[Mesa-dev] [PATCH] i965: Only emit 1 viewport when possible.

[Eric Anholt]

Kenneth Graunke <kenneth at whitecape.org> writes:

> In core profile, we support up to 16 viewports.  However, in the
> majority of cases, only 1 of them is actually used - we only need
> the others if the last shader stage prior to the rasterizer writes
> gl_ViewportIndex.
>
> Processing all 16 viewports adds additional CPU overhead, which hurts
> CPU-intensive workloads such as Glamor.  This meant that switching to
> core profile actually penalized Glamor to an extent, which is
> unfortunate.
>
> This patch tracks the number of relevant viewports, switching between
> 1 and ctx->Const.MaxViewports if gl_ViewportIndex is written.  A new
> BRW_NEW_VIEWPORT_COUNT flag tracks this.  This could mean re-emitting
> viewport state when switching, but hopefully this is offset by doing
> 1/16th of the work in the common case.  The new flag is also lighter
> weight than BRW_NEW_VUE_MAP_GEOM_OUT, which we were using in one case.
>
> According to Eric Anholt, this reduces the CPU overhead of scissor and
> viewport state changes n Glamor from 2.5% or so to .8% or so.
>
> Cc: Eric Anholt <eric at anholt.net>
> Signed-off-by: Kenneth Graunke <kenneth at whitecape.org>

If this is the same patch I tested on IRC earlier, x11perf -copypixwin10
performance improves by 11.5094% +/- 3.10841% (n=10) on my Skylake.
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