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

[Ian Romanick]

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?

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