i965/fs: Don't issue FB writes for bound but unwritten color targets.

We used to loop over all color attachments, and emit FB writes for each
one, even if the shader didn't write to a corresponding output variable.
Those color attachments would be filled with garbage (undefined values).

Football Manager binds a framebuffer with 4 color attachments, but draws
to it using a shader that only writes to gl_FragData[0..2].  This meant
that color attachment 3 would be filled with garbage, resulting in
rendering artifacts.  Now we skip writing to it, fixing rendering.

Writes to gl_FragColor initialize outputs[0..nr_color_regions-1] to
GRFs, while writes to gl_FragData[i] initialize outputs[i].

Thanks to Jason Ekstrand for tracking this down.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=86747
Signed-off-by: Kenneth Graunke <[email protected]>
Reviewed-by: Jason Ekstrand <[email protected]>
Cc: [email protected]
(cherry picked from commit e95969cd9548033250ba12f2adf11740319b41e7)

Conflicts:
	src/mesa/drivers/dri/i965/brw_fs_visitor.cpp

1 files changed, 9 insertions, 3 deletions

diff --git a/src/mesa/drivers/dri/i965/brw_fs_visitor.cpp b/src/mesa/drivers/dri/i965/brw_fs_visitor.cpp
index 0e4b814e73c..80488926d02 100644
--- a/src/mesa/drivers/dri/i965/brw_fs_visitor.cpp
+++ b/src/mesa/drivers/dri/i965/brw_fs_visitor.cpp
@@ -3520,7 +3520,7 @@ fs_visitor::emit_fb_writes()
          do_dual_src = false;
    }
 
-   fs_inst *inst;
+   fs_inst *inst = NULL;
    if (do_dual_src) {
       this->current_annotation = ralloc_asprintf(this->mem_ctx,
 						 "FB dual-source write");
@@ -3528,8 +3528,12 @@ fs_visitor::emit_fb_writes()
                                   reg_undef, 4);
       inst->target = 0;
       prog_data->dual_src_blend = true;
-   } else if (key->nr_color_regions > 0) {
+   } else {
       for (int target = 0; target < key->nr_color_regions; target++) {
+         /* Skip over outputs that weren't written. */
+         if (this->outputs[target].file == BAD_FILE)
+            continue;
+
          this->current_annotation = ralloc_asprintf(this->mem_ctx,
                                                     "FB write target %d",
                                                     target);
@@ -3542,7 +3546,9 @@ fs_visitor::emit_fb_writes()
                                      this->output_components[target]);
          inst->target = target;
       }
-   } else {
+   }
+
+   if (inst == NULL) {
       /* 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.