[Mesa-dev] [PATCH 06/12] i965/fs: Add a sampler program key for whether the texture is 16x MSAA

[Neil Roberts]

Ben Widawsky <ben at bwidawsk.net> writes:

> Hmm. As I read it, it sounded like you didn't have to send LOD it's
> implied to be 0 if you don't send it. If I am wrong about that, then I
> agree with you completely.

I'm a bit lost. You're right that it's not necessary to send the LOD
when it's zero. In fact Mesa never sends it for the TXF_MCS instruction
presumably because OpenGL doesn't provide a way to make a texture that
is both multisampled and mipmapped so the LOD will always be zero.
However I don't see how that relates to deciding whether to always use
the ld2dms_w instruction or not. The reason I am saying we shouldn't
always do it is because it makes the sampler message longer and thus
requires at least one extra instruction to initialise the extra argument
in the message. The extra argument is due to the second MCS argument
(mcsh), not the LOD. I don't understand the point you are trying to make
about the LOD.

Even without the LOD, the arguments are potentially up to 6
(si,mcsl,mcsh,u,v,r). On SIMD16 this takes up 12 registers which is
greater than the maximum of 11 so it has to be broken up into two SIMD8
messages. This only matters for sampler2DMSArray samplers because
otherwise the r argument is not needed.

Apologies if I'm being dumb.

Regards,
- Neil