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

[Martin Peres]

On 22/03/16 23:31, Ian Romanick wrote:
> On 03/17/2016 09:18 AM, Martin Peres wrote:
>> 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.
> Can you explain this result?  -10.45% of what?  Instructions?  FPS?  And
> this is comparing what to what?  Before this patch to after?

So sorry Ian, I completely missed your email!

Yes, it was just applying this patch and I was talking about FPS.