Re: [PATCH v2] libstdc++: add ARM SVE support to std::experimental::simd

[Matthias Kretz <m.kretz@gsi.de>]

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On Wednesday, 27 March 2024 11:07:14 CET Richard Sandiford wrote:
> I'm still worried about:
> 
>   #if _GLIBCXX_SIMD_HAVE_SVE
>   constexpr inline int __sve_vectorized_size_bytes = __ARM_FEATURE_SVE_BITS
> / 8; #else
>   constexpr inline int __sve_vectorized_size_bytes = 0;
>   #endif
> 
> and the direct use __ARM_FEATURE_SVE_BITS elsewhere, for the reasons
> discussed here (including possible ODR problems):
> 
>   https://gcc.gnu.org/pipermail/gcc-patches/2023-December/640037.html
>   https://gcc.gnu.org/pipermail/gcc-patches/2024-January/643734.html
> 
> Logically the vector length should be a template parameter rather than
> an invariant.  Has this been resolved?  If not, it feels like a blocker
> to me (sorry).

The vector length is always a template parameter to all user-facing API. Some 
examples

1. on aarch64 the following is independent of SVE flags (and status quo):

  simd<float> is an alias for
  simd<float, simd_abi::_VecBuiltin<16>

  fixed_size_simd<float, 4> is supposed to be ABI-stable anyway (passed via
  the stack, alignof == sizeof).

2. with -msve-vector-bits=512:

  native_simd<float> is an alias for
  simd<float, simd_abi::_SveAbi<64, 64>>

  simd<float, simd_abi::deduce_t<float, 4>> is an alias for
  simd<float, simd_abi::_SveAbi<16, 64>>

3. with -msve-vector-bits=256: 

  native_simd<float> is an alias for
  simd<float, simd_abi::_SveAbi<32, 32>>

  simd<float, simd_abi::deduce_t<float, 4>> is an alias for
  simd<float, simd_abi::_SveAbi<16, 32>>

Implementation functions are either [[gnu::always_inline]] or tagged with the 
ABI tag type and the __odr_helper template argument (to ensure not-inlined 
inline functions have unique names).

Does that make __ARM_FEATURE_SVE_BITS usage indirect enough?

Also for context, please consider that this is std::*experimental*::simd. The 
underlying ISO document will likely get retracted at some point and the whole 
API and implementation (hopefully) superseded by C++26. The main purpose of 
the spec and implementation is to gather experience.

Best,
  Matthias

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