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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>