[dpdk-stable] AVX512 bug on SkyLake

[Christian Ehrhardt]

> -    vmovdqu8 xmm0,XMMWORD PTR [rax*8+0x6]
> +    vmovdqu xmm0,XMMWORD PTR [rax*8+0x60]
>      vinserti128 ymm0,ymm0,XMMWORD PTR [rax*8+0x70],0x1
>      vmovups XMMWORD PTR [rsi+0x60],xmm0
>      vextracti128 XMMWORD PTR [rsi+0x70],ymm0,0x1
>
> > Not sure what causing compiler behaves that way.
> > BTW, looking though testpmd objdump output - it seems that only mlx5 driver
> > exhibits such problem (I didn't enable mlx4 actually, probably same problem here).
> > Which looks a bit weird to me.
>
> Yes it's weird. I don't see how the mlx5 code could influence
> the compiler to generate this bad code in AVX512 mode.

Thomas you have all this set up, do you have any chance to test this
on the GCC's in Ubuntu 18.10 and 19.04
If easy I'd love to see results wit hgcc-7 & gcc-8 as in Ubuntu 19.04
(current -dev).
If the above is too hard, at least could you try the gcc-8 in Bionic
is 8.2.0-1ubuntu2~18.04 that is rather close.

If you could share the simplified build options that you need to
reproduce that would help (at least me) - thanks in advance

-- 
Christian Ehrhardt
Software Engineer, Ubuntu Server
Canonical Ltd