[Mesa-dev] [PATCH 6/6] i965: Set prog_data->uses_kill if simulating alpha test via discards.

[Kenneth Graunke]

When using MRT on Gen4-5, we have to simulate GL's alpha test feature
by emitting discards in the fragment shader.  In this case, it makes
sense to set prog_data->uses_kill, which means the fragment shader may
kill pixels via the discard mechanism.

This saves us from having to look an extra key value in a couple of
places, including in the generator.

Signed-off-by: Kenneth Graunke <kenneth at whitecape.org>
---
 src/mesa/drivers/dri/i965/brw_fs.cpp           | 2 +-
 src/mesa/drivers/dri/i965/brw_fs_generator.cpp | 2 +-
 src/mesa/drivers/dri/i965/brw_wm.c             | 5 ++++-
 3 files changed, 6 insertions(+), 3 deletions(-)

This series was tested on 965, G45, Ironlake, Sandybridge, Ivybridge,
Baytrail, and Haswell.  965 had a few random changes that looked totally
unrelated, but everything else worked fine.  Not tested on Broadwell.

diff --git a/src/mesa/drivers/dri/i965/brw_fs.cpp b/src/mesa/drivers/dri/i965/brw_fs.cpp
index 0ffb07f..fdd1bb3 100644
--- a/src/mesa/drivers/dri/i965/brw_fs.cpp
+++ b/src/mesa/drivers/dri/i965/brw_fs.cpp
@@ -3601,7 +3601,7 @@ fs_visitor::run()
       bool alpha_test_func =
          (stage == MESA_SHADER_FRAGMENT) &&
          ((brw_wm_prog_key*) this->key)->alpha_test_func;
-      if (uses_kill || alpha_test_func) {
+      if (uses_kill) {
          fs_inst *discard_init = emit(FS_OPCODE_MOV_DISPATCH_TO_FLAGS);
          discard_init->flag_subreg = 1;
       }
diff --git a/src/mesa/drivers/dri/i965/brw_fs_generator.cpp b/src/mesa/drivers/dri/i965/brw_fs_generator.cpp
index f61e7a1..67dfe81 100644
--- a/src/mesa/drivers/dri/i965/brw_fs_generator.cpp
+++ b/src/mesa/drivers/dri/i965/brw_fs_generator.cpp
@@ -179,7 +179,7 @@ fs_generator::generate_fb_write(fs_inst *inst, struct brw_reg payload)
       /* On HSW, the GPU will use the predicate on SENDC, unless the header is
        * present.
        */
-      if (prog_data->uses_kill || key->alpha_test_func) {
+      if (prog_data->uses_kill) {
          struct brw_reg pixel_mask;
 
          if (brw->gen >= 6)
diff --git a/src/mesa/drivers/dri/i965/brw_wm.c b/src/mesa/drivers/dri/i965/brw_wm.c
index 5863573..34500e2 100644
--- a/src/mesa/drivers/dri/i965/brw_wm.c
+++ b/src/mesa/drivers/dri/i965/brw_wm.c
@@ -156,7 +156,10 @@ bool do_wm_prog(struct brw_context *brw,
       fs = prog->_LinkedShaders[MESA_SHADER_FRAGMENT];
 
    memset(&prog_data, 0, sizeof(prog_data));
-   prog_data.uses_kill = fp->program.UsesKill;
+   /* key->alpha_test_func means simulating alpha testing via discards,
+    * so the shader definitely kills pixels.
+    */
+   prog_data.uses_kill = fp->program.UsesKill || key->alpha_test_func;
 
    /* Allocate the references to the uniforms that will end up in the
     * prog_data associated with the compiled program, and which will be freed
-- 
2.1.3