[Development] Updating x86 SIMD support in Qt

[Alexandru Croitor]

> On 19. Jan 2022, at 04:01, Thiago Macieira <thiago.macieira at intel.com> wrote:
> 
> 1) assume all compilers support what we need
> 
> I propose we remove the tests for the intrinsics of each individual CPU 
> feature. Instead, let's just assume they all have everything up to 2016. 
> 
> The change https://codereview.qt-project.org/c/qt/qtbase/+/386738 is going in 
> this direction but retains a test (all or nothing). I'm proposing now we 
> remove the test completely and just assume.

I believe this will pessimize optimisations for certain platforms as pointed out in the linked change.

Specifically QNX and iOS simulator builds.

https://codereview.qt-project.org/c/qt/qtbase/+/386738/11#message-5015480c07228dd2088d6b1aba137927725a06b2

Perhaps WASM and Integrity as well.

This may or may not matter to those platform users.

> 3) add a way to have multi-arch glibc-based Linux builds

Who is going to implement this and how? 

CMake basically has no support for proper multi-arch builds.

For macOS / iOS we rely on Apple clang supporting it. That's not available for Linux.

One could try using ExternalProjects, but this very quickly gets hairy for multiple reasons, especially if it's not a complete Qt build (all libraries, and not just a selection of libraries).