[Mesa-dev] [PATCH] i965: Work around SIN/COS output range problem.

[Martin Peres]

On 16/03/16 19:33, Kenneth Graunke wrote:
> The SIN and COS instructions on Intel hardware can produce values
> slightly outside of the [-1.0, 1.0] range for a small set of values.
> Obviously, this can break everyone's expectations about trig functions.
>
> According to an internal presentation, the COS instruction can produce
> a value up to 1.000027 for inputs in the range (0.08296, 0.09888).  One
> suggested workaround is to multiply by 0.99997, scaling down the
> amplitude slightly.  Apparently this also minimizes the error function,
> reducing the maximum error from 0.00006 to about 0.00003.
>
> I chose to apply this only when not saturating, as saturate already
> clamps to 1.0.  This may or may not be a good idea.
>
> Fixes 16 dEQP precision tests
>
>     dEQP-GLES31.functional.shaders.builtin_functions.precision.
>     {cos,sin}.{highp,mediump}_compute.{scalar,vec2,vec4,vec4}.
>
> at the cost of making every sin and cos call more expensive.
>
> Signed-off-by: Kenneth Graunke <kenneth at whitecape.org>
> ---
>   src/mesa/drivers/dri/i965/brw_fs_nir.cpp   | 26 ++++++++++++++++++++------
>   src/mesa/drivers/dri/i965/brw_vec4_nir.cpp | 26 ++++++++++++++++++++------
>   2 files changed, 40 insertions(+), 12 deletions(-)
>
> This has been in the Vulkan tree for a while - we needed it to pass the
> Vulkan CTS, as it contains these same dEQP tests.
>
> I haven't run shader-db yet, but I don't expect we'll like the results.
>
> The patch is pretty sketchy, too.  I'm sort of tempted to hide it behind
> an INTEL_STRICT_CONFORMANCE=1 option, like we had way back in the day...

FYI, a quick run on hsw_gt2 shows -10.45% on Gputest:voplosion.