[Mesa-dev] Problems with accuracy of coeffs_init + attribs_update

Oded Gabbay oded.gabbay at gmail.com
Thu Oct 29 12:10:09 PDT 2015

On Thu, Oct 29, 2015 at 9:02 PM, Ilia Mirkin <imirkin at alum.mit.edu> wrote:
> On Thu, Oct 29, 2015 at 2:44 PM, Oded Gabbay <oded.gabbay at gmail.com> wrote:
>> However, I would hate to keep the situation as is, meaning the test
>> passes on x86-64 and fails on ppc64le.
> Sounds like it'd actually be a difference between AVX and SSE4.2 as
> well -- what happens if you run on your x86_64 chip with
> LP_NATIVE_VECTOR_WIDTH=128 ? It fails for me on my HSW chip, looking
> at the results visually it's mostly good but there's a sprinkling of
> red pixels.
>   -ilia

It fails on my Haswell chip - definitely sprinkling of red pixels.
Also the error seems to be greater than 5e-7. Even with 1.6e-6 as
failure point, it still fails, while on ppc64le it passes.
Only when I went for 2e-6, the test passes.

As I said and Roland explained, the calculation method is inherently
less accurate in the two-stages path. Although I don't know why on
SSE4.2 the deviation is a bit larger than on VMX


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