[Mesa-dev] [HACK] i965/fs: Fix ordering of src0 alpha and oMask in the framebuffer write payload.

[Francisco Jerez]

We were passing src0 alpha and oMask in reverse order.  There seems to
be no good way to pass them in the correct order to the new-style
LOAD_PAYLOAD (how surprising) because src0 alpha is per-channel while
oMask is not.  Just split src0 alpha in fixed-width registers and pass
them to LOAD_PAYLOAD as if they were part of the header as work-around
for now.

I've written a piglit test that demonstrates the problem by using
gl_SampleMask from a fragment shader with multiple color outputs [1].

[1] http://lists.freedesktop.org/archives/piglit/2015-July/016499.html
---
 src/mesa/drivers/dri/i965/brw_fs_visitor.cpp | 26 +++++++++++++++++---------
 1 file changed, 17 insertions(+), 9 deletions(-)

diff --git a/src/mesa/drivers/dri/i965/brw_fs_visitor.cpp b/src/mesa/drivers/dri/i965/brw_fs_visitor.cpp
index 94d6a58..304ae74 100644
--- a/src/mesa/drivers/dri/i965/brw_fs_visitor.cpp
+++ b/src/mesa/drivers/dri/i965/brw_fs_visitor.cpp
@@ -1535,6 +1535,19 @@ fs_visitor::emit_single_fb_write(const fs_builder &bld,
       length++;
    }
 
+   if (src0_alpha.file != BAD_FILE && color0.file != BAD_FILE) {
+      /* Neat, we need to chop the src0 alpha component and pass it as part of
+       * the header even though it has per-channel semantics, because the next
+       * optional field is header-like and LOAD_PAYLOAD requires all such
+       * fields to form a contiguous segment at the beginning of the message.
+       */
+      for (unsigned i = 0; i < exec_size / 8; i++) {
+         setup_color_payload(&sources[length], src0_alpha, 1, 8,
+                             use_2nd_half || i == 1);
+         length++;
+      }
+   }
+
    prog_data->uses_omask =
       prog->OutputsWritten & BITFIELD64_BIT(FRAG_RESULT_SAMPLE_MASK);
    if (prog_data->uses_omask) {
@@ -1561,19 +1574,14 @@ fs_visitor::emit_single_fb_write(const fs_builder &bld,
                              offset(this->outputs[0], bld, 3),
                              1, exec_size, false);
       length += 4;
-   } else if (color1.file == BAD_FILE) {
-      if (src0_alpha.file != BAD_FILE) {
-         setup_color_payload(&sources[length], src0_alpha, 1, exec_size, false);
-         length++;
-      }
-
-      setup_color_payload(&sources[length], color0, components,
-                          exec_size, use_2nd_half);
-      length += 4;
    } else {
       setup_color_payload(&sources[length], color0, components,
                           exec_size, use_2nd_half);
       length += 4;
+
+   }
+
+   if (color1.file != BAD_FILE) {
       setup_color_payload(&sources[length], color1, components,
                           exec_size, use_2nd_half);
       length += 4;
-- 
2.4.3