AArch64 vector ABI vs OpenMP

[Szabolcs Nagy <szabolcs.nagy@arm.com>]

Last time aarch64 libmvec was discussed, the OpenMP
declare variant syntax support was not ready in gcc
and there were open questions around how simd isa
variants would be supported.

https://gcc.gnu.org/pipermail/gcc-patches/2019-October/532940.html

The vector function ABI for aarch64 allows the declare
variant syntax and that is the only way to declare
vector math functions for a particular isa only.

https://github.com/ARM-software/abi-aa/blob/main/vfabia64/vfabia64.rst#aarch64-variant-traits

I would like to get feedback if there may be anything
preventing declare variant simd support on aarch64 like

  float64x2_t simd_cos (float64x2_t);

  #pragma omp declare variant(simd_cos) \
     match(construct={simd(simdlen(2), notinbranch)}, device={isa("simd")})
  double cos (double);

where isa("simd") means simd_cos can be used when
auto vectorizing cos calls with advanced simd.

Our hope is that this enables libmvec on aarch64
such that at least advanced simd variants of
some math functions can be declared in math.h
and implemented in libm, suitable for vectorization.
(Using the vector ABI names of those functions.)

Eventually we want to add isa("sve") support too,
but that may require further work on how scalable
vector length is represented.

Please let me know if there are outstanding issues
with this approach.

thanks.