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 <[email protected]>
Reviewed-by: Matt Turner <[email protected]>
Reviewed-by: Eric Anholt <[email protected]>

1 files changed, 6 insertions, 4 deletions

diff --git a/src/mesa/drivers/dri/i965/brw_fs.cpp b/src/mesa/drivers/dri/i965/brw_fs.cpp
index 65a4b66c6b9..a3268fb73bf 100644
--- 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.