[Mesa-dev] FLAG-DAY: NIR derefs

[Jason Ekstrand]

For those interested in following along, I've pushed a bunch of stuff to a
branch:

https://cgit.freedesktop.org/~jekstrand/mesa/log/?h=wip/nir-deref-instr

I will continue force-pushing that branch as I go.  My current approach to
doing things incrementally is to try and do it by type of dereference
initially.  It'll be tricky when I start using deref instructions for local
variables as that's where most of the passes want to use them.

--Jason

On Wed, Mar 14, 2018 at 1:32 PM, Jason Ekstrand <jason at jlekstrand.net>
wrote:

> All,
>
> Connor and I along with several others have been discussing for a while
> changing the way NIR dereferences work.  In particular, adding a new
> nir_deref_instr type where the first one in the chain takes a variable and
> is followed by a series of instructions which take another deref
> instruction and do an array or structure dereference on it.
>
> Much of the motivation for this is some of the upcoming SPIR-V stuff where
> we have more real pointers and deref chains don't really work anymore.  It
> will also allow for things such as CSE of common derefs which could make
> analysis easier.  This is similar to what LLVM does and it's working very
> well for them.
>
> The reason for this e-mail is that this is going to be a flag-day change.
> We've been talking about it for a while but this is going to be a major and
> fairly painful change in the short term so no one has actually done it.
> It's time we finally just suck it up and make it happen.  While we will try
> to make the change as incrementally and reviewably as possible but there is
> a real limit as to what is possible here.  My plan is to start cracking
> away at this on Monday and hopefully have something working for i965/anv by
> the end of the week or maybe some time the week after.  If anyone has
> something to say in opposition, please speak up now and not after I've
> spent a week straight frantically hacking on NIR.
>
> I would like everyone to be respectful of the fact that this will be a
> major change and very painful to rebase.  If you've got outstanding NIR,
> GLSL, or SPIR-V work that is likely to conflict with this, please try to
> land it before Monday so that we can avoid rebase conflicts.  If you have
> interest in reviewing this, please try to be responsive so that we can get
> it reviewed and landed before it becomes too painful.  I'll try to send out
> some preview patches as I go so that the data structures themselves can get
> some review before the rest of the changes have been made.
>
> I'm also asking for help from Rob, Bas, and Eric if there are changes
> needed in any of their drivers.  I suspect the impact on back-end drivers
> will be low because most of them don't use derefs directly, but it would be
> good of people were on hand to help catch bugs if nothing else.
>
> Thanks,
>
> --Jason Ekstrand
>
>
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