[RFC] libstc++: Implement gather and scatter

[Matthias Kretz]

Thank you! But please just call me Matthias :)

I already gave feedback at https://github.com/VcDevel/std-simd/pull/27. But 
I'd like to raise the issue of not-yet-proposed-but-we-want-to-write-a-paper-
soon features.

My understanding: If we define interfaces without using reserved names we're 
risking a source incompatible change (i.e. user code doesn't compile anymore 
after libstdc++ was modified to match the WG21 paper). I've seen non-standard 
extensions in libstdc++ use e.g. __member_function(...). I.e. without the _M_ 
or _S_ prefix. Is this the way to go? What about non-standard constructor 
overloads? We can't introduce a reserved name except via a tag type. 
Alternatively we could use a static function.

Cheers,
  Matthias

On Donnerstag, 18. Februar 2021 14:12:14 CET yaozhongxiao via Libstdc++ wrote:
> Memory load/store in gather and scatter mode with simd are common cases.
> I try to support the gather and scatter features in my workload.
> I send my draft to request for comment and suggestions before officially
> commits, Please do not hesitate to correct and comment, thanks.
> 
> Dr. Matthias Kretz, thank you very much for your work in simd,
> and hope to look forward to get suggestion from you.
> 
> --------------------------------------------------------------
>  libstc++: Implement gather and scatter
>     memory load/store in gather and scatter mode with simd are common
>     cases. Implement them in simd via call to _S_gather and _S_scatter
>     with it's Abi method.
> 
>     libstdc++-v3/ChangeLog:
> 
>             * include/experimental/bits/simd.h: Add simd::gather and
>             simd::scatter as the public methods.
>             * include/experimental/bits/simd_builtin.h: Add
>             _SimdImplBuiltin::_S_gather and _SimdImplBuiltin::_S_scatter
>             for simd native abi implementation.
>             * include/experimental/bits/simd_fixed_size.h: Add
>             _SimdImplFixedSize::_S_gather and _SimdImplFixedSize::_S_scatter
> for simd fix abi implementation;
>             _SimdTuple::_M_tuple_at for tuple accessing
>             * include/experimental/bits/ssimd_scalar: Add
>             _SimdImplScalar::_S_gather and _SimdImplScalar::_S_scatter
>             for simd scalar abi implementation.


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 Dr. Matthias Kretz                           https://mattkretz.github.io
 GSI Helmholtz Centre for Heavy Ion Research               https://gsi.de
 std::experimental::simd              https://github.com/VcDevel/std-simd
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