[Development] Updating x86 SIMD support in Qt

[Allan Sandfeld Jensen]

On Mittwoch, 19. Januar 2022 16:41:11 CET Thiago Macieira wrote:
> On Tuesday, 18 January 2022 22:43:40 PST Kevin Kofler via Development wrote:
> > Thiago Macieira wrote:
> > > By default, I'd like us to produce x86-64 v2 code, which is SSE4.
> > 
> > But v1 will still be available for distribution packaging? As long as that
> > is the case, I do not see a major issue, it will just be one more caveat
> > for distribution packaging. (Distributions still supporting v1, which I
> > think is most of the distros these days, will have to enable it
> > explicitly, possibly along with newer vn (n>1) if optimized builds are
> > desired.) But dropping support for v1 entirely causes headaches for
> > distributions.
> Yes, that's the idea. I'm just looking at raising our defaults in this case,
> not stop the older solutions, mostly because the compatibility we're
> talking about is with very old machines: the Intel Core line got SSE4.2 in
> 2008 with Nehalem, AMD got it with Bulldozer in 2011 and the Atom line got
> it in 2013 with Silvermont. Meanwhile, foregoing the SSE4 optimisations
> afforded by v2 leaves some performance on the table. Yes, most
> distributions still target v1 today but that's mostly for inertia reasons.
> As I said, I understand Red Hat 9 is going to up the minimum to v2.
> 
> Additionally, I'd like to make it easy to have both v1+v2 or, better yet,
> v1+v2+v3, so you can have your cake and eat it too. Some libraries like
> QtCore, QtGui and the Qt 3D ones make extensive use of math and would
> benefit from the extra operations, especially those of AVX.
> 
> > There are still (end) users of old hardware. E.g., my notebook is a Core 2
> > Duo that supports up to SSSE3 (so v1 + SSE3 + SSSE3), but no SSE4. So it
> > unfortunately falls one generation short of v2. (My desktop supports v2,
> > but not v3, because it is missing at least AVX2.) But as long as the
> > distribution packages work on it, I do not really care about what vn or
> > SSEn the Qt upstream binaries require.
> 
> I understand. I have one of those in a cabinet, but it doesn't power on (the
> PSU is bust). How much RAM do you have? How usable is a modern Linux
> desktop on it?

I have a ~10 year old Phenom II that I use as a media server, it also lacks 
SSE4 (only having AMDs so-called SSE4a). With 3 cores and 4GB of memory it 
runs a modern Qt5 based Linux desktop just fine, even if I don't regularly use 
it as such. So while it is no great loss for me if I needed to replace it with 
a 200€ NUC, it is certainly plausible people have such working old machines. I 
think it is fine to let the default not work on such machines, and let the 
distros that want to support it use v1.

'Allan