[Mesa-dev] [PATCH 0/2] Nir: Allow CSE of SSBO loads

[Iago Toral]

On Thu, 2015-10-22 at 09:39 -0400, Connor Abbott wrote:
> On Thu, Oct 22, 2015 at 7:21 AM, Iago Toral Quiroga <itoral at igalia.com> wrote:
> > I implemented this first as a separate optimization pass in GLSL IR [1], but
> > Curro pointed out that this being pretty much a restricted form of a CSE pass
> > it would probably make more sense to do it inside CSE (and we no longer have
> > a CSE pass in GLSL IR).
> >
> > Unlike other things we CSE in NIR, in the case of SSBO loads we need to make
> > sure that we invalidate previous entries in the set in the presence of
> > conflicting instructions (i.e. SSBO writes to the same block and offset) or
> > in the presence of memory barriers.
> >
> > If this is accepted I intend to extend this to also cover image reads, which
> > follow similar behavior.
> >
> > No regressions observed in piglit or dEQP's SSBO functional tests.
> >
> > [1] http://lists.freedesktop.org/archives/mesa-dev/2015-October/097718.html
> >
> > Iago Toral Quiroga (2):
> >   nir/cse: invalidate SSBO loads in presence of ssbo writes or memory
> >     barriers
> >   nir/instr_set: allow rewrite of SSBO loads
> >
> >  src/glsl/nir/nir_instr_set.c |  24 ++++++--
> >  src/glsl/nir/nir_opt_cse.c   | 142 +++++++++++++++++++++++++++++++++++++++++++
> >  2 files changed, 162 insertions(+), 4 deletions(-)
> >
> > --
> > 1.9.1
> >
> > _______________________________________________
> > mesa-dev mailing list
> > mesa-dev at lists.freedesktop.org
> > http://lists.freedesktop.org/mailman/listinfo/mesa-dev
> 
> NAK, this isn't going to work. NIR CSE is designed for operations
> which can be moved around freely as long they're still dominated by
> the SSA values they use. It makes heavy advantage of this to avoid
> looking at the entire CFG and instead only at the current block and
> its parents in the dominance tree. For example, imagine you have
> something like:
> 
> A = load_ssbo 0
> if  (cond) {
>    store_ssbo 0
> }
> B = load_ssbo 0
> 
> Then A and B can't be combined, but CSE will combine them anyways when
> it reaches B because it keeps a hash table of values dominating B and
> finds A as a match. It doesn't look at the if conditional at all
> because it doesn't dominate the load to B. This is great when you want
> to CSE pure things that don't depend on other side effects -- after
> all, this is the sort of efficiency that SSA is supposed to give us --
> but it means that as-is, it can't be used for e.g. SSBO's and images
> without completely changing how the pass works and making it less
> efficient.

Ugh! One would think that at least one of the 2000+ SSBO tests in dEQP
would catch something like this... I guess not :(.

> Now, that being said, I still think that we should definitely be doing
> this sort of thing in NIR now that we've finally added support for
> SSBO's and images. We've been trying to avoid adding new optimizations
> to GLSL, since we've been trying to move away from it. In addition,
> with SPIR-V on the way, anything added to GLSL IR now is something
> that we won't be able to use with SPIR-V shaders. Only doing it in FS
> doesn't sound so great either; we should be doing as much as possible
> at the midlevel, and combining SSBO loads is something that isn't
> FS-specific at all.

Yeah, agreed.

> There are two ways I can see support for this being added to NIR:
> 
> 1. Add an extra fake source/destination to intrinsics with side
> effects, and add a pass to do essentially a conversion to SSA that
> wires up these "token" sources/destinations, or perhaps extend the
> existing to-SSA pass.
> 
> 2. Add a special "load-combining" pass that does some dataflow
> analysis or similar (or, for now, only looks at things within a single
> block).
> 
> The advantage of #1 is that we get to use existing NIR passes, like
> CSE, DCE, and GCM "for free" on SSBO loads and stores, without having
> to do the equivalent thing using dataflow analysis. Also, doing store
> forwarding (i.e. replacing the result of an SSBO load with the value
> corresponding to a store, if we can figure out which store affects it)
> is going to much easier. However, #1 is going to be much more of a
> research project. I've thought about how we could do it, but I'm still
> not sure how it could be done feasibly and still be correct.

Thanks for sharing these ideas. #1 looks like the best way to go in
terms of benefits (although it looks rather artificial!), however I am
not sure that my understanding of NIR at this moment is good enough to
pursue something like that. Also, I would really like to see some sort
of support for this landing soon, even if limited, since having 16 SSBO
loads for a single matrix multiplication like the one I mention in the
commit log for my second patch is really bad, and even the simplest
version of #2 would address that, so I think I'll give #2 a try.

Iago