[Mesa-dev] [PATCH 0/5] Add ARB_derivative_control support

[Matt Turner]

On Wed, Aug 13, 2014 at 9:52 PM, Ilia Mirkin <imirkin at alum.mit.edu> wrote:
> I left all the variants as separate operations in the glsl ir. However for
> gallium I only added the fine version, as it seems like DDX can do pretty much
> whatever it wants. I was on the fence about adding coarse versions as well and
> then using the FragmentShaderDerivative hint to select one or the other in the
> glsl -> tgsi conversion.
>
> In the case of nv50/nvc0, doing the fine version is pretty much the only
> (easy) way of doing derivatives. I haven't traced the blob to see how it
> handles things yet. In any case, on nv50/nvc0 all this is completely moot, at
> least for now. Curious about what the situation with other hardware is.

i965 already implements coarse and fine derivatives, selectable by the
derivatives hint, coarse default.

The calculation of the derivative itself isn't faster for coarse
derivatives, but it was discovered that if all of the samples of a
sample_d are from the same LOD, it's a bunch faster on Haswell at
least. See commit 848c0e72. And with coarse derivatives they are.

Maybe other hardware has similar optimizations?

> Also, the extension spec claims to require GLSL 4.00, which seems a little
> extreme. Instead I restrict it to core contexts. Let me know if I should
> change this.

Making it core-only doesn't help, nor does it satisfy the GLSL >= 4.0
requirement in the spec. I'm not sure if we have a way to arbitrarily
limit an extension to being exposed under certain GLSL versions... ?