On Wed, May 21, 2025 at 11:01:31PM +0200, Alejandro Colomar wrote:
> This operator is similar to sizeof but can only be applied to an array,
> and returns its number of elements.
>
> FUTURE DIRECTIONS:
>
> - We should make it work with array parameters to functions,
> and somehow magically return the number of elements of the array,
> regardless of it being really a pointer.
>
> Link: <https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3550.pdf>
> Link: <https://gcc.gnu.org/bugzilla/show_bug.cgi?id=117025>
> Link: <https://inbox.sourceware.org/gcc/m8s4oqy--...@tutanota.com/T/>
> Link:
> <https://inbox.sourceware.org/gcc-patches/20240728141547.302478-1-...@kernel.org/T/#t>
> Link: <https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3313.pdf>
> Link: <https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3325.pdf>
> Link: <https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3369.pdf>
> Link: <https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3469.htm>
> Link: <https://github.com/llvm/llvm-project/issues/102836>
> Link: <https://thephd.dev/the-big-array-size-survey-for-c>
> Link: <https://thephd.dev/the-big-array-size-survey-for-c-results>
> Link: <https://stackoverflow.com/questions/37538/#57537491>
>
> gcc/ChangeLog:
>
> * doc/extend.texi: Document _Countof operator.
>
> gcc/c-family/ChangeLog:
>
> * c-common.h: Add RID_COUNTOF.
(enum rid): Add RID_COUNTOF.
> (c_countof_type): New function prototype.
> * c-common.def (COUNTOF_EXPR): New tree.
> * c-common.cc
> (c_common_reswords): Add RID_COUNTOF entry.
No newline in between the line with file and (c_common_reswords).
* c-common.cc (c_common_reswords): Add RID_COUNTOF entry.
fits just fine. And even if the description wouldn't fit completely,
you'd wrap on the first word that doesn't fit.
> (c_countof_type): New function.
>
> gcc/c/ChangeLog:
>
> * c-tree.h
> (in_countof): Add global variable declaration.
Ditto.
> (c_expr_countof_expr): Add function prototype.
> (c_expr_countof_type): Add function prototype.
> * c-decl.cc
> (start_struct, finish_struct): Add support for _Countof.
Ditto.
> (start_enum, finish_enum): Add support for _Countof.
> * c-parser.cc
> (c_parser_sizeof_expression): New macro.
Ditto.
> (c_parser_countof_expression): New macro.
> (c_parser_sizeof_or_countof_expression):
> Rename function and add support for _Countof.
> (c_parser_unary_expression): Add RID_COUNTOF entry.
> * c-typeck.cc
> (in_countof): Add global variable.
Ditto.
> (build_external_ref): Add support for _Countof.
> (record_maybe_used_decl): Add support for _Countof.
> (pop_maybe_used): Add support for _Countof.
> (is_top_array_vla): New function.
> (c_expr_countof_expr, c_expr_countof_type): New functions.
> Add _Countof operator.
>
> gcc/testsuite/ChangeLog:
>
> * gcc.dg/countof-compile.c: Compile-time tests for _Countof.
> * gcc.dg/countof-vla.c: Tests for _Countof with VLAs.
> * gcc.dg/countof-vmt.c: Tests for _Countof with other VMTs.
> * gcc.dg/countof-zero-compile.c:
> Compile-time tests for _Countof with zero-sized arrays.
> * gcc.dg/countof-zero.c:
> Tests for _Countof with zero-sized arrays.
> * gcc.dg/countof.c: Tests for _Countof.
>
> Suggested-by: Xavier Del Campo Romero <xavi....@tutanota.com>
> Co-authored-by: Martin Uecker <uec...@tugraz.at>
> Acked-by: "James K. Lowden" <jklow...@schemamania.org>
> Signed-off-by: Alejandro Colomar <a...@kernel.org>
> ---
> gcc/c-family/c-common.cc | 26 ++++
> gcc/c-family/c-common.def | 3 +
> gcc/c-family/c-common.h | 2 +
> gcc/c/c-decl.cc | 22 +++-
> gcc/c/c-parser.cc | 63 +++++++---
> gcc/c/c-tree.h | 4 +
> gcc/c/c-typeck.cc | 115 +++++++++++++++++-
> gcc/doc/extend.texi | 30 +++++
> gcc/testsuite/gcc.dg/countof-compile.c | 124 ++++++++++++++++++++
> gcc/testsuite/gcc.dg/countof-vla.c | 35 ++++++
> gcc/testsuite/gcc.dg/countof-vmt.c | 20 ++++
> gcc/testsuite/gcc.dg/countof-zero-compile.c | 38 ++++++
> gcc/testsuite/gcc.dg/countof-zero.c | 31 +++++
> gcc/testsuite/gcc.dg/countof.c | 120 +++++++++++++++++++
> 14 files changed, 609 insertions(+), 24 deletions(-)
> create mode 100644 gcc/testsuite/gcc.dg/countof-compile.c
> create mode 100644 gcc/testsuite/gcc.dg/countof-vla.c
> create mode 100644 gcc/testsuite/gcc.dg/countof-vmt.c
> create mode 100644 gcc/testsuite/gcc.dg/countof-zero-compile.c
> create mode 100644 gcc/testsuite/gcc.dg/countof-zero.c
> create mode 100644 gcc/testsuite/gcc.dg/countof.c
>
> diff --git a/gcc/c-family/c-common.cc b/gcc/c-family/c-common.cc
> index 587d76461e9e..f71cb2652d5a 100644
> --- a/gcc/c-family/c-common.cc
> +++ b/gcc/c-family/c-common.cc
> @@ -394,6 +394,7 @@ const struct c_common_resword c_common_reswords[] =
> {
> { "_Alignas", RID_ALIGNAS, D_CONLY },
> { "_Alignof", RID_ALIGNOF, D_CONLY },
> + { "_Countof", RID_COUNTOF, D_CONLY },
> { "_Atomic", RID_ATOMIC, D_CONLY },
> { "_BitInt", RID_BITINT, D_CONLY },
> { "_Bool", RID_BOOL, D_CONLY },
> @@ -4080,6 +4081,31 @@ c_alignof_expr (location_t loc, tree expr)
>
> return fold_convert_loc (loc, size_type_node, t);
> }
> +
> +/* Implement the _Countof keyword:
> + Return the number of elements of an array. */
> +
> +tree
> +c_countof_type (location_t loc, tree type)
> +{
> + enum tree_code type_code;
> +
> + type_code = TREE_CODE (type);
> + if (type_code != ARRAY_TYPE)
> + {
> + error_at (loc, "invalid application of %<_Countof%> to type %qT",
> type);
> + return error_mark_node;
> + }
> + if (!COMPLETE_TYPE_P (type))
> + {
> + error_at (loc,
> + "invalid application of %<_Countof%> to incomplete type %qT",
> + type);
> + return error_mark_node;
> + }
> +
> + return array_type_nelts_top (type);
> +}
>
> /* Handle C and C++ default attributes. */
>
> diff --git a/gcc/c-family/c-common.def b/gcc/c-family/c-common.def
> index cf2228201fad..0bcc4998afe6 100644
> --- a/gcc/c-family/c-common.def
> +++ b/gcc/c-family/c-common.def
> @@ -50,6 +50,9 @@ DEFTREECODE (EXCESS_PRECISION_EXPR,
> "excess_precision_expr", tcc_expression, 1)
> number. */
> DEFTREECODE (USERDEF_LITERAL, "userdef_literal", tcc_exceptional, 3)
>
> +/* Represents a 'countof' expression. */
> +DEFTREECODE (COUNTOF_EXPR, "countof_expr", tcc_expression, 1)
> +
> /* Represents a 'sizeof' expression during C++ template expansion,
> or for the purpose of -Wsizeof-pointer-memaccess warning. */
> DEFTREECODE (SIZEOF_EXPR, "sizeof_expr", tcc_expression, 1)
> diff --git a/gcc/c-family/c-common.h b/gcc/c-family/c-common.h
> index ea6c29750567..91fd120e77e1 100644
> --- a/gcc/c-family/c-common.h
> +++ b/gcc/c-family/c-common.h
> @@ -105,6 +105,7 @@ enum rid
>
> /* C extensions */
> RID_ASM, RID_TYPEOF, RID_TYPEOF_UNQUAL, RID_ALIGNOF,
> RID_ATTRIBUTE,
> + RID_COUNTOF,
> RID_C23_VA_START, RID_VA_ARG,
> RID_EXTENSION, RID_IMAGPART, RID_REALPART, RID_LABEL, RID_CHOOSE_EXPR,
> RID_TYPES_COMPATIBLE_P, RID_BUILTIN_COMPLEX, RID_BUILTIN_SHUFFLE,
> @@ -890,6 +891,7 @@ extern tree c_common_truthvalue_conversion (location_t,
> tree);
> extern void c_apply_type_quals_to_decl (int, tree);
> extern tree c_sizeof_or_alignof_type (location_t, tree, bool, bool, int);
> extern tree c_alignof_expr (location_t, tree);
> +extern tree c_countof_type (location_t, tree);
> /* Print an error message for invalid operands to arith operation CODE.
> NOP_EXPR is used as a special case (see truthvalue_conversion). */
> extern void binary_op_error (rich_location *, enum tree_code, tree, tree);
> diff --git a/gcc/c/c-decl.cc b/gcc/c/c-decl.cc
> index ad66d7d258b8..5bf638bfbd81 100644
> --- a/gcc/c/c-decl.cc
> +++ b/gcc/c/c-decl.cc
> @@ -8943,12 +8943,17 @@ start_struct (location_t loc, enum tree_code code,
> tree name,
> within a statement expr used within sizeof, et. al. This is not
> terribly serious as C++ doesn't permit statement exprs within
> sizeof anyhow. */
> - if (warn_cxx_compat && (in_sizeof || in_typeof || in_alignof))
> + if (warn_cxx_compat
> + && (in_sizeof || in_typeof || in_alignof || in_countof))
> warning_at (loc, OPT_Wc___compat,
> "defining type in %qs expression is invalid in C++",
> (in_sizeof
> ? "sizeof"
> - : (in_typeof ? "typeof" : "alignof")));
> + : (in_typeof
> + ? "typeof"
> + : (in_alignof
> + ? "alignof"
> + : "_Countof"))));
Why so many lines? Plus no idea why there are the ()s around, I see it is
preexisting for the outermost where it can help emacs formatting, but the
rest doesn't need that.
> "defining type in %qs expression is invalid in C++",
> (in_sizeof
> ? "sizeof"
> - : (in_typeof ? "typeof" : "alignof")));
> + : (in_typeof
> + ? "typeof"
> + : (in_alignof
> + ? "alignof"
> + : "_Countof"))));
Ditto.
> @@ -77,7 +77,17 @@ along with GCC; see the file COPYING3. If not see
> #include "asan.h"
> #include "c-family/c-ubsan.h"
> #include "gcc-urlifier.h"
> +
> +#define c_parser_sizeof_expression(parser)
> \
> +(
> \
> + c_parser_sizeof_or_countof_expression (parser, RID_SIZEOF)
> \
> +)
>
> +#define c_parser_countof_expression(parser)
> \
> +(
> \
> + c_parser_sizeof_or_countof_expression (parser, RID_COUNTOF)
> \
> +)
Up to Joseph, but I'd say these should just be inline functions, not macros,
and defined after the declarations.
Jakub
