[Bug 1702917] Re: [Feature] P9: Power9 CPU tuned libraries

[Adam Conrad]

As a general rule, I'd prefer not to have extra glibc builds.  The bug
report points out that most of the critical optimizations are handles by
IFUNC, is there a solid argument for why we should have two builds as
well?  Given that the followup request would almost certainly be to ship
both by default, we'd end up with a fair amount of filesystem bloat, and
it's also a bit of a packaging nightmare (there was great rejoicing from
my end when I killed libc6-i686, the last "tuned" build of glibc we
had).

For i386 versus i686, there were some very reasonable arguments.  The
mandatory FPU, the extra registers, cmov, etc, etc.  Generic compiler
tuninng of i386 versus i686 gave very obvious real world performance
increases.  I'm dubious that this would be true for p8 versus p9.

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https://bugs.launchpad.net/bugs/1702917

Title:
  [Feature] P9:  Power9 CPU tuned libraries

Status in The Ubuntu-power-systems project:
  New
Status in glibc package in Ubuntu:
  New

Bug description:
  glibc provides minimal support for POWER9 since version 2.23.
   glibc 2.25 is the first version to provide POWER9 optimizations via
   STT_GNU_IFUNC. The optimizations are enabled by default and will be disabled
   when using --disable-multi-arch (notice that multi-arch in glibc context is
   not the same thing as used by Debian).
   
   However, this feature request is proposing to provide a second glibc build,
   configured with --with-cpu=power9 and co-installed with the default glibc
   build, but under $(LIBDIR)/power9.
   Other distros usually install these under /lib64/power9.  Ubuntu may need to
   use a different directory due to supporting Debian's multiarch.
   
   The support for this is known to work upstream since glibc 2.23.
   So, I think this is just pending changes in the distro build system.

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