[Mesa-dev] [PATCH] i965/hsw: approximate DDX with a uniform value across a subspan

[Kenneth Graunke]

On 09/30/2013 07:16 PM, Ian Romanick wrote:
> On 09/11/2013 10:00 PM, Chia-I Wu wrote:
>> From: Chia-I Wu <olv at lunarg.com>
>>
>> Replicate the gradient of the top-left pixel to the other three pixels in the
>> subspan, as how DDY is implemented.  Before, different graidents were used for
>> pixels in the top row and pixels in the bottom row.
>>
>> This change results in a less accurate approximation.  However, it improves
>> the performance of Xonotic with Ultra settings by 24.3879% +/- 0.832202% (at
>> 95.0% confidence) on Haswell.  No noticeable image quality difference
>> observed.
>>
>> No piglit gpu.tests regressions.
>>
>> I failed to come up with an explanation for the performance difference.  The
>> change does not make a difference on Ivy Bridge either.  If anyone has the
>> insight, please kindly enlighten me.  Performance differences may also be
>> observed on other games that call textureGrad and dFdx.
> 
> After all the experiments and discussions with the hardware guys, lets
> go ahead and do this.  We should do a couple things, however.
> 
> 1. Disable the optimization if the application explicitly sets
> GL_FRAGMENT_SHADER_DERIVATIVE_HINT to GL_NICEST.

Urgh...I always hate adding more state-dependent recompiles...

To accomplish this, you'll have to:
- Add a new high_quality_derivatives flag to brw_wm_prog_key.
- In brw_wm_populate_key, add:
  /* _NEW_HINT */
  key->high_quality_derivatives =
     ctx->Hint.FragmentShaderDerivative == GL_NICEST;
- Add the _NEW_HINT dependency to brw_wm_prog's dirty flags.

> 2. Add a driconf option, as suggested by Chris, to disable the optimization.

...which means changing the key setup to:

  if (brw->disable_derivative_optimization) {
     key->high_quality_derivatives =
        ctx->Hint.FragmentShaderDerivative != GL_FASTEST;
  } else {
     key->high_quality_derivatives =
        ctx->Hint.FragmentShaderDerivative == GL_NICEST;
  }

and, in brw_fs_precompile, setting

key->high_quality_derivatives = brw->disable_derivative_optimization;

This all seems pretty awful to me...but I guess there's not really any
getting around it.  If the register had worked out, we could've just
added a Hint() driver hook that programmed it appropriately.  But alas.

> 3. Use the same DDX / DDY calculation on all platforms.
> 
> 4. Update the commit message and the comment in the code with the
> explanation of the optimization (the HSW sample_d instruction does some
> optimizations if the same LOD is used for all pixels, etc.).
> 
>> Signed-off-by: Chia-I Wu <olv at lunarg.com>
>> ---
>>  src/mesa/drivers/dri/i965/brw_fs_emit.cpp | 17 +++++++++++++----
>>  1 file changed, 13 insertions(+), 4 deletions(-)
>>
>> diff --git a/src/mesa/drivers/dri/i965/brw_fs_emit.cpp b/src/mesa/drivers/dri/i965/brw_fs_emit.cpp
>> index bfb3d33..c0d24a0 100644
>> --- a/src/mesa/drivers/dri/i965/brw_fs_emit.cpp
>> +++ b/src/mesa/drivers/dri/i965/brw_fs_emit.cpp
>> @@ -564,16 +564,25 @@ fs_generator::generate_tex(fs_inst *inst, struct brw_reg dst, struct brw_reg src
>>  void
>>  fs_generator::generate_ddx(fs_inst *inst, struct brw_reg dst, struct brw_reg src)
>>  {
>> +   /* approximate with ((ss0.tr - ss0.tl)x4 (ss1.tr - ss1.tl)x4) on Haswell,
>> +    * which gives much better performance when the result is used with
>> +    * sample_d
>> +    */
>> +   unsigned vstride = (brw->is_haswell) ? BRW_VERTICAL_STRIDE_4 :
>> +                                          BRW_VERTICAL_STRIDE_2;
>> +   unsigned width = (brw->is_haswell) ? BRW_WIDTH_4 :
>> +                                        BRW_WIDTH_2;
>> +
>>     struct brw_reg src0 = brw_reg(src.file, src.nr, 1,
>>  				 BRW_REGISTER_TYPE_F,
>> -				 BRW_VERTICAL_STRIDE_2,
>> -				 BRW_WIDTH_2,
>> +				 vstride,
>> +				 width,
>>  				 BRW_HORIZONTAL_STRIDE_0,
>>  				 BRW_SWIZZLE_XYZW, WRITEMASK_XYZW);
>>     struct brw_reg src1 = brw_reg(src.file, src.nr, 0,
>>  				 BRW_REGISTER_TYPE_F,
>> -				 BRW_VERTICAL_STRIDE_2,
>> -				 BRW_WIDTH_2,
>> +				 vstride,
>> +				 width,
>>  				 BRW_HORIZONTAL_STRIDE_0,
>>  				 BRW_SWIZZLE_XYZW, WRITEMASK_XYZW);
>>     brw_ADD(p, dst, src0, negate(src1));
>>
> 
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