[Development] Updating x86 SIMD support in Qt

[Thiago Macieira]

On Thursday, 20 January 2022 13:10:08 PST Lorn Potter wrote:
> well, from https://emscripten.org/docs/porting/simd.html
> "also turn on LLVM’s autovectorization passes, so no source
> modifications are necessary to benefit from SIMD."
> so emscripten's simd support is more than just sse2, sse3, etc.

True, but this is the autovectoriser. It operates on pure scalar C++ code and 
applies what it can to speed up. Conceivably, the compiler knows what is fast 
on the target environment and what isn't.

> well, CI doesn't build wasm simd, so in this respect, it doesn't concern me.
> 
> 256-bit intrinsics won't work in wasm/emscripten so don't enable them
> for those platforms that don't support them like wasm.
> 
> Just don't stop platforms from using 128bit intrinsics such as sse2.

Please see the code I'm submitting, like
https://codereview.qt-project.org/c/qt/qtbase/+/387217
https://codereview.qt-project.org/c/qt/qtbase/+/387414
https://codereview.qt-project.org/c/qt/qtbase/+/380895

That means:
* the new functions are inside an #ifdef __SSE2__ block
* the new functions' content and their calls are behind
    if constexpr (UseAvx2) {
or
    if constexpr (UseAvx256) {
* the new content uses __m256i among other things without #if

This is the request: if you #define __SSE2__, then you MUST provide __m256i 
and the 256-bit x86 intrinsics (up to and including the new ones added in 
AVX512). All or nothing.

You don't have to use them. They just need to be declared.

-- 
Thiago Macieira - thiago.macieira (AT) intel.com
  Software Architect - Intel DPG Cloud Engineering