[Mesa-dev] [PATCH] i965: Don't add barrier deps for FB write messages.

[Matt Turner]

On Mon, Feb 8, 2016 at 11:31 AM, Kenneth Graunke <kenneth at whitecape.org> wrote:
> There are never render target reads, so there are no scheduling hazards.
>
> Giving the extra flexibility to the scheduler makes it possible to do
> FB writes as soon as their sources are available, reducing register
> pressure.  It also makes it possible to do the payload setup for more
> than one FB write message at a time, which could better hide latency.
>
> shader-db results on Skylake:
>
> total instructions in shared programs: 9110254 -> 9110211 (-0.00%)
> instructions in affected programs: 2898 -> 2855 (-1.48%)
> helped: 3
> HURT:   0
> LOST:   0
> GAINED: 1
>
> A reduction in instruction counts is surprising, but legitimate:
> the three shaders helped were spilling, and reducing register
> pressure allowed us to issue fewer spills/fills.
>
> total cycles in shared programs: 69035108 -> 68928820 (-0.15%)
> cycles in affected programs: 4412402 -> 4306114 (-2.41%)
> helped: 4457
> HURT: 213
>

Nice!

Reviewed-by: Matt Turner <mattst88 at gmail.com>