[Mesa-dev] FLAG-DAY: NIR derefs

[Jason Ekstrand]

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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