[Mesa-dev] [PATCH 4/7] i965/blorp: Optimize manual_blend() for compressed multisampled surfaces.

[Anuj Phogat]

On Thu, Jul 12, 2012 at 10:43 AM, Paul Berry <stereotype441 at gmail.com> wrote:
> When downsampling a compressed multisampled surface, we can take a
> shortcut to downsample any pixels that were completely covered by a
> single primitive.  In this case, the first color value we fetch is the
> correct final color for the downsampled pixel, so we can skip the rest
> of the blending operation.
> ---
>  src/mesa/drivers/dri/i965/brw_blorp_blit.cpp |   23 +++++++++++++++++++++++
>  1 files changed, 23 insertions(+), 0 deletions(-)
>
> diff --git a/src/mesa/drivers/dri/i965/brw_blorp_blit.cpp b/src/mesa/drivers/dri/i965/brw_blorp_blit.cpp
> index 32fd48e..c8db662 100644
> --- a/src/mesa/drivers/dri/i965/brw_blorp_blit.cpp
> +++ b/src/mesa/drivers/dri/i965/brw_blorp_blit.cpp
> @@ -1143,6 +1143,26 @@ brw_blorp_blit_program::manual_blend()
>        }
>        texel_fetch(texture_data[stack_depth++]);
>
> +      if (i == 0 && key->tex_layout == INTEL_MSAA_LAYOUT_CMS) {
> +         /* The Ivy Bridge PRM, Vol4 Part1 p27 (Multisample Control Surface)
> +          * suggests an optimization:
> +          *
> +          *     "A simple optimization with probable large return in
> +          *     performance is to compare the MCS value to zero (indicating
> +          *     all samples are on sample slice 0), and sample only from
> +          *     sample slice 0 using ld2dss if MCS is zero."
> +          *
> +          * Note that in the case where the MCS value is zero, sampling from
> +          * sample slice 0 using ld2dss and sampling from sample 0 using
> +          * ld2dms are equivalent (since all samples are on sample slice 0).
> +          * Since we have already sampled from sample 0, all we need to do is
> +          * skip the remaining fetches and averaging if MCS is zero.
> +          */
> +         brw_CMP(&func, vec16(brw_null_reg()), BRW_CONDITIONAL_NZ,
> +                 mcs_data, brw_imm_ud(0));
> +         brw_IF(&func, BRW_EXECUTE_16);
> +      }
> +
>        /* Do count_trailing_one_bits(i) times */
>        for (int j = count_trailing_one_bits(i); j-- > 0; ) {
>           assert(stack_depth >= 2);
> @@ -1169,6 +1189,9 @@ brw_blorp_blit_program::manual_blend()
>                   brw_imm_f(1.0/num_samples));
>        }
>     }
> +
> +   if (key->tex_layout == INTEL_MSAA_LAYOUT_CMS)
> +      brw_ENDIF(&func);
>  }
>
>  /**
> --
> 1.7.7.6
>

This is a useful optimization.
Reviewed-by: Anuj Phogat <anuj.phogat at gmail.com>