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Re: [PATCH, i386] Enable option -mprefer-avx256 added for Intel AVX512 configuration


On 2017.09.14 at 14:36 +0200, Jakub Jelinek wrote:
> On Thu, Sep 14, 2017 at 12:10:50PM +0000, Shalnov, Sergey wrote:
> > GCC has the option "mprefer-avx128" to use 128-bit AVX registers instead
> > of 256-bit AVX registers in the auto-vectorizer.
> 
> > This patch enables the command line option "mprefer-avx256" that reduces
> > 512-bit registers usage in "march=skylake-avx512" mode.  This is the
> > initial implementation of the option.  Currently, 512-bit registers might
> > appears in some cases.  I have a plan to continue fix the cases where
> > 512-bit registers are appear.  Sergey
> 
> What is the rationale for this?  -mprefer-avx128 has been added because some
> (older) AMD CPUs implement AVX by performing 256-bit ops as two 128-bit uops
> and thus it is faster to emit 128-bit only code.
> Is that the case for any AVX512 implementations too?

You get a huge frequency drop when you run AVX512 code. There are
situations where this more than offsets the potential gains.
Glibc had to disable AVX512 memcpy because of this issue.

-- 
Markus


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