[Development] Updating x86 SIMD support in Qt

[Kevin Kofler]

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.

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.

        Kevin Kofler