i965: Only emit interpolation setup if there are actual FS inputs.

Dead code elimination would get rid of the extra instructions, but
skipping this saves iterations through the optimization loop.

From shader-db:

      N     Min     Max        Median           Avg        Stddev
x 14672       3      16             3     3.1334515    0.59904168
+ 14672       1      16             3     2.8955153    0.77732963
Difference at 95.0% confidence
        -0.237936 +/- 0.0158798
        -7.59342% +/- 0.506783%
        (Student's t, pooled s = 0.693935)

Embarassingly, the classic shadow mapping shader:

   void main() { }

used to require three iterations through the optimization loop.
With this patch, it only requires one (which makes no progress).

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Eric Anholt <eric@anholt.net>

diff --git a/src/mesa/drivers/dri/i965/brw_fs.cpp b/src/mesa/drivers/dri/i965/brw_fs.cpp
index 65a4b66c6b9548a698d33960ed79e17dacd84c89..a3268fb73bf84219ec27c074d684d721c8154624 100644 (file)
--- a/src/mesa/drivers/dri/i965/brw_fs.cpp
+++ b/src/mesa/drivers/dri/i965/brw_fs.cpp
@@ -3025,10 +3025,12 @@ fs_visitor::run()
          emit_shader_time_begin();
 
       calculate_urb_setup();
-      if (brw->gen < 6)
-        emit_interpolation_setup_gen4();
-      else
-        emit_interpolation_setup_gen6();
+      if (fp->Base.InputsRead > 0) {
+         if (brw->gen < 6)
+            emit_interpolation_setup_gen4();
+         else
+            emit_interpolation_setup_gen6();
+      }
 
       /* We handle discards by keeping track of the still-live pixels in f0.1.
        * Initialize it with the dispatched pixels.