[Mesa-dev] [PATCH] i965/vs: Gen4/5: enable front colors if back colors are written

[Kenneth Graunke]

On 07/10/2013 02:46 PM, Chris Forbes wrote:
> 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>
> ---
>   src/mesa/drivers/dri/i965/brw_vs.c | 6 ++++++
>   1 file changed, 6 insertions(+)
>
> diff --git a/src/mesa/drivers/dri/i965/brw_vs.c b/src/mesa/drivers/dri/i965/brw_vs.c
> index 60b40c5..5b8173d 100644
> --- a/src/mesa/drivers/dri/i965/brw_vs.c
> +++ b/src/mesa/drivers/dri/i965/brw_vs.c
> @@ -277,6 +277,12 @@ do_vs_prog(struct brw_context *brw,
>            if (c.key.point_coord_replace & (1 << i))
>               outputs_written |= BITFIELD64_BIT(VARYING_SLOT_TEX0 + i);
>         }
> +
> +      /* if back colors are written, allocate slots for front colors too */
> +      if (outputs_written & BITFIELD64_BIT(VARYING_SLOT_BFC0))
> +         outputs_written |= BITFIELD64_BIT(VARYING_SLOT_COL0);
> +      if (outputs_written & BITFIELD64_BIT(VARYING_SLOT_BFC1))
> +         outputs_written |= BITFIELD64_BIT(VARYING_SLOT_COL1);
>      }
>
>      brw_compute_vue_map(brw, &prog_data.base.vue_map, outputs_written,

I'm not terribly familiar with how this works, but it seems reasonable 
to me; those two usually come in pairs so we can use swizzling to select 
which we want.

Reviewed-by: Kenneth Graunke <kenneth at whitecape.org>