D_CXXONLY },
>    { "__reference_converts_from_temporary", RID_REF_CONVERTS_FROM_TEMPORARY,
>  					D_CXXONLY },
> -  { "__remove_cv", RID_REMOVE_CV, D_CXXONLY },
> -  { "__remove_reference", RID_REMOVE_REFERENCE, D_CXXONLY },
> -  { "__remove_cvref", RID_REMOVE_CVREF, D_CXXONLY },
> +#define DEFTRAIT(TCC, CODE, NAME, ARITY) \
> +  { NAME, RID_##CODE, D_CXXONLY },
> +#include "cp/cp-trait.def"
> +#undef DEFTRAIT
>  
>    /* C++ transactional memory.  */
>    { "synchronized",	RID_SYNCHRONIZED, D_CXX_OBJC | D_TRANSMEM },
> diff --git a/gcc/c-family/c-common.h b/gcc/c-family/c-common.h
> index d5c98d306ce..b306815c23b 100644
> --- a/gcc/c-family/c-common.h
> +++ b/gcc/c-family/c-common.h
> @@ -182,12 +182,12 @@ enum rid
>    RID_IS_TRIVIALLY_ASSIGNABLE, RID_IS_TRIVIALLY_CONSTRUCTIBLE,
>    RID_IS_TRIVIALLY_COPYABLE,
>    RID_IS_UNION,                RID_UNDERLYING_TYPE,
> -  RID_IS_ASSIGNABLE,           RID_IS_CONSTRUCTIBLE,
> -  RID_IS_NOTHROW_ASSIGNABLE,   RID_IS_NOTHROW_CONSTRUCTIBLE,
> -  RID_IS_CONVERTIBLE,		RID_IS_NOTHROW_CONVERTIBLE,
>    RID_REF_CONSTRUCTS_FROM_TEMPORARY,
>    RID_REF_CONVERTS_FROM_TEMPORARY,
> -  RID_REMOVE_CV, RID_REMOVE_REFERENCE, RID_REMOVE_CVREF,
> +#define DEFTRAIT(TCC, CODE, NAME, ARITY) \
> +  RID_##CODE,
> +#include "cp/cp-trait.def"
> +#undef DEFTRAIT
>  
>    /* C++11 */
>    RID_CONSTEXPR, RID_DECLTYPE, RID_NOEXCEPT, RID_NULLPTR, RID_STATIC_ASSERT,
> diff --git a/gcc/cp/constraint.cc b/gcc/cp/constraint.cc
> index ca73aff3f38..9323bb091e1 100644
> --- a/gcc/cp/constraint.cc
> +++ b/gcc/cp/constraint.cc
> @@ -3714,9 +3714,10 @@ diagnose_trait_expr (tree expr, tree args)
>      case CPTK_BASES:
>      case CPTK_DIRECT_BASES:
>      case CPTK_UNDERLYING_TYPE:
> -    case CPTK_REMOVE_CV:
> -    case CPTK_REMOVE_REFERENCE:
> -    case CPTK_REMOVE_CVREF:
> +#define DEFTRAIT_TYPE(CODE, NAME, ARITY) \
> +    case CPTK_##CODE:
> +#include "cp/cp-trait.def"
> +#undef DEFTRAIT_TYPE
>        /* We shouldn't see these non-expression traits.  */
>        gcc_unreachable ();
>      /* We deliberately omit the default case so that when adding a new
> diff --git a/gcc/cp/cp-objcp-common.cc b/gcc/cp/cp-objcp-common.cc
> index 2d3f206b530..bc397a5447f 100644
> --- a/gcc/cp/cp-objcp-common.cc
> +++ b/gcc/cp/cp-objcp-common.cc
> @@ -458,18 +458,13 @@ names_builtin_p (const char *name)
>      case RID_IS_TRIVIALLY_CONSTRUCTIBLE:
>      case RID_IS_TRIVIALLY_COPYABLE:
>      case RID_IS_UNION:
> -    case RID_IS_ASSIGNABLE:
> -    case RID_IS_CONSTRUCTIBLE:
> -    case RID_IS_NOTHROW_ASSIGNABLE:
> -    case RID_IS_NOTHROW_CONSTRUCTIBLE:
>      case RID_UNDERLYING_TYPE:
> -    case RID_IS_CONVERTIBLE:
> -    case RID_IS_NOTHROW_CONVERTIBLE:
>      case RID_REF_CONSTRUCTS_FROM_TEMPORARY:
>      case RID_REF_CONVERTS_FROM_TEMPORARY:
> -    case RID_REMOVE_CV:
> -    case RID_REMOVE_REFERENCE:
> -    case RID_REMOVE_CVREF:
> +#define DEFTRAIT(TCC, CODE, NAME, ARITY) \
> +    case RID_##CODE:
> +#include "cp/cp-trait.def"
> +#undef DEFTRAIT
>        return true;
>      default:
>        break;
> diff --git a/gcc/cp/cp-trait.def b/gcc/cp/cp-trait.def
> new file mode 100644
> index 00000000000..817951f3e42
> --- /dev/null
> +++ b/gcc/cp/cp-trait.def
> @@ -0,0 +1,37 @@
> +#ifdef DEFTRAIT
> +#define DEFTRAIT_EXPR(CODE, NAME, ARITY) DEFTRAIT(tcc_expression, CODE, NAME, ARITY)
> +#define DEFTRAIT_TYPE(CODE, NAME, ARITY) DEFTRAIT(tcc_type, CODE, NAME, ARITY)
> +#define DEFTRAIT_EXPR_DEFAULTED
> +#define DEFTRAIT_TYPE_DEFAULTED
> +#endif
> +
> +#ifndef DEFTRAIT_EXPR
> +#define DEFTRAIT_EXPR(CODE, NAME, ARITY)
> +#define DEFTRAIT_EXPR_DEFAULTED
> +#endif
> +
> +#ifndef DEFTRAIT_TYPE
> +#define DEFTRAIT_TYPE(CODE, NAME, ARITY)
> +#define DEFTRAIT_TYPE_DEFAULTED
> +#endif
> +
> +DEFTRAIT_EXPR (IS_ASSIGNABLE, "__is_assignable", 2)
> +DEFTRAIT_EXPR (IS_NOTHROW_ASSIGNABLE, "__is_nothrow_assignable", 2)
> +DEFTRAIT_EXPR (IS_CONSTRUCTIBLE, "__is_constructible", -1)
> +DEFTRAIT_EXPR (IS_NOTHROW_CONSTRUCTIBLE, "__is_nothrow_constructible", -1)
> +DEFTRAIT_EXPR (IS_CONVERTIBLE, "__is_convertible", 2)
> +DEFTRAIT_EXPR (IS_NOTHROW_CONVERTIBLE, "__is_nothrow_convertible", 2)
> +
> +DEFTRAIT_TYPE (REMOVE_CV, "__remove_cv", 1)
> +DEFTRAIT_TYPE (REMOVE_REFERENCE, "__remove_reference", 1)
> +DEFTRAIT_TYPE (REMOVE_CVREF, "__remove_cvref", 1)
> +
> +#ifdef DEFTRAIT_EXPR_DEFAULTED
> +#undef DEFTRAIT_EXPR
> +#undef DEFTRAIT_EXPR_DEFAULTED
> +#endif
> +
> +#ifdef DEFTRAIT_TYPE_DEFAULTED
> +#undef DEFTRAIT_TYPE
> +#undef DEFTRAIT_TYPE_DEFAULTED
> +#endif
> diff --git a/gcc/cp/cp-tree.h b/gcc/cp/cp-tree.h
> index 3cbcdf726ca..8c5a85ab5fe 100644
> --- a/gcc/cp/cp-tree.h
> +++ b/gcc/cp/cp-tree.h
> @@ -1405,17 +1405,12 @@ enum cp_trait_kind
>    CPTK_IS_TRIVIALLY_COPYABLE,
>    CPTK_IS_UNION,
>    CPTK_UNDERLYING_TYPE,
> -  CPTK_IS_ASSIGNABLE,
> -  CPTK_IS_CONSTRUCTIBLE,
> -  CPTK_IS_NOTHROW_ASSIGNABLE,
> -  CPTK_IS_NOTHROW_CONSTRUCTIBLE,
> -  CPTK_IS_CONVERTIBLE,
> -  CPTK_IS_NOTHROW_CONVERTIBLE,
>    CPTK_REF_CONSTRUCTS_FROM_TEMPORARY,
>    CPTK_REF_CONVERTS_FROM_TEMPORARY,
> -  CPTK_REMOVE_CV,
> -  CPTK_REMOVE_REFERENCE,
> -  CPTK_REMOVE_CVREF,
> +#define DEFTRAIT(TCC, CODE, NAME, ARITY) \
> +  CPTK_##CODE,
> +#include "cp/cp-trait.def"
> +#undef DEFTRAIT
>  };
>  
>  /* The types that we are processing.  */
> diff --git a/gcc/cp/cxx-pretty-print.cc b/gcc/cp/cxx-pretty-print.cc
> index b91615439e4..a2ddbcb899a 100644
> --- a/gcc/cp/cxx-pretty-print.cc
> +++ b/gcc/cp/cxx-pretty-print.cc
> @@ -2701,24 +2701,6 @@ pp_cxx_trait (cxx_pretty_printer *pp, tree t)
>      case CPTK_IS_LITERAL_TYPE:
>        pp_cxx_ws_string (pp, "__is_literal_type");
>        break;
> -    case CPTK_IS_ASSIGNABLE:
> -      pp_cxx_ws_string (pp, "__is_assignable");
> -      break;
> -    case CPTK_IS_CONSTRUCTIBLE:
> -      pp_cxx_ws_string (pp, "__is_constructible");
> -      break;
> -    case CPTK_IS_NOTHROW_ASSIGNABLE:
> -      pp_cxx_ws_string (pp, "__is_nothrow_assignable");
> -      break;
> -    case CPTK_IS_NOTHROW_CONSTRUCTIBLE:
> -      pp_cxx_ws_string (pp, "__is_nothrow_constructible");
> -      break;
> -    case CPTK_IS_CONVERTIBLE:
> -      pp_cxx_ws_string (pp, "__is_convertible");
> -      break;
> -    case CPTK_IS_NOTHROW_CONVERTIBLE:
> -      pp_cxx_ws_string (pp, "__is_nothrow_convertible");
> -      break;
>      case CPTK_REF_CONSTRUCTS_FROM_TEMPORARY:
>        pp_cxx_ws_string (pp, "__reference_constructs_from_temporary");
>        break;
> @@ -2728,15 +2710,12 @@ pp_cxx_trait (cxx_pretty_printer *pp, tree t)
>      case CPTK_UNDERLYING_TYPE:
>        pp_cxx_ws_string (pp, "__underlying_type");
>        break;
> -    case CPTK_REMOVE_CV:
> -      pp_cxx_ws_string (pp, "__remove_cv");
> -      break;
> -    case CPTK_REMOVE_REFERENCE:
> -      pp_cxx_ws_string (pp, "__remove_reference");
> -      break;
> -    case CPTK_REMOVE_CVREF:
> -      pp_cxx_ws_string (pp, "__remove_cvref");
> +#define DEFTRAIT(TCC, CODE, NAME, ARITY) \
> +    case CPTK_##CODE:			 \
> +      pp_cxx_ws_string (pp, NAME);	 \
>        break;
> +#include "cp/cp-trait.def"
> +#undef DEFTRAIT
>      default:
>        gcc_unreachable ();
>      }
> diff --git a/gcc/cp/parser.cc b/gcc/cp/parser.cc
> index d592d783250..138892716b4 100644
> --- a/gcc/cp/parser.cc
> +++ b/gcc/cp/parser.cc
> @@ -1147,9 +1147,10 @@ cp_keyword_starts_decl_specifier_p (enum rid keyword)
>        /* C++11 extensions.  */
>      case RID_DECLTYPE:
>      case RID_UNDERLYING_TYPE:
> -    case RID_REMOVE_CV:
> -    case RID_REMOVE_REFERENCE:
> -    case RID_REMOVE_CVREF:
> +#define DEFTRAIT_TYPE(CODE, NAME, ARITY) \
> +    case RID_##CODE:
> +#include "cp/cp-trait.def"
> +#undef DEFTRAIT_TYPE
>      case RID_CONSTEXPR:
>        /* C++20 extensions.  */
>      case RID_CONSTINIT:
> @@ -5923,14 +5924,12 @@ cp_parser_primary_expression (cp_parser *parser,
>  	case RID_IS_TRIVIALLY_CONSTRUCTIBLE:
>  	case RID_IS_TRIVIALLY_COPYABLE:
>  	case RID_IS_UNION:
> -	case RID_IS_ASSIGNABLE:
> -	case RID_IS_CONSTRUCTIBLE:
> -	case RID_IS_NOTHROW_ASSIGNABLE:
> -	case RID_IS_NOTHROW_CONSTRUCTIBLE:
> -	case RID_IS_CONVERTIBLE:
> -	case RID_IS_NOTHROW_CONVERTIBLE:
>  	case RID_REF_CONSTRUCTS_FROM_TEMPORARY:
>  	case RID_REF_CONVERTS_FROM_TEMPORARY:
> +#define DEFTRAIT_EXPR(CODE, NAME, ARITY) \
> +	case RID_##CODE:
> +#include "cp/cp-trait.def"
> +#undef DEFTRAIT_EXPR
>  	  return cp_parser_trait (parser, token->keyword);
>  
>  	// C++ concepts
> @@ -10998,30 +10997,6 @@ cp_parser_trait (cp_parser* parser, enum rid keyword)
>      case RID_DIRECT_BASES:
>        kind = CPTK_DIRECT_BASES;
>        break;
> -    case RID_IS_ASSIGNABLE:
> -      kind = CPTK_IS_ASSIGNABLE;
> -      binary = true;
> -      break;
> -    case RID_IS_CONSTRUCTIBLE:
> -      kind = CPTK_IS_CONSTRUCTIBLE;
> -      variadic = true;
> -      break;
> -    case RID_IS_NOTHROW_ASSIGNABLE:
> -      kind = CPTK_IS_NOTHROW_ASSIGNABLE;
> -      binary = true;
> -      break;
> -    case RID_IS_NOTHROW_CONSTRUCTIBLE:
> -      kind = CPTK_IS_NOTHROW_CONSTRUCTIBLE;
> -      variadic = true;
> -      break;
> -    case RID_IS_CONVERTIBLE:
> -      kind = CPTK_IS_CONVERTIBLE;
> -      binary = true;
> -      break;
> -    case RID_IS_NOTHROW_CONVERTIBLE:
> -      kind = CPTK_IS_NOTHROW_CONVERTIBLE;
> -      binary = true;
> -      break;
>      case RID_REF_CONSTRUCTS_FROM_TEMPORARY:
>        kind = CPTK_REF_CONSTRUCTS_FROM_TEMPORARY;
>        binary = true;
> @@ -11030,18 +11005,15 @@ cp_parser_trait (cp_parser* parser, enum rid keyword)
>        kind = CPTK_REF_CONVERTS_FROM_TEMPORARY;
>        binary = true;
>        break;
> -    case RID_REMOVE_CV:
> -      kind = CPTK_REMOVE_CV;
> -      type = true;
> -      break;
> -    case RID_REMOVE_REFERENCE:
> -      kind = CPTK_REMOVE_REFERENCE;
> -      type = true;
> -      break;
> -    case RID_REMOVE_CVREF:
> -      kind = CPTK_REMOVE_CVREF;
> -      type = true;
> +#define DEFTRAIT(TCC, CODE, NAME, ARITY) \
> +    case RID_##CODE:			 \
> +      type = (TCC == tcc_type);		 \
> +      kind = CPTK_##CODE;		 \
> +      binary = (ARITY == 2);		 \
> +      variadic = (ARITY == -1);		 \
>        break;
> +#include "cp/cp-trait.def"
> +#undef DEFTRAIT
>      default:
>        gcc_unreachable ();
>      }
> @@ -19882,9 +19854,10 @@ cp_parser_simple_type_specifier (cp_parser* parser,
>        return type;
>  
>      case RID_UNDERLYING_TYPE:
> -    case RID_REMOVE_CV:
> -    case RID_REMOVE_REFERENCE:
> -    case RID_REMOVE_CVREF:
> +#define DEFTRAIT_TYPE(CODE, NAME, ARITY) \
> +    case RID_##CODE:
> +#include "cp/cp-trait.def"
> +#undef DEFTRAIT_TYPE
>        type = cp_parser_trait (parser, token->keyword);
>        if (decl_specs)
>  	cp_parser_set_decl_spec_type (decl_specs, type,
> -- 
> 2.38.0.rc2
> 
>