[arch-dev-public] ON HOLD - RFC: Use x86_64-v2 architecture

[Allan McRae]

On 4/3/21 9:51 pm, Filipe Laíns wrote:
> On Thu, 2021-03-04 at 21:33 +1000, Allan McRae via arch-dev-public wrote:
>> On 3/3/21 10:54 am, Allan McRae via arch-dev-public wrote:
>>> On 2/3/21 9:51 pm, Allan McRae wrote:
>>>> Hi all,
>>>>
>>>> A new RFC has been opened here:
>>>> https://gitlab.archlinux.org/archlinux/rfcs/-/merge_requests/2
>>>>
>>>> Summary:
>>>> Make -march=x86_64-v2 the default for our packages.  This assumes the
>>>> following instruction sets which are essentially available on all but
>>>> the oldest AMD CPUs:
>>>>
>>>> CMPXCHG16B, LAHF-SAHF, POPCNT, SSE3, SSE4.1, SSE4.2, SSSE3
>>>>
>>>> Please visit the above link for discussion.
>>
>> Lets put discussion on this RFC on hold for a while.  Clearly there is a
>> reasonable amount of objection to making x86-64-v2 the default.  While
>> this mostly appears to be objection based on personal circumstances and
>> not on the basis of whether this change is good for the distro, I will
>> work within these limits.
>>
>> A lot of comments have suggested adding x86-64-v2 and -v3 as additional
>> architectures instead.  I will revamp the the proposal to take that
>> approach.  Though, to do this automated would require more work it may
>> be the push we need for a signing enclave to be set up.
>>
>> Allan
> 
> Thank you. Though, I find this a bit dismissive of my feedback arguing that this
> would very likely not have any significant effect whatsoever in performance, and
> that it fails to solve the ISA extensions issue we have. While there was some
> feedback based on personal circumstances, I provided objective argumentation
> about how the proposal as is is probably not a good idea and not the best path
> forward.

As I said, I am reworking the RFC, so your comments are not ignored but
instead just not addressed yet.

You are correct that a lot of software where the gains are biggest
already provide optimized paths.  But there are still gains to be had,
and not just in performance.  When I recompiled my system with something
similar to x86-64-v3, I gained significant battery life on my laptop on
daily use.  The power saving is likely more noticealbe than speed gains.
 As part of the updated RFC, I will get some benchmarks included.

And I know some packages have multiple variants provided with different
optimization level.  But the user needs to find them to use them.
Looking a pkgstats, that may happen for a decent proportion of
tensorflow users, but the other packages not so much.  And there are
issues with discoverability, and choice of optimization levels.

Allan