[PATCH] Generalized attribute support

[Richard Smith]

On Tue, Jan 14, 2014 at 5:47 PM, Richard Smith <richard at metafoo.co.uk>wrote:

> On Tue, Jan 14, 2014 at 5:23 PM, Alp Toker <alp at nuanti.com> wrote:
>
>> This patch generalizes C++11 attributes for use in C and C-like dialects,
>> and additionally enables the new syntax as an extension to C11.
>>
>
> Why do you allow this by default in C11? And conversely, why not in C90 /
> C99?
>

I think maybe this was unclear. I'd prefer one of these two options:

1) A -fcxx-attributes argument (or similar) to enable C++ attribute syntax
outside C++ (and either do or don't allow them by default in C++98), or
2) C++ attributes available by default in all language modes.

I'd prefer the first option (defaulting to following the language standard
more strictly) -- I think this would be the first time we enabled a C++
language feature in C modes, if we went with the second option.


> If we're going to allow these anywhere by default, C++98 would seem like a
> good place to start. I don't believe they introduce any ambiguities outside
> of Objective-C(++), and we already disambiguate those cases. (There's an
> ambiguity with lambdas, but we don't need to address that until/unless we
> allow lambdas in C++98.)
>
>
>> All features are carried forward from C++11, including usage on
>> declarations, attributed statements, scoped attribute names, GNU attribute
>> aliases and the clang-specific attribute namespace.
>>
>> A new feature detection macro is provided, breaking from the usual c/cxx
>> prefix convention in order to facilitate portable detection in C++ and C
>> modes:
>>
>> __has_feature(attributes) - 1 in C++11, otherwise 0.
>> __has_extension(attributes) - 1 in C++11 and C11, otherwise 0.
>>
>
> This is already available as __has_feature(cxx_attributes); using
> __has_extension(cxx_attributes) in C would seem to be the right approach
> here (we're allowing C++ attributes as an extension in C). This is what we
> already do for C99 and C11 features which we accept in C++.
>
> The new warning flag -W(no-)generalized-attributes suppresses the new
>> extension warning in C. The same flag can also be used to selectively
>> disable attribute compatibility warnings produced by the pre-existing
>> -Wc++98-compat option.
>>
>
> OK, so this is why you wanted us to pick a name for this feature; you're
> going to use it as a diagnostic name. I think this should be called
> -Wc++-attributes, to match our existing compatible-for-all-time feature
> name cxx_attributes. We can add an alias to a better name if we ever need
> one, but for now, we're pretty clearly allowing a C++ feature in C, so
> calling it "c++-something" makes sense to me.
>
> Newly added tests have been shared with C++11 where possible to ensure
>> consistency between language modes.
>
>
> Does this do the right thing for (for instance)
>
>   struct S {
>     [[ gnu::aligned(8) ]] int n;
>   };
>
> ? (Structs use different parsing code in C and C++. )
>
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