Mesa (master): i965: Always emit alpha when nr_color_buffers == 0.

[Kenneth Graunke]

Module: Mesa
Branch: master
Commit: 86e401b771ce4a6f9a728f76c5061c339f012d0a
URL:    http://cgit.freedesktop.org/mesa/mesa/commit/?id=86e401b771ce4a6f9a728f76c5061c339f012d0a

Author: Kenneth Graunke <kenneth at whitecape.org>
Date:   Thu Jul 12 11:25:58 2012 -0700

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 <eric at anholt.net>
Signed-off-by: Kenneth Graunke <kenneth at whitecape.org>

---

 src/mesa/drivers/dri/i965/brw_fs_visitor.cpp |   12 +++++-------
 src/mesa/drivers/dri/i965/brw_wm.c           |    4 +---
 src/mesa/drivers/dri/i965/brw_wm.h           |    1 -
 3 files changed, 6 insertions(+), 11 deletions(-)

diff --git a/src/mesa/drivers/dri/i965/brw_fs_visitor.cpp b/src/mesa/drivers/dri/i965/brw_fs_visitor.cpp
index 7224cbe..08c0130 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;
diff --git a/src/mesa/drivers/dri/i965/brw_wm.c b/src/mesa/drivers/dri/i965/brw_wm.c
index 8805646..7194a73 100644
--- a/src/mesa/drivers/dri/i965/brw_wm.c
+++ b/src/mesa/drivers/dri/i965/brw_wm.c
@@ -449,10 +449,8 @@ static void brw_wm_populate_key( struct brw_context *brw,
 
    /* Build the index for table lookup
     */
-   /* _NEW_COLOR */
-   key->alpha_test = ctx->Color.AlphaEnabled;
-
    if (intel->gen < 6) {
+      /* _NEW_COLOR */
       if (fp->program.UsesKill || ctx->Color.AlphaEnabled)
 	 lookup |= IZ_PS_KILL_ALPHATEST_BIT;
 
diff --git a/src/mesa/drivers/dri/i965/brw_wm.h b/src/mesa/drivers/dri/i965/brw_wm.h
index 2cde2a0..b976a60 100644
--- a/src/mesa/drivers/dri/i965/brw_wm.h
+++ b/src/mesa/drivers/dri/i965/brw_wm.h
@@ -65,7 +65,6 @@ struct brw_wm_prog_key {
    GLuint flat_shade:1;
    GLuint nr_color_regions:5;
    GLuint render_to_fbo:1;
-   GLuint alpha_test:1;
    GLuint clamp_fragment_color:1;
    GLuint line_aa:2;