Mesa (9.1): i965/vs: Gen4/5: enable front colors if back colors are written

[Chris Forbes]

Module: Mesa
Branch: 9.1
Commit: 4f26ea56120cffaa1a0c2337423cf08b835dffb8
URL:    http://cgit.freedesktop.org/mesa/mesa/commit/?id=4f26ea56120cffaa1a0c2337423cf08b835dffb8

Author: Chris Forbes <chrisf at ijw.co.nz>
Date:   Sun Jul  7 23:13:07 2013 +1200

i965/vs: Gen4/5: enable front colors if back colors are written

Fixes undefined results if a back color is written, but the
corresponding front color is not, and only backfacing primitives are
drawn. Results are still undefined if a frontfacing primitive is drawn,
but that's OK.

The other reasonable way to fix this would have been to just pick
the one color slot that was populated, but that dilutes the value of
the tests.

On Gen6+, the fixed function clipper and triangle setup already take
care of this.

Fixes 11 piglits:
spec/glsl-1.10/execution/interpolation/interpolation-none-gl_Back*Color-*

NOTE: This is a candidate for stable branches.

Signed-off-by: Chris Forbes <chrisf at ijw.co.nz>
Reviewed-by: Kenneth Graunke <kenneth at whitecape.org>
(cherry picked from commit 1ec66f2fb2fea0497efebc1db83c75ec95a816a4)

---

 src/mesa/drivers/dri/i965/brw_vs.c |    8 ++++++++
 1 files changed, 8 insertions(+), 0 deletions(-)

diff --git a/src/mesa/drivers/dri/i965/brw_vs.c b/src/mesa/drivers/dri/i965/brw_vs.c
index 0234ebb..950fca4 100644
--- a/src/mesa/drivers/dri/i965/brw_vs.c
+++ b/src/mesa/drivers/dri/i965/brw_vs.c
@@ -271,6 +271,14 @@ do_vs_prog(struct brw_context *brw,
 	 c.prog_data.outputs_written |= BITFIELD64_BIT(VERT_RESULT_TEX0 + i);
    }
 
+   if (intel->gen < 6) {
+      /* if back colors are written, allocate slots for front colors too */
+      if (c.prog_data.outputs_written & BITFIELD64_BIT(VERT_RESULT_BFC0))
+         c.prog_data.outputs_written |= BITFIELD64_BIT(VERT_RESULT_COL0);
+      if (c.prog_data.outputs_written & BITFIELD64_BIT(VERT_RESULT_BFC1))
+         c.prog_data.outputs_written |= BITFIELD64_BIT(VERT_RESULT_COL1);
+   }
+
    brw_compute_vue_map(brw, &c);
 
    if (0) {