i965: Always emit alpha when nr_color_buffers == 0.

If alpha-testing is enabled, we need to send alpha down the pipeline
even if nr_color_buffers == 0.  However, tracking whether alpha-testing
is enabled in the WM program key is expensive: it causes us to compile
multiple specializations of the same shader, using program cache space.

This patch removes the check for alpha-testing, and simply emits alpha
whenever nr_color_buffers == 0.  We believe this will also be necessary
for alpha-to-coverage, and it should add minimal overhead to an uncommon
case.  Saving the recompiles should more than make up the difference.

Reviewed-by: Eric Anholt <[email protected]>
Signed-off-by: Kenneth Graunke <[email protected]>

1 files changed, 5 insertions, 7 deletions

diff --git a/src/mesa/drivers/dri/i965/brw_fs_visitor.cpp b/src/mesa/drivers/dri/i965/brw_fs_visitor.cpp
index 7224cbe04f5..08c0130aa3e 100644
--- a/src/mesa/drivers/dri/i965/brw_fs_visitor.cpp
+++ b/src/mesa/drivers/dri/i965/brw_fs_visitor.cpp
@@ -2141,13 +2141,11 @@ fs_visitor::emit_fb_writes()
    }
 
    if (c->key.nr_color_regions == 0) {
-      if (c->key.alpha_test) {
-	 /* If the alpha test is enabled but there's no color buffer,
-	  * we still need to send alpha out the pipeline to our null
-	  * renderbuffer.
-	  */
-	 emit_color_write(0, 3, color_mrf);
-      }
+      /* Even if there's no color buffers enabled, we still need to send
+       * alpha out the pipeline to our null renderbuffer to support
+       * alpha-testing, alpha-to-coverage, and so on.
+       */
+      emit_color_write(0, 3, color_mrf);
 
       fs_inst *inst = emit(FS_OPCODE_FB_WRITE);
       inst->base_mrf = base_mrf;