[Mesa-dev] [PATCH 3/4] nir: support to flatten_all in peephole-select

[Rob Clark]

On Thu, Apr 2, 2015 at 1:24 PM, Jason Ekstrand <jason at jlekstrand.net> wrote:
> On Thu, Apr 2, 2015 at 1:20 AM, Kenneth Graunke <kenneth at whitecape.org> wrote:
>> On Wednesday, April 01, 2015 07:22:40 PM Connor Abbott wrote:
>>> I think it might be better here if we had a callback that backends can
>>> fill in that tells you when an instruction can be pulled out by the
>>> sel peephole. As Jason noted, you won't be able to do this for
>>> everything (notably, output writes and variable writes) and we'll
>>> probably need special handling for predicating discards if we want to
>>> be able to flatten everything. There are also a few cases where on
>>> i965 we aren't activating it when we could. Even then, I didn't think
>>> we'd need something this general, but with different backends with
>>> such varying needs I guess it makes more sense to go with the more
>>> general solution.
>>
>> NIR does have nir_intrinsic_discard_if, which performs a conditional
>> discard.  Assignments can be handled by conditional selects.  In the
>> absence of other memory writes (atomics, images, etc), that's probably
>> sufficient, no?
>
> Yes, that should be sufficient assuming array accesses can be
> flattened etc.  However, the NIR pass shouldn't flatten things that
> can't be flattened so we don't generate wrong NIR.  Instead, in those
> cases, we should just let the backend die on control-flow.

right..  I'll have a go at hoisting things out that can be moved (like
kill -> kill_if and change writes to output to writes to var and then
select before write to output).. for things that can't be moved, let
the backend fail.. doubtful a backend that doesn't support flow
control will support atomics and other 'fancy stuff' anyways ;-)

(which.. does makes me wonder how I'll eventually deal w/ atomics vs
instruction scheduling..  maybe I can split them out into a separate
unconditional successor basic block since at that point I'd be doing
instruction scheduling at the basic block level.  I think.)

BR,
-R


> --Jason