On Sun, Jul 16, 2023 at 6:50 AM Richard Sandiford <richard.sandiford@arm.com>
wrote:

> Jakub Jelinek <jakub@redhat.com> writes:
> > On Fri, Jul 14, 2023 at 04:56:18PM +0100, Richard Sandiford via
> Gcc-patches wrote:
> >> Summary: We'd like to be able to specify some attributes using
> >> keywords, rather than the traditional __attribute__ or [[...]]
> >> syntax.  Would that be OK?
> >
> > Will defer to C/C++ maintainers, but as you mentioned, there are many
> > attributes which really can't be ignored and change behavior
> significantly.
> > vector_size is one of those, mode attribute another,
> > no_unique_address another one (changes ABI in various cases),
> > the OpenMP attributes (omp::directive, omp::sequence) can change
> > behavior if -fopenmp, etc.
> > One can easily error with
> > #ifdef __has_cpp_attribute
> > #if !__has_cpp_attribute (arm::whatever)
> > #error arm::whatever attribute unsupported
> > #endif
> > #else
> > #error __has_cpp_attribute unsupported
> > #endif
>
> Yeah.  It's easy to detect whether a particular ACLE feature is supported,
> since there are predefined macros for each one.  But IMO it's a failing
> if we have to recommend that any compilation that uses arm::foo should
> also have:
>
> #ifndef __ARM_FEATURE_FOO
> #error arm::foo not supported
> #endif
>
> It ought to be the compiler's job to diagnose its limitations, rather
> than the user's.
>
> The feature macros are more for conditional usage of features, where
> there's a fallback available.
>
> I suppose we could say that users have to include a particular header
> file before using an attribute, and use a pragma in that header file to
> tell the compiler to enable the attribute.  But then there would need to
> be a separate header file for each distinct set of attributes (in terms
> of historical timeline), which would get ugly.  I'm not sure that it's
> better than using keywords, or even whether it's better than predefining
> the "keyword" as a macro that expands to a compiler-internal attribute.
>

With a combination of those approaches it can be a single header:

#ifdef __ARM_FEATURE_FOO
#define __arm_foo [[arm::foo]]
// else use of __arm_foo will fail
#endif