[Bug 1999551] Re: glibc: backport AArch64 memcmp improvements

[Simon Chopin]

A late follow up on that: I thought the Jammy package was broken because
I detected use of the non-SVE variant of memcpy. After a while I
realized it was due to it being used *before* the ifunc resolution was
done, probably within the dynamic linker itself.

I prepared a Focal package as well. Sadly, the patchset there is much
more involved than the one for Jammy. It's in the same PPA (which has a
decidedly inadequate name).

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Title:
  glibc: backport AArch64 memcmp improvements

Status in glibc package in Ubuntu:
  Fix Released
Status in glibc source package in Focal:
  In Progress
Status in glibc source package in Jammy:
  In Progress

Bug description:
  [impact]

  There have been relatively recent improvements to the memcmp and
  memcpy routines for server-grage AArch64 implementation, in particular
  AWS's Graviton3.

  We'd like to backport those improvements to Jammy and Focal when
  appropriate, under the HWE umbrella.

  The relevant patches are

  https://sourceware.org/git/?p=glibc.git;a=commit;h=9f298bfe1f183804bb54b54ff9071afc0494906c (Jammy & Focal)
  https://sourceware.org/git/?p=glibc.git;a=commit;h=b51eb35c572b015641f03e3682c303f7631279b7 (Focal only, already present in Jammy)

  In addition, to be able to actually test the changes and its impact on
  all architectures, we'll need the following fix:

  https://sourceware.org/git/?p=glibc.git;a=commit;h=311a7e0256975275d97077f1af338bc9caf0c837

  [test case]

  Since those are optimization patches, we'll be relying on the
  autopkgtests triggered by the upload for regression detection.

  However, we'll also benchmark the optimizations on Graviton AWS
  instances as well as various Raspberry Pi models to ensure there is no
  severe performance regression on those platforms.

  To do the performance test, first install the libc from this PPA:

  https://launchpad.net/~schopin/+archive/ubuntu/glibc-benchmark

  that is the current Jammy glibc with the extra fix for benchmarking.

  Then, untar the attached archive bench-timing.tar.xz on the target
  platform, and follow the instructions from the README.

  [Regression potential]

  This could potentially impact performance on other, non-server-grade
  arm64 platforms such as RPi. Furthermore, there could be unforeseen
  issues with the newly optimized routine in edge cases (a recent amd64
  optimization had issues on page boundaries, for instance).

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