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