Re: [PATCH 2/3] Compiler Attributes: add support for __copy (gcc >= 9)

[Nick Desaulniers <ndesaulniers@google.com>]

On Fri, Feb 8, 2019 at 4:09 PM Miguel Ojeda
<miguel.ojeda.sandonis@gmail.com> wrote:
>
> From the GCC manual:
>
>   copy
>   copy(function)
>
>     The copy attribute applies the set of attributes with which function
>     has been declared to the declaration of the function to which
>     the attribute is applied. The attribute is designed for libraries
>     that define aliases or function resolvers that are expected
>     to specify the same set of attributes as their targets. The copy
>     attribute can be used with functions, variables, or types. However,
>     the kind of symbol to which the attribute is applied (either
>     function or variable) must match the kind of symbol to which
>     the argument refers. The copy attribute copies only syntactic and
>     semantic attributes but not attributes that affect a symbol’s
>     linkage or visibility such as alias, visibility, or weak.
>     The deprecated attribute is also not copied.
>
>   https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html
>
> The upcoming GCC 9 release extends the -Wmissing-attributes warnings
> (enabled by -Wall) to C and aliases: it warns when particular function
> attributes are missing in the aliases but not in their target, e.g.:
>
>     void __cold f(void) {}
>     void __alias("f") g(void);
>
> diagnoses:
>
>     warning: 'g' specifies less restrictive attribute than
>     its target 'f': 'cold' [-Wmissing-attributes]
>
> Using __copy(f) we can copy the __cold attribute from f to g:
>
>     void __cold f(void) {}
>     void __copy(f) __alias("f") g(void);
>
> This attribute is most useful to deal with situations where an alias
> is declared but we don't know the exact attributes the target has.
>
> For instance, in the kernel, the widely used module_init/exit macros
> define the init/cleanup_module aliases, but those cannot be marked
> always as __init/__exit since they some modules do not have their
> functions marked as such.

Drop "they" from this sentence if there's a respin, otherwise looks helpful.
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>

>
> Suggested-by: Martin Sebor <msebor@gcc.gnu.org>
> Signed-off-by: Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>
> ---
>  include/linux/compiler_attributes.h | 14 ++++++++++++++
>  1 file changed, 14 insertions(+)
>
> diff --git a/include/linux/compiler_attributes.h b/include/linux/compiler_attributes.h
> index 19f32b0c29af..6b318efd8a74 100644
> --- a/include/linux/compiler_attributes.h
> +++ b/include/linux/compiler_attributes.h
> @@ -34,6 +34,7 @@
>  #ifndef __has_attribute
>  # define __has_attribute(x) __GCC4_has_attribute_##x
>  # define __GCC4_has_attribute___assume_aligned__      (__GNUC_MINOR__ >= 9)
> +# define __GCC4_has_attribute___copy__                0
>  # define __GCC4_has_attribute___designated_init__     0
>  # define __GCC4_has_attribute___externally_visible__  1
>  # define __GCC4_has_attribute___noclone__             1
> @@ -100,6 +101,19 @@
>   */
>  #define __attribute_const__             __attribute__((__const__))
>
> +/*
> + * Optional: only supported since gcc >= 9
> + * Optional: not supported by clang
> + * Optional: not supported by icc
> + *
> + *   gcc: https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#index-copy-function-attribute
> + */
> +#if __has_attribute(__copy__)
> +# define __copy(symbol)                 __attribute__((__copy__(symbol)))
> +#else
> +# define __copy(symbol)
> +#endif
> +
>  /*
>   * Don't. Just don't. See commit 771c035372a0 ("deprecate the '__deprecated'
>   * attribute warnings entirely and for good") for more information.
> --
> 2.17.1
>


-- 
Thanks,
~Nick Desaulniers