[Bug 92760] Add FP64 support to the i965 shader backends

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=92760

--- Comment #4 from Connor Abbott <cwabbott0 at gmail.com> ---
I've pushed my latest changes to the i965-fp64-v3 branch. It's based on the
patch series I sent out a little while ago to make glsl-to-nir use the visitor.
It's not cleaned-up, but it's got a decent amount of things working.

Sam and Iago: for the most part, the core NIR changes are pretty stable. The
one thing that Jason mentioned needed to be done there (the nir_algebraic
bitwidth verifier) is something that would be best for Jason or me to do, since
we've discussed it extensively in-person and know what it needs to look like.
It's also not essential to get it working (it's basically just a tool to
validate at compile-time that certain asserts/validator failures won't happen
when rewriting expressions with different bit-widths). What do you want to do
there?

Now, I think the main thing remaining to get the existing tests to pass
(besides implementing ftrunc and friends) is to make our handling of splitting
dvec3's and dvec4's into dvec2's and doubles more robust. Right now, it mostly
works, but falls over whenever we hit a case where we need to create a vec3.
There are two solutions I can see for this:

1. Special-case lower_vec_to_movs to skip over vec3/vec4 with doubles, and
handle vec3/vec4 specially in the i965 vec4 backend.

2. Add the ability to handle math on dvec3's and dvec4's, but with a writemask
that's never larger than 2 (i.e. we never have to emit more than one thing).
lower_vec_to_movs may coalesce movs away, so we would have to do handle this in
the backend in order to handle what NIR currently gives us.

#1 seems like a better solution to me, but unfortunately we'd have to modify
lower_vec_to_movs in a somewhat ugly, i965-specific way.

There's another problem with the vec4 backend and exec_masks, but I want to
send another email to see what other people who know about i965 have to say.

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