Determining maximum vector length supported by the CPU?

[Matthias Kretz]

Hi Martin,

I agree, we need more information from the compiler. Esp. whether the user 
specified `-mprefer-avx128` or `-mprefer-vector-width=none/128/256/512`.
OTOH `-msve-vector-bits=N` is reported as __ARM_FEATURE_SVE_BITS. So that's 
covered.

Related: PR83875 - because while we're adding things in that area, it'd be 
nice if they worked with target clones as well.

Are you aware of std::experimental::simd? It didn't make GCC 9.1, but you 
can easily patch your (installed) libstdc++ using https://github.com/VcDevel/
std-simd.

Cheers,
  Matthias

On Mittwoch, 22. Mai 2019 08:39:25 CEST Martin Reinecke wrote:
> [Disclaimer: I sent this to gcc-help two weeks ago, but didn't get an
> answer. Maybe the topic is more suited for the main gcc list ... I
> really think the feature in question would be extremely useful to have,
> and easy to add!]
> 
> Hi,
> 
> I'm currently writing an FFT library which tries to make use of SIMD
> instructions and uses a lot of variables with
>  __attribute__ ((vector_size (xyz))
> 
> The resulting source is nicely portable and architecture-independent -
> except for the one place where I need to determine the maximum
> hardware-supported vector length on the target CPU.
> 
> This currently looks like
> 
> #if defined(__AVX__)
> constexpr int veclen=32;
> #elif defined(__SSE2__)
> constexpr int veclen=16;
> [...]
> 
> This approach requires me to add an #ifdef for many architectures, most
> of which I cannot really test on ... and new architectures will be
> unsupported by default.
> 
> Hence my question: is there a way in gcc to determine the hardware
> vector length for the architecture the compiler is currently targeting?
> Some predefined macro like
> 
> HARDWARE_VECTOR_LENGTH_IN_BYTES
> 
> which is 32 for AVX, 16 for SSE2, and has proper values for Neon, VPX
> etc. etc.
> 
> If this is not provided at the moment, would it bo possible to add this
> in the future? This could massively simplify writing and maintaining
> multi-platform SIMD code.
> 
> Thanks,
>   Martin
//

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