[PATCH] Generalized attribute support

[Richard Smith]

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?

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++. )
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/cfe-commits/attachments/20140114/206dd216/attachment.html>