[Mesa-dev] [PATCH] i965/gen7: always lower textureGrad() on gen7

[Ian Romanick]

On 09/05/2013 08:57 AM, Chia-I Wu wrote:
> On Thu, Sep 5, 2013 at 5:12 PM, Chris Forbes <chrisf at ijw.co.nz> wrote:
>> A possible explanation for the perf change is that Xonotic uses
>> anisotropic filtering at this quality level. Lowering to txl defeats
>> it.
> I had a look at that.  gl_sampler->MaxAnisotropy is never greater than
> 1.0 in gen7_update_sampler_state() so there is no anisotropic
> filtering in this case.
> 
> It makes sense to me that avoiding punting to SIMD8 helps the
> performance.  But it is not clear to me why >10% performance change
> can still be observed when INTEL_DEBUG=no16 is specified.  A
> reasonable explanation is that the image quality is degraded in some
> way, which is why I am still nervous about the change.
> 
> An alternative approach to avoid punting seems to emulate SIMD16
> sample_d with two SIMD8 sample_d.  It will take longer to implement
> given my familiarity with the code, and may be less performant.  BUt
> that would allow things like anisotropic filtering to be honored.
> 
> 
>> It would be worth doing an image quality comparison before and after the change.
> Yeah, that is worth doing.  I will do that.

Any results?  Still waiting...

>> -- Chris
>>
>> On Thu, Sep 5, 2013 at 8:35 PM, Chia-I Wu <olvaffe at gmail.com> wrote:
>>> sample_d is slower than the lowered version on gen7.  For gen7, this improves
>>> Xonotic benchmark with Ultimate effects by as much as 25%:
>>>
>>>  before the change:                      40.06 fps
>>>  after the change:                       51.10 fps
>>>  after the change with INTEL_DEBUG=no16: 44.46 fps
>>>
>>> As sample_d is not allowed in SIMD16 mode, I firstly thought the difference
>>> was from SIMD8 versus SIMD16.  If that was the case, we would want to apply
>>> brw_lower_texture_gradients() only on fragment shaders in SIMD16 mode.
>>>
>>> But, as the numbers show, there is still 10% improvement when SIMD16 is forced
>>> off after the change.  Thus textureGrad() is lowered unconditionally for now.
>>> Due to this and that I haven't tried it on Haswell, this is still RFC.
>>>
>>> No piglit regressions.
>>>
>>> Signed-off-by: Chia-I Wu <olvaffe at gmail.com>
>>> ---
>>>  .../dri/i965/brw_lower_texture_gradients.cpp       | 54 ++++++++++++++--------
>>>  1 file changed, 36 insertions(+), 18 deletions(-)
>>>
>>> diff --git a/src/mesa/drivers/dri/i965/brw_lower_texture_gradients.cpp b/src/mesa/drivers/dri/i965/brw_lower_texture_gradients.cpp
>>> index 1589a20..f3fcb56 100644
>>> --- a/src/mesa/drivers/dri/i965/brw_lower_texture_gradients.cpp
>>> +++ b/src/mesa/drivers/dri/i965/brw_lower_texture_gradients.cpp
>>> @@ -34,8 +34,8 @@ using namespace ir_builder;
>>>
>>>  class lower_texture_grad_visitor : public ir_hierarchical_visitor {
>>>  public:
>>> -   lower_texture_grad_visitor(bool has_sample_d_c)
>>> -      : has_sample_d_c(has_sample_d_c)
>>> +   lower_texture_grad_visitor(bool has_sample_d, bool has_sample_d_c)
>>> +      : has_sample_d(has_sample_d), has_sample_d_c(has_sample_d_c)
>>>     {
>>>        progress = false;
>>>     }
>>> @@ -44,6 +44,7 @@ public:
>>>
>>>
>>>     bool progress;
>>> +   bool has_sample_d;
>>>     bool has_sample_d_c;
>>>
>>>  private:
>>> @@ -90,22 +91,33 @@ txs_type(const glsl_type *type)
>>>  ir_visitor_status
>>>  lower_texture_grad_visitor::visit_leave(ir_texture *ir)
>>>  {
>>> -   /* Only lower textureGrad with shadow samplers */
>>> -   if (ir->op != ir_txd || !ir->shadow_comparitor)
>>> +   if (ir->op != ir_txd)
>>>        return visit_continue;
>>>
>>> -   /* Lower textureGrad() with samplerCubeShadow even if we have the sample_d_c
>>> -    * message.  GLSL provides gradients for the 'r' coordinate.  Unfortunately:
>>> -    *
>>> -    * From the Ivybridge PRM, Volume 4, Part 1, sample_d message description:
>>> -    * "The r coordinate contains the faceid, and the r gradients are ignored
>>> -    *  by hardware."
>>> -    *
>>> -    * We likely need to do a similar treatment for samplerCube and
>>> -    * samplerCubeArray, but we have insufficient testing for that at the moment.
>>> -    */
>>> -   bool need_lowering = !has_sample_d_c ||
>>> -      ir->sampler->type->sampler_dimensionality == GLSL_SAMPLER_DIM_CUBE;
>>> +   bool need_lowering = false;
>>> +
>>> +   if (ir->shadow_comparitor) {
>>> +      /* Lower textureGrad() with samplerCubeShadow even if we have the
>>> +       * sample_d_c message.  GLSL provides gradients for the 'r' coordinate.
>>> +       * Unfortunately:
>>> +       *
>>> +       * From the Ivybridge PRM, Volume 4, Part 1, sample_d message
>>> +       * description: "The r coordinate contains the faceid, and the r
>>> +       * gradients are ignored by hardware."
>>> +       */
>>> +      if (ir->sampler->type->sampler_dimensionality == GLSL_SAMPLER_DIM_CUBE)
>>> +         need_lowering = true;
>>> +      else if (!has_sample_d_c)
>>> +         need_lowering = true;
>>> +   }
>>> +   else {
>>> +      /* We likely need to do a similar treatment for samplerCube and
>>> +       * samplerCubeArray, but we have insufficient testing for that at the
>>> +       * moment.
>>> +       */
>>> +      if (!has_sample_d)
>>> +         need_lowering = true;
>>> +   }
>>>
>>>     if (!need_lowering)
>>>        return visit_continue;
>>> @@ -154,7 +166,9 @@ lower_texture_grad_visitor::visit_leave(ir_texture *ir)
>>>                                expr(ir_unop_sqrt, dot(dPdy, dPdy)));
>>>     }
>>>
>>> -   /* lambda_base = log2(rho).  We're ignoring GL state biases for now. */
>>> +   /* lambda_base = log2(rho).  It will be biased and clamped by values
>>> +    * defined in SAMPLER_STATE to get the final lambda.
>>> +    */
>>>     ir->op = ir_txl;
>>>     ir->lod_info.lod = expr(ir_unop_log2, rho);
>>>
>>> @@ -168,8 +182,12 @@ bool
>>>  brw_lower_texture_gradients(struct brw_context *brw,
>>>                              struct exec_list *instructions)
>>>  {
>>> +   /* sample_d is slower than the lowered version on gen7, and is not allowed
>>> +    * in SIMD16 mode.  Treating it as unsupported improves the performance.
>>> +    */
>>> +   bool has_sample_d = brw->gen != 7;
>>>     bool has_sample_d_c = brw->gen >= 8 || brw->is_haswell;
>>> -   lower_texture_grad_visitor v(has_sample_d_c);
>>> +   lower_texture_grad_visitor v(has_sample_d, has_sample_d_c);
>>>
>>>     visit_list_elements(&v, instructions);
>>>
>>> --
>>> 1.8.3.1
>>>
>>> _______________________________________________
>>> mesa-dev mailing list
>>> mesa-dev at lists.freedesktop.org
>>> http://lists.freedesktop.org/mailman/listinfo/mesa-dev