Paul Berry <stereotype...@gmail.com> writes:
> Previously, we computed dFdy() using the following instruction:
>
> add(8) dst<1>F src<4,4,0)F -src.2<4,4,0>F { align1 1Q }
>
> That had the disadvantage that it computed the same value for all 4
> pixels of a 2x2 subspan, which meant that it was less accurate than
> dFdx(). This patch changes it to the following instruction when
> c->key.high_quality_derivatives is set:
>
> add(8) dst<1>F src<4,4,1>.xyxyF -src<4,4,1>.zwzwF { align16 1Q }
>
> This gives it comparable accuracy to dFdx().
>
> Unfortunately, for some reason the SIMD16 version of this instruction:
>
> add(16) dst<1>F src<4,4,1>.xyxyF -src<4,4,1>.zwzwF { align16 1H }
>
> Doesn't seem to work reliably (presumably the hardware designers never
> validated the combination of align16 mode with compressed
> instructions), so we unroll it to:
From the gen4 PRM vol4, page 340:
"A compressed instruction must be in Align1 access mode. Align16
mode instructions cannot be compressed."
Other than updating the comment about compressed due to the PRM cite,
this is:
Reviewed-by: Eric Anholt <e...@anholt.net>
Thanks for figuring this out.
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