[Development] Using SSE/NEON in Qt 6

[Lars Knoll]

> On 6 Feb 2020, at 21:54, Allan Sandfeld Jensen <kde at carewolf.com> wrote:
> 
> On Donnerstag, 6. Februar 2020 21:17:02 CET Lars Knoll wrote:
>>> On 6 Feb 2020, at 19:29, Allan Sandfeld Jensen <kde at carewolf.com> wrote:
>>> making the default SSE4.1 enabled but still offer users (linux distros 
>>> really), the option to force it down to only SSE2. 
>> 
>> 
>> We should at least default to SSE2 as the minimum requirement. We’ll anyway
>> need some pure C/C++ code as fallback, so if someone really needs to run it
>> on a CPU from 1999, he can compile things with SSE turned off.
> 
> Isn't that the current status?. The question is if we should now do the same 
> for SSE4 we have been doing for SSE2 the last several years. Or do you mean we 
> perhaps now can remove the option to non-SSE2?

The main difference is that I want to use SSE inline in public headers and unconditionally if enabled (which it is by default on x86/x64). The same for NEON on arm64.
> 
>>> 
>>> You could do the same with NEON, but I think we already use that 
>>> unconditionally if detected at configure time.
>> 
>> 
>> We do use NEON conditionally in some cases (e.g. qimagescale.cpp), but for
>> most embedded devices it simply makes most sense to do the choice at Qt
>> configuration time.
> 
> Hmm, all the NEON-runtime check got disabled long ago when we had issues with 
> instructions leaking from one compile unit to another through headers. That 
> issue later got solved when it also threatened x86 by leaking AVX 
> instructions, and while I never reinstated the removed runtime checks for NEON 
> in painting code, I do appear to have written new runtime switches where I 
> added new NEON code (at least in qimagescale.cpp) Didn't even remember I did 
> that, or why. That probably makes it limited to only a few places in image 
> handling, and it probably shouldn't be there either.

It could be ok changing that to always select at compile time. I don’t think we need the same level of runtime checking for NEON as we do for different SSE/AVX versions on the desktop.

Cheers,
Lars