On Mon, Aug 05, 2024 at 01:33:01PM +0000, Qing Zhao wrote:
> As discussed in
> PR116016:https://gcc.gnu.org/bugzilla/show_bug.cgi?id=116016#c48
>
> We should explicitly document this limitation and issue error messages for
> C++.
>
> The "counted_by" attribute currently is only supported in C, mention this
> explicitly in documentation and also issue error when see "counted_by"
> attribute in C++.
>
> The patch has been bootstrappped and regression tested on both aarch64 and
> X86,
> no issue.
>
> Okay for committing?
>
> thanks.
>
> Qing
>
> gcc/c-family/ChangeLog:
>
> * c-attribs.cc (handle_counted_by_attribute): Issue error for C++.
>
> gcc/ChangeLog:
>
> * doc/extend.texi: Explicitly mentions counted_by is available
> only for C.
>
> gcc/testsuite/ChangeLog:
>
> * g++.dg/flex-array-counted-by.C: New test.
> ---
> gcc/c-family/c-attribs.cc | 9 ++++++++-
> gcc/doc/extend.texi | 1 +
> gcc/testsuite/g++.dg/flex-array-counted-by.C | 11 +++++++++++
> 3 files changed, 20 insertions(+), 1 deletion(-)
> create mode 100644 gcc/testsuite/g++.dg/flex-array-counted-by.C
>
> diff --git a/gcc/c-family/c-attribs.cc b/gcc/c-family/c-attribs.cc
> index 685f212683f..f936058800b 100644
> --- a/gcc/c-family/c-attribs.cc
> +++ b/gcc/c-family/c-attribs.cc
> @@ -2859,8 +2859,15 @@ handle_counted_by_attribute (tree *node, tree name,
> tree argval = TREE_VALUE (args);
> tree old_counted_by = lookup_attribute ("counted_by", DECL_ATTRIBUTES
> (decl));
>
> + /* This attribute is not supported in C++. */
> + if (c_dialect_cxx ())
> + {
> + error_at (DECL_SOURCE_LOCATION (decl),
> + "%qE attribute is not supported for C++", name);
This should be sorry_at instead IMHO (at least if there is a plan to support
it later, hopefully in the 15 timeframe).
> + *no_add_attrs = true;
> + }
> /* This attribute only applies to field decls of a structure. */
> - if (TREE_CODE (decl) != FIELD_DECL)
> + else if (TREE_CODE (decl) != FIELD_DECL)
> {
> error_at (DECL_SOURCE_LOCATION (decl),
> "%qE attribute is not allowed for a non-field"
> diff --git a/gcc/doc/extend.texi b/gcc/doc/extend.texi
> index 48b27ff9f39..f31f3bdb53d 100644
> --- a/gcc/doc/extend.texi
> +++ b/gcc/doc/extend.texi
> @@ -7848,6 +7848,7 @@ The @code{counted_by} attribute may be attached to the
> C99 flexible array
> member of a structure. It indicates that the number of the elements of the
> array is given by the field "@var{count}" in the same structure as the
> flexible array member.
> +This attribute is available only for C.
And this should say for now or something similar.
> GCC may use this information to improve detection of object size information
> for such structures and provide better results in compile-time diagnostics
> and runtime features like the array bound sanitizer and
> diff --git a/gcc/testsuite/g++.dg/flex-array-counted-by.C
> b/gcc/testsuite/g++.dg/flex-array-counted-by.C
> new file mode 100644
> index 00000000000..7f1a345615e
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/flex-array-counted-by.C
Tests shouldn't be added directly to g++.dg/ directory, I think this should
go into g++.dg/ext/ as it is an (unsupported) extension.
> @@ -0,0 +1,11 @@
> +/* Testing the fact that the attribute counted_by is not supported in C++.
> */
> +/* { dg-do compile } */
> +/* { dg-options "-O2" } */
> +
> +int size;
> +int x __attribute ((counted_by (size))); /* { dg-error "attribute is not
> supported for C\\+\\+" } */
> +
> +struct trailing {
> + int count;
> + int field[] __attribute ((counted_by (count))); /* { dg-error "attribute
> is not supported for C\\+\\+" } */
> +};
Maybe it should also test in another { dg-do compile { target c++11 } } test
that the same happens even for [[gnu::counted_by (size)]].
Seems even for C23 there are no tests with [[gnu::counted_by (size)]].
The C++11/C23 standard attributes are more strict on where they can appear
depending on what it appertains to, as it applies to declarations, I think
it needs to go before the [] or at the start of the declaration, so
[[gnu::counted_by (count)]] int field[];
or
int field [[gnu::counted_by (count)]] [];
but I could be wrong, better test it...
Jakub
