Re: Why does glibc use AVX-512?

[Florian Weimer]

* Andy Lutomirski-alpha:

> glibc appears to use AVX512F for memcpy by default.  (Unless
> Prefer_ERMS is default-on, but I genuinely can't tell if this is the
> case.  I did some searching.)  The commit adding it refers to a 2016
> email saying that it's 30% on KNL.

As far as I know, glibc only does that on KNL, and there it is
actually beneficial.  The relevant code is:

      /* Since AVX512ER is unique to Xeon Phi, set Prefer_No_VZEROUPPER
         if AVX512ER is available.  Don't use AVX512 to avoid lower CPU
         frequency if AVX512ER isn't available.  */
      if (CPU_FEATURES_CPU_P (cpu_features, AVX512ER))
        cpu_features->preferred[index_arch_Prefer_No_VZEROUPPER]
          |= bit_arch_Prefer_No_VZEROUPPER;
      else
        cpu_features->preferred[index_arch_Prefer_No_AVX512]
          |= bit_arch_Prefer_No_AVX512;

So it's not just about Prefer_ERMS.

> I think we should seriously consider solutions in which, for new
> tasks, XCR0 has new giant features (e.g. AMX) and possibly even

I think the AMX programming model will be different, yes.

> AVX-512 cleared, and programs need to explicitly request enablement.
> This would allow programs to opt into not saving/restoring across
> signals or to save/restore in buffers supplied when the feature is
> enabled.

Isn't XSAVEOPT already able to handle that?

In glibc, we use XSAVE/XSAVEC for the dynamic loader trampoline, so it
should not needlessly enable AVX-512 state today, while still enabling
AVX-512 calling conventions transparently.

There is a discussion about using the higher (AVX-512-only) %ymm
registers, to avoid the %xmm transition penalty without the need for
VZEROUPPER.  (VZEROUPPER is incompatible with RTM from a performance
point of view.)  That would perhaps negatively impact XSAVEOPT.

Assuming you can make XSAVEOPT work for you on the kernel side, my
instincts tell me that we should have markup for RTM, not for AVX-512.
This way, we could avoid use of the AVX-512 registers and keep using
VZEROUPPER, without run-time transaction checks, and deal with other
idiosyncrasies needed for transaction support that users might
encounter once this feature sees more use.  But the VZEROUPPER vs RTM
issues is currently stuck in some internal process issue on my end (or
two, come to think of it), which I hope to untangle next month.