Re: [RFC 0/X] Implement GCC support for AArch64 libmvec

["Andre Vieira (lists)" <andre.simoesdiasvieira@arm.com>]

On 20/04/2023 17:13, Richard Sandiford wrote:
> "Andre Vieira (lists)" <andre.simoesdiasvieira@arm.com> writes:
>> On 20/04/2023 15:51, Richard Sandiford wrote:
>>> "Andre Vieira (lists)" <andre.simoesdiasvieira@arm.com> writes:
>>>> Hi all,
>>>>
>>>> This is a series of patches/RFCs to implement support in GCC to be able
>>>> to target AArch64's libmvec functions that will be/are being added to glibc.
>>>> We have chosen to use the omp pragma '#pragma omp declare variant ...'
>>>> with a simd construct as the way for glibc to inform GCC what functions
>>>> are available.
>>>>
>>>> For example, if we would like to supply a vector version of the scalar
>>>> 'cosf' we would have an include file with something like:
>>>> typedef __attribute__((__neon_vector_type__(4))) float __f32x4_t;
>>>> typedef __attribute__((__neon_vector_type__(2))) float __f32x2_t;
>>>> typedef __SVFloat32_t __sv_f32_t;
>>>> typedef __SVBool_t __sv_bool_t;
>>>> __f32x4_t _ZGVnN4v_cosf (__f32x4_t);
>>>> __f32x2_t _ZGVnN2v_cosf (__f32x2_t);
>>>> __sv_f32_t _ZGVsMxv_cosf (__sv_f32_t, __sv_bool_t);
>>>> #pragma omp declare variant(_ZGVnN4v_cosf) \
>>>>        match(construct = {simd(notinbranch, simdlen(4))}, device =
>>>> {isa("simd")})
>>>> #pragma omp declare variant(_ZGVnN2v_cosf) \
>>>>        match(construct = {simd(notinbranch, simdlen(2))}, device =
>>>> {isa("simd")})
>>>> #pragma omp declare variant(_ZGVsMxv_cosf) \
>>>>        match(construct = {simd(inbranch)}, device = {isa("sve")})
>>>> extern float cosf (float);
>>>>
>>>> The BETA ABI can be found in the vfabia64 subdir of
>>>> https://github.com/ARM-software/abi-aa/
>>>> This currently disagrees with how this patch series implements 'omp
>>>> declare simd' for SVE and I also do not see a need for the 'omp declare
>>>> variant' scalable extension constructs. I will make changes to the ABI
>>>> once we've finalized the co-design of the ABI and this implementation.
>>>
>>> I don't see a good reason for dropping the extension("scalable").
>>> The problem is that since the base spec requires a simdlen clause,
>>> GCC should in general raise an error if simdlen is omitted.
>> Where can you find this in the specs? I tried to find it but couldn't.
>>
>> Leaving out simdlen in a 'omp declare simd' I assume is OK, our vector
>> ABI defines behaviour for this. But I couldn't find what it meant for a
>> omp declare variant, obviously can't be the same as for declare simd, as
>> that is defined to mean 'define a set of clones' and only one clone can
>> be associated to a declare variant.
> 
> I was going from https://www.openmp.org/spec-html/5.0/openmpsu25.html ,
> which says:
> 
>    The simd trait can be further defined with properties that match the
>    clauses accepted by the declare simd directive with the same name and
>    semantics. The simd trait must define at least the simdlen property and
>    one of the inbranch or notinbranch properties.
> 
> (probably best to read it in the original -- it's almost incomprehensible
> without markup)
> 
I'm guessing the keyword here is 'trait' which I'm guessing is different 
from a omp declare simd directive, which is why it's not required to 
have a simdlen clause in an omp declare simd (see Jakub's comment).

But for declare variants I guess it does require you to? It doesn't 
'break' anything, just means I need to add support for parsing the 
extension clause as was originally planned.
> Richard