[11u] RFR 8245512: CRC32 optimization using AVX512 instructions

[Florian Weimer]

* Andrew Haley:

> We don't know the future of AVX-512. Rumours say it's not going
> to be included in Alder Lake-S, Intel’s first 10nm mainstream
> desktop processor. I don't think the 10900K Comet Lake supports
> it either. But I admit it does look like Xeons are going to
> support AVX-512 for some time, so perhaps.

AVX-512 support has started to appear in non-server CPUs with Tiger
Lake:

<https://ark.intel.com/content/www/us/en/ark/products/208656/intel-core-i3-1125g4-processor-8m-cache-up-to-3-70-ghz.html>

There certainly were issues around power consumption and the internal
turbo license for previous implementations of AVX-512.  But that's
largely a question for mainline defaults: If the change is not generally
beneficial and only looks good in certain micro-benchmarks, it's
probably not appropriate for mainline either, at least not by default.
I don't know to what extent it is necessary to reconsider mainline
optimization choices in a backport.

Thanks,
Florian
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