Re: [FFmpeg-devel] [PATCH v7] libavfilter/x86/vf_convolution: add sobel filter optimization and unit test with intel AVX512 VNNI

["Wang, Bin" <bin.wang-at-intel.com@ffmpeg.org>]

> -----Original Message-----
> From: ffmpeg-devel <ffmpeg-devel-bounces@ffmpeg.org> On Behalf Of James
> Almer
> Sent: Monday, November 14, 2022 9:36 PM
> To: ffmpeg-devel@ffmpeg.org
> Subject: Re: [FFmpeg-devel] [PATCH v7] libavfilter/x86/vf_convolution: add
> sobel filter optimization and unit test with intel AVX512 VNNI
> 
> On 11/14/2022 10:30 AM, Wang, Bin wrote:
> >> By using xmm# you're not taking into account any x86inc SWAPing, so
> >> this is using xmm0 and xmm1 where the single scalar float input
> >> arguments reside (at least on unix64), instead of xm0 and xm1 (xmm16
> >> and xmm17) where the broadcasted scalars were stored.
> >> This, again, only worked by chance on unix64 because you're using
> >> scalar fmadd, and shouldn't work at all on win64.
> >>
> >> Also, all these as is are being encoded as VEX, not EVEX, but it
> >> should be fine leaving them untouched instead of using xm#, since
> >> they will be shorter (five bytes instead of six for some) by using the lower,
> non callee-saved regs.
> >
> > Thanks for the help. I'm not familiar with WIN64 asm. So what I need to do is
> change the WIN64 swap from:
> > SWAP xmm0, xmm2
> > SWAP xmm1, xmm3
> > To:
> > VBROADCASTSS m0, xmm2
> > VBROADCASTSS m1, xmm3
> >
> > Is that correct?
> 
> Yes, that will ultimately broadcast the two scalars in xmm2 and xmm3 to
> zmm16 and zmm17.
> After that what you need to do is either change the fmaddss instruction to use
> xm0 and xm1 macros instead of xmm0 and xmm1 (so xmm16 and xmm17 with
> EVEX encoding is used), or much like the broadcast above use xmm2 and xmm3
> explicitly on win64, so it remains VEX encoded.

So, to fix the issue, does this 2 changes looks good for you?
First change the WIN64 swap from:
SWAP xmm0, xmm2
SWAP xmm1, xmm3
To:
VBROADCASTSS m0, xmm2
VBROADCASTSS m1, xmm3

Second change the fmaddss from:
fmaddss   xmm4, xmm4, xmm0, xmm1
To:
fmaddss   xmm4, xmm4, xm0, xm1


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