[Bug 89580] Implement a NIR -> vec4 pass

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=89580

--- Comment #6 from Eduardo Lima Mitev <elima at igalia.com> ---
(In reply to Jason Ekstrand from comment #5)
> (In reply to Eduardo Lima Mitev from comment #4)
> > 
> > - load_const: For this, we did the same as fs_nir, allocating a new register
> > and loading the constant on-demand, as part of get_nir_src(). We then
> > realized that this is not optimal for us, since we can emit vector
> > instructions and save a few movs. So we are planning to update the current
> > implementation to have a map with a register for each SSA variable, and pick
> > from that table when emitting. Thoughts on that?
> 
> The reason for re-allocating and re-eimitting every time is that the
> constant can be anywhere in the program (such as at the top) and we want the
> copy-propagation pass to be able to propagate them.  Having them in
> different control flow can cause problems for this.
> 

Hmm, yes we will have to take that into account. Pre-allocating registers for
every ssa variable will have to be done carefully (i.e, in a per-implementation
basis) to avoid unnecessary register spilling. I will give it some thought and
ask you before implementing it. For now, current (unoptimized) solution is
enough to move on.

>  I do have a couple of comments:
> 
> 1) There are two places so far, brw_type_for_nir_type and
> brw_conditional_for_nir_comparison, where we would like to put things in a
> common place.  Recently, I added a brw_nir.h file and I believe Ken added a
> brw_nir.c file which would be a perfect place to put those.  In fact, we
> could probably simplify some of the current FS NIR code with your
> conditionals function.
>

Yes, it is a good idea and will leave our code less cluttered. However, I will
turn brw_nir.c to c++ to add these methods. Any caveat with that?

> 2) We probably want to have emit_intrinsic be one big switch that does
> everything like we do in the FS.  I know that one big switch can be annoying
> at times, but so is having a pile of little 3-line helpers that we have to
> declare in the C++ class.  Not a big deal either way.
> 

Agree, will change that and take note.

> Thank you for the extremely prompt (given that it was the middle of the
> night there) reply!  Keep up the good work!

Well, it was totally worth the effort getting some code ready for you to look
at. Thanks a lot for the early review!

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