[FFmpeg-devel] [PATCH] pixblockdsp, avdct: Add get_pixels_unaligned

[Michael Niedermayer]

On Tue, May 12, 2020 at 10:04:13PM +0300, Martin Storsjö wrote:
> On Tue, 12 May 2020, Michael Niedermayer wrote:
> 
> >On Tue, May 12, 2020 at 11:25:34AM +0300, Martin Storsjö wrote:
> >>Use this in vf_spp.c, where the get_pixels operation is done on
> >>unaligned source addresses.
> >>
> >>This fixes fate-filter-spp on armv7.
> >
> >LGTM
> >
> >
> >>---
> >>People more familiar with the other assembly implementations of
> >>get_pixels (in particular, x86) can hook them up to
> >>get_pixels_unaligned if unaligned use explicitly is ok; as far as I
> >>can read at least ff_get_pixels_mmx, it looks like it expects the source
> >>to be aligned, but in practice it does seem to run fine even if
> >>ff_get_pixels_sse2 is disabled.
> >>---
> >
> >id suggest that code that has been tested with unaligned data and works
> >is enabled
> 
> Well ff_get_pixels_mmx uses "mova", which I'd presume is "aligned", but it
> still doesn't seem to crash when used with vf_spp today, even if that one
> uses it on unaligned addresses. So I'm not sure if I'm just being lucky, or
> is it guaranteed that this "mova" always will work on unaligned addresses?
> 
> As someone who doesn't do much x86 asm, I wouldn't base such an assignment
> just on "it seems to work for me".

you can look at libavutil/x86/x86inc.asm
for mmx both mova and movu are mapped to the same instruction

    %define mova movq
    %define movu movq

Also there is intels instruction set reference that should document alignment
requirements but that doesnt seem to
document alignment requirements in a way that i can easily find from
starting with a random instruction name

but likely someone reading has a link to nice short / table / summary of 
alignment requirements 

thx
    
[...]
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Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

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