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

[Roland Scheidegger]

Am 29.10.2015 um 20:10 schrieb Oded Gabbay:
> 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
> 

Does that have fma and does it auto-fuse mul/add to fma? Albeit I don't
think it should right now... Other than that, I'm not sure why the
results would be different - albeit on x86 we explicitly disable denorms
which could cause different results, for this example I don't think this
should be an issue.

Roland