[Intel-gfx] [PATCH 4/9] intel gen4-5: Fix backface/frontface selection when one one color is written to.

[Olivier Galibert]

Shaders, piglit test ones in particular, may write only to one of
gl_FrontColor/gl_BackColor.  The standard is unclear on whether the
behaviour is defined in that case, but it seems reasonable to support
it.

The choice done there to pick up whichever color was actually written
to.  That makes most of the generated piglit tests useless to test the
backface selection, but it's simple and it works.

Signed-off-by: Olivier Galibert <galibert at pobox.com>
---
 src/mesa/drivers/dri/i965/brw_fs.cpp     |    9 +++++++++
 src/mesa/drivers/dri/i965/brw_wm_pass2.c |    9 +++++++++
 2 files changed, 18 insertions(+)

diff --git a/src/mesa/drivers/dri/i965/brw_fs.cpp b/src/mesa/drivers/dri/i965/brw_fs.cpp
index 3f98137..3b62952 100644
--- a/src/mesa/drivers/dri/i965/brw_fs.cpp
+++ b/src/mesa/drivers/dri/i965/brw_fs.cpp
@@ -972,6 +972,15 @@ fs_visitor::calculate_urb_setup()
 	 if (c->key.vp_outputs_written & BITFIELD64_BIT(i)) {
 	    int fp_index = _mesa_vert_result_to_frag_attrib((gl_vert_result) i);
 
+            /* Special case: two-sided vertex option, vertex program
+             * only writes to the back color.  Map it to the
+             * associated front color location.
+             */
+            if (i >= VERT_RESULT_BFC0 && i <= VERT_RESULT_BFC1 &&
+                ctx->VertexProgram._TwoSideEnabled &&
+                urb_setup[i - VERT_RESULT_BFC0 + FRAG_ATTRIB_COL0] == -1)
+               fp_index = i - VERT_RESULT_BFC0 + FRAG_ATTRIB_COL0;
+
 	    /* The back color slot is skipped when the front color is
 	     * also written to.  In addition, some slots can be
 	     * written in the vertex shader and not read in the
diff --git a/src/mesa/drivers/dri/i965/brw_wm_pass2.c b/src/mesa/drivers/dri/i965/brw_wm_pass2.c
index eacf7c0..48143f3 100644
--- a/src/mesa/drivers/dri/i965/brw_wm_pass2.c
+++ b/src/mesa/drivers/dri/i965/brw_wm_pass2.c
@@ -96,6 +96,15 @@ static void init_registers( struct brw_wm_compile *c )
 	 if (c->key.vp_outputs_written & BITFIELD64_BIT(j)) {
 	    int fp_index = _mesa_vert_result_to_frag_attrib(j);
 
+            /* Special case: two-sided vertex option, vertex program
+             * only writes to the back color.  Map it to the
+             * associated front color location.
+             */
+            if (j >= VERT_RESULT_BFC0 && j <= VERT_RESULT_BFC1 &&
+                intel->ctx.VertexProgram._TwoSideEnabled &&
+                !(c->key.vp_outputs_written & BITFIELD64_BIT(j - VERT_RESULT_BFC0 + VERT_RESULT_COL0)))
+               fp_index = j - VERT_RESULT_BFC0 + FRAG_ATTRIB_COL0;
+
 	    nr_interp_regs++;
 
 	    /* The back color slot is skipped when the front color is
-- 
1.7.10.280.gaa39