On Monday, July 22, 2019, Ben Cotton <bcotton@redhat.com> wrote:

== Summary ==
Fedora currently uses the original K8 micro-architecture (without
3DNow! and other AMD-specific parts) as the baseline for its
<code>x86_64</code> architecture.  This baseline dates back to 2003
and has not been updated since.  As a result, performance of Fedora is
not as good as it could be on current CPUs.

This change to update the micro-architecture level for the
architecture to something more recent.

== Owner ==
* Name: [[User:fweimer| Florian Weimer]]
* Email: [mailto:fweimer@redhat.com fweimer@redhat.com]

== Detailed Description ==

After preliminary discussions with CPU vendors, we propose AVX2 as the
new baseline.  AVX2 support was introduced into CPUs from 2013 to
2015. See [https://en.wikipedia.org/wiki/Advanced_Vector_Extensions#CPUs_with_AVX2
CPUs with AVX2].

Along with AVX2, it makes sense to enable certain other CPU features
which are not strictly implied by AVX2, such as CMPXCHG16B, FMA, and
earlier vector extensions such as SSE 4.2.  Details are still being
worked out.

A test rebuild of a distribution largely based on Fedora 28 showed
that there is only a small number of build failures due to the
baseline switch. Very few packages are confused about the availability
of the CMPXCHG16B instruction, leading to linking failures related to
<code>-latomic</code>, and there are some hard-coded floating point
results that could change due to vectorization.  (The latter is within
bounds of the usual cross-architecture variation for such tests.)

== Benefit to Fedora ==

Fedora will use current CPUs more efficiently, increasing performance
and reducing power consumption.

Moreover, when Fedora is advertised as a distribution by a compute
service provider, users can be certain that their AVX2-optimized
software will run in this environment.

== Scope ==
* Proposal owners: Update the <code>gcc</code> and
<code>redhat-rpm-config</code> package to implement the new compiler
flags.  It is expected that the new baseline will be implemented in a
new GCC <code>-march=</code> option for convenience.

* Other developers: Other developers may have to adjust test suites
which expect exact floating point results, and correct linking with
<code>libatomic</code>. They will also have to upgrade their x86-64
machines to something that can execute AVX2 instructions.

* Release engineering: [https://pagure.io/releng/issue/8513 #8513]
** All Fedora builders need to be AVX2-capable.
** Infrastructure ticket:
[https://pagure.io/fedora-infrastructure/issue/7968 #7968]
* Policies and guidelines: No guidelines need to be changed.
* Trademark approval: N/A (not needed for this Change)

== Upgrade/compatibility impact ==
Fedora installations on systems with CPUs which are not able to
execute AVX2 instructions will not be able to upgrade.

== How To Test ==
General system testing will provide test coverage for this change.

== User Experience ==
User should observe improved performance and, likely, battery life.
Developers will benefit from the knowledge that code with AVX2
optimizations will run wherever Fedora runs.

== Dependencies ==
There are no direct dependencies on this change at this time.

== Contingency Plan ==
It is possible to not implement this change, or implement a smaller
subset of it (adopting the CMPXCHG16B instruction only, for example).

* Contingency mechanism: Mass rebuild with different/previous compiler glags.
* Contingency deadline: Final mass rebuild.
* Blocks release? No.
* Blocks product? No.

== Documentation ==
The new micro-architecture baseline and the resulting requirements
need to be documented.

== Release Notes ==
Release notes must mention how users can determine whether their
system supports AVX2 prior to upgrading, for example by running
<code>grep avx2 /proc/cpuinfo</code>.

Please just take back this change and come back at April first if it was supposed to be a joke - if not then submit again in about 10 years.