[Bug 1951032] Re: AArch64: Backport memcpy improvements

[Michael Hudson-Doyle]

Here's a version that compiles on arm64. Do note it expects a command
line argument, the size of the buffer to use in megabytes.

** Attachment added: "test_memcpy.c"
   https://bugs.launchpad.net/ubuntu/+source/glibc/+bug/1951032/+attachment/5557684/+files/test_memcpy.c

** Description changed:

  [impact]
  glibc 2.32 contained a number of improvements to the memcpy routines for server-grade AArch64 implementations (in particular, graviton2 & graviton3). They should be backported to focal, as the LTS releases are by far the most used on servers.
  
  [test case]
- Compile the test_memcpy.c that is attached to bug 1928508:
+ Compile the test_memcpy.c that is attached to this report:
  
  $ gcc -g -O3 test_memcpy.c -o test_memcpy64
  
  "./test_memcpy64 32" should be run before and after installing the libc
  packages from proposed. On graviton2 systems, this should show a
  substantial improvement. On other arm64 systems (raspberry pis of
  various vintage, thunderx2, xgene, etc etc) at least no significant
  regression should be seen.
  
  [regression potential]
  Rebuilding glibc is always a little risky (toolchain bugs and incompatibilities between the old and new versions can be surprising). But the autopkgtests and some manual general testing can help here.
  
  For this specific change, there is a potential risk that the new memcpy
  implementation could be used on a system where it is not in fact the
  fastest. We should run the test case not only on the systems where it is
  expected to help, but other systems such as the RPi4 and the launchpad
  build farm to ensure performance is not regressed there.

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https://bugs.launchpad.net/bugs/1951032

Title:
  AArch64: Backport memcpy improvements

Status in glibc package in Ubuntu:
  Fix Released
Status in glibc source package in Focal:
  Fix Committed

Bug description:
  [impact]
  glibc 2.32 contained a number of improvements to the memcpy routines for server-grade AArch64 implementations (in particular, graviton2 & graviton3). They should be backported to focal, as the LTS releases are by far the most used on servers.

  [test case]
  Compile the test_memcpy.c that is attached to this report:

  $ gcc -g -O3 test_memcpy.c -o test_memcpy64

  "./test_memcpy64 32" should be run before and after installing the
  libc packages from proposed. On graviton2 systems, this should show a
  substantial improvement. On other arm64 systems (raspberry pis of
  various vintage, thunderx2, xgene, etc etc) at least no significant
  regression should be seen.

  [regression potential]
  Rebuilding glibc is always a little risky (toolchain bugs and incompatibilities between the old and new versions can be surprising). But the autopkgtests and some manual general testing can help here.

  For this specific change, there is a potential risk that the new
  memcpy implementation could be used on a system where it is not in
  fact the fastest. We should run the test case not only on the systems
  where it is expected to help, but other systems such as the RPi4 and
  the launchpad build farm to ensure performance is not regressed there.

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