On Thu, Jul 12, 2012 at 10:43 AM, Paul Berry <stereotype...@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.pho...@gmail.com>
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