i965: compute DDX in a subspan based only on top row

Consider only the top-left and top-right pixels to approximate DDX in a 2x2
subspan, unless the application requests a more accurate approximation via
GL_FRAGMENT_SHADER_DERIVATIVE_HINT or this optimization is disabled from the
new driconf option disable_derivative_optimization.

This 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.

The improvement comes from faster sample_d.  It seems, on Haswell, some
optimizations are introduced to allow faster sample_d when all pixels in a
subspan have the same derivative.  I considered SAMPLE_STATE too, which allows
one to control the quality of sample_d on Haswell.  But it gave much worse
image quality without giving better performance comparing to this change.

No piglit quick.tests regression on Haswell (tested with v1).

v2: better guess for precompile program key

Signed-off-by: Chia-I Wu <olv@lunarg.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Chris Forbes <chrisf@ijw.co.nz>

1 files changed, 1 insertions, 0 deletions

diff --git a/src/mesa/drivers/dri/i965/brw_wm.h b/src/mesa/drivers/dri/i965/brw_wm.h
index f7a2c5f..aa786de 100644
--- a/src/mesa/drivers/dri/i965/brw_wm.h
+++ b/src/mesa/drivers/dri/i965/brw_wm.h
@@ -66,6 +66,7 @@ struct brw_wm_prog_key {
    GLuint render_to_fbo:1;
    GLuint clamp_fragment_color:1;
    GLuint line_aa:2;
+   GLuint high_quality_derivatives:1;
 
    GLushort drawable_height;
    GLbitfield64 input_slots_valid;