[Development] Updating x86 SIMD support in Qt

[Kevin Kofler]

Thiago Macieira wrote:
> Whether Fedora decides to up the minimum requirement for a future edition
> is something you'll be in a better position to answer than I am.

So far, I and few others have successfully blocked attempts at upping the 
minimum requirement for Fedora. (The requirement for the eln branch matches 
the EL one though. That was originally the rationale for the branch's 
creation, after a proposal to up the requirement throughout Fedora failed, 
but eln has since evolved to also include other EL-only changes to Rawhide 
before it branches to CentOS Stream.)

> The next question is whether you do (or would do) Qt 6 development on that
> laptop, while travelling and, if so, if you use (or would use) the binary
> packages from qt.io. If you build from source, it would not be a problem;
> if you use a distribution package, it's out of scope.

As already mentioned: I personally have no problem with you bumping the 
minimum requirement on the prebuilt upstream binaries as long as 
distribution packages can still be built for older x86 "versions", because I 
only use the latter anyway.

>> But it is not my primary computer, my primary
>> computer is the desktop on which I am typing this: Sandy Bridge Core
>> i7-2600K (released 2011, supports up to AVX(1)), 16 GiB RAM.
> 
> Sandy Bridges and Ivy Bridges are more like the previous generation
> (Westmere) than the next (Haswell). For example, even though SNB does have
> a 32-byte-load instruction and it can issue two load instructions per
> cycle, it can't do two 32-byte loads in the same cycle. That's something
> Haswell can and it is a cornerstone of the optimisations I've just made.
> So for those two, we'll keep the previous generation of optimisations,
> that focus on 16-byte (128-bit) loads and stores.

So both my computers are exactly one generation short of the next major 
improvement. Sad. :-(

> I used to have an SNB-based Mac Mini, but since Apple stopped providing
> updates for that generation, I was forced to upgrade.

Thankfully, we are not Apple! I am happy to be able to still use 10+ year 
old hardware without planned obsolescence.

> In my other $DAYJOB, I still need to supprt the SNB and IVB-based servers
> (Jaketown and Ivytown, see [1]).
> 
> [1] https://en.wikipedia.org/wiki/List_of_Intel_codenames

And that's good. :-)

        Kevin Kofler