[Mesa-dev] [PATCH] i965: Use NIR by default for VS

[Matt Turner]

On Thu, Aug 20, 2015 at 3:01 PM, Dave Airlie <airlied at gmail.com> wrote:
>> Shader-db results for vec4 on i965:
>
> reads like a Johnny Cash cover of NIN.

Agreed, so let me explain why we're enabling it now.

The merge window closed yesterday for Mesa 11.0. vec4 NIR isn't quite
where we'd like it to be in terms of code quality, but we want to
avoid vec4 NIR getting no external testing for a whole release cycle,
while still having to maintain both the NIR and non-NIR paths.

I've been mostly against enabling NIR for the vec4 backend until its
helped:hurt programs ratio dramatically improves, but I think that
leaving it disabled for another three months is probably worse.

>From what I've found, the largest code-quality regression is from the
combination vec4 backend's register coalescing optimization being
inadequate (a known problem regardless of NIR) and NIR generating more
instances of code that register coalescing cannot optimize.

I'm working on a new vec4 register coalescing pass, and I think that
there are some simple improvements we can make to improve the code the
backend gets from NIR. I'm planning to have these things in the 11.0
release.

If worse comes to worse, we can disable vec4 NIR before the release.