i965: Fix INTEL_DEBUG=shader_time for fragment shaders with discards.

"discard" instructions generate HALT instructions which jump to a final
HALT near the end of the shader.  Previously, fs_generator created this
final jump target when it saw the first FS_OPCODE_FB_WRITE, causing it
to jump right before the FB write epilogue.  This is normally good.

However, INTEL_DEBUG=shader_time also has an epilogue section which
records the final timestamp.  The frontend emits IR for this just before
FS_OPCODE_FB_WRITE.  Unfortunately, this led to the following ordering:

1. Shader Time Epilogue
2. Final HALT (where discards jump)
3. Framebuffer Write Epilogue

This meant that discarded pixels completely skipped the shader time
epilogue, causing no ending timestamp to be written.  This obviously
led to inaccurate results.

This patch adds a new FS_OPCODE_PLACEHOLDER_HALT in the IR stream just
before any epilogue sections.  This is where the final HALT should be
generated, and makes it easy to ensure the correct ordering:

1. Final HALT
2. Shader Time Epilogue
3. Framebuffer Write Epilogue

For shaders that don't discard, this opcode compiles away to nothing.
The scheduler adds barrier dependencies to make sure that it doesn't
get moved above any FS_OPCODE_DISCARD_JUMP instructions.

One 8-wide shader in GLBenchmark 2.7 dropped from 2291.67 Gcycles to
a mere 5.13 Gcycles.

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

1 files changed, 7 insertions, 6 deletions

diff --git a/src/mesa/drivers/dri/i965/brw_fs_emit.cpp b/src/mesa/drivers/dri/i965/brw_fs_emit.cpp
index ad1ca58f58c..a729569c840 100644
--- a/src/mesa/drivers/dri/i965/brw_fs_emit.cpp
+++ b/src/mesa/drivers/dri/i965/brw_fs_emit.cpp
@@ -102,12 +102,6 @@ fs_generator::generate_fb_write(fs_inst *inst)
    struct brw_reg implied_header;
    uint32_t msg_control;
 
-   /* Note that the jumps emitted to this point mean that the g0 ->
-    * base_mrf setup must be inside of this function, so that we jump
-    * to a point containing it.
-    */
-   patch_discard_jumps_to_fb_writes();
-
    /* Header is 2 regs, g0 and g1 are the contents. g0 will be implied
     * move, here's g1.
     */
@@ -1346,6 +1340,13 @@ fs_generator::generate_code(exec_list *instructions)
          generate_unpack_half_2x16_split(inst, dst, src[0]);
          break;
 
+      case FS_OPCODE_PLACEHOLDER_HALT:
+         /* This is the place where the final HALT needs to be inserted if
+          * we've emitted any discards.  If not, this will emit no code.
+          */
+         patch_discard_jumps_to_fb_writes();
+         break;
+
       default:
 	 if (inst->opcode < (int) ARRAY_SIZE(opcode_descs)) {
 	    _mesa_problem(ctx, "Unsupported opcode `%s' in FS",