On Tue, 5 Nov 2019, Szabolcs Nagy wrote: > GCC currently supports two ways to declare the availability of vector > variants of a scalar function: > > #pragma omp declare simd > void f (void); > > and > > __attribute__ ((simd)) > void f (void); > > However neither can declare unambiguously a single vector variant, only > a set of vector variants which may change in the future as new vector > architectures or vector call ABIs are introduced. So these mechanisms > are not suitable for libraries which must not declare more than what > they provide.
Rather, these mechanisms must imply a fixed set of variants defined at the time the ABI was set, with any new variants added in future requiring some different attribute / pragma to be specified, to avoid breaking existing libraries and headers. That's a requirement I made clear in the glibc review of the original libmvec addition, and is why the x86_64 vector ABI <https://sourceware.org/glibc/wiki/libmvec?action=AttachFile&do=view&target=VectorABI.txt> says "Compiler implementations must not generate calls to version of other ISAs unless some non-standard pragma or clause is used to declare those other versions are available.". -- Joseph S. Myers jos...@codesourcery.com
